Abstract
Slurry acidification with sulfuric acid is a commonly used measure to reduce ammonia emissions from animal waste in Denmark. However, high sulfate content is an inhibiting factor in anaerobic digestion. The objective of this study was to investigate the sulfate concentration thresholds for inhibition of thermophilic anaerobic digestion of animal slurry. We conducted a batch assay for 114 days using two types of slurry (pig and cattle) and ten different sulfate concentrations per type of slurry (0 to 5000 mg SO42- L-1). Added sulfate concentrations higher than 2500 mg SO42- L-1 in pig slurry and higher than 2000 mg SO42- L-1 in cattle slurry caused reductions of more than 40% in methane production and accumulations of volatile fatty acids throughout the study. The biogas produced contained large amounts of hydrogen sulfide (3.2% in pig slurry and 1.9% in cattle slurry) with concentrations peaking in the first 20 days and subsequently decreasing. Waste properties can affect the inhibitory threshold of the sulfate in anaerobic digestion. Our results provide specific sulfate inhibition thresholds for pig and cattle slurries, which differ from other sulfate-rich wastewaters.
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