Biostimulation and bioaugmentation with activated sludge as effective strategies for the remediation of chromium-containing tannery effluents

Tannery effluents contain high concentrations of organic matter, nutrients and toxic compounds, requiring efficient treatment before environmental discharge. This study evaluated the effectiveness of biostimulation (BS) and bioaugmentation (BA) with activated sludge for the remediation of tannery effluents from southern Córdoba (Argentina), which currently receive only primary physicochemical treatment. Under the experimental conditions that yielded the highest treatment performance (initial COD ≈3000 mg/L), BS, BA and their combined application achieved COD removal efficiencies of 83–85% after 5 days of treatment. Total nitrogen and phosphorus removals ranged from 16–21% and 13–18%, respectively. The influence of hexavalent chromium [Cr(VI)] on activated sludge performance was also assessed. Although exposure to 20 mg/L Cr(VI) reduced sludge quality during formation, COD removal remained close to 80% regardless of sludge quality or metal concentration (5 or 20 mg/L). Furthermore, activated sludge removed 42–62% of the added Cr(VI), with higher efficiencies observed in good-quality sludge. These findings demonstrate that activated sludge-based treatment is a robust and effective strategy for tannery wastewater remediation, maintaining high organic matter removal even under chromium stress. However, additional tertiary treatments may be required to further reduce nutrient and chromium concentrations and achieve full compliance with environmental discharge regulations.