How To Prevent Filter Media Loss During Backwashing Of Multi-media Filters?

There is no fixed standard for the air washing and water washing time of multi-media filters. It needs to be dynamically adjusted based on three core factors: filter media type, raw water pollution level, and washing effect . The core principle is "determine the time based on the effect" rather than relying on a fixed duration.

I. Determining the air washing time: based on the "filter media filling time"

The purpose of air washing is to agitate and break up the loose filter media by using air bubbles
The purpose of air washing is to agitate and break up adhering impurities by using air bubbles to loosen the filter media. The time is usually controlled between 3 and 8 minutes , and the specific judgment is based on the following:

Classified by filter media type

Conventional filter media (stone)
Activated carbon filter media: 5-8 minutes. Activated carbon surfaces easily adsorb sticky organic matter, requiring longer agitation to completely remove it; and the air washing intensity needs to be reduced (5-8 L/m²・s), which needs to be compensated for by extending the time.

Adjusted according to the degree of raw water pollution
Low turbidity water (such as groundwater, turbidity < 5 NTU): 3-4 minutes. Inorganic particulate impurities are easily agitated and do not require prolonged air washing.
High turbidity water (such as river water during the rainy season, turbidity > 50 NTU): 5-7 minutes. Viscous colloidal impurities tend to adhere, requiring a longer time to ensure they detach from the filter media.

Judging by visual effect
If the filter media is seen to be rolling evenly without localized caking through the observation hole on the tank wall, and only a small amount of impurities (no filter media particles) are carried out from the exhaust port, it indicates that the air washing is sufficient and can be stopped early.

若滤料局部不动(板结),需延长 1-2 分钟,或短暂提高气压(不超过 0.07MPa)辅助松动,避免板结杂质进入水洗阶段.
If the filter media remains stationary (clumped), extend the time by 1-2 minutes or briefly increase the air pressure (not exceeding 0.07MPa) to help loosen it and prevent clumped impurities from entering the washing stage.

II. Determining the washing time: based on the condition that the water drains clear and leaves no residue.

Water washing is divided into "combined air-water washing" and "separate water washing," with a total duration of 5-12 minutes . The core function is to remove impurities removed by air washing through water flow. The specific method for determining this is as follows:

1. Air-water combined washing stage (5-8 minutes)

Function: It works in conjunction with air washing to further flush out impurities, especially fine particles that were not removed during air washing.

Time judgment:

For the first 1-3 minutes: the drainage will be noticeably turbid (turbidity > 100 NTU), which is normal and requires no adjustment.

In the last 2-5 minutes: if the drainage gradually becomes clear (turbidity ≤ 10 NTU), it can be stopped early; if it is still turbid after 5 minutes (turbidity > 20 NTU), it needs to be extended by 1-2 minutes, and the backwash intensity needs to be checked to see if it meets the standard (e.g., 10-15 L/m²・s for quartz sand).

2. Separate water rinse stage (2-4 minutes)

Function: To flush out residual air bubbles and small impurities in the tank, preventing wastewater from entering subsequent systems after backflushing.

Time judgment:

Using "completely clear drainage" as the standard, it usually takes 2-3 minutes; if the drainage is still slightly turbid (turbidity 5-10 NTU), extend it for 1 minute until the turbidity is ≤5 NTU.

Multi-layer mixed filter media (e.g., quartz sand + anthracite + magnetite): requires 3-4 minutes. This ensures that residual impurities between filter media of different densities are thoroughly rinsed to prevent impurities from penetrating during subsequent filtration.

III. Time Adjustments for Special Circumstances

Severe filter media caking : When the pressure difference before backwashing is >0.15MPa, extend the air washing and water washing by 1-2 minutes each to ensure that the caking impurities are completely removed and discharged.

Seasonal water quality changes

During the rainy season, the turbidity of raw water increases sharply: extend the air washing and water washing time by 1-2 minutes each to cope with short-term high pollution loads.

In winter, when the water temperature is below 5℃, the viscosity of impurities in the water increases, so the water washing time should be extended by 1-2 minutes to prevent impurities from re-adhering to the filter media surface.

After using new filter media or after replacement : If dust remains on the surface of the new filter media, the water washing should be extended by 2-3 minutes until there are no dust particles in the drain water to avoid excessive turbidity in the effluent during initial filtration.