The MBR Book: Principles and Applications of Membrane

MLSS
sample
Centrifugation
5 min
5000 g
SMPp
Deionised
water
Filtration
1.2 µm
Mixing
10 min
Heating
10 min � 80ºC
�
Centrifugation
10 min
7000 g
SMP eEPS
SMPc
Fundamentals 79
eEPSp
Figure 2.30 Candidate method for EPS and SMP extractions and measurements
Filtration
1.2 µm
eEPSc
(Fig. 2.30). Regardless of the extraction method used, a distinction can be made
between EPS which derives directly from the active cell wall and that which is not
associated with the cell but is solublised in the mixed liquor. The former is usually
referred to as “EPS” in the literature, although a less ambiguous term would be
“eEPS” (extracted EPS, Fig. 2.29). The latter is normally termed SMP and invariably
refers to clarified biomass, although for some more recalcitrant feedwaters, clarified
biomass will inevitably contain feedwater constituents which remain untransformed
by the biotreatment process. SMP concerns soluble cellular components
released during cell lysis, which then diffuse through the cell membrane and are lost
during synthesis or are excreted for some purpose (Laspidou and Rittmann, 2002; Li
et al., 2005a). In MBR systems, they can also be provided from the feed substrate. It
is now widely accepted that the concepts of soluble EPS and SMP are identical (Jang
et al., 2005a; Laspidou and Rittmann, 2002; Rosenberger et al., 2005).
Protein and carbohydrate EPS fractions Typically, the EPS solution is characterised
according to its relative content of protein (EPSp) and carbohydrate (EPSc), measured
by the respective photometric methods of Lowry (Lowry et al., 1951) and
Dubois (Dubois et al., 1956). Reported data are summarised in Table 2.10. While
EPSp generally has hydrophobic tendencies, EPSc is more hydrophilic (Liu and Fang,
2003) and may therefore interact more strongly with the membrane. The EPS solution
can also be characterized in terms of its TOC level (Cho et al., 2005b; Nagaoka
and Nemoto, 2005) and, less frequently, its hydrophobicity by measurement of the
ultraviolet absorbance per unit TOC concentration, the specific UV absorbance (SUVA),
(Ahn et al., 2005). In many reported cases, EPSp (with a maximum concentration of
120 mg/gSS) is greater than EPSc (maximum concentration of 40 mg/gSS) and the
total concentration range reported is surprisingly narrow: 11–120 mg/L for EPSp
and 7–40 mg/L for EPSc. Sludge flocs have also been characterised in terms of protein
and carbohydrate levels, through colorimetric analysis carried out directly on
the washed biomass (Ji and Zhou, 2006), without any correlation evident between
these indicators and MBR fouling propensity. Finally, the measurement of humic
substances, generally overlooked for protein and carbohydrate, have revealed that
they arise in significant concentrations in activated liquors (Liu and Fang, 2003)
and may demand more attention in future research on MBR fouling.