Road Operations Division Vactor Decant Station Web Page

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Road Operations Division Decant Stations

Did you want to request a catch basin or storm drain be cleaned?

What is it?

When our Vactor trucks clean out storm drains, they pick up a dirty mess of solid and liquid street waste. The decant station provides separation and treatment of our street waste so that the materials collected can be reused or disposed of properly.

The decant station design provides state-of-the-art treatment of our waste stream. It uses an environmentally-sound process that is monitored and tested for compliance with state regulations.

Why did we build it?

Solid waste, such as sand, silt, and leaves, collected from our storm drainage system tends to have a higher than normal concentration of contaminates. Dissolved heavy metals (arsenic, chromium, and lead), heavy oil, and volatiles, such as gasoline, are commonly found. Large quantities of this waste, left in untreated stockpiles, could result in harm to the ground water and the environment.

Advantages to the Environment

Recent listing of Chinook salmon as an endangered species has required us to obtain approval from the National Marine Fisheries Service (NMFS) to perform our normal maintenance activities. Constructing these decant stations has helped us to get this approval.

Decant stations allows us to treat, monitor, and return the sand, silt, and gravel to the environment in a manner that meets or exceeds State and Federal environmental standards. Other benefits include lowering our disposal costs and our ability to monitor the quality and final destination of our sold waste.

How does it work?

Liquid and solid waste is brought to the station in one of our Vactor trucks where the fluid is released from the truck's tank into the decant bay. The solids remaining in the tank are dumped in the adjacent solids bay.

Liquid is held in the station while the suspended solids are allowed to settle out. During the settling process, molecules from dissolved metals attach themselves to the suspended solids, thus removing the metals and clearing the water.

The water is released from the station through a series of oil/water separators, sent through a bioswale (a grassy ditch that filters out impurities), and put through a sand filter, where clean water is returned to the environment.

The solids are air-dried, screened to remove litter, and tested to determine if they can be reused. Solids may be stored and aerated in the station and in large concrete block bins adjacent to the station for up to six months.

What do we do with the solid waste?

Once solids have been tested (and if contamination does not exceed state regulation limits) they can be reintroduced into the environment as backfill or made into a topsoil product.

If the contamination of the solids is outside acceptable limits, they are disposed of at an approved disposal facility.

What are the benefits?

The decant stations allow Pierce County to:

Save money on disposal costs.
Maintain our road and drainage infrastructure within national quality standards.
Meet requirements and guidelines for the protection of our environment.
Utilize and document an environmentally-safe disposal process.
Reuse the materials when applicable.