Ammonia recovery from digested dairy manure as nitrogen fertilizer

Manure management on large dairy farms has become a major issue due to air quality and water quality concerns caused by excess nitrogen and phosphorus. Prior research efforts have not been able to develop a practical solution beyond manure land application. In this study, a novel combined nutrient recovery and biogas upgrading system (combined system) was developed to mitigate the environmental concerns as well as to harvest value added by-products from the anaerobically digested manure. The combined system was proven both technically feasible and economically advantageous. Ammonia stripping is the main unit process of the combined system. Temperature and pH were identified as the most cost sensitive parameters of ammonia stripping. Economical optimization of the stripping process was studied and optimized parameters were provided for future application of the technology. The results suggest that the combined system can be a practical solution for the environmental problems associated with excess nitrogen and phosphorus on large dairy farms and can partially purify the biogas from the anaerobic digestion (AD).;The performance of the combined system is highly dependent upon the chemical equilibriums of the digested manure. Current physic-chemical process models for wastewater cannot explain slow crystallizing precipitants formed in the effluent of dairy AD with long hydraulic retention time (HRT). In addition, current chemical models need detailed chemical data as input which are difficult to obtain for dairy wastewater. In this study, a two-step, three-phase chemical equilibrium model was developed for the supernatant of dairy AD. The model input was reduced by introducing a new parameter Deltaions to account the difference between inert cations and anions. The model can differentiate the precipitates formed in the AD with long HRT from that in the post-AD treatment with short HRT. Model validation showed that crystallizing HAP (hydroxyapatite) formed slowly in the effluent of dairy AD with long HRT. The model was applied to the unit processes of the combined system and the results suggest that the two-step, three-phase chemical equilibrium model can be used to predict the performance of the combined system.