The MBR Book: Principles and Applications of Membrane

and immersed hybrid PAC–MBR (Kim and Lee, 2003). Decreased membrane fouling
has also been demonstrated in studies of the effects of dosing MBR supernatant at
up to 1 g/L PAC (Lesage et al., 2005) and dosing activated sludge itself (Li et al.,
2005c), for which an optimum PAC concentration of 1.2 g/L was recorded. In the
latter study, floc size distribution and apparent biomass viscosity were identified as
being the main parameters influenced, resulting in a reduced cake resistance, when
PAC was dosed into the bioreactor. Conversely, no significant improvement in performance
was recorded when a concentration of 5 g/L of PAC was maintained in the
bioreactor without sludge wastage (Ng et al., 2005). It was postulated that, under
these conditions, the PAC was rapidly saturated with organic pollutants and that fouling
suppression by PAC relies on its regular addition brought about by lower SRTs.
Experiments conducted with different system configurations based on immersed HF
membranes allowed direct comparison of hydraulic performances for pre-flocculation
and PAC addition. Under the operating conditions employed, pre-flocculation provided
higher fouling mitigation than that of PAC addition (Guo et al., 2004).
However, the use of both strategies simultaneously provided the greatest permeability
enhancement (Cao et al., 2005; Guo et al., 2004).
A detailed mathematical model has been proposed for predicting performances for
hybrid PAC–MBR systems (Tsai et al., 2004). The model encompasses sub-processes
such as biological reaction in bulk liquid solution, film transfer from bulk liquid phase
to the biofilm, diffusion with biological reaction inside the biofilm, adsorption equilibria
at the biofilm–adsorbent interface and diffusion within the PAC particles. Numerous
other studies in which the use of PAC has been reported for fouling amelioration have
generally been limited in scope and have not addressed the cost implications of reagent
usage and sludge disposal. Tests have been performed using zeolite (Lee et al., 2001b)
and aerobic granular sludge, with an average size around 1 mm (Li et al., 2005b) to
create granular flocs of lower specific resistance. Granular sludge was found to increase
membrane permeability by 50% but also lower the permeability recovery from cleaning
by 12%, which would be likely to lead to unsustainable operation.
A novel “membrane performance enhancer” MPE50, a cationic polymer-based
compound, has recently been developed by the company Nalco for use in MBRs. The
addition of 1 g/L of of the reagent directly to the bioreactor led to the reduction of
SMPc from 41 to 21 mg/L (Yoon et al., 2005). The interaction between the polymer
and the soluble organics in general, and SMPc in particular, was identified as being
the main mechanism responsible for the performance enhancement. In another
example, an MBR operated at an MLSS level as high as 45 g/L yielded a lower fouling
propensity when 2.2 g/L of polymer was dosed into the bioreactor. The product has
been tested at full scale (Section 5.2.1.4).
2.4 Summary
Fundamentals 99
Membrane separation processes applied to MBRs have conventionally been limited to
MF and UF for separation of the permeate product from the bioreactor MLSS. Other
processes, in which the membrane is used to support a biomass and facilitate gas
transfer into the biofilm (Section 2.3.2–2.3.3), have not reached the commercial