When Two Become One: Design and Planning Solutions for Trunk Sewer Consolidation

To address the challenge of population growth coupled with aging infrastructure, the City of Alvin, Texas, is embarking on its largest wastewater collection project ever: consolidation of two parallel trunk sewers into one 54-in. interceptor. Using this project as a case study, this presentation will give owners more tools in their toolbelt to improve the condition and increase the capacity of their wastewater collection system, minimize impact to their residents, and reduce capital and maintenance costs to their ratepayers. The City of Alvin has a single wastewater treatment plant (WWTP) that collects wastewater through two parallel trunk sewers (27- and 33-in. diameter). These pipes, installed in the 1960s and ’80s, show signs of increased deterioration and are at capacity to serve current demands. The City’s wastewater master plan identified a 54-in. trunk sewer, known as the Eastside Interceptor, to replace the existing parallel sewers. But rerouting sewer lines poses multiple hurdles, especially when a key goal is minimizing the disruption for residents. Sewer collection systems are built around the current location of the trunk sewers, but a reroute means redirecting the existing collection system to a new alignment. Alvin’s planning and design teams evaluated three different routes to reduce impacts to City residents. Using the City’s wastewater model developed as a part of the wastewater master plan, to evaluate up and downstream hydraulic impacts of each route alternative provided a dynamic analysis that allowed the City to choose a route based on a solid understanding of the construction and hydraulic implications of each option. Innovative design approaches addressed multiple challenges for the Eastside Interceptor. First, utilizing the existing sewer lines for reduced bypass pumping requirements reduces the bypass burden on the contractor during construction. Since there are existing parallel trunk lines, flow could be diverted into one trunk line and reduce pumping capacity necessary to replace the sewer lines. Additionally, a combination of rehab technologies will allow the selected alternative route to still convey wastewater through the existing sanitary sewers until they intersected with the new Eastside Interceptor.