How to prevent filter media loss during backwashing of multi-media filters?

To prevent filter media loss during backwashing of multi-media filters, the key is to control the backwashing parameters, ensure the integrity of the equipment structure, and standardize the operating procedures . This can be achieved by controlling the backwashing intensity at the source and intercepting the filter media at the end. Specifically, the following four points should be noted.

1. Precisely control backwashing parameters to avoid excessive flushing force.

Flushing parameters (pressure, intensity) are the main cause of filter media loss and must be strictly matched to the characteristics of the filter media.

.
Control the backwash water pressure : The pressure needs to be stable at 0.1-0.15MPa , and should not exceed 0.2MPa. Excessive pressure will directly break up the filter media (such as anthracite with low density, which is easily washed to the drain outlet). In particular, it is necessary to avoid a sudden pressure rise when the backwash pump is started (this can be achieved by slowly opening the valve or by installing a pressure buffer device).

Matching backwash intensity : Set the intensity according to the type of filter media: 10-15 L/m²·s for quartz sand, 8-12 L/m²·s for anthracite, and 6-10 L/m²·s for activated carbon. Insufficient intensity will result in incomplete rinsing, while excessive intensity will exacerbate media tumbling and wear, and may even wash away the support layer. Real-time monitoring and adjustment using a flow meter are necessary.

Control the air washing pressure (including air washing scenarios) : The air washing pressure needs to be kept stable at 0.05-0.06MPa to avoid the filter media from "boiling" and floating due to a sudden increase in pressure (especially light filter media such as anthracite and activated carbon). At the same time, the air washing intensity should not be too high to prevent the filter media particles from colliding and breaking and being lost with the water flow.

II. Ensure the integrity of the core structure of the equipment and build a solid "interception defense line" for the filter media.

Equipment structural defects (such as damaged support layers or malfunctioning water distribution/drainage devices) can directly lead to filter media loss, requiring regular inspection and maintenance.

Ensure the support layer is intact and effective : The support layer (usually pebbles) should be filled in layers with the particle size gradually decreasing from bottom to top (e.g., bottom layer 20-30mm, middle layer 10-20mm, top layer 5-10mm), with a thickness of not less than 200mm. If the pebbles in the support layer shift, break, or are insufficient in thickness, the machine must be stopped to replenish or replace them to prevent the filter media from penetrating the support layer and entering the drainage system.

Check the water distribution and drainage system :

Water distributors (such as perforated plates and filter caps) must ensure that the holes are not blocked or damaged to prevent excessive local flow velocity from dispersing the filter media;

A filter screen (with a pore size less than 1/2 of the minimum filter media particle size, such as 0.2mm for quartz sand with a minimum particle size of 0.5mm) should be installed at the drain outlet to directly intercept any filter media particles that may be washed out. The filter screen should be cleaned regularly to prevent clogging and ensure proper drainage.

Check the tank and valve seals : Ensure that the exhaust valve and drain valve on the top of the tank are tightly closed to prevent filter media from being lost through gaps due to internal leakage during flushing; the gaskets at the flange interfaces need to be replaced regularly to prevent filter media from being carried out with the leaking water.

3. Standardize backwashing procedures to reduce losses caused by human error.

Improper operation sequence can easily lead to filter media layer disorder; a fixed procedure must be strictly followed.

Before backwashing, drain and depressurize: Before starting backwashing, you must first close the raw water inlet valve and the clean water outlet valve, open the air vent valve and the upper drain valve to drain 1/3 to 1/2 of the upper clean water in the tank, reduce the water pressure above the filter layer, and avoid excessive water flow impact during backwashing that could disperse the filter media.

Air washing and water washing should be coordinated in a proper manner : in scenarios involving air washing, air should be introduced before water (air-water combined washing), and water should not be introduced before air. Introducing air first allows the filter media to tumble evenly in advance, resulting in a more stable water flow when water is introduced later; if water is introduced first, the air bubbles are easily suppressed by the water flow, causing localized turbulent water flow that disperses the filter media.

Avoid frequent or prolonged backwashing : Do not repeatedly start backwashing within a short period of time (such as within 1 hour). Frequent backwashing will aggravate filter media wear and tumbling, increasing the risk of loss. The total duration of a single backwash (air washing + water washing) should be controlled within 5-15 minutes. Do not wash for too long to avoid the filter media from becoming disordered due to prolonged tumbling.

IV. Regular monitoring and replenishment, and timely response to minor losses.

During routine maintenance, it is necessary to proactively monitor the filter media level to prevent small losses from escalating into major problems.

Regularly check the filter media layer thickness : Open the equipment observation port or manhole every 3-6 months to measure the actual thickness of the filter media layer. If it is more than 10% less than the initial filling thickness (e.g., the initial thickness is 1.2m, and it is now less than 1.08m), the corresponding type of filter media needs to be added in time to ensure that the filter layer height meets the filtration requirements.

Observe whether the drainage contains filter media particles : During backwashing, pay attention to the drain outlet. If obvious filter media particles (such as white quartz sand or black anthracite) are found in the water, stop washing immediately, check whether the support layer and filter screen are damaged, repair them before continuing the operation, and avoid further loss of filter media.