Up-rating underground sedimentation tanks subject to hydraulic overloading

The research in this project report is aimed at improving effluent q u a l ities of existing sedimentation tanks which are overloaded either because the incoming flow rate is higher than the design flow or because the incoming suspended solids concentration is higher than that which was predicted in the design stages of the clarifier. Conventional methods of up-rating tanks, usually in the form of rectangular tube modules, were discarded since they do not lend themselves to general design when the intention is to up-rate circular clarifiers. In stead a unique type of conical lamella settler was designed which comprises of one cone stacked on top of the other. The cones can be installed within an existing settler, and in so doing up-rate it. A conventional upward flow tank and tne up-rated version in the form of the rriniral lamella settler were tested in the laboratory. It was found that the conical lamella settler produces considerably better effluent qualities in comparison with the conventional cifirifior, especially at high overflow rates. In addition, the conical lameliti settler is comparatively insensitive to changes in hydraulic loading and influent s us pended solids concentrations making it ideal in absorbing shock loads. A conical lamella settler is modelled mathematically and it is shown that theoretically conical plates are a much more efficient form of settling than conventional rectangular lamella plates. The conical lamella settler can result in substantial savings in cost and space and therefore more research is needed to perfect it and to make it as generally applicable as possible.
A project report submitted to the Faculty of Engineering, University of the Witwatersrand, Johannesburg, in partial fulfilment of the requirements for the degree of Master of Science in Engineering. Johannesburg, 1987