The backwashing of active carbon filters shall be carried out according to the processof "preparation air flushing (optional) - flushing with water - rinsing - restoringfiltration". The main purpose of the washing is to eliminate impurities from thesurface and pores of activated carbon, to regain the adsorption capacity and preventfilter media loss or equipment damage. Here are the details:
I. Preparations for backflushing
Switching filter operation state: Close the filter inlet and outlet valves and open thebypass valve (if the system provides it) to ensure that raw water bypasses the filterand to prevent sewage from entering the outlet during backwashing.
Check equipment status: Ensure that the backwash pump and air compressor (if thecombined air and water flushing is adopted) are in standby, the backwash inlet valve,drain valve and exhaust valve are intact and can be opened and closed normally, andthe readings of the differential pressure transmitter and level gauge are accurate.
Open the vent valve at the top of the tank gradually to release the residual pressurein the tank (until the pressure gauge indicates OMPa) to avoid the water distributiondevice being damaged by pressure shock during backwashing.
Il. Air-flushing stage (optional, suitable for activated carbon with stickyimpurities/caking)
Turn on the air compressor, open the air flush valve and air flush the tank withcompressed air. Adjust the air flushing intensity to 18-25L/(m2-s) for 3-5 minutes.
Check the fluidisation state : Check the activated carbon layer from the tank sightglass to ensure that the filter layer expands evenly (expansion rate 10%~20%). Theclumped activated carbon is broken up and the oil, colloids and other impurities onthe surface are removed by means of airflow vibration.
Stop air flushing Close the air flushing valve and switch off the air compressor. Wait 1to 2 minutes for the impurities to fall to the bottom of the filter layer.
IIl. Stage of flushing with water (basic actions, divided into flushing and main flushing)
Backwash pump start-up: Open backwash inlet valve and drain valve, start backwashpump, low intensity flushing (8~10L/(m2 s)) for 2~3 minutes, gradually flush up thefilter layer, and do not have instantaneous high-intensity water flow impact to causeactivated carbon loss;
Increase flushing intensity: Adjust frequency of pump or open valve to increase waterflushing intensity to design value (10-15L/(m2 s) for ordinary granular activatedcarbon, 15~20L/(m2 s) for coconut shell activated carbon), keep filter bed expansionrate at 20%~40%, and continue for 8~12 minutes.
Watch the backwash drain outlet to check the quality of the backwash water. First ofall the drain water is brown and turbid. As the flushing progresses, it becomesgradually clearer. The rinsing phase begins when the rinse water is almost as clear asclean water.
IV. Rinse Step
Reduce rinsing intensity: Reduce the water rinsing intensity to 5~8L/(m2 . s), and carryout for 2~3 minutes, so that the activated carbon particles settle down slowly, theflatness of the filter bed is restored, and filter bed layering disorder is avoided.
To shut down the backwash system: 1. Turn off the backwash pump. 2. Turn off thebackwash inlet valve and the drain valve sequentially. 3. Open the vent valve torelease any residual air in the tank until the tank is full of water (close the vent valveafter water is released).
V. Status of Re-activation of Filter
Water filling and air venting: Open the water inlet valve slowly to fill the tank withwater, and at the same time open the top air vent valve to vent the air. When waterflows out of the air vent valve continuously without air bubbles, close the air ventvalve.
Open the outlet valve slowly and adjust the inlet valve opening to slowly reduce thetank pressure to normal working pressure (0.2~0.6MPa) to avoid the activated carbonlayer being disturbed by sudden pressure rise;
Sampling and testing of residual chlorine and turbidity in effluent. After confirmingthat the indicators meet the standards (residual chlorine < 0.1 mg/L, turbidity < 3NTU), close the bypass valve and return to normal filtration operation.
Key Considerations
During backwashing the filter bed expansion rate should not exceed 40% of the filterbed height to avoid excessive intensity and loss of activated carbon (if there are a lotof carbon particles at the outlet of the drain, reduce the intensity of backwashing).
In the case of only water rinsing (no air rinsing) which is the typical case, the air rinsingstep can be omitted and the process can start with a low intensity water rinse.
If the effluent is still turbid or the pressure difference is not reduced afterbackwashing, the air flushing + water flushing process needs to be repeated or theactivated carbon needs to be checked for the adsorption saturation (the filter medianeeds to be replaced).
In the case of low temperature (<5°C), backwashing intensity should be increased by10%~20% and the rinsing time should be appropriately extended, so as to avoid theincomplete rinsing caused by the increase of water viscosity.
