Influence of membrane pore size and material on membrane fouling in membrane bioreactors operated under different sludge retention time

Membrane bioreactors (MBRs) have several advantages such as high-quality effluent, small footprint and less sludge production over conventional activated sludge systems, and are therefore considered to be a promising technology for future wastewater treatment. For wider use of MBRs, however, high-energy consumption in the operation of MBRs, mainly attributed to membrane fouling, needs to be addressed. Membrane fouling can be divided into two categories: physically reversible fouling, which can be cancelled by physical cleaning (i.e., back wash, surface cleaning) and physically irreversible fouling, which cannot be cancelled by physical cleaning, and therefore chemical cleaning is required. Physically reversible fouling is considered to develop by accumulation of sludge particles whose particle sizes are larger than the membrane pore size. Physically irreversible fouling is considered to occur by the attachment of colloids and solutes inside the membrane pores [1]. Soluble microbial products (SMP), which is considered as major organic foulants, produced by mixed microbial populations, are of crucial importance for biological wastewater treatment systems because they have significant impacts on effluent quality and membrane fouling. Membrane fouling develops as a result of interaction between SMP and membrane surface, thus the membrane with less interaction to SMP should be developed to solve the fouling problem in MBR. Therefore, mechanisms of interaction between SMP and membrane surface should be investigated. However, the membrane materials always show different fouling propensity due to their different characteristics such as pore size, morphology and hydrophobicity, etc [2]. Especially, depending on the pore size and the type of biomass filtered, reported results in the literature have shown opposite trends [3]. Larger pore size membranes are advantageous in some cases, while smaller pore size membranes are advantageous in other cases. This discrepancy came from difference in experimental conditions. Especially, report that examined the influence of pore size with same material is few. In this study, influence of pore size on fouling was examined by using different pore size membranes with same material (polytetrafluoroethylene: PTFE), and the performance was compared with polyvinylidene fluoride (PVDF) membrane.