prepare a brief report on sewage disposal system
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Sewage disposal is a major form of coastal pollution throughout the world. Sewage outfalls near coastal communities release human waste as well as other organic matter, heavy metals, pesticides, detergents, and petroleum products. Nutrients from organic waste material may cause eutrophication; local waters may also be nutrient-enriched by detergents that contain phosphate and by agricultural and horticultural products entering from runoff. In addition, human sewage delivers pathogenic bacteria and viruses that are not necessarily killed by exposure to seawater; high concentrations of these microbes make local seafood unsafe to eat and contaminated waters unsafe for bathing. The chief health risk from sewage is through eating contaminated seafood, particularly filter-feeding clams or mussels which accumulate human pathogens on their gills. The cholera virus is a particular problem in some countries, and may be transmitted in just such a manner.
In urban areas of developed countries, sewage may receive special treatment to degrade organic matter or to remove nitrates and phosphates, but these processes are expensive. Usually no more sewage is treated than is deemed necessary, and in many places sewage is released into the sea without treatment. Generally the immediate area (within 100 m) around a large sewage outfall may be anoxic and dominated by anaerobic bacteria. At some greater distance from the outfall (within several km), nutrient enrichment typically leads to increased production of green macroalgae (Enteromorpha or Ulva) that form thick mats along the shoreline. A few opportunistic animals, such as the polychaete Capitella, are also indicative of sewage enrichment and may dominate affected benthic communities. At some ten kilometres from a major domestic outfall, there is usually sufficient attenuation of pollutants that community species diversity is not affected.
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