Flux loss by membrane fouling is the fundamental issue in MBR process. Since MBR relies on the absolute filtration by membrane, loss of flux can directly lead to the necessity of bypassing a portion of untreated wastewater to river, lake, or ocean.
It has been considered that microbial flocs are not the major cause of membrane fouling although they take a vast majority of MLSS. Instead, small particles and macromolecules, which are classified as either soluble microbial products (SMP) or extra-cellular polymeric substances (EPS), play a major role in membrane fouling. This notion is supported theoretically because small particles have greater tendency to deposit on membrane surface due to weaker back-transport mechanisms as discussed here.
In addition, it has been observed by many researchers independently that SMP and EPS concentrations are not directly correlated with membrane fouling rate (Drews, 2010). This suggests that the property of SMP and EPS is as important as their concentration. Further studies are required to elucidate the effect of SMP and EPS to the molecular level.
In the meantime, MLSS is generally not a good indicator of membrane fouling potential in a typical range, e.g. 6-12 g/L. Unless high MLSS causes poor mixing and slow oxygen dissolution, it does not necessarily cause negative impact on membrane performance. It is believed that biological flocs, which take a vast majority of MLSS, are easily scoured by air bubbles. In some cases, membrane fouling even decreases as MLSS increases perhaps due to the increasing solids retention time (SRT) that makes mixed liquor more filterable.
In this chapter, the factors affecting membrane fouling will be discussed along with fouling mitigation methods. In addition, various mixed liquor characterization methods that can be used to estimate the membrane fouling potential will be discussed.
© Seong Hoon Yoon