Towards microalgal tannery wastewater biorefineries: a review of biomass valorisation opportunities and remediation studies

Significant volumes of tannery wastewater (TWW) are generated during the processing of hides and skins to leather. Phyco-remediation is a promising technology for tertiary remediation of TWW because local discharge standards are often exceeded using conventional physicochemical and biological treatment processes. Wastewater biorefineries promote environmentally conscious circular rather than linear industrial processes. This review promotes the introduction of TWW biorefineries with a focus on tertiary phyco-remediation and biomass valorisation. Studies indicate that Tetradesmus and Chlorella species are particularly suited to growth in TWW and that good tertiary remediation may be expected in full-scale microalgal-bacterial (MAB) systems such as high-rate algal ponds. Removal of soluble chemical oxygen demand ranges from 36% to 93%, while removal of ammonia ranges from 36% to 100%. These results reflect inter-study differences in TWW and operational variables. From a valorisation perspective, the biomass from MAB systems is suited to low value applications such as fertilisers, or feedstocks for biofuels, biogas, or bioplastics. Production of biodiesel from TWW MAB biomass has been widely advocated due the high lipid and fatty acid methyl ester contents of up to 51% and 40%, respectively (>40%). The high protein (37%) and carbohydrates (22%) contents reported in other studies suggest that the biomass could be used as an animal or fish feed supplement. Pilot studies are now required to assess the valorisation efficiencies and biomass character and applications over time in-situ at tanneries in different environments.