Initial Infiltration and Inflow Reduction Project Alternatives Analysis Report

Issued April 2009

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Executive Summary

• Background
• Project description
• General findings
• Alternatives analysis
• Recommendations

Background

The Initial Infiltration and Inflow Reduction Project Alternatives Analysis report presents recommendations for projects to reduce infiltration and inflow (I/I) in portions of King County's regional wastewater conveyance system. Reducing I/I, which consists of stormwater and groundwater entering a sanitary sewer system from various sources, makes more capacity available for sewage in the county's wastewater system. This increased capacity helps prevent overflows and reduce the need for capital projects to add system capacity. King County is engaged in a long-term program to reduce I/I when cost effective to do so, and the projects outlined in this alternatives analysis represent an early test of the effectiveness of I/I reduction measures over a large area.

Previously, King County conducted a six-year study on I/I control, which included pilot-testing in several small areas of the county and a detailed benefit/cost analysis. The benefit/cost analysis outlined a general process for assessing whether the cost of I/I reduction measures can be recaptured through savings associated with elimination or reduction in size of capital projects that otherwise would be needed to increase system capacity. Specifically, the process looks at potential savings in the cost of projects planned under King County's Conveyance System Improvement (CSI) Program. The 2007 update to the CSI Program identifies 33 needed projects, assuming no broad I/I reduction across the service region. Successful I/I reduction projects may eliminate the need for an identified CSI project, reduce the required size of the project, or allow for delay in the implementation of the project. Any of these results can lead to cost savings in the CSI Program.

The initial I/I reduction projects fulfill the key components of the Executive's recommendation based on the six-year study:

• Select, implement, and evaluate two or three initial I/I reduction projects to test the costeffectiveness of I/I reduction on a larger scale than the pilot projects.
• After completion of the initial projects, make recommendations to the King County Council regarding long-term I/I reduction and control.

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Project description

Areas investigated and affected CSI projects

Mini-Basin—A drainage basin (geographic area encompassing all portions of the wastewater collection system draining to a single point) defined by King County's Regional I/I Control Program in order to establish manageable target areas for sewer system evaluation and rehabilitation. Mini-basins typically include about 20,000 feet of sewer lines.


Figure ES-1. Project Areas and Mini-Basins Evaluated for Initial I/I Reduction Project

Four project areas for the Initial I/I Reduction project were selected in October 2006 by the Engineering and Planning (E&P) Subcommittee of the Metropolitan Water Pollution Abatement Advisory Committee (MWPAAC), which represents all the jurisdictions served by the King County regional wastewater system. The four project areas—Eastgate, Issaquah, Renton and Skyway—were chosen from a list of nine that were originally identified as potentially cost-effective in the November 2005 Benefit/Cost Analysis Report. Each project area consists of one or more “minibasins,” which were previously delineated by King County 's Regional I/I Control Program. Figure ES-1 shows the locations of the four project areas.

The following CSI projects could be affected by I/I reduction in each project area:

• Eastgate Project Area (City of Bellevue)—The 2.33-million-gallon (MG) Eastgate Storage project
• Issaquah Project Area:
  • The 2.33-MG Eastgate Storage project
  • The 1.77-MG Issaquah Storage project
  • The 1.72-million-gallon-per-day (mgd) Issaquah Interceptor Parallel
• Renton Project Area—The South Renton Interceptor project
• Skyway Project Area—The 0.27-MG Bryn Mawr Storage project

Initial evaluations performed

For each mini-basin, the county has performed computer modeling to estimate the amount of I/I that would enter the sewer system within the mini-basin for a 20-year peak-flow event. These estimates are used as the baseline I/I flow for each mini-basin in this alternatives analysis. The evaluation of alternatives involved estimating how much each mini-basin's 20-year peak I/I flow could be reduced by implementing each alternative.

Investigations were conducted to assess the sewer system and surrounding environment for each project area. The sewer system evaluation survey (SSES) consisted of video inspections of sewers, smoketesting, new flow monitoring, and limited dye testing and field visits to evaluate the condition of the existing sewer system and the level of flows attributable to I/I. The environmental assessment used field visits and review of previous studies to determine whether I/I reduction projects would have the potential for negative impacts on groundwater, surface water, geotechnical features or local drainage systems.

Alternatives analysis approach

Following the initial evaluations of the sewer system and surrounding environment, the analysis of alternatives for the initial I/I reduction project consisted of the following steps:

• Rehabilitation unit costs were developed based on field conditions in each mini-basin. Each property in the project areas was assigned a rehabilitation difficulty rating (easy, medium or difficult) and associated unit cost for rehabilitation.
• I/I quantities were uniformly apportioned across each mini-basin and were equated to an average I/I per property in the mini-basin.
• Rehabilitation alternatives were developed that consisted of rehabilitation in single or multiple mini-basins, including alternatives that combine rehabilitation in the Issaquah and Eastgate project areas where there are mutual benefits in the reduction of regional downstream conveyance needs. The alternatives considered a range of I/I reduction effectiveness from 60 to 75 percent.
• Alternatives were rated using criteria developed by the E&P Subcommittee. To be considered cost-effective under these criteria, an I/I reduction project must reduce, delay or eliminate a recommended downstream CSI project; and the associated cost savings must equal or exceed the cost of the I/I reduction project. Rehabilitation scenarios found to be cost-effective were evaluated in greater detail to identify preferred alternatives.

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General findings

The following findings were made based on the analysis of project areas:

• The SSES revealed a moderate number of defects in mains, laterals and side sewers in each project area. I/I appears to be fairly uniformly allocated across each project area.
• The SSES generally confirmed the conventional wisdom that laterals and side sewers represent the major source of I/I in a system.
• New flow monitoring data collected in each project area during the 2007/2008 wet season was generally consistent with previous data; and both the new and old data suggest that a large percentage of I/I in the project areas originates from private property.
• The only significant difference between new and previous flow monitoring data was in the Renton project area, where the new data showed lower peak I/I levels. The SSES, including smoke tests and CCTV inspections, failed to identify the source of peak inflows in this project area. However, several manholes in a wetland within the project area were identified as becoming submerged during heavy rainfall events; water entering the system through these manholes is a likely significant source of inflow.
• Although geotechnical, groundwater and environmental conditions were found to vary widely across the project areas, no conditions were found that pose significant issues for potential rehabilitation projects.
• It is likely that instances of drainage-related problems will result from I/I reduction improvements. In order to address drainage complaints that could be caused by any initial I/I reduction projects, it is recommended that a portion of the project construction cost be allocated to fixing any associated drainage problems that occur following rehabilitation.
• For some of the basins, rehabilitation unit costs, which were developed based on actual field conditions in each project area, are substantially higher than previously estimated in King County's 2005 benefit/cost analysis because of the degree of difficulty in accessing mains, laterals and side sewers. This was particularly true for portions of the Eastgate and Issaquah project areas. Table ES-1 summarizes the unit costs developed for this project.
• A general finding of the analysis was that basins with an I/I allocation of less than 3 gallons per minute (gpm) per property were not good candidates for cost-effective removal of I/I because achieving the total targeted I/I reduction in those basins would require rehabilitation of too many properties.

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Alternatives analysis

Mini-basin scenarios

Mini-basin rehabilitation scenarios were developed consisting of varying combinations of rehabilitation of side sewers and laterals, and direct disconnections of roof drains and yard drains from the sewer system. The following are typical mini-basin rehabilitation scenarios:

• Rehabilitate laterals and side sewers on 50 percent of service parcels; rehabilitate only side sewers on 45 percent of service parcels; and implement direct disconnects on 4 percent of service parcels (this scenario, which does not distinguish between easy, medium and difficult parcels, was defined as “Technique 4” in the 2005 Benefit/Cost Analysis Report and was the recommended scenario in that report).
• Rehabilitate laterals and side sewers on all service parcels.
• Rehabilitate laterals and side sewers on 95 percent of service parcels.
• Rehabilitate laterals and side sewers on all easy and medium service parcels.

Variations of these scenarios were developed for each mini-basin. The goal was to establish and evaluate a reasonable range of I/I reduction approaches in order to find a suitable balance between construction cost and I/I reduction. Where smoke testing identified direct inflow sources, direct disconnects to eliminate the inflow sources are included in the rehabilitation scenarios. In all, 46 rehabilitation scenarios were developed and evaluated for nine mini-basins. Based on the evaluation, 20 scenarios in seven minibasis were selected to create initial I/I reduction alternatives.

I/I reduction alternatives

Mini-basin rehabilitation scenarios selected for use in alternatives were evaluated individually or in combinations, based on the downstream CSI project that could be affected by their implementation. Twenty-seven alternatives were developed from the selected scenarios.

The alternatives and their estimated impacts on I/I were provided to King County 's modeling group to assess the potential for reducing or eliminating downstream CSI projects due to the reduced I/I flows. Cost savings associated with CSI project reduction allowed by each initial I/I reduction alternative were estimated for comparison to the construction costs for the alternative. Alternatives with a benefit/cost ratio of 1.0 or greater may be recommended for implementation as initial I/I reduction projects.

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Recommendations

Figure ES-2. Eastgate/Issaquah Alternative BEL/ISS-B 


Figure ES-3. Skyway Alternative BLS-F

The alternatives analysis indicated that cost-effective rehabilitation is feasible in only four mini-basins. Cost-effective rehabilitation in all other mini-basins is limited due to a low I/I allocation per property (requiring a greater number of properties to be rehabilitated) and high unit costs for rehabilitation because of difficult field conditions. Two I/I reduction alternatives, consisting of rehabilitation in three of the four feasible mini-basins, are recommended for implementation:

• Eastgate/Issaquah Alternative BEL/ISS-B (see Figure ES-2)—This alternative includes rehabilitation of laterals and side sewers in Eastgate Mini-Basin BEL031 and Issaquah Mini- Basin ISS003. Components include the following:
  • 82 easy and 25 medium lateral and side sewer replacements in Mini-Basin BEL031 (50 percent of 213 properties in the mini-basin).
  • 37 easy and 76 medium lateral and side sewer replacements in Mini-Basin ISS003 (85 percent of 133 properties in the mini-basin).
  • The combined estimated range of I/I reduction for the two mini-basins is 1.04 mgd (75-percent reduction effectiveness) to 0.85 mgd (60-percent reduction effectiveness).
  • Reduction in the Eastgate Storage requirement is between 320,000 gallons (75-percent reduction effectiveness) and 260,000 gallons (60-percent reduction effectiveness).
  • Reduction in the Issaquah Storage requirement is between 450,000 gallons (75-percent reduction effectiveness) and 370,000 gallons (60-percent reduction effectiveness).
  • Reduction in the downstream CSI project costs is between $6.37 million (75-percent reduction effectiveness) and $5.12 million (60-percent reduction effectiveness).
  • The estimated project cost for the I/I rehabilitation is $5.23 million. Although this project is marginally below the cost-effectiveness threshold of 1.0 if only 60-percent I/I removal is achieved, past similar projects have shown I/I removal rates on average of 77 percent. The cost estimate for I/I reduction is also conservative, so the risk is minimized that the project would not achieve cost-effectiveness.
• Skyway Alternative BLS-F (see Figure ES-3)—This alternative includes rehabilitation of laterals and side sewers in Skyway Mini-Basin BLS002. Components include the following:
  • 292 easy and 51 medium lateral and side sewer replacements (89 percent of the 386 properties in the mini-basin).
  • The estimated range of I/I reduction is 2.29 mgd (75-percent reduction effectiveness) to 1.88 mgd (60-percent reduction effectiveness).
  • The rehabilitation eliminates the need for the 270,000-gallon Bryn Mawr Storage project, and the associated $5.37 million total project cost.
  • The estimated project cost for the I/I rehabilitation is $5.63 million, requiring costsharing of $260,000 by the Skyway Water and Sewer District to make the project costeffective.
  • The Skyway Water and Sewer District wants to add rehabilitation of mains and manholes to this project. The District has agreed to pay for the additional construction cost to add these components.

Estimates and project details will be refined through the predesign process in 2009 and the final design in 2010. The predesign will identify exact parcels for rehabilitation and confirm the preferred construction method. During final design, contract documents will be prepared, rights of entry will be acquired, and the public participation program will be carried out. Construction of the projects will take place in 2011 and 2012. Post-project evaluation and the King County Executive's submittal of recommendations for future work to the King County Council will occur in 2013.

For the Renton project area, flow monitoring performed for this alternatives analysis did not indicate the level of I/I that had previously been measured or predicted by modeling. It is possible that Washington State Department of Transportation construction on the SR-167 on-ramp resulted in changes to the surface water drainage patterns, diverting surface water away from the sewer line in the wetland area. Given the current levels of measured I/I in the mini-basin, it does not appear that rehabilitation in the mini-basin will meet the cost-effectiveness criteria established for this project. At the April 16, 2008 E&P Subcommittee meeting, a decision was made to remove the Renton project area from further consideration of large-scale rehabilitation under the Initial I/I Reduction project.

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For more information, please contact Erica Jacobs at (206) 684-1138 or Erica.Jacobs@kingcounty.gov.

Table of Contents

You will need a PDF reader to download and view these documents (.PDF).

Initial Infiltration and Inflow Reduction Project Alternatives Analysis Report, issued April 2009

• Report and appendices (88MB)
• Report, without appendices (60MB)
• Appendix A. Environmental Technical Memorandum (22MB)
• Appendix B. Skyway Project Area Drainage Complaints (450KB)
• Appendix C. Alternative Spreadsheets (450KB)
• Appendix D. Permitting Requirements (6MB)
• Appendix E. Risk Assessment Results (300KB)

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