Anaerobic digestion of chrome-tanned leather waste for biogas production

Abstract

The introduction of cleaner technologies through the reuse of wastes, producing a clean fuel (biogas) and its utilization for energy recovery, can improve the environmental performance of the tannery industry. The degradability assessment of collagenic substrates (tanned or not) is necessary to evaluate the possibility/need of using prior treatments or mixtures of tannery wastes before its use as a growing substrate for anaerobic degradation processes. In this work, the biodegradation mechanism was studied by comparing collagenous substrates containing different concentrations of chromium (tanning degree) which helps the comprehension of the interrelation effect between microorganisms and substrate. This work presents the results from assays carried out in biorreactors to generate biogas by protein-based substrates at bench scale under controlled conditions. In the bioreactors, four substrates (soybean meal, hydrolyzed collagen, hide powder and wet-blue leather shaving) were inoculated with three different biological sludges collected from wastewater treatment plants (sewage anaerobic sludge, slaughterhouse anaerobic sludge and tannery aerobic sludge). The mole fractions of methane, carbon dioxide, nitrogen and oxygen in the generated gases were evaluated by gas chromatography. The experiments showed a previous adaptation of some of the tested inocula (seeds) and allowed to visualize and discuss the effect of the Chromium III concentration in the substrates over the rate and total biogas production for each of the inocula. The rising of chromium concentration in the substrate significantly reduced the biogas and methane yield. The maximum rate of biogas production for chrome leather shaving occurred in periods between 3 and 36 days, reaching a biogas yield of 162.2 mL g−1 and methane fraction of 73.7%. Better results were obtained using slaughterhouse and tannery sludges due to its preadaptation to collagen based protein substrates. The results indicated a considerable gain in biogas production (74–181%), if the leather substrate is treated or mixed with readily degradable materials prior to its use as substrate for anaerobic digestion (destabilizing the chrome–collagen complexes, increasing of the water dispersion and lowering the stability of chrome–collagen complexes).