The MBR Book: Principles and Applications of Membrane

98 The MBR Book
expensive. As already stated (Section 2.3.7.1), much attention has been focused on
commercial development of efficient and effective aeration systems to reduce the
specific aeration demand, with possibly the most important publications arising in
the patent literature (Côté, 2002; Miyashita et al., 2000) and including cyclic aeration
(Rabie et al., 2003) and jet aeration (Fufang and Jordan 2001, 2002). The use of
uniformly distributed fine air bubbles from 0.5 mm ports has been shown to provide
greater uplift and lower resistance compared to a coarse aerator having 2 mm ports at
similar aeration rates (Sofia et al., 2004). In the same study, a bi-chamber (a riser and
down-comer) in an FS MBR has been shown to play a significant role in inducing
high CFVs. The use of a variable aeration rate to increase the flux during peak loads
has been reported for short-term tests (Howell et al., 2004). However, a recent study
from Choi and co-workers (Choi et al., 2005a) carried out with an sMBR indicated
tangential shear (imparted by liquid crossflow) to have no effect on flux decline when
pseudo-steady-state is reached (i.e. once the fouling layer governs permeability).
2.3.9.5 Chemically or biochemically modifying the mixed liquor
The biomass quality can be controlled biochemically, through adjustment of the
SRT (Section 2.3.6.3) or chemically. In practice, SRT is rarely chosen on the basis of
foulant concentration control, instead a target value is almost invariably based on
membrane module clogging propensity and biomass aeration efficiency. However,
studies have shown that a modicum of fouling control can be attained through the
addition of chemicals.
Coagulant/flocculant Ferric chloride and aluminium sulphate (alum) have both
been studied in relation to membrane fouling amelioration, most extensively for
potable systems but also for MBRs. In MBR-based trials, addition of alum to the reactor
led to a significant decrease in SMPc concentration, along with an improvement
in membrane hydraulic performances (Holbrook et al., 2004). Small biological colloids
(from 0.1 to 2 �m) have been observed to coagulate and formed larger aggregate
when alum was added to MBR activated sludge (Lee et al., 2001b). Although
more costly, dosing with ferric chloride was found to be more effective than alum.
Ferric dosing of MBRs has been used for enhancing the production of iron-oxidizing
bacteria responsible for the degradation of gaseous H 2S (Park et al., 2005a). In this
study, specific ferric precipitates like ferric phosphate and K-jarosite (K-Fe 3(SO 4) 2(OH) 6)
have been observed to foul the membrane. Pretreatment of the effluent by pre-coagulation/sedimentation
has revealed some fouling reduction (Adham et al., 2004), and
pre-clarification is employed at some sewage treatment works. In a recent example,
the ferric dosing was shown to control both irreversible fouling and suspension viscosity
(Itonaga et al., 2004). Precoating of MBR membranes with ferric hydroxide
has also been studied as a means of increasing permeability and improving permeate
quality (Zhang et al., 2004). In this study, additional ferric chloride was added to
remove non-biodegradable organics which accumulated in the bioreactor.
Adsorbent agents Addition of adsorbents into biological treatment systems decreases
the level of organic compounds. Dosing with PAC produces biologically activated
carbon (BAC) which adsorbs and degrades soluble organics and has been shown to
be effective in reducing SMP and EPS levels in a comparative study of a sidestream