Bottled Water Free Day

How a Water Treatment Plant Works

Water treatment is required in most cases with very few exceptions. Drinking water sources need to be treated or purified prior to distributing to homes.

Natural rainwater tends to be pure but the need for water treatment is because the rainwater may pick up contaminants as it drops, or as the overland water flows to lakes, rivers, and wells.

Under the Safe Drinking Water Act, the US Environmental Protection Agency (EPA) sets maximum allowable levels of several contaminants found in source water supplies.

Under the Act, public water suppliers must monitor their water to make sure it complies with science-based public health standards. However, the Act does not cover private wells so each good owner must be responsible to find out what contaminants are in their drinking water.

Because some contaminants are colorless and odorless, testing is recommended for well water to ensure it is safe to drink.

Contaminants in source water may come naturally or are from human activities such as combined sewer overflows, concentrated animal feeding operations, failed septic systems, constructions.

Contaminants may come in the form of dissolved organic and inorganic substances. The source water may also smell and look bad, and may also contain bacteria and other microbiological organisms at levels that threaten public health.

Microbial contaminants that may be of special concern are pesticides, minerals, and solvents which may cause gastrointestinal problems, skin irritations, cancer, reproductive and developmental problems, and other chronic health effects.

If contamination poses an immediate health threat, water suppliers are required by law to immediately notify consumers.

To remove these contaminants, source water may go through the following basic treatment steps: preliminary screening, coagulation, sedimentation, filtration, and disinfection.

Some water treatment plant may require additional treatment steps because of the extent or types of contaminants in the source water such as aeration, ion exchange, distillation, and other steps as determined by the water treatment operators.

Preliminary Screening

Screening is necessary to remove floating debris from the source water. This debris may be dead woods, leaves, rags, or from runoffs from storm events or snow melts. Some water treatment plants also use very fine screen materials or micro-strainers to remove suspended algae and plankton.


While the screening process is to remove small suspended particles, the coagulation process is to remove dirt and other suspended particles missed by screening. Coagulation requires the addition of alum and other chemicals. These contaminants flow toward the chemicals to form “flocs” which attract other impurities. The flocs become heavy and are separated from the water through the next process – sedimentation.


Think about sedimentation in terms of muddy water scooped into a clear jar and allowed to sit for a few hours. You will notice the water at the top becomes clear (maybe not fit to drink) as a sheet of impurities settles at the bottom of the jar. Sedimentation on a large scale is also the separation of water from the flocs. The combination of alum and contaminants will settle at the bottom leaving the clear water on the surface. Water from the coagulation process is allowed to settle for several hours (typically three hours) and the clear water is then transferred to the filtration unit.


Water filtration is a process whereby water passes through filters, some made of layers of sand, gravel, and charcoal that help remove even smaller particles that may have passed through the preceding process units. These particles may pose significant threats to human health; therefore, filtration is the surest way to remove them prior to distribution by water suppliers. In fact, the Surface Water Treatment Rule under the Safe Drinking Water Act requires some source water to be filtered. The most common filtration methods are sand filtration and rapid gravity filtration. Others may include packaged filtration, membrane filtration, and diatomaceous earth filtration.


Disinfection is the most important part of water treatment. This process is to kill or inactivate any pathogenic bacterial in source water such as algae, spores, and viruses. Ninety-nine percent of Giardia cysts must be inactivated to provide safe and healthy water. The basic disinfection methods are chlorination, chloramines, ozone, ultraviolet light, and nanofiltration.


After the water is fully treated, it is now ready to be distributed to consumers. However, treated water is placed in a closed tank or reservoir and then flows through pipes to homes and businesses in the community. Similarly, some multi-stage home water filter systems come with a storage tank. For instance, a 5-stage reverse osmosis water filter comes with a storage tank. This tank stores the treated water and ready to be dispensed.

Why Home Water Filters?

You may wonder why people still buy water purifiers or filters after all the regulations and treatment methods. A disinfectant must be effective in killing or inactivating all pathogenic bacteria and should not leave any residual.

But residual chlorine often remains in the water supply leaving unpleasant tastes and odors which must be removed by consumers. Concerns about water qualities have driven people to buy portable water purifiers when they are away from homes.

These home water treatment devices can remove these unwanted tastes and odors. Home water filters that come with activated carbon and reverse osmosis filters are capable of removing tastes and odors.

Also, water distributed to homes may be fit to drink; however, it may pick up contaminants within the home plumbing fixtures. Lead in old pipes or in solders may leach into the water.

Therefore, it may be necessary to have the water tested to determine if water filters purifiers are necessary. It is even advisable for well water to be tested for possible contaminants, and to determine if a water purifier or filter is necessary.

During water treatment, UV radiation is generated by a special light that penetrates the cell wall of an organism. This water treatment technology is also used in some home water treatment systems to kill the bacteria in the water.

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Solid carbon block filters effectively treat the age-old contaminants such as lead, TCE, chlorine, benzene and several contaminants found in the nation’s drinking water supplies today like MTBE, chloramines, and PCBs.…