Navigating the Rapids: Overcoming Obstacles in the Rehabilitation of a 90-inch Sewer

Abstract Summary: This presentation will discuss the approach to design for rehabilitation of 2,300 LF of 90-inch sanitary sewer main under the EPA consent decree, and the associated challenges, including flows up to 70 MGD, parallel large diameter sanitary sewer and recycled water mains, and a downstream wastewater treatment plant. Many aspects of the project required unique approaches to design and lessons learned which can be applicable for other utilities and design engineers navigating rehabilitation of wastewater systems. Abstract: For the past decade, the San Antonio Water System (SAWS) has been under a Consent Decree with the United States Environmental Protection Agency (EPA). The Consent Decree required SAWS to make improvements to their wastewater system within a ten-year timeframe, in an effort to reduce sanitary sewer overflows throughout the system. As a part of the Consent Decree, SAWS was required to implement a Condition Assessment Program to identify sewer infrastructure in poor condition and prioritize rehabilitation. As the 10-year deadline for rehabilitation of sewers approached, Kimley-Horn was contracted by SAWS for the design of one of the consent decree rehabilitation projects. The project included 6,500 LF of sanitary sewer mains in need of rehabilitation, ranging in diameter from 48-inches to 90-inches and in various locations throughout San Antonio. With the project being a part of the EPA consent decree and due to be constructed by the end of 2023, time was of the essence. As with many large diameter sewer mains within the SAWS system, all of the project sites were in rural areas, traversing through private property. Obtaining access, acquiring easements, and finding water for curing of CIPP liner were just a few of the challenges encountered on the project. Additionally, each sewer main to be rehabilitated was paralleled by another large diameter sewer line. The existing sewers were aging and in poor condition, which posed risk of collapse during construction with heavy equipment traversing over and excavating near the mains. To mitigate this risk, tee-base manholes were specified instead of traditional manholes due to the reduced area required for excavation. The parallel sewer mains also provided a potential benefit — flow diversion. Diverting flows to the parallel mains reduced the bypass pumping required and saved SAWS money on costly bypass setups, freeing up funding which could be used for other critical projects. But not all of the parallel lines had capacity for flow diversion, including the line parallel to 2,300 LF of 90-inch sanitary sewer. Flow meters on the 90-inch sewer observed flows as high as 70 MGD. But a traditional approach to bypass could not be taken. The sewer main upstream of the segment was a siphon, and the bypass piping would have to cross a roadway and railroad. Downstream of the 90-inch main was another siphon, which led to the site of an existing wastewater treatment plant that had been decommissioned. To further complicate matters, the treatment plant's headworks were still in operation and bypassing the headworks posed concerns regarding debris accumulation in downstream siphons. Through careful coordination with various departments within SAWS regarding operation of the treatment plant, Kimley-Horn was able to develop a bypass plan that allowed for rehabilitation of the 90-inch sewer and reduced risks associated with impacts to downstream sewer structures and the treatment plant headworks. The design team utilized sluice gates in the existing siphon structures immediately upstream of the segment to isolate flows and serve as bypass suction points. Removal of the siphon lids for additional suction points was also proposed, thus reducing excavation required for doghouse manholes and risks associated with the adjacent, live sewer main. Review of CCTV for the 90-inch main indicated that it was in suitable condition for rehabilitation for CIPP, but with the sewers located in rural areas, no potable water was available for curing of the liner. Kimley-Horn proposed use of the parallel recycled water main as a water source for curing. These inherent challenges required unique approaches to design of the sanitary sewer rehabilitation plans. These approaches could benefit other engineers and utilities undergoing similar improvements in their system. The project is 75% complete and construction will be completed in January 2024.