Modeling of ammonia speciation in anaerobic digesters

Anaerobic digestion of high-nitrogen wastes such as animal manure can be inhibited by high concentrations of un-ionized ammonia, NH3 (aq). Understanding the toxicity of NH3 (aq) to anaerobic digestion requires a method for determining its concentration. Previous work on ammonia toxicity in anaerobic digesters has utilized a simple equilibrium calculation for estimating NH3 (aq) concentration from total ammonia, temperature, and pH. This approach is not appropriate for concentrated solutions. In this work, a speciation model for major solutes in anaerobic digesters, based on Pitzer's ion-interaction approach, is presented. Model simulations show that the simple equilibrium calculation (without corrections for non-ideal behavior) substantially overestimates NH3 (aq) concentration for all but dilute digesters. This error in concentration determination increases with total solids content and is estimated to be greater than 40% for a digester fed dairy manure with 5% total solids or swine manure with 3% total solids. However, including an estimate of the activity coefficient for NH4+ in the simple equilibrium calculation results in much more accurate estimates of NH3 (aq) concentration. In this case, the estimated error is less than 10% in the absence of struvite precipitation at the highest total solids contents considered.