City may be forced to replace sewer line under tank farm
Beverly Bryant - May 17, 2017 3:11 pm
Ponca City’s Board of Commissioners heard Monday about a problem with a long section of concrete pipe which carries sewage from the west side of the Phillips 66 tank farm to the wastewater treatment plant on the east side of Ponca City.
The official name of the project is the “Bois d’Arc Intercepter Replacement Project.”
The problem was discovered during a sewer overflow at the wastewater treatment plant. The Environmental Services Department said that what they saw in that event made them want to investigate further.
Hong Fu, the director of Environmental Services, gave a presentation on the project.
She said this section of sewer line was installed about 45 years ago – about 1971-72 – and is a concrete pipe. At the time it was installed, she said, thre was a strong push to replace vitrified clay pipes with concrete.
Using concrete had some advantages, but over time proved to have a major disadvantage, she said.
“It is not corrosion resistant in a wastewater collection system,” she said.
She said this concrete sewer has piping in three sizes, including 18-, 24-, and 27-inch diameter sections. The manhole where the problem was first discovered is outside the west gate of the tank farm, on Waverly. The sewer line turns north, the goes east parallel to the power lines across the tank farm to the wastewater treatment plant.
When staff looked at the sewer line, Fu said, “We saw that the section had suffered serious corrosion due to acid attack.
“Under certain conditions, the bacteria in the sewer will break down nutrients and create sewer gas with hydrogen sulfite, which will combine with water to attack the concrete,” Fu said. “We have serious corrosion because of the acid attack. What we saw was serious enough to believe some corrective action must be taken as soon as possible. We cannot wait for another budget cycle to start before addressing this.
Fu’s department is seeking a supplemental budget amendment to fund engineering services, a survey and engineering report to the Oklahoma Department of Environmental Quality for approval.
The replacement of the sewer line will call for a consistent size in the pipeline. Fu said the project will use an open trench method to replace the 18-inch sections and a trench for the larger sections.
City Manager Craig Stephenson and Finance Director Marc LaBossiere said they believe the project can be funded through existing wastewater funds.
Fu said the sewer does not go under the refinery’s production area, but does go under the tank farm. Because of that, there may be issues involving contaminated soil. Such contamination would not be the responsibility of the City of Ponca City, but there would be special considerations.
Fu said the west end of the sewer line extends another 3,600 feet which goes under the Bois d’Arc Creek upstream to Highland on the west side of Waverly. It is buried under a field which is so wet right now that it cannot be checked.
“We don’t know the condition or how quickly action would be required,” Fu said.
A contract on the project will be on the City Commission’s agenda for its meeting on Monday.
Fu estimated engineering will take three to six months, with construction lasting another six months to a year.