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

Globally, waste stabilization ponds (WSPs) in warm climates have demonstrated high treatment efficiencies for organic matter and suspended solids. This study evaluates the stage-wise performance and overall treatment efficiency of the Kalobe WSPs in Mbeya city, Tanzania, which treat approximately 15,000–18,000 m3/day of mixed domestic and industrial wastewater, compared with a design capacity of 28,800 m3/day. The system comprises anaerobic, facultative, and maturation ponds operating in parallel. Field sampling and laboratory analysis were conducted to determine pollutant concentrations across each treatment stage, focusing on key parameters including biochemical oxygen demand (BOD), 5-day, chemical oxygen demand (COD), total suspended solids (TSSs), ammonia, nitrite, and total dissolved solids (TDS). Results revealed that while the WSP system demonstrated high removal efficiencies for organic and solid pollutants, achieving reductions of 58.1% for BOD, 87.3% for COD, and 87.2% for TSS, 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 Tanzania Breweries Limited, introduced intermittent hydraulic shocks, which may impair treatment consistency. The findings highlight the need for system upgrades, enhanced industrial pre-treatment enforcement, and the integration of post-treatment units to improve nutrient polishing. Overall, while Kalobe WSPs remain a cost-effective solution, strategic interventions are necessary to ensure sustained regulatory compliance and environmental protection.