Sewage System

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In the year 2000, it was estimated that 70 percent of the U.S. population lived in areas where domestic wastes pass through a sewage treatment plant before being discharged back into a water source. Sewage treatment systems and chlorination of water have made major contributions to the reduction of the incidence of waterborne diseases such as cholera and typhoid fever. Sewage treatment is used to improve the quality of wastewater so that it can be released into a waterway without causing damage to aquatic species or causing waterborne diseases among humans. Several levels of sewage treatment are used.

Primary sewage treatment removes larger floating objects through screening and sedimentation. The incoming wastewater flows through one or more screens and then enters a grit chamber where it slows down enough to allow sand, gravel, and other inorganic matter to settle out. In treatment plants where only primary treatment occurs, the effluent is chlorinated and discharged into circulation in a water source. The sludge, or sedimentation of larger solids, is removed, dried, and disposed of. Primary treatment removes 50 to 65 percent of suspended solids and decreases biological oxygen demand (BOD) by 25 to 40 percent. Primary treatment alone is not considered adequate for protection of the environment or people's health.

Secondary treatment relates to processes similar to natural biological decomposition. Aerobic bacteria and other microorganisms are used to break down organic materials into inorganic carbon dioxide, water, and minerals. Trickling filters, which are made from a bed of rocks with a microbial covering, are used to absorb the organic material present in the water. Activated sludge processes can be used in place of trickling filters. The level of suspended solids and BOD in wastewater after primary and secondary treatment has been decreased by 90 to 95 percent. This level of treatment is not effective in removing viruses, heavy metals, dissolved minerals, or certain chemicals.

Tertiary treatment is an advanced level of treatment. This form of treatment can decrease the level of suspended solids and BOD to approximately 1 percent of what was present in the raw sewage prior to primary treatment. Advanced treatment processes consist of several biological, chemical, or physical mechanisms.

Sewage treatment aims to destroy pathogenic organisms. Since primary and secondary treatments do not destroy a significant number of organisms, chlorination, which is effective in killing bacteria, is used to disinfect treated effluent.

Private sewage treatment, usually a septic system, is constructed on-site and is maintained by the private homeowner. In this case, the septic tank holds the solid materials while the water goes to a leach field or absorption field. The solids undergo decomposition, and on a regular basis, generally every three years, are pumped from the holding tank. This will vary according to use and capacity.

Mark G. Robson

(see also: Biological Oxygen Demand; Sanitation; Wastewater Treatment; Water Quality )


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