By knowing the time required for treated soil particles to settle out of a water column, we can calculate the required settling basin Surface Area (SAmin) for a given flow rate, as follows:
Step 1. Convert design flow rate from gallons per minute to cubic feet per second Q=500gpm x 0.00222801= 1.11 cf/s
Step 2.
Mix soil from your site with the Polyacrylamide (PAM) chosen for your site in clean water. Agitate the mixture for the time specified in the soil test results as “Reaction Time”. Stop agitating the mixture, and simultaneously start a timer, to measure the time it takes for the water column (of known height) to clear. Use that duration to calculate the settling rate, using the equation:
Vall=Y÷T
Where:
Vall = Velocity for all visible particles to settle (feet/sec.)
Y = Height of water column (feet)
T = Time for particulates to fall distance Y (seconds)
Using a 1-foot high water column, the time for all visible soil particles to settle to the bottom is tested 3 times. For our example, a silt loam, with a pinch of SiltStop 712 Floc Log took 240 seconds to clear a 1-foot water column.
Vall=1/(240 seconds)= .004167ft/sec
Step 3.
In this example, the required Surface Area (SAmin) is calculated as:
Area of Settling Basin=1.2 x 1.11cfs/.004167=
320 sq.ft.
Note: Once stuck to a surface, flocculated soil particles will stay there.
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