The purification of surface water from sources such as reservoirs or rivers usually requires several phases of treatment. A municipal surface water treatment plant must first screen out large objects such as trash and leaves. Next, the raw water may be flash-mixed with various chemicals to alter its pH, encouraging the flocculation (clumping) and settling of smaller suspended solids. After an additional settling stage, the water can be forced through filter beds composed of sand, garnet, and anthracite to remove even smaller particles. The finished water is then disinfected with chlorine gas or chlorine compounds, ozone, or ultraviolet light, before it is pumped into the distribution system of water mains and storage tanks on its way to consumers. Some plants also pre-chlorinate their raw water influent after the screening phase. Water utilities may choose to further boost chlorine levels in the distribution system to counteract any pathogens that may occur.
Groundwater from an aquifer not immediately influenced by surface runoff is generally considered to require less rigorous treatment, but must meet the same standards of safety and palatability. Soil and rock layers will have naturally filtered the groundwater to a high degree of clarity even before it is pumped to the treatment plant, but the facility may need to reduce the iron or manganese content of this water to make it pleasant for drinking, cooking, and laundry use. Disinfection is also required.
Many environmental and cost considerations affect the siting and design of water purification plants. Groundwater is cheaper to treat, but aquifers once depleted can take thousands of years to recharge. Surface water sources must be carefully monitored for the presence of unusual types or levels of contaminants. The treatment plant itself must be kept secure from vandalism or terrorism and the presence of large quantities of dangerous chemicals mandates special training for workers and emergency personnel. The facility must responsibly dispose of its settled and filtered solids and prevent them from contaminating the treatment components or the source waters. All facilities disinfect finished water, but the exact method of disinfection can be controversial, and the costs and benefits of different methods must be evaluated.