Aeration Induced pH Elevation of Anaerobically Digested Wastewater for Enhanced Ammonia Stripping and Recovery
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During mesophilic or thermophilic anaerobic digestion, a considerable fraction of organic nitrogen is mineralized to ionic ammonia, producing an effluent with higher concentrations of ammonia nitrogen as well as elevated temperatures, both of which can be useful in dowstream processing of ammonia via ammonia stripping. Unfortunately, release of the ammonia also requires a suitable increase in pH well above that normally existing in digested wastewaters. Chemical dosing for pH elevations at or above 10 is possible but economically problematic. Our research has shown that limited, non-biological aeration at laboratory conditions of 0.5-1 L/min of the digested wastewater (dairy and poultry manure) at varying temperature regimes (35, 55, 70C), results in intriguing chemical shifts strongly related to the high levels of carbon dioxide, bicarbonates and carbonates present in digested effluent. These chemical shifts, ultimately release carbon dioxide and raise the pH of the effluent to levels near 10, which combined with elevated operating temperatures (from waste engine heat in commercial scenario), can lead to 70-90% shift from ionic to free, gaseous form of ammonia and subsequent recovery of the ammonia through acid contact. Length and extent of aeration time, percent recovery of ammonia, and final pH attained is influenced by wastewater source and in particular its levels of suspended solids, chemical composition, buffering capacity, and level of ammonia binding to struvite or other salt compounds. The resultiing stripped effluent shows an important enhancement in degree of suspended solids (TSS) and total phosphorus (TP) settling, with 50 and 80% reductions, respectively. This oral presentation is a summary of laboratory results with dairy manure wastewater, with particular emphasis on the chemical relationships and equilibrium shifts associated with the aeration process and its subsequent release of gases. Additional information will also summarize scale-up and demonstration of the process with industrial partners.