Sewage flows into Cedar from overwhelmed pipeline; no penalty likely for spill

Published 2:51 pm Monday, December 13, 2010

An undetermined amount of sewage overflowed into the Cedar River last weekend when stormwater overwhelmed a pipeline under the river, forcing sewage through two locked manhole covers.

The sewage overflowed for several hours, starting late Saturday or early Sunday; it was reported Sunday morning by a nearby resident, said Annie Kolb-Nelson, a spokeswoman for the King County Department of Natural Resources.

“That is significant,” she said.

It’s the same spot along the Cedar where sewage gushed late last May from a manhole serving a 14-inch pipeline that clogged with debris, then collapsed near the Royal Hills Apartments.

The City of Renton sent a worker familiar with its surface water management division who is familiar with the sewerline collapse to inspect the situation, said Gregg Zimmerman, the city’s public works administrator.

The worker saw signs that wastewater had flowed from the county manhole but saw no discharge from a Renton manhole farther away from the river up a steep hill, Zimmerman said.

“We saw no problems with our system,” he said.

Renton officials have been talking with King County about the overflow.

The location is upriver from the Riverview Park on State Route 169.

The county reported the spill to Public Health – Seattle and King County and the Department of Ecology, which will determine whether to impose any penalties against county.

Larry Altose, an ecology department spokesman, said Tuesday it’s unlikely the state would impose a penalty on the county because the overflow was caused by the storm.

The county is reviewing how much sewage might have flowed from the manhole covers, Kolb-Nelson said.

The 14-inch pipeline is maintained by the City of Renton. The county takes over responsibility for the sewage line when it reaches the Cedar. There, it narrows to 12 inches and uses a syphoning action to pull the sewage through the U-shaped pipeline under the river.

A record rainfall “overwhelmed” the pipeline, said Kolb-Nelson. The pipeline is not a dual-purpose line, meaning it doesn’t carry sewage and stormwater, she said. However, it’s still possible for stormwater to enter the line through cracks, tree roots in the pipeline or downspots connected to a sewer line.

Zimmerman said water enters a closed sewer lines through a process called inflow or infiltration.

Sewage can leak out from pipelines, especially in the summer, but in a much smaller amount, he said. Groundwater, which is under pressure, is forced into some sort of crack in a sewage line. That pressure also helps keep the liquid sewage in the line, along with the fact it’s flowing.

The record rainfall also had King County’s sewage-treatment plant in Renton operating at or near its capacity – 300 million gallons a day, said Kolb-Nelson.

“The plant was able to maintain normal operations,” she said.

The county plans for such major events and has procedures in place to bring in backup personnel when needed, which was necessary last weekend to respond to a number of sewage overflows in the county.