Environmental Engineering Reference
In-Depth Information
6.7
Case studies
Three case studies are now presented to show how the anaerobic process can
be optimised for different purposes. The examples have been chosen to
illustrate how the process can be optimised to
.
meet the metabolic needs of the anaerobic consortium
.
obtain the best overall energy yield within defined system boundaries
.
maximise overall environmental benefits.
6.7.1 Case study 1. Optimising the volumetric methane
production in food waste digestion
Food wastes and residues from food processing and animal slaughter are
attractive feedstocks because of their high biogas potential, but AD of
substrates with a high nitrogen content has always posed difficulties because
of the build-up of ammonia to inhibitory concentrations. This case study
shows how the digestion of source-segregated food waste has been optimised
by increasing the metabolic capacity of the digester, through stimulating the
more ammonia-tolerant hydrogenotrophic methanogens by selective trace
element addition. Early studies (Banks et al., 2008; Climenhaga and Banks,
2008) showed that mesophilic digestion of food waste could be achieved at
moderate loadings but with an accumulation of VFA. Although undesirable
for operational reasons, this did not result in a significant reduction in
biogas production because the system was strongly buffered by ammonia,
giving a pH greater than 7.8. The first full-scale treatment plant built in the
UK to treat source-segregated domestic food waste also showed similar
symptoms of VFA build-up, and the loading in this case was restricted to
around 2 kg VSm
3
day
1
(Banks et al., 2011). During these and other
studies, a pattern was identified in which food waste digesters first
accumulated acetic acid which reached a peak then declined, followed by
a longer term accumulation of propionic acid. The timespan over which
these changes occur depends on the process loading, but typically it was
more than a year before the accumulation of acid products overcame the
digester buffering capacity, leading to process failure. A theory was put
forward to explain this (Banks and Zhang, 2010): the peak in acetic acid was
a consequence of inhibition of the acetoclastic methanogens as the ammonia
concentration reached a threshold value (Karakashev et al., 2006; Schnurer
and Nordberg, 2008). The subsequent decline in acetic acid, despite a
continuing rise in ammonia concentrations, and its stabilisation at a low
value were considered to indicate a shift in the dominant methanogenic
activity from acetoclastic to hydrogenotrophic. The non-reversible accu-
mulation of propionic acid was thought to occur due to deficiencies in the
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