Enhancing thermophilic co-digestion of nitrogen-rich substrates by air side-stream stripping

High ammonia concentrations can inhibit thermophilic anaerobic digestion, thus limiting the advantageous treatment of wastes rich both in carbon and nitrogen. In the present paper, an air side-stream stripping column was coupled to two thermophilic digesters to control ammonia content. The effects of exposing an increasing biomass fraction to the harsh conditions inside the column (pH above 9, O₂ at saturation and high T) on digester performance were tested by treating and recirculating 21% of reactor volume 3 or 5 times per week. Neither biomass/liquid separation before stripping nor addition of chemicals to control pH were required. Ammonium nitrogen concentration was lowered from 2.4 to 1.1±0.1gN-TANL^-1 and from 4.5 to 2.0±0.1gN-TANL^-1 without compromising process stability. The air side-stream stripping process was successfully implemented to maintain ammonia concentration below 0.6±0.1gN-NH₃L^-1 while boosting methane production by doubling organic (and therefore nitrogen) loading rate.