Sulfate addition as an effective method to improve methane fermentation performance and propionate degradation in thermophilic anaerobic co-digestion of coffee grounds, milk and waste activated sludge with AnMBR

• Thermophilic anaerobic co-digestion of coffee wastes was studied with AnMBR.
• Propionate accumulation was a problem to limit the loading rate of process.
• Sulfate addition was found as effective tool to improve operation performance.
• Effect of sulfate on propionate degradation was analyzed.

This study was conducted to investigate the effects of sulfate on propionate degradation and higher organic loading rate (OLR) achievement in a thermophilic AnMBR for 373 days using coffee grounds, milk and waste activated sludge (WAS) as the co-substrate. Without the addition of sulfate, the anaerobic system failed at an OLR of 14.6 g-COD/L/d, with propionate accumulating to above 2.23 g-COD/L, and recovery by an alkalinity supplement was not successful. After sulfate was added into substrates at a COD/SO42− ratio of 200:1 to 350:1, biogas production increased proportionally with OLR increasing from 4.06 to 15.2 g-COD/L/d. Propionic acid was maintained at less than 100 mg-COD/L due to the effective conversion of propionic acid to methane after the sulfate supplement was added. The long-term stable performance of the AnMBR indicated that adding sulfate was beneficial for the degradation of propionate and achieving a higher OLR under the thermophilic condition.

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