Primary Settlement Tanks In Water Treatment: Why Are They Used?

Clarification or primary settlement tanks are used for a wide variety of applications in the wastewater treatment process ensuring effective removal of suspended solids from wastewater providing a large settlement area within a small footprint. A site settlement tank is used in the primary treatment of sewage is the removal of floating and settleable solids through sedimentation. Primary clarifiers reduce the content of suspended solids as well as the pollutant embedded in the suspended solids.

Because of the large amount of reagent necessary to treat domestic wastewater, preliminary chemical coagulation and flocculation are generally not used, remaining suspended solids being reduced by following stages of the system.  Sedimentation tanks are used in water treatment as “secondary settlement clarifiers” to remove flocs of biological growth created.

How Primary Settlement Tanks Work

The water to be treated is often mixed with the chemicals and flows down to the base of the coagulation vessel via the central chamber. The water then flows up the outside of the chamber where the suspended solids and precipitates settle out. The solids (sludge) collected by the unit settle within the unit’s single hopper and are easily removed by the opening of a gate valve, eliminating the need for the unit to be taken offline for emptying thus allowing continuous operation.  The treated water is collected at the top of the vessel.

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Silt whilst technically referring to a specific grain size is more often a broad term applied to fine particles of soil carried by water giving it that muddy or dirty discolouration. The most common source of silt pollution is the result of rainwater runoff from topsoil stripped areas of construction sites. When washed off into nearby watercourses, silt pollution is highly visible and easily traceable back to the site. It can travel a long way, causing significant environmental harm including suffocating fish by blocking gills and settling on river beds killing bottom-dwelling organisms. There are legal limits for the concentration of suspended solids in discharged water and breaching these is a common reason for construction companies being taken to court and fined.

Chemicals used in Primary Treatment

Some primary settlement treatment plants require the addition of chemicals to the water being treated to form precipitates and settle suspended solids in the raw water. Applications include the addition of lime to raw water in the cold lime softening process, which also reduces the alkalinity of the raw water, and the addition of lime or other alkalis to effect precipitation of insoluble hydroxides.

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Settlement Tanks on Construction Sites

A site settlement tank is used on construction sites to allow silt to drop out the water in the tank. Settlement/sedimentation tanks are used as sedimentation water treatment, primary sedimentation tank or settling tank.

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