Background:
Rapid population growth coupled with a drastic expansion of industrial production has led to a water crisis (Ciu, 2021). Many freshwater sources are getting contaminated with harmful pollutants such as heavy metals, fertilizers, pesticides, dyes, etc., making the water crisis even worse (Ghiorghita, 2020). Earth is majority water, but, unfortunately, less than 3% of that water is potable, and 2% of the potable water is frozen, so a mere 1% of the world's water can be safely ingested (Kumar, 2021). Climate change is exacerbating the water crisis. Arnell et al. found that climate change will negatively impact freshwater biodiversity, quality and quantity of potable water, and weaken the effectiveness of current water purification techniques (2015). Additional concerns include a decrease in the number of water sources caused by declining rainfall and the destruction of water treatment infrastructure as a result of flooding (Howard, 2016).
The dire need for clean water has led to the invention of many new water purification techniques (Gheorghita, 2020). Each method of cleaning polluted water has the potential for success, but some may pose more challenges in the future. These include flocculation, which produces toxic residue; membrane filtration which generates excessive waste; advanced oxidation, which is extremely expensive; electrocoagulation, that requires a plethora of power to function; and more. Certain water purification methods require non-renewable energy sources which can lead to pollution; some may require extensive funding, and some may be cumbersome (Gheorghita, 2020). For example, modern day water treatment plants have implemented a process called sludge digestion that is used to break down solids in water and can be used as a fertilizer. A major drawback, though, is that methane gas production is a byproduct of sludge digestion. Methane gas is extremely flammable and can lead to destructive fires and, if released into the atmosphere before it is burned, can be extremely hazardous for the environment (Capstone Fire and Safety Management, 2019). Scientists are looking into non-hazardous, cheap, renewable means of purifying and desalinating water. Using the heat from the sun’s rays to clean ocean water is an extremely viable option considering that this is a renewable energy source (Kumar, 2021).
The goal of water desalination is to remove dissolved salts to a point where it has an electrical conductivity greater than or equal to 0.7 mS/cm and less than 2 mS/cm (Naser, 2019). Conductivity is defined as the ability to conduct an electrical current, which is controlled by the cations and anions present in an aqueous solution (Qian, 2017). Consuming large amounts of sodium from drinking water can have negative health outcomes. A study by Abu Mohd Naser et al. found that ingesting water with an electrical conductivity greater than or equal to 0.7 mS/cm and less than 2 mS/cm lowers blood pressure and decreases a patient’s risk of developing hypertension (Naser, 2019). For these reasons, drinking ocean water is not a solution to the water crisis and thus having a reliable means to remove dissolved salts and create potable drinking water is necessary for public health.
Scientists are currently looking into multiple viable water purification methods; according to a study conducted by scientist Indra Mohan et al., the solar still, a device that utilizes heat from the sun to desalinate water, could provide that solution. Solar stills can function for longer than all other desalination devices invented thus far for a few reasons. First, the solar still utilizes a non-exhausting power source (the heat from the sun). Secondly, once a solar still is built, it requires little to no maintenance and can be easily cleaned. There are no conventional electric connections or intricate wiring loops that could potentially hinder the maintenance process, they can be picked up and moved to a water source for proper cleaning, and it costs little to nothing to build. Thirdly, they can be made on smaller scales making the transportation and storing of the device simple. Finally, a solar still does not require any parts aside from a basin, glass cover sheet, outlet pipe, and bucket for the collection of the purified water once it exits the still (Mohan, 2017). As previously mentioned, other methods of water purification can cause undue harm to the environment; once a solar still is running, it causes negligible harm to the environment (Edoja, 2015). The main drawback of using a solar is the brine (water that is strongly saturated with salt) that it produces. The disposal of brine is a tricky process. One may think that dumping it into the ocean is an obvious solution, but dumping large amounts of brine will raise the salinity of the ocean causing undue harm to marine life and oceanic ecosystems. Kenigsberg et al. found that even an increase of 2-3 salinity units can have a negative effect on ecosystems (2020). On average, brine that is leftover from desalination processes doubles the salinity levels of ocean water (Kenigsberg, 2020). The disposal of brine continues to be a major knowledge gap as scientists still do not know how to adequately dispose of it.
Further research is needed to mitigate the negative effects of brine. One such option is converting the leftover brine into a deicing solution. Deicing is a solution designed to prevent the collection of snow and the emergence of frost on pavement (roads) prior to a storm. The liquid chemical (made up of mostly brine) is poured across roads thus preventing the ice from bonding to the snow (Galaza, 2012). Deicing functions as a deterrent because of its main ingredient: salt. After the water in the anti-icing solution evaporates, the salt left over inhibits the formation of hydrogen bonds between the ice and the road--it’s high boiling point lowers the freezing point of the water on the roads thus decreasing the odds of the water freezing. Anti-icing is an environmentally friendly process, but could become even more sustainable if it utilized the brine from solar stills as its main material. As mentioned previously, the disposal of brine provides a major drawback for solar still usage as there is no way to dispose of brine in an environmentally friendly manner. By using it as a material in an anti-icing solution, it would not only produce a usable solution, but also provide a means of sustainably disposing of the brine.
Alternatively, solar stills do pose many benefits. After a solar still is set up, they can function without external assistance. When the sun shines down onto the surface of a solar still, solar irradiation concentrates at the surface causing the temperature to increase inside the still. From here, heat is absorbed by the salt water, which causes increased evaporation: a phase change of a liquid solution (ie. the salt water) to a gas (ie. water vapor) below its boiling point (Liu, 2021). Solar stills are capable of removing salts, heavy metals, and killing waterborne pathogens (Edoja, 2015). The purification process that takes place within the solar still is a cyclic process, meaning that even though desalination occurs in the still, the effluent produced is in the same state as the contaminated solution (ie. changing from a liquid to a gas and then back to a liquid). When the salt water is heated, the water evaporates, turning into a gas, and leaves behind contaminants with a higher boiling point. When the vapor rises within the solar still and comes into contact with the slightly cooler cover sheet, the water molecules condense turning back into a liquid (Snustad, 2018). The condensed, purified liquid then rolls down the inclined cover sheet, builds up in the trough, and exits through the outlet pipe to be collected as drinking water.
Maximum yield of desalinated water is achieved when the cover sheet is set on an incline of 14.6° (Kaushal 2009) and when the solar still is lined with the color black (Kaviti, 2015). When the cover sheet is inclined, the sun’s rays come in more direct contact with the solar still’s cover sheet, and the effective area (portion of the still in direct contact with the sun) increases (Kaviti 2015). With more direct contact comes more exposure to heat, which increases the overall efficiency and yield of the solar still. The color black increases heat absorption more than any other color because it is capable of absorbing all wavelengths of light (Kaviti, 2015; UCSB, 2018). A blackened base will also increase the still’s efficiency, as it increases the temperature within the still more quickly and with less solar irradiation. A study conducted by B.B. Sahoo et al., in which they tested the effects of lining the inside of a solar still with a blackened base liner, found that the still efficiency (amount of desalinated water produced over time) was increased by over 4.69% when the blackened base liner was used compared to a still with no modifications (ie. no blackened base liner) (Sahoo, 2007). Together, an ideal angle and lining can increase the yield and efficiency of the still.
One stark difference between seawater and pure water is their distinct boiling points. As more and more water evaporates from within the still, the salt concentration of the water increases. This increase in salt concentration lowers the vapor pressure (how likely matter is to change from a liquid to a gas) which simultaneously increases the boiling point of the solution. Thus, as the evaporation of the salt water continues, and the concentration of salt in the solution increases, the boiling point steadily increases as the still desalinates the water. Eventually, the brine will reach a point where it will no longer evaporate because its boiling point will be too high, effectively stopping the still’s desalination process (Sharqawy, 2011). As a result, solar stills always produce leftover brine which alters their efficiency.
A crucial issue and knowledge gap of desalination using solar stills is the still’s maximum yield, which is approximately 60% (Edoja, 2015). Numerous experiments have been conducted in an attempt to increase still efficiency and decrease the amount of brine produced. According to Edoja et al., scientists have attempted to decrease the volumetric heat capacity (amount of heat needed to increase the temperature of one unit of the water) of the basin of the still, attach sub systems (self contained systems that extend from the solar still meant to increase performance), and take other steps that alter the typical still design to increase efficiency. It is widely known that the temperature difference between the water and the glass cover sheet affects a still’s yield (Edoja, 2015). Knowing that the thickness of the glass cover sheet affects the temperature inside the solar still, Edoja et al. decided to measure the effects of altering the thickness of the cover sheet on the overall yield (2015). They created five stills: single panned, double panned, double panned with a 4 mm space between each layer, triple panned, and triple panned with a 4 mm space between each layer (Edoja, 2015). After one day, single panned solar stills (4 mm thick) were capable of achieving the maximum yield of 306.3 cm^3 while the triple panned solar still had no yield of purified water. As mentioned previously, condensation occurs when vapor comes in contact with a cooler surface causing the particles to condense into a liquid. Edoja et al.’s study discovered that when the cover sheet is too thick (three layers of 4 mm thick glass), it absorbs all of the heat. The temperature difference between the inner glass surface and water in the single panned solar still was 4.2 ℃ (the water had a higher temperature) whereas the temperature difference in the triple panned solar till was -0.9 ℃ (a negative value indicating that the inner glass cover sheet had a higher temperature than the water). This prevented a majority of the water within the still to heat up and evaporate. Additionally, had the water evaporated, it would not have condensed back into a liquid because the cover sheet would have been too hot (Edoja, 2015). Though these discoveries are crucial towards increasing still efficiency, they do not provide a solution for how to dispose of brine.
The current water crisis has created a desperate need for renewable sources of potable water as well as means of purifying contaminated sources. Furthermore, this dire need for potable water requires solutions that do not further harm the environment. Solar stills offer an approach to addressing this heightened need as they quickly purify salt water (which is plentiful) using renewable energy from the sun. Nonetheless, they still produce a waste byproduct in the form of brine. Considering the yield of the solar still continues to be a major drawback and altering the thickness of the plexiglass cover sheet affects the yield, I decided to replicate Edoja et. al’s experiment. My experimental question is: How does the thickness of the plexiglass cover sheet affect the salinity levels and conductance of the effluent and the amount of effluent produced? Results from this experiment will help find the optimal thickness for the plexiglass cover sheet, which will aid efforts to combat increased potable water shortages. Based on results obtained from Edoja et. al’s study, I hypothesize that solar stills with the thinnest plexiglass sheets are the most effective at desalination and water purification (measured by rate and salinity/conductance) because thinner panels increase the rate of evaporation due to an increase in still temperature. As an extension to this study, I will attempt to mitigate the negative environmental effects of the brine produced by these solar stills. Brine and road anti-icing solutions share similar properties, so, theoretically, brine can be repurposed and used as an anti-icing solution. This will not only create a means to safely dispose of the brine, but provide an environmentally friendly solution for road deicing. These results will serve as crucial next steps for solving the ever evolving water crisis.