Profiling and modelling of thermal changes in a large waste stabilisation pond

Abstract

A thermal profiling study was undertaken at four depths at each of nine sites, and at the inlets and outlets of a large waste stabilisation pond (WSP). Results were collected simultaneously using a network of 42 thermistors and dataloggers. Profiles at each site were categorised as either "stratified" or "unstratified", and persistence analysis was used to determine the frequency and persistence of stratification events at each of the nine sites. Stratification was found to persist most strongly at the site furthest upwind in the WSP, with respect to prevailing wind during the study, leading to the conclusion that stratification induced short-circuiting will be greatest in this region of the WSP. A computational fluid dynamics (CFD) model was constructed of the WSP, including an energy balance to predict the bulk stratification gradient in the pond. Environmental conditions and WSP inlet temperature during one day in June 2001 were used as boundary conditions. The pond thermal profiles measured during the profiling study, together with outlet temperature during the day, were used to validate the CFD model results. The model predicted mean pond temperature with a high degree of accuracy (r2 = 0.92). However it was evident that even modest winds (> or = 1.5 m/s) partially broke down stratification, leading to poor prediction of the gradient by the CFD model, which did not directly account for the impact of wind shear stress on mixing in the WSP.

Nitrification potential in waste stabilisation ponds: comparison of a secondary and tertiary pond system

“…The Bolivar WSPs are located north of Adelaide, South Australia (34845 0 23 00 S 138834 0 15 00 E) and comprise six ponds operating in two parallel trains covering a total area of around 350 ha; treating effluent from a population equivalent of 800,000 people. The first and largest pond (Pond 1) was selected for this research (see Sweeney et al (2005a) for detailed description). With a nominal operating depth of 1.3 m and an hydraulic residence time of 12 -15 days, the tertiary-level facultative WSP receives effluent from an AS plant.…”

“…Samples of sediment and overlying wastewater (5 L) were also collected using a tubular sediment corer (3 cm £ 130 cm) and transported to the laboratory. In the laboratory, samples (wastewater or wastewater plus sediment) were incubated in water bath acclimated microcosms under dark conditions at 258C-a temperature similar to that at the sample sites during summer (Herdianto 2003;Sweeney et al 2005a). Microcosm inorganic N concentrations were augmented either with 10 mg NO 2 -N L 21 or 100 mg NH 4 -N L 21 .…”

Significance of buoyancy in turbulence closure for computational fluid dynamics modelling of ultraviolet disinfection in maturation ponds

“…Computational fluid dynamics (CFD) modelling of hydrodynamic flows and the implementation of disinfection models to maturation ponds have been developing (Wood et 3 al., 1995(Wood et 3 al., , 1998Shilton and Harrison, 2003;Shilton and Mara, 2005;Sweeney et al, 2005;Abbas et al, 2006;Shilton et al, 2008;Badrot-Nico et al, 2009;Alvarado et al, 2012a,b;Dahl et al, 2017b) in parallel with the continual improvements to sunlight kinetic rate models (e.g.…”

“…Turbulent flow modelling of thermal energy was only undertaken by Dahl et al (2017b) for maturation ponds and by Sah et al (2011) for facultative ponds and possibly by Sweeney et al (2005) and Badrot-Nico et al (2009). Laminar flow models have been employed by Wood et al (1995) and Olukanni and Ducoste (2011).…”

“…The most widely used turbulence model is the   k model and has been used by many researchers (e.g. Wood et al, 1998;Peterson et al, 2000;Sweeney et al, 2005;Shilton et al, 2008;Alvarado et al, 2012a,b). At this point, it is noted that the short-comings of unsteady Reynolds-averaged Navier-Stokes (URANS) turbulence models have been discussed in the context of chemical UV reactors (Wols et al, 2010).…”