Preliminary Treatment

Preliminary treatment at a wastewater treatment plant begins with a screening device such as a bar screen, drum screen, traveling screen or hydro sieve. The screens help to protect downstream equipment from damage and keep floating material off the surface of the clarifiers. These screening devices can have automatic cleaners that remove the trapped material from the screen. If your device does not have a screen, then it will need to be manually cleaned multiple times each day. The material removed by the screens is normally trash, plastics, rags and other large untreatable objects that get into the waste stream.

Following the screening device is the grit removal system. This portion removes the heavier inorganic material from the waste stream by slowing the flow to less than 1 ft. per second. This allows the solids to settle into an area of the treatment process provided for that purpose. Another method of grit removal is through the use of centrifugal force. The wastewater enters a cylindrical tank tangentially creating a vortex-flow pattern, and the centrifugal and gravitational forces cause the grit to separate and settle out.

Grit can be organic in nature and produces odor as it decomposes. To combat this problem, some of the automated grit collection systems have a washer to remove some of the organic material from the grit before disposal to the landfill. If your system does not have the automated removal system, then the grit will have to be manually removed. The grit removal system is there to protect the downstream piping and pumping equipment, such as RAS and WAS pumps.

The comminutor or grinder is an essential piece of equipment at a treatment plant. It cuts the organic and inorganic material into more manageable pieces for the treatment process. As it reduces the size of the organic material, it allows more surface area for the bacteria to attack the BOD in the wastewater. This allows for the BOD to be removed more quickly. Inorganic material (plastics) should be cut into small enough pieces that it will settle out with the sludge in the secondary clarifiers and be disposed of as waste sludge. If the teeth of the comminutor are not properly adjusted, the material will not be properly reduced in size and may cause problems in the treatment process.

Another section of the preliminary treatment process may be an equalization basin (EQ basin). An EQ basin is used to smooth out wide variations in flow or organic load so that a constant or nearly constant flow rate or loading rate can be achieved to the plant. Flow equalization improves the performance of the downstream processes and may reduce the size and cost of downstream treatment facilities.

The mixing equipment used in EQ basins has to be adequate to prevent deposition of solids in the basin. Aeration is needed to prevent the wastewater from becoming septic and odorous. Two types of aeration are surface type aeration or a manifold type system.

The equipment used to carry out the functions of preliminary treatment is some of the most important in a wastewater treatment plant. It is in constant need of repair and cleaning due to the aggressive nature of its environment. Preventive and periodic maintenance is required to keep it in working order.


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