Micro-straining

In some cases, algae, aquatic plants, and other very small debris may be removed by still finer screening.
Chemical Pretreatment. The water is conditioned for removal of natural organics, primarily algae and other aquatic microorganisms, as well as their by- products. This may precede micro-straining.

Main Treatment Processes Chemical Feed and Rapid Mix. Chlorine and other chemicals, such as alum or lime, are added to the water to help remove impurities, destroy any taste or odor, raise pH, disinfect, and sometimes remove excess minerals such as iron that may cause rust or staining problems. The water is then mixed rapidly to distribute the chemicals evenly.

Since the early 1900s, chlorine (as a solid, liquid, or gas) has been the primary disinfectant used in the United States because it is effective and inexpensive and can provide a disinfectant residual in the distribution system. Ozone and ultraviolet radiation can also be used as primary disinfectants, but chlorine or an appropriate substitute must also be used as a secondary disinfectant after the main treatment processes to prevent re-growth of microorganisms in the distribution system.

Chemicals may be added to oxidize ferrous iron (Fe++), which is relatively high in some groundwater, to the ferric state (Fe+++). If pH of the water is above 7 (either naturally or by adding lime), the insoluble compound of ferric hydroxide is precipitated.