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A settling basin, settling pond or decant pond is an earthen or concrete structure using sedimentation to remove settleable matter and turbidity from wastewater. The basins are used to control water pollution in diverse industries such as agriculture,Sunnyside Valley Irrigation District. Sunnyside, WA. "Settling Basins." Accessed 2009-10-02. aquaculture,Western Regional Aquaculture Center, University of Washington. Seattle, WA (2001). "Settling Basin Design." WRAC Publication No. 106. and mining.Government of British Columbia. Ministry of Energy, Mines and Petroleum Products. Victoria, BC (2002). "Settling Pond." Aggregate Operators Best Management Practices Handbook for British Columbia. Chapter 7: Best Management Practices. April 2002.U.S. Environmental Protection Agency (EPA). Seattle, WA (2003). "EPA and Hardrock Mining: A Source Book for Industry in the Northwest and Alaska. Appendix E: Wastewater Treatment." January 2003. Turbidity is an optical property of water caused by scattering of light by material suspended in that water. Although turbidity often varies directly with weight or volumetric measurements of settleable matter, correlation is complicated by variations in size, shape, refractive index, and specific gravity of suspended matter.Franson, Mary Ann Standard Methods for the Examination of Water and Wastewater 14th edition (1975) APHA, AWWA & WPCF p.131 Settling ponds may be ineffective at reducing turbidity caused by small particles with specific gravity low enough to be suspended by Brownian motion.
Settling basins used in dairy production reduce the nutrient-loading on a vegetative filter strip from lot runoff, thus decreasing the required lagoon volume for a new facility. Moreover, settling basins are useful to remove unwanted solid materials, such as hay, straw and feathers from the waste stream before flowing to the lagoon, aid to reduce smell and avoid crust formation on the lagoon surface. A baffle may be used to retain the floating solids removed. There are two types of settling basins, based on the method of removing solids. With one type, the solids are removed mechanically (after the free water has drained away), usually with a front-end or skid-steer loader. The other type uses hydraulic (pump) removal of the solids. Typically, pumping is initiated when the basin is half full of solids and the remainder is water. Vigorous agitation is needed to mix the liquid and the solids, preferably by propeller-type agitators or pumps with agitation nozzles.Fulhage, C. D., Pfost D. L. &Feistner J. W., Settling Basins and Terraces for Cattle Manure, University of Missouri Extension, EQ386, new March 2002, accessed date 12 October 2013, http://extension.missouri.edu/p/EQ386
However, settling basins can introduce new kinds of water contamination, particularly if the water supply is from a well. The basin can catch windblown contaminants, and if the water is retained for a long period, algae grows in the pool, leading to greater filtration problems. Settling ponds may also be ineffective at reducing turbidity caused by small particles with specific gravity low enough to be suspended by Brownian motion. Usually, it can only remove particles ranging from sand (2 mm in diameter) to silt (0.002 mm in diameter).
Translation of required settling time surface area to settling basin geometry requires consideration of short circuiting and turbulence induced by wind, bottom scour, or inlet and overflow design. Settling basin geometry is important because effective time of settling within the basin will be the time a volume of water spends in non-turbulent conditions before reaching the settling basin overflow. Median time is always less than the mean time calculated by dividing available volume by anticipated flow. The median time of passage through a short, wide settling basin may be significantly less than that through a long, narrow settling basin. Settling basins with overflow structures near the entrance points may hold a large volume of stagnant water while newly admitted water rapidly reaches the overflow point before settling can occur. Effective surface area for settling seldom extends perpendicularly more than a tenth the distance of a flow line from basin entrance to overflow unless baffles are installed. Effective surface area and geometry may change as accumulating sediment fills part of the originally constructed volume. Short cut channels may rapidly form through heavier sediment accumulations near the entrance to the settling basin. Flow through shallow portions of the settling basin may cause turbulence resuspending sediment from the bottom of the basin. Two feet has been recommended as a minimum settling basin depth to avoid bottom scour.
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