Types Of Screening In Wastewater Treatment

18th April, 2024.

In this post, we will see the types of screening chambers in wastewater treatment.

When you are treating sewage waste water, the first thing that obstructs the treatment equipment are big particles. Objects like packets, plastic, and other larger particles are a hindrance to finer treatment equipment present. So, it is necessary to first of all remove them. The way to remove them is called screening in wastewater treatment. In this post, we will see the concept of screening in wastewater treatment.

What is screening in wastewater treatment?

Refer to the below image. Basically, screening can be simply defined as a filter which chokes solids and bigger objects, and allows only water with smaller waste particles to pass. Only particle sizes can pass which cannot be trapped inside. A rack bar system is used which is used to trap the particles. The rack is nothing but an inclined conveyor which moves when a large amount of debris gets stuck. 

Means, suppose water with waste is flowing for an hour and the solid waste gets stuck near the rack bar as shown. When the bar gets choked, it means less water will pass. Less water means more debris will start to choke and water level will start to rise. When this happens, the conveyor where the debris is stuck starts to move upwards. This will throw the debris out and again make the rake clean for operation. Then once again, the debris will start to get stuck and the cycle continues. This is a screening operation. It is also called as preliminary treatment as it is the very first stage of the process.

Design of screening in wastewater treatment:

The design will be clear to you from the above image. A screen consists of parallel bars, rods or mesh wires which have openings on it. The opening size allows only the solids designed for smaller than it to pass. In this process, larger particles get stuck on the screen. Then, this bar conveyor is driven electrically after some frequent interval of time or when the debris level increases. It travels to the top and at the top, there is another small conveyor, which acts as a bucket type and runs whenever this screen conveyor runs, to throw the debris out.

If you program this screen system through PLC, the logic is pretty simple. The screen has a level transmitter in the inlet side, for monitoring water level. When water flow increases or if the debris quantity stuck on the screen has increased, water level will increase. A setpoint will be there in PLC for high level and when this level has been achieved, then the conveyor will start to run. Alternatively, a secondary program will be there on a timer on-off basis. So, the conveyor will run either through level or timer, with level being the first priority.

Types of wastewater screens:

1. Coarse Screen:

This is the first type of screen which removes major big particles and solids. It can be said to be the first stage of sewage water treatment. All the large particles are choked and trapped in this conveyor rack. The screen has an opening of 6 to 150 mm. So, the largest items can get choked here.

2. Fine Screen:

This is the second type of screen which removes small particles and solids. It can be said to be the second stage of sewage water treatment. All the smaller particles are choked and trapped in this conveyor rack. The screen has an opening of less than 6 mm. 

3. Drum Screen:

This is the third type of screen which removes the smallest particles and organisms. It can be said to be the third stage of sewage water treatment. All the smallest and micro particles are choked and trapped in this conveyor rack. The screen has an opening of 10 to 35 µm. Drum is a rotary part which rotates as per demand, and as the water passes through it, waste is stuck on the drum surface and clean water passes through it. As the drum rotates continuously, it throws the debris out. Occasionally, the drum is cleaned with clean water, also known popularly as backwash process. This step removes any stuck debris on the drum screen and makes it again reusable for treatment.

I have covered some general principles related to screen technology in wastewater treatment. I have also not attempted to cover all the methods, as it can vary from case to case and there is no end to it. Once you are familiar with this technique, you can easily tackle all types of problems in it.

Thank you for reading the post. I hope you liked it and will find a new way in this type of technology.