Coarse screens in wastewater – What you need to know

The first step in the wastewater treatment is the wastewater screening in which the wastewater screw pump is used for lifting large quantities of water/wastewater. Wastewater screens are used to keep the large solid particles out away from the wastewater going into the treatment plant or wastewater clarifier.

The purpose of the screening process is to remove particles that

• Can contaminate water
• Can reduce the efficiency of the system
• Can cause damage to the equipment

The classification of wastewater screens-

The wastewater screens are classified into three types on the basis of size of the opening and removal mechanism. This includes

• Coarse screens
• Fine screens
• Micro-screens

But, what is coarse screens?

Coarse screens have a size of opening ranging from 6mm to 150mm. They have components like parallel bars, wires and rods and the plates with openings generally of rectangular and circular shapes. These screens are used to remove coarse solid components. For example rags. Based on the type of cleaning method, coarse screens are defined into two parts:

Hand-cleaned: Hand-cleaned screens are used in small wastewater treatment plants. Most of the time they are used as standby screens during high flow periods. They are also used for a short period when mechanical screens are out of service or in case there is a power failure. For convenient hand Ranking, the length of the bar rack should be approximately 3m. The design of screens channels should be prepared in such a way that it should avoid the accumulation of large solid particles. The channel’s approach should be perpendicular towards the bar screen which allows uniform distribution throughout the flow.

• Mechanical cleaned screens: The main goal of designing mechanically cleaned screens is to reduce maintenance issues. This way the efficiency of the screening process increases. Mechanically cleaned screens are of four types. These are:

Chain driven screens: Chain driven screens use automatic chains for cleaning the screens. They are further classified into two types depending on how the screen is raked.

Catenary screens: Catenary screens have rake which is held by a chain. In case there is any solid particle then rakes will manage to stop further jamming.

Climber screens: Climber screens uses only one rake. That’s why they have limited capacity in handling screen loads.

Continuous belt screens: Here multiple rakes are connected to drive chains.