Ammonia recovery from anaerobic digestion effluent using membrane distillation: Membrane fouling at different water vapor transfer scenarios

•The interaction between water vapor transfer and membrane fouling was investigated.•Higher water vapor transfer led to greater decline and more severe fouling in DCMD.•Inorganic and organic fouling dominated PP and PTFE treatments, respectively.•Stable ammonia recovery performance was obtained during 25-d isothermal DCMD.•The absence of water vapor transfer contributed to a low propensity for fouling. Membrane fouling and its effect on ammonia recovery performance during anaerobic digestion (AD) effluent treatment were systematically investigated under a high water vapor transfer (i.e., conventional direct contact membrane distillation, DCMD) and a low water vapor transfer (i.e., isothermal DCMD) conditions. For the conventional DCMD scenario, three commercial hydrophobic membranes including 0.22 µm pore-sized PP (PP-0.22), 0.22 µm pore-sized PTFE (PTFE-0.22), and 0.65 µm pore-sized PTFE (PTFE-0.65), were evaluated to determine the influence of membrane properties on the mass transfer and membrane fouling behaviors. In a 4-h DCMD experiment, PTFE-0.65 treatment achieved the highest water and ammonia mass transfer, followed by PTFE-0.22 and PP-0.22 treatments. However, during a 12-d AD effluent treatment, the PTFE-0.65 treatment exhibited the largest decline in water flux (62.22 %), compared to PTFE-0.22 treatment (41.56 %) and PP-0.22 treatment (36.54 %). A similar trend was observed for the ammonia mass transfer coefficient, with the greatest reduction in PTFE-0.65 treatment (93.28 %), followed by PP-0.22 treatment (84.87 %), while PTFE-0.22 treatment showed a significantly smaller decrease (40 %). This smaller reduction in PTFE-0.22 treatment was attributed to its high porosity, which enhanced transmembrane ammonia transfer. The results showed that under a high water vapor transfer condition, a high initial water flux resulted in a high flux decline, indicating more severe membrane fouling, which was further supported by the highest concentration of foulants in the cleaning solution from PTFE-0.65 treatment, followed by PTFE-0.22 and PP-0.22 treatments. Comparatively, under an isothermal DCMD scenario, ammonia recovery remained stable over 25 days with a minimal fouling, highlighting the effectiveness of reduced water vapor transfer in mitigating the membrane fouling.