The MBR Book: Principles and Applications of Membrane

Table 2.12 Dynamic effects
Determinants Variables
Fundamentals 89
Flow rate Ultimate flux and rate of change
Feedwater quality Ultimate composition and rate of change
MLSS dilution Dilution factor and rate of concentration change
(Partial) aeration loss Percentage and period of reduction
Backflush/cleaning loss Period of loss
Hydraulic shock Rate and level of flow increase
Saline intrusion Ultimate concentration factor and rate of
concentration change
in feeding patterns: the addition of a pulse of acetate in the feedwater has been
shown to significantly decrease the MBR biomass filterability due to the increase in
SMP levels produced (Evenblij et al., 2005b).
The effects of starvation conditions on the biological suspension have been
assessed by incorporating different substrate impulses in batch tests (Lobos et al.,
2005). Exogenous phases were followed by starvation periods, both characterized by
the S:X (substrate to biomass concentration ratio) where high ratios led to multiplication
of bacteria cells whilst at low ratios MLVSS decreased, SMPp production was
absent and bacteria lysis ceased. S:X closely relates to F:M ratio (Equation (2.11)),
and the low F:M values generally used in MBRs are thus theoretically close to starvation
conditions which are in turn likely to be beneficial to MBR operation on the
basis of the reduced SMPp production and correspondingly reduced fouling.
The principal period of unsteady-state operation is during start-up when the system
is acclimatising. Cho et al. (2005b) reported temporal changes of the bound EPS
levels when the MBR was acclimatised at three different SRTs (8, 20, 80 days). As
expected from general trends described in Section 2.3.6.5, the EPS concentration was
lower at the longer SRT (83 vs. 26 mgTOC/gSS for SRTs of 8 and 80 days, respectively).
An initial latent phase was observed in which EPS concentration did not vary
significantly. However, EPS levels increased exponentially after 40 days of operation
at an SRT of 8 days, and after 70 days when the MBR was operated at 20-day SRT. No
change in EPS levels was observed during the 80 days of operation at 80-day SRT. For
another MBR operated at infinite SRT, no significant changes in SMP concentration
during 100 days of operation were observed, over which time period the MLSS
increased from 1.8 to 4.5 g/L ( Jinhua et al., 2004). In a further study, following a latent
phase of 30 days, MLSS and SMP levels started to significantly increase and stabilised
after 140 days of operation at infinite SRT, whereas EPS levels increased continuously
from the start but also stabilized after 140 days (Gao et al., 2004a). Nagaoka and
Nemoto (2005) observed an increase in MLSS concentration from 4 to 14 g/L over
100 days along with a steady increase in EPS (from 50 to 250 mgTOC/L). There
therefore appears to be no distinct pattern regarding foulant species generation and
start-up, other than a general trend of more stable foulant levels at longer SRTs.
The generation of foulants arising from changes in salinity have been studied
by Reid (2006). According to established literature on the ASP extending back to
the 1960s (Ludzack and Noran, 1965; Tokuz and Eckenfelder, 1979), changes in
salinity have a greater impact on biotreatment efficacy, as manifested in the outlet