Clarifier and filter loading rates
surface overflow rateSORdetention timefilter loading ratesolids loadingbackwash
Detention time, surface overflow rate, solids loading, filter rate and backwash water — the loading rates that decide plant capacity.
Hydraulic Detention Time
Theoretical detention time of a tank, clarifier or contact basin: the working volume divided by the flow passing through it.
Surface Overflow Rate
Surface overflow (surface loading) rate of a settling basin — flow divided by plan area, reported here in metres per day.
Clarifier Solids Loading Rate
Solids applied to a secondary clarifier per unit of surface area, in kg of MLSS per square metre per day, including the return flow.
Filtration Rate (Filter Loading Rate)
Filtration rate through a granular media filter: flow divided by filter bed area, the approach velocity reported in metres per day.
Backwash Water Volume
Water consumed by one filter backwash, from the backwash rise rate, the filter bed area and the duration of the wash.
Percent Backwash Water
Share of a filter's production consumed by its own backwash, the housekeeping ratio that flags short runs and poor coagulation.
How they fit together
These are all flow divided by something — by volume for detention time, by surface area for overflow and filtration rate, by area again for solids loading with the solids weight included. Surface overflow rate has a physical meaning worth knowing: expressed in gpd/ft² it is a velocity, the upward water velocity a particle must out-settle to be captured. Any particle settling slower than the overflow rate leaves over the weir no matter how long the tank is.
Size a primary or a water plant clarifier on overflow rate, size a secondary clarifier on both overflow rate and solids loading — a secondary usually fails on solids first, because it has to thicken as well as clarify. Detention time is the weakest of the three and should be treated as a description rather than a design basis, since it assumes plug flow that no real tank achieves; short-circuiting means the water that matters spends far less time in the tank than the arithmetic says. On filters, watch percent backwash water: above about 4 to 6% of production you are either running the wash too long or the filter is blinding early, and that water is treated water you are sending back to the head of the plant.