Water Treatment Fundamentals
US drinking water is generally assumed to be safe and healthful. Of those Americans served by municipal water systems, 70 percent derive their water from surface water sources, e.g., rivers, lakes and reservoirs, after it is ushered through a series of critical treatment steps.
The remaining 30 percent are served by municipal groundwater systems that tap underground water sources using wells. Although the majority of US municipal water systems use groundwater as a raw water source, the majority of people in the US are served by surface water sources.1
While groundwater supplies may produce safe drinking water at the well, most warrant treatment with a disinfectant to protect the water from contamination as it flows through distribution pipes. This is an increasingly important consideration in areas of the US with compromised, aging water infrastructure (see Pain at the Pipe – Part 1: Why the US Should Respond to Drinking Water Infrastructure Needs).
Municipal drinking water systems are required by the US Environmental Protection Agency (EPA) to monitor drinking water quality regularly and to adjust treatment to provide a consistently healthful water supply for their customers. The roughly 15 percent of Americans who derive their water from private wells are responsible for monitoring and adjusting their own water quality (see “Is Your Well Drinking Water all Well and Good?”).
A Word about Cryptosporidium
Image from US Centers for Disease Control and Prevention website, Parasites – Cryptosporidium
Cryptosporidium is a single-celled, chlorine-resistant parasite that causes gastrointestinal illness. It is spread through the feces of mammals, and may infect surface water supplies. Cryptosporidium was the cause of the largest reported drinking water outbreak in US history, sickening over 400,000 and killing at least 100 people in Milwaukee in 1993. Because some surface water supplies are particularly susceptible to Cryptosporidium, EPA has ruled that these systems must either cover their finished water reservoirs or impose additional treatment.
Cryptosporidium treatment is carried out by disinfection and/or physical removal. Effective disinfectants include chlorine dioxide, ozone and UV light. Filtration is used for physical removal and includes microfiltration, in which water is passed through a membrane containing microscopic openings that prevent the passage of the parasite. Treatment combined with watershed protection, ranging from prohibiting septic system discharges to controlling beaver activity, constitutes a “multi-barrier approach” to protecting water supplies from Cryptosporidium.
Cleaning up Water for Drinking
Transforming raw surface water into the clean, clear liquid flowing from your tap involves a series of steps at the water treatment plant. These are described in a general way below. The particular “treatment train,” or the order of steps adopted by each water system, is carefully designed to maximize water quality, and is based primarily on the quality of the source water (see: Protecting the Watershed: Step #1 for Clean Drinking Water). Other considerations include cost and the safety and security of treatment chemical delivery, storage and use.
Coagulation: Surface water is full of environmental debris, including bits and pieces of natural organic matter, sediment particles and living organisms. Following the removal of coarse debris by filtering through screens, coagulation removes finer particles that are suspended in water. Alum and other chemicals are added to water to form tiny sticky particles called “floc,” which attract suspended particles. The combined weight of these particles and the alum (floc) become heavy enough to sink.
Sedimentation: Coagulated particles fall by gravity through water in a settling tank and accumulate at the bottom of the tank, clearing the water of much of the solid debris.
Filtration: Water from the sedimentation tank flows through sand, gravel, coal, activated charcoal or other media to remove solid particles not previously removed by sedimentation.
Disinfection: Disinfecting chemical is added or a disinfection method is applied to filtered water to destroy harmful microorganisms. A “chlorine residual” is added to the water as it leaves the treatment plant to help protect treated water from re-contamination as it travels through the distribution system.
Storage: Water may be pumped to a closed tank or protected reservoir. The water is released to flow to homes and businesses in the community.
We gratefully salute our nation’s dedicated water treatment professionals!