Biological Solubilisation of Leather Industry Waste in Anaerobic Conditions: Effect of Chromium (III) Presence, Pre-Treatments and Temperature Strategies

Collagen-based polymers and their blends have attracted considerable interest for new materials development due to their unique combination of biocompatibility, physical and mechanical properties and durability. Leather, a modified natural biopolymer made from animal rawhide and the first synthetic collagen-based polymer known since the dawn of civilization, combines all these features. Rawhide is transformed into leather by tanning, a process in which the collagen is cross-linked with different agents to make it stronger and more durable and to prevent its decay. Research on the development of environmentally friendly procedures and sustainable materials with higher efficiency and lower costs is a rapidly growing field, and leather industry is not an exemption. Chrome-tanned and vegetable-tanned (chromium-free) shavings from the leather industry present a high content of organic matter, yet they are considered recalcitrant waste to be degraded by microbiological processes like anaerobic digestion (AD), a solid technology to treat organic waste in a circular economy framework. In this technology however, the solubilisation of organic solid substrates is a significant challenge to improving the efficiency of the process. In this context, we have investigated the process of microbial decomposition of leather wastes from the tannery industry to search for the conditions that produce optimal solubilisation of organic matter. Chrome-tanned and chromium-free leather shavings were pre-treated and anaerobically digested under different temperature ranges (thermophilic–55 °C-, intermediate–42 °C- and mesophilic–35 °C) to evaluate the effect on the solubilisation of the organic matter of the wastes. The results showed that the presence of chromium significantly inhibited the solubilization (up to 60%) in the mesophilic and intermediate ranges; this is the fastest and most efficient solubilization reached under thermophilic conditions using the chromium-free leather shaving as substrates. The most suitable temperature for the solubilization was the thermophilic regime (55 °C) for both chromium-free and chrome-tanned shavings. No significant differences were observed in the thermophilic anaerobic digestion of chromium-free shavings when a pre-treatment was applied, since the solubilisation was already high without pre-treatment. However, the pre-treatments significantly improved the solubilisation in the mesophilic and intermediate configurations; the former pre-treatment was better suited in terms of performance and cost-effectiveness compared to the thermophilic range. Thus, the solubilisation of chromium-free tannery solid wastes can be significantly improved by applying appropriate pre-treatments at lower temperature ranges; this is of utter importance when optimizing anaerobic processes of recalcitrant organic wastes, with the added benefit of substantial energy savings in the scaling up of the process in an optimised circular economy scenario