Stages of Purified Water

Water purification is the process of removing unwanted materials from water to make it safe for human consumption. All sources of drinking water contain some type of contamination, and the water must meet specific health standards before it can be consumed. Contaminants include parasites, bacteria, chemicals or sediment. Municipal water suppliers rely on the same basic procedures for water purification all across the country. Some steps may be skipped depending on the initial quality of the water being treated.

Pre-treatment

• Water must be moved from a source, such as a lake, to a holding tank.

  The first stage of purifying water is pretreatment. Included in this step is the movement of water from its source to a storage area, screening to remove large debris such as trash and sticks, storing the water to allow for some natural biological filtration, preconditioning of hard water, possibly prechlorination to discourage growth of organisms in pipes and holding tanks, and adjustment of the pH of water to prepare it for additional purifying steps.

Floccutation

• The process of flocculation removes dirt and discoloration from drinking water.

  Next, flocculation improves the clarity of water by removing dirt and color when an agent is added to form a sticky substance called "floc". This floc attracts and traps dirt, which is then filtered out of the water.

Sedimentation

• After flocculation, drinking water goes through a step called sedimentation. Water slowly flows into a tank, allowing the flocculated particles to settle out of the water. As particles settle to the bottom of the tank, a sludge is formed that must be removed and treated.

Filtration

• After sedimentation has taken place, water goes through a filtration process to remove any remaining floc and suspended particles. Filtration helps to clarify the water and prepare it for disinfection. Filtration may be achieved by rapid or slow sand filtration, membrane filtration, ultrafiltration, ion exchange or electrodeionization.

Disinfection

• The water is then disinfected before entering the distribution system. The process of disinfection destroys many pathogens, such as bacteria, viruses and protozoa that are harmful if consumed. It typically involves the use of chlorine or a chlorine derivative such as chloramine and chlorine dioxide. Ozone, ultraviolet light and hydrogen peroxide may also be used.

Additional Treatments

• After going through the process of purification, drinking water may also undergo one or more optional treatments. Fluoridation is the most widely used additional water treatment. It is done to prevent tooth decay. Water conditioning may also be used in areas of overly hard water. Plumbosolvency may be performed to prevent water from leaching lead from the pipes that carry it. Radium and fluoride removal may be used to remove excess amounts of these elements.