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

Abstract 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 ...