How To Determine The Air Rinsing And Water Rinsing Times For Multi-media Filters?

There is no fixed value for the air washing and water washing time of multi-media filters. It needs to be dynamically adjusted according to the type of filter media, the degree of pollution of raw water, and the washing effect . The core principle is "determine the time based on the effect" rather than strictly following a fixed duration.

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

The purpose of air washing is to agitate and loosen the filter media with air bubbles, breaking up attached impurities.
The purpose of air washing is to agitate and break up adhering impurities in the loosened filter media using air bubbles. The time is usually 3-8 minutes , but the specific time needs to be determined based on the following factors:

Classified by filter media type
Conventional filter media (quartz sand + anthracite): 3-5 minutes. This type of filter media has high hardness and impurities are easily removed. Excessive air washing will increase energy consumption and provide no additional benefits.
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
High turbidity water (such as river water during the rainy season, turbidity > 50 NTU): 5-7 minutes to ensure that viscous colloidal impurities do not remain in the gaps between 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.

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

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 determination method is as follows:

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

Function: It works in conjunction with air washing to remove impurities, targeting 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), no adjustment is needed, just continue flushing.
After 2-5 minutes: Observe the turbidity of the drainage. If it gradually becomes clear (turbidity ≤ 10 NTU), you can stop early. If it is still turbid after 5 minutes (turbidity > 20 NTU), you need to extend it for 1-2 minutes and check whether the backwash intensity 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, preventing wastewater from entering subsequent systems.

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 (such as quartz sand + anthracite + magnetite) require 3-4 minutes to ensure that impurities in the gaps between filter media of different densities are thoroughly flushed out.

For 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.

Seasonal water quality changes :

Turbidity rises sharply during the rainy season: extend air washing and water washing by 1-2 minutes each to cope with high pollution loads.

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

After using new filter media or after replacement : If there is residual dust in the new filter media, wash with water for an extended period of 2-3 minutes until there are no dust particles in the drain water to avoid exceeding the initial filtered water standard.