(505c) pH of Reverse Osmosis Permeate Water and the Chemistry Leading to It's Variability

Every form of life on this Earth is dependent on water. Water covers 70% of our planet, and it is easy to think that it will always be plentiful. However, freshwater—the water we drink, bathe in, irrigate our farm fields with—is incredibly rare. Only 3% of the world’s water is fresh water, and two-thirds of that is tucked away in frozen glaciers or otherwise unavailable for our use. According to WHO and Unicef’s Progress on Drinking Water and Sanitation 2014 update, there are still 748 million people who lack access to proper drinking water source. Around 173 million people rely on untreated surface water for consumption. Though efforts are being made to address this issue, with everyday increase in world population, changes in climate and rising temperature, this problem may soon get out of hand. One way to solve the freshwater scarcity is desalination of brackish groundwater using Photovoltaic Reverse Osmosis. In places where the population has easy access to brackish groundwater, and abundant solar insolation, desalination with the help of Photovoltaic Reverse Osmosis is most suitable. But Reverse Osmosis has many problems associated with it. It needs high pressure pump and it is susceptible to membrane fouling. Another major problem is the lowering of permeate pH. Reverse osmosis is a very simple membrane separation process. Still, for some reason it lowers the pH when water is passed through it. The pH of permeate water can get as low as 5. Given that the pH scale is logarithmic, it means that water with pH 5 is 100 times more acidic than neutral water at pH 7. The rate of decrease of pH is not constant. It varies depending on many factor. Carbon dioxide-bicarbonate equilibrium in the feed water is one of the main factors affecting the pH.

In the present study Photovoltaic Reverse Osmosis system was used for removal of chlorine salts from different types of water and the change of permeate pH were noted. pH of the feed water were changed to notice the change in the permeate pH. Samples were collected from each of these experiments. Studies like Ion Chromatography and Total Inorganic Carbon were performed on these samples to better understand the reactions leading to the change of pH. Relationship between feed water chemistry and permeate pH were noted.