Previous studies have suggested the use of digester biogas mixing systems for in situ ammonia removal from anaerobic digestates. The feasibility of this was tested at moderate and complete gas mixing rates at mesophilic and thermophilic temperatures in a 75-L digester. Experimental results showed that at gas mixing rates typical of full-scale commercial digesters the reduction in total ammonia nitrogen concentrations would be insufficient to allow stable acetoclastic methanogenesis in mesophilic conditions, or to prevent total inhibition of methanogenic activity in thermophilic food waste digestion. Simulation based on batch column stripping experiments at 55°C at gas violent flow rates of 0.032 m3 m-2 min-1 indicated that ammonia concentrations could be reduced below inhibitory values in thermophilic food waste digestion for organic loading rates of up to 6 kg VS m-3 day-1. These mixing rates are far in excess of those used in full-scale gas-mixed digesters and may not be operationally or commercially feasible.
Keywords: Ammonia removal; anaerobic digestion; food waste; in situ stripping; mixing rate.