Coupling thermal stripping and acid absorption for ammonia recovery from dairy manure: Ammonia volatilization kinetics and effects of temperature, pH and dissolved solids content

Both liquid dairy manure and manure digestate have high concentrations of ammonia. This study coupled thermal stripping with acid absorption to recover ammonia from dairy manure as ammonium sulfate. Ammonia volatilization kinetics and effects of temperature, pH, and dissolved solids content on thermal stripping were investigated in batch experiments. Statistically insignificant differences were found in ammonium sulfate formation and liquid mass transfer of ammonia between the experiments with initial feed pH values of 11 and 9 and between the experiments with filtrate of undigested and digested manure, which have different dissolved solids contents. Temperature had significant effects on thermal stripping of ammonia. When temperature was set at the boiling points (101-102 °C) of manure filtrate, ammonia was completely stripped in 3-4 h. It took 5 h to strip 50% of ammonia at 97 °C. Little ammonia was stripped at 86 and 76 °C. The saturation concentrations of ammonia in manure filtrate was zero at temperatures 76-102 °C. Ammonia mass transfer coefficient upon boiling was 30.3-31.4. mm/h in manure filtrate. Ammonia mass transfer coefficient increased exponentially with temperature, indicating a temperature coefficient of 1.164. High-purity ammonium sulfate crystals (≥98%) were produced when sulfuric acid solutions were pre-saturated with ammonium sulfate. When ammonia was completely stripped, ammonium sulfate was formed at 3.70-3.87. g/L filtrate. A preliminary economic analysis shows that this novel technology is economically promising for practical application, especially when it is applied to a side-stream of anaerobic digesters.