Water Treatment of the Hetch Hetchy Water Supply

After just finishing a class on water treatment, I found it extremely interesting that Hetch Hetchy water was designated with “filtration avoidance” status. I had just spent 11 weeks studying all the ways that we, as humans, have come up with to clean out all the contaminates (that we as humans typically put there) that might be in our water. There is everything from heavy metals, suspended solids, bacteria, protozoa, fecal material, pathogens..... and the list goes on and on. I was already amazed that Hetch Hetchy water did not need to be filtered, but I was even more surprised when I heard that it did not need to undergo any coagulation or flocculation. The only requirement was disinfection! The only difference between the water sitting in the afterbay at Moccasin Power House and the water coming out of the taps in San Francisco is a tablet of chlorine (fluoride and a couple other chemicals are added as well). I laughed when I saw the ducks and swallows leaving their droppings in the water and how all the unsuspecting San Franciscans will be filling their cups with the same water. An interesting fact that I found was how a new requirement that will be implemented soon involves having two disinfection systems for water plants. UV disinfection will be added which is something that is almost never found at water treatment plants. In this process, bacteria cells are deactivated to prevent them from reproducing. Unfortunately, there is nothing preventing new bacteria to form. Chlorine is almost always used because it leaves a residual in the pipes that prevents any future growth.