Pond-in-Pond's anaerobic pit performance over conventional anaerobic ponds – A computational fluid dynamics comparison

Waste stabilization ponds (WSP) are recommended to treat wastewater in small communities. However, due to lack of pond hydraulics knowledge and uncertainties in conventional pond designs, a novel design, the Pond-in-Pond (PIP), was proposed. This design combines an anaerobic pit surrounded by berms into an integrated aerobic pond. Although this design promotes a more reliable operation, the unknown factor is the most appropriate configuration. Thus, to improve the anaerobic pit's design, a series of computational fluid dynamics (CFD) simulations were evaluated. In total, 13 configurations were compared, consisting of three different length-to-width ratios (LWR), 1:3, 1:1, and 3:1. Each LWR was simulated with the outflow exiting the pit through 1, 2, 3, or 4 sides; and, a conventional anaerobic pond with a 3:1 LWR was simulated for reference. The assessment was based on 1) the hydrodynamic velocity profile; 2) a computational tracer test using an Eulerian model to identify the short circuit index, hydraulic retention time and hydraulic efficiency; and 3) the solids' retention capacity in the critical path using Helminths eggs as a reference. The CFD simulations showed the 1:1 LWR (square pit) performed better overall when compared to the 1:3 and 3:1 LWR, and much better than the anaerobic pond. The square pit presented lower velocities, larger hydraulic efficiency, and higher solids' retention capacity, settling particles as small as 50 μm, such as Hookworms. Despite the velocity profiles presented statistically significant differences in the number of sides outflowing, the tracer test demonstrated that the differences were negligible. [Display omitted] •Computational Fluid Dynamics (CFD) was successfully applied in the analysis of ponds to compare different configurations.•The Pond-in-Pond’s (PIP) anaerobic pit, proved to perform better than traditional flat-bottom anaerobic ponds.•Different designs for the anaerobic pit were analyzed in terms of number of outflowing sides and length-to-width ratios.•The short-circuit index highlighted that the number of outflowing sides do not represent a significant difference (under 10%).•The 1:1 LWR performed hydrodynamically better and is the most appropriate shape for the PIP’s anaerobic pit.