The MBR Book: Principles and Applications of Membrane

Fundamentals 77
in the influent. Abundance of actinomycetes such as Nocardia or Microthrix are
commonly related to foaming in activated sludge plants, and have been identified in a
full-scale MBR plant subject to variable OLRs (Smith, 2006). However, foam in MBR
plants has been observed in the absence of actinomycetes. The degree of foaming is
reported as being related to the protein EPS concentrations (Nakajima and Mishima,
2005). Foaming sludges also appear to yield lower membrane permeabilities (Chang
and Lee, 1998), attributed to the higher hydrophobicity of foaming activated sludge
(Section 2.3.6.4). Foaming thus provides an indication of sludge fouling propensity.
2.3.6.4 Floc characteristics
Floc size Comparison of the aggregate size distribution of ASP and MBR sludges
(Cabassud et al., 2004) revealed a distinct difference in terms of mean particle sizes
(160 and 240 �m, respectively). A bimodal distribution was observed for the MBR
sludge (5–20 and 240 �m), the high concentration of small colloids, particles and
free bacteria being caused by their complete retention by the membrane. In another
study where partial characterisation of the MBR flocs up to 100 �m was carried out,
floc sizes ranging from 10 to 40 �m were reported with a mean size of 25 �m (Bae
and Tak, 2005). Floc size distributions reported for three MBRs operated at different
SRTs were similar, although the mean floc size increased slightly from 5.2 to 6.6 �m
for SRTs increasing from 20 to 60 days (Lee et al., 2003).
Given the large size of the flocculant solids compared to the membrane pore size,
pore plugging by the flocs themselves is not possible. Flocs are also to some extent
impeded, by drag forces and shear-induced diffusion, from depositing on the membrane
surface. They nonetheless contribute to fouling through production of EPS and
also directly affect clogging of the membrane channels. The interaction between EPS
levels and floc size is discussed in Section 2.3.6.5, and the use of ancillary materials to
suppress fouling through floc size and structure discussed in Section 2.3.9.5.
Hydrophobicity and surface charge A number of reports can be found in the literature
providing evidence of membrane fouling by highly hydrophobic flocs. Relative
floc hydrophobicity can be directly measured by bacterial adhesion/partition using
hydrocarbons such as hexane (Jang et al., 2005b), or estimated by contact angle
determination (Yu et al., 2005b). Although the direct effect of floc hydrophobicity
on MBR fouling is difficult to assess, hydrophobicity measurement of sludge and EPS
solutions has been carried out and briefly reported by Jang and co-workers (Jang
et al., 2005a; Jang et al., 2005b). EPS level and the filamentous index (a parameter
related to the relative presence of filamentous bacteria in sludge) directly influence
biomass floc hydrophobicity and zeta potential. Excess growth of filamentous bacteria
has been reported to yield higher EPS levels, lower zeta potentials, more irregular
floc shape and higher hydrophobicity (Meng et al., 2006). Sludge of higher foaming
propensity, attributed to its hydrophobic nature, has been shown to produce a flux
decline 100 times greater than that of non-foaming sludge (Chang and Lee, 1998).
The anionic nature of the functional groups of natural organic materials means that
charge and zeta potential of activated sludge flocs (and EPS) tend to be in the �0.2
to �0.7 meq/g VSS and from �20 to �30 mV regions, respectively (Lee et al., 2003;