Optimisation of biogas yields from anaerobic digestion by feedstock type - The Biogas: Science, Production and Applications - page 135

6.3.2 Biogas and methane productivity

Although the specific methane yield of a substrate is a function of the
substrate itself, the methane productivity is dependent on the rate at which it
is produced and, importantly, on the digester volume that produces it.
Productivity is therefore often expressed as m 3 CH 4 m 3 digester day 1 .
From a kinetic perspective, the most important factors determining this in a
continuous digestion system are the food-to-biomass ratio and the net
specific growth rate of the biomass. In a simple single-pass digester without
solids recycle, the net specific growth rate is equal to the dilution rate, which
is the reciprocal of the hydraulic retention time (HRT). The food-to-mass
ratio, also known as the biomass organic loading rate (OLR), is difficult to
determine and is therefore often expressed simply on a volumetric basis,
using either volatile solids (VS) or chemical oxygen demand (COD) as the
measure of organic matter. Of these parameters, the OLR is paramount in
determining the methane productivity provided that the HRT is not reduced
to a point where the methanogens are washed out.
The effect of increasing the OLR on a digester of the continuous stirred
tank reactor (CSTR) design is shown in Fig. 6.3. The specific methane yield
(SMY) remains relatively constant as the loading is increased, resulting in a
linear increase in the volumetric methane production (VMP). During this
time, the HRT decreases because the volume of material added must
increase in order to satisfy the organic load, assuming that the feedstock VS
content remains constant. In the hypothetical example shown in Fig. 6.3(a),
which is based on a slurry input with a VS of 60 g l 1 , the HRT would reach
the 'safe limit value' of 12 days at an OLR of 5 kg VSm 3 day 1 . Higher
loadings would start to wash out the methanogens, with a fall in specific
methane yield as both contact period and quantity of biomass are reducing.
When the feedstock VS content is increased as in Fig. 6.3(b), the only
parameter that changes is the HRT. In this case a safe limit value for HRT is
not reached even when the highest loading is applied and, in fact for this VS
content, the loading could theoretically be increased to 20 kg VSm 3 day 1
before the HRT reaches the critical 12-day value. Yet the hypothetical curve
still shows a drop in specific methane yield and volumetric methane
production at about the same loading: this is based on typical empirical
data, but depends on a number of factors discussed later in the chapter.
What is certain is that the loading to a single-pass digester cannot be
increased indefinitely as there comes a point when the metabolic capacity of
the digester is reached; that is, the amount of biomass present can no longer
consume all the food applied - in this respect bacteria are not much different
to humans! Little information is available as to what this maximum loading
rate is, because to produce a real graph for even a single substrate in place of
the hypothetical Fig. 6.3 represents a large experimental effort even at

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