Does the addition of proteases affect the biogas yield from organic material in anaerobic digestion?

Only very high concentrated enzymes caused an increase in biogas production in batch experiments. In semi-continuous biogas tests no positive long-term effects (100 days) were observed. Higher enzyme-dosage led to a reduced biogas-yield (13% and 36% lower than the reference). Phenylacetate and -propionate increased (up to 372 mg l−1) before the other volatile fatty acids did. Volatile organic acids rose up to 6.8 g l−1. The anaerobic digestion process was inhibited.