Implementation Challenges of Public and Private Source Control to Eliminate SSOs
The Sanitation District No. 1 of Northern Kentucky (SD1) is under a Watershed Consent Decree (CD), which calls for the elimination of separate sanitary sewer overflows (SSOs) by 2025. Plans are to be submitted every five years indicating the projects that will be implemented to meet the requirements of the CD. Many of SD1's SSOs are located in older communities with aging, failing infrastructure where the overflows are attributed to the inflow and infiltration (I/I) of storm water or groundwater into the sanitary sewer system.
SD1 undertook a comprehensive approach in evaluating the alternatives for overflow elimination such as convey/storage and treat options as well as scenarios targeting I/I removal to help reduce or eliminate overflows. In two study areas where I/I reduction options were evaluated, the life cycle analysis found that I/I reduction in the form of comprehensive public sewer rehabilitation, private property source removal, and applicable storm sewer improvements would likely be more cost effective than upsizing conveyance piping or building equalization with subsequent treatment. In addition, other non-quantitative benefits would be realized with the I/I reduction approach such as long term asset management, water quality, and SD1's green infrastructure initiative. There are also opportunities to coordinate with SD1's storm water program. Based on this planning level pilot analysis, SD1 is developing a system-wide sanitary sewer evaluation survey (SSES) program to identify and remove excess I/I where it has a likelihood of being more cost effective than convey/storage and treatment. The analysis of these areas also helped inform the need and approach for locating and removing private I/I sources.
This presentation will summarize a system-wide strategy analysis for I/I removal and will include a focus on the alternatives analysis for elimination of SSOs in the Vernon Lane and Lakeside Park study areas and how I/I removal was selected as the preferred alternative. Questions that will be discussed include:
What I/I reduction measures on public and/or private properties will be necessary to achieve the reduction goals?
How do we measure effectiveness of I/I reduction?
How do we engage/enforce private homeowners to reduce their amount of I/Icontribution?
Vernon Lane Case Study
The Vernon Lane study area is primarily residential and consists of approximately 70 acres with a sanitary sewer system made up of 1.9 miles of pipe. SD1 has identified 2 SSOs in the area through a combination of field characterization and modeling. Based on smoke and dye testing results, a significant portion of the peak flow is coming from identified I/I sources. Therefore, it is reasonable to believe that removing these I/I sources could significantly reduce the peak flows from this area. By eliminating 50% of the I/I in the Vernon Lane area SD1 can eliminate the two recurring high public health risk SSO locations and the corresponding 0.26 million gallons of overflow volume in the typical year. By removing I/I at the source, downstream overflows in a downstream SSO, which is the largest SSO in the District, can also be reduced by 3.3 million gallons in the typical year.
Costs estimates were developed to perform the comprehensive I/I removal and were based on removing identified private I/I sources and comprehensive rehabilitation/replacement of all 1.9 miles of sanitary sewer in the area to achieve at least 50% I/I removal in the Vernon Lane area. This cost was then compared to the cost to convey all peak flows downstream to the nearest trunk sewer, an approach that would eliminate the local overflows in the Vernon Lane area but convey more flows to downstream SSOs. Source I/I removal is more cost effective in a direct comparison with conveyance and becomes even more cost effective when downstream impacts are considered.
Consistent with SD1's holistic approach, the capacity of Vernon Lane's storm water system was preliminarily evaluated, and where appropriate innovative green infrastructure (e.g., rain gardens, rain barrels) to manage the additional runoff that will no longer be discharged to the sanitary system was included in the cost estimate.
Lakeside Park Case Study
The Lakeside Park study area is primarily residential and consists of approximately 235 acres and 750 properties, with a sanitary sewer system made up of 11 miles of pipe and 339 manholes, with an average dry weather flow of 0.2 MGD. SD1 has identified seven SSOs in the area through a combination of field characterization and modeling. Six different alternatives were compared that included conveyance, rehabilitation by chemical grouting and Cured-in-Place pipe to remove infiltration, private source I/I removal with storm water management, and various combinations of these corrective measures. Fifty percent I/I reduction is necessary to eliminate the 0.33 million gallons of overflow in a typical year. In order to capture the full sustainability and operating costs, the present values for all scenarios were determined over a time period of 25 years.
Based on the extensive assessment of source identification from SSES and the 50% I/I reduction needed to achieve the SSO elimination goals, SD1 determined that comprehensive public sewer rehabilitation (including laterals up to edge of pavement) and private source control of disconnecting illegal downspouts, driveway drains and area drains was going to be the more cost-effective solution. Storm water system assessments were also evaluated to ensure complementary efforts to reduce and improve runoff quantity and quality.
To monitor effectiveness of the I/I reduction program, this project is being phased with on-going flow monitoring within the project area and in control areas. Using control areas greatly reduces the variability of antecedent conditions and rain fall from year to year, and provides a method for determining the average percent I/I reduced to compare to the goals.
SD1 Private I/I Source Reduction Program Implementation
SD1's private source removal program is being defined District-wide for use and will be piloted in Lakeside Park and Vernon Lane. SD1 decided to implement a 50/50 cost share for homeowners up to a maximum of $2,500 paid by SD1. The cost share is only in place for the next year while SD1 gauges effectiveness and response. The presentation will discuss the decision making process for engaging private homeowners.
Keywords: Infiltration and inflow (I/I) reduction; SSES; collection system models; green infrastructure; private property I/I source control; separate sanitary sewer overflows (SSOs); sewer rehabilitation
Document Type: Research Article
Publication date: January 1, 2010
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