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Nitrogen and Phosphorus Recovery from Anaerobic Digested Dairy Wastewater

Published by the American Society of Agricultural and Biological Engineers, St. Joseph, Michigan www.asabe.org

Citation:  Paper number  131618745,  2013 Kansas City, Missouri, July 21 - July 24, 2013. (doi: http://dx.doi.org/10.13031/aim.20131618745) @2013
Authors:   Quanbao Zhao, Craig Frear, Cynthia Alwine, Jingwei Ma, Shulin Chen
Keywords:   Aeration anaerobic digestion nitrogen phosphorus dairy manure

Abstract. In the anaerobic digestion (AD) process, a considerable fraction of organic nitrogen is converted to inorganic nitrogen, producing an effluent with higher concentrations of ammonium as well as elevated temperatures, both of which can be useful in downstream processing of N recovery via ammonia stripping. Unfortunately, release of the ammonia also requires a suitable increase in pH to convert ionic ammonium to free ammonia. Chemical dosing for pH elevations at or above 10 is possible but economically problematic. On the other hand, the supersaturated CO2 and high concentration of bicarbonate present as a result of the digestion process is an important resource for elevating pH by stripping out the acid gas CO2. Our research has demonstrate that limited aeration is an effective way to elevate pH of the AD effluent because of 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. Moreover, the removal of supersaturated CO2 and elevated pH could further enhance the phosphorus-rich solids precipitation, resulting a better total suspended solids (TSS) and total phosphorus (TP) removal. This manuscript 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.

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