Impact of chemical cleaning and air-sparging on the critical and sustainable flux in a flat sheet membrane bioreactor for municipal wastewater treatment

The paper discusses the experimental optimisation of both chemical and mechanical cleaning procedures for a flat-sheet submerged membrane bioreactor fed with municipal wastewater. Fouling was evaluated by means of the critical flux concept, which was experimentally measured by short-term flux-stepping tests. By keeping constant most important parameters of the biological process (MLSS, sludge age), two different chemical cleaning protocols ( 2,000mgL(-1) NaOCl and 200mgL(-1) NaOCl) were applied with different frequency and, after approximately 9 months of operation, the criticality threshold was determined under different values of SAD(m) ( specific aeration demand per unit of membrane surface area). The weaker and more frequent chemical cleaning regime ( 200mgL(-1), monthly) proved much more effective than the stronger and less frequent strategy ( 2,000mgL(-1), once every three months). The improvement of performances was quantified by two TMP-based parameters, the fouling rate and the DTMP ( difference between TMP values during the increasing and decreasing phase of hysteresis). The best performing configuration was then checked over a longer period by running four long-term trials showing an exponential trend of the sub-critical fouling rate with the imposed flux.