Thermophilic anaerobic digestion of coffee grounds with and without waste activated sludge as co-substrate using a submerged AnMBR: System amendments and membrane performance

•Lack of nitrogen and trace metals caused the failure of coffee grounds digestion.•A long-term operation was established by using sludge as co-substrate.•Additions of NH4HCO3 and trace metals overcame the unstable problem.•Stable operation of membrane was successful with TS in reactor up to 75g/L. Coffee grounds are deemed to be difficult for degradation by thermophilic anaerobic process. In this research, a 7L AnMBR accepting coffee grounds was operated for 82days and failed with pH dropping to 6.6. The deficiency of micronutrients in the reactor was identified. The system was recovered by supplying micronutrient, pH adjustment and influent ceasing for 22days. In the subsequent 160days of co-digestion experiment, waste activated sludge (15% in the mixture) was mixed into coffee grounds. The COD conversion efficiency of 67.4% was achieved under OLR of 11.1kg-COD/m3d and HRT of 20days. Tannins was identified affecting protein degradation by a batch experiment. Quantitative supplements of NH4HCO3 (0.12g-N/g-TSin) were effective to maintain alkalinity and pH. The solid concentration in the AnMBR reached 75g/L, but it did not significantly affect membrane filtration under a flux of 5.1L/m2h. Soluble carbohydrate, lipid and protein were partially retained by the membrane.