Performance evaluation and mathematical modelling of coliform die-off in tropical and subtropical waste stabilization ponds

The paper investigates the coliform removal in 33 facultative and maturation ponds in Brazil. The ponds were located in different parts of the country, with climates ranging from tropical to subtropical and latitude from 7 to 24°S. The ponds had different physical configurations, temperature and detention times. The total number of data used in the study, mostly comprising long-term averages, was 66. Two flow regimes were investigated: CSTR and dispersed flow. In the dispersed-flow model, the dispersion number was estimated using the formulae by Agunwamba et al. and Yanez, and both were found to give similar results. The coliform die-off coefficient Kb was correlated with the pond depth and hydraulic detention time in the dispersed-flow model (R2=0.847). A regression equation, using theoretically generated data, was proposed, correlating Kb for the CSTR model with Kb for dispersed-flow model, using the length-to-breadth ratio and detention time of the pond (R2=0.997). Use of this equation yields virtually the same removal efficiency values as those obtained directly by the dispersed-flow model. Utilisation of the Kb values calculated by the proposed regression models gave a very good prediction of the log effluent coliform concentration of the 33 ponds (R2=0.951 for the CSTR model and R2=0.959 for the dispersed-flow model). Applying the proposed model for dispersed flow, it was concluded that, for a given removal efficiency, a shallow pond, due to its larger Kb, requires less surface area, compared to a deep pond, even though the latter has a higher detention time.