Desalination and Purification of Water using a Solar Powered Hydrogel Multistage
Abstract:One third of humanity is without access to clean drinking water. In addition, over 90% of clean drinking water is contaminated with microplastics. 71% of the earth’s surface is water but we are without a cheap, efficient and versatile means of making water safe to drink. This study aims to design, build and test a highly efficient, solar powered and portable water purification method that will cleans water of most contaminants, including microplastics, salt and pathogens, and can be used across the globe. In this study a water vaporization enthalpy decreasing chitosan and PVA hydrogel was synthesized via repeated freeze drying at -80°C to stimulate the expansion of pores within the hydrogel decreasing the vaporization of the water within the hydrogel from 2260Jg-1 to 1200 J g-1. Additionally, a solar tracking mylar coated nested paraboloidal solar collector has been designed to power a multistage of these hydrogels which increases the efficiency of the solar power for vaporization by ~450%. The unity of these industry leading concepts enables the proposed system to reliably function at not only a highly efficient rate but also a reduced energy cost. This design’s solar tracking ability collects up to 223.8W which, when partnered with its ability to float, makes it functional nearly anywhere there is water. This design is capable of sustaining a purification rate of 1.84L per hour and an evaporation rate of 180L hr-1 m-2 enabling it to purify water indefinitely.
The United Nations has a goal to supply clean water and sanitation for all . This goal originates from the fact that one in three people do not have access to safe drinking water . Clean water is an essential resource for our survival, yet of the 3% of water on Earth that is fresh only 0.5% is drinkable . As well as this, our already meager water resources are being threatened by climate change as weather patterns change and sea levels rise . For example, San Diego’s water is supplied by the Colorado River but, due to a change in weather patterns, the water level is dropping resulting in a need to look elsewhere for clean water . This lack of clean water will likely become more widespread and even amplified as our climate becomes less stable due to catastrophes, such as the deforestation of the Amazon.
To tackle water shortages desalination plants are being built. However, the process is inhibited by high operating costs. For example, plants spend 1,000 to 2,000 US dollars per acre-foot (of water), $10’s to $100’s of millions per year in maintenance, and billions in construction costs . The cost of these plants make them simply out of the question for many impoverished developing countries. The most widely used methods of desalination are reverse osmosis and thermal evaporation . Thermal desalination is typically not commercially viable due to its intensive energy requirement resulting in reverse osmosis plants becoming the favored design. However, reverse osmosis plants have many consequences such as toxic waste pollution and killing of local wildlife . Even our supposedly safe drinking water is at risk as microplastics have been found in over 90% of all drinkable water. Microplastics have the potential to cause health problems such as cancer, weakened immune systems and reproductive problems . The water industry is crucial to humanity’s survival, yet it has so much room for improvement.
 “Water and Sanitation – United Nations Sustainable Development.” United Nations, United Nations, www.un.org/sustainabledevelopment/water-and-sanitation/.
 California-Great Basin, Bureau of Reclamation. “Central California Area Office.” Water Facts - Worldwide Water Supply | ARWEC| CCAO | Area Offices | California-Great Basin | Bureau of Reclamation, 4 Nov. 2020,www.usbr.gov/mp/arwec/water-facts-ww-water-sup.html#:~:text=3%25%20of%20the%20earth's%20water,extracted%20at%20an%20affordable%20cost.
 Gabriel Filippelli, Indiana University – Purdue University Indianapolis. “Climate Change Threatens Drinking Water Quality Across the Great Lakes.” Discover Magazine, Discover Magazine, 29 Apr. 2020, www.discovermagazine.com/environment/climate-change-threatens-drinking-water-quality-across-the-great-lakes.
 Robbins, Jim. “Desalination Is Booming as Cities Run out of Water.” Wired, Conde Nast, 26 June 2019, 9:00 AM, www.wired.com/story/desalination-is-booming-as-cities-run-out-of-water/.
 “Climate Change.” Amazon Aid Foundation, 25 Feb. 2018, amazonaid.org/threats-to-the-amazon/climate-change/#:~:text=The%20Amazon%20forest%20is%20a,much%20of%20the%20world's%20biodiversity.
 May, Sue Chen 26, et al. “How to Filter and Remove Microplastics from Tap Water? - TAPP Water.” USA, 10 Feb. 2021,
 Reports, Consumer. “You're Literally Eating Microplastics. How You Can Cut down Exposure to Them.” The Washington Post, WP Company, 7 Oct. 2019, www.washingtonpost.com/health/youre-literally-eating-microplastics-how-you-can-cut-down-exposure-to-them/2019/10/04/22ebdfb6-e17a-11e9-8dc8-498eabc129a0_story.html.
 Zhao, X., et al. (2019) Architecting highly hydratable polymer networks to tune the water state for solar water purification. American Association for the Advancement of Science, vol. 5, no. 6, doi:10.1126/sciadv.aaw5484.
 Lara, R., et al. (2017) Influence of freezing temperature and deacetylation degree on the performance of freeze-dried chitosan scaffolds towards cartilage tissue engineering. Science Direct, vol. 95, pp. 232-240, ISSN 0014-3057.
 Xu, Z., et al. (2020) Ultrahigh-Efficiency Desalination via a Thermally-Localized Multistage Solar Still. Energy & Environmental Science, vol. 13, no. 3, pp. 830– 839., doi:10.1039/c9ee04122b.
 Boundless. “Boundless Physics.” Lumen, courses.lumenlearning.com/boundless-physics/chapter/specific-heat/#:~:text=The%20heat%20capacity%20and%20the,phase%20of%20a%20given%20substance.
“Department of Health.” Boil Water Response-Information for the Public Health Professional, Nov. 2018, www.health.ny.gov/environmental/water/drinking/boilwater/response_information_public_health_professional.htm.
Additional Project Information
One third of humanity doesn’t have access to clean drinking water. Over 70% of the earth's surface is water yet only 0.5% of water is drinkable. This makes a system capable of purifying surface water in an efficient and cheap way invaluable.
This study aims to design, build and test a system capable of producing 1.84 liters per hour. This study utilizes hydrogels that half the evaporation enthalpy by water, a multistage that increases the heat efficiency of the system by 450% and is powered by a nested paraboloid solar tracker.
850 mg PVA and 150 mg chitosan + DI water (10 mL) + 1 mL HCl (1.2 M) sonicated at room temperature for 24 hours for full dissolution of PVA and chitosan.
Glutaraldehyde (50 μL, 50% wt in DI water) was added to 4 mL of PVA/chitosan precursor solution for h-LAH4 and the solution was left to gelate for 5 days.
The gelated gel was immersed into DI water to obtain the pure hydrogel and was frozen in -80°C and then reheated at a temperature of 60°C ten times over with microscopic images at 40x zoom being taken every second freeze.
After the hydrogels finished the freeze drying process they were tested.
The evaporation enthalpy testing was done using a peltier plate mounted on a heatsink that was powered by a controlled power source. The heat sink was placed on top of a scale and the hydrogels were hydrated before being weighed and placed on the peltier plate. The weight of the hydrogel was recorded at minute intervals during the heating of the hydrogel by the peltier plate for six minutes.
The correlation between evaporation rate and surface area was tested using the same setup of a controlled power source powering a peltier plate mounted on a heat sink and the heat sink being placed on a scale. Three different hydrated hydrogels of differing size were tested by being weighed before and during the heating by the peltier plate at minute intervals for 3 minutes.
The evaporation rate of the hydrogel at 200°C was tested by placing the hydrated hydrogel on an aluminum plate that was mounted to a hotplate. The hydrogel's mass was recorded before being placed on the hotplate, after 30 seconds of heating and after 1 minute of heating.
The function of a single stage was also tested by heating the bottom aluminum plate of the stage holding hydrated hydrogels using a peltier plate. The top aluminum plate was then monitored for condensation to determine the functionality of the GORE-TEX. The GORE-TEX was also tested to see if it was hydrophobic by playing water droplets on its surface and monitoring the GORE-TEX for leakage.
The nested paraboloidal mirror system was made by 3D printing 15 individual parts and welding them together by hand using a 3D print pen. Mylar sheets were then heatgunned onto the surface of the paraboloids. The turntable top and base was made by laser cutting ¼ inch HDPE sheets. Motors and pillow blocks were mounted to the cut sheet which held the 80 20 aluminum arms enabling the movement of the nested paraboloidal system and the rotation of the turntable. The geared motor heads were resin printed.
The entire system is controlled by an arduino that controls 3 tic t500s while are connected to the three stepper motors for arm rotation, table rotation and slider control. The arduino is also connected to a solar tracker that is attached to the secondary paraboloid which informs the movement of the turntable and arm stepper motors. The arduino is also connected to the thermometer of the central absorber that powers the multistage which informs the temperature regulating slider.
The prototype of the multistage was made by 3D printing in PLA but the final model is 3D printed in thermally resistant resin. The 1mm thick aluminum plates were cut using a guillotine shear and the GORE-TEX was cut using the laser cutter used for the HDPE.
Four 1ft pool noodles were attached to the bottom of the turntable base using zip ties and a 24inch diameter lifesaver was attached to the bottom of the turntable base using zip ties for floatation.
Evaporation rate was recorded while being charged by a controlled power source enabling the calculation of evaporation enthalpy. The evaporation rate per mm2 was also recorded at 200°C which is the max temperature of function within the multistage. Using the 200°C evaporation rate of 1.11x10-5 g s-1 mm-2 the required power of the design was calculated as 137W and the system’s overall evaporation rate was calculated as 1.84 L per hour.
Questions and Answers
1. What was the major objective of your project and what was your plan to achieve it?
The major objective of this project was to design, build and test a highly efficient, solar powered and portable water purification method that will cleanse water of most contaminants, including microplastics, salt and pathogens, and can be used across the globe. The plan to achieve this goal was to use enthalpy decreasing hydrogels in a multistage configuration which would increase the heat efficiency greatly and to power this setup using solar power collected via a solar tracking nested paraboloidal reflector.
a. Was that goal the result of any specific situation, experience, or problem you encountered?
Yes. I grew up in Ireland where it rains most days a year and water is something I took for granted. However, I moved to Australia while they were in the Millennial drought and I got quite a shock. My friends cried when they saw rain for the first time. From this experience I gained a respect for water that was awoken yet again when I visited San Diego in 2017 while they were also in a drought. At the time, not understanding the energy intensive process it is, I couldn't understand how a city so close to the ocean wouldn't just desalinate water. So when I finally got the opportunity to work in a research program in my school it was a no brainer that I would choose water desalination and purification.
b. Were you trying to solve a problem, answer a question, or test a hypothesis?
I am trying to solve a problem. This problem is that over 71% of the earth's surface is water but only 0.5% of the water on earth is clean and of that 0.5%, 90% contains microplastics. I plan to solve this problem by building a cheap and portable device with a purification rate capable of sustaining multiple people.
2. What were the major tasks you had to perform in order to complete your project?
The major tasks were synthesizing and testing the hydrogel, designing and testing a functional multistage and designing and building the solar tracker. These tasks each had their own challenges but the primary challenge was gaining lab access during COVID-19 in order to complete these tasks.
a. For teams, describe what each member worked on.
3. What is new or novel about your project?
The concept of combining the ideas of a multistage, a hydrogel and a solar tracker is novel. The temperature at which the hydrogel was freeze dried and the effect the temperature has on the evaporation enthalpy is novel. The design of the multistage and the fact it enables a 2.5mm gap which was determined as the ideal gap distance for a multistage is also novel. Additionally the solar tracker concept and design is original however a large amount of work and research has been done in the field of paraboloidal concentrators regarding the amplification of sound waves so how novel the design is is unknown.
a. Is there some aspect of your project's objective, or how you achieved it that you haven't done before?
I haven't done most of the things described in the research plan including the use of many tools and the design of such a large and complex project. I have never synthesized a hydrogel before my research nor have I done proper research and read articles in the way this level of investigation has required.
b. Is your project's objective, or the way you implemented it, different from anything you have seen?
I have never seen anything like my project before. I've seen the concepts such as hydrogels and a multistage which I have based my project upon, but my research has it's own unique spin on these concepts and I have not seen any research that combines these concepts.
c. If you believe your work to be unique in some way, what research have you done to confirm that it is?
I have done extensive research in the fields of both hydrogels and multistages and haven't found a combination of the two for the function of water purification. Hydrogels utilized for water purification itself is a very new idea and not much research has been published on the topic especially for the effect of differing temperatures on the evaporation enthalpy.
4. What was the most challenging part of completing your project?
The freeze-drying process for the hydrogels was very demanding as it required me to schedule 12 days in which I could use my schools Chemistry lab under supervision. Due to COVID-19 restrictions this was very awkward and I am grateful for my chemistry teacher's, Dr. Chen's, flexibility and understanding.
a. What problems did you encounter, and how did you overcome them?
The design of the multistage required water vapor to heat the next stage while also condensing and collecting that water vapor without allowing either the hydrogel above or below to absorb the condensed water. Achieving this process required a lot of thought and eventually I realized a semipermeable membrane would work perfectly so I bought GORE-TEX and incorporated it into my design which enabled the multistage to function. The schematic of this design is in my paper or can be viewed in the Onshape document I have linked to this application.
I also realized that the nested paraboloidal solar collector would collect over 5 times the required amount of energy to sustain the multistage so I designed a slider capable of controlling the amount of energy powering the system.
I also used a 3D printer to construct the nested paraboloidal solar collector which had multiple problems throughout the manufacturing process which required a lot of fine tuning and fixing.
b. What did you learn from overcoming these problems?
I learned a great deal about patience and communication. The best way to solve a problem is to communicate and take your time.
5. If you were going to do this project again, are there any things you would you do differently the next time?
I would reduce the size of the nested paraboloid as the energy it collects is equivalent to the necessary power of 5 multistages rather than a single multistage. However, at the time of designing the solar collector the required energy was unknown as the enthalpy of the hydrogel and the evaporation rate were unknown. I am generally quite happy with the methods and actions I took as they were justified and well reasoned.
6. Did working on this project give you any ideas for other projects?
This project gave me the idea for future research concerning this project. This future research would include the building and testing of the finalized design by populating the multistage with hydrogels. Additionally, the use of 5 multistages would have a very high production rate which would also be worth proving. In researching water purification I also discovered clathrates which I have begun to learn about and am interested in which may lead me to do an investigation and project on clathrates in the future.
7. How did COVID-19 affect the completion of your project?
COVID-19 limited my progress substantially. The speed at which I could synthesize and freeze-dry hydrogels without COVID-19 would have enabled me to complete my build and test the full model.