In an MBR system with multiple aeration and membrane tanks, liquid flow distribution among tanks must be rigorous enough to guarantee successful operation. If mixed liquor distribution is not performed properly, MLSS can be different tank by tank, which can cause excessive MLSS accumulation in some membrane tanks. If the membranes in those tanks cannot produce design permeate flow in spite of the rising TMP, the loss of flux must be compensated by other membrane tanks, which eventually causes accelerated membrane fouling across all membrane tanks.
Fig. 1 shows one hypothetical example, where mixed liquor from the three aeration tanks are distributed to four membrane tanks. After extracting equal amount of permeate from the four membrane tanks, concentrated mixed liquor merge again in the concentrate header before recycled back to the front end of the aeration tanks. In this system, the inlet and the outlet of each membrane tank are open to the headers to allow free mixed liquor flow driven by the water level difference between aeration tanks and membrane tanks.
In this system, for the flow from aeration tank #1, it is a shortcut for mixed liquor to pass through membrane tank #1 rather than membrane tank #2. Likewise, for the flow from aeration tank #2, flowing through membrane tank #2 is preferred to other route to go through membrane tank #3. For the flow from aeration tank #3, same phenomenon occurs. Overall, mixed liquor must travel the longest distance when it goes through membrane tank #4.
As a result, membrane tank #4 would have least amount of mixed liquor circulation while losing same amount of permeate as other membrane tanks assuming fluxes are all same in the four membrane tanks. Due to the least amount of mixed liquor circulation and the same amount of permeate loss, membrane tank #4 would have the highest MLSS. Depending on the extent of the imbalance, this can be a critical operational issue.
This imbalanced flow distribution problem occurs more often than not in practical situations especially in retrofit plants, where existing tanks must be reused to save capital costs. Appropriate care must be taken to avoid unintended imbalance in mixed liquor circulation.
Fig. 1. A hypothetical example of imbalanced mixed liquor circulation.
© Seong Hoon Yoon