Ammonia removal in anaerobic digestion by biogas stripping: an evaluation of process alternatives using a first order rate model based on experimental findings

The feasibility of biogas stripping to remove ammonia in the anaerobic digestion of source segregated food waste was investigated. It was found in batch experiments that ammonia could be removed from digestate and that the removal followed 1st order kinetics with respect to total ammonia nitrogen concentration. Increasing temperature, biogas flow rate and initial pH all increased removal rates. Using kinetic data gathered in these experiments allowed the integration of ammonia stripping with an anaerobic digestion plant to be modelled for different configurations. Four scenarios were identified: post digestion, in situ, side-stream and pre-digestion ammonia removal relating to where in the process the ammonia stripping was performed. The modelling showed that in situ ammonia removal may be best able to reduce in-digester ammonia concentrations over a wide range of organic loading rates whereas pre-digestion showed most promise in terms of application due to the flexibility to control each part of the process separately. Further experimental work is required into these scenarios to confirm their viability.