Advanced modelling to determine free ammonia concentrations during (hyper-)thermophilic anaerobic digestion in high strength wastewaters

Anaerobic digestion is an attractive treatment technology for concentrated waste streams. However, high ammonia concentrations cause inhibition of methanogenesis, especially when operated at elevated temperatures like (hyper-)thermophilic (55 and 70 °C) anaerobic digestion. These emerging (hyper-)thermophilic technologies are beneficial due to high conversion rates and pathogen removal, but are more susceptible for ammonia toxicity as consequence of a temperature-induced pKa shift. Determination of NH3-N (free ammonia nitrogen (FAN); toxic form) concentrations is conventionally based on an equilibrium model and the total ammonia nitrogen concentration (TAN). However, the conventional equilibrium model overestimates the FAN concentration and therefore we developed an Ionic Activity Model which takes the ionic strength and organic matter interactions into account. The difference between the two models could mainly be attributed to the high ionic strength of the waste stream, whereas interactions with organic matter had a smaller effect. Based on this Ionic Activity Model and batch experiments at hyper-thermophilic conditions, we found that acetoclastic methanogenesis was completely inhibited at FAN concentrations exceeding 588 mg/L, whereas hydrogenotrophic methanogenesis could produce methane up to 925 mg/L. During thermophilic and hyper-thermophilic black water treatment, the ionic strength and organic matter interactions resulted in NH3 concentrations below the inhibitory threshold.