The Napa Sanitation District (NapaSan) oversees a sewer collection system in Napa, California consisting of 270 miles of mains, including a 66-inch diameter trunk sewer that parallels the eastern bank of Napa River for about three miles through wetlands and threatened species habitat. This pipeline connects the City to NapaSan’s Soscol Water Recycling Facility (SWRF), is the backbone of the collection system, and conveys up to 58 MGD peak wet weather flow. That is more than 90 percent of the wastewater flow generated within NapaSan’s service area. Installed in the 1960s, the trunk sewer is constructed entirely of reinforced concrete pipeline (RCP) without a protective lining or coating and the system has no redundancy for this asset.
NapaSan partnered with Woodard & Curran to conduct an inspection and condition assessment of the three-mile stretch of 66-inch diameter trunk sewer. The project team performed CCTV and sonar inspections, assessed the data, and developed remaining useful life projections, project prioritization and recommendations for repair. The team identified more than one mile of the inspected trunk sewer had reached the end of its useful life and required structural rehabilitation. With pertinent data on hand, the project team identified rehabilitation alternatives and developed a preliminary and final design to rehabilitate the compromised section, including the design challenge of routing more than 20 MGD through a bypass pumping system along the Napa River. The design opted for use of trenchless technology in the form of cured-in-place pipe (CIPP) lining.
The complexity of this $5.6 million project was featured by Trenchless Technology Magazine in October 2022. It will also be recognized during the 2023 NASTT No Dig Conference in Portland, Oregon, as the 2022 Trenchless Rehabilitation Project of the Year – Runner Up.