The Safe Drinking Water Act is a federal law, but like most federal laws, implementation is on a regional or local scale, often by surface water treatment plants.
Surface Water Treatment Plants
The Safe Drinking Water Act is a federal law, but like most federal laws, implementation is on a regional or local scale, often by surface water treatment plants. These plants take water from polluted sources and clean it for residential use using a variety of methods.
Water is cleaned in a sequence of five treatment steps: coagulation, flocculation, sedimentation, filtration, and disinfection. These steps are standard for all surface water treatment plants, though the equipment and chemical reactions used may vary by specific areas. Below is an overview of each step:
This is the process of chemically treating water to remove dirt and other suspended solids. Coagulation is often grouped with flocculation because they employ similar processes. However, coagulation processes differ from flocculation processes in:
- Particle size
- Chemicals used
- Speed of mixing
Coagulants are chemicals added to water to attract dirt/other particles. Theya are inorganic, with a positive charge to attract negatively charged target molecules. After a coagulant is added to the water, the solution is mixed rapidly to cause collisions between the target particles and coagulant. When the target particles and coagulant collide, they bind to each other. Binding happens because negatively charged targets and positively charged coagulants are electronically attracted. The charges are neutralized when binding is complete, and the bound particles are known as “microflocs.”
This is the process of binding microflocs to one another to create larger floc particles. In flocculation, water is slowly mixed to promote collisions and binding between microflocs. Polymers (different coagulating agents than used in the prior step), are added to further bind the growing flocs, add weight, and facilitate settling. As flocs grow larger, the speed of mixing decreases so they do not break apart.
In sedimentation, water is moved from flocculation to a settling chamber. Stagnation of the water in this step causes floc to settle to the bottom, creating a “sediment layer,” which can be separated from the liquid (water) layer. This process typically takes 1-4 hours.
In filtration, water is moved from the sedimentation chamber, passing through a variety of filters and pore sizes to remove some pathogens, chemicals, and dissolved particles not removed in prior steps. Filtration methods vary from place to place, and some methods and materials will remove more pollutants than others. At the minimum, filtration should bring pollutant levels of inorganics, organics, and radionuclides into compliance with the EPA’s standards.
In disinfection, water is treated to kill any remaining pathogens. Typically, this is done chemically with chlorine. Chlorine leaves a residual, which can help protect water from contamination with bacteria, viruses, or parasites in transport from the plant to a home or business.
Other disinfection methods include UV treatment, ozonation, and over-liming. Chlorine is the most commonly used method of disinfection in the United States because it is generally more cost-effective compared to other methods.
- Minnesota Rural Water Association. (2009). Coagulation and Flocculation. Retrieved July 12, 2017, from https://www.mrwa.com/WaterWorksMnl/Chapter%2012%20Coagulation.pdf
- Minnesota Rural Water Association. (2009). Filtration. Retrieved July 12, 2017, from https://www.mrwa.com/WaterWorksMnl/Chapter%2018%20Filtration.pdf
- Minnesota Rural Water Association. (2009). Disinfection. Retrieved July 12, 2017, from https://www.mrwa.com/WaterWorksMnl/Chapter%209%20Disinfection.pdf
- Centers for Disease Control and Prevention. (2015, January 20). Drinking Water. Retrieved July 12, 2017, from https://www.cdc.gov/healthywater/drinking/public/water_treatment.html