The leather industry converts hides (by-product of the meat industry) into value-added products. However, processing of leather generates collagen-based solid wastes such as shavings and offcuts, which consist mostly of collagen and 3-5% chromium oxide. Chromium shavings and leather offcuts are stable towards temperatures of up to 100 degrees C and enzymatic degradation, thus making them hard to digest in a biogas plant. This stability is caused by the structure of collagen and chemical cross-links achieved by Cr3+ salts. These wastes are responsible for the main ecological problem caused by tanneries. In order to enable enzymatic degradation for producing biogas, a pre-treatment of the wastes was carried out by extrusion to denature collagen. Biogas production of untreated shavings, gelatin, extruded shavings, and extruded offcuts were investigated with different substrate to inoculum ratios. In some batches, diauxie was observed. Since this could be a consequence of excessive production of acids, volatile fatty acids were measured. It could be shown that extruded chromium leather wastes can produce biogas. Reduced loads of substrate in the bioreactors lead to higher biogas yields, and diauxie in biogas production may be caused by the accumulation of acetic acid and an excess of propionic acid. (C) 2019 Elsevier Ltd. All rights reserved.