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Home > Sewage Treatment > Primary Sewage Treatment

Primary Sewage Treatment

The first step in sewage treatment is known as primary treatment. This stage of sewage treatment concentrates mainly on the removal of solid waste products from water. First, the sewage water is screened and larger solid objects are removed, then grit is removed, and finally the rest of the solids are removed using gravity in large sedimentation tanks. The entire process is discussed below :
Primary Sewage Treatment
Removal of Larger Solids

The process starts with the sewage water poured into the sewage treatment plant and made to pass through the bar screens or bar racks. The large size solid objects present in the water could not pass through the bar screen and get stuck and are thereby removed. Ideally, there are three different types of screens that are used - trash racks, manually cleaned racks, and mechanically cleaned racks.

Trash racks have the largest openings, almost 40 to 150 mm and are used to remove the largest objects present in the wastewater
Manually cleaned racks have comparatively smaller openings ranging from 25 to 50 mm.
Mechanically cleaned racks are used to collect the finest contaminants as they have the smallest openings, about 1 to 40 mm.
Generally, a sewage treatment plant is always equipped with at least two channels of bar screens, so that one can be operational while the other undergoes cleaning and maintenance.

Grit Removal

Small but dense material such as sand, dirt or broken glass present in the wastewater are called as Grit. It is necessary to remove the grit from sewage because their presence can wear and cause damage to the mechanical devices in a treatment plant. For removing the grit, the sewage is sent through a channel where the speed of the water flow is such that it easily passes to the next level of treatment but the grit settles down. Like the bar screen channels, each treatment plant possess two grit removing channels, so that one can be operational while the other undergoes cleaning and maintenance.
Primary Sewage Treatment

After the grit is removed, the sewage water is pumped to the sedimentation tanks, called clarifying tanks. Clarifying tanks are either of circular or square shape and they remove the remaining suspended materials in the sewage water using gravity. Usually, circular tanks are built with diameters ranging from 3 to 90 meters across so that these tanks are more energy efficient. The size of the square tanks varies from 15 to 100 meters in length and 3 to 24 meters in width and are used when there is constrain of space. Water in the sedimentation tanks are allowed to sit for a few hours so that the suspended particles either settles to the bottom or floats at the surface. The high density sediments settle down and they are called as sludge which are later pumped out of the tank and is forwarded for the secondary treatment. The light density sediments like oil and grease floats in the surface are also sent to undergoe secondary treatment. The left out clarified water goes on for tertiary treatment.
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