Study On The Membrane Fouling Control Of Self-forming Dynamic Membrane Bioreactor

In recent years, an innovative and promising wastewater treatment technology, termed self-forming dynamic membrane bioreactor (SFDMBR) system, has attracted increasing attention worldwide. The idea is to utilize the membrane, sludge layer essentially, self-formed on certain coarse materials (e.g. non-woven fabric, nylon mesh, industrial filter-cloth) with pore size ranging from 10-100μm for effective solid-liquid separation. It has been widely reported that self-forming membrane (SFM) can be quickly shaped up and function similarly as commercial microfiltration (MF) membranes, evidenced by the comparable effluent quality free of suspended solids.The costs of SFMBR systems are significantly lower than MBR systems due partly to the replacement of MF membranes by cheap coarse materials, and partly to the less energy consumptions of SFM operation. This makes SFDMBR systems highly potential alternatives of MBR systems especially in developing countries where MBR systems are usually unaffordable. However, as with MBR systems, membrane fouling indicated by the undesirable increase of filtration resistance remains the most serious problem greatly impairing the performance of SFDMBR systems.The filter material is the key factor of dynamic membrane fouling. In this study, we chose three-dimensional filter-cloth and nonwoven material to build dynamic membrane bioreactors (DMBR) for wastewater treatment. Three-dimensional filter-cloth have three-dimensional structure, hydrophobic surface and little pore size, which made foulants difficult to adhere to filter-cloth surface, and decreased the probability of membrane fouling. In a word, three-dimensional filter-cloth is a better material for DMBR, resisting membrane fouling efficiently.Moreover, for better controlling membrane fouling and identification of appropriate operating conditions, the concept of critical flux was proposed. It was found that fouling nearly did not occur under sub-critical flux for MBR. However, in sharp contrast to the numerous investigations in MBR systems, critical flux has not been seriously studied in SFMBR systems. This is partly because of the relative novelty of SFMBR systems, and partly because of the lack of appropriate measurement methods. The critical flux in MBR systems is commonly obtained from short-term flux-transmembrane pressure (TMP) profiles by either TMP or flux stepping methods. These methods, however, cannot be directly applied in SFMBR systems because of the distinctive compressible nature of SFM. Unlike commercial MF membranes having rigid configuration, SFM is indeed a sludge layer with relatively loose structure and thus expected to be considerably compressed as TMP or flux rapidly increases during short-time critical flux determination tests. In other words, the properties of SFM would significantly change during the course of measurement when applying traditional TMP/flux-step methods, resulting in inaccurate critical flux values.In this study, a new method (i.e. intermittent TMP-step method) was developed for more accurate determination of critical flux in SFMBR systems. Relaxation phases are incorporated in the new method to minimize the variation of SFM properties mainly resulted from the continuous compression during the stepwise increase of TMP. A series of comparative tests were carried out in the specially designed lab-scale SFMBR system for more systematic and rigorous evaluations of the intermittent TMP-step method. The results reported here may contribute to a better understanding of SFM properties, and consequently the establishment of the proper method for critical flux determination in SFMBR systems.