Effluent from a mesophilic liquid AD system fed with food wastes, fats, oils and greases (FOG), and sewage sludge (operated by quasar energy group in Columbus, OH, USA) was used as inoculum. Due to the low TS content (7.7%), the effluent was centrifuged (Thermo Scientific Sorvall Legend T+) at 3500 rpm (2634g) for 30 min to obtain the required TS content of 15%. The decanted liquids were removed from the solids by turning the plastic containers (600 ml for each) upside down and letting the liquid portion run out. The solids attached at the bottom of the container were collected to be used as inoculum for SS-AD. Effluent was kept in air-tight buckets at 4 °C in a walk-in cooler. Prior to use, the inoculum was starved for 1 week and incubated at 37 °C to reactivate microbiological activity and remove the easily degradable VS.