Waste Water Treatment Efficiency of Selected Aquaculture Farms and Its Implications for Receiving Water Bodies in Oyo State

Aquaculture effluent discharge is a growing source of nutrient enrichment and biological oxygen demand loading in freshwater bodies across Nigeria's inland states. This study assessed the wastewater treatment efficiency of ten commercial catfish and tilapia farms in Oyo State, South West Nigeria, and evaluated the water quality impact on receiving rivers and streams. Influent and effluent water quality parameters including total suspended solids, biochemical oxygen demand, ammonia-nitrogen, total phosphorus, and coliform counts were measured at each farm over a three-month period. Treatment systems assessed included settling ponds, constructed wetland filters, and direct discharge configurations. Downstream and upstream water quality at receiving water bodies was compared using standard statistical tests. Farms with constructed wetland treatment systems achieved 62 to 78 percent reduction in biochemical oxygen demand and 55 to 70 percent reduction in total phosphorus. Farms practising direct discharge without treatment showed effluent BOD values exceeding FEPA standards by a factor of four. Receiving water bodies adjacent to untreated discharge farms recorded dissolved oxygen depression, elevated nitrate, and elevated coliform counts that persisted up to 500 metres downstream. Regression analysis confirmed that farm stocking density was the strongest predictor of effluent nutrient load. Nine of ten farms lacked formal environmental management plans. The study recommends mandatory effluent treatment as a condition of farm licensing in Oyo State, with constructed wetland systems prioritised as cost-effective treatment solutions for small and medium-scale aquaculture. Keywords: aquaculture effluent, wastewater treatment, Oyo State, water quality, constructed wetlands.