Anaerobic cultivation of waste activated sludge to inoculate solid state anaerobic co-digestion of agricultural wastes: Effects of different cultivated periods.

•Waste activated sludge was a potential inoculum of SSAD.•Anaerobic cultivation was indispensable for activated sludge as inoculum of SSAD.•Different anaerobic cultivation time of activated sludge affected SSAD performance.•Ratio of substrate to inoculum affected stability of SSAD process.•Methanosarcina abundance was corresponded with high methane yield.