Membrane distillation process for concentration of nutrients and water recovery from digestate reject water

During low-solids anaerobic digestion (AD) of substrates with a high solids content, substrate dilution can be obtained by recycling the liquid phase (reject water) of dewatered digestate. However, this can lead to accumulation of compounds, which inhibits the AD process. This laboratory-scale study assessed the potential of thermally driven air gap membrane distillation (AGMD) for concentrating nutrients in the reject water and recovering process water suitable for use in AD. The results showed that the removal rate of COD, P, S, and K was >98% and the removal rate of total ammonia nitrogen (TAN) reached nearly 100%. The corresponding yield of recovered permeate water was >56%. The concentration of TAN and PO4-P in the recovered permeate was <0.05 mg/L, while the COD concentration was 400-500 mg/L. The flux was 3.3 L/(m2 h) at 60 °C inlet temperature and showed a 28% decline by the end of the experiment. There was no leaking, wetting, or fouling of the membrane over the entire three consecutive day's test duration. Specific heat demand for AGMD ranged from 900 to 1300 kWh/m3 without heat recovery and was as low as 66-170 kWh/m3 with heat recovery. The performance results reported here highlight the potential and robustness of the AGMD process.