Activity, life time and effect of hydrolytic enzymes for enhanced biogas production from sludge anaerobic digestion

- Majority of enzymes were active in both sludge environments tested.
- Enzyme life time and effect was limited due to endogenous protease activity.
- Several enzymes had zero effect on biogas production rate and yield.
- Only subtilisin provided a significant effect on biogas production rate and yield.
- For practical application, enzymes with higher longevity are desirable.

As an alternative to energy intensive physical methods, enzymatic treatment of sludge produced at wastewater treatment plants for increased hydrolysis and biogas production was investigated. Several hydrolytic enzymes were assessed with a focus on how enzyme activity and life time was influenced by sludge environments. It could be concluded that the activity life time of added enzymes was limited (<24 h) in both waste activated sludge and anaerobic digester sludge environments and that this was, for the majority of enzymes, due to endogenous protease activity. In biogas in situ experiments, subtilisin at a 1% mixture on basis of volatile solids, was the only enzyme providing a significantly increased biomethane production of 37%. However, even at this high concentration, subtilisin could not hydrolyze all available substrate within the life time of the enzyme. Thus, for large scale implementation, enzymes better suited to the sludge environments are needed.

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