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, NH(3) (aq). Understanding the toxicity of NH(3) (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 NH(3) (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 NH(3) (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 NH(4)(+) in the simple equilibrium calculation results in much more accurate estimates of NH(3) (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.

Ranking of refrigerants.

Environmental ranking of refrigerants is of need in many instances. The aim is to assess the relative environmental hazard posed by 40 refrigerants, including those used in the past, those presently used, and some proposed substitutes. Ranking is based upon ozone depletion potential, global warming potential, and atmospheric lifetime and is achieved by applying the Hasse diagram technique, a mathematical method that allows us to assess order relationships of chemicals. The refrigerants are divided into 13 classes, of which the chlorofluorocarbons, hydrofluorocarbons, hydrochlorofluorocarbons, hydrofluoroethers, and hydrocarbons contain the largest number of single substances. The dominance degree, a method for measuring order relationships among classes, is discussed and applied to the 13 refrigerant classes. The results show that some hydrofluoroethers are as problematic as the hydrofluorocarbons. Hydrocarbons and ammonia are the least problematic refrigerants with respect to the three environmental properties.