Thermal Drying of Digestate with Acid Scrubbing of Exhaust Air – A Full Scale Study

Anaerobic digestion of organic wastes and/or livestock manure represents a considerable opportunity for the production of renewable energy, for the creation of jobs and, most of all, for its contribution to a cleaner environment. However, in many cases digestate management still presents challenges: where surplus nitrogen is a concern, dedicated solutions must be implemented. Treatment options could allow to obtain a reduction of volume and concentration of nutrients in added value end products. The present research was conducted on the first full-scale digestate drying plant realized in Italy, in order to assess the efficiency of the process and its environmental sustainability. The system processes part of the digestate from a 1 MWe biogas plant, fed with bovine manure and other biomasses. The thermal energy for the evaporation is derived from the combined heat and power unit (CHP). Air temperature, pH and nitrogen concentration in digestate are favorable to the stripping of ammonia: the exhaust airflow from the evaporator is treated in an acid scrubber resulting in the recovery of ammonium sulfate. The inputs and outputs of the process, mass flows and energy consumption, and exhaust emissions were monitored. The throughput of the system resulted variable: an average of 46.7%, of digestate (mass basis) was treated, with a diversion of 46.7% of nitrogen and phosphorus in dry digestate (4.3%N, 46.3%P) and in ammonium sulfate (4.9-19.7%N, negligible P). These fractions present interesting agronomical characteristics, and could find a market for their content of nutrients, organic nitrogen and phosphorus for the dry fraction (31 gN/kg, 23.5 gP/kg) and ammonium sulfate (31% solution, 31-124gN/kg).