Series: How do they clean my water?

Other posts in this series: Bottled water, what’s in my water, and where does it come from?

All water treatment plants have a process for cleaning out your drinking water. They use one or, in most cases, multiple of these techniques. You can find out which of these techniques your treatment plant uses by reading your Annual Water Quality Report, or in some cases checking out your municipalities website.

Flocculation/Sedimentation
After screening out large objects like fish and sticks (if pulling from a surface water source), coagulant chemicals are added to the water to cause the tiny suspended particles that make the water cloudy to be attracted to each other and form “flocs.” Alum and iron salts or synthetic organic polymers (used alone or in combination with metal salts) are generally used to promote coagulation.

Filtration
Many water treatment facilities use filtration to remove all particles from the water. Those particles include clays and silts, natural organic matter, precipitates from other treatment processes in the facility, iron and manganese, and microorganisms. Filtration clarifies water and enhances the effectiveness of disinfection. Conventional, direct, slow sand, and diatomaceous earth filtration systems all do a good job of removing most protozoa, bacteria, and viruses.

Conventional filtration: After flocculation, the mixture is then allowed to settle out of the water. Once this is complete, water is passed through filters so that remaining particles attach themselves to filter material.

Direct filtration is different from conventional filtration only because the water is not allowed to settle before being filtered.

Slow sand filtration systems have no coagulation step. Water is allowed to pass slowly downward through a bed of sand two to four feet deep. A biologically active layer forms along the upper surface of the sand bed, trapping small particles and degrading some organic contaminants. Diatomaceous earth filtration uses the fossil shells of tiny marine organisms as the filter through which raw source water is fed. The earth physically filters particle contaminants from the water.

Membrane Filtration
Membrane water treatment systems were originally used only in desalination projects. But improvements in membrane technology have made them an increasingly popular choice.

Water treatment membranes are thin sheets of material that are able to separate contaminants based on properties such as size or charge. Water passes through a membrane; but depending on their size, larger particles, microorganisms, and other contaminants are separated out.

Some of these systems are pressure driven, depending on water pressure to separate the particles based on size. Microfiltration employs the largest pore size, and can remove sand, silt, clay, algae, bacteria, Giardia, and Cryptosporidium. Ultrafiltration can also remove viruses. Nanofiltration systems provide nearly complete protection against viruses, remove most organic contaminants, and can reduce hardness in water. Reverse osmosis systems are dense membranes that remove almost all inorganic contaminants and all but the smallest organic molecules.

Adsorption
Organic contaminants, unwanted coloring, and taste-and-odor-causing compounds can stick to the surface of granular or powder activated carbon and are thus removed from the drinking water.

Disinfection
Water is often disinfected before it enters the distribution system to ensure that potentially dangerous microbes are killed. Chlorine, chloramines, or chlorine dioxide are most often used because they are very effective disinfectants, not only at the treatment plant but also in the pipes that distribute water to our homes and businesses. Ozone is a powerful disinfectant, and ultraviolet radiation is an effective disinfectant and treatment for relatively clean source waters, but neither of these are effective in controlling biological contaminants in the distribution pipes.

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