Tannery wastewater as nutrient supply in production of biogas from solid tannery wastes mixed through anaerobic co-digestion

•Anaerobic co-digestion of tannery solid and liquid waste simultaneously.•Tannery wastewater as a nutrient supply.•Low biogas production of 1.9±0.3ml/g of VSS.•Reduction in TOC of 66 %, showing the high efficiency of waste treatment.•Reduction in the cost of wastewater treatment and solid waste disposal.