ABSTRACT
Surface overflow rate is used to determine loading on clarifiers. It is similar to hydraulic loading rate-flow per unit area. However, hydraulic loading rate measures the total water entering the process (plant flow plus recirculation) whereas surface overflow rate measures only the water overflowing the process (plant flow only). As indicated in the diagram to the right, surface overflow rate calculations do not include recirculated flows. This is because recirculated flows are taken from the bottom of the clarifier and hence do not flow up and out of the clarifier (overflow). Since surface overflow rate is a measure of flow (Q) divided by area (A), surface overflow rate is an indirect measure of the upward velocity of water as it overflow S the clarifier: *
This calculation is important in maintaining proper clarifier operation since settling solids will be drawn upward and out of the clarifier if surface loading rates are too high.**
Other terms used synonymously with surface loading rate are:
Surface Loading Rate, and
Surface Settling Rate
sq ft area sq ft area
Surface Overflow = Rate Area, sq ft 1
Example 1: (Surface Overflow Rate) O A circular clarifier has a diameter of 55 ft. If the primary effluent flow is 2,075,000 gpd, what is the surface overflow rate in gpd/sq ft?
