Transparent exopolymer particles (TEP) formation from alginate blocks and their effects on membrane fouling

Abstract

Transparent exopolymer particles (TEP) have drawn ample attention since they were introduced into the field of membrane fouling. TEP have been described as a class of transparent particulate acidic polysaccharides that can be stained by alcian blue. So far, it has been reported that TEP are involved in the fouling development in various membrane systems including membrane bioreactor (MBR), microfiltration (MF), ultrafiltration (UF) and reverse osmosis (RO). The extensive involvement of TEP in membrane fouling is probably due to following four reasons: (i) TEP are typically stickier than other particles in water, leading to their easy accumulation and difficult removal in membrane systems; (ii) TEP are deformable, thus they could pass through membrane with a pore size smaller than their dimension; (iii) TEP can flex, fragment and disperse through membrane pores of MF and UF and then reassemble in the filtrate to form large size network before reaching the RO membrane modules and (iv) Highly colonized by microbes, TEP could transport bacteria from water phase to membrane surface by adhesion, accelerating the formation of biofilm on the membrane. It should be realized that the formation of TEP mainly depends on the type of precursor materials and chemistry conditions of bulk solution. Therefore, this study was aimed to investigate the TEP formation from alginate blocks under different chemistry conditions and the role of TEP in membrane fouling development. In the first phase of study, alginate blocks and their effect on membrane fouling was investigated. Alginate as a common and widespread polysaccharide has been reported to generate TEP-like particles that can be stained by alcian blue. Alginate is composed of two different monomers, namely M ((1→4) linked β-D-mannopyranuronic acid) and G ((1→4) linked α-L-gulopyranuronic acid) which are randomly arranged into MG-, MM- and GG-blocks. Results showed the severest fouling in the filtration of MG-block, and the least flux decline in the filtration of MM-block. The initial pore blocking was found to be responsible for the fouling observed in MG-block filtration, while the cake layer formed on membrane surface during the MM-block filtration could serve as a pre-filter that prevented membrane from further pore blocking. TEP were found to form through aggregation or cross-link of alginate blocks. It was observed that more TEP were produced from MM-blocks than from MG-blocks in solutions. As TEP were bigger than original alginate blocks, they could facilitate the formation of cake layer on membrane surface, which explained why cake resistance was dominant in the filtration of MM-blocks as compared to MG-blocks. The analysis by the extended Derjaguin-Landau-Verwey-Overbeek (XDLVO) theory further revealed that MM-blocks had lowest cohesive interaction energy among all three alginate blocks, which favoured aggregation of MM-blocks, and ultimately leading to the formation of more TEP. This study provided insights into the TEP formation from different alginate blocks and revealed the…