Lessons Learned from Operating Large Sewage Tunnels

Tunnels for both conveyance and storage of wet weather flows are often evaluated as the greenest of the gray alternatives for overflow control in ongoing combined sewer overflow (CSO) control plans and frequently in sanitary sewage capacity assurance plans. However, standard wastewater engineering texts and manuals offer limited guidance on how the tunnels should be configured to minimize operation costs and maximize sustainability. This paper summarizes the experiences from several operating conveyance/storage tunnels to share lessons learned and recommendations for future design and operation of such systems. The paper is an update of a similar paper presented at WEFTEC 04 (Schultz, et al., 2004). The 2004 paper summarized lessons from 12 operating conveyance/storage tunnels, the oldest of which had a 24-year history.For the current paper, the authors expanded the subject to include large conveyance-only sewage tunnels. It is apparent that some of the older conveyance-only tunnels were constructed with similar access restrictions and operation concerns. Designer and operator questions commonly focus on features that make tunnels different from the facilities commonly operating within the collection systems. From an operational point of view, a conveyance tunnel is a sewer with extremely challenging and often limited access. A storage tunnel is a limited-access sewer with a higher than normal solids deposition potential, leading to concerns about the odor and corrosion associated with solids deposition. The questions addressed in the 2004 paper were expanded to include those asked by designers and future operators of tunnel systems from projects around the world, from Edmonton, Canada, to Auckland, New Zealand. The questions cover a broad range, from hydraulic transient control through noise at drop shafts to emergency repair options in a continuous service tunnel.Answers to the design and operation questions were initially developed from engineering theory. As more of the large sewer tunnels are commissioned, the body of knowledge expands, and the answers are refined. This 2012 update summarizes lessons learned through operation of more than 20 operating tunnel systems. The authors gathered the information from literature, personal experience, and numerous discussions with other operators. The summary is offered in hopes that further discussion and documentation of the lessons learned will encourage continued growth in the efficiency of operating tunnels and improved design of tunnels currently in planning.This paper summarizes experienced-based recommendations for design and operation of sewage conveyance and storage tunnels, and provides lessons on how tunnels avoid the energy and man-hour-intensive machinery and operations. It will also identify experienced tunnel systems from which more information can be obtained.