Regional Infiltration and Inflow Control Program

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Benefit/Cost Analysis Report

View document table of contents.

November 2005
Prepared for King County by
Earth Tech Team, Seattle, WA

What is the Benefit/Cost Analysis?

As part of its Regional Needs Assessment, King County developed a list of conveyance system improvement (CSI) projects. These projects will help accommodate the increasing wastewater flows brought about by growth. To make the most effective use its resources, the County evaluated whether it is cost effective to eliminate or delay projects on the CSI Project List by reducing the amount of I/I in the conveyance system. The benefit/cost analysis compared the estimated costs of constructing conveyance system improvement projects with the estimated costs of I/I reduction projects.

What Information was Used for the Benefit/Cost Analysis?

To conduct the benefit/cost analysis, information was needed that could be used to address:

• The anticipated effort and cost necessary to reach target levels of I/I reduction.
• The capacity and cost-savings effects of proposed I/I reduction on the regional conveyance system.
• The cost effectiveness of implementing I/I reduction projects compared with the costs of regional conveyance system improvements.

The County and local agencies worked together to obtain this information and to develop assumptions about I/I reduction. The benefit/cost analysis used the information collected and tools developed for the Regional I/I Control Program between 2000 and 2005, including:

• Physical characteristics of local agency collection systems – including the size, age, material and location of pipes; points of connection between local agency and regional conveyance systems; boundaries and acreage served; topography; and land use.
• Rainfall data – to help understand patterns in I/I flows after storms, as well as the relationship between measured rainfall and wastewater flows.
• Flow monitoring – to determine the geographic distribution of I/I throughout the local agency facilities tributary to the County’s collection system, to quantify I/I levels, and to subdivide the entire system of local agency sewer lines into geographic areas called minibasins and model basins.
• Hydrologic model – to simulate the physical process of how rainfall ends up as I/I.
• Hydraulic model – to simulate the actual pipes that convey wastewater flows and I/I, and to evaluate how the system performs under existing and future demands.
• Pilot projects – to demonstrate the County’s success in finding and reducing I/I and to obtain “lessons learned” information.
• Regional needs assessment – to establish the extent of required capacity improvements and to estimate the costs associated with planning, design, and construction of conveyance system improvements.
• Assumptions – to establish target I/I reduction levels and to agree upon what I/I reduction levels could be achieved and the associated costs.
• Techniques – to develop a means of decreasing I/I by replacing or rehabilitating selected components of the sewer system (for example, disconnecting and re-routing downspouts that connect to the sewer system).
• Alternatives – to develop a recommended I/I program for defining a target level of I/I, to determine how cost-effectiveness is measured, and to address funding options.

What is Cost Effectiveness and How was it Determined?

To evaluate cost effectiveness, a benefit/cost ratio was calculated for each of the planned conveyance system improvement projects:

Benefit/Cost Ratio =

(CSI Project Savings after I/I Reduction) /
(Cost of Proposed I/I Reduction Project)

A proposed I/I project was considered cost effective if the CSI savings resulting from the I/I reduction project were greater than the cost of the I/I reduction. All cost-effective projects had a benefit/cost ratio greater than 1.

A database analysis tool, the Benefit/Cost Analysis Tool, was specifically developed for the Regional I/I Control Program. It was used in association with the County’s TABULA cost estimating software to compare reductions in capital costs (if any) to the cost of I/I rehabilitation. Inputs into these tools included information about the physical characteristics of the collection system, technique selected for reducing I/I, cost assumptions, results of hydraulic modeling, and information about upstream and downstream facilities.

Other factors that affected the cost effectiveness of a project included the level of confidence in the data and whether or not a threshold level of flow reduction was achieved.

What were the Results of the Benefit/Cost Analysis?

As shown in the table (refer to page 4 of Chaper 1 -- PDF file), nine I/I reduction projects were identified by the benefit/cost analysis as cost effective. For these projects:

• The estimated cost of implementing the I/I reduction projects is approximately $73 million.
• The anticipated I/I reduction achievable is estimated at 22 million gallons per day (mgd), or approximately 5 percent of the I/I present in the entire regional service area.
• As a result of reducing I/I flows, it is estimated that the capital cost for nine impacted regional conveyance facility improvement projects could be reduced from approximately $268 million to $164 million, resulting in regional conveyance facility improvement savings of nearly $104 million.
• The net overall savings realized from implementing the nine cost-effective I/I reduction projects is estimated at approximately $31 million.

View the entire report.

Table of Contents

If you need a free copy of the Adobe Acrobat Reader to view these documents (.pdf), visit the King County Acrobat help page.

Entire Report (without appendices), issued November 2005 (4.8 MB)

Title Page, Acknowledgements (18 KB)
Acronyms and Abbreviations (11 KB)
Contents (17 KB)
Chapter 1. Executive Summary (33 KB)
Chapter 2. Introduction (101 KB)
Chapter 3. Data Development (2.5 MB)
Chapter 4. Benefit/Cost Analysis (1.6 MB)
Chapter 5. Summary (358 KB)
Glossary (20 KB)

Appendices (Available on CD upon request)

Appendix A1 – Select List Cost-Effectiveness Analysis Package per MWPAAC E&P Planning Assumptions
Appendix A2 – Regional Cost Effectiveness Analysis Package per MWPAAC E&P Planning Assumptions
Appendix A3 – 30-Percent I/I Removal Cost Effectiveness Package per MWPAAC E&P Planning Assumptions
Appendix B1 – Sensitivity Analysis Select List-Cost Effectiveness Analysis Packages per Initial Planning Assumptions

For questions about the I/I Control Program Web site, please contact Valerie Garza.

Department of Natural Resources and Parks
Wastewater Treatment Division

Updated: Jan. 19, 2006

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