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Primary Treatment

Primary sewage treatment involves the removal of biosolids and other settleable material in domestic wastewater. Prior to this, the wastewater entering the facility goes through a screen to remove large floating objects that can clog pipes and pumps. The screens can vary from removal of coarse material to the removal of very fine material while the rest of the wastewater continues on to the primary treatment process. Grinding mechanisms known as comminutors can cut and shred material which can then be prepared for further processing. The wastewater then flows into a grit chamber where sand, grit, cinders, and small stones can settle out, and then into a primary sedimentation tank where the wastewater is slowed down and suspended solids can be settled to the bottom of the tank by use of gravity or also with the addition of a chemical coagulant. The settled materials or primary sludge is then removed either manually or continuously depending on the age of the plant. The sludge is removed to a sludge digester where the process is carried out by anaerobic bacteria, occurring in the absence of air, that are present in the solids. The fluid that is left over should be introduced into secondary treatment. The material that settles should be considered to be biosolids and dealt with accordingly.

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