Unit: 3 

Design of Coarse Screen and Fine Screen for water treatment 

  • Design Criteria 
  • Design of Coarse Screen 
  • Design of Fine Screen 

Design criteria for coarse and fine screens include the following:

Screening efficiency: The ability of the screen to remove unwanted particles from the water. This is typically measured by the percentage of particles that are removed by the screen.

Head loss: The amount of pressure drop that occurs across the screen. This should be kept to a minimum to minimize energy consumption and maintain flow rate. 

Screening capacity: The amount of water that can pass through the screen per unit of time. This is typically measured in cubic meters per second (m3/s).

Screening size: The size of the particles that the screen can remove. This is typically measured in millimeters (mm).

Durability: The ability of the screen to withstand the wear and tear of daily use. This includes factors such as corrosion resistance, abrasion resistance, and the ability to withstand high flow rates.

Maintenance: The ease of cleaning and maintaining the screen. This includes factors such as the design of the screen, the materials used to construct it, and the ability to easily access and remove debris.

Cost: The initial and ongoing costs associated with the screen, including purchase price, installation costs, and maintenance costs.

The location of the screen, if it is installed in an open channel, the screen should be placed in an area with a stable flow and no turbulence.

If it is installed in a closed pipe, the screen should be placed in an area where it is easy to access and maintain, and where the water flow is not too fast, so that debris can be easily removed.

The environmental impact, the screen should not cause any negative environmental impact such as fish or aquatic life entrapment, or chemical discharge. 

Design of Coarse Screen : 

The design of a coarse screen involves determining the appropriate dimensions and materials to effectively remove large debris from the water. Some key considerations for designing a coarse screen include:

Screening size: The size of the debris that the screen is designed to remove. This is typically measured in millimeters (mm) and should be larger than the debris that is likely to be present in the water. 

Bar spacing: The distance between the bars or grates of the screen. This should be small enough to effectively remove debris, but large enough to prevent clogging and maintain flow rate.

Bar thickness: The thickness of the bars or grates of the screen. This should be thick enough to withstand the weight and impact of large debris, but not so thick that it increases head loss.

Materials: The materials used to construct the screen. Common materials include stainless steel, aluminum, and high-density polyethylene. The materials should be corrosion-resistant and able to withstand high flow rates.

Screen angle: The angle at which the screen is installed. A steeper angle can increase the screening efficiency, but can also increase the head loss.

Grit channels: Grit channels are the channels that are placed before the coarse screen to remove heavy debris, such as sand and gravel, before they reach the screen.

Screen cleaning: The design should include a mechanism for cleaning the screen, such as a brush or water jet system.

Location: The coarse screen should be located in an area with a stable flow and no turbulence, such as a straight section of an open channel, or a location in a closed pipe where it is easy to access and maintain.

The coarse screen can be of two types: mechanical screens such as bar screens, and coarse screens such as trash rack screens. The choice of the type of coarse screen is determined by the site condition, flow rate, and type of debris.

Lastly, the design should be compliant with the relevant regulatory requirements, such as safety and environmental regulations. 

Design of Fine Screen : 

The design of a fine screen involves determining the appropriate dimensions and materials to effectively remove small particles and organisms from the water. Some key considerations for designing a fine screen include: 

Screening size: The size of the particles that the screen is designed to remove. This is typically measured in micrometers (µm) and should be smaller than the particles that are likely to be present in the water.

Slot size: The width of the openings in the screen. This should be small enough to effectively remove particles, but large enough to prevent clogging and maintain flow rate. 

Materials: The materials used to construct the screen. Common materials include stainless steel, aluminum, and high-density polyethylene. The materials should be corrosion-resistant, non-toxic, and able to withstand high flow rates.

Screen angle: The angle at which the screen is installed. A steeper angle can increase the screening efficiency, but can also increase the head loss.

Screen cleaning: The design should include a mechanism for cleaning the screen, such as a brush or water jet system.

Location: The fine screen should be located downstream of the coarse screen, in an area with a stable flow and no turbulence, such as a straight section of an open channel, or a location in a closed pipe where it is easy to access and maintain.

The Design of fine screen can be of two types:

Mechanical screens such as band screens, and fine screens such as drum screens. The choice of the type of fine screen is determined by the site condition, flow rate, and type of particles.

The Fine screen should be designed to minimize the entrapment of fish and aquatic life.

The design should be compliant with the relevant regulatory requirements, such as safety and environmental regulations.

Lastly, the fine screen should be designed to minimize the maintenance and operation costs, as fine screens can be prone to clogging and require frequent cleaning.