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What is a sedimentation tank?
A sedimentation tank, or settling tank, holds water to allow solids to settle. The treatment helps prevent solids like sand and grit from reaching and blocking the sewer system.
Water from construction sites must be treated in sedimentation tanks before going to the sewer. Wastewater discharged into the sewer must contain less than 7 milliliters of solids per liter of water.
Materials that are not allowed in King County sewers:
- Ashes
- Sand
- Grass
- Gravel
Circular sedimentation tanks
Smaller construction sites that have occasional process wastewater can use circular sedimentation tanks. Water collects in these tanks over time and is treated in batches. The maximum sediment accumulation allowed is 25 percent of the tank's capacity.
The batch sedimentation process requires a minimum 60 minute quiescent settling time. Process wastewater cannot be added to or discharged from the sedimentation tank during this time. Depending on site-specific needs, circular sedimentation tanks can be set up as gravity or pumped discharge.
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 |
Minimum length-to-width-ratio
Rectangular sedimentation tanks need at least a length-to-width ratio of 4 to 1. A tank's length must be at least four times as long as it is wide.
Maximum overflow rate
The maximum overflow rate is 1.0 gallons per minute (gpm) per square foot of the surface area (1.0 gpm/ft2).
Calculate a tank's overflow rate by dividing instantaneous flow rate (gpm) over the surface area of the sedimentation tank (gallons per minute/surface area).
Surface area is calculated by multiplying the length (feet) by the width (feet) of the tank (tank length (feet) x width (feet)).
Maximum sediment accumulation
The maximum sediment accumulation, or the level of the sedimentation tank water column, must be 25% or less.
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.
Additional resources
Information about King County wastewater discharge limits and regulations