Predictions made more realistic

Rob Harwood, principle CFD engineer, environmental, with Fluent Europe looks at the way new CFD modelling techniques are helping advance the understanding of CSO performance

For more than a decade computational fluid dynamics (CFD) simulation techniques have been applied to the design and performance assessment of combined sewer overflow (CSO) chambers.

The increase in the use of CFD-based analysis techniques over this period has been driven by, and coupled with, several factors. These include increasingly stringent performance standards imposed by the regulator which need to be implemented within specific time scales. Such standards have demanded an understanding of the performance of existing and proposed structures and assessments of the impact on performance of remedial measures retrofitted to unsatisfactory CSO chambers. Business-based drivers include the need to 'sweat the assets' which has placed increased pressure on existing chambers to deal with ever higher flow rates whilst still being able to satisfy the demands of the regulator.

IT developments have seen the emergence of specialised, user-friendly commercial CFD software packages coupled with the appropriate hardware at realistic costs. This has brought the technology to a wider client base. Models have also developed. The implementation of more robust, accurate and appropriate physical models (for example Eulerian-Eulerian and Eulerian-Granular models) within existing commercial software has enabled ever more realistic problems to be addressed.

Early work using CFD focused principally on the hydraulic performance of CSOs and the ability of these chambers to retain discrete particles within the sewer system. The latter approach, known as particle tracking, whilst limited to non-directly interacting particles, has yielded design information of significant value and is still widely used in rapid comparative design performance studies. Indeed, the approach has been validated against experimental data and the results published at conferences and in technical journals.

Often, particle tracking is used to predict the path and final destination of a statistically significant number of particles through the CSO chamber simulated, and this data is then used to determine a retention efficiency performance for the chamber. The efficiency data, when derived for a range of particle sizes and chamber inflow rates, can be used to generate a non-dimensional performance cusp for the chamber, and in this way the performance of alternative chamber designs can be compared across the spectrum of operating conditions.

Hydro International, a designer, developer and supplier of non-powered, non-moving part systems for use in various wastewater treatment and storm flow control-related applications, makes extensive use of the Fluent CFD software throughout the design cycle for many of its products. Typical analyses include hydraulic simulation and retention efficiency performance using a particle tracking based approach. Much of this work has been published, for example Faram and Harwood's A Method for the Numerical Assessment of Sediment Interceptors, Water Science and Technology, Vol 47, No. 4, 2003. Pictured above left is an example of how Hydro modelled the performance of baffles to cut blinding on a Hydro-Static screen.

Benefits to be gained by adopting a CFD-based CSO design approach, as opposed to more traditional methods like physical modelling, include:

  • compressed design cycle and, hence, rapid time-to-market for the CSO design,
  • many alternative designs can be considered - including several outside the conventional design envelope,
  • comprehensive quantitative data on the hydraulic performance of the device,
  • movie clips and graphics communicate the design and fundamental flow properties.

In recent years, following advances in software technology and robustness, more complex models have been used to examine the performance of CSO structures. These include the Eulerian-Eulerian multiphase models. Unlike the particle tracking approach, the simulations solve the motion of multiple and interacting phases and hence, are a closer approximation to the behaviour of real sewage particles. In this way problems involving sedimentation and the effect of high particle loading rates on flow patterns can be addressed. It should be noted, however, that these models are still approximations and do not yet account directly for all effects.

Screen tests

Such a modelling approach is of particular relevance to screen design and screen performance as many CSO chambers are now being designed to include (or be retrofitted with) a proprietary screen to reduce the quantity of pollutants discharged to the receiving watercourse.

Haswell, as a consultant to Severn Trent Water, contracted Fluent Europe to carry out a CFD analysis to determine the relative performance of two proposed configurations of non-standard screened CSO chambers, the primary performance criteria being the blinding rate of the screen and the identification of any areas of premature blockage. To fully satisfy the needs of the client, Fluent adopted a Eulerian-Eulerian multiphase modelling approach to represent the water and sewage phases. In the CFD model, the screen was defined such that any sewage particles with a diameter greater than the diameter of the screen hole aperture, were prevented from passing through the screen. A transient analysis was carried out to monitor the build up of sewage particles on the screen with time. A typical result is illustrated (below left) in conjunction with the chamber geometry considered (left). Contours showing solids build up on the screen with time (red is high and blue is low).

The performance of many standard designs of CSO chamber and CSO chamber and screen combinations is well understood. However, little information is available on designs outside the 'standard' envelope. Over the last decade CFD-based simulation has been used on an ever-increasing basis to fill this gap in knowledge. As the industry moves forward, the emphasis on post-project appraisal and on-site performance monitoring is likely to increase. Here too, CFD will be able to aid the designer in addressing any issues identified post-construction



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