When operating MBR for a long time, there exists an irreversible and polluted solid layer. Backwash and online cleaning methods may not work, and the TMP did not reduce. It is necessary to do offline chemical cleaning. Its flux can recover by offline chemical cleaning.
Step 1: Remove the suction tube and the aeration tube from the MBR module. Then take the MBR module out of the tank.
Step 2: Use a nozzle to clean and remove the activated sludge attached to the surface of the fibers.
Step 3: Take the MBR elements out of the module and immerse the elements in a prescribed chemical solution tank. Stay for 6-12 hours.
Step 4: Take out the MBR element from the immersion cleaning tank and wash it thoroughly with water to remove the chemical solution.
Step 5: Install the MBR on the MBR module after cleaning.
Step 6: Connect the aeration pipe and put the module in the tank. Stop the filtration, just let the aeration run for more than 30 minutes, and then start the filtration.
Step 7: Before discharging the chemical solution, reduce the sodium hypochlorite with sodium thiosulfate (Na2S2O3 NH2O), and then discharge.
Step 8: If there is a large amount of inorganic scale on the surface of the MBR after sodium hypochlorite washing, acid washing is also needed.
The chemicals used in offline chemical cleaning are shown in the below chart.
| Main pollutant | Detergent | Use Conditions | Cleaning time |
| Metal oxide, calcium-containing scale | 0.5% Citric acid / 0.2% Hydrochloric acid | pH=2 | 4～8h |
| Fatty acids, proteins, polysaccharides | Alkali (0.5mol/L NaOH) and 200 mg/L oxidizing agent | pH=10 | 4～8h |
| Bacteria, biomacromolecule | Alkali (0.1mol/L-0.5mol/L NaOH) and 200mg/L oxidizing agent / Enzyme preparation, surfactant (0.1%-2%) / 1000-2000mg/L NaClO, 1%-3% Alkali | pH=10 | 4～8h |
