Innovative ammonia recovery and biogas enhancement via direct contact membrane distillation in thermophilic anaerobic digestion of mixed sludge

Abstract

The effect of membrane distillation (MD) on both NH₃ recovery and anaerobic digestion during the treatment of urban wastewater mixed sludge was studied in a 3 L thermophilic continuous stirred tank reactor combined with a membrane-based module for extraction. A hydraulic retention time of 20 days was used to operate the ther- mophilic anaerobic digester at 55 ◦C, while the flat sheet PTFE membrane module was continuously operated at 0.25 L min 1 of liquid recirculation rates. MD was able to progressively reduce the total ammoniacal nitrogen (TAN) concentration from 0.4 ± 0.2 to 0.1 ± 0.1 g TAN L 1 after 40 operating days. The CH4 yield increased by 3-fold as a result of NH₃ extraction. Similarly, chemical oxygen demand and volatile solids removal efficiencies increased by 1.8-fold and 1.4-fold, respectively. Interestingly, the reduction in TAN concentration led to a complete assimilation of acetic and propionic acid.