Periodic backflushing can be performed in hollow fiber membrane processes to remove a portion of the cake layer formed on membrane surface. In practice, backflushing is performed for 10-60 seconds periodically every 10 minutes to a few hours at 1.5 to 2.0 times higher flow rates than the permeate flow rate using the permeate stored in permeate tank. However, the efficacy of backflushing is often not evident in long-term operation. Removing the thin and dense cake layer formed on membrane surface becomes harder over time. Moreover, the growing microorganisms in membrane lumen and permeate storage tank can foul internal membrane surfaces during backflushing. Due to the low long-term efficacies, backflushing with permeate is no longer considered as an essential part of operational procedure in MBR.
Less frequent backflushing with chlorine (or bleach), so called maintenance cleaning, has replaced the conventional backflushing over the last decade. The frequency of maintenance cleaning varies widely depending on membrane fouling propensity, but it is typically performed once or twice a week. Typically 100-600 mg/L of bleach (or chlorine) solution is back flowed through the permeate line at 15-20 LMH for 10-20 minutes while scouring air is off. Total volume of cleaning solution required ranges 3-6 L/m2 each time maintenance cleaning is performed.
Maintenance cleaning is known to aim flux recovery, but it is very effective mean to disinfect membrane lumen and permeate pipe. Without the maintenance cleaning, hollow fibers are vulnerable to lumen blockage that causes not only the loss of effective membrane surface area, but also the unbalanced filtration among fibers that expedite flux loss. The effect of lumen plugging by microorganismsshould be similar to the effect of lumen plugging by air bubbles as discussed here.
Maintenance cleaning can be also performed in flat sheet membrane systems in a similar manner, but net head pressure of cleaning solution must remain less than 0.3-0.6 meter H2O (or 3-6 kPa) to prevent the burst of membrane sheets. Since flat sheet modules typically have bigger lumen spaces in the module with an exception of the modules withintegrated membrane and support frames, microbial plugging of the lumen is much less likely than for hollow fiber modules. Therefore, maintenance cleaning is less compelling for flat sheet membranes than for hollow fiber membranes.
Recovery cleaning is performed to bring the membrane permeability back to the original level. The criteria of permeability limit depends on membrane manufacturer and plant situation, but it can be performed when permeability drops below 0.8-1.0 LMH/kPa. It must be noted that flux recovery becomes increasingly harder as the recovery cleaning is postponed until high TMP, e.g. 30 kPa, is observed. In general, cake layer becomes more compact as TMP increases especially in the bottom layer that contacts with membrane as discussed here. Therefore, recovery cleaning must be performed as soon as either permeability hits the minimum or TMP hits the maximum.
In immersed hollow fiber membrane processes, membrane modules/cassettes are hoisted out from aeration tank tank and cleaned with water jet either in right above the tank or in other designated cleaning tank. Fibrous materials and other large junks must be hand picked before or after the water jetting. Then the modules/cassettes are immersed in the cleaning tank filled with 1,000-2,000 mg NaOCl/L for 1-4 hours or longer. If scales such as CaCO3, Fe(OH)3, CaSO4, etc. are concern, the cleaning tank can be drained after the NaOCl cleaning and filled up with 1-2% oxalic or citric acids solution. These organic acids provides moderately low pH, e.g. 2-3 that is enough to dissolves scales, but safe to membranes. Simultaneously the high buffer capacity of the organic acids provides stable pH that does not change much during the scale-dissolving process. Spent cleaning solution can be sent to the upstream to mix with feed wastewater, but the chlorine shock and/or organic loading shock must be carefully reviewed beforehand to determine the mixing ratio of spent cleaning solution and wastewater.
If membranes are immersed in a separate membrane tank as shownhere, modules/cassettes can be cleaned without being removed. Mixed liquor is transferred to aeration tanks followed by cleaning the tank walls and membranes with water jet. After draining the contaminated water, cleaning solution is filled up. Rest of the procedures are similar to those in above paragraph.
EDTA can be also used to tackle scaling issues, but the environmental impact must be carefully reviewed in conjunction with local government regulations. For persisting organic foulants, cleaning can be performed at high pH (12-13) using NaOH solution for PVDF membranes. Various surfactants can be also tested at 20-35 oC especially for industrial MBR.
For GE membranes (ZW500), a maximum cumulative chlorine exposure is recommended at 500,000 ppm.hr (Fenu, 2012)