When precipitation occurs in undeveloped areas a number of abstractions catch and absorb the precipitation. Abstractions include plants and trees, which catch a portion of the water, pervious soils that infiltrate stormwater until saturated and natural changes in topography where stormwater accumulates. During more intense, longer storm evens the abstraction limit in a watershed can be reached and runoff occurs. In the process of developing areas for cities, towns, industrial sites and transportation the watershed characteristics of that area are changed and the quantity of abstraction in the developed area is reduced. Pervious soils are replaced with impervious surfaces and plants that abstract, absorb and release moisture through evapotranspiration are removed. As a result a significantly larger quantity of surface runoff is generated in these areas. There are a number of methods used to manage the quantity of runoff. Two popular options are the combined sewer system and the separate sewer system. The combined sewer system conveys stormwater runoff and sanitary sewage from households to a wastewater treatment plant. After treatment water is released into a receiving body and the quality of the effluent is monitored to comply with water quality regulations in general, or for water quality regulations at a particular receiving body of water. The separate sewer system, also known as the municipal separate storm sewer system (MS4) conveys stormwater separate from sanitary waste. The MS4 generally discharges stormwater to a receiving body of water through a system of outfalls while sanitary sewage is conveyed to a wastewater treatment plant. Because stormwater picks up and transports pollutants deposited in urban areas, and is then discharged to a receiving body, or treated along with sanitary waste it is necessary reduce pollutants and/or reduce quantity of runoff for water quality and economic reasons. Within stormwater management methods that reduce runoff volume and or runoff pollution are called best management practices (BMP).
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Gorski, Jacob, "Modeling Liquid Solid Separation in a Coalescing Plate Oil Water Separator" (2013). Engineering and Technology Management Student Projects. 470.