Solar still for saline water desalination for low-income coastal areas

Applied Water Science (2019) 9:104 https://doi.org/10.1007/s13201-019-0986-9 ORIGINAL ARTICLE Solar still for saline water desalination for low‑income coastal areas Asiful Hoque1 · Ashif Hasan Abir1 · Kironmoy Paul Shourov1 Received: 16 September 2018 / Accepted: 15 May 2019 © The Author(s) 2019 Abstract Solar still is a simple device that utilizes the evaporation–condensation technique to convert impure saline water into the distilled water by eliminating dissolved salts as well as other dissolved impurities and suspended solids. This study aims to produce fresh drinking water from saline water with solar still in the context of Bangladesh. For this study, a pilot-scale solar still with an effective surface area of 0.214 m2 is fabricated with mild steel sheet. At first, solar still productivity is evaluated by varying basin water amount by 3 L, 3.5 L, 4 L and 4.5 L synthetic water. Experimental investigations show a decrease in water production with an increase in basin water amount. The optimum basin water amount is found to be 3.5 L at which distillate production is maximized. Then, the effect of salt concentration is assessed by synthetic solutions with 2000 ppm, 5000 ppm and 8000 ppm total dissolved solids (TDS). An inverse relation is found between salt concentration and freshwater production. Lastly, real seawater is fed to the basin and an average freshwater production of 2.38 L/m2-day is obtained with a removal efficiency of 99.87%, 99.83%, 99.78% and 99.81% for turbidity, chloride, TDS and electrical conductivity respectively. Keywords Saline water · Solar still · Solar desalination · Drinking water Introduction Water is the source of all forms of lives on earth which is equally important to the human as well as entire wildlife. It is estimated that about 3.6 billion people are faced with water crisis at present and by 2050, the number may increase to 4.8–5.7 billion (UN-Water 2018). In the coastal regions of Bangladesh, about 15 million people are compelled to drink saline water and 30 million people are deprived of drinking water for lack of safe water sources (Hoque 2009). In this context, desalination of water using solar still is a viable option to provide drinking water in remote and arid regions of Bangladesh, especially for smaller communities and domestic use. Solar still is a device that harvests solar energy to produce fresh drinking water from saline water by utilizing the evaporation–condensation technique. The process is entirely dependent on solar radiation and does not involve any supply of external energy, which makes it an attractive * Asiful Hoque 1 Civil Engineering Department, Chittagong University of Engineering and Technology, Chittagong, Bangladesh choice for the regions that receive abundant solar energy throughout the year. A greater portion of coastal regions of Bangladesh is affected by the salinity of both groundwater and surface water. Many coastal regions around the world, especially in the Middle East and North Africa, are now dependent on treated saline water (Buros 2000). According to the International Desalination Association (IDA), 18,426 desalination plants around the world are in operation at present. The desalination plants use conventional desalination processes, namely reverse osmosis (RO), multi-stage flash (MSF), multi-effect distillation (MED), etc., which are both energy- and cost-intensive, require huge initial investment, skilled workforce, regular maintenance and expertise for smooth operation. Moreover, these plants are not favorable for small-scale water supply for their large capital cost of plant and coverage pipeline. In such cases, where the power supply is not available, water sources are saline and demand is less than 200 m3/day. Solar still desalination remains the only process that can be utilized for freshwater production (Tiwari et al. 2003). However, the main drawbacks of solar stills involve the requirement of large installation areas, lower productivity and higher land cost associated with the larger land requirement (Ayoub and Malaeb 2012). So, the studies mainly focus 13 Vol.:(0123456789) 104 Page 2 of 8 on enhancing the water production from solar still to negate the limitations as well as on keeping the unit cost of water as low as possible. According to Abujazar et al. (2016), the productivity of a solar still is influenced by ambient conditions (solar radiation, ambient temperature, wind velocity, etc.), operating conditions (brine water depth, saline concentration, etc.) and design conditions (cover angle, insulation, etc.). The behavior of solar stills with different configurations and different working conditions has been widely studied over the past few decades. Since the distribution of solar radiation around the world is non-uniform, the performance studies of solar still are required to be replicated under the climatic condition of Bangladesh. According to the meteorological data obtained from Bangladesh Meteorological Department (BMD), Bangladesh receives an average 3.125 kW h/m2-day solar radiation during summer (March–June) with sunshine hour of 4.4–7 h while an average 2.31 kW h/m2-day solar radiation during winter (October–March) with 6–9 sunshine hours. Thus, the condition of Bangladesh is in favor of the solar still application for drinking water production. In addition, solar still is easy to construct with local labor force using low-cost readily available materials and even illiterate people can operate solar still due to its very simple operations, which make it an excellent choice in the context of Bangladesh. This study aims to evaluate the effect of brine water depth and the effect of salt concentration on the productivity of a single slope passive solar still under the climatic condition of Chittagong, Bangladesh. It is an established fact that water production from solar still decreases with an increase in brine water depth (Khalifa and Hamood 2009). The effect of brine depth is studied to select an optimum water depth for the fabricated solar still. With the optimized water depth, the effect of basin water salt concentration on productivity is also studied using synthetic saline water. Later, the solar still is provided with real seawater to compare the productivity obtained from synthetic water. Finally, water parameters are tested to determine the removal efficiencies of dissolved impurities as well as suspended particles and to ensure the quality of distilled water upon comparing with WHO guideline. Applied Water Science (2019) 9:104 with the horizontal, which is nearly equal to the latitude of the experimental site (22.5°), as suggested by Singh and Tiwari (2004), to minimize radiation loss due to reflection. Moreover, this optimized angle ensures to receive solar radiations normal to the glass surface throughout the year (Srivastava and Agrawal 2013). A distillate collection trough is built integrally with the setup by bending of mild steel sheet (...truncated)