Fouling in a MBR system with rotating membrane discs: Influence of concentration and shear

Membrane bioreactors (MBR) are an attractive alternative solution for municipal and industrial wastewater treatment. The MBR, which is a combination of a bioreactor for sludge degradation and a membrane for separation, has the advantages of a low footprint, ability to handle high sludge concentrations and a clear effluent with no bacteria present in the permeate [1]. However, the process performance is limited by membrane fouling, which results in a lower productivity and higher energy demand and hence places demands for limitation of fouling and/or cleaning of the membranes. One way to do this is to enhance the shear on the membrane surface by either air scouring or by generation of a crossflow [1], e.g. by membrane rotation. However, this is associated with high energy consumption, and the gain in permeability should compensate for the additional energy requirements in order to be competitive to conventionally activated sludge systems. Therefore, it is essential to understand the impact of enhanced shear on filtration performance.
The influence of shear and sludge concentration on the limiting flux is investigated using the limiting flux as a measure of the degree of fouling. The utilized MBR system uses rotating ceramic membrane discs for creation of shear, which can be changed by controlling the membrane rotation speed of the membrane. Furthermore, the influence of shear on fouling is studied at different radii from the center of rotation, by dividing membranes into different concentric rings.
An empirical model is proposed to describe limiting flux as a function of shear stress and concentration. The shear stress is calculated from the local shear rate on the membrane disc, expressed as a function of rotation speed and radial distance [2]. The activated sludge is considered to have non-Newtonian shear-thinning properties in the calculation of shear stresses from the shear rates [3].
The modeled limiting flux fits the experimental limiting flux well at varying sludge concentration, rotation speed and radial distance on the disc. The limiting flux increases with rotation speed and radial distance and decreases with increasing sludge concentration. From the proposed model, the filtration performance can be estimated as a function of shear (rotation speed) and concentration in order to improve filtration performance and productivity.

References
1. A. Drews, J. Membr. Sci., 363, 1–28 (2010).
2. R. Bouzerar, L. Ding, M. Jaffrin, J. Membr. Sci., 170, 127-141 (2000).
3. S. Rosenberger, K. Kubin, M. Kraume, Eng. Life Sci., 2, 269-275 (2002).