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Water from a construction site must be treated in a sedimentation tank before it can be sent to the sewer. This prevents solids like sand and grit from getting in the sewer, settling, and blocking the flow.

A sedimentation tank, also called a settling tank, holds the water and allows the solids to settle out before the water is released to the sewer. Facilities that send solids to the sewer are liable for any damages caused by sewage backups.

Wastewater that goes into the sewer must contain less than 7 milliliters of solids per liter of water capable of settling in a one hour period. Materials such as ashes, sand, grass, and gravel are not allowed in the sewer. King County has developed the following standards for sedimentation tanks.

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Circular sedimentation tanks

Circular sedimentation tanks are used for smaller construction sites that produce process wastewater intermittently. This water can be collected and treated in batches in circular sedimentation tanks. Tanks can be set up as gravity discharge or as pumped discharge depending on site-specific needs and limitations. The maximum sediment accumulation allowed is 25 percent of the tank capacity. The batch sedimentation process requires a minimum 60 minute quiescent settling time. During this settling time, no additional process wastewater can be added to the sedimentation tank or discharges occur from the sedimentation tank.

Rectangular sedimentation tanks

Rectangular sedimentation tanks are used for continuous discharge. They are flow-through tanks. They need to be sized to allow 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 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 multiply the maximum instantaneous flow rate by the 90-minute hydraulic retention time to calculate the minimum sedimentation tank volume required.

Example

Maximum instantaneous flow rate Minimum hydraulic retention time Minimum required sedimentation tank volume 
500 gallons per minute (gpm)  X  90 minutes   45,000 gallons

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.

For this example, if an 18,000 gallon sedimentation tank were used, this would require the use of three tanks operated in parallel. If a 21,000 gallon sedimentation tank were used, this would also require the use of three tanks operated in parallel, as two tanks would have an insufficient volume (42,000 gallons).

Minimum length-to-width ratio

The rectangular sedimentation tank needs to be at least four times (4x) longer than it is wide, for a ratio of 4:1.

Maximum overflow rate

The maximum overflow rate must be equal or less than 1.0 gallons per minute per square foot of surface area (1.0 gpm/ft2).

This is calculated by dividing the maximum instantaneous flow rate (gpm) and by the surface area of the sedimentation tank. Surface area is calculated by multiplying the length (feet) by the width (feet) of the tank. 

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.

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