This study assessed the thermo-economic performance of membrane distillation (MD) for concentrating nutrients and recover process water from digestate at a thermophilic biogas plant. The input data were derived from mapping the Uppsala Vatten och Avfall biogas system, present knowledge on anaerobic digestion process management and technologies for biogas system operating conditions in Sweden. The study evaluated the potential for recovering waste heat from the digestate effluent and boiler flue gas for use in the MD system. The thermal energy requirement, size, and separation efficiency of the MD unit were based on a previous laboratory study. The study assessed the overall energy efficiency and costs estimation of a full-scale codigestion plant with thermally integrated MD. Presented results shows that the proposed model of integrated MD system has the best thermal performance. The recovered waste heat contributed total thermal energy demand of MD and additionally it could save 19% boiler energy by heating incoming slurry. The results showed that the MD product water permeate was 3.5 L/(m2 h) at 65°C digestate inlet temperature. Specific heat demand for MD was 100 kWh/m3 with internal heat recovery. Cost estimation showed that the unit cost of MD permeate water was 3.6 €/m3 at a digestate feed temperature of 65°C. The economic assessment indicated that thermal integration of a biogas plant with MD could be economically feasible. However, long-term continuous studies are needed to determine impact of fouling and membrane lifetime.
Keywords biogas plant, digestate, membrane distillation, concentrated nutrients and water recovery, thermo-economic analysis