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

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 mu m pore-sized PP (PP0.22), 0.22 mu m pore-sized PTFE (PTFE-0.22), and 0.65 mu m 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.