NASHVILLE CUTS WET WEATHER SSOs USING SEWER REHABILITATION

Over the past several years, there has been a national debate on EPA's proposed policy to control SSOs (Sanitary Sewer Overflows). EPA is also placing an increased emphasis on auditing sewer maintenance programs and developing enforcement tools for improving streams affected by SSOs. Rehabilitation, using various techniques, is one component of I/I control, SSO reduction and long-term sewer maintenance. Despite the growing number of sewer rehabilitation processes and products, few reports have been available in the literature about their effectiveness for I/I removal, and SSO reduction. This may stem from an attitude of complacency by municipalities. However, an analysis of NPDES reports from two states show that over a third of the systems experienced dilute influent sewage, likely due to excessive I/I. This paper presents results from flow monitoring studies, and overflow reporting over the past nine years in Nashville, Tennessee.To develop a data base of objective measurements of sewer rehabilitation effectiveness, twenty seven project area in the Nashville, Tennessee, OAP (Overflow Abatement Program) were analyzed which included 83,781 m (274,871 ft) of sewer lining or replacement. This represents about half of the rehabilitation work conducted through this program since 1990. The goal of the program is to improve system characteristics, recapture capacity, and reduce I/I so the sewers can carry the peak flow from a design storm with a specified recurrence probability, without overflows. The most stringent design criteria used a 5-year, 24-hour storm. Linear regression analyses were used to predict the I/I expected from the selected design rainfall. Some of the projects showed sufficient I/I reduction to allow a five-year payback from treatment costs avoidance. A cumulative annual I/I reduction of 10.36 × 106 m3 (2,737 million gallons) has been documented for these projects. The trend of results indicate that an I/I removal rate of approximately 10,000 m3 /year/100m of lining (8.2 million gallons/year/1000 ft of lining) may be expected from rehabilitating deteriorated sewers in the Nashville area.I/I removal rates will likely vary in different locations due to groundwater levels, annual rainfall, previous construction techniques, soil types, etc. Accordingly, sewer rehabilitation programs should include procedures to measure I/I removal effectiveness as a program progresses. For I/I control programs that are just beginning and which lack performance data, the rehabilitation experience in Nashville illustrates a procedure to develop performance measures and procedures for long range program planning. Once defined for a given area, a removal rate can be coupled with rehabilitation construction costs and I/I quantities from flow monitoring. Knowing the relationship between I/I removal effectiveness and the quantity of sewer rehabilitation by lining required can assist planners and municipal officials in projecting the long-term program costs for sewer rehabilitation.State regulatory actions are increasingly focused on streams that have not achieved water quality levels appropriate for their designated uses. Lists of these streams have been published as required by Section 303d of the Clean Water Act. In many cases these streams have been listed because of SSOs or CSOs (separate sanitary sewer overflows or combined sewer overflows) that discharge during wet weather conditions. However, long-term water quality studies on tributary streams in Nashville have indicated that background or non-point pollution sources may be significant. In some cases, removal of wet weather discharges may not be sufficient to achieve compliance. TMDLs (total mass discharge limits) may be needed.