A potential solution for clean water supply: multi-effect solar still and water quality analysis

The detrimental impact of untreated wastewater on major industrial and urban populations has reached an alarming level in various regions of the world. Both domestic and industrial wastewater, when seeped into underground water reserves, pose a serious threat (Khan et al. 2019). Furthermore, the discharge of untreated wastewater into surface water bodies, including rivers, lakes, and canals, not only contaminates these water sources but also poses a global threat to wildlife and human inhabitants (Anh et al. 2021). In developing nations, where access to safe and treated drinking water is lacking for at least one-third of the population, urban and suburban communities often rely on untreated water sources. This reliance contributes to the spread of life-threatening diseases, including hepatitis, typhoid, and diarrhea (Qadri et al. 2020). Gastrointestinal illnesses alone are estimated to claim the lives of approximately 2.5 million individuals annually, with a significant portion of the victims being children under 5 years of age.

In the quest for sustainable solutions, historical precedents, such as the use of solar irradiance for water disinfection dating back to 2000 BC, have paved the way for innovative approaches to water treatment (Reed 2004). However, the first systematic study on this subject was conducted by Downing & Blunt (1878). Their research demonstrated that bacteria could be eradicated from nutrient broth and urine through exposure to sunlight for several hours. Almost a century later, Acra et al. (1980) presented the first practical approach to sunlight disinfection, which is now commonly known as solar water disinfection (SODIS). SODIS has remained a popular disinfection process due to its low cost and ease of operation in rural areas. Various traditional techniques for purifying drinking water exist, encompassing basic settling, the use of chlorination or iodination for disinfection, reverse osmosis (RO) water systems, ion exchange for water softening, activated carbon filtration, and ultraviolet disinfection. These methods have served as reliable means to ensure safe drinking water for many years. Despite the continuous progress in water treatment approaches, like settling, chlorine disinfection, RO systems, and ion exchange water softeners, they come with their own set of challenges. For instance, while RO can effectively eliminate contaminants, it is not without drawbacks, such as incomplete removal of certain pollutants and the high costs associated with installation. These limitations, particularly the financial barriers, pose significant obstacles to their extensive implementation, especially in regions with limited resources (Hanson et al. 2004).

Amid these persistent challenges, solar distillation systems have gained prominence as a compelling solution for providing clean drinking water in rural and suburban areas. Leveraging solar radiation, these systems employ evaporation to separate impurities from water, followed by condensation of the purified water. Solar distillation, primarily through stills, thrives in sunlight-rich areas, offering a cost-efficient route to water purification (El-Bialy et al. 2016; Younis et al. 2022). Solar stills' practicality has been demonstrated over decades, offering an accessible, durable, and economical means of water distillation, particularly advantageous in arid and saline-prone regions (Rajvanshi 1981; Murugavel et al. 2008, 2010). These advancements in solar distillation technologies have led to their increased recognition as a promising avenue for safe water production.

As researchers and innovators explore the potential of solar distillation, one critical aspect that has garnered attention is the optimization of multi-effect solar stills. The demand for cost-effective and energy-efficient solutions has spurred investigations into the shape and number of stages for condensation of water in these stills. Recent studies have shed light on the design considerations of multi-effect solar stills (Xiao et al. 2013). These stills are categorized into active and passive modes, and the number of stages varies, with options for single, multiple, and multi-effects. However, limited work has been conducted to optimize multi-effect solar stills for enhanced water distillation. Researchers like Schwarzer et al. (2009), Adhikari et al. (2000), Shatat & Mahkamov (2010), Estahbanati et al. (2015), and Abdessemed et al. (2019) have explored various aspects of multi-effect solar still design, encompassing considerations such as the number of stages, heat recovery systems, and condensing tray shapes. Their findings have shown promise in improving the performance and economic viability of these stills. These active solar stills, which employ concentrating techniques, have also emerged as cost-effective sources of fresh water. Solar distillation systems excel in meeting production requirements up to 50,000 L/day, making them a superior choice when compared to alternative water purification technologies. For even higher water demands, reaching 200,000 L/day, solar stills seem to be the obvious solution (Goosen et al. 2000). In comparison to other water treatment technologies, distillation has been found to be not only cost-effective but also energy efficient.

This study aims to build upon the historical foundation of solar-driven water treatment and the contemporary challenges of untreated wastewater. By exploring the performance and significance of multi-effect solar stills, this research endeavors to advance our understanding of solar distillation technologies and their potential to address the critical issue of providing safe drinking water, especially in resource-constrained communities. Furthermore, in alignment with the UN Sustainable Development Goals, this paper examines the role of energy-efficient decentralized solutions, such as solar wastewater treatment, in achieving accessible water and sanitation in low-income countries, offering a comprehensive approach to wastewater treatment within a single solar still.

The methodology comprehensively encompasses the entire process, which includes the construction and operation of the solar still, the systematic collection of samples, and the thorough analysis of a diverse array of parameters including bacteriological parameters. All these aspects are presented collectively within this section, each under its respective subheading.

The solar collector efficiently captured sunlight and directed it toward a contaminated water container, while the distiller featured multi-effect trays serving as condensing surfaces. These trays, each measuring 800 cm², were stacked closely with a vertical spacing of 9 inches. Slanted collectors, composed of the same material, were attached beneath these trays to collect the distilled water. The distiller unit has a wall thickness of 1.5 inches and was insulated with polystyrene to minimize heat loss. After optimizing various tray shapes, U-shaped condensing trays were selected for experimentation. The distiller unit was fabricated using galvanized iron steel, and the properties of the materials used are listed in Table 1. The fabrication of these components involved processes such as cutting, welding, and drilling, which were carried out at the Mechanical Engineering Departmental workshop, Bahauddin Zakariya University, Multan.

The solar still was operated in a batch mode, with four distillate units used for experimentation. Different input waters were introduced into the system. The temperature of the U-shaped multi-effect surfaces was maintained between 40 and 55 °C by continuously supplying running water from the upper sides of the condensing effects, thereby enhancing the condensation of water vapor. Water for distillation purposes was stored in 200-L tanks positioned on the roof of the mechanical engineering department building and connected through suitable piping to the focus point containers where solar radiation was concentrated. Steam emanating from the central focus point container was channeled into the distillation unit of the still. Upon reaching the underside of each condensing tray, the steam condensed back into purified water. The resulting distillate was carefully collected in graduated cylinders via a system of well-positioned conduits connected to the inclined collectors attached underneath each condensing tray. The solar stills were able to operate for up to 14 h each day during Multan's summer, thanks to the extended daylight hours. Each unit demonstrated an impressive hourly yield of approximately 2.5 L/m².

Experiments were conducted in the laboratory of the Mechanical Engineering Department at BZU, Multan, using an experimental setup as shown in Figure 1. Ten different water samples from various locations in Multan (see Table 5) were collected before and after the distillation procedure. The collection of samples for physio-chemical analysis was carried out in polystyrene bottles of different capacities. Proper washing and rinsing with the same water to be sampled were ensured. For trace elements and nitrate analysis, preservatives such as hydrochloric acid and boric acid were added. For bacterial analysis, samples were collected in sterilized containers. The samples were obtained from clean taps, tube wells, water distribution networks, hand pumps, dug wells, and water streams, with proper precautions taken to avoid unwanted materials entering the bottles. Production samples were taken from the distillate produced from the graduated cylinders attached to each condensing tray immediately after determining the quantity of distillate produced. The comprehensive methodology involved the operation of the multi-effect solar still, performance monitoring, and the analysis of various physio-chemical and bacteriological parameters. This allowed for a thorough evaluation of the solar still's efficiency and its impact on water quality.

Table 2 outlines the complete schedule of testing, detailing the nature of input water and the analyses performed. Several key parameters were analyzed. (1) Conductivity: total dissolved solids (TDS) and inorganic materials such as chlorides, carbonate compounds, alkalis, and sulfides contribute to the ionic concentration (Wong et al. 2022). TDS measurements were obtained by assessing conductivity using the Calculator-Based Laboratory (CBL) system connected to a TI-85 calculator. (2) pH value: the pH meter was employed to measure the pH value of the water. The pH of water is temperature-sensitive, with pure water displaying a high pH value at low temperatures and vice versa (Binks et al. 2005). (3) Hardness of water: the concentration of calcium and magnesium, indicative of water hardness, was determined through titration with EDTA. EDTA serves as a chelating agent that captures metal ions and binds them together. (4) Fluoride concentration: fluoride, a common water contaminant, should ideally be maintained at levels below 1.5 mg/L according to World Health Organization (WHO) guidelines. Fluoride concentration was determined using the SPADNS method developed by Marques & Coelho (2013). (5) Nitrate concentration: the method employed for assessing nitrate (NO₃) levels in water involved the use of TI CBL instruments with nitrate probes (Li & Tabassum 2021). Elevated nitrate levels exceeding 50 mg/L in water consumption have been associated with various health issues, including blue baby syndrome.

Fecal coliform bacteria assessment utilized a presence/absence test and the most probable number (MPN) method for quantification (Cho et al. 2010). Fecal coliforms were chosen as indicators for overall microbial water quality. Conventional analyses for disease-causing bacterial pathogens were avoided due to complexity and cost. The efficacy of the multi-effect distillation method was evaluated by conducting a presence/absence test for fecal coliform in both supply and production waters. Wastewater was introduced into 200 L supply tanks, and samples were incubated at 35 ± 0.50 °C for 24 h with lactose fermentation. The experimental protocol involved combining domestic wastewater from kitchen waste pipes and bathroom drains, followed by distillation in the multi-effect solar still.

The study also examined the multi-effect solar still's ability to remove different pesticides from water using the Environmental Protection Agency (EPA) method 505. This method determined the presence of organochlorine pesticides in finished drinking water, during treatment stages, and in raw water samples. Samples for this analysis were extracted in 35 mL containers along with 2 mL of hexane and analyzed using a gas chromatograph with a linearized electron detector. The quantity of organochlorine pesticides was estimated through calibration.

One of the main causes of organic contamination of ground water resulted from the use of pesticides in agricultural fields. Volatilization and their penetration into the product water are considered the main concerns of the organic contamination. Henry's law constant (HCL) is an important parameter used to describe the volatility of different materials in air/water partitioning. It tells the material's concentration ration during gaseous phase and the liquid phase after its dissolution in water. Table 3 shows a few pesticides of the organochlorine group that were analyzed with a desire to have high values of HCL and their maximum contaminant limit (MCL).

In this study, the multi-effect solar still (capable of producing an average per hour yield of 2.519 L/m²/h) was rigorously evaluated for its effectiveness in treating water contaminated with inorganic, organic, and volatile pollutants. Ten water quality variables (Table 4) were selected for analysis, with their acceptable/recommended values aligned with international standards, including those established by the WHO, US-EPA, and IBWA (Pourfadakari et al. 2022).

These findings are consistent with previous literature studies, which have also underscored the importance of adhering to international guidelines to ensure water quality. At first, 10 samples of different locations on Multan were collected, for organic parameters analyzation, and wastewater samples were collected for microbiological parameters. Samples and their contamination percentages have been listed in Table 5. Out of four distillate units, the first three were used to analyze the solar still performance against the removal of nitrate, conductivity (salinity), hardness, fluoride, and pH.

It was observed that the input fluoride concentration has shown a decreasing trend from day 1 to day 4 from where it started rising and reached to a maximum value of 15.1 mg/L, and the tap water input variation showed high values for first 3 days and became under 4 mg/L from the 4th day of the experimentation. Output distillate has shown that variation is near 0 or 0.2 mg/L for all input waters. The input fluoride concentrations decreased steadily over the experimental period, with the output distillate consistently maintaining fluoride levels near 0 or 0.2 mg/L, well within safe limits.

For tap water, the value of nitrate content was within the acceptable limit. The solar still demonstrated effective control over nitrate levels in both nitrate-spiked water and tap water, with a maximum observed concentration of 0.2 mg/L in the distillate, well below the acceptable limit.

Technically, pH is called the measure of the activity of hydrogen ion (H⁺), and in aqueous solutions, pH value can be measured as the negative logarithm of hydrogen ion concentration of the solution (Figure 5).

In this study, fecal coliform measurements were adopted as an efficient surrogate for identifying the presence or absence of these contaminants (Fiello et al. 2014). We conducted a presence/absence test to assess the microbial contamination in both the wastewater mixture samples and the distillate samples. The results were strikingly clear: all wastewater mixture samples were found to be 100% contaminated with fecal coliform. In contrast, the distillate samples exhibited an absolute absence of fecal coliform, registering a contamination rate of 0%. This outcome underscores the remarkable efficacy of the distillation process in eliminating these microbial contaminants from the water source. It reinforces the practical application of distillation as a robust method for ensuring the safety and purity of water.

Organochlorine pesticides represent a class of chlorinated hydrocarbon compounds widely employed for mosquito control and agricultural purposes. These substances, upon short-term exposure, can induce a range of adverse effects, including convulsions, discomfort, dizziness, nausea, tremors, cognitive impairment, muscle weakness, and speech difficulties.

In summary, this study underscores the remarkable potential of the multi-effect solar still as an effective and versatile water treatment technology. The results align with and contribute to the existing literature, highlighting the technology's ability to address diverse water quality parameters and contaminants. Offering valuable insights into its practical applicability, the study underscores the technology's importance in ensuring access to safe and clean drinking water, especially in regions facing water quality challenges. However, it is worth mentioning that the study's scope is confined to water reuse for irrigation and washing, with recycled water explicitly not recommended for drinking. To fully unlock the potential of this innovative water treatment approach, additional research and real-world applications are necessary. This aligns with recommendations and insights from prior studies in the field, emphasizing the ongoing need for exploration and implementation.

In conclusion, this study has presented a promising solution to the water scarcity challenge faced by the suburban and remote areas of Pakistan through the development of a multi-effect solar still. The still has demonstrated exceptional efficiency in removing various pollutants from contaminated water, with nearly 99% efficacy observed in removing conductivity, total hardness, pH, fluoride, and nitrate. Furthermore, the disinfection process proved to be effective in eliminating microbiological pollutants, with no detectable fecal coliforms or Escherichia coli present in the samples. The still's ability to produce safe drinking water was further supported by the satisfactory results obtained in organic parameter testing, including alachlor, lindane, and endrin. As a result, this proposed technology represents a viable and accessible solution to the challenge of providing clean drinking water to underserved communities.

We would like to express our sincere gratitude to the Mechanical Engineering Department and the Agricultural Engineering Department at Bahauddin Zakariya University, Multan, for their invaluable support and resources throughout the course of this research.

Data will be available upon reasonable request.

The authors declare there is no conflict.