Microfaunal and microalgal community dynamics during bioaugmentation of tannery effluents with activated sludge

Activated sludge systems are widely used for the treatment of effluents with high organic loads. In this study, tannery effluents were treated through bioaugmentation with activated sludge under mesocosm conditions. After 10 days of treatment, significant removal efficiencies were achieved for key physicochemical parameters, including COD (80%), BOD₅ (85%), total chromium and Cr(VI) (87% and 83%, respectively), sulfates (78%), and phenols and total suspended solids (100%). Overall removal efficiencies ranged from 70 to 100%, with several parameters approaching or reaching regulatory limits. The structure and temporal dynamics of microfaunal and microalgal communities were analyzed in both activated sludge and clarified effluent. Community composition changed over time, with an initial dominance of swimming ciliates and Cyanobacteria under high organic load conditions, followed by an increase in flagellates, sessile and crawling ciliates, and Euglenophyta as physicochemical conditions improved. Principal component analysis (PCA) revealed that most physicochemical parameters were associated with swimming ciliates and Cyanobacteria during the initial stages of treatment, whereas dissolved oxygen and pH were correlated with flagellates, amoebae, and Euglenophyta at later stages. These findings suggest that microbial community dynamics were mainly driven by treatment time and the associated environmental changes rather than by the treatment condition itself. This study provides a comprehensive evaluation of bioaugmentation efficiency for tannery effluent treatment and demonstrates the relevance of microfaunal community dynamics as indicators of treatment progression and system stabilization. In addition, it offers novel insights into the structure and potential bioindicator role of microalgal communities, which remain poorly documented in activated sludge systems treating tannery effluents.