Seasonal variation in membrane fouling in membrane bioreactors (MBRs) treating municipal wastewater.

We investigated seasonal variation in membrane fouling in membrane bioreactors (MBRs) treating municipal wastewater regarding the difference between physically reversible and irreversible fouling. Two separate MBRs with different solid retention times (SRTs) operated in parallel for about 200 days including high- and low-temperature periods to evaluate the effect of operating conditions on seasonal variation of membrane fouling. Seasonal variations of both types of membrane fouling (i.e., physically reversible and irreversible fouling) were observed for the MBR with short SRT (13 days). However, in the MBR with long SRT (50 days), there were no significant seasonal variations in both types of membrane fouling. In the MBR with short SRT, the trends in the seasonal variation in the development rates of physically reversible and irreversible fouling were different. Physically reversible fouling was more significant in the low-temperature period, while physically irreversible fouling developed more rapidly in the high-temperature period. The development rates of physically reversible fouling can be related to the concentration of dissolved organic matter in the mixed liquor suspension of MBRs; whereas those of physically irreversible fouling could not be explained by the concentration of dissolved organic matter. The characteristics of dissolved organic matter differed depending on the temperature period, and the trends of dissolved organic matter variation in mixed liquor were similar with those of foulants that caused physically irreversible fouling. The results obtained in this study indicated that seasonal variation in physically reversible and irreversible fouling is related to changes in quantity and quality of organic matter, respectively.