Many pollutants are present in urban wastewater, including oxygen-demanding substances, pathogens, inorganic and synthetic organic chemicals, and nutrients. Dissolved oxygen in water is necessary to support aquatic life, consumed by aerobic bacteria and other aquatic organisms for living purposes. Biochemical oxygen demand (BOD) is the amount of oxygen that would be consumed if all organic substances in water were oxidized by bacteria and protozoa (Interlab Supply, 2010). Microorganisms decompose organic matter by consuming dissolved oxygen present in wastewater, and BOD is normally used to measure wastewater treatment plant performance. (United States Environmental Protection Agency, 2004) The wastewater effluent produced by the treatment plant has a high content of organic pollutants such as ammonia which will require more oxygen. These substances are destroyed and converted into other compounds by bacteria when the water has sufficient oxygen. Large quantities of bacteria widely distributed in wastewater are also known as pathogens and dangerous to our health, while waste products in wastewater are often liquid or solid and can be biologically, chemically or radioactively dangerous. In addition to having negative health effects, wastewater contamination can also have natural and ecological effects, including the degradation of ecosystems that include the decline of aquatic plants that help maintain the condition of waterways or the loss of biodiversity. For example, the loss of aquatic life affects fish and crustaceans which constitute an important part of both animal and human diets. Disinfection of wastewater and chlorination of drinking water supplies can reduce the occurrence of waterborne diseases such as typhoid fever... middle of paper... associated with screening. However, shredders and shredders can also create problems for downstream processes, such as increased plastic buildup in digestion tanks or rag buildup on air diffusers. Additionally, solids from grinders and shredders do not break down during the digestion process. If these synthetic solids are not removed, they can cause biosolids to be rejected for reuse as a soil amendment. Large quantities of grit and grit entering a treatment plant can cause serious operational problems necessary for treatment. Sand includes sand, gravel, ash, or other heavy solid materials that are "heavier" than the biodegradable organic solids found in wastewater. Sand removal prevents unnecessary abrasion and wear of mechanical equipment, sand buildup in pipes and channels, and sand buildup in anaerobic digesters and aeration basins.