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

The influence of the presence of the tanning agent in co-digestion of leather shavings mixed with sludge before and after the tanning stage was examined. Chromium and vegetable tannin are the main tanning agents used and the presence of both was evaluated in two orthogonal arrays. The energy efficiency was evaluated through biogas and methane production. The efficiency in waste treatment was evaluated through the variation of TOC, IC, TN and the concentration of the tanning agents. Sludge with chromium proved to be significantly more suitable for AD than sludge without it, producing 27.9 mL of biogas/g of VSS with 60% of methane; a reduction of 62.8% of TOC; and a 325.0% increase of IC, showing that the initial 7.6 mg/L of chromium in these tests was beneficial. Shavings with chromium also proved to be significantly more suitable for AD, even though with a higher initial 16.4 mg/L of chromium, producing 10.7 mL of methane/g of VSS and a 55.2% reduction of TOC, confirming that the concentrations of chromium in the residues were not inhibitory. Shavings with vegetable tannins were unfavorable to the mineralization of the wastes through anaerobic digestion, showing significantly poorer results than shavings without it. They produced 4.1 mL of methane/g of VSS, confirming the establishment of anaerobic digestion. However, they increased the TOC by 23.7%, showing that the initial hydrolytic activity occurred, but the waste was not mineralized by the other phases of the process due to the high initial concentration of 590 mg/L of tannins.