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P-2 <br />only 0.1% pollutants which need to be removed. Wastewater treatment is <br />difficult because it is necessary to maintain a very high removal efficiency <br />while dealing with large volumes of water. <br />The 0.1% in wastewater that must be removed or modified consists mostly of <br />organic material, bacteria, and nutrients. The organic material, which is <br />sometimes called the "biochemical oxygen demand" (BOD), comes from a variety of. <br />sources in a domestic wastewater system. Food preparation, dish and clothes <br />washing, and wastes from the relatively inefficient human .digestive system all <br />discharge into the domestic sewage system. Table B.1. shows the concentration <br />of BOD, suspended solids, and other materials in raw sewage and in septic tank <br />effluent. BOD and suspended solids are measures of the percentage of organic <br />material found in wastewater. <br />Pathogens are microorganisms which cause disease and are usually present in <br />domestic sewage. Fecal coliforms are indicators of the presence of pathogens <br />and Table B.1. shows that in 100 ml (about 112 cup) of septic tank effluent, <br />there are between 1,000 and 1 million. There are about the same number of <br />viruses in this quantity of effluent. <br />Nutrients are part of the materials in septic tank effluent and are of concern <br />because of possible eutrophication of ground or surface waters and possible <br />contamination of water supply wells. Nitrogen and phosphorus are the nutrients <br />of major concern in sewage tank effluent. Quantities of these nutrients are <br />listed at various locations in the soil treatment system in Table B.1. <br />Table B.1. Treatment Efficiencies of Soils <br />One Foot <br />Three Feet <br />Raw <br />Septic Tank <br />Below Trench <br />Below Trench <br />Parameter <br />Waste <br />Effluent_ <br />Bottom <br />Bottom <br />BOD5 (mg/1) <br />270 - 400 <br />140 - 175 <br />0 <br />0 <br />TSS (mg/1) <br />300 - 400 <br />45 - 65 -_ <br />0 <br />0 <br />Fecal Coliform <br />�1 <br />1 million to <br />thousand to <br />0 - 100 <br />0 <br />(MPN/100 ml) <br />100 million <br />million <br />Viruses <br />(Unknown) <br />11 <br />100 thousand to <br />0 - 1000 <br />0 <br />(PFU/ml) <br />10 million <br />Nitrogen <br />Total (mg/1) 100 - 150 50 - 60 - _ <br />NH4 (mg/1) 50 - 120 30 - 60 *B - 60 B <br />NO3 (mg/1) < 1 t B - 40 B - 40 <br />Total Phos- <br />phorus (mg/1) 10 - 40 10 - 30 B - 10 B - 1 <br />* B = c3ackground <br />