Recovery of methane from tannery sludge: the effect of inoculum to substrate ratio and solids content

Anaerobic batch experiments were conducted at mesophilic temperatures to investigate the possibility of biogas recovery from tannery sludge originating from a tannery wastewater treatment process. According to preliminary findings, the highest biogas yield was obtained as 0.172 L biogas/g VSadded within an inoculum to substrate (I:S) ratio of 1:10 and at solids content of 10.6 %. The highest methane yield of 0.089 L CH4/g VSadded was achieved within an I:S ratio of 1:2 during the experiments. Relatively lower biogas and methane yields could be attributed to the low organic content of raw sludge cake. Potential inhibitors of anaerobic digestion, such as heavy metals, particularly chromium, chloride and sulfate available in raw sludge cake did not seem to influence the process adversely. Despite lower methane yields, it seemed possible to employ anaerobic digestion process for stabilization and energy recovery from tannery sludge, since the biogas obtained could further be used in the wastewater treatment plant as a source of energy. Co-digestion of tannery sludge with another type of substrate available (e.g., tannery solid wastes) can be an alternative to achieve higher biogas and methane yields considerably.