The MBR Book: Principles and Applications of Membrane

82 The MBR Book
surface where they provide a possible nutrient source for biofilm formation and
a hydraulic resistance to permeate flow (Rosenberger et al., 2005). SMP materials
appear to be retained at or near the membrane. Biomass fractionation studies conducted
by Lesjean and co-workers (Lesjean et al., 2005) revealed levels of carbohydrates,
proteins and organic colloids to be higher in the SMP than in the permeate, a
finding similar to those previously reported (Brookes et al., 2003a; Evenblij and van
der Graaf, 2004).
Three methods of separating the water phase from the biomass, so as to isolate
the SMP, have been investigated. Simple filtration through filter paper (12 �m) was
shown to be a more effective technique than either centrifugation or sedimentation
(Evenblij and van der Graaf, 2004). It is likely that removal of colloidal material
would demand more selective pre-filtration, e.g. 1.2 �m pore size (Figure 2.30). As
with EPS, the SMP solution can be characterised with respect to its relative protein
and carbohydrate content (Evenblij and van der Graaf, 2004), TOC level (Gao et al.,
2004b) or with SUVA measurement (Shin and Kang, 2003), as well as MW distribution.
HPSEC analysis conducted on SMP solutions has revealed the SMP MW
distribution to be different significantly across a range of full-scale reactors operated
under different conditions, unlike the MW distribution for the eEPS fraction (Brookes
et al., 2003b). However, the SMP solution fingerprint was largely unchanged in
weekly analysis conducted on a single reactor, indicating no significant change in
SMP characteristics for biomass acclimatised to specific operating conditions. When
compared to eEPS MW distribution, the SMP solution featured generally larger
macromolecules.
Comparison between acclimatised sludges obtained from MBR and ASP pilot
plants revealed similar levels of EPSp, EPSc and EPS humic matter (Cabassud et al.,
2004). The membrane did not seem to affect the floc EPS content. However, corresponding
levels of the SMP fractions were significantly higher for the MBR sludge.
Critical flux tests carried out under the same conditions for both MBR and ASP
sludge revealed a higher fouling propensity of the MBR sludge over that of the ASP;
critical flux values were around 10–15 and 32–43 LMH, respectively. Since the
measured levels of EPS were unchanged, it was surmised that the higher fouling
propensity related to the SMP level. During this study, Cabassud and co-workers
observed significant biological activity in the MBR supernatant, indicating the presence
of free bacteria which may have contributed to fouling.
A number of different studies have indicated a direct relationship between the
carbohydrate level in SMP fraction and MBR membrane fouling directly (Lesjean
et al., 2005), or fouling surrogates such as filtration index and CST (Evenblij et al.,
2005a; Grelier et al., 2005; Reid et al., 2004; Tarnacki et al., 2005), critical flux
(Le-Clech et al., 2005b) and permeability (Rosenberger et al., 2005). The hydrophilic
nature of carbohydrate may explain the apparently higher fouling propensity of
SMPc over that of SMPp, given that proteins are more generally hydrophobic than carbohydrates.
Strong interaction between the hydrophilic membrane generally used in
MBRs and hydrophilic organic compounds may be the cause of the initial fouling
observed in MBR systems. However, the nature and fouling propensity of SMPc has
been observed to change during unsteady MBR operation (Drews et al., 2005) and,
in this specific study, it was not possible to correlate SMPc to fouling. Thus far,