The MBR Book: Principles and Applications of Membrane

40 The MBR Book
Rotifers,
Nematodes
Protozoa
Heterotrophic
bacteria
Suspended and
dissolved organic matter
Figure 2.16 Ecology of activated sludge systems
and insect larvae may contribute to the consumption of particulate organic matter,
especially in TF systems.
There is evidence to suggest that higher organisms, protozoa, filamentous organisms,
nematodes and ciliates, are present at lower concentrations in MBRs than in conventional
activated sludge systems (Cicek et al., 1999; Witzig et al., 2002). However,
higher concentrations of protozoa, particularly flagellates and free ciliates, have been
reported for MBRs compared with an activated sludge operating at the same SRT
(Ghyoot and Verstraete, 2000). These experiments were performed on a system with
long HRT (20–74 h), hence the shorter HRT associated with MBRs may be responsible
for the absence of protozoa in other studies. Predatory organisms have a negative
effect on nitrification (Lee and Welander, 1994) and overgrowth of protozoa have
been shown to create a complete breakdown of nitrification (Bouchez et al., 1998). This
grazing in activated sludge is accounted for in the death coefficient (k e), and recent
research suggests that this effect has a greater impact on sludge concentration than
previously thought in an activated sludge system (van Loosdrecht and Henze, 1999).
In contrast, the sludge concentration in an MBR is limited by the energy provided
and cell decay (Low and Chase, 1999). Higher organisms, such as nocardia, have been
shown to develop in full-scale MBRs and produce significant foaming problems
(Section 2.3.6.3).
Conditions are created in an MBR by allowing the sludge to accumulate to a maximum
biomass concentration where all of the energy available is used for cell maintenance.
The high sludge concentration compared to the food available creates an
environment where bacteria are facing starvation conditions so the bacteria are not in
a physiological state for cell growth (Muller et al., 1995). Oxygen uptake rates in an
MBR system compared with a conventional activated sludge system are lower, indicating
that the MBR is carbon rather than oxygen limited (Witzig et al., 2002). Even
if the cells in an MBR system are not growing, new bacteria are constantly being
introduced with the influent wastewater; since no grazing organisms exist, there must
be cell decay to keep the biomass concentration constant.
The microbial community in any biological system comprises a large number of
different bacterial species. In both an MBR and activated sludge system, the dominant