Thermal integration of membrane distillation in an anaerobic digestion biogas plant – A techno-economic assessment

Digestate reject water from biogas production is often recirculated for dilution of source-separated organic waste to yield a suitable feedstock for the digestion process. The total solids (TS) content of the recycled reject water has a large impact on the potential added amount of organic substrate, and thus on the efforts to maximize the capacity of the plant without exceeding the capacity of the pumps. This study assessed the potential to improve the overall efficiency of a full-scale co-digestion plant using thermally integrated membrane distillation (MD) and the possibility of recovering waste heat from the substrate sanitization process for use in the MD system. The results showed that reducing the TS content of recirculated reject water by MD could increase the loading, thus increasing biogas production by 45–50%, and that thermal integration with MD improved the overall energy efficiency of the integrated system. The thermal energy demand for the MD process was supplied by the low-temperature waste heat from the sanitization process (21%) and additional heat from a district heating (DH) network (79%). On using waste energy recovery, the energy demand to heat the reject water for MD was lower than the energy in the additional biogas production. Specific thermal energy demand for the MD system tested ranged from 800 to 1050 kWh/m3 without coolant-side heat recovery, but was only 116 kWh/m3 when heat recovery was possible. The concentration of nutrients from highly diluted reject water (>90% water content) reduced the storage requirement and transportation costs of the bio-fertilizer. The economic assessment indicated that thermal integration of a biogas plant with MD could be economically feasible. However, the lifetime of the MD modules and the impact of fouling need further study.