The MBR Book: Principles and Applications of Membrane

88 The MBR Book
of the anti-fouling properties of the air scouring, since severe fouling has been
reported when aeration ceases (Jiang et al., 2005; Psoch and Schiewer, 2005b).
2.3.7.2 Solid retention time (SRT)
SRT impacts on fouling propensity through MLSS concentration, which increases
with increasing SRT, and in doing so reduces the F:M ratio (Equation (2.11)) and so
alters the biomass characteristics. Extremely low SRTs of �2 days have been shown
to increase the fouling rate almost 10 times over that measured at 10 days, with the
F:M ratio correspondingly increasing from 0.5 to 2.4 g COD/(g VSS/day) and the MLSS
increasing only slightly from 1.5 to 1.2 g/L ( Jang et al., 2005b). In practice, the F:M
ratio is generally maintained at below 0.2/day.
Operation at long SRTs minimises excess sludge production but the increase in
MLSS level which inevitably takes place presents problems of clogging of membrane
channels, particularly by inert matter such as hair, lint and cellulosic matter
(Le-Clech et al., 2005a), membrane fouling and reduced aeration efficiency, as manifested
in the �-factor (Fig. 2.19). Even after increasing membrane aeration by 67%,
fouling of an HF sMBR has been reported to almost double on increasing the SRT
from 30 to 100 days, producing a corresponding increase in MLSS levels from 7 to
18 g/L and a decrease in F:M ratio from 0.15 to 0.05 kg COD/kg MLSS/day (Han et al.,
2005). At infinite SRT, most of the substrate is consumed to ensure the maintenance
needs and the synthesis of storage products. The very low apparent net biomass generation
observed can also explain the low fouling propensity observed for high SRT
operation (Orantes et al., 2004). In such cases sludge production is close to zero.
Scientific studies indicate that SRT is a key parameter in determining fouling
propensity through MLSS and EPS fraction concentrations. On this basis, an optimum
SRT can be envisaged where foulant concentrations, in particular in the SMP
fraction, are minimised whilst oxygen transfer efficiency remains sufficiently high
and membrane clogging at a controllable level. In practice, SRT tends not to be rigorously
controlled. Moreover, SRT probably has less of an impact on fouling than
feedwater quality and fluctuations therein.
2.3.7.3 Unsteady-state operation
Unsteady-state operation arising from such things as variations in feedwater quality
(and so organic load), feedwater and/or permeate flow rate (and hence hydraulic
load) and aeration rate are all known to impact on MBR membrane fouling propensity,
along with other dynamic effects (Table 2.12). In an experiment carried out
with a large pilot-scale MBR in which the effects of unstable flow and sludge wastage
were assessed (Drews et al., 2005), it was established that the level of carbohydrate
in the supernatant before and after each sludge withdrawal increased. Whilst the
increase following wastage was thought to be due to the sudden stress experienced
by cells due to biomass dilution (which in extreme cases is known to lead to foaming
in full-scale plant), increase before sludge withdrawal was attributed to the high
MLSS concentration and the resulting low DO level in the bioreactor. It was concluded
that unsteady-state operation changed the nature and/or structure (and
fouling propensity) of the carbohydrate rather than the overall EPS formation. These
findings corroborated results previously reported on effects of transient conditions