Solar Distillation of Impure Water from Four Different Water 1 Sources under South-Western Nigeria Climate

. The enormous problems caused by scarcity of potable water and transmission of water-borne diseases 10 such as Cholera, Dracunculiasis, Hepatitis, Typhoid and Filariasis in some parts of Nigeria have created a public 11 health concern. Thousands of lives are wasted daily due to contact with water-borne diseases. The insufficient 12 medical resources available in developing countries are deployed towards the treatment of water-borne diseases that 13 can easily be avoided if potable water can be made available. This study seeks to investigate purification of four 14 different water samples (namely, water from flowing river, freshly dug well or groundwater, rainwater from the 15 rooftop, and heavily polluted dirty water) consumed by the people in the local community using solar desalination 16 method. A single basin solar still was constructed and experimental studies were carried out to determine the 17 influence of solar insolation and temperature variations on the yield of the distillate for both passive and active solar 18 still tested. The quality of the distillate was tested by measuring the total dissolved solid (TDS) and electrical 19 conductivity (EC) and later compared to World Health Organization (WHO) standard for drinkable water. The 20 values obtained after desalination falls within the acceptable/tolerable range for TDS and EC in accordance with the 21 World Health Organization standard for quality drinkable water. This analysis provides an indigenous distillation


Introduction
Naturally, most water exists in a polluted or non-purified form with lots of microorganisms capable of causing Cholera, Dracunculiasis, Hepatitis, Typhoid, Filariasis and so on (Rab, M. A., Bile, M. K., Mubarik, M. M., Asghar, H., Sami, Z., Siddiqi, S., ... & Burney, M. I., 1997).Most times, the water consumed by people in sub-Sahara Africa, Nigeria specifically is sourced from a flowing river, freshly dug well or groundwater, run-off water from rusted iron or asbestos rooftops and heavily polluted water (dirty stagnant surface water) without any further purification (Onwujekwe et al., 2009;Smith et al., 2004).The drinking of water from these sources without further purification poses health challenges to different rural settlements in this category.For instance, rainwater collected from rusted rooftops are not only dirty but may be harmful (Abbasi and Abbasi, 2011;Bennamoun et al., 2013;González, 2012;Lye, 2009;Meera and Ahammed, 2006;Mendez et al., 2010;Norman et al., 2019; University of Texas at Austin, n.d.).Many water purification processes exist including desalination technology with over 10,000 desalination plants in the world having a total desalted water capacity of over 5 billion gallons a day.Saudi Arabia is the largest user of desalination with about 25 percent of the world capacity, and the United States is the second largest user with Sundaram, 2013;Taghvaei et al., 2015).This quantity produced varies largely with the available solar radiation, cloud conditions, atmospheric humidity, wind speed and ambient temperature, which are meteorological parameters that cannot be altered by human beings.Other design parameters that affect productivity are the orientation of the still, depth of water, inclination of the glass cover, slopes of the cover, insulation materials, area of absorber plate, the inlet temperature of water and the temperature difference between the glass cover and the basin water (Sivakumar and Sundaram, 2013).
The salinity of any water strongly depends on the electrical conductivity (EC) of the water.The lower the EC of a water, the purer the water.Low levels of salts are found naturally in waterways and are important for plants and animals to grow.High salt levels in freshwater causes problems for aquatic ecosystems and becomes complicated in human organs.
Apart from the coastal region of Nigeria where people are forced by circumstance to process salty water for domestic use, the commonly available water in some rural areas is not pure due to dissolved organic and inorganic materials.In some locations (e.g Ile-Ife, Osun state: 7.4905°N, 4.5521°E) the salt content of water fetched from dug wells, rivers and even bore hole is very high and requires treatment.Hence, an affordable, yet very efficient process that requires little or no technical know-how and maintenance for the purification of water from these sources will be a welcome idea in such rural settlement.Therefore, the main objective of this study is to design, construct and test a solar desalinating plant made with locally sourced materials for the purification of water from the following sources; rainwater, freshly dug well water, river water and heavily polluted water which are peculiar to the site where this research is carried out and most rural areas in Nigeria.
The effects of the solar radiation intensity, inner glass surface temperature and the absorber plate temperature as they affect the hourly distillate yield was examined for both passive and active solar still configurations.The performance and efficiency of the solar desalinating plant was evaluated based on its distillate yield.Finally, the water samples were tested after desalination to ascertain suitability of the water for drinking purpose based on the WHO standard for drinking water.This is with a view to mitigate the widespread of water-borne diseases in rural settlements in Nigeria as a result of indiscriminate drinking of untreated or impure water due to unavailability of drinkable water.

Materials and Methods
This research work was carried out in the Department of Mechanical Engineering, Obafemi Awolowo University, Nigeria (Latitude 7.5175° N and longitude 4.5270° E) between the month of July and September.Two sets of experiments were prepared: the conventional solar still (CSS) and conventional solar still with a flat plate collector (CSS-FPC).For CSS-FPC type, a pressure valve was used to prevent water inlet into the still until the desired water temperature and the pressure was reached to sufficiently force the pressure valve opened to allow the flow of water to the still basin from the flat plate solar collector.
In this experimental work, the CSS was fabricated with a square stainless-steel sheet of 1 m 2 and 2 mm thickness.
The surface area of the solar collector that receives the heat from the sun measures 1 m 2 .The solar still basin was coated with a black paint in order to increase the solar radiation absorptivity of the still.The black body absorbs the heat energy from the sun to raise the temperature and the vapor pressure of the water (Cowling, T. G., 1950;Manabe, S., & Wetherald, R. T., 1967).Figure 1 shows the isometric and the exploded view of the experimental setup, while Figure 2 shows the photo of the experimental setup.A single slope CSS was used basically because it is a good recipient of higher levels of solar radiation at both low and high latitude stations compared to its doubledsloped counterpart (Sivakumar and Sundaram, 2013).Stainless steel was used to construct the basin principally due to its higher heat retaining capacity and higher resistance to corrosion that could further contribute to the water salinity.The CSS exterior walls (the sides and the bottom) were thermally insulated using 5 cm fibreglass to prevent heat energy loss from the solar still to the surroundings.Silicon sealant was used to prevent water leakage within the system and to create an air-tight environment in the interior.
The solar still was covered with a condensing glass having 5 mm thickness.The glass selected was a tempered glass of high tensile strength capable of withstanding high solar radiation intensity, wind and rain load with very low solar reflectivity (El-samadony et al., 2016).Morad et al. ( 2015) showed that increasing the glass cover thickness reduces the amount of solar radiation that passes through it into the air gap then to the basin water hence reduction in SS thermal retention ability and efficiencies as the glass cover thickness increases.The glass inclination is one of the major parameters that determine the CSS performance.SS productivity was found to increase with a decrease in glass inclination (Edlin, 1973;Garg and Mann, 1976).In the present experiment, the tilt angle of the glass cover was kept at 17°52'', that is, the latitude, ∅ of the research location (7.5175° N) plus 10° (Chinnery, 1971;Elsayed, 1989;Felske, 1978;Heywood, 1971;Khorasanizadeh et al., 2014;Qiu and Riffat, 2003;Stanciu and Stanciu, 2014).A float valve was used to maintain a constant water level in the basin as the water flows from either the storage tank or the flat plate collector.Water productivity has been found to be inversely proportional to the water depth (Elango et al., 2015a(Elango et al., , 2015b;;Kabeel et al., 2014aKabeel et al., , 2012;;Manokar et al., 2014;Muftah et al., 2014;Nafey et al., 2000).Also, a depth of 5 cm was found to be the optimum water depth for an improved SS performance according to Kabeel et al. [28,29].In addition, the higher the distance between the glass cover and the basin's water surface, the more the energy and the time required of the vapor to travel to the inner glass surface (Tiwari, A. K., & Tiwari, G. N., 2005, 2008;Tiwari et al., 1994).Hence, the gap was reduced to 2 cm.

Experimental Design
Four different water samples (rainwater, freshly dug well water, river water and heavily polluted water) commonly consumed by people in rural settlements in Nigeria due to unavailability of clean drinkable water were selected for the purpose of this research.The water from these sources has been found to be dirty and unhygienic for human consumption (Onwujekwe et al., 2009;Smith et al., 2004).The villagers, even passers-bye have their bathe, urinate, defecate and even dispose-off their refuses or dirt in the river water and the heavily polluted water.Following the solar still design and set up above, the experiments were conducted for a period of thirty days between 8 a.m. and 6 p.m. while readings were taken on an hourly basis.One water sample was chosen for each day and was filled into the solar still basin to the required depth (5 cm as mentioned above).The basin was subsequently tilted to angle 17°52'' based on the geographical location of the research.The experiment set up was left outside in the sun to run between 8 a.m. and 6 p.m daily.During this period, the heat from the sum evaporates the water in the basin and later condenses on the inner surface of the glass which is later channeled and collected.The temperature of the inner surface of the glass (condensing surface), the outer surface of the glass, absorber plate (evaporating surface) of the solar still, basin water temperature and temperature of the glass of the flat plate collector were measured and recorded intermittently on hourly basis through a data logger while the experiment is ongoing.Five pieces of Copper-constantan thermocouples (Type T) with temperature readout were strategically mounted on different parts of the experimental set up to measure temperatures at specific locations.Also, the most important meteorological parameters for efficient performance of the CSS such as solar radiation, ambient temperature and wind velocity were subsequently measured and recorded.By the law of nature, these parameters cannot be controlled/altered; however, they were measured using a weather station positioned at the research location (Figure 2).A transmitter was incorporated into the weather station.This was used to download and record necessary meteorological parameters such as the amount of rainfall, ambient temperature, relative humidity, wind velocity and luminous intensity.Dust deposition and shade coverage on the glass surface reduces the transmittance power of the solar radiation which could affect the distillate yield and the efficiency of the solar still.Hence, these were controlled by placing the experimental set up at a height of clean environment, a little above the ground cleared of anything that could constitute shade coverage and the glass surface also was cleaned intermittently using a wet towel.
The basin was washed and made ready for another water sample after each experiment.These experiments were conducted for both conventional and the single slope solar still with flat plate collector following the same steps discussed above.

Performance Evaluation
The TDS in the water sample was measured using a digital conductivity meter by Mettler Toledo with ±0.5 % conductivity accuracy.The digital meter was used to measure both the TDS and the EC.It consists of a mode which is usually interchanged/switched when either the TDS or the EC measurement is required.This digital meter consists of a probe.For each time, each water sample was to be tested, the probe was immersed into the water sample up to the maximum manufacturer's immersion level after the protective cap was removed while the temperature of the water sample is maintained at room temperature..The water sample was thoroughly agitated to dislodge air bubbles and evenly distribute the particulate matter present in the water.The TDS and the EC level for the sample were taken after the reading stabilizes.After each measurement, the probe was thoroughly cleansed as prescribed in order to eliminate the interference of the previous sample particle with the current sample.The digital meter also displays the temperature of the water sample to be measured.The reading gives us the salinity estimate of the produced fresh water from the solar desalination unit.
The percentage reduction in TDS and EC was be calculated using Eq. ( 1):: (1) P = Parameter under consideration (TDS or EC).
Subscript a and b represent after and before respectively.
The solar still instantaneous efficiency,   was calculated using Eq. ( 2): Also, the daily production efficiency,   of the solar still system was be calculated using Eq. ( 3): where,  ℎ = distillate productivity per hour   = Absorber Area C = Concentration ratio that is     ⁄   = Aperture Area ℎ , = latent heat of vaporization daily average.

Results and Discussion
Experiments were conducted for a period of thirty days between 8 a.m. and 6 p.m. while readings were taken on an hourly basis.

Solar radiation and temperature variations in solar still
Solar radiation is the radiant energy emitted and deposited by the sun in an area every second from a nuclear fusion reaction that creates electromagnetic energy with a temperature of about 5800 K.It is one of the most important factors that determines the solar still productivity (Sharshir et al., 2016).Figure 3 (a-d) shows the variation of solar radiation intensity, ambient temperature, glass temperature, absorber plate temperature and water temperature with time for some randomly selected days.The graphs and results for other days shared some similarities.It was observed that the temperature keeps increasing until maximum point around 3 pm in the afternoon for all days of the experiment.This is due to a consistent daily increase in the solar radiation intensity until 3 pm in the afternoon.The temperatures began to drop as soon as the solar radiation intensity started dropping, and vice versa.This shows that the solar radiation intensity determines the temperatures of the elements in the still.It is also observed that the ambient temperature is always lower than all other temperatures for all days of the experiments in the research location.The solar radiation was maximum on the first day of the experiment with the intensity of about 1128 W/m 2 at 3 pm in the afternoon and the lowest value obtained was 27.2 W/m 2 on the second day of the experiment at 7 am in the morning.The solar radiation intensity was measured with Eppley precision spectral pyrometer (PSP) with an accuracy of ±0.5% from 0 to 2800 W/m 2 .
It was observed that the evaporation rate and consequently the distillate yield increased as a result of an increase in the temperature difference between the temperature of the inner surface of the glass (condenser) and the temperature of the absorber plate (evaporator).From the graphs in Figure 4, it can be depicted that the glass temperatures are far lower than the temperature of the water.The minimum condensation glass temperature obtained was 25 o C and the maximum was 40 o C. The wind speed of the environment at the moment under consideration affects the rate of condensation by the glass.The faster the wind speed the faster the vapour loses its latent heat of vaporization to the surroundings.The increased wind speed yields a rapid drop in the condensing glass temperature and hence a wide temperature difference between the condensing glass and water.This enhanced the heat transfer performance and hence the distillate yields because heat transfer rate is directly proportional to temperature difference.This is in good agreement with some similar past studies (El-Sebai, 2000;El-Sebaii, A. A., 2004;Stonebraker et al., 2010;Winfred Rufuss et al., 2017).
The temperature increase in the absorber shows that the absorber and the black body material is a good absorber and retainer of heat.This property was responsible for evaporation even in off-peak periods when there was no sunlight and little or no solar irradiance.The stored heat in the black body raised the temperature of the water in the basin and with the corresponding saturation pressure, evaporation occurs.The maximum temperature obtained for the absorber was 63 o C

Effect of temperature variation on distillate yield
Figure 4(a-d) shows the effect of temperature variation on distillate yield.Figures 4 a and d gives the distillate yield for the active solar still while Figures 4b and c represent the distillate yield for the passive still.The graphs also justify that temperature difference (that is the difference between the glass cover and absorber plate temperatures) is the major factor responsible for evaporation.This trend was also observed by several authors but to mention a few (Ahsan et al., 2013;Ali et al., 2019;Edeoja et al., 2015;Kumar and Bai, 2008;Murugavel et al., 2010;Onyegegbu, 1986;Ozuomba et al., 2017;Sathyamurthy et al., 2015).Preheating the feed water to the solar still basin plays an important role in increasing the productivity of the still (Ahmadi et al., 2017;Badran and Abu-khader, 2007;Delgado-Torres et al., 2007;Kalogirou et al., 2016).Comparatively, huge distillate yield was experienced when the flat plate collector was used on days 8, 1, 9 and 2 as shown in Figures 6 (a and b).The solar still was used alone without the flat plate collector in the remaining days.It was observed that continuous deposition of hot water into the basin from the Flat Plate Collector resulted into higher production rates in all operation periods and mainly between 2-4 pm daily.This is due to higher internal convective, evaporative and radiative heat transfer from the water to the glass cover as the preheated water from the flat plate solar collector is deposited to the basin.Higher temperature differences were observed in solar still with the flat plate collector compared with that of no flat plate collector throughout the working hours and under all conditions of the experiment.

Effect of solar radiation on distillate yield
Figure 5 (a-d) shows the variation of solar radiation intensity and the distillate yield with time.Like the temperatures, the solar radiation intensity had a similar effect on the distillate yield.However, the differences between the effects with and without the flat plate collector cannot be easily detected using the solar radiation intensity curve alone.The temperature curves clearly show the differences between the glass temperature and the water temperatures and their consequential effects on the solar still productivity.Furthermore, the graphs (Figure 5 a-d) clearly indicate that the incident solar radiation strongly determines the increase in the Still productivity.The graphs clearly show the significant differences between the cumulative yield and the distillate yield per hour of the still incorporated with the Flat Plate Collector and the ones without the Flat Plate Collector.Day 8 shows significant cumulative distillate yield not only because of the second largest solar radiation intensity recorded for the day (965 W/m 2 ) but basically because of the comparative huge temperature difference between the glass cover (condensation surface) and the water in the basin and the consistently higher solar radiation intensity recorded for the other hours of the day.As earlier discussed, the variations observed in the distillate yield are due to the condensation glass-water temperature difference, wind speed variations and relative humidity of the research location per time.The contents of the polluted/saline water and the extents at which the water is polluted also affects the evaporation rates and hence the solar still productivity because the presence of impurities increases the boiling point of a fluid (or any substance) (Cengel, Yunus A., and A. J. Ghajar., 2011).Details of this are not explored in this research.

Laboratory Examination of the Water Samples before and after Desalination (Quality of the distillates from the raw water samples)
Table 1 shows the results of the water analyses conducted before and after the solar distillation process.Observation shows that water quality lies within the acceptable range for good and drinkable water according to WHO prescription for EC.Also, the physical appearance of the distillate/desalinated water shows good turbidity (water looks so clear and colorless) appealing for human consumption.Also, the repulsive and the irritating odor of the heavily polluted water was drastically reduced.

Comparison of the distillate yield in the present studies against that which exist in the literature
Several authors have worked on solar still of different configurations.Table 2 shows the comparison of performance evaluation of earlier results and the results of the present studies.With the understanding that the performance of any solar still is dependent on the location under consideration viz-a-vis the inherent/current climatic and atmospheric condition, diurnal irradiance and other specified experimental conditions, however, it can be noticed that the performance of the solar still in consideration is relatively comparable with those existing in the literature and in some cases of better performance despite the simple design.

Solar Still Efficiency
The average of the overall daily efficiencies of the CSS with flat plate collector and the single slope solar still with flat plate collector are 13.906 % and 16.298 % respectively.This shows an improvement of 14.67 % with the inclusion of the single slope design compared with the conventional type.Since these values are dependent on the weather, climate and the atmospheric conditions with the diurnal irradiance coupled with the still design, hence it is difficult to compare with existing designs in the literature.
The daily production efficiency,   of the still are 15.85 % and 26.25 % respectively for the CSS with flat plate collector and the single slope solar still with flat plate collector.

Cost
It is important to estimate the cost of solar still basically for the purpose of improvement both in terms of production and efficiency.Kabeel et al. (2010) listed the running and capital costs that affects the cost of production of a solar still such as design and size of the unit, climatic condition of the site, the properties of the feed water, the required quality of the distilled water to be produced and the cost of wages for available staff.The adopted design in this research is tailored towards cost effective and simple infrastructure produced from locally sourced material which are readily available, easily produced, operate and maintained.This is ensured so that the set up can easily be acquired by an average family in the rural areas to make portable water readily accessible.
The solar still in this present study is made with locally sourced materials and as at the time of the construction the average cost is approximately $ 150.The analysis for the cost per liter of distilled water based on Kabeel et al. [62] Annual Comparing the cost per litre of distilled water by the present design with earlier designs by Kumar and Tiwari, (2009), Abdallah and Badran, (2008), the present design showed a significant reduction in cost of production and can be adopted by rural communities that have shortage of drinkable water.

Conclusion
The possibility of using the renewable energy from the sun in providing potable drinkable water from saline or heavily polluted water in areas where potable water is scarce has been explored using solar desalination technology.
Solar desalination method has been found to be a clean energy and eco-friendly, readily accessible, affordable, easy and renewable method of purifying water.A single slope rectangular basin was designed and constructed with low cost, lightweight, available locally sourced materials.The effects of solar radiation intensity, ambient temperature, condensing inner glass cover temperature, water temperature and absorber temperature on the water distillate yield from the solar still were observed based on the climatic condition of Ile-Ife, Nigeria.Results show the direct relationship and huge dependency of solar still daily distillate yield on the solar radiation intensity and the temperature difference between the condensing inner glass cover and the water.A high distillate yield was recorded when the solar radiation intensity was at the peak accompanied with temperatures increase for all the solar still components at the same time in the day.The temperatures increased as the solar radiation intensity increased, however, the larger increase was experienced for water and the absorber in the basin, this was primarily due to the heat retaining ability property of the black body used.The wind speed of the research station also was a contributing factor to the drop in the glass temperature, hence constituting a huge temperature difference between the condensing inner glass cover and the water for higher heat transfer and evaporation rate and larger distillate yield.The impact of the flat plate collector on the distillate yield was also investigated.The incorporation of the flat plate collector produced higher distillate yield.The preheated water it supplied created a huge temperature difference between the condensing inner glass cover and the water which consequentially produced more distillate yield compared to a solar still without flat plate collector.The desalination product quality was analyzed based on its EC and the amount of total dissolved solid present in it.The distilled water was found to be within the acceptable range for drinkable water according to the World Health Organization standard and guidelines.This shows the potential of water desalination using solar energy most especially in areas where water-borne diseases are imminent due to the scarcity of potable drinkable water.It could be predicted from the results trends that the distillate yield would be higher during the dry season characterized by higher solar radiation intensity compared to the solar radiation intensity recorded during the raining season during the period in which the experiment was performed.still, Heat Recover.Syst.CHP, 14(4), 447-455, doi:10.1016/0890-4332(94)90048-5, 1994. Tiwari, G. N., Dimri, V., Singh, U., Chel, A. andSarkar, B.: Comparative thermal performance evaluation of an active solar distillation system, Int.J. Energy Res., 31, 1465-1482, doi:10.1002/er, 2007. Tiwari, G. N., Dimri, V. and Chel, A.: Parametric study of an active and passive solar distillation system : Energy and exergy analysis, DES, 242(1-3), 1-18, doi:10.1016/j.desal.2008.03.027, 2009.University of Texas at Austin: Rainwater harvest study finds roofing material affects water quality, ScienceDaily, doi:Retrieved June 30, 2020 from www.sciencedaily.com/releases/2011/03/110307142229.htm,n.d.Velmurugan, V., Deenadayalan, C. K., Vinod, H. and Srithar, K.: Desalination of effluent using fin type solar still, Energy, 33, 1719Energy, 33, -1727Energy, 33, , doi:10.1016Energy, 33, /j.energy.2008Energy, 33, .07.001, 2008 , M = mass of the desalinated water at the output ℎ  = latent heat of vaporization of the fluid A = Area of the flat plate collector (1 m 2 ) I = Average solar irradiation for the time under consideration ∆ = Time under consideration (usually 1 hr).

Figure 6 a
Figure 6 a and b present the cumulative distillate yield and the distillate yield per hour for the 9 days, respectively.

Figure 1 :Figure 2 :Figure 5 :
Figure 1: Isometric diagram and the exploded view of the experimental setup.

Table 2 :
Performance comparison of the solar still in terms of maximum daily productivity 502