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

Abstract

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.