Wastewater screening is a crucial part of the primary water treatment. It is used to clean wastewater from larger solids like rags, sticks, plastic, etc., before subjecting it to other treatment processes. The purpose of screening in wastewater treatment is to:
- protect the equipment downstream from clogging and damage;
- remove large debris that can adversely affect the efficiency of treatment processes;
- shield final discharge waterway from contamination.
Governments ensure that most wastewater, including stormwater, sewage, and water generated from factories and municipal facilities, gets treated before it is reused or released back to the environment.
Screening process in wastewater treatment
Screening is the first step in the wastewater treatment process. It is an efficient and cost-effective way of removing solids from sewage. Treatment plants are equipped with suitable wastewater screening systems to handle both municipal and industrial wastewater screening.
Primary screening in wastewater treatment begins when wastewater arrives at treatment plants and is channeled through mechanical screens.
Primary screening in wastewater treatment begins when wastewater arrives at treatment plants and is channeled through mechanical screens. As the effluent passes the screens, solid wastes that are larger than the screen’s spacing are trapped on its surface. These solids are then removed or washed off into a sludge container. Then, the screened effluent continues its way into an inlet tank for the subsequent treatment process.
Types of screening in wastewater treatment
The choice of wastewater screening equipment is determined by various factors, such as:
- The size of the treatment plant;
- Solid contaminants to be filtered;
- The approach velocity of wastewater.
These factors also influence the design of a screen, particularly the size of its opening. Based on the size of this inlet, there are two significant types of wastewater screens: coarse screens and fine screens.
These screens typically have about 0.25 in (6’’) openings and comprise of parallel vertical bars or wires, grating, wire mesh, or perforated plates with circular or rectangle-shaped openings. They are also called “bar screens” and they prevent the clogging of other screens downstream.
The simplicity of the screen’s design makes it efficient in handling debris. It is also a cost-effective wastewater screening solution. The screen is categorized into two types:
Manually cleaned screens. This screen is generally mounted at a vertical angle, between 30 to 45 degrees. It has a spacing of 1 to 2 inches between its bars. The bars capture screenings when effluent flows through the channel. These screenings are then manually removed with rakes, after which they are removed via the discharge chute. Small wastewater pumping stations typically choose this screen.
Mechanically cleaned screens. Most medium-sized and large treatment plants favor mechanically cleaned screens. They can capture a larger amount of debris and tolerate heavier effluent flow. There are 4 types of mechanically cleaned screens:
- Chain or cable-driven screens: This screen comprises a set of bar rakes, all of which have chains connected on both sides. These rakes move in a looping circuit, from the bottom of the bar rack to the top. They push solids caught on the bars to the top of the rack for disposal. These screens require little headroom and last for a long time. However, some of its components are submerged and can suffer wear and tear.
- Reciprocating rake screens: This screen uses a single rake mounted on a gear, unlike other bar screens. During cleaning, the gear turns, moving the rake up and down the bar rack to remove the solids caught on the bar. None of its components are submerged, but its screening loading rate is low.
- Continuous belt screen: This self-cleaning screen has a dense mesh of specially-shaped teeth that can filter coarse and fine solids. The solids are lifted to the top of the screen to be disposed of through the discharge chute. Its pivot design allows personnel to service the unit above the channel efficiently. However, the presence of several moving components makes it susceptible to wear and tear.
- Catenary screen: This screen also uses a chain-driven rake mechanism. Its bearings, shafts, and sprockets are above effluent level, lowering the potential for corrosion.
Fine screening is the next treatment phase in the wastewater screening process. These screens have an opening of 0.02 to 0.25 inches. They filter coarse screenings and a higher amount of debris like rags, wood, and organic matter. Their cleaning mechanism is more elaborate than their coarse counterparts since organic matter is harder to clean from smaller openings. There are three types of fine screens:
Fixed screens. With rounded openings of 0.010 to 0.5 inches, these screens are typically found in small wastewater treatment plants.
Rotary drum screens. This screen is placed on a rotating cylinder in the flow channel, and it traps solids on its surface. As the screen spins, a collective of cleaning brushes or water spray cleans debris off the drum.
Step screens. This machine has two bars, a stationary and a movable one, which sit parallel to each other. The bars are self-cleaning and efficient in lifting debris to the top of the screen for disposal. Its hydraulic capacity is higher than other fine screens, and it can be installed in existing channels.
The screening process is a crucial part of the treatment process that ensures the removal of visible and large floating solids. Hence, the screening equipment is present in almost all industrial and municipal water treatment facilities. However, when water quality evaluations demand an even more thorough filtration, screening alone won’t do. Additional treatment downstream would be required to enhance the effect.