In the water treatment process, filters play a crucial role. They intercept and adsorb impurities and stains in the water through a filter media layer, ensuring that the effluent meets the required standards. However, over time, the filter media layer becomes clogged, leading to a decline in the quality of the effluent. At this point, backwashing of the filter is necessary to restore its filtering performance. This article will provide a detailed introduction to the principles, methods, and precautions of filter backwashing to help you better understand and operate the process.
During the use of a filter, the filter media layer traps and adsorbs a large amount of impurities and stains. These impurities and stains gradually clog the pores of the filter layer, resulting in a decline in the quality of the effluent, mainly manifested in the following aspects:
Declining Water Quality: The normal filtered water quality deteriorates, with an increasing amount of suspended solids in the effluent, leading to a gradual decline in water quality.
Increased Pressure Differential: The pressure difference between the inlet and outlet pipes gradually increases, indicating an increase in the resistance of the filter layer.
Reduced Flow Rate: The flow rate of a single filter gradually decreases, failing to meet the design requirements.
When these situations occur, it is necessary to backwash the filter to restore its filtering performance.
The principle of backwashing involves reversing the flow of water through the filter media layer, causing the filter layer to expand and suspend. By leveraging the shear force of the water flow and the collision and friction forces between particles, the filter media layer is cleaned, and the contaminants are detached and expelled with the backwash water. Specifically, the backwashing process includes the following steps:
Reversed Water Flow through the Filter Media Layer: In normal filtration, water flows from top to bottom through the filter media layer. During backwashing, the flow direction is reversed, flowing from bottom to top. This reversed flow causes the filter media layer to expand and suspend, increasing the gaps between filter media particles, facilitating the removal of contaminants.
Shear Force of Water Flow and Particle Collision and Friction: The shear force of the water flow can strip contaminants from the surface of the filter media, while the collision and friction between particles further clean the surface of the filter media, detaching the contaminants.
Contaminant Discharge: Contaminants detached from the filter media are expelled from the filter along with the backwash water, restoring the cleanliness of the filter media layer.
There are several methods of filter backwashing:
Air washing uses compressed air to rinse the filter media layer. First, the water level in the filter pool is lowered to 100 mm above the surface of the filter layer, and then air is introduced for several minutes. As the air rises, it forms bubbles within the filter media layer, creating a scrubbing action on the surface of the filter media. This method is suitable for filters with heavy surface contamination and light internal contamination.
Precautions:
The corresponding valves must be properly closed; otherwise, when the water level drops below the surface of the filter layer, the upper part of the filter layer will not be immersed in water. During the agitation of the particles, contaminants will not be effectively expelled and may instead move deeper into the filter layer.
The intensity of the backwash air is generally 15 m³/(m²•h), and the pressure of the backwash air should be ≤0.15 MPa.
Water backwashing uses the reversed flow of water through the filter media layer to rinse it. This method can directly flush out contaminants from the filter media layer, restoring its cleanliness.
Precautions:
The intensity of the backwash water is generally 40 m³/(m²•h), and the pressure of the backwash water should be ≤0.15 MPa.
During the backwashing process, the introduced backwash water collects at the top of the filter and should mostly be discharged through a dual-port exhaust valve. In daily production, it is necessary to frequently check the patency of the exhaust valve, mainly by observing the freedom of movement of the valve ball.
Combined air and water backwashing involves introducing air and water simultaneously into the filter media layer for rinsing. This method combines the advantages of both air and water, resulting in better backwashing performance.
Working Principle:
Air and backwash water are introduced simultaneously from the bottom of the stationary filter layer. As the air rises, it forms large bubbles within the sand layer, which then break into smaller bubbles upon encountering the filter media, creating a scrubbing action on the surface of the filter media.
The backwash water loosens the filter layer, suspending the filter media and facilitating the scrubbing action of the air on the filter media.
The expansion effect of the backwash water and air is combined, resulting in a stronger action than when used individually.
Precautions:
The backwash pressure and intensity of water and air are different, so attention should be paid to the sequence to prevent backwash water from entering the air pipeline.
After the combined air and water backwashing is completed, the air supply should be stopped, and the backwash water should continue to flush for an additional 3 to 5 minutes at the same flow rate to remove any residual air bubbles in the filter bed.
To ensure effective backwashing, it is necessary to strictly control the parameters of the backwashing process.
The expansion rate refers to the degree of expansion of the filter media layer during backwashing. The expansion rate should be moderate; too high or too low will affect the backwashing effect.
For dual-layer filter media: the expansion rate is 40% to 50%.
The volume and pressure of the backwash water are important parameters that affect the backwashing effect.
Intensity of backwash water: generally 40 m³/(m²•h).
Pressure of backwash water: ≤0.15 MPa.
Intensity of backwash air: 15 m³/(m²•h).
Pressure of backwash air: ≤0.15 MPa.
During production operations, it is necessary to randomly inspect the filling height of the filter media and the expansion height. During normal backwashing, some filter media may be lost or worn and need to be replenished. A relatively stable filter layer has the following advantages:
Ensuring stable filtered water quality: A stable filter layer can ensure the stability of the filtered water quality, avoiding fluctuations in water quality.
Ensuring effective backwashing: A stable filter layer can ensure effective backwashing, restoring the filter media layer to good filtering performance.
When to perform backwashing can be determined based on the characteristics of the raw water quality and the requirements of the effluent quality, using criteria such as head loss, effluent quality, or filtration time. Generally, backwashing is required under the following circumstances:
When the head loss reaches a certain limit: Backwashing is necessary when the head loss of the filter reaches the upper limit specified in the design.
When the effluent quality declines: Backwashing is required when the effluent quality of the filter deteriorates and fails to meet the design requirements.
When the filtration time reaches a certain cycle: Based on the design and operating experience of the filter, backwashing is necessary when the filtration time reaches a certain cycle.
During the backwashing process, the following points should be noted:
Valve Operation: The corresponding valves must be properly closed; otherwise, it will affect the backwashing effect.
Uniformity of Air Distribution: The distribution of backwash air must be uniform; otherwise, it will lead to uneven expansion heights and affect the backwashing effect.
Supplement of Filter Media: During normal backwashing, some filter media may be lost or worn and need to be replenished regularly.
Patency of Exhaust Valves: During backwashing, the introduced backwash water collects at the top of the filter and should mostly be discharged through a dual-port exhaust valve. In daily production, it is necessary to frequently check the patency of the exhaust valve, mainly by observing the freedom of movement of the valve ball.
In actual operation of filter backwashing, the following problems may be encountered.
Filter media compaction occurs due to the presence of oil in the raw water, which is trapped in the filter and, after backwashing, leaves residual parts that accumulate over time, leading to compaction. The solution is to perform regular backwashing and pay attention to the backwash pressure and intensity of air and water during the process to prevent backwash water from entering the air pipeline.
Non-uniform distribution of backwash air is mainly caused by perforations in the bottom air distribution pipes, blockage or damage to local filter caps, or deformation of the grid spacing. The solution is to regularly inspect and maintain the air distribution pipes and filter caps, and promptly repair any damaged parts.
Fine filter media particles on the surface have fewer opportunities for collision during backwashing, resulting in low momentum and difficulty in cleaning. Attached sand particles are prone to forming small mud balls. The solution is to regularly check the filling height and expansion height of the filter media and promptly replenish the fine particles.
Filter backwashing is an important step in maintaining the filtering performance of a filter. By using appropriate backwashing methods and controlling parameters, the cleanliness of the filter media layer can be effectively restored, ensuring the normal operation of the filter. In actual operation, attention should be paid to valve operation, uniformity of air distribution, replenishment of filter media, and the patency of exhaust valves to ensure the effectiveness of backwashing. It is hoped that this article will help you better understand and operate filter backwashing, ensuring the efficient operation of the filter.
