Water Purification

Steps in Water Purification

The steps used in treating raw water to make it safe and desirable for drinking vary widely between communities, due to the wide variability of raw water. The following procedure is typical of a treatment plant processing raw water containing large amounts of impurities. Such might be the case for a community that relies on a river as its water source. Such water may vary widely in amount and temperature from season to season, may contain trace amounts of fertilizer from farm runoff and may contain some treated wastewater from upstream communities. Water obtained from wells is generally of higher quality than river water and usually requires fewer treatment steps. Both Battle Creek and Marshall rely on well water for producing all of their drinking water.

Aeration

In some water treatment plants, the first step is aeration. The raw water first goes to aeration tanks where large quantities of air are injected into and bubble through the raw water. Aeration can reduce tastes and odors and can oxidize soluble iron.

Flash Mixing

The next step after aeration is flash mixing. If the raw water is not first aerated, then the first step usually is flash mixing.

One or more chemicals are mixed into the raw water to neutralize or remove specific impurities found in the raw water. The types and amounts of chemicals added depend on the types and concentrations of impurities found in the raw water. If the water is hard (high in dissolved calcium and magnesium), lime and sodium carbonate may be added. If fine particulates are present, alum may be added. If bacteria and other organics are present, chlorine may be added as a disinfectant. The raw water and the added chemicals are mixed by agitation in flash mixing chambers. The goal is instantaneous mixing and accurate measuring of the added chemicals.

Flocculation

After flash mixing, the mixing of the chemicals and the raw water continues but at a slower pace. The added chemicals react with the impurities in the raw water and remove the dissolved impurities by reacting with the impurities and forming small particles of solid matter suspended in the water. During this slow and gentle mixing, the solid particles (called floc) accumulate in feathery white agglomerations.

Settling

Gravity and time are all that is needed to remove the floc from the water. The water is sent to a clarifier (settling basin) where the floc is allowed to settle out of the water. These settling basins are typically twelve to eighteen feet deep and hold the water for two to four hours at a forward velocity of 0.5 to 3 feet per minute. The settled sludge is often disposed of in a sewer.

Filtration

Allowing the partially treated water to settle may remove some, but not all, of the floc. The water usually is then filtered through sand to remove remaining solids. Sand filtration further purifies the water by permitting additional impurities to be adsorbed (adhere) to the surfaces of the sand particles.

During filtration, the sand particles acquire a sheath of floc. From time to time, the sand filter is cleaned by shutting off the flow of partially treated water and then forcing purified water up through the sand bed, in a direction of flow opposite from that used by partially treated water. The water used for cleaning is then disposed of (usually, in a sewer) and the sand is allowed to settle back down. The sand filter is then ready for reuse.

Additives

Additional chemicals may be mixed in the water before it leaves the water treatment plant. Chlorine is usually added at this time, not only to disinfect the water but to provide a level of chlorine sufficient to help prevent future growth of pathogens. Lime and glassy phosphates may be added to reduce corrosion of pipes carrying the water. Fluoride may be added to reduce tooth decay.


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Copyright 2002, 2003 City of Battle Creek, Michigan
Last modified: March 02, 2005