A wall or plate is placed in an open channel to measure or control flow, from settling tanks and clarifiers, to ensure a uniform flow rate and avoid short-circuiting. The Weir loading rate is the flow rate of clarifier effluent, divided by the length of the overflow weir.
It is argued that if weir loading rates are higher than recommended, the velocity of currents approaching the weirs may be high enough, to carry excessive solids over the weir.
What is weir loading rate in Wastewater Treatment Plants?
A weir is a device that measures the flow of wastewater. It is a type of obstruction that is used to raise the level of flow, and distribute it evenly. It enables flow measurement to be easily calculated.
Weirs are commonly used in wastewater treatment plant processes to "skim off" the flow of water, from the sedimentation process before the water travels to the filters. Overflow rates for weirs are commonly expressed in gallons per minute per foot of weir. They are also given in million gallons per day per foot of weir.
How to calculate weir overflow?
The amount of water leaving the settling tank per linear foot of water is referred to as the weir overflow rate (or weir loading rate). This calculation's output can be compared to design.
A settling tank is typically designed with weir overflow rates ranging from 10,000 to 20,000 gpd/ft2. The weir overflow rate can be calculated as follows:
Weir loading rate in gpd/ft = Flow in gpd / weir length in ft
Use total weir feet to calculate weir circumference. = 3.14 x weir diameter, ft
Let’s understand it with an example!
A circular clarifier has a weir around its circumference and a diameter of 100 feet. The flow rate of the effluent is 2.22 MGD. What is the overflow rate of the weir in gallons per day per foot (gpd/ft)?
1: Determine the length of the weir first:
· Total feet weir = 3.14 x Diameter, ft
· Total feet weir = 3.14 x 100 ft
· Total feet weir = 314 ft
2: Convert the flow from MGD to gpd:
2.22 MGD x (1,000,000 gal/1MG) = 2,220,000 gpd
Now determine the weir overflow rate in gpd/ft:
Weir loading rate, gpd/ft = flow, gpd
Weir length, ft
Weir loading rate, gpd/ft = 2,220,000 gpd/314
Weir loading rate, gpd/ft = 7006.36 gpd/ ft
Let’s understand with one more example!
A sedimentation basin has 125 feet of weir. When the flow is 2,055,000 gpd, what is the weir overflow rate in gpd/ft?
There are no pre-conversions required with this one; simply plug and play.
Weir loading rate in gpd/ ft = 2,055,000 gpd
125 ft
Weir loading rate, gpd/ft = 16440gpd/ ft
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