The effect of incorporating different ratios of acidified pig slurry on methane yield was
evaluated in two scales of anaerobic digesters: Thermophilic (50
C) pilot scale digester
(120 l), operating with an average hydraulic retention time of 20 days and thermophilic (52
C)
full-scale digesters (10 and 30 m
3
), operating with an average hydraulic retention time of 30
days. In the lab-scale digester, different inclusion levels of acidified slurry (0e60%) were
tested each 15 days, to determine the maximum ratio of acidified to non-acidified slurry
causing inhibition and to find process state indicators helping to prevent process failure. In
the full-scale digesters, the level of inclusion of the acidified slurry was chosen from the ratio
causing methane inhibition in the pilot scale experiment and was carried on in a long-term
process of 100 days. The optimal inclusion level of acidified pig slurry in anaerobic codigestion
with conventional slurry was 10%, which promoted anaerobic methane yield by
nearly 20%. Higher inclusion levels caused methane inhibition and volatile fatty acids
accumulations in both experiments. In order to prevent process failure, the most important
traits to monitor in the anaerobic digestion of acidified pig slurry were found to be: sulfate
content of the slurry, alkalinity parameters (especially partial alkalinity and the ratio of
alkalinity) and total volatile fatty acids (especially acetic and butyric acids
