Effects of various backwash scenarios on membrane fouling in a membrane bioreactor

The main objective of this work was to determine the impact of various backwash scenarios on fouling in a pilot-scale submerged membrane bioreactor (MBR) aerobically treating domestic wastewater. A total of seven different backwash scenarios were tested. The degree of membrane fouling was determined employing the flux-step method and using the resistance-in-series model for each backwash scenario. Fouling rates and total membrane resistances Generally decreased with more frequent backwashing; decreasing of filtration duration improved fouling control. A higher rate of fouling was observed above critical flux for all scenarios. As the backwashing scenarios approached "no-backwash-continuous filtration", permeabilities deviated more from linearity indicating a higher degree of fouling. At a flux of 20 L/m(2)-h, fouled membrane resistance was reduced by 1.63-fold with 9 min 45 s of filtration followed by 15 s of backwash compared to continuous filtration. The negative impacts of filtration duration on total membrane resistances were more pronounced than the positive impacts of backwash duration, i.e., as the ratio of filtration/backwash duration increased resistances also increased linearly (R-2 : 0.96). The strong correlation found among this ratio and the total membrane resistance suggests that the use of this ratio may be an effective tool in the optimization of fouling control in submerged MBRs employing routine backwashing. Scenarios 4 (9 min 55 s of filtration followed by 5 s of backwash) and 5 (9 min 45 s of filtration followed by 15 s of backwash) were found to be optimum based on fouling control and net daily volume of permeate production. The reversible fouling caused mainly by pore blocking and cake layer was effectively removed by these filtration/backwash scenarios as long as the operating flux was less than the critical flux.