Hydraulic assessment of waste stabilization ponds : Comparison of computational fluid dynamics simulations against tracer data
The overall performance of a wastewater treatment plant is in addition to the effectiveness of the biokinetic processes controlled by the flow dynamics. A wide used technique to characterize the water flow dynamics is the use of tracers. At full-scale tracer experiments in wastewater treatment plants provide insufficient information to completely grasp the flow dynamics. Additionally results are often biased by uncontrolled external factors. As alternative, computational fluid dynamic (CFD) models for performance analysis of existing and the design of new wastewater treatment plants have been increasingly used over the last decade. The paper presents the results of a tracer experiment against the output of a CFD model with application to a 7 ha large maturation pond. The tracer study was conducted with the fluorescence dye Rhodamine WT using the stimulus response technique. The three dimensional CFD model was built considering a variable density distribution of spatial grids based on the specific characteristics of the system. For the unsteady flow simulations the turbulence model k- was adopted. Residence time distribution (RTD) generated by both approaches were compared and showed fairly good agreement. The velocity and turbulence profiles of the CFD model were compared with experimental data confirming, although the high computational demand, the robustness of the used CFD model.
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 Lb to the width of the pond W: Lb/W, and the ratio of baffle spacing Δb to the length of
the pond L: Δb/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 λ (hydraulic
efficiency) = 0.83 as oppose to λ = 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.
Computer-Aided System of Virtual Testing of Gas Turbine Engines
The article describes the concept of a virtual lab that includes subsystem of gas turbine engine simulation, subsystem of experiment planning, subsystem of measurement errors simulation, subsystem of simulator identification and others. The basis for virtual lab development is the computer-aided system of thermogasdynamic research and analysis “ASTRA”. The features of gas turbine engine transient modes simulator are described. The principal difference between the simulators of transient and stationary modes of gas turbine engines is that the energy balance of the compressor and turbine becomes not applicable. The computer-aided system of virtual gas turbine engine testing was created using the developed transient modes simulator. This system solves the tasks of operational (throttling, speed, climatic, altitude) characteristics calculation, analysis of transient dynamics and selection of optimal control laws. Besides, the system of virtual gas turbine engine testing is a clear demonstration of gas turbine engine working process and the regularities of engine elements collaboration. The interface of the system of virtual gas turbine engine testing is described in the article and some screenshots of the interface elements are provided. The developed system of virtual gas turbine engine testing provides means for reducing the laboriousness of gas turbine engines testing. Besides, the implementation of this system in the learning process allows the diversification of lab works and therefore improve the quality of training.