Membrane Bioreactor (MBR) as an Advanced Wastewater Treatment Technology
65
Aeration flow is also one of the main factors that affect the biochemical
process of BOD and COD removals. The right amount of oxygen needs to be
provided to the microorganisms in response to their three specific demands:
1. carbonaceous BOD (conversion of the carbonaceous organic matter in
wastewater to cell tissue and various end products),
2. nitrogenous BOD (in the process of nitrification ammoniacal nitrogen is
oxidized to the intermediate product nitrite, which is then converted to
nitrate),
3. inorganic COD (oxidation of reduced inorganic compounds within the
wastewater) [117].
Biomass characteristics such as SMP and EPS strongly influence the
oxygen transfer, so therefore they will determine the rate of organics re-
moval. These compounds are also widely recognized as the main membrane
foulants [118]. SMP is soluble and thus is in the liquid phase and EPS is
bounded to cells and makes a part of the solid phase. In order to reach the
active sites on the bacterial cell membrane, the oxygen needs to penetrate
the liquid film surrounding the flocs and then diffuse through the floc matrix
(EPS) [117]. EPS amounts differ with changes in microbial state and operating
conditions of the bioreactor. In the intermittently aerated MBR they increase
in proportion to the non-aeration time [119]. Nevertheless, Ujang et al. [89]
reported no significant difference in COD removal efficiency when varying
aeration and non-aeration time, indicating that in intermittently aerated MBR
organic matter can be degraded both under aerobic and anaerobic conditions.
Also, over-aeration can bring about poor sludge characteristics such as bad
floc structure and rather low sludge volume index (SVI), which can then be
related to fouling [101].
In conclusion, immersed MBR is strongly capable of resisting shock-
loadings, and variations in the inflow turbidity and organic matter content
have no effect on their removal efficiencies. The removal of organic pollutants
in terms of COD and SS has been proven to be very high and a good-quality
effluent can be achieved during long-term operation. However, how to op-
erate MBR systems efficiently remains a topic of argument because there is
a lack of information on the development of microbial community structure
in MBRs during nitrification [57]. It is important to distinguish among these
different contributions to the overall COD removed, in order to better under-
stand the dynamics of the process and to build-up a useful basis for process
designing and modelling.
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