On population balance modelling of membrane bio-reactor operation with periodic back-washing, A.I.Ch.E. Journal.
Year : 2011
Author : Kostoglou M. and Karabelas A.J.
The membrane bioreactor (MBR) is a system that combines biological treatment with membrane ﬁltration into a single process. A basic type of this bioreactor consists of a tank, where the activated sludge biological process takes place, with a wastewater feed and clean water efﬂuent stream, which is obtained through a membrane module immersed in the tank. The efﬂuent water is drawn through the membrane by the application of mild suction. In addition, air in the form of bubbles is fed in the liquid to provide oxygen (electron acceptor) for the bioreactions and to facilitate the circulation of the mixed liquor. Major advantages of the membrane bioreactor system over the conventional activated sludge process include the independent control of residence times of sludge particles and pure liquid. However, during membrane bioreactor operation, solids tend to be accumulated on the membrane causing clean water ﬂux reduction. For this reason, the common operating mode involves a periodic back-washing, aiming at breaking the deposited layer (into ‘‘particles’’), to avoid excessive ﬂux reduction, thus, effectively controlling the membrane bioreactor operation. In parallel, the inherent process limitation, related to the diffusive transport of the reactants into the sludge particles, is expected to be affected by the particle-size distribution, which is considered an important parameter inﬂuencing the bioreaction efﬁciency. In this connection, air bubbling is beneﬁcial as it tends to increase the turbulence level in the tank, in addition to the aforementioned beneﬁts. However, despite the back-washing, an irreversible bioﬁlm tends to slowly buildup on the membranes leading to a long-term reduction of the bioreactor efﬁciency. Therefore, considerable effort has been devoted in the literature to the investigation and modeling of the bioﬁlm development.