Waste heat promoted ammonia recovery from sludge lysate using a MES-TMCS system: thermal-electric-microbial synergies

Sludge lysate from low-temperature pyrolysis is rich in high ammonia and waste heat (<100 °C), yet its conversional treatment is energy- and chemical- intensive. Microbial electrochemical systems (MES) coupled with ammonia recovery technologies have shown considerable potential for nitrogen recovery, but remain untested for such thermally impacted wastewaters. Here, an MES coupled with a transmembrane capture system (MES-TMCS) was applied to sludge lysate to investigate the ammonia recovery performance and underlying mechanisms under controlled temperature (25–45 °C). The best performance was achieved when both chambers were maintained at 45 °C. The NH₄⁺-N removal efficiency reached 81.26 ± 11.01%, representing a 31.77-percentage-point increase over the room-temperature group. The recovery ratio reached 0.94 ± 0.09, corresponding to a 1.65-fold improvement. Meanwhile, current generation was enhanced by 2.46-fold, and the system completely eliminated external alkali addition by exploiting cathodic OH⁻ production. Analysis of FT-ICR-MS indicated that elevated temperature likely promoted to the partial biotransformation of nitrogen-containing organics (CHON) during microbial electrochemical conversion. The high temperature selectively enriched thermophilic electroactive bacteria (Desulfurella and Thermincola) and anaerobic degraders (Caldicellulosiruptor), which could enhance the electron transfer and refractory organics breakdown, thereby increasing NH₄⁺ electromigration. Moreover, the elevated temperature lowered the pKa of the NH₄⁺/NH₃ equilibrium, reduced the mass-transfer resistance of NH₃, and facilitated subsequent NH₃ transfer and transmembrane capture. Overall, these finding suggest a potential thermal-electric-microbial coupling mechanism, in which elevated temperature enhances electrochemical reactions, promotes ammonia phase conversion, and enhances transmembrane capture, ultimately boosting ammonia recovery. The MES-TMCS offers an alkali-free strategy coupled with residual heat reuse for carbon-nitrogen separation and ammonia recovery from waste heat contained complex wastewaters, supporting the low-grade thermal energy utilization.