Strategies for optimizing recovery of the biogas process following ammonia inhibition

Strategies for recovery of ammonia-inhibited thermophilic biogas process, were evaluated in batch and lab-scale reactors. Active methane producing biomass (digested cattle manure) was inhibited with NH4Cl and subsequently, 3–5 days later, diluted with 50% of water, or with 50% digested manure, or with 50% fresh manure or kept undiluted. Dilution with fresh cattle manure resulted in the highest methane production rate during the recovery period while dilution with digested cattle manure gave a more balanced recovery according to the fluctuations in volatile fatty acids. Furthermore, the process recovery of a 7600 m3 biogas plant suffering from ammonia inhibition was observed. The ammonia concentration was only gradually lowered via the daily feeding with cattle manure, as is the normal procedure at Danish full-scale biogas plants. Recovery took 31 days with a 40% methane loss and illustrates the need for development of efficient process recovery strategies.