Minimum Standards for Rectangular Sedimentation Tank Design
The King County Industrial Waste Program evaluated the technology readily available for construction dewatering discharges and determined that use of rectangular sedimentation tanks is the minimum required treatment when settleable solids are encountered. The findings of this evaluation are summarized in the technical memorandum Minimum Standards for Sedimentation Tanks Used at Construction Dewatering Sites.
This technical memorandum also identifies circumstances when exemptions to the rectangular sedimentation tank standard are allowed.
Minimum standards for rectangular sedimentation tank design:
The sedimentation tank design must address each of the following:
Minimum Hydraulic Retention Time: ≥ 90 minutes
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 capable pumping rate to 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 use the maximum instantaneous flow rate and the 90 minute hydraulic retention time to calculate the minimum sedimentation tank volume required. For example, if a maximum instantaneous flow rate of 500 gallons per minute (gpm) is expected, then the minimum required sedimentation tank volume would be the following: 500 gpm x 90 minutes = 45,000 gallons
For this example, if an 18,000 gallon sedimentation tank were used, this would require the use of three (3) tanks operated in parallel. If a 21,000 gallon sedimentation tank were used, this would also require the use of three (3) tanks operated in parallel, as two (2) tanks would have an insufficient volume (42,000 gallons).
Minimum Length-to-Width Ratio (length:width): ≥ 4:1
The rectangular sedimentation tank needs to be at least four times (4x) longer than it is wide.
Maximum Overflow Rate (gallons-per-minute per square foot of surface area): ≤ 1.0 gpm/ft2
This is calculated by taking the maximum instantaneous flow rate (gpm) and dividing this value by the surface area of the sedimentation tank (length (ft) x width (ft)).
Maximum Sediment Accumulation (level of sedimentation tank water column): ≤ 25%
For the purpose of evaluating the sediment volume requirement, the 25% criterion is calculated as an average of three values taken from locations that represent three equally-spaced sections of a rectangular sedimentation tank: Inlet Section, Middle Section, Outlet Section. The liquid level height in the tank is determined by the lowest static water level elevation, which for most gravity discharges is determined by the elevations of the outlet pipe invert or of an overflow weir.
Requirement to Add Additional Sedimentation Tanks In Parallel For Higher Flow Rates:
When discharge rates exceed the capacity of a sedimentation tank, one or more additional sedimentation tanks will be needed to accommodate the higher flow rates. For these circumstances, additional tanks must be added in parallel (i.e., side by side) to split the flow.
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