Answer: to design two equally sized rectangular settling basins for treating a total flow rate of 2.5 MGD using an overflow rate of 500 gpd/ft2 and a detention time of four hours, each basin should have dimensions of 20.91 ft by 83.64 ft and a weir loading rate of 55.32 gpd/ft.
Step-by-step explanation:
To design the settling basins, we can use the following steps:
Step 1: Calculate the surface area of each basin
The total flow rate is 2.5 MGD, which is equivalent to 3,472.2 ft3/hr. The detention time is 4 hours, so the volume of each basin is:
Volume = Flow rate × Detention time
Volume = 3,472.2 ft3/hr × 4 hr
Volume = 13,888.8 ft3
Assuming a length to width ratio of 4:1, we can write:
Length = 4 × Width
The surface area of each basin can be calculated as:
Surface area = Volume / Depth
Surface area = Volume / (Length × Width / 500)
Surface area = 13,888.8 ft3 / (4 × Width × Width / 500)
Surface area = 173.61 / Width
Step 2: Calculate the dimensions of each basin
Since we want both basins to be equally sized, we can set the surface area of each basin to be the same:
173.61 / Width = 173.61 / Width
Width = 20.91 ft
Length = 4 × Width = 83.64 ft
Therefore, each basin should have dimensions of 20.91 ft by 83.64 ft.
Step 3: Calculate the weir loading rate
The effluent weir length in each basin is three times the basin width. Therefore, the effluent weir length is:
Weir length = 3 × Width = 62.73 ft
The weir loading rate can be calculated as:
Weir loading rate = Flow rate / Weir length
Weir loading rate = 3,472.2 ft3/hr / 62.73 ft
Weir loading rate = 55.32 gpd/ft
Therefore, the weir loading rate for each basin is 55.32 gpd/ft.
In summary, to design two equally sized rectangular settling basins for treating a total flow rate of 2.5 MGD using an overflow rate of 500 gpd/ft2 and a detention time of four hours, each basin should have dimensions of 20.91 ft by 83.64 ft and a weir loading rate of 55.32 gpd/ft.