Year

1990

Degree Name

Doctor of Philosophy

Department

Department of Civil and Mining Engineering

Abstract

A study of the behaviour of sedimentation basins was conducted using both numerical and experimental model simulations. Two specific effects to be considered were the effect of wind action, and the effect of density stratification. The aim was to confirm that these effects are detrimental to the ability of a basin to remove particles from a suspension by the gravitational settlement process.

The numerical model developed for this study used a penalty function technique incorporated in a finite difference scheme, This allowed the system to be solved explicitly, thus obviating the need to store and solve matrices, as is the case with implicit schemes. The model was validated by comparisons of the results with standard flows and experimental data.

An experimental wind-water tank facility was constructed so that experimental simulations could also be carried out. Moreover, instrumentation (Laser Doppler Anemometer) was designed and assembled so that accurate measurements could be taken. The tank facility is approximately a one-sixteenth scale model of a typical field size sedimentation tank, as would commonly be found in a water treatment plant. By changing the fan around, the wind direction could be reversed, so that both co-current and counter-current wind cases could be modelled.

To determine "point" suspended solids concentrations, an optical fibre turbidity probe was employed. It was found that the diatomaceous earth used as sediment gave an excellent linear correlation between turbidity and suspended solids. Thus it was possible tc accurately convert the point turbidity readings into suspended sediment concentration.

The results indicated that, as wind speed increased, sediment removal efficiency decreased, with the counter-current case slightly worse than the co-current case, The trends were fairly linear for all the test runs made, indicating uniform reductions in efficiency for increasing wind speeds. As sediment concentration was increased the removal efficiency increased. Removal efficiency also increased as the flow rate was decreased.

When no wind was blowing removal efficiency dropped noticeably in the presence of stratification, When wind was present the induced flow circulation prevented the formation of stratification, and so the results were very similar to those when no stratification was attempted at all.

The overall comparison of the numerical and experimental values was good, indicating that the numerical model can give reasonable estimates of basin performance even with the presence of wind and/or stratification.

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