Solid–liquid separation as a pre-treatment before
anaerobic digestion was tested in a pilot-scale digester,
in which part of the liquid manure was substituted with
the solids from separation (high-solids digester) and
compared with anaerobic digestion using liquid manure
alone (reference digester).
The ratio of CH4 in the biogas averaged 71% in the
reference digester and 65% in the high-solids digester.
Substitution of slurry with up to 60% solid manure
resulted in a higher gas production per digester volume at
52 °C than can be achieved by liquid manure alone, thus
the CH4 yield in terms of digester volume was almost
twice that in the reference digester during the period with
60% substitution.
However, this high ratio of solid matter also increased
the NH4–N level beyond the normal inhibitory level, to
more than 5 g L− 1, giving rise to VFA levels higher than
normally associated with a stable process, and to a reduced
yield in terms of VS; the yield in the high-solids digester in
terms ofVSwas on average slightly above 200 LCH4 kg− 1 VS, compared to the level of 320 L CH4 kg− 1 VS in the
reference digester, which indicated an inhibition of the
process in the high-solids digester. When the high-solids
digester was coupled with an efficient post-digestion,
the total yield in terms of VS was comparable for both
digesters, indicating that the reduced yield caused by
inhibition can be compensated for by coupling an efficient
post-digestion to the nitrogen-inhibited process.