Filtration Filtration is an important process that is needed to make water safe to drink. Filtration is used in the water coming to NYC from the Croton Watershed but not so much with the water coming from the Catskills Watershed because it's cleaner. The filtration process starts with aeration, this is when water is oxygenated. Then the water is coagulated the idea of this is to neutralize impurities into larger, heavier compounds. After this the water moves on to sedimentation, this is when water is put in sedimentation tanks to allow the heaviest particles to settle at the bottom. The water is then pumped through valves, also chemicals are added at this point to adjust the pH. The second to last step is clarifying. This is when the water is passed through a series of filters, each filter being finer than the previous to remove smaller particles. The final step in this process of filtration is disinfection. This is when chlorine is introduced to get rid of any harmful organisms that may have survived till this point.
Filtration is used to clean water so people don't get sick. Most reservoirs go through filtration before becoming drinking water. The Catskill Watershed, where 90% of NYC's water comes from, is exempted from filtration because the forest ecosystems in the Catskills are healthy enough to filter the water naturally.