Floc Roll-Up and Its Implications for the Spacing of Inclined Settling Devices

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Inclined plate and tube settlers are commonly used to make sedimentation tanks more compact. Conventional design equations for inclined settling devices are based on obtaining a desired particle capture velocity; these equations suggest that a suitable capture velocity can be achieved by reducing plate settler spacing or tube settler diameter below that specified in conventional design guidelines. Smaller spacing would reduce capital costs by decreasing the sedimentation tank volume. However, existing literature does not explain why smaller values of plate or tube spacing cannot be used, and failure mechanisms that set the minimum spacing recommended in design practice have not been documented. This research shows that the fluid velocity gradient at the tube or plate surface is the limiting constraint for spacing, and for very small spacing, particles that settle on the solid surface are carried up the incline. This failure mode, termed “floc roll-up,” occurs when the terminal velocity of a floc along the incline is less than the upward velocity of the fluid at the center of the floc. This article presents an experimental and theoretical investigation of the floc roll-up failure mode and its implications for plate settler design. A model was developed to explain the physics of floc roll-up, and experiments in a bench-scale water treatment apparatus demonstrated its effects on sedimentation performance. When subject to an increasing velocity gradient between the surfaces, turbidity removal in an experimental tube settler decreased from 90% to near zero, even when the traditional design equations would predict no degradation in performance. Both the model predictions and experimental results show that under some conditions, inclined settlers can employ closer spacing than the typical 5-cm design value without sacrificing turbidity removal performance.