Field and laboratory studies of mixing tubes for marine outfalls

Gowda, Ramdas N. (1992) Field and laboratory studies of mixing tubes for marine outfalls. Masters thesis, Memorial University of Newfoundland.

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Abstract

Increase in land costs due to urbanization has led many coastal communities towards marine outfalls as a convenient means of disposing of domestic and non toxic wastes. It has been common practice to use submerged outfalls for small discharges to take advantage of the immediate dilution available as the effluent rises to the surface. The prime objective of the marine outfall installation is to maximize the total dilution of the waste and to minimize impact on the marine environment. Jet pumps or mixing tubes may be used to improve the initial dilution and to promote plume submergence in small outfalls. Theoretical and experimental studies have been carried out but field trials have been very limited. -- Studies have shown that very limited improvement in dilution is obtained with conventional mixing tubes. The basic reason for this is that the combination of increased flow and increased diameter of the mixing tube modify the values of density difference, relative depth and densimetric Froude number. Due to these changes less mixing takes place between the end of the mixing tube and the surface than would have taken place in the original jet discharged without a mixing tube. The combination of increased dilution within the mixing tube and decreased dilution in the buoyant jet has been found to limit the improvement factor of overall dilution to a factor of about two. -- In this thesis experiments on a novel shape or mixing tube are described. This was designed to overcome the problems discussed above and used a transition from a square section at entry to a two dimensional slot at the exit. The study showed that better performance can be achieved at low values of the Froude number. The improvement of overall dilution with a slot mixing tube compared to the circular mixing tube was however, fairly limited. -- Little information is available to describe the field performance or mixing tubes. This lack was rectified by a field study run parallel to the laboratory work. In this study a circular mixing tube was built and installed on a small outfall located at Spaniards bay on the east coast of the island of Newfoundland. Dye studies were done at this installation to check the dilution achieved in the field. Comparisons were made between the performance of a horizontal jet with and without the mixing tube but little improvement in overall dilution was achieved with the mixing tube. It was found that the mixing tubes performed better in the field than was theoretically predicted.

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