Assessment of Cyanotoxin Contamination in Treated Wastewater and Its Toxicological Impact on Fish

Abstract

This study explores the application of Oxygenic Photo-Granulation (OPG)-based bioreactors for wastewater treatment (WWT) and evaluates the potential ecological risks of cyanotoxin release. Cyanotoxin contamination in treated wastewater poses a growing environmental and public health risk in Pakistan, where wastewater treatment plants (WWTPs) are often inefficient in removing biological contaminants. This study addresses this research gap by exploring the potential of Oxygenic Photo-Granulation (OPG)-based bioreactors as an eco-sustainable wastewater treatment (WWT) strategy capable of both nutrient recovery and cyanotoxin mitigation. Two 1.8 L sequencing batch OPG bioreactors (R1 and R2) were operated under controlled light/dark cycles without external aeration. The bioreactors contained mixed algal–bacterial granules derived from municipal wastewater, facilitating self-aerated treatment and granule formation. Samples were collected periodically for physicochemical analysis, while toxin extraction was performed using solid-phase extraction followed by GC–MS for organic pollutant identification. Both reactors achieved stable biomass growth (TSS/VSS ≈ 2000 mg/L, VSS/TSS = 0.6–0.8), effective COD removal (85 ± 9 %), and moderate nutrient reduction. Hematological and histopathological analyses in fish exposed to treated and untreated effluents revealed increased RBCs, Hb, and PLTs, alongside decreased WBCs and tissue necrosis in untreated groups. Reactive Oxygen Species (ROS) biomarkers showed higher oxidative stress in untreated fish, confirming toxicity reduction post-treatment. Overall, the study demonstrates that OPG bioreactors not only enhance wastewater treatment efficiency but also significantly reduce cyanotoxin-associated toxicity. These findings highlight the potential of OPG systems as a low-energy, sustainable alternative for wastewater management in Pakistan, emphasizing the need for integrated ecotoxicological monitoring to ensure environmental safety.