Anaerobic co-digestion of tannery effluents: Process optimisation for resource recovery, recycling and reuse in a circular bioeconomy

Abstract

The anaerobic treatment of tannery effluents from the different process stages are limited by the various toxicants that are mainly added as feed chemicals. The segregated effluents present an opportunity for co-treatment to abate inhibition, supplement deficient nutrients and/or promote resource recovery using anaerobic digestion. This study investigated the feasibility of anaerobic co-digestion of beamhouse (BH) and pre-treated tanyard (TY) effluents using the standardised biochemical methane potential (BMP) protocol. It was established that all reactors were active, while those with higher TY compositions and operating at very high/low inoculum to substrate ratios (3<ISR ≤ 2) suffered severe methanogenesis inhibition. Process efficiency and kinetics (maximum CH4 production rate and reaction rate constant) improved with increasing BH composition and/or ISR. The logistic, modified Gompertz and cone model showed a better fit to the experimental cumulative CH4data (0.827 ≤ Adj R2 ≤ 0.999), respectively. The optimal operating conditions (ISR = 2.5, 100% BH and 20 days retention time) demonstrated the feasibility of a circular bioeconomy and net positive tannery operations where 639 mL biogas/gVS (59% CH4), 13% and 18% of the inlet sulphur and nitrogen, respectively, are recoverable as products. The process also recovered reusable process/irrigation water, recyclable digestate as a biofertiliser and/or ceramic aggregate with energy recovery.