The Advantages and Challenges of Trenchless Rehabilitation - Rehabilitation of a one-of-a-kind Vacuum sewer system in Portsmouth, Virginia

An unusual condition exists in the Southeastern Virginia City of Portsmouth. In the early 1930's the city was required to construct a comprehensive sewer treatment system. While other cities in the area decided on gravity lines leading to traditional pump stations, Portsmouth went with a more unique solution. Due to it's low elevation, the city decided to begin installation of a vacuum sewer system.A typical Portsmouth collection sub-system consists of gravity lines leading to strategically placed collection manholes. These collection wells are connected, via vacuum lines of varying sizes, to a vacuum pump station. From these pump stations the sewage is piped to local treatment plants. At this time, Portsmouth's system is the only one of it's kind in America. The system is unique because of the way the pump stations were designed. Within the pump stations a large vacuum standpipe is used, in conjunction with vacuum transducers, to maintain an equilibrium state between the collection wells. When sewage is input into the collection manholes the system is brought out of equilibrium. When the input causes the water level within the standpipe to reach a given elevation, transducers turn on the pumps. Water is drawn from the collection manholes and the system is brought back to equilibrium. Some of these systems have been in place for 70 years, and while the collection manholes and pump stations have been periodically upgraded throughout the years, the vacuum lines are in need of rehabilitation. It has been reported that in some areas the lines have been constricted to 75% of their original inside diameters. In these lines the flow has reached scour velocity.The City is developing a capital improvement program for the rehabilitation of the lines connecting the collection wells to the pump stations. Using one of the smaller systems as a test (demonstration) project, the City will evaluate various rehabilitation methods and use the findings to develop the CIP. The purpose of the CIP is to extend the life of the systems while maintaining capacity, however the systems are so unique that special analysis tools had to be devised. To ensure that the eventual rehab technique would provide the required capacity, a computer model of the systems has been developed by HDR Engineering, Inc. and the Old Dominion University Department of Civil and Environmental Engineering. Using this model to simulate various rehabilitation methods, the City can choose the techniques that best meet the needs of each sewer sub-system. With the proper rehabilitation, these lines will serve the City of Portsmouth for many years to come, and we will have helped to preserve an elegant and creative example of wastewater engineering.In order to illustrate some of the unique advantages and challenges of trenchless rehabilitation, this technical paper and presentation will provide an overview of the technical issues related to the implementation of citywide collection system rehabilitation. Topics to be discussed will include utility construction in downtown and historic districts, innovative trenchless technologies and the challenges of rehabilitating negative pressure pipe. Also to be discussed will be project specific design, construction, maintenance and cost issues.