Water containing human waste and excreta is generally termed "wastewater." Usually, wastewater consists of 99.9 percent water and 0.1 percent waste. In the United States, each state has a law that requires the disposal of human waste in a sanitary manner. Treatment of wastewater is required to prevent the pollution of surface waters, the pollution of groundwater, and to prevent pathogenic and microbial contamination from the use of excreta as fertilizer. Also, wastewater should be disposed of in a sanitary manner to make it inaccessible to insects that transmit disease.
Wastewater treatment consists of physical, chemical, and biological processes—either aerobic or anaerobic. The aerobic process is used most frequently. In the activated sludge process, air has to be forced into the liquid in a tank that is used to maintain aerobic microbial activity and to prevent odor. Additionally, temperature and pH must be maintained for the microbial activity.
In a municipal system the flow moves as follows: from sanitary sewer to screening and grinding process, to primary clarification, to activated sludge or trickling filter, to secondary clarification, to chlorine treatment, and finally to a water body such as a river or stream. Wastewater from the home enters a domestic or sanitary sewer—a system of pipes that collect the wastewater. The waste is then transported to a wastewater treatment plant. As it enters the plant, it flows through a bar screen, which strains out large materials. It then continues into a grit basin or chamber, where the water is slowed down enough to allow heavy or dense particles to settle out. These particles are then removed and taken to a landfill. The materials that do not settle out are ground up to prepare them to be digested by microorganisms in the treatment plant.
The wastewater then enters the primary clarifier, which allows materials to settle out. The flow of water through the clarifier is slow, allowing large amounts of suspended solids to settle at the bottom in the form of sludge. The sludge is then scraped and pumped away to allow the process to continue.
From the primary clarifier, the wastewater enters activated sludge tanks or trickling filters. Trickling filters are large areas of biological decomposition consisting of rocks that host biological organisms on their surfaces. These organisms metabolize most of the suspended solids that did not settle in the primary clarifier. The buildup on these rocks eventually sloughs off. The activated sludge tank is also used to remove waste from the wastewater. In this process, water from the primary clarifier is pumped into an aeration tank and combined with a mixture rich in bacterial growth. Pure oxygen is pumped through, allowing the decomposition of the organic materials in the wastewater. The remaining water is moved from the top of the tank, leaving sludge at the bottom.
Water from the trickling filter moves to a secondary clarifier, which settles any remaining suspended solids. The solids are then pumped into a digester, while the effluent is chlorinated and released back into a water channel, river, or stream.
MARK G. ROBSON
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