Investigation of the Main Parameters Influencing the Kinetics of an Ammonia Stripping Plant Treating Swine Digestate

Ammonia stripping is a consolidated technology that can be adopted to reduce the excessive content of nitrogen in the digestate, thus allowing its reuse as fertilizer within the maximum nitrogen loads set by regulations. In this work, the role of the main operational parameters (airflow and temperature) was assessed on a pilot bubble reactor treating a swine-based digestate, under batch conditions. A kinetic model was developed to correlate temperature, airflow and the ammonia removal yield. Varying the temperature from 60 to 70 °C and the airflow rate from 30 to 60 m3air h−1 m−3digestate, the performance of the plant was measured at different reaction times. The process was favored by the spontaneous pH increase over 10, without basifying agent addition. The lowest removal efficiency (50%) was recorded at 60 °C and 30 m3air h−1 m−3digestate. A removal efficiency higher than 80% was reached running the process at 70 °C and 45 or 60 m3air h−1 m−3digestate. Performance curves were than developed as a practical tool to identify the operating conditions leading to a desired nitrogen removal rate: for instance, under the most severe conditions, up to 80% ammoniacal nitrogen could be removed in about 10 h. Though the obtained results are valid for the treated digestate, the experimental methodology as well as the mathematical model could be replicated in other case studies, thus allowing the amelioration of the process operation.