Vivek Kumar Nair, Vivek Dalvi, Pooja Ghosh, Anushree Malik
Polishing anaerobically treated textile effluent remains a significant challenge because residual organic matter, nutrients, and colour often persist after primary biological treatment. In this context, photogranule-based systems offer a promising low-aeration approach for simultaneous effluent treatment and biomass recovery. Here, we present a rapid mixed algal-cyanobacteria-assisted photogranulation strategy coupled with optimised photoperiods as a promising approach for sustainable effluent treatment and biomass valorisation. Inoculation of activated sludge with a mixed cyanobacterial consortium reduced the granulation time from the conventional 40–70 days to just 25 days, achieving 100% success. These seed photogranules were then applied in batch reactors under variable light–dark regimes. A balanced 3.5 h light: 2.5 h dark cycle emerged as optimal, removing 100% of nitrate, 95% of ammoniacal nitrogen, 90% of phosphate, and 58% of COD, while maintaining high dissolved oxygen without external aeration. Continuous illumination caused photoinhibition, whereas extended dark phases limited treatment efficiency, underscoring the importance of photoperiod balance. Biomass harvested under the optimal cycle exhibited 22% lipid (TS) and 17.6 µg mL -1 chlorophyll, demonstrating clear potential for biofuel and pigment production. Moreover, ICP-MS screening of harvested photogranule biomass revealed systematic enrichment of trace and mineral elements, with preferential accumulation of transition metals such as iron, zinc, manganese, and copper, and negligible levels of heavy metals. Trace element retention was maximised under the balanced photoperiod regime, indicating a strong coupling between photosynthetic activity and metal immobilisation. Overall, the findings highlight a promising batch-scale approach for energy-efficient polishing of anaerobically treated textile effluent and support further validation under continuous-operation and scale-up conditions, while advancing the UN Sustainable Development Goals 6, 12, and 13 through integrated water reuse, pollution reduction, and the valorisation of residual biomass.