Integrated Planning in Action
Seattle, Washington
Elliott Bay with Seattle skyline. Photo courtesy of
Seattle Parks and Recreation.
Seattle, Washington, is the largest city in the Pacific Northwest, with a
population of more than 700,000. This seaport city is located in King County,
sandwiched between Puget Sound—the second largest estuary in the United
States—and Lake Washington. Seattle operates a combined sewer system
and a separate storm sewer system.1 The combined sewer system brings
stormwater and sewage to one of the six wastewater treatment facilities owned
and operated by King County. The storm sewer system discharges about 13
billion gallons of stormwater per year. These facilities and systems discharge
to Puget Sound, Elliott Bay, Lake Washington, and the Lower Duwamish
Waterway.
Challenges
Between 2007 and 2010, about 200 million gallons of sewage entered
Seattle's local water bodies every year through combined sewer overflows
(CSOs) and unauthorized discharges. Both CSOs and stormwater discharges
add metals, total suspended solids, nutrients, bacteria, and organic compounds
to local waterways. In 2013, the city agreed to reduce CSO discharges to meet
the Washington Department of Ecology's limit of one overflow per outfall per
year. The consent decree required Seattle to develop a CSO long-term control
plan (LTCP) and complete construction of CSO projects by 2025. It also gave Seattle an alternative: develop an
integrated plan and potentially extend the CSO project construction deadline, but only if the integrated plan
results in significant water quality improvements beyond what the CSO projects under the LTCP would have
achieved alone.
Integrated Planning in Action
In 2013, Seattle began to develop two plans: an LTCP with CSO projects and an integrated plan with both CSO
and stormwater projects. The city engaged the public throughout the planning process. Seattle made information
available through community updates, briefings, animations, visualizations, website videos and updates,
and an email listserv. The city solicited input through public information meetings, scoping sessions, online
questionnaires, and emails.
Seattle identified potential stormwater projects to include in the integrated plan, then ranked these projects based
on water quality impacts and other criteria (see details in the box below). The city then compared the highest-
ranking stormwater projects with the lowest-ranking CSO projects.
Using this analysis, Seattle developed an integrated plan with three stormwater projects that it determined
1 Storm sewers and storm sewer systems can also be referred to as municipal separate storm sewer systems (MS4s). Stormwater
discharge permits can be referred to as MS4 permits.
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Utilities
EPA Region 10
700,000 population
CSO II WIS4

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would provide better public health and environmental
benefits than the CSO projects alone. Modeling
showed that these stormwater projects would remove
larger quantities of PCBs (polychlorinated biphenyls),
fecal coliform, phosphorus, and other pollutants. They
include:
¦	Reconstructing city rights-of-way to include
bioretention basins (a green infrastructure practice)
that infiltrate stormwater to reduce the amount
discharged and remove pollutants.
¦	Building a facility to treat stormwater from a largely
industrial area.
¦	Increasing street sweeping on major roads to
minimize stormwater contamination.
The integrated plan also included several large,
more effective CSO projects—such as sewer system
improvements, CSO storage facilities, and a new
tunnel—that were expected to lead to significant
reductions in pollution. The plan deferred completion of
six other small CSO projects beyond 2025.
Seattle's analysis concluded that the integrated plan
would achieve greater water quality benefits than the
LTCP. Even with certain CSO projects deferred, the
stormwater projects would treat a much larger volume
of stormwater than the deferred CSO projects, resulting
in greater reductions of total suspended solids, metals,
bacteria, and other pollutants. For example, Seattle
estimated that the integrated planning projects would
remove 110 more pounds of zinc per year than the LTCP
projects alone. The city projected that enhanced street
sweeping would keep an estimated 40 tons of total
suspended solids out of waterways every year.
Seattle estimated that the integrated plan would
cost a total of $592 million over 20 years, including
both capital and operation and maintenance costs.
Stormwater projects accounted for $88 million—
about 15 percent of the total cost. The integrated plan
included $450 million in non-deferred CSO projects
and proposed to defer $54 million in CSO projects
until 2028-2030, which is later than the consent
decree and LTCP. The integrated plan was ultimately
more expensive than the LTCP option, but it extended
CSO project implementation by four to five years, and
the proposed stormwater projects were predicted to
Seattle's Selection Process for Integrated
Plan Projects
To choose projects for the integrated plan, Seattle:
¦	Modeled pollutant reduction of each project
¦	Estimated each project's effectiveness at reducing human
and animal exposure to bacteria and other harmful pollutants
¦	Determined how close each project would be to planned
stormwater projects
¦	Ranked stormwater and CSO projects based on water
quality impacts, proximity to existing stormwater projects,
performance risk, operation and maintenance costs, and
community values
¦	Compared the benefits of prioritized stormwater projects and
lower-volume CSO projects to ensure that the stormwater
projects would achieve significantly higher benefits
achieve greater water quality benefits than the deferred
CSO projects.
Results
EPA and the Washington Department of Ecology
approved the Plan to Protect Seattle's Waterways in
2015. The city's CSO discharge permit, issued in 2016,
required two of the three proposed stormwater projects
(i.e., bioretention in city rights-of-way and street
sweeping) and deferred the six small CSO projects in
accordance with the schedule identified in Seattle's
integrated plan.
As of 2018, the city reduced CSO discharges by 41
percent. During 2018, the street sweeping program
removed nearly 60 tons of total suspended solids.
Seattle finished constructing right-of-way bioretention
in one area in 2017 that was designed to reduce
CSO discharge volume by one million gallons per
year. This green infrastructure project also benefits
the community by increasing pedestrian activity,
calming traffic, improving aesthetics, and increasing
public awareness of how impervious surfaces affect
stormwater. In April 2020, Seattle Public Utilities
received a $192 million Water Infrastructure Finance
and Innovation Act loan to help finance an underground
storage tunnel recommended in the plan. This storage
tunnel is designed to reduce CSOs at 6 outfalls, and its
construction is expected to create over 1,000 jobs.
For more information, visit EPA's integrated planning Report to Congress webpage at:
https://www.epa.gov/npdes/inteqrated-planninq-municipal-stormwater-and-wastewater
SEPA
EPA-832-F-21-017 I June 2021

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