Modeling Faecal Coliform Inactivation Time in Waste Stabilization Ponds Using Cox Proportional Hazards Regression

This study applies Cox proportional hazards regression to model faecal coliform inactivation in a pilot-scale waste stabilization pond (WSP) system, focusing on the influence of environmental covariates, including solar radiation, air temperature, and wind speed, on retention time required for complete disinfection. Experimental data were collected over a five-month period from a treatment facility located in southeastern Nigeria, and faecal coliform counts of the pilot pond were analyzed using the Most Probable Number (MPN) technique. A series of Cox models were fitted to determine the statistical significance and predictive strength of each environmental factor. The results show that air temperature was the most significant predictor of survival time (p < 0.0001), with a Nagelkerke pseudo R² of up to 0.972, indicating high explanatory power. Survival curves derived from the model reveal that retention times required for >99% confidence of complete faecal coliform inactivation range from 5.4 days at 24°C to 2.4 days at 29°C, illustrating the critical role of ambient temperature in driving disinfection efficiency. Solar radiation and wind speed, while individually significant in some models, exhibited inconsistent effects when combined with temperature. These findings align with previous literature emphasizing temperature-dependent microbial activity and highlight the practical value of integrating survival analysis into WSP design. The study underscores the need to consider local climate conditions in optimizing pond retention time and improving public health protection through wastewater treatment.