I. Preparations before backflushing
Switching operating status : Close the filter inlet valve and outlet valve, and open the bypass valve (if the system is configured) to ensure that raw water bypasses the filter and prevents sewage from entering the outlet during backwashing.
Check equipment status : Confirm that the backwash pump and air compressor (when using combined air and water flushing) are in standby mode, the backwash inlet valve, drain valve, and exhaust valve are intact and can be opened and closed normally, and the differential pressure transmitter and level gauge readings are accurate.
Slowly open the vent valve on the top of the tank to release the remaining pressure inside the tank (until the pressure gauge shows 0MPa) to prevent pressure shock during backwashing from damaging the water distribution device.
II. Air-flushing stage (optional, suitable for activated carbon adhering to sticky impurities/caking scenarios)
Start the air compressor : Open the air flushing valve and introduce compressed air into the tank. Control the air flushing intensity at 18~25L/(m²・s) for 3~5 minutes.
Observe the fluidization state : Observe the activated carbon layer through the tank sight glass to ensure that the filter layer expands evenly (expansion rate 10%~20%). Use airflow vibration to break up the clumped activated carbon and remove oil, colloids and other impurities adhering to the surface.
Stop air flushing : Turn off the air flushing valve and air compressor, and let it stand for 1 to 2 minutes to allow the detached impurities to settle to the bottom of the filter layer.
III. Water Flushing Stage (Core Steps, Divided into Initial Flushing and Main Flushing)
Start the backwash pump : Open the backwash inlet valve and drain valve, start the backwash pump, initially use low intensity flushing (8~10L/(m²・s)) for 2~3 minutes, gradually flush up the filter layer, and avoid instantaneous high-intensity water flow impact that causes activated carbon loss;
Increase flushing intensity : Adjust the pump frequency or valve opening to increase the water flushing intensity to the design value (10~15L/(m²・s) for ordinary granular activated carbon, 15~20L/(m²・s) for coconut shell activated carbon), maintain the filter bed expansion rate at 20%~40%, and continue for 8~12 minutes.
Monitor the quality of the backwash water : Observe the backwash drain outlet. Initially, the drain water is dark brown and turbid. It gradually becomes clearer as the flushing progresses. When the transparency of the drain water is close to that of clear water, the rinsing stage begins.
IV. Rinsing Stage
Reduce the rinsing intensity : Reduce the water rinsing intensity to 5~8L/(m²・s) and continue for 2~3 minutes to allow the activated carbon particles to settle slowly, restore the flatness of the filter bed, and avoid the filter bed layering disorder.
To shut down the backwash system : First, turn off the backwash pump, then turn off the backwash inlet valve and drain valve in sequence, and open the vent valve to release any residual air in the tank until the tank is full of water (close the vent valve after water is released).
V. Restore Filtering Status
Water filling and air venting : Slowly open the water inlet valve to fill the tank with water, and at the same time open the top air vent valve to vent the air. When water comes out of the air vent valve continuously and there are no air bubbles, close the air vent valve.
Gradual pressurization : Slowly open the outlet valve and adjust the inlet valve opening to restore the tank pressure to the normal working pressure (0.2~0.6MPa) to avoid sudden pressure rise that could disturb the activated carbon layer;
Sampling and testing of residual chlorine and turbidity in the effluent; after confirming that the indicators meet the standards (residual chlorine < 0.1 mg/L, turbidity < 3 NTU), close the bypass valve and resume normal filtration operation.
Key Considerations
Throughout the backwashing process, the filter bed expansion rate must be kept below 40% to avoid excessive intensity leading to activated carbon loss (if a large number of carbon particles are found at the drain outlet, immediately reduce the backwashing intensity).
When using a single water rinse (without air rinsing), the air rinsing stage can be skipped, and the process can begin with a low-intensity water rinse.
If the effluent is still turbid or the pressure difference does not decrease after backwashing, the air flushing + water flushing process needs to be repeated, or the activated carbon needs to be checked for adsorption saturation (the filter media needs to be replaced).
In low-temperature environments (<5℃), the backwashing intensity needs to be increased by 10%~20%, and the rinsing time should be appropriately extended to avoid the increase in water viscosity leading to incomplete rinsing.
