|Bisogni, James J|
Submitted to: Water Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/31/2009
Publication Date: 6/6/2009
Citation: Hafner, S.D., Bisogni, J. 2009. Modeling of ammonia speciation in anaerobic digesters. Water Research. 43:4105-4114. Interpretive Summary: Anaerobic digestion is a process that can be used to treat animal manure. This process reduces odor and recovers energy in the form of methane gas. When ammonia concentrations are high in manure, a condition known as ammonia toxicity can occur, where microbial conversions slow. The toxic form of ammonia, free ammonia, makes up only a fraction of the total ammonia in anaerobic digesters. Understanding and controlling ammonia toxicity requires a method for determining free ammonia concentration. Since it is difficult to measure free ammonia directly, researchers measure total ammonia and use a simple equation to estimate free ammonia. However, this equation is not expected to be accurate for manure, due to the presence of many other chemicals at high concentrations. The objective of this work was to present a new comprehensive model for predicting free ammonia in anaerobic digesters by combining data from previous studies, and to use the resulting model to test the simple equation. Results show that the simple equation is very inaccurate for anaerobic digesters treating manure. However, its accuracy can be greatly improved by incorporating an additional term. The comprehensive model presented in this work can be used for other tasks as well, such as predicting the composition of gas produced by an anaerobic digester.
Technical Abstract: Anaerobic digestion of high-nitrogen wastes can be inhibited by high concentrations of un-ionized ammonia, NH**3 (aq). Understanding the toxicity of NH**3 (aq) to anaerobic digestion requires an understanding of the mechanisms controlling its concentration. Previous work on ammonia toxicity in anaerobic digesters has utilized a simple equilibrium calculation for estimating NH**3 (aq) concentration from total ammonia, temperature, and pH. This approach is not appropriate for concentrated solutions. In this work, a comprehensive 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 NH**3 (aq) concentration for all but dilute digesters. However, including an estimate of the activity coefficient for NH4+ in the simple equilibrium calculation results in much more accurate estimates of NH**3 (aq) concentration (estimated error is less than 10% in the absence of struvite precipitation).