Possibilities for enhanced nitrogen recovery from digestate through thermal drying

Ammonia release during thermal drying of digestate originating from a thermophilic anaerobic digestion process was continuously measured using Fourier transform infrared spectroscopy analysis, and the total amount of ammonia released was calculated. Batch thermal drying was performed at temperatures of 70, 100, 130, and 160 A degrees C without pH adjustment. Increased drying temperature facilitated higher peak concentrations of ammonia and significantly reduced digestate drying time. However, it was only when the temperature was increased from 70 to 100 A degrees C that a statistically significant increase in the total amount of ammonia released, from 8990 to 9860 mg kg(-1) dry matter (DM), was recorded. Further gradual increases in the drying temperature, up to 160 A degrees C, did not significantly facilitate nitrogen release. Depending on the drying temperature, the share of nitrogen released during drying was 27.3-29.9% of total nitrogen contained in digestate. The total nitrogen content in the condensate of the off-gas ranged 600-1280 mg kg(-1) DM at different temperatures, which is equivalent to 6.7-13.2% of nitrogen released into off-gases during drying. Stripping could be applied to release the nitrogen contained in the condensate back to the gas phase, where both adsorption and absorption could be applied for nitrogen recovery and the consequent production of nitrogen-rich fertilizers.