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Wastewater screening is a key part of primary water treatment. It is used to remove waste water from larger solids (such as rags, sticks, plastics, etc.) before proceeding to other treatment processes. The purpose of screening in sewage treatment is:

The government ensures that most wastewater, including rainwater, sewage, and water from factories and municipal facilities, is treated before being reused or released back to the environment. 

Screening is the first step in the wastewater treatment process. This is an effective and cost-effective method of removing solids from sewage. The treatment plant is equipped with a suitable wastewater screening system to deal with the screening of municipal and industrial wastewater. 

When the wastewater reaches the treatment plant and is guided through a mechanical screen, the primary screening in wastewater treatment begins.

When the wastewater reaches the treatment plant and is guided through a mechanical screen, the primary screening in wastewater treatment begins. When the effluent passes through the screen, solid waste larger than the spacing between the screens is trapped on its surface. These solids are then removed or washed into the sludge container. Then, the sieved effluent continues to enter the inlet tank for subsequent processing.

The selection of wastewater screening equipment is determined by many factors, such as:

These factors will also affect the design of the screen, especially the size of its opening. According to the size of the inlet, there are two important types of wastewater screens: coarse screens and fine screens.  

These screens usually have openings of approximately 0.25 inches (6") and consist of parallel vertical bars or wires, grids, wire mesh, or perforated plates with circular or rectangular openings. They are also called "bar screens" and can prevent other screens downstream from clogging. 

The simplicity of the screen design allows it to effectively dispose of debris. It is also a cost-effective wastewater screening solution. There are two types of screens:

Manually clean the screen. This screen is usually installed at a vertical angle, between 30 and 45 degrees. There is a spacing of 1 to 2 inches between its rods. When the sewage flows through the channel, the bar fence will capture the screened material. Then use a rake to manually remove these sieves, and then remove them through the discharge chute. Small sewage pumping stations usually choose this screen.

Mechanically clean the screen. Most medium and large processing plants prefer mechanically cleaned screens. They can capture larger amounts of debris and withstand heavier sewage streams. There are 4 types of mechanical cleaning screens:

Fine screening is the next treatment stage in the wastewater screening process. The openings of these screens are 0.02 to 0.25 inches. They filter coarse sieves and large amounts of debris such as rags, wood, and organic matter. Their cleaning mechanism is more complicated than rough counterparts because organic matter is more difficult to clean from smaller openings. There are three types of fine screens:

Fixed screen. These screens have circular openings ranging from 0.010 to 0.5 inches and are commonly used in small sewage treatment plants. 

Rotating drum screen. The screen is placed on a rotating cylinder in the flow channel and traps solids on its surface. When the screen rotates, a set of cleaning brushes or water spray will remove debris on the roller. 

Step screen. This machine has two rods, one fixed and one movable, which are parallel to each other. These rods have a self-cleaning function, which can efficiently lift the debris to the top of the screen for processing. Its hydraulic capacity is higher than other fine screens and can be installed in existing channels. 

The screening process is a key part of the treatment process, ensuring that visible and large floating solids are removed. Therefore, screening equipment exists in almost all industrial and municipal water treatment facilities. However, when water quality assessment requires more thorough filtration, screening alone is not feasible. Additional downstream processing is required to enhance the effect. 

Guest author published on November 4, 2021

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