Experimental Investigations of a Solar Water Treatment System for Remote Desert Areas of Pakistan

: Pakistan is among the countries that have already crossed the water scarcity line, and the situation is worsened due to the recent pandemic. This is because the major budget of the country is shifted to primary healthcare activities from other development projects that included water treat-ment and transportation infrastructure. Consequently, water-borne diseases have increased drastically in the past few months. Therefore, there is a dire need to address this issue on a priority basis to ameliorate the worsening situation. One possible solution is to shift the focus/load from mega-projects that require a plethora of resources, money, and time to small domestic-scale systems for water treatment. For this purpose, domestic-scale solar stills are designed, fabricated, and tested in one of the harshest climatic condition areas of Pakistan, Rahim Yar Khan. A comprehensive overview of the regional climatology, including wind speed, solar potential, and ambient temperature is presented for the whole year. The analysis shows that the proposed system can adequately resolve the drinking water problems of deprived areas of Pakistan. The average water productivity of 1.5 L/d/m 2 is achieved with a total investment of PKR 3000 (<$20). This real site testing data will serve as a guideline for similar system design in other arid areas globally.


Introduction
The global water demand has been continuously increasing by ~1% per year since the 1980s [1], thus stressing the existing freshwater resources [2,3].About one-fifth of the world's population is already living in water-scarce areas, with many regions lying below the availability level of ~500 m 3 per capita per year [4,5].Therefore, many of these regions are already using water desalination to augment their water supplies to cope with the growing demands [6,7].Meanwhile, it is important to mention that the conventional commercial-scale desalination systems [8][9][10] can only reach 10-13% of their thermodynamic limits [7,11], resulting in high energy consumption.However, recently, some hybrid desalination technologies have shown improvement of the conventional processes' limitations [12][13][14][15].The overall water cost for these systems is leveled-off at 0.9 ± 0.3 $/m 3 [16], which is high for the low-income countries, where the average salary is close to $1-2/day [17].
Further, the recent COVID-19 global pandemic has also unveiled many issues like food supply in remote areas, immediate budget shifts to primary healthcare, and frequent handwashing requirements [18,19].These measures worsened the water scarcity in many countries by increasing water consumption by 20% and wastewater generation by 15-18% [20].Therefore, the affordable freshwater supply in remote areas is a serious dilemma for both the residents and the authorities.Moreover, large-scale investments (i.e., millions of $) [21] on industrial-scale water treatment plants are not viable for technologically backward countries.Therefore, there is no time more urgent to address the world's water crisis than now when people are constantly being reminded to use water to combat the virus spread [22].Therefore, it is the right time to focus on the implementation of small-scale water treatment systems operating on alternative energy sources [23], especially for remote areas in water-stressed countries like Pakistan where most of the population is residing in less developed regions.In this regard, first, the country's water situation is assessed comprehensively to summarize the water availability, consumption, quality, issues, and management plans as presented in the following sections.

Pakistan's Water Situation
Pakistan is the 5th most populated country worldwide, and its population is expected to increase from 207 million in 2017 to 333 million by 2050 [24,25].According to the United Nations Development Program (UNDP-2012) report, Pakistan is likely to face a 14% increase in water demand by 2025, which could be even higher, depending upon the industrial and economic growth rate [26].On the other hand, it is ranked 19th on the list of water-stressed countries by the World Resources Institute [27], and the freshwater supply is highlighted as a formidable challenge by the Pakistan Council of Research in Water Resources (PCRWR) [28].Moreover, around 19% of Pakistan's GDP is associated with the agriculture sector [29], which shares about 91.6% of the total water consumption in the country [30,31].Therefore, water shortage directly damages economic growth.Meanwhile, the quality of groundwater is another issue, and the country is ranked 80th in terms of drinking water quality [32].From 16.71 million hectares (mha) of groundwater (especially from the Indus basin), only 5.75 mha is available for drinking and agricultural activities due to low salt concentration <1000 ppm.The remaining ~1.84 mha is slightly more concentrated at 1000-3000 ppm, and ~4.28 mha is highly concentrated at >3000 ppm, which cannot be used without treatment [33][34][35].
Furthermore, almost 80% of the country's water is contaminated with bacteria and other organic/inorganic pollutants causing waterborne diseases, which constitute about 80% of all diseases and are responsible for 33% of deaths [36].Moreover, arsenic contamination is another major issue in almost all parts of the country, and ~47.5 million people are residing in regions with arsenic levels well above the WHO's permissible limits [37,38].Therefore, overall, around 62% of the urban and 84% of the rural population in Pakistan do not have access to fresh water, resulting in high water-related health issues [39,40].The issue is particularly severe among infants, as the country is facing around 0.06 million children deaths annually because of water-borne diseases, which cost around $1.3 billion/y [41][42][43].Keeping in view the current and forthcoming water crises, Pakistan is looking forward to creating a solid platform to enhance cooperation and partnerships globally [44].It will help to adopt the United Nations' Decade of Action (2018-2028) "Water for Sustainable Development" [45].In line with this action plan, Pakistan's first-ever National Water Policy was approved in April 2018 [46].A comprehensive summary of all water-related activities in the country is presented in Tables A1-A4 in Appendix A.
Furthermore, the government and NGOs are collaboratively working on establishing a small water treatment filtration system in rural areas to provide clean water at the town level [47].However, these projects are facing halts in terms of electricity supply in the remote areas because around 40% of the rural and 7% of the urban population do not have electricity facilities [48,49].The expansion of solar photovoltaic PV systems around the country can help to address the power requirement issues for water treatment systems, but it is time-consuming and requires large investments.Secondly, the diverse groundwater composition is an even bigger challenge (i.e., turbidity, total dissolved solids (TDS), pH, microbiology, etc.), which makes membrane selection, maintenance, and replacement complicated and expensive [50].A current market survey [51] suggests that a domesticscale photovoltaic reverse osmosis PV-RO unit with a capacity of 100 gallons/day needs a capital investment of PKR 21,000.Additional PKR 12,000/year are also required for operation and maintenance of the overall system for smooth operation (chemical dosing, filters, and membrane replacement, etc.).Meanwhile, the evaporation-based water treatment systems can remove the impurities more effectively and can handle a variety of feed water without affecting the operation and produced water quality.The simplest evaporation-based water treatment system is the solar still, which works on the natural evaporation-condensation cycle [52].Many recent studies have shown the scope of this system for remote areas with different modifications such as the use of phase change materials, nanoparticles, integration of active equipment, and augmenting the productivity through parallel systems [53,54].It is important to emphasize that, despite technical suitability, severe feed water quality handling capability, fewer maintenance requirements, and economic affordability, the solar still system has not yet been developed and tested for the climatic conditions in Pakistan due to lack of design data.Therefore, the current study is focused on the fabrication and experimental investigation of two different passive solar stills (single-slope and pyramid shape) to provide detailed design specifications for future applications.In the first step, a single-slope solar still is fabricated and tested to investigate the effect of solar irradiance, wind velocity, and 24 h operation.Then, a pyramid shape system is developed for higher productivity that can be stacked in parallel to increase the capacity to support different family sizes.The proposed solar still will provide clean water at low maintenance and affordable investment requirements to the targeted community in remote hot and dry areas in Pakistan such as South Punjab and Interior Sindh.

Climatology and Solar Potential
The analysis was done in Rahim Yar Khan, situated in the South Punjab of Pakistan, i.e., an arid area with a relatively warm climate.It is located at 28.4° N latitude and 70.3° E longitude, at an elevation of 84 m.The highest average daily solar radiation (Global Horizontal Irradiance, GHI) ambient air temperature, and wind speed in June are 6.86 kWh/m 2 /d, 33.2 °C, and 4.2 m/s respectively [55].The daily solar GHI from March to September is recorded as >5 kWh/m 2 /d [55].The solar GHI, temperature, and wind speed for the whole year are presented in Figure 1.The proposed site not only has good GHI, but is also suitable due to the poor groundwater quality in the area, low precipitation rate, high ambient air temperature, and high solar intensity, with ~10 h long sunshine duration in summer.
The productivity of the solar still system strongly depends upon the solar radiation, ambient air temperature, wind speed, dust, and cloud cover [56,57].Aburideh et al. [58] examined the effect of climatic conditions on double-slope plane solar still, and they concluded that an increase in productivity strongly depends on solar radiation.Hinai et al.'s [59] investigation showed that a 10 °C increase in the ambient air temperature improves production by 8.2%.El-Sebaii [60] analyzed the effect of wind speed on the outputs of solar stills and found that the output was boosted with increasing wind speed.The rate of evaporation rises with increasing wind speed.As reported in many studies, the performance of the solar still system increases with the increase in solar radiation, ambient air temperature, and wind speed [57].Thus, the Rahim Yar Khan region has the rich potential of the solar effective operation of solar stills.

Proposed System and Methodology
The proposed system consists of a solar still that was fabricated and tested in two different configurations.In the first step, a single slope solar still (a simple standard configuration as shown in Figures 2 and 3) was developed and tested to investigate the performance under local operating conditions.This was followed by the development of a high-productivity advanced pyramid shape passive solar still as shown in Figures 4 and  5.A square-shaped basin of 1 m 2 area and 0.06 m height at the bottom was used to store the feed water.The feed water evaporates by absorbing solar thermal energy.The vapors rise and condense after striking the glass cover, and due to the adhesion effect, the condensate is collected in a rectangular channel (2 inch wide food grade plastic sheet) attached at the end of the glass cover.The freshwater is collected in a container attached to the channel.The product water sample collected was tested for important water quality parameters, and the results are presented in the subsequent sections in the subsequent sections.The system works in a batch process, and the brine is collected at the end of each cycle (24 h).The water basin has two apertures for feed and brine.Due to high productivity, the results are discussed in detail for pyramid shape configuration only, as the single slope system may be considered a special case as a single side of the pyramid system with lower productivity.
Meanwhile, it is important to mention that the system is sealed externally using silicon to avoid penetration of dust or water from fog or rain.However, it is also worth mentioning that the system gives productivity during sunshine because the system is designed for hot and humid areas with very rare rain seasons, since, in the other case, rainwater can be collected and used for drinking purposes.Moreover, the maintenance of the system only involves the removal of suspended particles from the basin, which can be done by removing the glass cover for which the glass lifting supports are installed as shown in Figures 3 and 5.After that, the sample collected is tested using water testing kit (TDS meter) as depicted in Figure 6.

Governing Equations
The above-proposed system is also analyzed theoretically to calculate productivity.The governing equations used for this purpose include mass, material, and energy balances as given below.
The energy balance within the solar is still based on the thermal radiation incident, transmitted through the glass cover to the water surface, the heat losses by convection, evaporation, and radiation back to the glass cover from the water surface to cover [52].By considering the thermal capacitance of seawater, the energy balance is given as where αw is absorptivity of water, is the transmissivity of glass, global irradiance, qga is heat loss from glass to air, qba is heat loss from the basin, m is the mass of water, and Tw is the temperature of feed water.The energy losses from the body to ambient due to convection, radiation, and conduction is given as [61]    ga r c e q q q q (4) The heat flux from the water surface to glass cover by radiation is given as [52]   where Fs is the radiation shape factor and depends upon the geometry and solar radiations.For the case of a pyramid-shaped solar still, the shape factor is taken as 0.9 [62]: Energy loss from water body to base of the solar still is given as The heat flux from the water to glass cover by natural convection and evaporation is calculated as The heat flux from water to the cover by natural convection and evaporation is given as The heat loss from solar still to the outside environment depends upon the convection and radiation heat transfer coefficients, while, the radiation to the sky depends upon the effective sky temperature, which is generally taken as 11 °C less than the ambient temperature [52].The equation for combined radiation and convection heat transfer coefficient is given as [52].
The above equation is not valid for all types of plate types, so the modified form of the formula is given by Sparrow et al. [63], which is more reliable to determine the heat loss from glass to air.It is given by Equation (15).
The heat transfer coefficient inside the solar still due to natural convection is calculated using the correlation modified by Antar and Zubair [52] as follows.
where + sign in exponent shows that the term will only be considered if it is calculated >0.Ra is Raleigh number, which is calculated as where T is the modified temperature difference, calculated as [52]    ' ( ) 273 Finally, the mass of distillate (freshwater) is estimated based on the above-calculated heat transfer coefficient (hc) and partial pressure of water at Tw and Tg.

Results and Discussion
The proposed system was analyzed experimentally as well as theoretically considering the freshwater productivity as an output parameter.Table 1 summarizes the operational data and shows that the proposed system satisfactorily removed the dissolved solids from the brackish feed.Moreover, it is also observed that the system can concentrate brine to achieve a TDS of ~35,000 ppm, thus approaching zero liquid discharge.However, in the current case, the system is operated to fulfill the drinking water needs, which requires a continuous washing of brine for higher productivity.The developed system was operated continuously for a week to investigate the variations in productivity against temperatures, wind velocity, and solar irradiations.The data for a complete day (productive hours) are presented in Table 2.It is observed that the noticeable productivity starts between 7:00 and 8:00 a.m., when evaporation occurs.Before that, a small amount of condensate is observed because of condensation during the night due to temperature drop.The peak hourly productivity is observed between 12:00 to 16:00 due to maximum evaporation at solar noon.After that, the evaporation rate declines; however, the condensation occurs at a rather higher rate due to temperature drop.The overall per day productivity is estimated as an arithmetic means of weekly productivity as 2.5 L/day.The variations in ambient, glass, and feed temperatures during the day are presented in Figure 7.It is observed that these temperatures (particularly TF) show a significant rise between 11:00 to 16:00 because of peak solar irradiance.In the meantime, the maximum productivity is observed as shown in Figure 8.A close agreement between theoretical and experimental productivity is also observed in Figure 9, which confirms the model validity as well.Moreover, it is also noticed that the wind velocity varies randomly over the day; however, the productivity is observed to be more sensitive to the temperature variations than Vwind.The hourly water production for three consecutive days is presented in Figure 9.This shows that productivity is almost the same for all days, with some sunshine and temperature.Therefore, it is expected to reasonably have the same productivity for the months with the same climatic conditions (i.e., February to November).3 shows the suitability of distillate water for drinking purposes.Most importantly, it decoupled the water production from the grid connection using solar energy, so it can be applied in remote areas where there is either no power supply or high load shedding due to limited electricity generation.The overall cost of the system is estimated as PKR 3000 (<$20) with minimal maintenance.The proposed solar still can be designed per family member with a small increase in cost.In addition, due to simple operations, local illiterate people can easily operate without any training.

Conclusions
The solar-based passive pyramid solar still with a basin area of 1m 2 has been designed, fabricated, and tested in Rahim Yar Khan city of Pakistan.The effect of feed water temperature, wind speed, and ambient temperature on the experiment productivity of the solar still has been analyzed with the following findings.
The yearly average value of daily solar radiation-horizontal is >5.2 kWh/m 2 /d, which is suitable for the solar desalination system.The feedwater temperature increases to a maximum of 72 °C due to the highest ambient air temperature and solar radiation in the summer season; therefore, the water productivity rises to the maximum value.The wind speed increases the external heat transfer coefficient, which is also increased due to the increase in the temperature difference between glass and feed water.The peak ambient temperature is recorded at 42 °C at noontime, and it is the maximum-productivity time.The different days of the experiment have shown different productivity, which is close to the theoretical productivity.The average daily productivity obtained was 2.5 L/m 2 /day with 20 TDS and up to 7.5 pH and no arsenic content.This project will help to increase the use of renewable energy resources to overcome water scarcity in remote and off-grid areas.It is a sustainable alternative to conventional energy-based systems and can help the implementation of National Water Policy in true spirit.It can also boost up the research in the water sector and complete the projects related to water treatment plants.It

Acknowledgments:
The authors acknowledge the support provided by Northumbria University, Newcatsle upon Tyne UK, and Khwaja Fareed University of Engineering and Information Technology (KFUEIT).Also acknowledged is the support provided by Engr.Muhammad Adeel (ex Undergraduate student at KFUEIT) for organizing the literature.

Conflicts of Interest:
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Figure 2 .
Figure 2. Schematic diagram of the single-slope solar still.

Figure 7 .
Figure 7. Temperature variation during the day (23 June 2020) and the abbreviations in the figure are as follows: Tamb = ambient temperature, Tg = glass temperature, and TF = feed temperature.

Figure 8 .
Figure 8. Productivity variation during the day (reading taken 23 June 2020) and the circle and arrow in the figure refers to the secondary axis for wind speed also the abbreviations in the figure are as follows: md = theoretical productivity, mac = actual productivity, and Vwind = Wind velocity.

Figure 9 .
Figure 9. Productivity variations in three typical days and the abbreviations in the figure are as follows: Th = theoretical and Exp = experimental. 

Table 2 .
Hourly data of variation in temperatures, wind speed, and productivity.

Table 3 .
Chemical composition test for water samples (The AQUA Solutions, RYK).

Table A2 .
[66][67][68]lans related to water issues[66][67][68].Adoption of "Water for Sustainable Development" and IWRM model, firm implementation of National Standards for Drinking Water Quality (2008), Canal and Drainage Act of 1873, and use of media for awareness of water-related challenges  Purification of saline water, recycling of sewerage and industrial water using local technologies, solar-based water desalination for remote areas, and low cost  Incorporation of proper pretreatment systems for better plant performance, use of nonchemical water softening techniques (magnetic treatment), improvements in the investments by central and provincial departments for sewerage water recycling and freshwater safety, suitable treatment of private housing schemes' wastewater before disposal, excavation of water wells where subsurface hydrogeology permits (seaside) Ministry of Environment (2005)  Improve water treatment facilities, water quality monitoring, facilitate municipal authorities for pure water supply, suitable measures for rainwater collection, artificial recharge of groundwater in arid areas, metering of water consumption to discourage unnecessary water usage, implementation of Water Conservation Act  Integrated watershed management, continued freshwater flows in the marine ecosystems, programs for clean-up and up-gradation of water bodies, and standards for the classification of surface and groundwater Supreme Court of Pakistan (2018) [68]  Initiation of Dam Fund by Chief Justice (CJ) and Prime Minister of Pakistan for the construction of two new dams Diamer Bhasha and Mohmand Dam