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Wastewater Treatment

King County, Washington

For questions about the Wastewater Treatment Division website, please send an e-mail message or contact us at:

King Street Center
201 S. Jackson St., Suite 500
Seattle, WA 98104-3855
Phone: 206-477-5371
Fax: 206-684-1741
Telecommunication device for the deaf (TTY): 711

Get Directions to our office location in Seattle, Washington.

Staff Contacts

Facts about the King County regional wastewater conveyance and treatment system

All data current for 2015,
the most recent year for which data is available, unless otherwise noted.

-- means "not applicable" or "does not apply"
XX means "information not available at this time"
mgd means "million gallons per day" and
mg/yr means "million gallons per year."

System and treatment plant data

Customers and service area

  System West Point South Vashon Carnation Brightwater 
34 local agencies
(includes 17 cities, 16 sewer districts, and the Muckleshoot Indian Tribe)
-- -- 1 sewer district City of Carnation  --
Population Served (estimated)
Approximately 1.7 million residents 683,000
(based on census data)
2,000 226,000
424 square miles 113 square miles 241 square miles 660 acres
(by 2050)
832 acres
(in city and surrounding Urban Growth Area)
67 square miles

Sewage treated

  System West Point South Vashon Carnation  Brightwater
Sewage Treated
(Volume expected in Average Rainfall Year)
176 mgd 95 mgd 66 mgd 0.12 mgd 0.09 mgd 15 mgd
Sewage Treated (2015)
178 mgd 92 mgd 69 mgd 0.14 mgd 0.09 mgd 17 mgd
24.3 mg/yr -- 24.3 mg/yr
(2,025 dry tons) 
-- -- --

Facilities and Equipment

  System West Point South Vashon Carnation Brightwater
Treatment plants
5 West Point Treatment Plant,
South Treatment Plant,
Vashon Treatment Plant,
Vashon Island
Carnation Treatment Plant,
Brightwater Treatment Plant, Snohomish County north of Woodinville
Pump Stations
47 20 19 4 --
Regulator Stations
26 26 -- -- -- --
CSO wet weather treatment facilities
Alki, Carkeek, Mercer/Elliott West, Henderson/Norfolk
CSO storage facilities 2:
South Magnolia,
North Beach 
Conveyance storage 2:
North Creek
King County sewer conveyance lines
about 391 miles -- -- 3.2 miles 1.6 miles --
Sewer pipe diameter
(smallest to largest)
2.5 inches to 14 feet, 8 inches 8 inches to 14 feet, 8 inches 8 inches to 14 feet, 8 inches 2.5 inches to 8 inches 8 inches to 12 inches 8 inches to 14 feet, 4 inches
Outfalls, length and depth
-- 3,600 ft long;
240 ft deep;
600 ft diffuser

Discharge to Puget Sound.
2 parallel pipes,
10,000 ft long ea;
625 ft deep;
4 diffusers @ 500 ft L ea.

Discharge to Puget Sound.
2,850 ft long;
200 ft deep;
no diffuser

Discharge to Puget Sound.
Primary discharge: Chinook Bend Wetland Enhancement Project.
Backup: Snoqualmie River at Carnation Farm Road Bridge.
2 parallel pipes, 4,600 ft long ea;
600 ft deep;
2 diffusers @ 250 ft L ea

Discharge to Puget Sound.

Design criteria

  West Point South Vashon Carnation Brightwater
Average wet-weather flow (non-storm):
This number is often used to describe the size of the treatment plants. It is based on average wet-season flow (November through April) but does not include extraordinarily large storms in the average. It is based on an average that does not show the maximum flows that can be handled at a plant.
133 mgd 115 mgd 0.26 mgd (est.) 0.22 mgd 30 mgd (2012-2016)
Maximum monthly average flow
215 mgd 144 mgd 0.52 mgd 0.27 mgd 40.9 mgd (2012-2016)
Average dry-weather flow
110 mgd 96 mgd -- 0.21 mgd 25.2 mgd (2012-2016)
Biological oxygen demand (BOD) influent loading (annual average)
168,000 lbs/day 220,000 lbs/day 526 lbs/day 314 mg/L 66.063 lbs/day (max month 2012-2016)
Total suspended solids (TSS) influent loading (average annual)
181,000 lbs/day 201,000 lbs/day 526 lbs/day 314 mg/L  61.400 lbs/day (max month 2012-2016)
Instantaneous maximum capacity:
Maximum flows that can be handled at the plants--often for short periods; some portions of these flows get minimal treatment.
440 mgd 325 mgd 1.14 mgd
(2.05 mgd with on-site storage)
1.4 mgd 100 mgd
(until 2025 or until fifth IPS pump set is installed)

Permit information

  System West Point South Vashon Carnation  Brightwater
Permit (NPDES) renewal dates:
View permit limits (BOD, TSS, fecal coliform, chlorine residual) for each treatment plant.
-- NPDES permit 1/31/2020 NPDES permit 7/31/2020.

Reclaimed Water permit 7/31/2020.
NPDES permit 8/31/2016 NPDES and Reclaimed water permit (combined) 12/31/2018  NPDES permit 7/31/2016

Reclaimed Water permit 8/1/2016

Resource recovery

  System West Point South Vashon Carnation Brightwater
Electricity produced (2015)
21.918 million kilowatt hours 17.992 million kilowatt hours 3.926 million kilowatt hours -- --  --
Methane gas sold (2015)
(Both West Point & South Plant also use digester gas to power plant equipment, such as boilers and digesters)
1.825 million therms Not applicable 1.825 million therms
-- --  --
Loop Biosolids produced (2015)
114,957 wet tons
(or 28,262 dry tons) 
39,758 wet tons at 29.29% solids (or 11,644 dry tons) 

58,615 wet tons at 22.83% solids (or 13,384 dry tons)

Note: South Plant receives untreated solids from Vashon and Carnation plants.

1,101,918 gallons at 2% solids (or 91.9 dry tons). Untreated solids hauled to South Plant for further treatment 574,820 gallons at 2% solids (or about 47.9 dry tons). Untreated solids hauled to South Plant for further treatment  16,585 wet tons at 19.62% solids (or 3,254 dry tons )

Carbon sequestration (2015)

(From Loop biosolids land application. MtCO2e = metric tons of carbon dioxide equivalents)

37,263 MtCO2e (the equivalent of taking 8,000 cars off the road) 16,177 MtCO2e 16,553 MtCO2e --  -- 4,533  MtCO2e
Reclaimed/Recycled water (filtered) used at treatment plants (2015)
761.6 million gallons per year 161 million gallons per year

Note: Additional lower quality (non-Class A) water is also used for in-plant processes.
80.6 million gallons per year

Note: Additional lower quality (non-Class A) water is also used for in-plant processes.

-- -- 520 million gallons per year
Reclaimed/Recycled water (Class A) distributed offsite (2015) About 85.6 million gallons per year    -- 3.2 million gallons per year (offsite use for irrigation, street cleaning, & sewer flushing) --  32.1 million gallons per year (distributed to Chinook Bend Natural Area wetland enhancement site) 50.3 million gallons per year (distributed for irrigation, toilets/urinals, and public art).

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Financial and budget information

Wholesale sewer rate $42.03 per month (beginning 1/1/2016)
Sewage treatment capacity charge $58.70 per residential customer equivalent (RCE) per month (beginning 1/1/2016) 
Annual budget (2016)
Operating: $141.4 million 
Capital-related: $167.2 million
Number of employees 
(as of Jan 2016)
About 606 full-time equivalents (FTEs); 11 term-limited temporary (TLT) employees, and 10 interns
WTD expenditures for capital facilities through 2030 
>> view projects under construction
$3.7 billion (2015-2030)

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Other data (rainfall, industrial pretreatment, combined sewer overflows)

Total Rainfall 
(measured at SeaTac)
Total rainfall 2015 Water Year, 36.82 inches; 
Long-term average annual rainfall 38.35 inches (1949-2015)
Industrial Pretreatment Program
Significant industrial users 116
Industrial/Commercial facilities with discharge authorizations 483
Approximate number of compliance samples taken 1,570
Inspections conducted 356
Combined Sewer Overflows (CSOs) 
CSO Control Program
Baseline volume 
2.3 billion gallons per year
Current volume 
(Jan.-Dec. 2015)
1.47 billion gallons per year 
(view CSO reduction graph)
Number of King County CSOs 38; (view CSO location map)

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Ratepayer Report, April 2016
Ratepayer Report
, April 2016