The MBR Book: Principles and Applications of Membrane

68 The MBR Book
density in overall MBR performance and the maintenance of MLSS in particular
(Vigneswaran et al., 2004).
The effects of other membrane characteristics, including HF orientation, size and
flexibility, have been reviewed (Cui et al., 2003). For HF membranes used for yeast filtration,
higher critical fluxes were measured for membranes of smaller diameter
(0.65 mm) and greater length (80 cm) (Wicaksana et al., 2006), though contradictory
results showing slightly higher permeabilities for shorter membranes (0.3 cf.
1.0 m) have also been reported (Kim and DiGiano, 2004). A significant impact of
membrane length is the pressure drop due to lumen-side permeate flow. Ideally, the
resistance to permeate flow from the membrane to the outlet of the element should
be small compared with that offered by the membrane itself. If this is not the case,
then the hydraulic losses across the permeate side of the membrane element may be
sufficient to produce a significant flux distribution across the membrane surface.
Significant pressure losses (up to 530 mbar) have been measured for fibres longer
than 15 cm; below this critical length, pressure loss was reported as being at less
than 110 mbar (Kim et al., 2004). Further discussion of fouling distribution in HFs
can be found elsewhere (Chang and Fane, 2002; Chang et al., 2002b; Lipnizki and
Field, 2001; Ognier et al., 2004; Zheng et al., 2003; Zhongwei et al., 2003).
Anaerobic MBRs Research into membrane fouling of anaerobic membrane bioreactors
(anMBRs) has been limited. Whilst fouling mechanisms may be similar to aerobic
MBRs, the nature of the foulants can be expected to be different and, as with the
aerobic systems, to change with feedwater characteristics, membrane surface and
membrane module properties and process operating conditions. A recent review by
Bérubé et al. (2006) has been conducted, and aspects of this review are summarised
below and in Table 2.7.
Cake layers on membranes in anMBRs have been reported to contain both organic
matter and precipitated struvite (Choo and Lee, 1996a), though fouling of organic
membranes appears to be governed by biological/organic interactions with the
membrane rather than by struvite formation. Choo et al. (2000) observed no difference
in fouling rate when ammonia, a component of struvite, was removed from the mixed
liquor prior to filtration using an organic membrane. It also appears that internal
Table 2.7 Anaerobic MBR studies: optimal membrane pore size
Mixed Mode Mean Reactor Material Area Pore Optimal Reference
liqour 1 / particle volume type (m 2 ) size
feed 2 dia. (L) (µm)
(�m) Tested
Anaerobic Batch 3.28– 0.18 PVDF 0.09 0.015–1 0.1 Choo and Lee
digestion a,1 23.3 (1996b)
Sludge Continuous 13 10 ZrO 2–TiO 2 N/a 0.05– 0.14 Elmaleh and
digester b,1 , sMBR 0.2 Abdelmoumni
Synthetic c,2 (1998)
Synthetic 2 Continuous N/a N/a N/a N/a 0.45 0.6 Chung et al.
iMBR. & 0.6 (1998)
aBroth was fractionated into constituent parts (dissolved, fine colloids, cell suspension).
bSupernatant. cAcetic acid/nutrients in tap water.