Membrane fouling control through the change of the depth of a membrane module in a submerged membrane bioreactor for advanced wastewater treatment

Membrane fouling is a principal limitation of wide application of membrane bioreactors (MBR) to wastewater treatment. In this study, to control the membrane fouling the position of a membrane module in a submerged MBR was elevated from bottom to top of the reactor. This could divide the reactor into two different zones: upper and lower. Air was not supplied at the lower zone whereas aeration was given to the upper zone where the membrane filtration was carried out. Biosolids concentration was reduced in the upper zone because the mixed liquor could be settled down to the lower zone. Therefore, membrane fouling could be lessened in the upper zone due to the reduced biosolids concentration. In this study, to verify if this newly designed MBR configuration could mitigate membrane fouling, the effect of the vertical position of the membrane module in a lab-scale MBR on membrane fouling was investigated. Furthermore, a pilot plant (50 m3/d) of the membrane coupled biological nutrients removal (BNR) process was designed based on the above configuration and was run for 5 months. In the lab test, the higher the membrane was located in the bioreactor, the less membrane fouling was observed. With the pilot plant operation, MLSS concentration in the upper part was lessened to 16~33% than that in the lower part of the tank where air was not supplied. It indicates that two different zones were successfully formed. The interval of periodical chemical cleanings with NaOCl was extended from 2 to 4 months, indicating that the membrane fouling was mitigated. DO concentration at the upper part was 5.3 mg/L, whereas DO at the lower part was 0.4 mg/L. Therefore, this may result in better denitrrification efficiency in the anoxic tank because the recycled sludge to the anoxic tank has low DO concentration. Nitrate concentration at the lower part of the MBR was 2.8 mg/L, whereas that at the upper part was 5.8 mg/L, suggesting that partial denitrification of nitrate was carried out in the lower part of the MBR. Moreover, overall TN removal was 75%, which was higher than that of the conventional BNR processes, indicating that denitrification rate in the anoxic tank increased due to the low DO level of the returned sludge. Consequently, this newly designed MBR could make it possible to control membrane fouling and to get a better TN removal.