Improving hydrodynamic performance of waste stabilisation ponds using three-dimensional numerical models

Waste stabilisation ponds (WSPs) are widely used for wastewater treatment throughout the world. They are shallow constructed basins, typically located at the end of a treatment plant, that use natural microbiological, photosynthetic, biochemical, physico-chemical and hydrodynamic processes to generate a reduction of organic matters and pathogenic organisms in wastewater (Watters et al., 1973). They require little technical attention during operation, and are less demanding in terms of construction cost and energy consumption than other engineered wastewater treatment systems. Practical experience and research over the past few decades have established that hydrodynamics is one of the crucial factors determining WSP's overall treatment performance. The Department of Environment and Planning (1992) in the state of Tasmania surveyed 39 wastewater treatment systems and reported that 74% of the pond systems failed to achieve the discharge requirements, and it is due to the hydraulic problems including short-circuiting, stratification in hot Australian climates, and stagnant fluid in dead zones. Therefore, it is of primary importance that WSP's hydrodynamic performance be improved before WSP treatment efficiency can be guaranteed. A substantial number of numerical modelling studies have been undertaken to look into WSP hydrodynamics, both two-dimensionally and three-dimensionally. It is the ultimate goal of this study to use numerical modelling techniques to investigate measures to improve WSP hydrodynamic performance, consequently to propose retrofitting design. A validated three-dimensional numerical model using MIKE 3 by DHI (Danish Hydraulic Institute) was developed to study a typical pond with a dimension of 50 m (length) by 20 m (width) by 1.5 m (depth). The retrofitting scheme was proposed by placing baffles in the pond with different geometric ratios: the ratio of baffle length L_b to the width of the pond W: L_b/W, and the ratio of baffle spacing ?_b to the length of the pond L: ?_b/L. For generalised design guidance, baffles positively contribute to the hydraulic efficiency for ponds with a relatively small L/W ratio. Placing 8 baffles in a pond with L/W = 1.6 results in a ? (hydraulic efficiency) = 0.83 as oppose to ? = 0.23 if the pond is not baffled. However, ponds do not benefit from retrofitting baffles if their L/W ratios are large. Ultimately, this study is to provide regulators, decision makers, water managers and operators with information and tools to best operate and manage WSPs, to protect public and environmental health and optimise uses of the treated water.