Optimisation of biogas yields from anaerobic digestion by feedstock type - The Biogas: Science, Production and Applications - page 148

trace elements required for synthesis of the enzymes needed in syntrophic
hydrogenotrophic methane production (see Chapter 5). In particular,
selenium (Se) was thought to be important because of its role as a co-
factor in formate dehydrogenase for formate oxidation (Bo¨ ck, 2006).
Accumulation of formate, a breakdown product of propionic acid, had been
reported as possibly triggering feedback inhibition in propionic acid
oxidation (Dong et al., 1994).
A series of batch and semi-continuous tests was undertaken at laboratory
scale to determine the effects of trace element addition in these high-
ammonia food waste digesters (Banks et al., 2012). Initial batch screening
tests showed that addition of selenium could stimulate the breakdown of
propionic acid in digestate from acclimated digesters operating at high
ammonia concentrations. These batch experiments were followed by an
extended semi-continuous digestion study in which trace elements (TEs)
were tested singly and in combination against controls without TE
supplementation at different OLRs (Fig. 6.5(a)). As expected, the control
digesters without TE supplementation showed a slow accumulation of VFA
over the study period and, in order to maintain biogas production, it was
necessary to limit the loading to 1.6 kg VSm 3 day 1 (Fig. 6.5(b)). An
increase in loading from 2 to 3 kg VSm 3 day 1 led to a rapid increase in
VFA, resulting in acidification and digester failure. In digesters supple-
mented with TE combinations that included selenium, it was possible
sequentially to raise the loading from 2 to 3, 4 and 5 kg VSm 3 day 1 at low
VFA concentrations, and with a small increase in specific methane yield
relative to the unsupplemented control at 1.6 kg VSm 3 day 1 .
The digesters supplemented only with selenium began to show signs of
VFA accumulation at a loading of 5 kg VSm 3 day 1 , and at this point it
was found that the addition of cobalt (Co) was necessary to reduce the VFA
concentration and maintain stable digestion. The microbial population
structure was confirmed using fluorescent in situ hybridisation (FISH)
techniques to show that only hydrogenotrophic methanogens were present:
later work using a 14C radio-labelling technique has confirmed this as the
route to methane production, with a complete lack of acetoclastic activity.
The interpretation of these results in terms of the metabolic capacity of the
digester is that there was a lack of the co-factors needed to allow expansion
of the hydrogenotrophic population when the acetoclastic organisms
became inhibited by increasing ammonia concentrations. The addition of
selenium unblocked the metabolic pathway, allowing uneven carbon chain
length VFA to be converted to acetate, hydrogen and CO 2 . Sufficient co-
factors and TE were present in the food waste to allow acetate oxidation via
the reverse Wood-Ljungdahl pathway until higher loadings were applied,
when this pathway is thought to have become limited by the availability of
cobalt, resulting in the acetate accumulation seen in Fig. 6.5(b) until cobalt

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