Post-treatment of biogas digestate – An evaluation of ammonium recovery, energy use and sanitation

Abstract

The utilization of digestate from anaerobic digestion (AD) processes offers the possibility to recycle nutrient from organic waste materials back to the food chain. However, digestates are characterized by a high water content resulting in high storage capacities and transportation costs. Various organic wastes such as organic municipal solid waste (oMSW) require sanitation with at least one hour hydraulic retention time (HRT) at 70°C to inactivate pathogens for the safe release of the material. Consequently, the sanitation process is one of the largest energy consumers in the whole AD process chain. In this study, a post-treatment of the biogas digestate was evaluated regarding the potential for energy savings and nutrient recovery via nitrogen stripping in comparison to the conventional pre-sanitation of the organic waste. With increasing HRT (one to three hours) and aeration flow (0.0-5.3 L air per L digestate and minute) more condensate and ammonia was removed from the digestate. The total ammonia removal from the digestate after one and three hours with the highest aeration flow rate reached 42% and 80%, respectively. However, energy requirement for aeration exceeded energy savings from the lower volume for sanitation after the digestion substantially. On a system level, a positive energy balance could still be achieved by taking energy savings from the replacement of mineral fertilizer (36 GJ per ton NH4) into account. Moreover, the digestate as fertilizer could be applied in a more demand-oriented way by adding ammonium sulphate obtained by the ammonia stripping during the post-treatment.