Environmental Engineering Reference
In-Depth Information
5.2.6 Sulphate-reducing bacteria
Sulphate-reducing bacteria (SRB) can utilise multicarbon compounds and
methanogenic substrates: carbon dioxide, hydrogen and acetate. With high
additions of sulphate, the SRB metabolise unicarbon compounds and
hydrogen, more effectively to the detriment of the methanogens. Due to the
syntrophic nature of anaerobic processes, this affects all trophic groups
(Zeikus, 1979).
5.3 Microbial environment
5.3.1 Reducing effects
By its very definition, anaerobic digestion takes place in the absence of
molecular oxygen and the environment changes from an oxidising state to a
reducing one. This may be explained in chemical terms by noting that
alternate electron acceptors must be found to replace molecular oxygen.
Usually, carbon atoms associated with organic compounds will become
electron acceptors and will be reduced while other organic compounds will
be oxidised to carbon dioxide and volatile acids. The end product of this
reaction still contains large amounts of energy (potential to accept electrons)
in the form of methane.
For the bacterial cell to yield enough energy for cell growth and
maintenance, a large quantity of substrate needs to be processed
(Eckenfelder et al., 1988). Thus, bacterial production is much less than
would occur in aerobic conditions with oxygen as the electron acceptor
(Colleran, 1991b). The sludge/bacteria produced from the aerobic conver-
sion of glucose is 450 kg of dry microbial biomass per tonne of chemical
oxygen demand (COD) converted. This may be compared with 45 kg of dry
microbial biomass per tonne of COD converted in the anaerobic reaction
(Colleran, 1991b) as follows:
Δ
G (kJ/reaction)
Aerobic:
Glucose
þ
6O
2
= 6CO
2
þ
6H
2
O
2826
Anaerobic:
Glucose = 3CO
2
þ
3CH
4
403
For industrial wastewater treatment this offers significant advantage to
anaerobic processes over aerobic processes. Dissolved oxygen as low as
0.1mg/l can completely inhibit methanogenic growth (Casey, 1981). Not
only oxygen but other highly oxidised materials, which are electron
acceptors, may inhibit methanogenesis (Pfeffer, 1979). Nitrites, nitrates
and sulphates are examples of such oxidised materials.
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