Hydraulic Modeling and Optimization of Waste Stabilization Pond Design for Developing Nations

Wastewater stabilization ponds (WSPs) have been identified and are used extensively to provide wastewater treatment throughout the world. It is often preferred to the conventional treatment systems due to its higher performance in terms of pathogen removal, its low maintenance and operational cost. A review of the literature revealed that there has been limited understanding on the fact that the hydraulics of waste stabilization ponds is critical to their optimization. The research in this area has been relatively limited and there is an inadequate understanding of the flow behavior that exists within these systems.

This work therefore focuses on the hydraulic study of a laboratory-scale model WSP, operated under a controlled environment using computational fluid dynamics (CFD) modelling and an identified optimization tools for WSP. A field scale prototype pond was designed for wastewater treatment using a typical residential institution as a case study. This was reduced to a laboratory-scale model using dimensional analysis. The laboratory-scale model was constructed and experiments were run on them using the wastewater taken from the university wastewater treatment facility. The study utilized Computational Fluid Dynamics (CFD) coupled with an optimization program to efficiently optimize the selection of the best WSP configuration that satisfy specific minimum cost objective without jeopardizing the treatment efficiency.

This was done to assess realistically the hydraulic performance and treatment efficiency of scaled WSP under the effect of varying ponds configuration, number of baffles and length to width ratio. Six different configurations including the optimized designs were tested in the laboratory to determine the effect of baffles and pond configurations on the effluent characteristics. The verification of the CFD model was based on faecal coliform inactivation and other pollutant removal that was obtained from the experimental data. The results of faecal coliform concentration at the outlets showed that the conventional 70% pond-width baffles is not always the best pond configuration as previously reported in the literature. Several other designs generated by the optimization tool shows that both shorter and longer baffles ranging between 49% and 83% for both single and multiobjective optimizations could improve the hydraulic efficiency of the ponds with different variation in depths and pond sizes.

The inclusion of odd and even longitudinal baffle arrangement which has not been previously reported shows that this configuration could improve the hydraulic performance of WSP. A sensitivity analysis was performed on the model parameters to determine the influence of first order constant (k) and temperature (T) on the design configurations. The results obtained from the optimization algorithm considering all the parameters showed that changing the two parameters had effect on the effluent faecal coliform and the entire pond configurations.

This work has verified its use to the extent that it can be realistically applied for the efficient assessment of alternative baffle, inlet and outlet configurations, thereby, addressing a major knowledge gap in waste stabilization pond design. The significance of CFD model results is that water and wastewater design engineers and regulators can use CFD to reasonably assess the hydraulic performance in order to reduce significantly faecal coliform concentrations and other wastewater pollutants to achieve the required level of pathogen reduction for either restricted or unrestricted crop irrigation.