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  • Published: 27 October 2021

Global potential for harvesting drinking water from air using solar energy

  • Jackson Lord   ORCID: orcid.org/0000-0002-8242-3060 1 ,
  • Ashley Thomas   ORCID: orcid.org/0000-0003-2098-3801 1 ,
  • Neil Treat 1 ,
  • Matthew Forkin 1 ,
  • Robert Bain   ORCID: orcid.org/0000-0001-6577-2923 2 ,
  • Pierre Dulac 3 ,
  • Cyrus H. Behroozi   ORCID: orcid.org/0000-0002-4067-3707 1 ,
  • Tilek Mamutov 1 ,
  • Jillia Fongheiser 1 ,
  • Nicole Kobilansky 1 ,
  • Shane Washburn 1 ,
  • Claudia Truesdell 1 ,
  • Clare Lee 1 &
  • Philipp H. Schmaelzle   ORCID: orcid.org/0000-0002-5773-596X 1  

Nature volume  598 ,  pages 611–617 ( 2021 ) Cite this article

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  • Computational science
  • Developing world
  • Water resources

Access to safely managed drinking water (SMDW) remains a global challenge, and affects 2.2 billion people 1 , 2 . Solar-driven atmospheric water harvesting (AWH) devices with continuous cycling may accelerate progress by enabling decentralized extraction of water from air 3 , 4 , 5 , 6 , but low specific yields (SY) and low daytime relative humidity (RH) have raised questions about their performance (in litres of water output per day) 7 , 8 , 9 , 10 , 11 . However, to our knowledge, no analysis has mapped the global potential of AWH 12 despite favourable conditions in tropical regions, where two-thirds of people without SMDW live 2 . Here we show that AWH could provide SMDW for a billion people. Our assessment—using Google Earth Engine 13 —introduces a hypothetical 1-metre-square device with a SY profile of 0.2 to 2.5 litres per kilowatt-hour (0.1 to 1.25 litres per kilowatt-hour for a 2-metre-square device) at 30% to 90% RH, respectively. Such a device could meet a target average daily drinking water requirement of 5 litres per day per person 14 . We plot the impact potential of existing devices and new sorbent classes, which suggests that these targets could be met with continued technological development, and well within thermodynamic limits. Indeed, these performance targets have been achieved experimentally in demonstrations of sorbent materials 15 , 16 , 17 . Our tools can inform design trade-offs for atmospheric water harvesting devices that maximize global impact, alongside ongoing efforts to meet Sustainable Development Goals (SDGs) with existing technologies.

Ensuring reliable access to safe drinking water for all remains a global challenge, and is formally recognized as an international development priority by 2030 in the United Nations framework for global development priorities, the Sustainable Development Goals 6.1 18 . Progress towards this target is measured by the WHO/UNICEF Joint Monitoring Programme (JMP) as the percentage of population using safely managed drinking water (SMDW), where ‘safely managed’ is defined as “an improved source located on the premises, available when needed and free of fecal and priority chemical contamination” 1 , 2 . Traditional routes to bring SMDW on premises to currently unserved populations are estimated to cost US$114 billion per year (from 2015), more than three times the historical financing trend 19 . Moreover, there is increasing global interest in solutions that provide safe drinking water without the environmental consequences of increasing reliance on bottled water and that do not require household-level intervention, which has limited adherence 20 , 21 . Atmospheric water harvesting (AWH) shows promise to accelerate decentralized access to underserved communities if a cost-effective, off-grid device can be designed and scaled 6 .

Several classes of off-grid AWH designs exist or are being explored 8 , 12 , 22 , 23 , as summarized in Table 1 . AWH devices are categorized by energy source—active devices use external energy sources whereas passive devices rely solely on atmospheric conditions that allow for pre-condensed dew or fog to be harvested. Passive devices are thus limited to geographic niches where dew or fog can be systematically harvested 7 , 12 , 24 . Active, sorbent-based AWH devices extract water using primarily solar thermal energy in one of two operational modes: diurnal-mode devices extract at night (when RH is higher) and condense during the day (when solar energy is available) in a single daily cycle, requiring a large sorbent bed. By contrast, continuous-mode devices are not limited to a single daily cycle, and need only hold a small amount of water vapour in-process 3 , 4 , drastically reducing sorbent mass and device size. This, however, requires extraction at lower RH when solar energy is available, raising questions about performance 7 , 8 , 9 , 10 , 11 . Cooler–condenser devices use work (typically electric energy) to actively cool air below its dew point and collect condensation and—if solar-driven—call for photovoltaic (PV) panels. Unlike solar–thermal devices, solar-driven cooler–condenser devices suffer from a steep loss in electric energy conversion. In the context of specific yield, we use kWh to denote primary solar energy prior to thermal and other losses, and kWh PV to denote electrical energy supplied to the device from PV panels after conversion. Unless stated otherwise, ranges of SY refer to RH between 30% and 90% at 20 °C.

Here we present an assessment of solar-driven, continuous-mode AWH (SC-AWH) using global data. AWH has much lower SY than infrastructural water sources such as desalination 25 (approximately 200 l kWh −1 ). However, SC-AWH devices sized to produce sufficient daily drinking water output for an individual or family could address both the water quality and the water access dimensions of SMDW solutions at the household level.

Geography of the global challenge

To estimate the impact potential of SC-AWH, we first mapped the distribution of the approximately 2.2 billion people without SMDW 2 . Recent studies have used geostatistical techniques to estimate subnational inequalities of safe water and sanitation from a variety of data sources reporting metrics of facility type 26 , 27 . Here we use a deterministic method based exclusively on JMP data on drinking water service levels. In this study, we assume that SC-AWH is for drinking water only and does not replace water for other domestic uses such as hygiene, cooking and sanitation 14 , 28 .

The overall percentage of the population in regions reported by the JMP at the lowest respective available regional hierarchy is shown in Fig. 1a . This seamless fabric of national and subnational survey regions gives a spatially continuous picture of the global distribution of people living without SMDW. Sub-Saharan Africa contains the highest total number of people without SMDW, in alignment with previous reports 2 , 29 , followed by regions in South Asia and Latin America.

figure 1

a , Percentage share of total population in survey region living without SMDW as reported by the WHO/UNICEF JMP. b , Log population density of people without SMDW from WorldPop at 1 km resolution adjusted by JMP proportions at 1 km resolution. Produced in ArcGIS 10.

The regional proportions from Fig. 1a were applied as a linear weight to each pixel of the WorldPop (2017) 1 km-resolution residential population counts image ( https://www.worldpop.org ). This gives an estimate of the distribution of people without SMDW to a spatial resolution that more closely matches the scales at which climate variables relevant for AWH vary owing to physical geography, such as topography and land cover. The resulting weighted population distribution is shown in Fig. 1b .

Geospatial toolset for AWH assessment

We present a geospatial tool (AWH-Geo) for assessing the global potential for notional SC-AWH devices given available climatic resources. AWH-Geo was built in Google Earth Engine 13 and is extensible across climate data. For this study, AWH-Geo uses the ERA5-Land climate reanalysis over the 10-year period 2010–2019 (inclusive). ERA5-Land was chosen for its fine resolution (9 km at hourly intervals), global coverage and ability to represent historical synoptic conditions. This period is sufficient to account for interannual variability, although decadal trends are explored in brief in Extended Data Fig. 9 . For shorter computation times running their own analysis, the user can adjust the analysis period within the tool.

AWH-Geo takes as input the instantaneous rate of water output as a function of the three dominant environmental variables: (1) global horizontal irradiance from sunlight (GHI (in W m −2 )), (2) RH (%) and (3) air temperature ( T (°C)). Secondary climate variables could be incorporated later (for example, downwelling infrared and surface wind speed). We propose an output table with water yield values as a function of binned climate inputs GHI, RH and T , as a way to connect AWH device models or experimental characterizations with geospatial analyses. Water output can be entered in areal harvesting rates (in l h −1  m −2 ) for abstractions, or as the expected yield of a real device with known collection areas (in l h −1 ). Across all data points of a multi-year climate image time series, AWH-Geo uses the given output table to look up yield values and aggregates water outputs for display as global maps or derived plots. Whereas previous assessments have been limited to relatively small numbers of locations with on-site meteorological data 7 , 30 or limited the analysis to a region 31 , the approach presented here is global and spatially continuous. Figure 2 shows a conceptual workflow of AWH-Geo and adjacent processes to produce results in this study.

figure 2

Cylinders indicate data stores from Google Earth Engine, the WHO/UNICEF JMP or open online content. Shown are processes (rectangles), geo-images (parallelograms) and outputs (circles).

We first used AWH-Geo to map theoretical upper bounds of solar-driven AWH by constructing output tables from the literature as specific water yields SY (in l kWh −1 ). SY is an evaluative metric for AWH sensitive to RH 32 , and is the inverse of specific energy consumption (SEC), which is commonly used for other water and desalination systems. Resulting maps are overlaid with a dot-density representation of the distribution of people without SMDW for visual comparison in Fig. 3 .

figure 3

a – c , Mean daily water output of solar-driven AWH given overall thermodynamic limits of any process 33 ( T hot  = 100 °C) ( a ), cooler–condenser processes driven by PV 32 ( b ) and example of active sorbent device types (TRP gels from ref. 15 ) ( c ). Callout charts in a show select seasonal profiles in bi-weekly intervals of mean output and primary climate drivers: GHI, RH and temperature. Output (in l d −1  m −2 ) normalized to horizontal device area in sunlight. Real devices will perform below maximum theoretical potentials. Overlaid dot density (red) of 2.2 billion people without SMDW. Placement of dots is spatially arbitrary across the survey region. Produced in ArcGIS 10.

Source data

Recently, Kim et al. have described the fundamental thermodynamic limits for AWH 33 . This model gives the minimum thermal energy required (at a given hot-side temperature level) per unit water output of a black box AWH, corresponding to SY values between 5 and 50 l kWh −1 . Kim’s thermodynamic limits are mapped in Fig. 3a . Mapping thermodynamic limits is useful to set maximum expectations for SC-AWH output globally and to assess the improvement potential that may exist between existing device performance and fundamental physical limits. Similar analytic approaches have been used to assess condenser-based devices, diurnal devices and dew collectors applied to a specific location or region 7 , 12 , 30 , 31 . The geographic patterns of output closely follow time-averaged humidity values generally, modified by the availability of sunlight. Notably, the results show significant water production potential throughout much of the world, particularly in the tropics.

Next, we mapped the maximum output of two basic design types. Peeters describes the maximum yield for active cooler–condensers 32 , giving SYs of 1–30  \({\rm{l}}\,{{\rm{kWh}}}_{{\rm{PV}}}^{-1}\) (0.2–6 l kWh −1 ), plotted using AWH-Geo in Fig. 3b . For sorbent designs, metal organic frameworks (MOFs) and thermo-responsive polymer (TRP) gels 17 show the highest yields at low and high RH, respectively. Zhao et al. demonstrated exceptional performance of a TRP 15 at high RH (0.2–9.3 l kWh −1 (converted to SY by Peeters 32 )), generally outperforming MOFs (whose reported maximum 32 SYs are around 1 l kWh −1 ). Global projections for Zhao’s TRP are mapped in Fig. 3c .

In addition to annual means, AWH-Geo is capable of deriving metrics useful for analysing seasonal variability of output. Optionally, AWH-Geo exports 90% availability (P90) values across a set of time windows (Methods).

Assessing the global potential

Our coincidence analysis calculates the mean hours per day during which GHI and RH are simultaneously above parametric thresholds. Fig. 4a maps annual means for such daily coincidence hours for the given threshold pairs, interpreted as the operational hours per day (ophd) for a hypothetical device. Important transition areas between tropical and desert regions show the expected trade-off between sunlight and humidity, which generally vary inversely. Very low RH thresholds of 10% increase ophd potential by only 1–2 h from the ophd at 30% RH in arid regions in the Sahel across GHI thresholds, but ophd then falls sharply at higher RH thresholds. This indicates a diminishing return to devices operating below 30%. Coastal areas show promise for consistent 2–4 ophd worldwide above 50% RH.

figure 4

a , b , Geographic distribution ( a ) and sum ( b ) of population without SMDW living in areas meeting parametric thresholds relevant to operation of SC-AWH devices. Operational hours per day (Ophd) is the mean daily duration of both sunlight (GHI) and RH thresholds exceeded simultaneously. Usage example: a device requiring more than 5 h d −1 of sunlight above 400 W m −2 must operate down to 40% RH to reach approximately 700 million users. c , d , People without SMDW reachable in relation to mean daily output normalized to horizontal device area in sunlight ( c ) and SY profile ( d ). Target curves are hypothetical SY profiles capable of providing 5 l d −1 for a given solar collection area. Water output and SY targets scale linearly with device area in sunlight. For demonstration we therefore show that, for a given RH, doubling the area of a device from 1 m 2 to 2 m 2 halves the target SY requirement to achieve SMDW for a target population. ZMW Source profile approximated from the manufacturer’s technical specifications sheet 35 . Note that the full ZMW panel is approximately 3 m 2 . Experimental values for MOFs and sorbents are taken from experiments 3 , 36 (0.19 l kWh −1 and 0.84 l kWh −1 ), and TRP is taken from ref. 15 , all converted as in ref. 32 . Values for the Bagheri device 34 assume work instead of heat input; therefore photovoltaic efficiencies were applied when converting from GHI. Maps are produced in ArcGIS 10.

Next, we summed the population without access to SMDW segmented by threshold pair using the weighted population image, grouped cumulatively by ophd at whole intervals and shown in Fig. 4b . Inflections of diminishing user potential occur between values of RH between 30 and 50%, GHI between 400 and 600 W m −2 and ophd between 3 and 5 h. These reflect key spatio-demographic patterns along similar climatic transitions in the tropics, where the bulk of those living without SMDW live—particularly in the tropical savanna of sub-Saharan Africa and the Ganges River Valley in India. A device that could operate above these values has the theoretical potential to serve more than half the world’s remaining population lacking access to SMDW.

Next we ran the SY profiles of a collection of SY curves through AWH-Geo, including commercial cooler–condenser devices evaluated by Bagheri 34 and a data sheet for the SOURCE panel, a sorbent-based device from company SOURCE, formerly known as Zero Mass Water 35 (ZMW).

Figure 4c shows resulting outputs normalized by area (in l d −1  m −2 )—a performance metric advocated by LaPotin et al. 11 —as a function of the population without SMDW reached. Steep gradients of the human impact of the output mirror those in the coincidence analysis. Linear SY profiles prioritize performance at low RH, but cap output even in resource-rich climates. The target curves are based on hypothetical SY values similar to those characteristic of sorbent or device profiles that reach 1 billion users at an average of 5 l d −1  m −2 . Comparing the two target curves demonstrates the expected trade-off between serving more users at low output (linear) and fewer users at high output (logistic).

To further explore trade-offs of the SY curve across different values of RH, we plotted SY values from materials and devices in relation to target curves for reaching 0.5–2.0 billion people without SMDW at 5 l d −1 , the approximate daily drinking water requirements of an individual 14 (Fig. 4d ). We based the target curves on a 1 m 2 device unless otherwise noted, although water output and SY targets scale linearly with device area in sunlight. To demonstrate this, we plotted a version of the 1.0-billion target based on 2 m 2 —this doubling of the device area halves the SY requirements for the target impacts. The existing devices both follow approximately linear yields across RH below the 0.5-billion impact target curves. MOFs and other sorbents show varied results 3 , 36 , although they remain roughly linear. Zhao’s exceptional yields at high RH make up for low performance at low RH (logistic profile), and show the most promise for reaching the largest user base (2.0 billion). Figure 4d compares material and device performance side-by-side to show the gap between present capabilities and theoretical limits, although real devices will be subject to losses that will prevent them from fully reaching idealized material performance or theoretical limits.

Closing the gap

This study presents initial conclusions—developing detailed SC-AWH design criteria will require further work. A device with a 1 m 2 solar collection area and a SY profile of 0.2–2.5 l kWh −1 (0.1–1.25 l kWh −1 for 2 m 2 ) can serve the SMDW needs of about 1 billion people, assuming continuous harvesting of 2–3 h per day of coincident sunlight of more than 600 W m −2 and RH above 30%. The shape of the SY curve is critical for SC-AWH to take advantage of coincident humidity and solar energy during key periods of the day, typically during morning and evening hours. A trade-off exists between increasing yields at lower RH (around 30%) for those in climate transition zones (northern sub-Saharan Africa and western India), versus focusing on exponentially higher yields in humid regions such as Bangladesh and equatorial regions.

Researchers and device inventors can cross-reference Fig. 4 when making trade-off decisions between sets of technical specifications and servable regions and people. Recent experiments 4 , 5 , 37 show rapid improvements in multi-cycled sorption material yield, ranging from 0.1 to more than 8.0 l d −1  kg −1  sorbent in outdoor conditions (RH 10–60%, GHI < 1,000 W m −2 ), and show inflections in performance along similar ranges as population distributions 11 , 31 (RH 30–50%, GHI 400–600 W m −2 ). Advancements in device efficiencies from innovative design architectures 38 and novel high-performance physical sorbents 15 , 17 , 39 , 40 , 41 show promise for increasing SC-AWH output. Individual specific yields from materials experiments or prototypes can be plotted in Fig. 4d for benchmarking against target impacts. Validated device performance in outdoor field conditions and published output tables and are needed for global researchers to advance progress of AWH.

The long-term averaged output of an AWH device is an important but limited metric. Seasonal, weekly and diurnal variability in output will influence user adoption and market viability. Some seasonal profiles are explored in Extended Data Figs. 4 – 8 . Short periods of shortfall may be supplemented by storage from previous surpluses. Rainfall collection or alternative sources would be required for seasonal shortfall periods, such as those in monsoon climates. Use of multiple water sources and seasonal switching are well established in the literature, although there may be trade-offs with respect to water quality and contamination 42 , 43 , reinforcing the need for in-depth knowledge of existing water access practices when deploying AWHs, with a focus on household water treatment and safe storage.

The hydro-ecological impacts of AWH for drinking water are probably negligible given the scale of the global atmospheric water budget. Serving all 2.2 billion people without SMDW at 10 l d −1 sums to approximately 8 km 3  yr −1 , a mere 0.20% of the net water extraction of global cropland (4,000 km 3  yr −1 ) and 0.01% of total evapo-transpiration over land 44 (65,500 km 3  yr −1 ).

SC-AWH devices have the potential to be low-cost. Most design architectures have few moving parts (for example, a slowly rotating sorbent wheel 8 ), and can be constructed from widely available components. Advanced sorbent materials (for example, MOFs or TRP) will need to be mass manufactured to reach cost targets. New high-volume manufacturing methods for MOFs 45 , 46 have the potential to drastically reduce costs.

Technology development is only one part of the complex problem of safe water access; user-centric formative research with a wide variety of end users is critical for ensuring that devices are adopted widely. Similar to bottled water 21 SC_AWH devices could paradoxically undermine efforts to develop permanent piped infrastructure. Product affordability and adoption require parallel financial and socio-cultural efforts such as scaling availability of loans, promoting awareness of waterborne disease risk and increasing women’s influence over community decisions 47 , 48 , 49 .

Our analysis demonstrates that daytime climate conditions may in fact be sufficient for continuous-mode AWH operation in world regions with the highest human need. This assessment suggests that focusing device design criteria on maximum impact and reducing costs of off-grid production of drinking water at the household scale is a worthwhile effort.

Water access data processing

Data on drinking water coverage by region was acquired from the WHO/UNICEF JMP. The JMP acts as official custodian of global data on water supply, sanitation and hygiene 2 and assimilates data from administrative data, national census and surveys for individual countries, and maintains a database that can be accessed online through their website. We accessed data tables for national and subnational drinking water service levels from https://washdata.org .

JMP datasets are not geographically linked to official boundary files. We joined the tables to GIS boundaries obtained from the following open-source collections: GADM ( https://gadm.org ), the Spatial Data Repository of the Demographic and Health Surveys Program of USAID (DHS) and the Global Data Lab of Radboud University (GDL) 2 , 50 , 51 , 52 , 53 . Subnational regions reported by the JMP are unstructured, representing various regional administrative levels (province, state, district and others).

The JMP national and subnational data were joined to GIS boundaries using a custom geoprocessing tool built in Python and ArcGIS 10. The tool joins the available JMP subnational-level survey data to the closest name match of regional boundary names from a merged stack of GADM (admin1, admin2 and admin3), DHS and GDL boundaries worldwide. The JMP national-level survey data is then joined to GADM national (admin0) boundaries for countries which have no subnational data available. Finally, the two boundary-joined datasets (national and subnational) are merged, processed and exported as a seamless global fabric of water-stressed-population data at the highest respective spatial resolutions available (Fig. 1a ).

JMP does not report the breakdown between the SMDW and basic service level within subnational regions, and instead reports a combined category called ‘at least basic’ (ALB). To estimate the SMDW values in subnational regions, a simple cross-multiplication was performed using the splits at the national level:

where ALB national , ALB subnational and SMDW national are known values.

Validation of the cross-estimation of share of SMDW from ALB for subnational regions was conducted on a reference dataset of nationally representative household surveys that collected data on all criteria for SMDW 54 , shown in Extended Data Fig. 2 . We report regression results of R 2  = 0.87 and a standard error of 3.67, indicating a bias which over-reports SMDW share and a probable underestimate of people living without SMDW in our study. This discrepancy comes from JMP calculations of SMDW that rely on the minimum value of multiple drinking water service criteria (free from contamination, available when needed and accessible on premise) rather than considering whether individual households meet all criteria for SMDW 55 .

The fraction of population without SMDW was multiplied by residential population values in the WorldPop top-down unconstrained global mosaic population count of 2017 at 1 km spatial resolution 56 ( https://www.worldpop.org ). WorldPop was accessed online as a TIF image and imported to Google Earth Engine. The year 2017 was chosen to more closely match water access data from JMP. The percentages reported by JMP are probably not uniform within most regions 57 , introducing an unknown error to Fig. 1b , but represent the best estimate available to us given the limitations of these regionally reported data.

Climate input and conversion approximations

Ghi and reference plane.

We used GHI (in W m −2 ) as solar energy input data. GHI has good availability in climate datasets and introduces the fewest number of assumptions. Since GHI describes the irradiance in a locally horizontal reference plane, this approximation is only exact for devices having a horizontally oriented solar harvesting area. Annually averaged comparisons between horizontal and optimal fixed-tilt panels show negligible differences in direct plus diffuse radiation in tropical latitudes, and ratios below 25% in locations within 50° north and south latitudes 58 . Those seeking precise absolute predictions for tilted devices or higher latitudes are encouraged to adapt the provided code to their specific assumptions.

Conversion from SY to AWH output

As discussed in the main text, solar-driven AWH devices typically have one of two predominant energy inputs: thermal (converted directly from incident sunlight on the device) or electrical (from PV). Here, the energy units used to calculate yield in l kWh −1 are incident solar energy directly from GHI. The various assumptions are made in relation to the reported values based on their source. The thermal limits 33 , target curves, and experimental results reported by TRP 15 and MOFs were assumed to have direct (100%) conversion from sunlight to heat. For the ZMW device, the table provided by the manufacturer accounts for system losses, so the table values were directly converted in our model 35 . For ref. 34 and the cooler–condenser limits from ref. 32 , which both assume work input instead of heat, we applied a typical PV conversion efficiency of 20% to convert from sunlight kWh (GHI) to kWh PV (electrical work) input to the device 59 .

Sufficiently short sorbent cycling times

AWH-Geo assumes continuous or quasi-continuous AWH. AWH-Geo considers each 1-h timestep independently and is thus stateless. Aside from edge cases, this is a safe assumption for mass efficient SC-AWH devices, which typically have time constants shorter than 1 h, both for sorbent cycling and for most of the thermal time constants. For devices with longer time constants, batch devices or processes with slow (de)sorption kinetics, this assumption may introduce increased error, and may require further adaptation of the provided code.

Climate time-series calculation

AWH-Geo is a resource-assessment tool for AWH. It consists of a geospatial processing pipeline for mapping water production (in litres per unit time) around the world of any solar-driven continuous AWH device that can be characterized by an output table of the form output =  f (RH, T , GHI).

Output tables show AWH output values in l h −1 or l h −1  m −2 across permutations of the 3 main climate variables in the following ranges: RH between 0 and 100 % in intervals of 10%, GHI between 0 and 1,300 W m −2 in intervals of 100 W m −2 , and T between 0 and 45 °C in intervals of 2.5 °C (2,145 total output values). The tables are converted into a 3D array image in Google Earth Engine and processed across the climate time-series image collection for the period of interest. Finally, these AWH output values are composited (reduced) to a single time-averaged statistic of interest as an image.

Climate time-series data was acquired from the ERA5-Land climate reanalysis from the European Centre for Medium-Range Weather Forecasts (ECMWF) 60 , accessed from the Google Earth Engine data catalogue. ERA5-Land surface variables were used in 1-h intervals and 0.1°× 0.1° (nominal 9 km). The 10-year analysis period (2010–2019, inclusive) was used for this work, and represents a period long enough to provide a reasonable correction for medium-term interannual climatic variability.

Climate variables GHI and T were matched to ERA5-Land parameters ‘Surface solar radiation downwards’ (converted from cumulative to mean hourly) and ‘2 metre temperature’ (converted from K to °C), respectively. RH was calculated from the ambient and dew point temperature parameters in a relationship derived from the August–Roche–Magnus approximation 61 rearranged as:

where a is 17.625 (constant), b is 243.04 (constant), T is the ERA5-Land parameter ‘2 metre temperature’ converted from K to °C, and T d is the ERA5-Land parameter ‘2 metre dewpoint temperature’ converted from K to °C.

Spot validation of the climate parameters and the mapped output was performed manually in Google Earth Engine across several timesteps in 2016 in Ames, Iowa (using the Iowa Environmental Mesonet AMES-8-WSW station 62 ) and showed insignificant error (< 5%).

Mapping upper bounds

Figure 3a maps thermodynamic upper bound outputs for SC-AWH based on an equation from Kim et al. 33 , reproduced below.

where \({\dot{Q}}_{{\rm{hot}},{\rm{in}},{\rm{\min }}}\) is the minimum input heat flux (in W heat ) required to drive the process, \({T}_{{\rm{hot}}}\) is the temperature (in K) at which the input heat is delivered, \({T}_{{\rm{ambient}}}\) is the ambient temperature (in K) at which heat is rejected and water and air exit the process, \({\dot{m}}_{{\rm{water}},{\rm{out}}}\) is the production rate of liquid water by mass, \(\omega \) denotes humidity ratios in kg of water per kg of dry air, \({e}\) denotes specific exergies, which can be looked up for given temperatures and humidities, subscript air,in denotes ambient air drawn in at \({T}_{{\rm{ambient}}}\) from which to extract moisture, subscript air,out denotes air exiting the process at \({T}_{{\rm{ambient}}}\) after extracting some moisture from it, subscript water,out denotes liquid water exiting the process at \({T}_{{\rm{ambient}}}\) as the desired product.

Parameters not present in this formula, but that are in Kim’s underlying derivation: this upper limit is obtained for a small recovery ratio (RR ~ 0) chosen for numerical stability and for reversible process conditions (entropy generation,  S gen  = 0).

Kim’s model assumes an AWH in which the fundamental energies required are driven by input heat supplied at a temperature \({T}_{{\rm{hot}}}\) . The limit it represents applies independent of the process, number of stages, sorbent choice, and so on, as long as heat drives the process.

We adapt Kim’s model to solar energy input, assuming an idealized conversion efficiency from solar irradiance to usable heat of 100%. This idealization retains a robust upper bound without bringing in additional parameters. Literature values for theoretical sun-to-heat efficiency limits range from >99.99 to 95.80% for thermal absorbers, depending on the level of angular selectivity 63 .

Rearranged, Kim’s model yields

where, in addition, \({\dot{V}}_{{\rm{water}},{\rm{out}}}\) is the production rate of liquid water by volume, \({A}\) is the area harvesting sunlight (see approximation section below), \({E}_{{\rm{GHI}}}\) is GHI in W sun  m −2 , and \({\rho }_{{\rm{water}}}\) is the density of water.

This is now a function of the three key climate variables: GHI (in the first term), ambient temperature (in the second and hidden in the third term) and RH (entering the third term). This was converted to an output table and processed through the AWH-Geo pipeline and presented in Fig. 3a . While this can be run for any choice of parameter \({T}_{{\rm{hot}}}\) , we present figures here for \({T}_{{\rm{hot}}}\)  = 100 °C, a temperature still achievable in low-cost (non-vacuum) practical devices without tracking or sunlight concentration. Higher driving temperatures increase the upper bound for water output. For the limits analysis, values of RH above 90% are clamped to prevent unrealistically high theoretical outputs as Kim’s equation goes to infinity at 100% RH. A further assumption is made that new ambient air is efficiently refreshed.

Figure 3b maps the maximum yield for active cooler–condensers without recuperation of sensible heat—all given work input and an optimum coefficient of performance of the cooling unit at a condenser temperature that maximizes specific yield as modelled by Peeters 32 , which we digitized from their fig. 11. Peeters chose to set yield to zero whenever frost formation would be expected on the condenser. Since Peeters assumes work input, we convert from solar energy (GHI) to kWh PV as discussed above.

Figure 3c maps Zhao’s experimental results from a TRP using a logistic regression curve fit to their reported SYs of 0.21, 3.71 and 9.28 l kWh −1 at 30, 60 and 90% RH, respectively 15 . The terms of the curve fit are reported in the next section.

Custom yellow to blue map colours are based on www.ColorBrewer.org , by C. A. Brewer, Penn State 64 .

Specific yield and target curves

Two simple characteristic equations, linear and logistic, were used to fit a limited set of SY and RH pairs from laboratory experiments or reported values and plotted through AWH-Geo using calculated output tables. Hypothetical curves of similar form whose terms were adjusted iteratively in AWH-Geo to goal-seek a target output (5 l d −1 ) and user base, and are reported here (for 1-m 2 devices). In the following equations, RH in % is taken as a fraction (for example 55% is equivalent to 0.55).

The linear target curve is a simple linear function which crosses the y -axis at zero:

where a is set to 1.60, 1.86 and 2.60 L/kWh to reach targets of 0.5, 1.0, and 2.0 billion people without SMDW, respectively, and RH is input RH (fractional).

The logistic target curve is a logistic function:

where L is set to 1.80, 2.40 and 4.80 L kWh −1 to reach targets of 0.5, 1.0 and 2.0 billion people without SMDW, respectively, k is the growth rate set to 10.0, and \({\rm{RH}}\) and \({{\rm{RH}}}_{0}\) are input RH (fractional), and 0.60, respectively.

The SY values reported by Zhao for TRPs (which they term ‘SMAG’) were fit to a logistic function of the same form with the following parameters: L set to 9.81 L kWh −1 , k set to 11.25 and RH 0 set to 0.645.

The resulting fitted SY profile is expanded into an output table. As with all reports providing SY values instead of full output tables, this forces an assumption of linearity in heat rate (approximately equal to GHI), which may introduce error at lower GHI levels. Zhao reports SY of the TRP material is consistent across temperature below 40 °C—the material’s lower critical solution temperature—above which its performance drops precipitously. Accordingly, we set the SY to 0 l kWh −1 for temperatures ≥40 °C in the output table.

Bagheri reported performance of three existing AWH devices across several climate conditions using an ‘energy consumption rate’ in kWh/L, which can be considered to be the SEC, and the simple reciprocal of SY. Instead of fitting a logistic curve to the reciprocals, we fit an exponential function to the average SEC of the three devices in conditions above 20 °C of the equation:

where SEC is specific energy consumption in kWh PV  l −1 and RH is fractional.

This was applied to RH and taken as reciprocal in an output table and run through AWH-Geo. Since Bagheri reports the equivalent of kWh PV , we scale to adapt to GHI input with a photovoltaic conversion efficiency as discussed above.

For performance of the ZMW device (the company’s ~3 m 2 SOURCE Hydropanel), we used values from the panel production contour plot in the technical specification sheet available from the manufacturer’s website 35 . The decision for inclusion was made owing to the importance as an early example of a SC-AWH product with commercial intent. Values in l per panel per day were taken at each 10% RH step at 5 kWh m −2 , assumed to represent kWh m −2  d −1 , and divided by 15 kWh (~3 m 2  × 5 kWh m −2 ) to convert to SY in l kWh −1 . From the resulting SY curve, an output table was generated and processed with AWH-Geo.

Coincidence analysis and population sums

The coincidence analysis was run through AWH-Geo across 70 threshold pairs given the full permutation set of RH from 10 to 100% and GHI from 400 to 700 W m −2 threshold intervals, using binary image time series. The resulting mean multiplied by 24 represents average hours per day thresholds are met simultaneously, giving ophd. Below is a functional representation of this time-series calculation:

where \({{\rm{RH}}}_{t,{\rm{px}}}\) is the RH in the map pixel \({\rm{px}}\) at time \(t\) , \({{\rm{RH}}}_{{\rm{threshold}}}\) is the threshold of RH above which the device is assumed to operate, \({{\rm{GHI}}}_{t,{\rm{px}}}\) is the GHI in the map pixel \({\rm{px}}\) at time \(t\) , and \({{\rm{GHI}}}_{{\rm{threshold}}}\) is the threshold of GHI above which the device is assumed to operate.

The population calculation was then conducted on these images in Google Earth Engine.

Zonal statistics were performed on the mean ophd images as integers (0–24) using a grouped image reduction (at 1,000-m scale) summing the population integer counts on the population without SMDW distribution image created previously (derived from WorldPop). This reduction was performed at 1,000 m. Validation was performed in Google Earth Engine on single countries within single ophd zones and showed insignificant error (<2%). The population results were collected as a table (feature collection) and population was summed cumulatively within stacked ophd zones. These were exported to R for plotting in Fig. 4b .

To assess the sensitivity of results to the choice of climate and population dataset, we performed a coincidence analysis (Fig. 4b ) with alternative datasets and provide those results in Extended Data Fig. 1 .

As an alternative climate dataset to ERA-5 (1 h, 9 km), we used NASA’s Global Land Data Assimilation System (GLDAS) 2.1 at 0.25° × 0.25° spatial resolution (nominally 30 km) and 3 h temporal resolution 65 during the period concurrent with the main results, 2010–2019. As an alternative population dataset to WorldPop 2017, we used Oak Ridge National Laboratory’s LandScan 2017 ambient population counts at 1 km spatial resolution 66 . Two results comparisons were calculated: (1) GLDAS calculated with WorldPop 2017 for direct comparison of climate data input, and (2) GLDAS calculated with LandScan for comparison of climate and population dataset substitution.

The intercomparisons suggest there is negligible sensitivity to the population dataset used, but substantial and systematic sensitivity to the climate dataset used, while all intercomparisons agree in main features and qualitative conclusions. The spatially and temporally (3×) coarser GLDAS dataset consistently results in lower predictions of water output and impact than the finer ERA-5 climate reanalysis. We speculate that the 3-h timesteps of GLDAS are insufficient to capture the performance-critical humidity and GHI dynamics throughout the day (probably morning and evening hours), and, similarly, the 30-km pixels are insufficient to resolve fine-scale climate patterns driven by topographic and other microscale physiographic effects. This illustrates the importance of using high-resolution climate datasets.

Variability statistics of AWH output

To go beyond annual averages and study availability, we introduce a set of metrics we named moving average density 90th percentile (MADP90).

The MADP90-t represents a device’s average output rate (l d −1  m −2 ) that will be exceeded for 90% of periods lasting t days at the given location. MADP90 is calculated from the derived P90 value across a probability density function (PDF) of daily mean output during each t -day window in the time series (2010−2019). The result is a scalar that can be mapped spatially. Moving-window periods of 1, 7, 30, 60, 90 and 180 days were examined in this study. MADP90-results are available as additional results and map layers in AWH-Geo.

Extended Data Fig. 3 provides an example set of PDFs for a location in southwest Tanzania. Each of the P90 values correspond to a version of the MADP90 metric corresponding to a moving window period. The P90 value naturally increases with t in most geographic locations as the PDF tightens its dispersion about the natural (P50) mean.

Data availability

The software and datasets generated during and/or analysed during the current study are available in the following repositories. GitHub: https://github.com/AWH-GlobalPotential-X/AWH-Geo ; Figshare:  https://doi.org/10.6084/m9.figshare.c.5642992.v1 ; JMP Geoprocessor package (Python and ArcGIS geoprocessing model); JMP Geofabric dataset (shapefile); population without SMDW image data layer (geoTiff); upper limit AWH output data layers (geoTiff); coincidence analysis results data tables (Sheets); and output tables used in this study (Sheets).  Source data are provided with this paper.

Code availability

The software used during the current study is available as follows. GitHub: https://github.com/AWH-GlobalPotential-X/AWH-Geo ; AWH-Geo application: processor and output viewer with source code; population and result data processing scripts.

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Acknowledgements

We acknowledge contributions from many colleagues, including A. Aron-Gilat, D. Youmans, G.L. Whiting, M. Eisaman, S. Lin, J. Sargent, S. McAlister, S. Chariyasatit, B. Dixon, E. St Jean Duggan, F. Carlsvi, K. Stratton, M. McCoy, R. Hessmer, J. Hanna, H. Riley, P. Watson, M. Day, B. Quintanilla-Whye, A. Ramadan, A. Little and D. Moufarege. We thank the WHO/UNICEF JMP team for guidance on drinking water service estimates, in particular T. Slaymaker, R. Johnston and F. Mitis; the team at Google Earth Engine, in particular S. Ilyushchenko, S. Agarwal, T. Erickson, N. Gorelick, M. Hancher, M. Dixon, M. DeWitt, J. Conkling, N. Clinton, K. Reid, E. Engle, W. Rucklidge and the entire Earth Engine development community for advice; C. Caywood for code review; B. Schillings and J. Gagne for internal sponsorship at X. Funding was provided by Google LLC.

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Contributions

P.H.S. and J.L. conceived the study. J.L., P.D., T.M. and N.T. performed analysis and plots. A.T., N.T., J.L., P.H.S., R.B. and C.H.B. developed arguments. J.L., P.H.S., A.T. and R.B. wrote the paper. This study was conducted as a subset of a larger effort at X, led by P.H.S., M.F., N.T. and A.T., to develop a household AWH as a commercial product, which informed the current study: M.F., N.T. and S.W. led prototype development and experimentation, C.H.B. conducted physical modelling, M.F., S.W., C.T., C.L. and others built devices and conducted experiments, A.T., J.F. and N.K. conducted market and user research.

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Correspondence to Jackson Lord or Philipp H. Schmaelzle .

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We disclose the following potential competing interests. This work was funded by X, The Moonshot Factory (formerly known as Google[x]). X is a part of Alphabet. Both are for-profit entities. X has filed for patent protection for water-from-air devices, on which multiple authors are listed as inventors. Water-from-air devices may represent significant commercial opportunities upon meeting certain metrics. This work may be pursued further in various ways, including as a possible spinout company in which one or more authors may become founders, officers, shareholders, employees or otherwise involved with a financial interest.

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Extended data figures and tables

Extended data fig. 1 comparison of coincidence analysis results to input datasets..

Main results from coincidence analysis (Fig. 4b , people without SMDW served by opH/d of coincident climate threshold) with ERA5-Land and WorldPop 2017 datasets compared with results from (a) GLDAS 2.1 climate and WorldPop 2017 population, and (b) GLDAS 2.1 climate and LandScan 2017 population datasets. Operational hours per day ( opH/d ) shown across global horizontal irradiance ( GHI ) and relative humidity ( rH ) thresholds.

Extended Data Fig. 2 Validation of SMDW using household surveys reporting SMDW at household-level.

(a) Charted and (b) tabulated validation of cross-estimation of percentage safely managed (SM) from at least basic (ALB) drinking water ladders at sub-national (SN) level from national (N) breakdowns using known reference data set at SN level from WHO/UNICEF JMP data. Reference values from nationally representative Multiple Indicator Cluster Surveys integrating water quality testing (ref. SM) compared with our estimates from the JMP Geoprocessor combining JMP sub-national estimates for ALB and national estimates for safely managed drinking water services (est. SM). Ordinary least squares regression (OLS) resulted in standard error (stdErr) as reported. Sample size n  = 15. Table (b) shows main results (ERA5-Land) population counts after adjustment from regression. Population without safely managed drinking water (SMDW) shown across global horizontal irradiance ( GHI ) and relative humidity ( rH ) thresholds.

Extended Data Fig. 3 Visual representation of MADP90 concept from location in Tanzania.

Histograms of moving-averaged output (L/d/m 2 ) across window periods (indicated in days) for a location in Manda, Tanzania. P90 availability value increases as averaging window period increases. P90 values are estimated and for illustrative purposes only.

Extended Data Fig. 4 Select MADP90 metrics of AWH upper bounds.

(a) MADP90-90day, and (b) MADP90-7day values (measure of availability through time) globally for AWH thermodynamic upper bounds (Kim 2020), during ten year 2010–2019 (inclusive) analysis period.

Extended Data Fig. 5 Bi-weekly timeseries of AWH output and climate drivers for equatorial profile in Davao, Philippines.

Bi-weekly mean output (L/d/m 2 ), and climate inputs global horizontal irradiance ( GHI , plotted from 0–1000 W/m 2 ), relative humidity ( rH , plotted from 0–100 %), and temperature (plotted from 0–100 °C) of (a) AWH thermodynamic upper bounds (Kim 2020) during ten year 2010–2019 (inclusive) analysis period for each bi-weekly interval and (b) averaged by bi-weekly period annually during this period, and (c) for the 1 billion user linear target curve for each bi-weekly interval. Example of a steady, low variability output profile characteristic of equatorial tropics.

Extended Data Fig. 6 Bi-weekly timeseries of AWH output and climate drivers for tropical savanna profile in Accra, Ghana.

Bi-weekly mean output (L/d/m 2 ), and climate inputs global horizontal irradiance ( GHI , plotted from 0–1000 W/m 2 ), relative humidity ( rH , plotted from 0–100 %), and temperature (plotted from 0–100 °C) of (a) AWH thermodynamic upper bounds (Kim 2020) during ten year 2010–2019 (inclusive) analysis period for each bi-weekly interval and (b) averaged by bi-weekly period annually during this period, and (c) for the 1 billion user linear target curve for each bi-weekly interval. Example of a seasonal wet-dry tropical savanna climate with moderate semi-annual fluctuations of AWH output driven by rH .

Extended Data Fig. 7 Bi-weekly timeseries of AWH output and climate drivers for tropical savanna profile in Dhaka, Bangladesh.

Bi-weekly mean output (L/d/m 2 ), and climate inputs global horizontal irradiance ( GHI , plotted from 0–1000 W/m 2 ), relative humidity ( rH , plotted from 0–100 %), and temperature (plotted from 0–100 °C) of (a) AWH thermodynamic upper bounds (Kim 2020) during ten year 2010–2019 (inclusive) analysis period for each bi-weekly interval and (b) averaged by bi-weekly period annually during this period, and (c) for the 1 billion user linear target curve for each bi-weekly interval. Example of a seasonal wet-dry tropical savanna climate with pronounced semi-annual fluctuations of AWH output driven by rH .

Extended Data Fig. 8 Bi-weekly timeseries of AWH output and climate drivers for mid-latitude profile in Ulaanbaatar, Mongolia.

Bi-weekly mean output (L/d/m 2 ), and climate inputs global horizontal irradiance ( GHI , plotted from 0–1000 W/m 2 ), relative humidity ( rH , plotted from 0–100 %), and temperature (plotted from 0–100 °C) of (a) AWH thermodynamic upper bounds (Kim 2020) during ten year 2010–2019 (inclusive) analysis period for each bi-weekly interval and (b) averaged by bi-weekly period annually during this period, and (c) for the 1 billion user linear target curve for each bi-weekly interval. Example of a mid-latitude climate with pronounced semi-annual fluctuations of AWH output driven by temperature.

Extended Data Fig. 9 Decadal anomaly of AWH output with logistic SY profile between 2000–2009 and 2010–2019.

(a) Overall mean output (L/d/m 2 ) of 1 billion user target logistic curve at 5 L/d/m 2 during ten year 2010–2019 (inclusive) period. (b) Ratio (%) anomaly of output of same specific yield ( SY , in L/kWh) profile averaged over ten year 2000–2009 (inclusive) period. Red colors indicate increasing AWH output with time between the two decades. Blue colors indicate decreasing AWH output.

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Lord, J., Thomas, A., Treat, N. et al. Global potential for harvesting drinking water from air using solar energy. Nature 598 , 611–617 (2021). https://doi.org/10.1038/s41586-021-03900-w

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Sustainability of Rainwater Harvesting System in terms of Water Quality

Sadia rahman.

1 Department of Civil Engineering, Faculty of Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia

M. T. R. Khan

2 Department of Architecture, Faculty of Built Environment, University of Malaya, 50603 Kuala Lumpur, Malaysia

Shatirah Akib

Nazli bin che din, s. k. biswas.

3 Department of Civil Engineering, Bangladesh University of Engineering & Technology, Dhaka 1000, Bangladesh

S. M. Shirazi

4 Institute of Environmental and Water Resources Management (IPASA), Faculty of Engineering, Universiti Teknologi Malaysia, 81310 Johor, Malaysia

Water is considered an everlasting free source that can be acquired naturally. Demand for processed supply water is growing higher due to an increasing population. Sustainable use of water could maintain a balance between its demand and supply. Rainwater harvesting (RWH) is the most traditional and sustainable method, which could be easily used for potable and nonpotable purposes both in residential and commercial buildings. This could reduce the pressure on processed supply water which enhances the green living. This paper ensures the sustainability of this system through assessing several water-quality parameters of collected rainwater with respect to allowable limits. A number of parameters were included in the analysis: pH, fecal coliform, total coliform, total dissolved solids, turbidity, NH 3 –N, lead, BOD 5 , and so forth. The study reveals that the overall quality of water is quite satisfactory as per Bangladesh standards. RWH system offers sufficient amount of water and energy savings through lower consumption. Moreover, considering the cost for installation and maintenance expenses, the system is effective and economical.

1. Introduction

Dhaka is a densely populated city with an area of 1425 km 2 [ 1 ] which is already labelled as a mega city [ 2 – 4 ]. This significant population craves a larger amount of water for different purposes. Therefore, there is always a shortcoming of supplied water due to an imbalance between demand and supply. Dhaka Water Supply and Sewerage Authority (DWASA) is the only authoritative organization available to deliver consumable water to Dhaka City dwellers. DWASA [ 1 ] provides 75% of total demand of water in which about 87% is accumulated from groundwater sources, and the remaining 13% is collected from different treatment plants. Dhaka presently relies heavily on groundwater, with approximately 80 to 90% of demand coming from this source. Overreliance on groundwater sources is depressing the water level. Every year the groundwater table is dropping down around 1 to 3 m due to the extreme amount of withdrawal. Figure 1 shows the groundwater level depletion trend for Dhaka City. Moreover, scientific studies on the groundwater revealed that excessive exploitation has been lowering the aquifer level, thus limiting natural recharge [ 5 , 6 ]. Additionally, overexploitation for longer periods may account for several natural hazards such as unexpected landslides, sustained water logging, reduction in soil moisture, and changes in natural vegetation [ 2 , 7 – 9 ].

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Groundwater depletion in Dhaka City [ 1 ].

Conjunctive use of groundwater and surface water would be one potential solution to reduce heavy reliance on groundwater. Surface water treatment plants are treating polluted water before delivering it to a supply pipeline. But the level of pollution of surface water has limited the applicability of the treatment process. DWASA supplies 2092.69 million liters of water daily against the current demand for 2815.7 million liters [ 1 ], which indicates that the city is facing a huge shortage of water daily. All the scenarios between water demand and supply prevail the immediate need for adopting alternative solutions to release the pressure on water sources. Moreover, current water practices have limited attention to the climate change impacts on water availability [ 10 ]. Surveys on climate projections provide evidence on critical impacts of climate on natural water sources that eventually affect human societies and ecosystems [ 11 ].

Rainwater harvesting (RWH) could be the most sustainable solution to be included in the urban water management system. It could mitigate the water crisis problem, reduce the burden on traditional water sources, alleviate nonpoint source pollutant loads, control water logging problems, prevent flooding, help in controlling climate change impacts, contribute to the storm water management, and so forth [ 12 – 16 ]. Water scarcity and the limited capacity of conventional sources in urban areas promote RWH as an easily accessible source [ 17 ]. The system could be utilized locally and commercially for securing water demand in water-scarce areas all around the world. Harvested rainwater could be idealized and used like supply water if the water-quality parameters satisfy the desired level. The monitoring of collected rainwater is of great concern as it is the potential for health risk because of the presence of chemical and microbiological contaminants [ 18 ]. Therefore quality assessment of collected water is essential before use. This paper is mainly focused on scrutinizing and assessing water-quality parameters as per allowable limit and also on the financial benefit acquired by using this technique. Finally this paper suggests a rainwater harvesting system as a potential source of water supply in Dhaka City.

2. Water Scenario in Dhaka City

About 75% of total demand of water in Dhaka is supplied by DWASA, and the rest comes from privately owned tube wells. At present DWASA can yield about 2092.69 million liters (ML) [ 1 ] per day in which about 1840.04 MLD is collected from 586 deep tube wells (DTW), and the remaining 252.65 MLD is supplied by two surface water treatment plants [ 1 ]. More details are given in Figure 2 .

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Water production per day in Dhaka city [ 1 ].

Buriganga, Balu, Turag, and Tongi Khal are the main four water bodies surrounding the city and could be an ideal sources of water supply [ 19 , 20 ]. But these water bodies already lost their potentiality as sources of supply due to the huge pollutions. Untreated municipal and industrial wastes make the river water so contaminated that most of the water quality parameters surpassed their allowable level. However, the water supply authority mainly relies on groundwater sources and needs to install more tube wells to fulfill demand [ 21 , 22 ]. Installation of more tube wells must lower the groundwater level. Therefore it is urgent to find a sustainable solution that could alter the usage of groundwater. Rainwater harvesting would be one of the most conceivable and viable solutions to release the pressure on the groundwater table as the system utilizes natural rainwater without affecting groundwater sources.

3. Water Supply and Demand Variation

In order to understand the variation between demand and supply, the total demand needs to be known. That could be calculated through population data and per capita demand. According to Bro [ 23 ], per capita demand for 2006 was about 200 liters, including 10% provisions for commercial use and 40% due to system loss during supply. As per capita demand will be assumed to be decreased in the future by proper inspection and management, for 2015 the total per capita demand will stand at 180 liters per day and for 2025 and 2030 at 160 liters per day. According to DWASA, 2011 [ 24 ], the water supply is about 1356.67 MLD (considering service flow with 40% leakages), and the total demand is 2200 MLD (assuming 85% service area). So the deficit is about 843.33 MLD. As demand is more than just supplied water, deficit prevails, which is increasing every day. Therefore the water crisis becomes a normal issue due to this huge deficit in Dhaka City during the dry period. The trend of deficit is due to difference in demand and supply as shown in Figure 3 . In 1963 the total demand was 150 million liters (ML), which turned into 2240 million liters in 2011 due to the augmentation of the population. Within 48 years demand became 15 times more than expected. In a similar way, the deficit also crosses predicted values. In 1963 the deficit was 20 ML, and in 2010 it became 190 ML, which was more than calculated. But after that, the shortage became something better than in the previous year. This indicates that supply capacity is improving, and authorities are trying to reduce the shortages. The overall deficiency of supplied water triggers the need for augmentation and improvement of the water supply system to meet the increased demand in future [ 5 ].

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Relation among water demand, supply, and deficit in Dhaka City [ 1 ].

Figure 4 shows the variation of the water deficit with the present supply and variation of the population for the projected years. If the present supply prevails for the coming years, the deficit of water will be increasing to a high amount that could not be alleviated within the allowable limit.

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Present water supply, shortage, and population variation for projected years.

Dhaka is located in a hot and humid country, and its annual temperature (25°C) categorizes the city as monsoon climate zone. The city is blessed by a huge amount of rainfall during the monsoon period, which poses ample opportunity to use this rainwater in a sustainable manner [ 25 ]. Figures ​ Figures5, 5 , ​ ,6, 6 , and ​ and7 7 show the monthly rainfall pattern, monthly average relative humidity, and the maximum and the minimum monthly temperature trend, respectively, for Dhaka City.

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Monthly average rainfall in mm in Dhaka City.

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Monthly average relative humidity (%) in Dhaka City.

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Maximum and minimum temperature (°C) trend in Dhaka City.

The common practices of recharging natural aquifers are by direct rainfall, river water, and direct infiltration and percolation during floods [ 26 ]. Overpopulation makes these options inappropriate by reducing the recharge area. Covering the vertical recharge inlets with pavement materials or other construction materials can cause water logging for even small duration heavy rainfall in most areas of Dhaka City. Inadequate storm water management infrastructures and improper maintenance of storm sewer systems further aggravates the scale of this problem. Harvesting of this storm water in a systematic way thus prevents water logging. Furthermore, utilization of collected rainwater highly releases the dependency on groundwater sources.

4. Rainwater Harvesting

Rainwater harvesting is a multipurpose way of supplying usable water to consumers during a crisis period, recharging the groundwater and finally reducing the runoff and water logging during the season of heavy rainfall. Traditional knowledge, skills, and materials can be used for this system. During the rainy season, an individual can collect water on his rooftop and manage it on his own. Reserved rainwater on rooftops can be used for self-purposes or domestic use. Water from different rooftops of a lane can also be collected through a piped network and stored for some time. This water can be then channeled to deep wells to recharge groundwater directly, to ponds to replenish groundwater slowly, and to reservoirs to dilute reclaimed water for nonpotable use. Figure 8 shows the schematic view of a rainwater harvesting system.

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Schematic of a rainwater harvesting system.

Unless it comes into contact with a surface or collection system, the quality of rainwater meets Environmental Protection Agency standards [ 27 ], and the independent characteristic of its harvesting system has made it suitable for scattered settlement and individual operation. If needed, a chemical treatment such as chlorination can be used to purify the water. The acceptance of rainwater harvesting will expand rapidly if methods are treated such as building services and if designed into the structure instead of being retrofitted [ 28 ].

5. Benefits of Rainwater Harvesting

Rainwater harvesting is a simple and primary technique of collecting water from natural rainfall. At the time of a water crisis, it would be the most easily adaptable method of mitigating water scarcity. The system is applicable for both critical and normal situations. It is an environmentally friendly technique that includes efficient collection and storage that greatly helps local people. The associated advantages of rainwater harvesting are that

  • it can curtail the burden on the public water supply, which is the main source of city water;
  • it can be used in case of an emergency (i.e., fire);
  • it is solely cost effective as installation cost is low, and it can reduce expense that one has to pay for water bills;
  • it extends soil moisture levels for development of vegetation;
  • groundwater level is highly recharged during rainfall.

6. Quality of Rainwater

The quality of harvested rainwater is an important issue, as it could be utilized for drinking purposes. Quality of captured water from roof top depends on both roof top quality and surrounding environmental conditions, that is, local climate, atmospheric pollution, and so forth [ 11 ]. Tests must be performed to check its viability and applicability before using as drinking water. Previous researches [ 29 – 31 ] showed that water quality of collected water did not always meet standard limits due to unprotected collection. Local treatment of harvested water could easily make water potable. Again rainwater could be also identified as non-potable sources for the purpose of washing, toilet flushing, gardening, and so forth, where quality is not a great concern. In this respect, treatment of collected water is of no such importance; rather it is used for household purposes. In this paper an assessment has been made on the quality of rainwater collected through a well-maintained catchment system.

7. Methodology

Rainwater harvesting is a more effective technology that could be easily undertaken through normal equipment during a water crisis. Qualitative assessment is important before introducing collected rainwater as potable water. In this paper, a case study has been made to check rainwater quality to identify its acceptability and suitability as household water. Water samples were collected from the selected residential building where a rainwater harvesting system was introduced successfully using laboratory prepared plastic bottles to collect samples. The samples were bottled carefully, so that no air bubble is entrained in the bottle. All parameters were measured in the environment laboratory of Bangladesh University of Engineering Technology (BUET).

The maximum amount of rainwater that could be encountered from a roof top is

where V is the amount of harvestable water, A is catchment area, R is total amount of rainfall, and C is the runoff coefficient.

Equation ( 1 ) was used to calculate the amount of harvested water from a residential building located at Dhaka, Bangladesh. The system was designed for meeting water requirements of 60 persons living in the entire building. Total area was about 3600 sq. ft. (square feet). Maximum ground coverage would be around 2250 sq. ft. (considering the floor area rule of RAJUK, the city development authority), and within this area 1850 sq. ft was used as catchment area where rainwater was collected. Per capita water consumption is about 135 lpcd for conservative use. The total demand for this building stands at about 8100 liter per day and 243,000 liters per month. In a practical case, the size of the catchment area is taken from maximum ground coverage. To get an overview of the amount of collected rainwater, monthly average rainfall data from January to December has been considered, including the dry and monsoon periods. The runoff coefficient value was taken as 0.85. For analysis purpose, a one-year rainfall data were considered. Volume of collected rainwater was also an important aspect in introducing rainwater for domestic purposes. In the selected time frame, maximum volume of water was collected during June, 2012, which was about 4.5 m 3 and a minimum was collected during October, 2011. Significant amount of water could be collected during heavy rainfall. From this point of view, it could be said that, with larger catchment area, amount of harvested water would be significant to be used in household works.

8. Results and Discussion

The main focus of this paper relies on several aspects, such as examining the quality of water with respect to standard values, analyzing associated financial benefits in terms of cost, and considering water and energy conservation and lastly suggesting the system as a potential source of water both in normal and critical situations.

In this section, the quality of harvestable water was checked considering several parameters such as pH, fecal coliform, total coliform, total dissolved solids, turbidity, NH 3 –N, lead, and BOD 5 . The time period for analysis was from October 2010 to October 2011. Two different collecting points were considered: water collected before entering into the storage tank (called first flush water) and water collected from the storage tank (tank water). Figure 9 shows the variation of pH over time. According to Bangladesh standards for drinking water [ 32 ], the allowable limit for pH is 6.5 to 8.5. Results showed that pH value for both flash and tank water was very near to this range during the tested time period. Therefore, the pH level of collected water did not pose any threat to water quality and conformed to the standard limit.

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Variation of pH over time.

Figure 10 shows the variation of total coliform over time. The number of total coliforms present in the water was quite low until June 2011. After that a large number of total coliform grew in both flash and tank water. Figure 11 shows the variation of fecal coliform over time. In the case of drinking water, it is expected that water should be free from all types of fecal and total coliforms. In the present case, at first in October 2010, few fecal coliforms were found in water. It remains zero until March 2011. But after that there was an increasing trend in the number of fecal coliform. In October 2011, there was huge number of fecal coliform, which is not expectable for drinking water. In both cases (fecal and total coliform), at first when rainwater was harvested, growth of coliform was lower but with time those increased to a large quantity. From June 2011, rainfall was not adequate and maintenance was not proper, which is why coliform grew to a huge quantity in the stored unused water. As pure water should be free from all kinds of coliforms, proper maintenance of tank and catchment areas could minimize coliform level and make rainwater safe for household purposes.

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Variation of total coliform over time.

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Variation on fecal coliform with time.

Figure 12 shows the variation of total dissolved solids over time. The allowable limit for total dissolved solids (TDS) in drinking water is about 1000 (mg/L) according to Bangladesh standards for drinking water [ 32 ]. For all the selected periods, the total dissolved solids in collected water were quite lower than the standard limit. Therefore total dissolved solids did not pose any threat to water used for drinking purposes. Figure 13 shows the variation of turbidity over time. The standard limit for turbidity is 10 NTU. The measured turbidity level in collected water was below this standard limit. Therefore rainwater could be considered satisfactory from an aesthetic point of view. In a similar way, the NH 3 –N level was quite below the standard limit (0.5 mg/L) during the collection period ( Figure 14 ).

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Variation of total dissolved solids over time.

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Variation of turbidity over time.

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Variation of NH 3 –N over time.

Figure 15 shows the variation of BOD 5 in the collected flash and tank water. In all of the selected time period, BOD 5 is less than the Bangladesh standard for drinking water [ 32 ]. Another thing, BOD 5 became less in flash water than in tank water. Due to the lack of proper maintenance, BOD 5 increased in the tank water. Further treatment may make water more usable for household work. In order to analyze the water quality in terms of lead concentration in collected water, tests were performed, which found that lead concentration always remained below the allowable limit according to the Bangladesh standards for drinking water [ 32 ]. Figure 16 shows the variations of lead concentrations with time.

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Variation of BOD 5 over time.

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Variation of lead over time.

9. Cost Effectiveness Analysis

Thefinancial benefit associated with a rainwater harvesting system is solely connected with cost. The associated costs of a rainwater harvesting system are for installation, operation, and maintenance. Of the costs for installation, the storage tank represents the largest investment, which can vary between 30% and 45% of the total cost of the system dependent on system size. A pump, pressure controller, and fittings in addition to the plumber's labor represent other major costs of the investment. A practical survey showed that (in Dhaka) the total cost related to construction and yearly maintenance of a rainwater harvesting system for 20 years' economic life is about 30000 BDT. This cost includes construction cost of tanks, gutters, and flushing devices and labor cost [ 33 ]. In the present case study, about 313.80 thousands liter water can be harvested from rain over one year. This amount of water could be collected within 1850 sq. ft catchment area and considering monthly rainfall data. The yearly consumption of this selected building stands at 2916 thousands liters. Therefore utilizing harvested rainwater for this building can save up to 11% of the public water supply annually. This volume of rainwater can serve a building with 60 members for about 1.5 months in a year without the help of traditional water supply. Figure 17 shows the month-wise harvestable amount of rainwater and the associated amount of cost savings. Furthermore, considering DWASA current water bill, about 8359.70 BDT can be saved per year, and about 125395.30 BDT can be saved in 15 years if rainwater is used for daily consumption. So, within three to four years, the installation cost of a rainwater harvesting system can be easily returned. Moreover, the building owner would be exempted from paying large amount of water bill as well as additional taxes and fees charged by the city authority with the water bill if rainwater is utilized for daily consumption. Cost comparison and associated benefit between a rainwater harvesting system and traditional water supply system encountered and revealed a rainwater harvesting system as a cost-effective technology.

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Month-wise harvestable amount of rainwater and the associated cost savings.

10. Water Savings Strategy

Rainwater harvesting system plays an important role in developing sustainable urban future [ 24 ]. Availability of water of serviceable quality from conservative sources is becoming limited day by day due to huge demand. Rainwater provides sufficient quantity of water with small cost. Hence, the system can promote significant water saving in residential buildings in many countries. Herrmann and Schmida [ 35 ] studied that potential saving of roof captures water was about 30–60% of potable water demand in a house depending on the demand and catchment area. Coombes et al. [ 36 ] analyzed 27 houses in Australia with rainwater harvesting system and found that about 60% of potable water could be saved. Ghisi et al. [ 37 ] performed investigation on collected rainwater in Brazil and found that about 12–79% of potable water could be saved depending on the size of roof tank. Most of the researches on rainwater harvesting systems (RWHS) revealed that water conservation achieved through RWHS is quite significant especially in places where water is not easily available to consumers.

11. Energy and Climate

Conventional use of water imparts critical impacts on natural resources. Water collection from ground and surface sources, treatment, and distribution are closely associated with energy consumption, however, being related to climate consequences. The extraction of water from the sources, the treatment of raw water up to the drinking standards and the delivery of water to the consumers require high energy. Moreover, there should be some energy losses during performing extracting, treating, and delivering of water. Therefore, the water sector consumes a huge amount of electricity from local and national grid. Approximately 300 billion kilowatt hours of energy could be saved if potable water demand could be reduced by 10% [ 38 ]. Adoption of RWHS is one of the most potential solutions that could save energy directly by reducing potable water demand. Table 1 represents the estimated energy required to deliver potable water to consumers. Reduction of water demand by 1 million gallons can result in savings of electricity use by 1,500 kWh. In the present case study, with an 1850 sq. ft. catchment area, about 69,026 gallons (313.8 thousands liters) could be harvested over one year. However, this amount could reduce potable water demand and approximately 100 kWh electricity could be saved in the selected residential building by introducing rainwater capturing system. Integrating rainwater harvesting system with the conventional water collection and distribution approach in residential as well as large scale, nonresidential applications suggest a potential method of reducing energy use. However, limiting energy demand has critical impact on carbon dioxide emissions, as release of carbon dioxide is closely associated with electricity generation. There should have sufficient reduction in carbon dioxide emissions when fossil fuel is used for power generation. Hence, limited contribution is to be expected from lower carbon release in climate change concept. Table 2 showed the carbon dioxide emissions from electric power generation.

Energy consumption in conventional water resources system [ 34 ].

Carbon dioxide emission from water treatment and distribution system [ 39 ].

However, water use should be critically judged from availability, safety, and sustainability of natural resources. Energy conservation is a critical component in sustainability concern. Decreased use of conventional potable water reduces energy demand that in turn reduces emission of carbon dioxide. Integrated water management approach with rainwater harvesting along with gray water and reclaimed water reuse could limit contributions to climate change and conserve limited water and energy resources.

12. Future Action Plan

Rainwater is one of the advantageous methods of using natural water in a sustainable manner. Rain is a blessing of nature. Densely populated cities with a water crisis and adequate rainfall should adopt this technology. Cities like Dhaka, where water is a major concern during dry periods, should introduce this system along with its traditional water supply system. Pressure on groundwater tables thus could be prevented, and natural recharging would also be proceeded through this system. Regular maintenance of harvested water might make it suitable for daily consumption. Water shortages will become the most concerned issue all around the world in the future. Therefore city planners should rethink of the possibilities, outcome, and benefits of a rainwater harvesting system and should create policies to make the system easily available to everyone. The following research could be made in future.

  • This study focused only on rainwater harvesting system on a small scale basis. Further research could be performed on large scale residential, commercial or industrial sector.
  • Comparisons could be made with rainwater harvesting systems to conventional ground water system on the basis of quality, quantity, environmental impacts, energy saving, water conservation, economy, and so forth.
  • Case studies could be investigated to evaluate energy consumption in rainwater system with ground water system in a large scale. In a more applied setting, energy efficiencies of large scale rainwater harvesting systems should be analyzed to help determine the future of rainwater harvesting as a valuable technology for providing water, a crucial resource that is becoming more depleted with the ever increasing population and water demand.
  • A comprehensive cost-benefit analysis should be performed on different climate regions to get essential insight on the economic viability of rainwater harvesting system (RWHS).
  • More detailed and advanced research on impacts on climate factors, human health risk, and potential ecological aspects should be performed in a large scale.
  • More comprehensive studies for better quantification of energy and climate factors should be made for proper development of the system.
  • Rainwater could be highly polluted by pesticides in any agricultural region. Hence, biological and chemical analysis should be done before adopting harvested rainwater as a source of daily water.

13. Conclusion

Water shortage is one of the critical problems in Dhaka City. This problem is not new one, and it cannot be solved overnight. As DWASA relies on groundwater abstraction through deep tube wells to overcome the excessive demand, the water table is lowering day by day, and the recharge of groundwater table is facing difficulties. Rainwater harvesting is an effective option not only to recharge the groundwater aquifer but also to provide adequate storage of water for future use. This paper tried to focus on the sustainability and effectiveness of a rainwater harvesting system in terms of quality. Water was collected in a well maintained catchment system from rain events over one year and chemical analysis was performed regularly to observe the quality of collected water. The overall quality of rainwater was quite satisfactory and implies that the system could be sustained during critical periods as well as normal periods. Additionally, the system is cost effective as large amounts of money can be saved per year. Energy conservation and related reduced emissions are crucial parts of this system. Moreover, increased awareness on water crisis has led rainwater harvesting to be proposed as a community facility. The small and medium residential and commercial construction can adopt this system as sustainable option of providing water. It is almost the only way to upgrade one's household water supply without waiting for the development of community system. The system could become a good alternative source of water supply in Dhaka City to cope up with the ever-increasing demand and should be accepted and utilized by the respective authorities as well as by the city dwellers.

Acknowledgments

The authors gratefully acknowledge the support of Bangladesh University of Engineering and Technology (BUET). This research is financially supported by University Malaya Research Grant (UMRG) RP009/2012 and High Impact Research Fund, Project no. UM.C/625/1/HIR/MOHE/ENG/61.

Conflict of Interests

The authors declare that there is no conflict of interests regarding the publication of this paper.

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Issue Cover

Article Contents

1. introduction, 2. atmospheric fog harvesting, 3. dew water harvesting, 4. discussions and conclusions, acknowledgments.

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Review of sustainable methods for atmospheric water harvesting

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Hasila Jarimi, Richard Powell, Saffa Riffat, Review of sustainable methods for atmospheric water harvesting, International Journal of Low-Carbon Technologies , Volume 15, Issue 2, May 2020, Pages 253–276, https://doi.org/10.1093/ijlct/ctz072

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The scope of this paper is to review different types of sustainable water harvesting methods from the atmospheric fogs and dew. In this paper, we report upon the water collection performance of various fog collectors around the world. We also review technical aspects of fog collector feasibility studies and the efficiency improvements. Modern fog harvesting innovations are often bioinspired technology. Fog harvesting technology is obviously limited by global fog occurrence. In contrast, dew water harvester is available everywhere but requires a cooled condensing surface. In this review, the dew water collection systems is divided into three categories: i) dew water harvesting using radiative cooling surface, ii) solar-regenerated desiccant system and iii) active condensation technology. The key target in all these approaches is the development of an atmospheric water collector that can produce water regardless of the humidity level, geographical location, low in cost and can be made using local materials.

Globally, the number of people lacking access to water is 2.1 billion, while 4.5 billion people have inadequate sanitation and clean water source [ 1 ]. The latter, has led to risk of infected by diseases, such as cholera and typhoid fever and other water-borne illnesses. As a result, the world has witnessed 340 000 children under five die each year from diarrheal diseases alone [ 1 ]. Clearly, water scarcity is an issue requiring urgent action. The situation is exacerbated by climate change causing rainfall patterns to change with some areas already experiencing prolonged droughts.

Worldwide, many methods have been used to harvest water such as through water desalination, ground water harvesting and rain water collection and storage. Obviously, for these to work liquid water must already be available, but when such supplies are limited, harvesting atmospheric water becomes essential. Therefore, not surprisingly, it is now receiving considerable attention from researchers worldwide. This paper reviews this work, discussing the various water harvesting technologies and their performance, both theoretical and experimental. Commercialized atmospheric water harvesting technologies are also described. We hope this review will help new workers wishing to enter this important field by providing introduction to state-of-the-art technologies and inspire them to develop their own ideas for innovative and sustainable atmospheric water harvesting technology. We believe that general readers, with an interest in the welfare of ‘water poor’ people, will also find this paper useful by showing how emerging water harvesting systems can contribute to improve living standards.

Figure 1 shows how atmospheric water harvesting technologies may be classified. The first category is harvesting water from fog, i.e. a visible cloud water droplets or ice crystals that are suspended in the air at or near the Earth’s surface [ 2 ]. It normally occurs due to added moisture in the air or falling ambient air temperature. Methods may be usefully divided into ‘traditional’ and ‘modern’.

 Categories of atmospheric water harvesting techniques.

 Categories of atmospheric water harvesting techniques.

The second collection category is the collection of water vapour. While fog is visible to our naked eyes, water vapour is invisible and is generated by the evaporation of liquid water or the sublimation of ice. When water vapour condenses on a surface cooled temperature below the dew point temperature of the atmospheric water vapour, ‘dew water’ will be formed [ 3 ]. While fog water harvesting system are more related to traditional concept using a mesh-like structure, there are various technologies related to dew water harvesting technique. The early studies involved passive systems using radiative condenser, but their low efficiencies resulted in, researchers introducing solar-regenerated desiccant methods to enhance the moisture sorption and desorption, however, still has not proved on its own to be sufficient. Thus, research in dew water harvesting also covers integration with active cooling condenser technology that covers the use of typical vapour compression air conditioning system and most recently, thermoelectric cooler. Due to the high in efficiency of active cooling condenser systems, at the end of this paper, readers will be presented with selected commercially available technology on water harvesting technology involving active cooling condensing system.

2.1. Fog collector inspired by traditional concept

Illustrated in Figure 2 , the traditional fog collecting method is very simple, comprising a mesh exposed to the atmosphere over which the fog is driven by the wind. Two posts on guy wires are used to support the mesh and cables to suspend the mesh. Water droplets trapped by the mesh accumulate and drain under gravity into the channels of the water collection system. Collectors can be usefully classified as standard fog collectors (SFCs) and large fog collectors (LFCs) [ 2 ]. SFCs are typically used in a small scale exploratory studies to evaluate the amount of water that can be collected for a specific condition. The collector has a typical size of (1 × 1) m 2 surface with a base of 2 m above the ground [ 4 ]. LFCs, typically 12 m long and 6 m high has mesh covers the upper 4 m of the collector giving ~48 m 2 of water collection area. They are mainly used for actual harvesting installation. For maximum efficiency, fog collectors should be positioned perpendicularly to the prevailing wind. Typically, LFSs produce 150 l to 750 l of water a day depending on the site [ 5 ]. Reported in 2011, the cost for a unit of 48 m 2 fog collectors is US$400 meanwhile, the 1 m 2 SFCs cost from US$100 to US$200 to build depending on the country and the materials [ 5 ].

 The basic concept of fog collector. Adapted with permission from [6] Copyright (2013) American Chemical Society.

 The basic concept of fog collector. Adapted with permission from [ 6 ] Copyright (2013) American Chemical Society.

 An example of Raschel mesh used in a project by Fog Quest [10].

 An example of Raschel mesh used in a project by Fog Quest [ 10 ].

where f is defined as the ratio of mesh openings area to the total screen area.

Along the longitudinal direction, the mesh filament is tied up continuously, meanwhile transversely, we can see that the filaments are not continuous but knotted to the longitudinal one [ 8 ]. A leading developer of fog harvesting technology based on Raschel-weave shading mesh is the non-profit registered Canadian charity, FogQuest, ( www.foqguest.com ), which ‘is dedicated to planning and implementing water projects for rural communities in developing countries’. Their first fog water harvesting experience dates from 1987. In addition to innovative fog collectors, they have also included rainfall collectors to make optimum use of natural atmospheric sources of water.

2.1.1. Selected projects from the past 30 years to current

Fog harvesting is common in arid and semi-arid areas close to the ocean where clouds are formed over the sea and pushed by the prevailing winds towards the mainland. The clouds would become fog when they intercept with the surface of highlands near to the sea. There are various fog collector installation, both for research and real applications in different places such as Namib Desert, Africa. The desert is well known for its potential in harvesting water through fog collection. Mtuleni et al. [ 11 ] conducted an interesting research to find out the quality of the Namibian fog water. Fourteen SFCs were studied at three Topnaar villages in Namib Desert [ 11 ]. The highest water collection was 2.122 l/m 2 at Klipneus village. In terms of the water quality, after a non-foggy period, the initial rinse of SFCs give turbid, brackish water that contains 1630 mg NaCl [ 11 ]. The water was considered as marginally fit for human consumptions. Nevertheless, the subsequent water collected after the initial rinse was found fairly cleaner and has low salt content. In the Coquimbo region of Chile, in 1980s, a research project involving fifty 48 m 2 fog collectors was conducted [ 12 ]. Forty-one new large fog collectors were installed to provide fresh water supply for 100 families benefited, supported initially by the foreign partners and then given over to the local population in the 1990s [ 12 ]. However, due to the incompetency of the local non-governmental organisation(NGO) in terms of technical skills, the project was reported degraded. Large fog collectors were also developed from 1995–99 utilized mainly for reforestation and restoration of degraded coastal ecosystems near the town of Mejia, Peru [ 13 , 14 ]. In Pachamama Grande, Ecuador a large scale project was developed such that 40 LFCs were constructed throughout 1995–97 with the collection efficiencies are as high as 12 l per square metre per day [ 15 ]. Also in the 1990s, in Oman, a major fog collector study was conducted. Daily average collection rates were reported to be as high as 30 l/m 2 . However, the large amount of water collected only happens during monsoon season that occurs about only 2 months in a year. This was considered as a huge limitation to the use of fog collectors in that region [ 16 ]. The following Table 1 listed more fog collection projects carried out worldwide.

The selected fog collector projects worldwide.

2.1.2. Fog collectors design

For LFCs, the prevailing wind imposes pressures on the mesh which then imposes forces on the supporting structures and finally weakening/break the foundation. Meanwhile, the mesh and other components of LFCs can be damaged by UV radiation and also other environmental factors. Lacking in rational or engineered design process of LFCs being the main reason to the collapse of LFCs under extreme weather. This apparently explains the maintenance issue faced by the local people in managing fog collectors [ 8 ]. In order to suit different environmental conditions for examples for very windy sites, robust materials for the fog collectors were made using stronger stainless steel mesh, co-knitted with poly material. See Figure 4 [ 2 ].

 The examples of robust materials. Left: is a robust material with a stainless mesh, co-knitted with poly material. Right: a 3D net structure (1 cm thickness) of poly material [2].

 The examples of robust materials. Left: is a robust material with a stainless mesh, co-knitted with poly material. Right: a 3D net structure (1 cm thickness) of poly material [ 2 ].

Various collector designs have also been researched by Lummerich and Tiedemann [ 22 ] in a field study on the outskirts of Lima (Peru) to address crucial aspects of economic competitiveness of fog water harvesting. Prior to the field testing, five small scale prototypes with different shapes and materials were tested in selecting the most effective fog collector structure. Following the small scale testing, three different types of large scale fog collector were investigated termed ‘Eiffel’, ‘Harp; and ‘Diagonal Harp’. The ‘Eiffel Collector’ is an example of a 3D collector that is used at places with a rare condition with no unique wind direction associated with the occurrence of fog. In their report, a three-winged screen called astropod was introduced as an improved means to evaluate the amount of water yield by fog water harvesting. The use of astropod allowed the measurement of the favourable wind direction and absolute amount of collected fog at the same time. The fog collector designs and the description are summarized in Table 2 .

Selected fog collector designs [ 22 ].

A unique design of fog collector called cloud harvester has been designed by Choiniere-Shields [ 23 ], see Figure 5 . The concept of cloud harvester is based on a fog catcher that turn the condense fog into water droplet. In comparison to the current model available on the market, the unique part of in the design of cloud harvester is that it uses stainless steel mesh instead of the polypropylene nets with an extra sheet under the net for the water collection. The cloud harvester is expected to have a better condensing efficiency and much smaller than the similar products that are currently on the market. The cloud harvester has a potential water harvesting output of 1 l of fresh water per hour for each 10 square feet of mesh [ 23 ].

 The concept of the cloud harvester. The harvester is designed to catch and condense fog into water droplets that in turn run down on a stainless steel mesh into a gutter type extrusion leading to a water storage container [23].

 The concept of the cloud harvester. The harvester is designed to catch and condense fog into water droplets that in turn run down on a stainless steel mesh into a gutter type extrusion leading to a water storage container [ 23 ] .

Aiming to harvest water from the atmosphere to supply fresh drinking water to the community in the developing world, a unique wooden atmospheric water harvesting project called Warka Water has been founded by Arturo Vittori [ 24 ]. The project won the World Design Impact Prize 2015–16 at World Design Capital(R) Taipei 2016 Gala [ 25 ]. Arturo and his team have developed 12 different prototypes since 2012. Figure 6 shows an example of the prototype and its working principle.The team’s target is to develop a prototype that is lightweight (about 80 kg), easy and quick to build using local materials without using scaffolding and power tools. They intend to use bamboo for the frame structure, while the water catchment system will be made from biodegradable mesh 100% recyclable materials. Fog and dew, and also rainwater, will be collected when they strike the mesh and then trickle down a funnel into a reservoir at the base. To prevent water evaporation, a fabric canopy will be used to cover the lower section of the water collector. There is no indication of the amount of water that can be produced by the prototype since the project is still in the exploratory phase. However, the aim of the project is to produce water from fog or highly humid places between 50 to 100 l per day [ 26 ].

Water bamboo tower top: the working prototype and bottom: the concept [24].

Water bamboo tower top: the working prototype and bottom: the concept [ 24 ].

2.1.3. Fog collector efficiency and feasibility studies

(i) Fog passing around the fog water collector.

(ii) Fog passing through the openings of the mesh.

(iii) Droplets bouncing back into the airflow.

For the fraction of the fog that is captured by the fog water collector, we call this fraction as fog interception efficiency [ 9 ].The captured water droplet merged, move to the lower part of fog collector, reached the water gutter and transported to the water tank. However, at water gutter, there is a potential of re-entrainment or water can return back to the air flow or some water from the mesh slack, wrinkles and folds, may be entering the gutter and collected at the water tank.

Where |${{\dot{W}}_{coll}}^{"}$| ( ⁠|$\frac{kg/s}{m^2}$|⁠ ) is the water flow rate collected in the gutter per unit screen area, |${v}_o\left(\frac{m}{s}\right)$| is the unperturbed wind velocity of the incoming fog/air flow and LWC |$\left(\frac{kg}{m^3}\right)$| is the liquid water content of the incoming fog/air flow.

The aerodynamic collection efficiency |${\eta}_{AC}$|⁠ , calculated based on the amount of unperturbed fog droplets that would collide with the fog’s mesh.

The capture efficiency |${\eta}_{capt}$|⁠ , to account for the fraction of the aforementioned intercepted droplets that are actually captured by the mesh wire.

The draining efficiency |${\eta}_{dr}$|⁠ , to account for the fraction of the water captured by the mesh that is collected by the gutter since some of the water can spill or re-enter the air flow.

Where RH is relative humidity measured by weather station, WH is the potential water harvested (litres per square metre per day) and the subscript 3 represents for every 3 hours, an input value chosen because data at the representative stations was recordered 3 hourly, and they assumed stable conditions were achieved after this period is achieved. |${U}_2$| is the wind velocity at 2 m height above the ground, |${M}_t$| is the absolute humidity that is defined as the humidity in grams per cubic meter of air in a specific temperature (g/cm 3 ). The values of wind speed for eight different wind directions were then investigated. Their analysis have shown promising results for water collection at Abadan and Chahabar station with the amount of potential collected water is 6.7 l/m 2 /day and 156.3 l/m 2 /day, respectively [ 27 ].

2.1.4. Studies on mesh topology

To improve fog collector performance, understanding the effects of fog collector topology is a key as defined especially by the mesh radius and mesh diameter. Collectors can be categorized based on their fibre radius R and the half spacing of the fibres D [ 28 ], values that are important in the calculation of Stokes coefficient that is related to the collector efficiency. Stokes number typically determines the inertia of the moist air and its migration across the streamline and thus indicates the effectiveness of the fog collector design, thus a large Stokes number implies a higher rate of water droplet collection [ 28 ]. However, this paper will not further elaborate the equation used for the calculation of Stoke coefficient. Interested readers may refer to [ 29 ] for further description. As previously discussed, Rivera [ 9 ] investigated aerodynamic collection efficiency (ACE). Rivera [ 9 ] considered that two important characteristics of the mesh were the shade coefficient and the characteristics of the fibres used to weave or knit the mesh. He also discussed a simple superposition model in analyzing the influence of these parameters to Regalado and Ritter [ 29 ] the ACE of the fog water collectors. Rivera [ 9 ] concluded that the ACE value can be increased by introducing concave shape to the fog water collector and improving the aerodynamics of the mesh fibres. Regalado and Ritter [ 29 ] have performed a theoretical analysis on wind catchers in the form of cylindrical structures equipped with several screens of staggered filaments to determine their efficiency. Like Rivera [ 9 ], these researchers also assessed the aerodynamic effects of the water/fog impacting on the mesh.

2.1.5. Studies on surface wettability of a fog harvester

While most researchers focussing on the mesh topology, Park et al. [ 6 ] have investigated the influence of surface wettability characteristics, length scale and weave density on the fog harvesting capability of woven meshes. In their research, Park et al. [ 6 ] have developed a model that combined the hydrodynamic and surface wettability characteristics of a fog water collector in predicting the overall fog collection efficiency. From their modelling, later validated against experimental results and depicted in Figure 7 , there are two limiting factors that will effect fog harvesting and reducing the collection efficiency; first is the re-entrainment of collected droplets into the prevalent wind, and second one is the blockage of the mesh opening. However, they have concluded appropriate tuning of the wetting characteristics of the surfaces, reducing the radius of the wire and optimizing the wire spacing will lead to more efficient fog collection. Additionally, they have introduced family of coated meshes that have demonstrated enhancement in the fog collecting efficiency as high as five times of the conventional polyolefin mesh. To coat the mesh, quoted from the researchers’ paper [ 6 ], ‘a 1.7 wt.% 1H,1H,2H,2H-heptadecafluorodecyl polyhedral oligomeric silsesquioxane (fluorodecyl POSS) 98.3 wt.% poly(ethyl methacrylate) (PEMA, MW = 515 kDa, Sigma Aldrich) solution in a volatile hydrochlorofluorocarbon solvent (Asahiklin AK-225, Asahi Glass Company) at a concentration of 10 mg/m’ was used by the researchers. They first dipped the mesh in the solution for 5 minutes and then air dried to evaporate the solution. To check the uniformity of the coating, they have used scanning electron microscopic method and also by contact angle measurements at several locations on the coated surface. The aim of the coating is to decrease the contact-angle hysteresis of the mesh wires that allows small droplets to easily slide down into the collecting gutter when they were captured by the mesh wires. Even in a mild fog with a droplet radius of 3 μm, wind speed of 2 m/s and liquid water content of 0.1 g/m 3 , the use of optimal dip-coated mesh surface can collect ~2 l of water over an area of 1 m 2 in a day [ 6 ].

Factors affecting fog harvesting and reducing the collection efficiency are (a) the re-entrainment of collected droplets in the wind and (b) blockage of the mesh. Adapted with permission from [6] Copyright (2013) American Chemical Society.

Factors affecting fog harvesting and reducing the collection efficiency are ( a ) the re-entrainment of collected droplets in the wind and ( b ) blockage of the mesh. Adapted with permission from [ 6 ] Copyright (2013) American Chemical Society.

(a) The schematics of the experimental arrangement and (b) the photos of different materials used to the test surface wettability in fog harvesting with the water droplets [30].

( a ) The schematics of the experimental arrangement and ( b ) the photos of different materials used to the test surface wettability in fog harvesting with the water droplets [ 30 ].

Seo et al. [ 30 ] have investigated the effects of surface wettability for both fog and dew harvesting. Their approach to fog harvesting involves different test surfaces. A commercially available copper was used in various wetting characteristics, see Figure 8b . The wettability of surface is determined by the contact angle of the liquid on the surface where the liquid-vapour meets the surface. When a droplet is flowing, the contact angle ( Figure 9 ) can be classified as advancing or receding. The researchers showed that the moisture harvesting performance was determined by the combination of the moisture capture at the surface and the removal of the captured water from the surface. In their study, they found out that a large receding contact angle is a determining factor in performance. Among all the surfaces tested, the oil-infused surfaces with their large receding contact angle at a high super-saturation condition exhibit the best fog harvesting performance.

Schematics represent advanced and receding angles from Weistron [31].

Schematics represent advanced and receding angles from Weistron [ 31 ].

(a) Copper comb sample and (b) polyolefin mesh (double layered) scale bar 1 cm [32].

( a ) Copper comb sample and ( b ) polyolefin mesh (double layered) scale bar 1 cm [ 32 ].

Illustration and experimental results of mist flow (optical images) on two rectangular Rachel meshes with cylindrical fibres (real images) conducted by Rajaram et al. [33].

Illustration and experimental results of mist flow (optical images) on two rectangular Rachel meshes with cylindrical fibres (real images) conducted by Rajaram et al. [ 33 ].

Fog-basking behavior of a Namib desert beetle. Courtesy of James Anderson/NSF/Creative Commons BY-NC-SA 2.0.

Fog-basking behavior of a Namib desert beetle. Courtesy of James Anderson/NSF/Creative Commons BY-NC-SA 2.0.

Surfaces with fine microstructures and different coatings can have markedly different wetting behaviours than smooth surfaces. Therefore, in their research, they have investigated smooth and microgrooved copper wire with a diameter of 1.2 mm. They created the microgroove surface using a sandpaper. Then, microgrooves were implemented on the wire surface using Korn 80 sandpaper that contains particles with the diameter of 190–265 μm. Illustrated in Figure 10a , the copper wires (10 of them, with smooth and microgrooved structure) were soldered electrically on a wire stick.

Polyolefin mesh samples that comes in three types, hydrophilic mesh (attract water), superhydrophilic mesh that was dip coated with an aqueous TiO2 solution and dried at room temperature for 48 hours and ‘hydrophobic mesh’ (repel water) that were prepared by dip coating the polyolefin mesh with a hydrophobizing agent and dried at room temperature for 48 hours.

Epoxy replication (replica) to replicate surface microstructures of Gunnera and Dendrocalamus under leaf surfaces and a smooth glass (microscope slide). The glass replica had a smooth surface, the Gunnera replica had a convex shape microstructure and random channels with hairs inside of the channel and the Dendrocalamus replica had microgroove surface.

Moloch horridus lizard and the hydrophilic surface [36].

Moloch horridus lizard and the hydrophilic surface [ 36 ].

Water droplets on spider web [38].

Water droplets on spider web [ 38 ].

It was found that the amount of collected water by superhydrophilic mesh was five times higher than the hydrophilic polyolefin mesh. Whereas water collection by hydrophobic mesh was 2.5 times higher than the hydrophilic mesh. In the micro-structured replica, water dripped 2–3 times higher than unstructured replica and smooth surface. In addition, the water was collected more quickly for the micro-grooved copper wire than smooth wires [ 32 ].

Rajaram et al. [ 33 ] studied ways to improve the capacity of fog water collection by modifying the surface and geometrical shapes of Raschel mesh structure as shown in Figure 11 . The surface modification includes coating the mesh using superhydrophobic coating such as Teflon, ZnO nanowires, NeverWet and hydrobead. In general, when compared with the uncoated Raschel mesh, the use of the coatings gives about 50% enhancement in the collection efficiency given by equation ( 3 ). Meanwhile, in terms of the modification to the geometrical shapes, they have increased the shade coefficient of the Raschel mesh by developing a new manufacturing method via a punching process. That has resulted in reduction in the pore size and also the increase in the distance between two inclined filaments. The change in the geometrical shape leads to another 50% of enhancement. In general, both methods have collected water about two times that of a typical Raschel mesh.

2.2. Biomimicry-inspired fog water harvesting

2.2.1. animals and plants with special characteristics in harvesting water from the ambient.

In parts of the world, despite extreme water shortages resulting from the low annual rainfall, animals have evolved to survive in such conditions by acquiring special characteristics that allow them to collect water from the fog or the atmosphere. Namib desert beetles, such as Stenocara gracilipes ( Figure 12 ), for instance, survive by collecting water although the annual rainfall is only 12 mm [ 34 , 35 ].The surface of the beetle’s back is covered with a random array of smooth hydrophilic bumps and microgrooves ~0.5 mm in diameter and arranged at 0.5–1.5 mm intervals. These bumps on the forewings are micro size (in micron dimension) allowing water to condense and trickle directly to their mouth. Both fog and dew water harvesting efficiency are said to increase with the combination of hydrophilic (water attracting) and hydrophobic (water repelling) areas.

Other water harvesting animals are a lizard species known as Moloch horridus [ 36 ] ( Figure 13 ).The lizard species is native to hot and arid regions, which drinks water droplets collected over its hydrophilic skin and that reach to its mouth by capillary action. In contrast, a spider, Uloborus walckenaerius uses its web ( Figure 14 ) to collect water. A special structure formed a combination of its spindle-knot structure and the web joints. As seen in Figure 15 , the spindle knots have rough surface and the joints have nanofibrils that make it less rough. The transportation of the water droplets towards the rough spindle-knots from the joints is promoted by the driving force resulting from the Laplace pressure gradient and surface structural anisotropy [ 37 ].

Plants are also able to survive in arid climates by harvesting water. An example is the endemic Namib desert grass called Stipagrostis sabulicola. The round shape of the plants’ stem are covered with leaves whose surfaces are hydrophilic and have an irregular construction. The water droplets travel from the leaves onto the roots ( Figure 16 ) via grooves along its cone-shaped structure. A combination of surface roughness, prickle hairs and wax prevent the scattering of water droplets [ 39 ].

Structure of the spindle-knot and joint [37].

Structure of the spindle-knot and joint [ 37 ].

Stipagrostis sabulicola in their natural habitat [39].

Stipagrostis sabulicola in their natural habitat [ 39 ].

Many of the cactaceae (cactus) family living in hot and arid regions also show great tolerance to water scarcity and capable of water harvesting [ 40 ]. One species, Opuntia microdasys , from the Chihuahuan Desert, has several characteristic with properties that provide effective fog collection [ 41 ]. It has hair-like needles (glochids) instead of spines on its large green leaves, thus reducing exposure to sunlight, which limits the evaporation of water, thus causing more storage of water. In this way, more water is stored for longer survival [ 42 ].

The water collection mechanism of Lychnis sieboldii , a plant species from dry grassland in Japan has surface hairs that show morphological changes when in contact with water, [ 43 ]. The microfibres in the hairs play a vital role in absorbing and releasing water by becoming cone-shaped when exposed to water but changed to a perpendicularly twisted shape under dry conditions as shown in Figure 17 .

The morphology changes of hairs on the leave of Lychnis sieboldii [43].

The morphology changes of hairs on the leave of Lychnis sieboldii [ 43 ] .

A small desert moss, Syntrichia caninervis from the Great Basin in the western United States and the Gobi Desert in China, also survives arid conditions by condensing water using its hairs.The water condensation and the droplet formation are promoted by the grooves and barbs on the hair surfaces. The condensed water droplets will then travel from the tip to their base [ 44 ].

2.2.2. Biomimicry approach in atmospheric water harvesting

In recent decades, reports on bioinspired water harvesting have emerged rapidly [ 45 ]. Inspired by the Namib beetles, Garrod et al. [ 35 ] have investigated the influence in the degree of hydrophilicity/hydrophobicity of beetle backs in determining their overall micro-condensation efficiency. In this research, the micro-condensation efficiency of fog water harvesting units has been explored in terms of the chemical nature of the hydrophilic ‘pixels’ and their dimensions. Imitating the pattern on the back of the beetle, they have applied plasma deposition method to make a hydrophilic polymer array on a superhydrophobic background. The performance of the surfaces as microcondensors were investigated by measuring the amount of water collected from a fine mist in 2 hours. The bumpy array patterns of the hydrophilic and hydrophobic surfaces are concluded to be more efficient at collecting suspended water droplets than a pure hydrophilic or hydrophobic surface. The amount of water collected by surfaces with bumpy array is more than 50% higher than the smooth surfaces.

To imitate the hairs of the cactus and its surface, Cao et al. [ 46 ] investigated a large-scale fog collector through integrating cactus spine-like, hydrophobic, conical micro-tip arrays. The tip arrays were arranged on a spherical hydrophobic cotton matrix, see Figure 18 a–d. For the fog collector, about 30–40 micro tips were placed at each edge of the artificial cactus at 4~5 mm distance, see Figure 18 a and b. The experimental set up is shown in Figure 18 d. The distance between the fogging jet and the collector was set at 3 cm. At fog velocity of 45~50 cm/s, the biomimetic cactus-inspired fog collector was reported to harvest ~3 ml of water in 10 minute. The results imply that at this wind speed, 100 cactus-like fog collectors will be able to collect the water in 1.5 hours, sufficient drinking water for human survival. Clearly, a promising device for collecting water in foggy regions.

The illustration of the (a) cactus-inspired device and (b) the water transportation pathway in the device. (c) The photographs of the cactus-inspired continuous fog collector and (d) the photographs of collection process of the device [46].

The illustration of the ( a ) cactus-inspired device and ( b ) the water transportation pathway in the device. ( c ) The photographs of the cactus-inspired continuous fog collector and ( d ) the photographs of collection process of the device [ 46 ].

More research on bio-inspired plants was conducted by Gürsoy et al. [ 47 ] who replicated the surface of the Eremopyrum orientale leaf, which displays an asymmetric-anisotropic directional mist collection behavior underpinned by macroscale grooves, microscale tilted cones (tilted in the direction of water flow) and nanoscale platelets to harvest water. The surface replication, achieved using soft lithography combined with either nanocoating deposition or functional nanoimprinting, was shown to be highly-efficient for directional mist collection, compared to mist water harvesting by flat surfaces. In a different study, Gürsoy et al. [ 48 ] have reported that non-woven and cotton fibrous materials are shown to mimic the fog harvesting behaviour of Salsola crassa hairs, see Figure 19 . In order to enhance the overall mist collection efficiency, they incorporated multiple length scale (hierarchical) channel structures and tune the surface wettability by introducing hydrophobic functionalization of the fibres (in order to mimic the leaf waxes of the plant Salsola crassa ) using initiated chemical vapor deposition surface coatings or plasma-enhanced chemical vapor deposition. The overall mist collection efficiency can be enhanced by over 300%.

‘Fog collection mechanism of salsola crassa plant species and bioinspired fibrous water harvesting’ [48].

‘Fog collection mechanism of salsola crassa plant species and bioinspired fibrous water harvesting’ [ 48 ].

An interesting fog water harvesting concept has been demonstrated by Park et al. [ 49 ] on the design of the fog water harvesting surface bioinspired by combining three different elements from different species: Namib desert beetles, cacti and pitcher plants. Inspired by the bumpy surface of Namib desert beetles, they have performed modelling to optimize the radius of curvature and cross-sectional shape of the water harvester surface to promote condensation. Then, inspired by cactus spine, they integrated the geometry with a widening slope in facilitating water droplet to the collector in a faster rate to avoid a decrease in the droplet size. Finally, they integrated the optimized bump radius and the wide slope structures with a slippery nano-coated surface that is inspired by pitcher plants. The role of the slippery surface is to promote coalescence droplet growth.

Shang et al. [ 50 ] mimic the special characteristics of the spider web silk in order to harvest water. In their research, they have developed a novel microfluidic technology that can control the size and spacing of the spindle knots in order to adjust the flow rates. In this way, the size and spacing of the spindle knots can be controlled and thus, the function of humidity-responsive water capture can be obtained. As a result, some features are gained such as thermally triggered water convergence, humidity-responsive water capture that can be used for many applications.

In fog water harvesting, the collection of water will occur when the fog droplets impact and intercept with the collection surfaces. However, the main limiting factor of harvesting water from the fog droplets is the global fog occurrence that is highly dependent on the geographical and metrological factors or conditions. Only limited number of places experience environmental conditions whereby the temperature of moist air could naturally drop below its saturation temperature thus form fog. Not surprisingly therefore, on a global scale, fog is reported to be even less accessible than seawater as an alternative source of freshwater [ 51 ]. Water vapour is ubiquitous in the atmosphere, so, if condensed by cooling, freshwater can be harvested at many locations. Nevertheless, the condensation process is more thermodynamically complicated than fog harvesting and as reported in Gido et al. [ 51 ], the process involves a significant release of heat.

Water droplets that are formed due to the condensation of water vapour on a surface at temperature below its dew point temperature are called dew water [ 3 , 52 ]. In this paper, dew water harvesting processes are divided into three categories: i) passive (radiative) cooling condenser, ii) solar-regenerated desiccant and iii) water harvesting from air using active cooling condensation technology. This review includes dew water collection under both high and low humid air conditions.

3.1. Water harvesting using radiative cooling condenser (passive systems)

(i) maximize the infrared wavelength emitting properties of the surface to allow surface cooling at night;

(ii) increase the reflectivity of the condensing surface to ensure that the surface will not trap heat that will warm the condenser and resulting in evaporation during the day;

(iii) reduce the wind effect to the condenser by tilting the condenser surface;

(iv) increase the hydrophilic property of the surface, and this can be achieved by applying hydrophilic coating to the surface and lastly;

(v) reduce the heat inertia of the condensing surface to promote change in temperature difference and also as a means to avoid heat transfer from the ground.

Studies on passive cooling system include investigation on materials with low emissivity surfaces. Early study on the influence of condensing surface materials to the dew formation has been investigated for Bahrain climatic condition [ 54 ]. Three materials: aluminium, glass and polyethylene foils were investigated as the condensation surfaces. From their study, aluminium surfaces were reported to have the highest amount of average dew collected at 3 kg/m 2 per hour, followed by glass and polyethylene foils at 0.8 and 0.3 kg/m 2 per hour, respectively. Three different types of condensing surface namely: i) galvanized iron (GI) sheet with emissivity 0.23 and thickness 1.5 mm, ii) commercial aluminium sheet with emissivity of 0.09 and thickness 1.5 mm and iii) PETB film (polyethylene mixed with 5% TiO2 and 2% BaSO4) UV stabilized with emissivity 0.83 and thickness 0.3 mm have been investigated, see Figure 20 [ 55 ]. The condensing surfaces were tested as a radiative condenser at 1 m × 1 m in size installed at the village of Kothara (23° 14 N, 68° 45 E, 21 m a.s.l.) that is a part of the semi-arid coastal region of northwest India. The aim of the project was to use the water harvesting system as a solution to drinking water problem in that region that is well known with poor groundwater quality. From the daily data collected over 2-year period in 2004 and 2005, the quantity of water collected on most (60%) nights varied more or less uniformly between 0.05 and 0.25 mm and there were two peaks. The peaks that one of them centred over March–April (summer) and the other over October (fall) shows water collection of 0.55 mm. From all the three surfaces being tested, the highest collection was in the PETB units (19.4 mm) followed by GI (15.6 mm) and aluminium (9 mm).

Different types of condenser surfaces investigated by [55].

Different types of condenser surfaces investigated by [ 55 ].

Kothara village in the Kutch region now has India’s first potable large-scale water production plant designed to harvest atmospheric moisture and process it into drinking water. The condensers were made of planar panels using high emissivity plastic film insulated underneath that promotes cooling. In addition to dew water harvesting, the condenser are also capable to collect rainwater. It was reported that the expected cost of 1 l of bottled water is 0.5 rupee with the expected yield of filtered, treated potable water from the plant is 150 000 litres a year [ 56 ].

Another important surface parameter that influences the performance of the passive system is the shape of the radiative condenser. As reported in Khalil et al. [ 3 ], among the early researchers who investigated various shapes of these passive condenser surfaces were Jacobs et al. [ 57 ] who investigated an inverted pyramid shape. Investigated at the grassland of the Netherlands, the authors concluded that their collector collected water 20% more that the planar shape at angle 30°. Researchers [ 58 ] have performed a CFD simulation Computational Fluid Dynamics using PHOENIXS to simulate the innovative designs proposed in their study.

Reported in 2011, the world’s largest dew and rain water collecting system was constructed in 2006 at Panandhro in the semi-arid area of Kutch (NW India). Ridge-and-trough shape modules have been chosen as the shape of the dew water collector [ 59 ]. The performance of the large dew condenser at 850 m 2 net total surface with 10 ridge-and-trough modules had a total output for 2007 of 6545 l, corresponding to 7.7 mm/day on average. The maximum collection rate reported was 251.4 l/night (0.3 mm). In addition to dew, the designed condenser could also collect rain (and, to a lesser extent, fog).

In a passive system, natural convection between the condenser surface and the air flow is not favoured since it will reduce the condensing efficiency of the condenser system. Thus, a condenser in a hollow form such as a funnel will reduce the free convection along the surface since the heavier cold air will remain at the bottom of the funnel due to gravity regardless of the wind direction [ 53 ]. The researchers have performed both simulation and field studies. From their simulations, cone angle ≈ 60° give the best condenser cooling efficiency. Based on experimental work and field testing, a repetitive pattern of hollow shapes to pave a planar or weakly curved roof surface, have been considered, providing pleasing aesthetics and construction cost advantages. The egg-box and origami types were specifically investigated. The prototypes were fabricated and installed at Les Grands Ateliers (Villefontaine - France) during the ‘Chaleurs urbaines’ project (ENSA de Grenoble - Métro).

3.2. Solar regenerated desiccant in water harvesting (passive system)

Low yield is a key issue for the passive, radiative condenser system because of its dependency on certain parameters, notably the sky emissivity, the amount of water vapour in the air (relative humidity), wind speed and topographic cover [ 3 ]. Desiccant materials such as silica gel, zeolites and CaCl 2 are hygroscopic and can absorb moisture through adsorption and absorption process thus increasing the amount of the dew water collected. As a result, desiccant beds are now commonly being used in atmospheric water harvesting applications. Figure 21 presents the generic process of atmospheric water harvesting using desiccant. The process may be explained as follows: the first stage is water absorption stage at night where the desiccant bed will absorb moisture from humid air. The second stage is water desorption during the day by heating the bed with solar radiation, which will regenerates the desiccant by driving out water vapour. In the third and final stage, the evaporated water will then condensed into water droplets and collected in a tank.

Wet desiccant technique for water production from atmospheric air [60].

Wet desiccant technique for water production from atmospheric air [ 60 ].

The advantages of a desiccant system over radiative condensers include the hygroscopic capacity of the desiccant that enables more efficient water collection, achieving low dew points without the risk of freezing thus reducing operational cost [ 51 ]. Early studies on solar regenerated systems involve desiccants such as saw dust [ 61 ], silica gel [ 62 ] and recycled newspaper [ 63 ]. In a patent, Ackerman [ 64 ] claimed a spiral water harvester containing hydrophilic particles such as silica gel and tilted at an angle that optimized water collection. To improve the atmospheric water harvester performance, various collector designs have been investigated by researchers and several are described below.

3.2.1. Glass pyramid collector

Kabeel [ 65 ] described a glass pyramid collector ( Figure 22 ) comprising: i) desiccant beds on shelves, ii) a slanting wall cover, iii) a collection cone and iv) a condenser section mounted on top of the pyramid, shading it from solar radiation. Sawdust and cloth, saturated with CaCl 2 , were investigated as the desiccants. The covers over the beds are open overnight so the desiccant can absorb water vapour from the air. During the day, the covers are closed so the beds are heated by solar radiation driving off the absorbed water, which condenses on the sides and especially at the pyramid apex water, where it is collected by a central cone and flows through a tube to an external container. The reported water yield is 2.5 l/day/m 3 ; the cloth bed showed better performance than the sawdust bed system.

(a) Photograph of the system used. (b) Pyramid with glass covers open at night (right) [65].

( a ) Photograph of the system used. ( b ) Pyramid with glass covers open at night (right) [ 65 ].

3.2.2. Corrugated surface

Based on the principle of desiccant moisture absorption at night and simultaneous desorption (regeneration using solar energy) and water vapour condensation during the day, Gad et al. [ 66 ] introduced the use of an integrated desiccant/solar collector to harvest water from humid air. In their study, a small air circulation fan was used to force the ambient air to enter the glass-enclosed solar collector during the evening ( Figure 23 ). In the collector, a thick layer of corrugated cloth was used as the desiccant bed. The use of corrugated surface was meant to increase the heat and mass transfer area during the absorption/desorption mechanism. During the day, water vapour condensation will occur on the inner surface of the glass enclosing the solar collector. According to the researchers, the solar driven system could produce 1.5 l of fresh water per square meter per day.

Schematic diagram of the experimental apparatus and the corrugated desiccant bed [66].

Schematic diagram of the experimental apparatus and the corrugated desiccant bed [ 66 ].

3.2.3. Trapezoidal prism

William et al. [ 67 ] designed a trapezoidal prism with CaCl 2 as the desiccant ( Figure 24 ) supported on sand and on dark cloth. For the prism wall, transparent fibre glass bolted to aluminium frames was used while the top of the prism was an opaque material that acted as a condenser and to facilitate collecting the condensate water, the walls were slanting. The trapezoidal prism worked in essentially the same way as the pyramidal system described above in that moisture absorption occurred at night time and the solar radiation driven desorption occurred during the day with the evaporated water forming water droplets that collected in the water tank. The system efficiency was computed by considering the total heat of evaporation to the total incident solar radiation during the day time. The recorded daily total evaporated water for cloth and sand bed achieved a maximum of 2.32 and 1.23 l per m 2 at system efficiency of 29.3% and 17.76%, respectively.

Schematic diagram of experimental test rig [67].

Schematic diagram of experimental test rig [ 67 ].

3.2.4. Solar glass desiccant box type system

In India, an atmospheric water harvesting system that named ‘solar glass desiccant box type system’ (SGDBS) with a capture area of 0.36 m 2 was developed and investigated. The box was made of a 3 mm single glaze glass; the desiccant bed was fixed at 0.22 m at inclination of 30°. The desiccant bed was a composite material using sawdust impregnated with CaCl 2 ( Figure 25a , absorption and Figure 25b , desorption). Three boxes were tested under the Indian climatic conditions at NIT Kurukshetra, India [29° 58′ (latitude) north and 76° 53′ (longitude) east] in October. The researchers observed that the performance depend mainly on the concentration of CaCl 2 , which generated 180 ml/kg/day at a loading of 60% on the sawdust.

(a) and (b) the design of the SGDBS and (c) the experimental setup [68].

( a ) and ( b ) the design of the SGDBS and ( c ) the experimental setup [ 68 ].

3.2.5. MOF porous metal-organic framework-801

Recently, the potential of harvesting water from humid air as low as 20% have been investigated by researchers from Berkeley and MIT [ 69 ]. Based on the same principal of introducing hygroscopic element to improve moisture uptake, the researchers have developed an hygroscopic sheet using a kilogram of dust-sized MOF porous metal-organic framework-801 [Zr6O4(OH)4(fumarate)6] crystals pressed into a thin sheet of porous copper metal positioned between a solar absorber plate (at the top) and a condenser plate (see Figure 26 ), both placed in a chamber [ 70 ].

The experimental setup [70].

The experimental setup [70].

The device is shown in Figure 26 . At night flaps are open, allowing ambient air to enter the chamber. Water vapour diffuses into the porous MOF and is absorbed on its internal surface in clusters of eight molecules, essentially tiny ‘cubic droplets’. In the morning, with the chamber closed, natural sunlight (~1 kW/m 2 ) heats MOF causing the water to desorb as vapour, which then condenses on the bottom of the chamber [ 70 ] and the resulting liquid drains to a collecting tank. Published results suggest that MOF-801 is superior to other absorbents, being capable of generating 2.8 l of water per kg and with the ability to operate a relative humidity level as low as 20% [ 70 ].

3.3. Water harvesting from air using active cooling condensation technology

The water harvesting systems described previously can be described as ‘passive’, i.e. they are driven simply by solar heating and do not require the input of electric or other high-grade power. In contrast, ‘active’ systems typically require electrically powered compressors or vacuum pumps and the quantity of water harvested in directly related to the input energy [ 3 ]. Active harvesters range in scale from those suitable for domestic drinking water (15–50 l per day) to industrial scale units for irrigation (2000 l per day), outputs typically significantly larger than passive systems. The power consumption per kilogramme of water collected is a major concern for active systems and will be affected by the ambient temperature, humidity and efficiency of ‘coolth’ recovery in the equipment. Leading active technologies are described below.

3.3.1. Dehumidifier using selective membrane

Water vapour is only a minor component of air in the atmosphere, even at 30°C/100% RH only 30.4 g is present, while at 10°C/RH 100% the moisture content is 9.4 g/m 3 , so the maximum quantity of water that can be recovered by cooling between these temperatures is 21 g/m 3 . However, this requires cooling 1 m 3 of air by 20 K that requires the removal of 24 kJ of heat plus 52.5 kJ of latent heat to condense the water. If the coolth of the outgoing air after condensation is not recovered, it represents a significant inefficiency. To minimize the power requirement of the dehumidification process, as shown in Figure 27 , researchers [ 71 , 72 ] have used water vapour selective membranes to separate the water vapour component prior to cooling and condensation, thus avoiding cooling the other atmospheric gases. The key element of the system is the water-selective membrane that allows only water vapour to pass through driven by a concentration gradient imposed by the vacuum pump. The concept underlying themembrane system is shown in Figure 28 in a different study by Woods [ 73 ]. The researchers [ 74 ] found that with a 62 kW power input, the harvester produce water at the rate of 9.19 m 3 /day, a 50% better efficiency than the equivalent system without the membrane. In addition to improved energy efficiency, the selective membrane generated fresh water that cleaner than water condensed directly out of the air. Other than selective membranes, some researchers also use desiccants systems (liquid and/or solids) to absorb the water vapour from an incoming air stream. However, these methods require regeneration steps and cyclic operation conditions reduce the rate of water production. Furthermore, the use of spatially separated liquid desiccant dehumidification methods results in energy-intensive regeneration and condensation processes [ 75 ].

The representation of the water vapour selective membrane in an atmospheric water harvesting system [74].

The representation of the water vapour selective membrane in an atmospheric water harvesting system [ 74 ].

The concept of water vapor selective membrane [73].

The concept of water vapor selective membrane [ 73 ].

Various selective membranes have been investigated. A Singapore group investigated water vapour permeation through membranes fabricated by impregnating poly(vinyl alcohol) (PVA) with LiCl [ 76 ]. They concluded that higher LiCl contents and lower temperature optimizes the water vapour permeance of the membrane. With respect to humid condition, the tests showed that the membrane was suitable for dehumidifying air at high humidity conditions.

In a separate publication, the group compared two different membranes, one containing LiCl and the other triethylene glycol (TEG) supported on PVA. The researchers concluded that the water vapour permeability of the membranes increased with increasing amounts of the hygroscopic component (LiCl or TEG), because it lowered the diffusion energy and thus the barrier to permeation. The researchers further claimed that a membrane with PVA/TEG is highly durable, has less corrosive problems and more environmentally friendly in comparison to the membrane with LiCl as the hygroscopic component [ 77 ].

3.3.2. Atmospheric water harvesting integrated with air conditioning system and condensing coil

Active condensing systems, using the conventional reverse Rankine cycle, operate in the same way as a dehumidifier where passage of moist air passed over a coil cooled by a refrigerant, causes the water vapour to condense. The rate of the water production depends mainly on the relative humidity and the air temperature. Versions of the technology have been described in various academic papers and patents. For example, Lukitobudi [ 78 ] claimed a mobile dehumidifier unit that simultaneously produced drinking water. Sawyer and Larson [ 79 ] who presented a disclosure unified system that provides both air conditioning and atmospheric water harvesting. Magrini et al. [ 80 ] have discussed in their paper the advantage of water harvesting from the integration with an HVAC system that also serves as the air conditioning system for a hotel in a sub-tropical arid climate. Rather than having the condensate water from an HVAC system wasted, the water is collected and utilized. The researchers found that the integrated system water produce ~56% of the hotel water daily demand.

Ecolo Blue EB30 [82].

Ecolo Blue EB30 [ 82 ].

Atlantis H2O Elite Atmospheric Water Generator [83].

Atlantis H2O Elite Atmospheric Water Generator [ 83 ].

Another study into water harvesting from an air conditioning system has been recently conducted by Dalai et al. [ 81 ] to maximize the amount of water vapour captured by a window air conditioner, a process termed ‘atmospheric water vapour processing’ (AWVP). The water was claimed to be sufficiently good quality for human consumption. With a power input of 160 watt and air flow rate of 0.00623 m 3 /s, the amount of water collected was reported to be as high as 1025 ml.

Ecolo Blue, a United States company, produces the EB30 commercial unit based on dehumidifier circuit to harvest atmospheric water ( Figure 29 ). To minimize contamination of the water by the metals of the cooling coils, they are treated with a food grade coating. The EB30 can generate up to 30 l of water from air over a 24 hour cycle with a unit cost of 1300 US dollars.

Another company, Atlantis Solar, offer the Atlantis H2O Elite range of units providing atmospheric water harvesting from 100 l up to 10 000 l per day ( Figure 30 ) (Atlantis [ 83 ]).

3.3.3. Thermoelectric cooling in atmospheric water harvesting

The application of thermoelectric cooling (TEC) is being actively investigated as an alternative approach to conventional Rankine cycle for water harvesting for example by Joshi et al. [ 84 ] who constructed a prototype containing 10 Peltier components ( Figure 31 ).

Left and middle: diagram of prototype and right: actual water harvester prototype [84].

Left and middle: diagram of prototype and right: actual water harvester prototype [ 84 ].

To enhance the cooling performance, the researchers have introduced an internal heat sink on the cold side to increase the cooling rate and thus the condensation rate. Over a 10 hour run, the TFWG with internal heat sinks showed 81% improvement over in amount of water collected compared to the TFWG without the heat sinks. Other parameters being investigated are electric current, air mass flow rate and air humidity.

Liu et al. [ 85 ] have investigated a portable water generator, with two TECs. In their system, air is forced into the mixing chamber and then humidified. The humidified air is then flow through the TECs via the inlet air channel. At TECs, the temperature of the inlet air was reduced by the cool surface of the TECs to the dew point temperature and water condensation occurs. The researchers investigated the relationships between inlet relative humidity and air flow rates with the amount of the water generated/condensed. They concluded, not surprisingly, that the higher the air relative humidity the higher the amount of water generated, while increasing the air flow rate lowered the condensation rate, possibly because the reduced contact time between the air flow and the TEC degraded the heat transfer rate. Lui et al. [ 85 ] showed that the maximum amount of generated water was ~25.1 g/h with 0.216 m 2 of condensation surface and 58.2 W power input.

3.3.4. Innovative cooling condensation technology: concept and prototype development

Exciting developments integrate cooling condensation technology with wind energy source element. The water harvesting billboard (2013) designed by University of Engineering and Technology of Peru ( Figure 32 ) contains five generators that extract moisture from air using an inverse osmosis filtration system [ 86 ]. The water flows through the small ducts to a central holding tank at the billboard’s base. Although the billboard requires power supply, it could provide as much as 100 l of drinking water per day.

Water harvesting billboard [86].

Water harvesting billboard [ 86 ].

EOLE WATER have introduced the WMS1000 wind turbine ( Figure 33 ) that harnesses wind energy to simultaneously drive the compressor of a Rankine cycle dehumidifier-type system and create an airflow over the cold coil. With an electrical output of 30 kW, the WMS 1000 can produce up to 1000 l of drinking water per day and requires no additional external electrical input [ 87 ].

The WMS1000 wind turbine from EOLE WATER [87].

The WMS1000 wind turbine from EOLE WATER [ 87 ].

Over the past decade, Australia has suffered severe droughts causing considerable economic hardship to its famers. To alleviate their plight, Edward Linacre has therefore invented the airdrop water harvester [ 88 ]. Airdrop comprises a mast-like tube above ground through which air is sucked and driven into an underground metal coil by a wind-powered turbine. Since the earth is at a lower temperature, it cools the air below its dew point resulting in water vapour condensation. Liquid water collects in a reservoir from where it is pumped to a network of irrigation tubes to the plant roots, a very efficient method of distribution since it minimizes water loss. The airdrop can harvest 11.5 ml of water for every cubic meter of air in the driest deserts such as the Negev in Israel, which typically has a relative humidity of 64%, and can produce 1 l of water per day [ 88 ]. The airdrop is a low-tech solution that could be installed and maintained easily and it is self-contained, using a combination of wind and solar power. The turbine is generally wind powered, but when wind speeds are low it is powered by solar PV buffered by a battery.

At least 2.7 billion people worldwide experience water scarcity, a problem that is increasing and has the potential to cause conflicts between countries as they compete for an increasingly short resource. Clearly, this crisis needs tackling urgently and will be compounded as climate change causes profound shifts in rainfall patterns. Although traditionally arid regions, such as the Middle East will suffer, developed countries are certainly not immune as prolonged droughts in parts of Australia and California have demonstrated. Not surprisingly therefore, harvesting atmospheric water has received considerable attention from researchers worldwide since starting with the traditional method of capturing water from fog 50 years ago.

This review has described various technologies in rapidly developing field we expect more to appear in the near future. All have their merits and disadvantages with some being more suited than others to specific situations. Fog harvesting systems are simple, relying upon simple, relatively cheap materials that may be obtained from indigenous natural resources. However, fog only occurs in a limited number locations where rainfall is low, so can only make a modest contribution to alleviating water shortages.

Atmospheric water vapour is a world-wide resource and is available even in the driest climates. Passive harvesting devices relying upon radiative heat loss, and, like fog collectors, also have advantage of being simple and not requiring an external power source. The surface energies and topographies can be modified to facilitate the collection of water and facilitating drainage. However, long term testing is required to check whether fouling, either natural or man-made, might compromise performance over a time scale of several years. Will regular cleaning be required? The quantities of water that can be harvested by passive systems are limited and are perhaps limited to providing drinking water to small communities rather than large-scale applications such as agricultural irrigation.

Desiccant-based water collection systems are more sophisticated than radiation-based systems, but can collect more water for a given size of unit. Although cheap absorbents can be fabricated from sawdust and calcium chloride, recently developed modern metal organic framework (MOF) materials are able to operate with relative humidities as low as 20%, but will be more expensive. The choice of absorbent will be determined by economics versus technical efficiency. The desiccant systems described in this review rely upon thermal solar energy to drive the desorption process, which is not a problem since most arid areas have plentiful sunshine. Desiccant systems would benefit from fans to drive moist air over the beds on windless nights, which require solar PV cells and batteries. All the systems reviewed rely upon flaps to opened and closed manually. Obviously, this is not a problem for an experimental system, but for a production unit an automatic vent opener typically used for greenhouses would allow water harvesting with minimum of attention. Of course, it would need to be installed to close the vent during the day and open at night, the reverse of its normal operation.

‘Active’ water harvesting units that require the cooling of air by the input of electric or mechanical energy are capable of operating from scales of few litres to 1000s litres per day and can be used for domestic water to agricultural irrigation. Whether fossil fuel or nuclear, provide the power for condensation, it is questionable whether this makes technical or economic sense since such stations require large quantities of cooling water. If such water is available why not use it directly. However, solar or wind power is readily available in an arid area, using it harvest water is potentially attractive. Furthermore, water can be readily stored; a renewable energy installation might be scaled to supply both the power and the water for an arid locality, with water harvesting continuing when power demand was low. Water can also be used for evaporative air conditioning systems so conceivably integrated power and a/c systems might be designed. Maybe in arid climates, we shall see the construction of fully self-contained dwellings that do not rely upon any connections to public utilities? Of course, there may more than one system installed, so that the house derives its power and water from PV cells, while the garden is watered by several ‘airdrop’ units scattered around the grounds. For public buildings and facilities such as golf courses and where adequate land is available, the EOLE WATER WMS1000 water unit might be attractive because of its large scale.

Water harvesters based on the reverse Rankin cycle, operating on the same principle as present-day dehumidifiers, require a conventional refrigerant. Over the past 25 years, the major refrigerants have been the Hydrofluorocarbon (HFCs), but these are now being phased-down and ultimately phased out because of the high global warming potentials (GWP). The low GWP replacements are the so-called ‘natural’ refrigerants, carbon dioxide, ammonia and hydrocarbons and the so-called ‘synthetic’ refrigerants the HFOs (hydrofluoroolefins), notably R1234yf and R1234ze(E). Ammonia and hydrocarbons have well-known hazards so increasing their applications in close proximity with the public means they must be treated with caution. Carbon dioxide is non-flammable and has low toxicity, but of necessity has to operate at high pressure supercritical conditions for part of the cycle, which presents significant thermodynamic efficiency problems. The two HFOs have low toxicity, are only marginally flammable and can operate on a conventional reverse Rankine cycle. However, they attract considerable opposition from campaigning environmentalists who strongly advocate the ‘natural’ refrigerants, although, as presently sourced, these are just synthetic as the HFOs being manufactured in large chemical plants. Any future work on active reverse Rankine cycle harvesters should consider what refrigerants will be available in the future. The ‘airdrop’ system does not rely upon refrigerants or external power, so is possible to develop a large-scale version? Maybe this is the way forward? The TEC cooling systems also avoid the need to choose a refrigerant, but are they as efficient and can they be operated at large scales?

Several of the technologies we described above are essentially laboratory studies; water harvesting technology is only now being to be commercialized. If water is being collected for drinking water then attention must be paid to potential contamination. Fog nets, passive radiation and even desiccant collectors may be fouled with algal and bacterial growth and bird droppings, so the water obtained may need to be treated before being drunk. The problem of legionnaire’s disease in a/c water tanks is well known. Atmospheric pollution, such as soot particles, might also be a hazard. Comparable problems might occur with active collection devices.

Dalai et al. [ 81 ] recognized the need to treat the water collecting plates of their AWVP windowbox device with a coating that prevented potential contamination of the water with metals to ensure it was drinkable. This is an important point; chemical as well as natural contaminants must be considered. Standard horticultural Raschel fabric may contain additives, such as plasticisers and UV stabilisers, that would contaminate collected fog water. A food grade material might be specified, but would this survive sufficiently long in the open air? In any case, natural contamination accumulating during use might nullify the value of food grade material. Fluorochemical coatings provide the highest water repellency so they would seem to potentially useful for water harvesting devices. However, it has been known for over 20 years that they slowly release non-biodegradable perfluoroalkylsulfonic salts that can accumulate in the fats within organisms. The use of fluorochemical coatings is therefore best avoided. For crop irrigation, potable quality water is not required so these problems are not issues, apart perhaps from the fluorinated coatings.

Water harvesting is a technology whose time has come. Clearly, considerable challenges remain to optimize efficiency and ensure the delivery of water with a quality appropriate to its end use at cost the customers can afford. These problems can be solved.

This work was supported by Newton Fund Institutional Links [grant number 261839879]

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Urban rainwater harvesting systems: Research, implementation and future perspectives

Affiliations.

  • 1 Department of Civil Engineering and Architecture, University of Catania, Viale A. Doria, 6, 95125, Catania, Italy. Electronic address: [email protected].
  • 2 Centre for Water Systems, College of Engineering, Mathematics and Physical Sciences, University of Exeter, Exeter, EX4 4QF, UK.
  • 3 Waterway Ecosystem Research Group, School of Ecosystem and Forest Sciences, University of Melbourne, Burnley, Australia.
  • 4 Department of Environmental, Water & Agricultural Engineering, Faculty of Civil and Environmental Engineering, Technion-Israel Institute of Technology, Haifa, 32000, Israel.
  • 5 Biological and Agricultural Engineering, North Carolina State University, Campus Box 7625, Raleigh, NC 27695, USA.
  • 6 Department of Civil Engineering, University of Cape Town, Private Bag X3, Rondebosch, South Africa.
  • 7 Federal University of Santa Catarina, Department of Civil Engineering, Laboratory of Energy Efficiency in Buildings, Florianópoli, SC, Brazil.
  • 8 School of Computing, Engineering and Mathematics, University of Western Sydney, Sydney, Australia.
  • 9 Research Center for Water Environment Technology, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo, 113-8656, Japan.
  • 10 Department of Civil and Environmental Engineering, Seoul National University, 1 Gwanak-ro, Gwanak-gu, Seoul, South Korea.
  • PMID: 28279940
  • DOI: 10.1016/j.watres.2017.02.056

While the practice of rainwater harvesting (RWH) can be traced back millennia, the degree of its modern implementation varies greatly across the world, often with systems that do not maximize potential benefits. With a global focus, the pertinent practical, theoretical and social aspects of RWH are reviewed in order to ascertain the state of the art. Avenues for future research are also identified. A major finding is that the degree of RWH systems implementation and the technology selection are strongly influenced by economic constraints and local regulations. Moreover, despite design protocols having been set up in many countries, recommendations are still often organized only with the objective of conserving water without considering other potential benefits associated with the multiple-purpose nature of RWH. It is suggested that future work on RWH addresses three priority challenges. Firstly, more empirical data on system operation is needed to allow improved modelling by taking into account multiple objectives of RWH systems. Secondly, maintenance aspects and how they may impact the quality of collected rainwater should be explored in the future as a way to increase confidence on rainwater use. Finally, research should be devoted to the understanding of how institutional and socio-political support can be best targeted to improve system efficacy and community acceptance.

Keywords: Rainwater harvesting; Stormwater management; Sustainable urban water systems; Water conservation; Water efficiency.

Copyright © 2017 Elsevier Ltd. All rights reserved.

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Rainwater Harvesting for Water Security in Campus (case study Engineering Faculty in University of Pancasila)

D. Ariyani 1 , A. Wulandari 1 , A. Juniati 1 and R. Nur Arini 1

Published under licence by IOP Publishing Ltd Journal of Physics: Conference Series , Volume 1858 , The 7th International Conference on Engineering, Technology, and Industrial Application (ICETIA 2020) 8-9 December 2020, Indonesia Citation D. Ariyani et al 2021 J. Phys.: Conf. Ser. 1858 012020 DOI 10.1088/1742-6596/1858/1/012020

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1 Civil Engineering Department in University of Pancasila Jakarta Indonesia

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Global climate change is a worldwide issue affecting rain and groundwater supplies. To realize the SDGs' 6th goal regarding clean water and proper sanitation, Rainwater Harvesting (RWH) is one solution for pure water requirements and useless groundwater. RWH, which is equipped with infiltration wells, can overcome water scarcity during the dry season and reduce flooding in the rainy season. This research was conducted to harvest rainwater that falls on the roof into the reservoir to the ground, so it is necessary to calculate the rainwater availability and water demand to design RWH building plan. This research was conducted at the Faculty of Engineering, Pancasila University, which uses groundwater for essential water requirements. Two RWH reservoirs are planned in different locations, with the dimensions of the RWH reservoir are 4 m (length) x 3.5 m (width) x 3 m (height). Based on the flood discharge from the roof of 84 m3, the amount of water needed is 19.4 m3. For that, it is necessary to be equipped with 4 infiltration wells with a diameter of about 1.5 m and a height of 3 m to store groundwater around 6.402 m3. RWH application can also be applied to campus that use groundwater to reduce groundwater use.

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W ater Harvesting Research is an online, single blind peer-reviewe, free and open access journal published semiannually by University of Birjand with the cooperation of Iranian Rainwater Catchment Systems Association   . With our broad scope, we welcome research from areas across the water resources engineering.  All submitted manuscripts are checked for similarity through  iThenticate plagiarism detection software to verify the originality of written work. 

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    Hiroaki, Mooyoung Add to Mendeley Share https://doi.org/10.1016/j.watres.2017.02.056 Get rights and content Highlights • A review of practical, theoretical and social aspects or urban rainwater harvesting systems. • Much of the implemented systems do not consider the multi-purpose nature of RWH. •

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    Key findings are: (1) despite RWH being aimed to provide social benefits, the present literature is constrained in supporting social policy-making and management of rainwater as a public utility system; (2) policy and governance frameworks that include socio-economic and socio-environmental pragmatism are needed to achieve RWH programs realistic...

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