Water Reuse

Drinking Water Treatment Process

Summary of common drinking water treatment processes (in typical order)

Flocculation/Sedimentation
This involves coagulating, or combining, small particles in the water to form larger particles. A coagulant such as alum is typically added to catalyze this process. After coagulation, the sediments naturally settle to the bottom of the tank and the remaining water can be moved to the next step in the process.


Filtration
Filtration is also used to remove particles from the water. Filters can be made out of various substances such as sand and anthracite coal. Some systems are required by EPA regulations to use filtration. Filtration not only removes dangerous particles, but can also make disinfection processes more effective.

Ion Exchange/Adsorption
Some contaminants, particularly metals, cannot be removed using only filtration and flocculation. Ion exchange methods can remove these chemicals. This process is also used to treat hard water. Adsorption refers to the use of activated carbon or similar items to remove organic contaminants.

Disinfection
Disinfection is required for many PWSs under several of EPA's rules and can be accomplished in a variety of ways. The most common is the addition of chlorine, chloramines or chlorine dioxide to the water. This kills many (though not all) pathogens and has the advantage of staying in the water as it passes through the distribution system to continue disinfecting. Other disinfection methods such as ozone or UV radiation can also be used, but these do not have the same lingering effects. Many aspects of disinfection are regulated by EPA's disinfection byproducts rules.

Distribution
The distribution system of a PWS is not part of the treatment process, but it does play a role in the safety of a PWS's water. No treatment is applied to water after it leaves the treatment facility, but there is a danger of pathogens being introduced via pathways such as main breaks, storage containers and biofilms. A well operated and sanitary distribution system is essential to the safety of a water supply.


Sources:
Drinking Water Treatment. EPA circular 810-F-99-013. December 1999.
http://www.everettwa.org/WFPVirtTour/WFPSchematic.htm
Pontius, Frederick W., ed. Water Quality and Treatment: A Handbook of Community Water Supplies. American Water Works Association. McGraw Hill. 1990.