Rectangular sedimentation tanks
Construction sites with continuous process wastewater discharge use rectangular sedimentation tanks. Rectangular sedimentation tanks are flow-through tanks. Tanks must be large enough for a minimum hydraulic retention time of 90 minutes.
Minimum hydraulic retention time
The working volume of the sedimentation tank must have sufficient volume to allow for a minimum hydraulic retention time of 90 minutes under peak instantaneous flow conditions. The peak instantaneous flow rate must be the lower value of the maximum allowable discharge rate from the sedimentation tank or the maximum discharge rate allowed by the local sewer utility.
A different way to evaluate the minimum hydraulic retention time is to multiply the maximum instantaneous flow rate by the 90-minute hydraulic retention time to calculate the minimum sedimentation tank volume required.
|Maximum instantaneous flow rate||Minimum hydraulic retention time||Minimum required sedimentation tank volume|
|500 gallons per minute (gpm)||X||90 minutes||=||45,000 gallons|
Additional tanks must be added in parallel (side by side) when discharge rates exceed the capacity of a sedimentation tank. Below are examples of multiple tanks running in parallel for a 45,000 gallon sedimentation tank requirement:
|Tank volume||Number of tanks required||Total tank volume|
|18,000 gallons||►||3||►||54,000 gallons|
|21,000 gallons||►||3||►||63,000 gallons|
|24,000 gallons||►||2||►||48,000 gallons|