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Screening and Grit Removal for the Protection of Downstream Equipment

Updated: Apr 25

In the initial stages of the wastewater treatment process, screening and grit removal are vital steps that help to remove large objects and coarse materials from the influent wastewater. The primary purpose of these processes are to protect downstream equipment such as pumps and pipes. 

Screening is the first stage in the physical treatment process, designed to remove large objects and debris from the wastewater. It also helps to prevent damage to the subsequent treatment processes.

The process begins with wastewater flowing through a series of bars or grids, typically made of steel or iron, with openings ranging from 6 to 38 mm (0.25 to 1.5 inches). The bars are spaced in such a way that they catch large objects such as branches, rags, plastics, and other debris. The bars are placed at an angle so that the debris can be easily removed and collected. In some treatment plants, especially those that handle combined sewage, fine screens with smaller openings are employed to capture smaller debris, such as wipes, paper, and other small solids. These fine screens usually have openings ranging from 2 to 6 mm (0.08 to 0.25 inches).

Equipment used for screening includes mechanical bar screens, which are designed to physically remove large objects from the wastewater, and automatic screens, which are self-cleaning screens that use mechanical rakes or rotating drums to remove debris and carry it to a collection point. Regular maintenance is necessary to prevent the accumulation of debris and to ensure the screens are functioning properly. Debris collected on the screens needs to be removed manually or automatically to prevent blockages.

Grit removal is the process of removing heavy, inorganic solids such as sand, gravel, eggshells, and coffee grounds from the wastewater. These materials are usually larger and denser than organic particles and can cause abrasion and damage to pumps, pipes, and other equipment. Removing grit also helps prevent the accumulation of sediment in the treatment process, ensuring efficient operation and preventing wear and tear on the equipment.

After screening, the wastewater enters a grit chamber, where the flow velocity is reduced to allow the settling of heavy materials to the bottom of the chamber. Some treatment plants use detritus tanks, which are similar to grit chambers but operate on a longer detention time, allowing for more effective settling. The settled grit is then removed from the bottom of the tank.

Equipment used for grit removal includes grit chambers, which can be horizontal or vertical and are designed to allow the settling of heavy inorganic materials, and detritus tanks, which have a longer detention time and are designed to allow settling of heavier grit. Regular cleaning and removal of the collected grit are essential to prevent buildup and maintain the efficiency of the treatment process. Periodic inspection and adjustment of the grit removal equipment is also necessary to ensure optimal performance.

Proper operation and maintenance of screening and grit removal equipment are crucial for the overall efficiency and effectiveness of the treatment plant.

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