In early days of MBR technology, MLSS was highlighted as a major factor affecting membrane fouling (Yamamoto, 1989). But, many experimental evidences were revealed that biopolymers were better correlated with membrane fouling rather than MLSS since the middle of 1990’s (Chang, 1998; Chang, 1999). Biopolymers are either attached on microorganisms, i.e. extracellular polymeric substances (EPS), or free in mixed liquor, i.e. soluble microbial products (SMP). The EPS and the SMP are again divided to polysaccharides and proteins in each category. Though there are no complete consistencies among reported studies, polysaccharides are known to play more significant role in membrane fouling than proteins in most cases. One example is shown in a literature (Yigit, 2008)
It has been also reported that TOC and COD of supernatant/filtrate also have a reasonable correlation with membrane fouling rate in a given site. It is because TOC/COD of supernatant tends to be proportional to SMP. Some researchers also use colloidal TOC of mixed liquor as an indicator of membrane fouling potential, where colloidal TOC is defined as a difference between the TOC of membrane permeate and the TOC of filter paper filtrate. Transparent exopolymer particles (TEP) are also used as an indicator of biofouling potential (De La Torre, 2008). Although various methods were introduced, all of these are the measure of cell debris, soluble polysaccharides and proteins, any other colloidal materials, macromolecules, etc. in mixed liquor.
One remarkable aspect of EPS and SMP is that these do not represent specific molecules with specific structure or configuration. Instead EPS and SMP are operationally defined. Any molecules, colloids, or particles can be detected as EPS or SMP as long as they are qualified by the method used by reacting with the reagents used. Therefore, depending on the analytical methods used, the measured SMP and EPS can be quite different for a same mixed liquor sample. Unless the methods used are identical, comparing values from two different studies is not valid.
The positive correlations between membrane fouling rate and one of the above parameters had been in fact quite convincing. As shown in Fig. 1, the time required for TMP to reach 100 mmHg became shorter as total SMP level increases. Here, mixed liquor was filtered by 0.45 micron membrane filter and the filtrate TOC was regarded as total SMP (Cha, 2003). Fig. 2 also shows a similar trend, but, in this study, colloidal TOC of the filtrate was used as an indicator of biopolymer concentration.
Fig. 1. Relation between total SMP and the time requires for TMP to reach 100 mmHg (Cha, 2003)
Despite the wide acceptance of the direct correlation between SMP/EPS concentration and membrane fouling, there are many experimental evidences that tell otherwise (Drews, 2008; Drews, 2010). The SMP and membrane fouling rate data collected from three different studies are plotted in Fig. 3. Though sample preparation, analytical methods, and even the spectrophotometer used were identical in all studies, no apparent correlations were observed unlike to popular belief that SMP/EPS are proportionally correlated with membrane fouling rate.
Some explanations for the discrepancies are as follow.
- The experimental methods used to detect SMP and EPS do not necessarily detect only the colloids/macromolecules defined as SMP and EPS. They also detect terrestrial humic substances, non-biological polymeric substances, etc. (Judd, 2008). A fraction of these extra substances may or may not contribute to membrane fouling.
- A fraction of SMP and EPS detected may not strongly contribute to membrane fouling. If this fraction increases preferentially, higher SMP and EPS levels would not necessarily cause faster membrane fouling. There have been attempts to elucidate what are the actual molecules that foul membrane, but it is still not clear about the identity of the molecules and when those molecules are predominantly secreted by microorganisms.
- Some colloids and macromolecules not detected by the methods used to quantify SMP and EPS may contribute membrane fouling. If the concentration of these substances increase, membrane fouling rate can increase without EPS and SMP increases.
The above arguments can be visualized as shown in Fig. 4.
Fig. 3. Fouling rate against SMP concentration from three different studies. Sample preparation, analytical methods, and spectrometer were identical in all three studies (Modified from Drews, 2008).
Fig. 4. Conceptual diagram of relation between detectable biopolymers and actual membrane foulant.
© Seong Hoon Yoon