Ammonia recovery from food waste digestate using solar heat-assisted stripping-absorption

The highest costs of stripping-absorption processes for ammonia recovery are related to energy (for heating and air supply) and chemical addition (for pH adjustment). In this paper, a simplified system that used no chemicals, and a renewable source of energy for heating, was tested to recover nitrogen as ammonium sulfate from food waste digestate. pH adjustment was achieved by CO₂ stripping, and vacuum tube solar collectors were used to provide heating. The effect of different temperatures (25 °C and 45 °C) and gas to liquid ratios (1700 and 2600) on ammonia removal and recovery were assessed. Ammonia removal efficiencies higher than 91% were achieved for all evaluated experimental conditions. The solar heater showed adequate capacity to increase the temperature of the liquid digestate by 21 °C and maintain the temperature at 45 °C throughout the experiment. Tests carried out at 45 °C achieved the highest ammonia removal efficiency (98%) at the lowest evaluated G/L ratio (1700). Better absorption efficiencies could potentially have been achieved if lower inlet airflow rates and packing material had been used in the absorption column.