Enhancing the co-digestion efficiency of sewage sludge and cheese whey using brewery spent grain as an additional substrate

•The co-digestion of sewage sludge and cheese whey was not sufficiently efficient.•Using an additional substrate (brewery spent grain) enhanced the biogas yields.•The highest biogas yield of 0.54 m3 kg−1 VS was found in three-component system.•In BSG presence, kinetic was reduced as compared to the two-substrate co-digestion.•The energy profit increased by 160% in three-component system at HRT of 16.7 d. This study examined the influence of the application of brewery spent grain (BSG) on biogas production efficiency as well as its kinetics in the co-digestion of acid cheese whey (ACW) and sewage sludge (SS). The experiment was conducted in semi-flow anaerobic reactors under mesophilic conditions (35 °C) with different hydraulic retention times (HRT) of 16.7 d, 18 d and 20 d. The results indicate that the addition of BSG significantly enhanced the biogas yields, ensuring good process stability. The highest value of 0.54 m3 kg−1 VSadded was obtained at HRT of 16.7 d, while for ACW and SS it was only 0.50 m3 kg−1 VSadded at HRT 18 d. However, the decrease in the rate constant k occurred (0.07 h−1) as compared to the two-component system (0.096 h−1). The highest energy profit of 160% was enhanced for the three-substrate co-digestion, indicating it as a cost-effective solution.