Evaluation of an integrated ammonia stripping, recovery, and biogas scrubbing system for use with anaerobically digested dairy manure

Anaerobically digested animal manure contains appreciable levels of nitrogen, particularly ammonia that can lead to potential air and water quality problems. In this study, a process for ammonia removal through air stripping of digested dairy manure was developed and optimised, and feasibility was demonstrated. Temperature and pH were identified as the most cost sensitive parameters for the process. Ammonia stripping efficiency was tested at different pH levels and temperatures, allowing for development of economic optimisation. When the temperature of the digester effluent was maintained at mesophilic (35 °C) level during the stripping process, the optimised effluent pH for ammonia stripping was 10.3 with a corresponding lime dosage of 3.3 g l−1, resulting in 90% ammonia removal. Stripped ammonia was stabilised as ammonium sulphate through contact with sulphuric acid. This absorption process was nearly complete when suitable acid flow rate and pH protocols were maintained. The high pH of the effluent was later readjusted to a lower level by passing the biogas produced from the digester, which at the same time purified the biogas by scrubbing H2S and CO2. With available biogas/liquid ratios of greater than 20 to 1 from dairy digesters, there was sufficient supply of acidic gas to reduce the pH of the final effluent leaving the digester.