Stage-wise performance evaluation of the Kalobe waste stabilization ponds in Mbeya, Tanzania: Organic load and nutrient removal efficiency

This study evaluates the stage-wise performance and overall treatment efficiency of the Kalobe Waste Stabilization Ponds (WSP) in Mbeya City, Tanzania, which treat approximately 15,000–18,000 m³/day of mixed domestic and industrial wastewater against a design capacity of 28,800 m³/day. The system comprises anaerobic, facultative, and maturation ponds operating in series. Field sampling and laboratory analysis were conducted to determine pollutant concentrations across each treatment stage, focusing on key parameters including BOD, COD, TSS, ammonia, nitrite, and TDS. Results revealed that while the WSP system demonstrated high removal efficiencies for organic and solid pollutants—achieving 58.1% BOD, 87.3% COD, and 87.2% TSS reductions—its performance in nutrient and dissolved solids removal remained limited. Ammonia was moderately reduced (80.5%), whereas nitrite and TDS exhibited low removal efficiencies of 17.4% and 13.7%, respectively. The final effluent BOD (39 mg/L) and COD (78 mg/L) exceeded Tanzania’s national discharge standards, indicating partial non-compliance. The study also found that flow variations, particularly from industrial contributors such as Pepsi and TBL, introduced intermittent hydraulic shocks, which may impair treatment consistency. The findings highlight the need for system upgrades, enhanced industrial pretreatment enforcement, and integration of post-treatment units to improve nutrient polishing. Overall, while Kalobe WSP remains a cost-effective solution, strategic interventions are necessary to ensure sustained regulatory compliance and environmental protection.