Biogas production for anaerobic co-digestion of tannery solid wastes under presence and absence of the tanning agent

•Co-digestion of leather shavings mixed with sludge before and after the tanning.•Wastes anaerobically biodegradable with a methane potential of 4.1–11.3mL/g.•Low concentrations of chromium produced 3 times more biogas than its absence.•Inhibitory effect of vegetable tannin where the residue underwent only hydrolysis.