For those of us trained in wastewater, I find the phrase “rise rate” to be a little confusing. Rise rate is a water term but there are terms we use in wastewater that have the same meaning.

Rise rate is defined as the volume of flow per unit of tank area and has the dimensions of gpm/ft^{2 }where gpm is gallons per minute. Rise rate is used to evaluate the performance and loading on clarifiers. But for wastewater operators we use terms with similar units, such as the hydraulic loading rate, to describe the loading on a trickling filter or rotating biological contactor, for example. The units for the hydraulic loading rate on a trickling filter are gpd/ft^{2} where gpd is gallons per day as compared to the gallons per minute term used in rise rate.

In wastewater treatment we also use the term surface overflow rate (SOR) to determine the loading on clarifiers. The SOR uses the same units (gpm/ft^{2}) as the rise rate and in this comparison we need to realize that both terms apply only to clarifiers.

But, and I think this contributes to some of the confusion, because the SOR can also be calculated in units of gpd/ft^{2}. So the distinction we need to make is this: rise rate always refers to water clarifiers and the units are gpm/ft^{2}. The surface overflow rate refers to wastewater clarifiers and the units can be either gpm/ft^{2} or gpd/ft^{2}.

^{}The area of a circular tank cross section is π × r^{2} where r is the tank radius in feet. The area of a circle can also be calculated as 0.785 × D^{2} where D is the diameter of the tank in feet. The equations for rise rate and surface overflow rate are shown below.

For conventional, gravity water clarifiers, whether circular or rectangular, the typical rise rate is 1.0 gpm/ft^{2}.

For downflow water clarifiers the rise rate should not exceed 2.0 gpm/ft^{2}.

For upflow (sludge blanket) water clarifiers the rise rate should not exceed 1.8 gpm/ft^{2}.

And for inclined plate clarifiers the rise rate should not exceed 2.0 gpm/ft^{2}.

For wastewater clarifiers the typical surface overflow rate is approximately 1,000 gpd/ft^{2}.

The maximum wastewater clarifier surface overflow rate is 1,500 gpd/ft^{2}.