Intermittent suction is performed in virtually all MBR employing immersed membranes to improve net permeate flow. It consists of repeated cycles of suction and relaxation. During relaxation time, some of the membrane foulants formed during suction time can be removed by air scouring. Although some productivity loss is inevitable during the pause time, net permeate volume improves in vast majority of the situations. There are no predominant rules in cycle time, but the relaxation time (or pause time) typically takes 10-20% of the total cycle time depending on membrane fouling propensities in the specific location, e.g. any combination of 8-11 minutes on and 1-2 minutes off. Most full scale plants have a capability of modifying the cycle time depending on the membrane fouling rate and wastewater flow rate.
Periodic backflushing, which is also called backwashing or backpulsing, has been used to control membrane fouling more actively than the simple intermittent suction especially in hollow fiber membrane processes. In this method, a portion of permeate is flowed backward through membrane lumen for short period of time, e.g. 5-30 seconds every 10-15 minutes. This method can be effective for improving net flux in many situations, but the complexities of the system and the durability issue of the valves have been pointed out as drawbacks like inintermittent aeration. In addition, the advantage of periodic backflushing over intermittent aeration has been found not impressive. As a result, periodic backpulsing is performed less commonly than a decade ago in hollow fiber membrane processes.
Backflushing with chemical can be also performed periodically, but not as frequent as intermittent suction or periodic backwashing, e.g. once a few hours to once a day or two. This procedure is also called maintenance cleaning and typically employing 100-1000 mg/L bleach solution. The purpose of this procedure is known to be cleaning the membrane surface by oxidizing some of the deposit, but it appears more effective in sanitizing and reducing organic deposits in the hollow fiber lumen. Without periodic chemical backflushing, hollow fibers are vulnerable to partial lumen blockage that can cause effective membrane surface area loss and unbalanced filtration among fibers. Air bubbles attached in hydrophobic domain of hollow fiber lumen can cause similar problems.
© Seong Hoon Yoon