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Undergraduate Project Report - Reducing Mixing Effects In Water Storage Tanks

D. B. Manson (2000)

Clean drinking water is becoming more and more of a world wide problem, particularly in developing countries. Rain water harvesting is a technique which is creating growing interests as it is decentralised, locally manageable and relatively cheap to implement. However one barrier to its widespread use, is the issue of health, particularly with regard to bacteria washed in from the roof. There is substantial evidence that this bacteria die off after a period of storage in dark conditions so it is advisable to add water to one part of a tank and take it from another, while not allowing the water to mix.

This report looks at the methods of mixing in water tanks and describes experiments performed on a cylindrical model tank with a negatively buoyant water source discharging through various input arrangements.

Mixing was found to be confined to the bottom of the tank with the height of the mixing front proportional to the product of the Froude number and the inlet diameter, the constant of proportionality changing with different input arrangements. Recommendations are made regarding input and output arrangements based on these results and on observations made during the experiments.

 

Full Document: (54pp, 360kb) (PDF Document)