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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 2014,
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)
1.6 million residents 670,000
(varies summer to winter)
(varies summer to winter)
(based on census data)
2,000 221,000
424 square miles 113 square miles 240 square miles 660 acres
(by 2050)
821 acres
(in city and surrounding Urban Growth Area)
69 square miles

Sewage treated

  System West Point South Vashon Carnation  Brightwater
Sewage Treated
(Volume expected in Average Rainfall Year)
181.21 mgd 95 mgd 69 mgd 0.12 mgd 0.09 mgd 17 mgd
Sewage Treated (2014)
185.92 mgd 95.3 mgd 72.0 mgd
0.128 mgd 0.089 mgd 18.4 mgd
-- -- 27.1 million gallons -- -- --

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
-- 25 -- -- -- --
CSO wet weather treatment facilities
Alki, Carkeek, Mercer/Elliott West, Henderson/Norfolk
-- -- -- -- --
King County sewer conveyance lines
about 391 miles -- -- 3.2 miles 1.6 miles --
Sewer pipe diameter
(smallest to largest)
2.5 inches to 17 feet 8 inches to 14 feet, 8 inches 8 inches to 17 feet 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 mdg (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 administratively extended until a new permit is issued.
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 annually
-- 18.963 million kilowatt hours 0.286 million kilowatt hours -- --  --
Methane gas sold annually
(Both West Point & South Plant also use digester gas to power plant equipment, such as boilers and digesters)
-- Not applicable 1.88 million therms
-- --  --
Biosolids produced annually
119,325 wet tons
(or 29,675 dry tons) 
44,086 wet tons at 28.4% solids (or 12,528 dry tons) 

60,163 wet tons at 23.3% solids (or 14,054 dry tons)

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

595,856 gallons at 2% solids (or 42 dry tons). Untreated solids hauled to South Plant for further treatment 544,565 gallons at 2% solids (or about 43 dry tons). Untreated solids hauled to South Plant for further treatment  15,074 wet tons (or 3,092 dry tons at 20.5% solids)
Reclaimed/Recycled water (Class A) used at treatment plants 
698.9 million gallons per year

184.9 million gallons per year (used for industrial process at treatment plant).

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

95 million gallons per year (industrial use at the treatment plant)

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

-- -- 419 million gallons per year            
Reclaimed/Recycled water (Class A) distributed offsite  About 75.3 million gallons per year   --  2.7 million gallons per year (off-site customer use for irrigation, street sweeping, and sewer flushing).  --  32.6 million gallons per year (distributed to Chinook Bend Natural Area wetland enhancement site) 40 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 1/1/2015)
Sewage treatment capacity charge $57.00 per residential customer equivalent (RCE) per month (beginning 1/1/2015) 
Annual budget (2014) 2015:
Operating: $135 million 
Capital-related: $162.5 million
Number of employees 
(as of Jan 2015)
About 569 full-time equivalents (FTEs); 24 term-limited temporary (TLT) employees, and 9 temporaries (7 interns and 2 Customer Service Specialists).
Brightwater Treatment Plant estimates Cost: $1.859 billion (with inflation) 
Serves: South Snohomish and North King counties 
Capacity: 30 mgd, 2013; 54 mgd, 2040
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 2014 Water Year, 39.11 inches; 
Long-term average annual rainfall 38.37 inches (1949-2014)
Industrial Pretreatment Program
Significant industrial users 121
Industrial/Commercial facilities with discharge authorizations 514
Approximate number of compliance samples taken 1,700
Inspections conducted 347
Combined Sewer Overflows (CSOs) 
CSO Control Program
Baseline volume 
2.3 billion gallons per year
Current volume 
(Jan.-Dec. 2014)
1.14 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 2014
Ratepayer Report
, April 2014