Environmental Engineering Reference
In-Depth Information
amount entering the digester as part of the feed. For this reason an optimal
ratio between C/N is often referred to in the scientific literature and values
quoted are almost always in the range 20-30:1. Too little N is also
potentially a problem as there is a minimum requirement for growth: values
of C/N over 40 are likely to be insufficient, leading to restrictions in the
growth of the microbial consortium and reduction in the metabolic capacity.
Nitrogen in the form of ammonia also provides an important buffering
component. This is well demonstrated in the digestion of abattoir and food
wastes, where high ammonia concentrations allow stable digester operation
with high VFA; in contrast, digestion of low-nitrogen products such as
paper wastes can be problematic, as relatively small changes in VFA
concentration can lead to drastic swings in pH. Too much N, however, will
lead to ammonia concentrations known to be inhibitory to methanogens.
Surprisingly, despite the number of reported studies that have looked at
ammonia inhibition, it is still difficult to give exact values for the
concentration at which it becomes toxic (see also Chapter 5). This is partly
due to the fact that ammonia in digesters exists in the form of the more toxic
free ammonia and less toxic ammonium ions, with the equilibrium strongly
dependent on pH and temperature. A number of equations have been put
forward to describe this relationship, such as that proposed by Østergaard
and quoted by Hansen et al. (1998)
0
1
1
10
pH
FreeNH
3
Total NH
3
¼
@
A
1
þ
2729
92
10
0
:
09018
þ
:
T
ðÞ
where T(K) is the temperature in Kelvin.
It is now accepted that acetoclastic methanogens are more sensitive to
ammonia inhibition than hydrogenotrophic methanogens (see Section
6.7.1), and the C/N ratio may therefore start to dictate the biochemical
pathway through which methane can be formed. Based on the authors'
experience in food waste digestion, acetoclastic methanogens are replaced by
hydrogenotrophic at around 500mg l
1
of free ammonia in both mesophilic
and thermophilic systems. Phosphorus is also needed for cell growth and the
requirements can usually be satisfied at a C/P ratio of 120. In some cases it
may be necessary to supplement the feed to achieve this.
At the C/S ratio of
600 that is required for microbial growth, it is
unlikely that sulphur will become limiting for most feedstocks, but it may
have an impact on metabolic capacity of the system. This is due firstly to the
competition for acetate, a methane precursor, as an energy source by
sulphate-reducing bacteria. These bacteria work at a higher redox potential
than methanogens and can outcompete them for intermediate product
energy sources. Secondly, soluble sulphides are themselves inhibitory to
~
Search WWH ::
Custom Search