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Primary/physical/preliminary Treatment

	Primary/physical/preliminary Treatment The primary treatment process consists of screening and sedimentation processes. Screening The purpose of screening is to remove larger floating solids and larger organic solids which do not aerate and decompose (become septic), and to skim grease and fatty oils. This process is accomplished by screeners which are of three types: Rock or coarse screens which are made of parallel bars 75 mm apart. It involves more area and hence is more effective.
Medium screens having bars or mesh with a width of 12.5-40mm. Fine screens made of fine wire of perforated metal with openings less than 13 mm. They get easily clogged. To prevent overloading with collected solids, the screen is slowly rotated so that solids can be removed at regular intervals. Screenings are usually disposed by burial or incineration or they are passed through a macerator which shreds them to small sizes.
Disposal of screenings By disintegration after which they are either dumped into sea or buried. By returning the screening back into the sewage after maceration. By burying the screenings in shallow trenches covered with dry earth. This method requires lot of land. By comminuters (mechanical appliances used in USA) which combines a screen and a disintegrator in a single mechanism. Both are done in closed chamber. By mixing the screenings with house refuse composting. By incineration (this method is costly).

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