Economic Analysis of King County’s Failing On-site Septic Systems (OSS)

Equitable Wastewater Infrastructure Analysis map. Source: King County

A cost and equity analysis to guide wastewater infrastructure funding decisions

King County, Washington, has more than 85,000 on-site sewage systems (OSS), and roughly two-thirds are more than 30 years old, near the end of their working life. Failing systems can send untreated sewage into homes and yards and pollute nearby streams, lakes, and beaches, creating public health and environmental hazards. Cost is the biggest barrier to replacing failing OSS or connecting aging systems to sewer, and it falls hardest on low-income and communities of color historically left out of public and private sewer investment.

King County engaged us to quantify what it will cost to address failing OSS countywide and to weigh the equity, regulatory, and funding challenges that shape each community's options.

We characterized the county's OSS areas into four tiers that share similar conditions, costs, and strategies, distinguishing where OSS will remain the long-term solution, where sewer is already available, and where sewer expansion is needed. Drawing on data from the King County Wastewater Treatment Division, the Valley View Sewer expansion, and Craft3, we estimated the cost of each option, modeled homeowner costs over 20 years, and translated the findings into policy and financing recommendations the county can act on.

The analysis puts the scale of the challenge into perspective. Addressing OSS more than 20 years old could cost King County homeowners an estimated $3.7 billion, including nearly $3 billion for the systems already past 30 years. Because a neighborhood that replaces failing OSS with new OSS rarely connects to sewer afterward, the county has a short window to act. With these findings, King County can set funding goals, incorporate OSS-to-sewer conversion into sewer planning, and prioritize sewer expansion and financial assistance where costs and infrastructure gaps are greatest.

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Read our executive summary, The Cost of Addressing Failing Septic Systems in King County. Explore King County’s Equitable Wastewater Futures Program to see how the County is addressing wastewater equity and infrastructure challenges.