The dynamic change of specific cake resistance in membrane bioreactor due to periodical cake relaxation

Abstract

Activated sludge is highly deformable. Within membrane bioreactor, the cake layer formed on the membrane surface can be compressed by the trans-membrane pressure (TMP), resulting in more compressed cake and high specific cake resistance. As the deposited cake mass is rarely measured, it is not clear how the specific cake resistance change during the course of filtration. Many studies related specific cake resistance to TMP by linear or power-law function with little experimental verification. In this study, the temporal change of specific cake resistance (RS) during the course of filtration was experimentally determined. The specific cake resistance varied between 2−5 × 1012 (m−1/kg) and 0.4−2 × 1012 (m−1/kg) for the continuous mode and filtration/pause mode, respectively. In both cases, the RS increased sharply initially, then decreased to their initial values. The specific cake resistance was found to be in a linear relationship with the ratio between TMP and cake mass (P/m). The dynamic change of specific cake resistance due to the periodical relaxation of the cake layer under the filtration/pause mode was also derived. The results indicated that, overall, periodical cake relaxation reduced the specific cake resistance compared to the continuous operation. However, the effective ness of cake relaxation in reducing the RS decreased significantly after the initial stage of filtration of 4 h only.