Napa Sanitation District is planning a $15 million project to rehabilitate a deteriorating pipe that carries 90 percent of local sewage to the wastewater treatment plant and has no backup.
Even without a backup, the relentless stream of sewage from the city of Napa must be kept flowing while work is done. NapaSan is making plans on how to handle the project in the summer of 2021, possibly using a system of temporary, above-ground bypass pipes.
“It’s not difficult to keep (sewage) flowing,” NapaSan General Manager Tim Healy said. “It’s expensive to keep it flowing.”
The half-century-old, 66-inch-diameter concrete pipe transports raw sewage three miles from the city of Napa to the wastewater treatment plant near the airport industrial area. A district report calls it the “backbone” of the sewer system.
NapaSan officials say a mile-long section of this sewage-transporting lifeline is reaching the end of its useful life. They clearly don’t relish the prospect of it possibly failing some day.
“That’s not an option we really have,” said Andrew Damron of NapaSan. “It really cannot fail.”
A 2017 closed-circuit television inspection inside the pipe showed significant deterioration since 2012. A more-comprehensive 2018 inspection confirmed that a section is structurally compromised and concluded it should be rehabilitated “in the short term,” district reports said.
NapaSan recently released a draft environmental study for the project required under the California Environmental Quality Act. The study gives details of the planned project.
Workers won’t have to dig up the old pipe and bury a new one. Rather, they could use a method called cured-in-place pipe. They would pull a synthetic fabric liner through the existing pipe to form a pipe within a pipe.
Sewage can’t flow in the pipe while this work is being done. The plan calls for setting up a system of temporary, above-ground, parallel pipes and pumps for a bypass system.
How complicated the bypass might be remains to be seen. A report said the district could set up a single, long bypass to serve the entire project or a shorter bypass that is moved as different sections of pipe are worked on. Workers would monitor the bypass 24 hours a day for leaks.
There is another option – insert a spiral-wound lining that snaps into place within the pipe. That can be done while the pipe is operating, as long as flow remains below 20 percent capacity, the draft environmental report said. No bypass system would be needed.
NapaSan will choose an option later based on such considerations as cost. A January report listed the cured-in-place pipe with a bypass system as the preferred option.
The 66-inch pipe runs near the Napa River and past sections with marshes, Bedford Slough and Soscol Creek. Approvals for the rehabilitation work are needed from the U.S. Army Corps of Engineers, state Department of Fish and Wildlife, San Francisco Bay Regional Water Quality Control Board and state Water Resources Control Board.
NapaSan plans to borrow money for the project and pay for the loan using sewer service charges. Damron said refurbishing the pipe has been in the budget for the last two years.
When the pipe was built some 55 years ago, the wastewater treatment plant was located on Imola Avenue along the Napa River. The pipe carried treated wastewater to district facilities in the airport industrial area.
In the late 1990s, NapaSan consolidated its wastewater treatment at the plant near the airport industrial area and subsequently demolished the Imola Avenue plant. At that point, the 66-inch pipe started carrying raw sewage that is more corrosive, district reports said.
You can reach Barry Eberling at 256-2253 or firstname.lastname@example.org.