A two-stage stripping treatment of anaerobic digestion effluent for ammonia recovery and scale prevention

In anaerobic digestion of food waste, ammonia inhibition and scale formation present challenges for steady operation. Herein, a two-stage stripping technology was developed to address this issue, exemplifying the actual anaerobic digestion effluent. Firstly, air stripping was performed at 55 °C with a gas flow rate of 1.5 L/min to remove ammonia and elevate the pH, facilitating Ca²⁺ and Mg²⁺ precipitation from the wastewater. After settling, biogas was introduced at an ambient temperature to reduce the pH with a flow rate of 1.5 L/min, maintaining the stability of Ca²⁺ and Mg²⁺ and preventing scaling in the second stage. The results demonstrated that air stripping for 480 min elevated the pH of the anaerobic digestion effluent from 8.15 ± 0.01 to 9.83 ± 0.01, achieving an ammonia removal efficiency of (90.69 ± 0.84) %. Additionally, the removal rates for Ca²⁺ and Mg²⁺ were (79.80 ± 2.62) % and (43.55 ± 3.58) %, respectively. Subsequent biogas stripping reduced the pH of the anaerobic digestion effluent from 8.37 ± 0.02 to 7.13 ± 0.01 while maintaining consistent levels of Ca²⁺ and Mg²⁺ during the stripping process. Furthermore, the MINEQL+ simulation results showed that in the anaerobic digestion effluent treated by air stripping, the concentrations of calcite (CaCO₃) and dolomite (CaMg(CO₃)₂) were 0 and 54.9 %, respectively. Following biogas stripping treatment, the concentrations of both compounds were further decreased to 0 %. These findings demonstrated that this strategy could successfully lower carbonate scale production, avoiding crusting and obstruction in the anaerobic system and minimizing scale formation.