Optimisation of biogas yields from anaerobic digestion by feedstock type - The Biogas: Science, Production and Applications - page 132

This is usually carried out under the most favourable conditions of
temperature, inoculum type, nutrient balance and test duration (Angelidaki
et al., 2009). In practice, the BMP or maximum specific methane yield may
not be achieved in a continuously or semi-continuously operated digester for
a number of reasons: in a mixed system such as a stirred tank reactor, a
proportion of the substrate will bypass the system; the nominal average
retention time of the digester may be insufficient for degradation to proceed
to completion; or the balance of nutrients and minerals may be
unsatisfactory to maintain an active anaerobic consortium.
There are examples where almost all of the maximum specific methane
yield of the substrate is achieved and these relate primarily to substrates with
rapid degradability or systems with a long retention time, or a combination
of the two. Many of the new generation of energy crop digesters fed on
maize silage achieve specific methane yields approaching the BMP value. On
the other hand, a digester treating municipal wastewater biosolids or animal
slurries is unlikely to achieve the BMP value if operated at an economic
loading rate. This is because the degree of dilution of the substrate reduces
the retention time to a point where more recalcitrant components of the
substrate cannot be degraded.
Examples of some BMP test results for selected substrates are shown in
Fig. 6.1. As can be seen from the figure, the rates of reaction at which
degradation occurs are variable, with substances like food waste breaking
down rapidly while others (e.g. cattle slurry) have a much slower gas
production rate. It is clear that the final BMP values vary considerably, from
around 0.47 l CH 4 g 1 VS added for food waste to 0.26 l CH 4 g 1 VS added
for cattle slurry and card packaging (Fig. 6.1(a)). The shapes of the gas
production curves also differ quite markedly: substrates with a high
proportion of fibre, such as cellulose and biodegradable municipal waste
(BMW), show a distinct lag period lasting 1-4 days, which corresponds to
the period needed for hydrolysis of the long-chain polymers before
fermentation and methanogenesis can begin. The other substrates all
showed a rapid initial rate of gas production which, in the case of food
waste, continued until almost all of the methane potential had been realised.
In the case of cattle slurry, this slowed quite rapidly and the final methane
potential had still not been achieved after 100 days, while maize silage and
potato waste lay between these two extremes. In all cases, the data shown
were taken from BMP tests carried out in stirred mesophilic digesters with
inoculum taken from the same municipal wastewater biosolids digester, and
carried out using the method described in Banks and Zhang (2010).
Gas production curves of the type shown in Fig. 6.1 are sometimes
modelled using a simple first-order rate equation

￿ ￿ ￿ ￿ ￿ ￿

e kt Y ¼ Y max 1 ½ 6 1

: