Primary Water Treatment is the primary sedimentation stage, sewage flows through large tanks, commonly called “pre-settling basins”, “primary sedimentation tanks” or “primary clarifiers”. The clarification tanks are used to settle sludge while grease and oils rise to the surface and are skimmed off.
Primary settling tanks are usually equipped with mechanically driven scrapers that continually drive the collected sludge towards a hopper in the base of the tank where it is pumped to sludge treatment facilities. Grease and oil from the floating material can sometimes be recovered for saponification (soap-making).
Primary water clarification is designed to remove suspended and floating solids from raw sewage or effluent wastewaters. It includes screening or a weir to trap solid objects and sedimentation by gravity to remove suspended solids. This level is sometimes referred to as “mechanical treatment“, although chemicals are often used to accelerate the sedimentation process.
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Primary water treatment removes material that will either float or readily settle out by gravity including the physical processes of screening, grit removal, and sedimentation. Screens are made of long, closely spaced, narrow metal bars that block floating debris such as wood, rags, and other bulky objects that could clog pipes or pumps. In modern plants, the screens are cleaned mechanically, and the material is promptly disposed and shredded material is removed later by sedimentation or flotation processes.
After going through a clarifier water is often put through flocculation or sedimentation practices. Flocculent chemicals are used in water treatment processes for the removal of suspended particles that are too small to settle out or will take too long to settle on their own, Chemicals such as ferric chloride (FeCl3), PAC (poly aluminium chloride or aluminium sulphate are often used as they can get these particles agglomerate during the flocculation stage and settle faster as the mass of the solids increases.