Causal model to describe the variation of faecal coliform concentrations in a pilot-scale test consisting of ponds aligned in series

In this study, the application of path analysis was tested to validate a causal model developed to verify the influence on several factors of the variation of faecal coliform concentration in pilot-scale ponds that treat domestic wastewater under distinct physical and operational characteristics. The ponds, aligned in series, are located in the city of Campina Grande, Northeastern Brazil. The model proved to be efficient for describing the variation in both secondary facultative and maturation ponds. The coefficient of determination (R2) ranged between 0.66 and 0.84 for the facultative ponds and between 0.54 and 0.95 for the maturation ponds. Effluent faecal coliform concentration was negatively affected by solar radiation in shallow ponds and positively affected by influent faecal coliform concentration in deep ponds. Typically the effects related to phytoplankton photosynthetic activity were well characterized in all shallow maturation ponds. Faecal coliform concentration was negatively affected by pH in almost all the cases. This model is not suitable for anaerobic ponds.