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Facts about the King County regional wastewater system

All data current for 2023, the most recent year for which data are available, unless otherwise noted.

Customers and service area

  System West Point South Brightwater Vashon Carnation
Wholesale customers King County has sewage disposal contracts with 18 cities, 15 sewer districts, and the Muckleshoot Tribe to provide wholesale wastewater treatment services. -- -- -- 1 sewer district City of Carnation
Population Served (estimated) Approximately 2 million residents 785,300 866,200 293,100 1,100 (based on
2020 census data)		 2,167
Area served 424 sq mi 113 sq mi 240 sq mi 69 sq mi 660 ac (by 2050) 821 ac (in city and surrounding
urban growth area)
Key:
-- = not applicable or does not apply
sq mi = square miles
ac = acres

Sewage treated

  System West Point South Brightwater Vashon Carnation
Sewage treated (volume expected in average rainfall year)
178 mgd 95 mgd 65 mgd 18 mgd 0.15 mgd 0.100 mgd
Sewage treated (2023)
182.3 mgd1 91.9 mgd 71.1 mgd 18.5 mgd 0.154 mgd 0.104 mgd
Peak day flow (2023)
 731.1 mgd2 411 mgd3 195.4 mgd4 38.2 mgd5 0.779 mgd6 0.157 mgd7
Septage treated (2023)
23.8 mg/yr
(1,810 dry
tons solids)8		 -- 23.8 mg/yr
(1,810 dry
tons solids)		 -- -- --
Key:

-- = not applicable or does not apply
1System average includes 0.5 mgd from the CSO Treatment Plants.
2System peak flow includes 85.55 mgd from the CSO treatment plants; System Peak day occurred on 12/5/2023.
3West Point max day flow occurred on 12/5/2023.
4South Plant max day flow occurred on 12/5/2023.
5Brightwater max day flow occurred on 12/5/2023.
6Vashon max day flow occurred on 12/5/2023.
7Carnation max day flow occurred on 12/4/2023.
8Septage flow and dry tons includes septic haulers plus flow and solids from other treatment plants. 
CSO = combined sewer overflow
mgd = million gallons per day
mg/yr = million gallons per year

Facilities and pipelines

  System West Point South Brightwater Vashon Carnation
Treatment plants 5 West Point Treatment Plant, Seattle South Treatment Plant, Renton Brightwater Treatment Plant, Snohomish County north of Woodinville Vashon Treatment Plant, Vashon Island Carnation Treatment Plant, Carnation
Pump stations 48 20 19 4 5 --
Regulator stations 26 24 2 -- -- --
CSO wet weather treatment facilities 5: Alki, Carkeek, Georgetown Henderson/Norfolk,
Mercer/Elliott West		 -- -- -- -- --
CSO storage facilities 4: Murray, North Beach, Rainier Valley, South Magnolia -- -- -- -- --
Conveyance storage 2: Logboom, North Creek -- -- -- -- --
King County sewer conveyance lines About 383 mi sewer lines (and 15 mi reclaimed water lines) -- -- -- 3.9 mi 1.75 mi
Sewer pipe diameter (smallest to largest) 2.5 in. to 14 ft, 8 in. 8 in. to 14 ft, 8 in. 8 in. to 14 ft, 8 in. 8 in. to 14 ft, 4 in. 2.5 in. to 8 in. 8 in. to 12 in.
Outfalls, length and depth -- 3,600-ft long; 240-ft deep; 1 diffuser at 600 ft-long

Discharge to Puget Sound.		 2 parallel pipes, 10,000-ft long each; 625-ft deep; 4 diffusers at 500-ft long each

Discharge to Puget Sound. 		2 parallel pipes, 4,600-ft long each; 600-ft deep; 2 diffusers at 250-ft long each

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
Key:
-- = not applicable or does not apply
CSO = combined sewer overflow
ft = feet
in. = inches
mi = miles

Design criteria

  West Point South Brightwater
 Vashon Carnation
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). For West Point, the average does not include flow on days with rainfall. For all plants, this number is based on an average that does not show the maximum flows that can be handled at a plant. 		133 mgd 115 mgd 30 mgd
 0.26 mgd 0.37 mgd
Maximum monthly average flow: 215 mgd 144 mgd 40.9 mgd
 0.52 mgd 0.48 mgd
Average dry-weather flow: 110 mgd 96 mgd 25.2 mgd
 -- 0.37 mgd
Biochemical oxygen demand (BOD) influent loading (annual average) 168,000 lb/day 220,000 lb/day 66,063 lb/day (maximum month)
 526 lb/day 321 pounds/day
Total suspended solids (TSS) influent loading (annual average) 181,000 lb/day 201,000 lb/day 61,400 lb/day (maximum month)
 526 lb/day 321 pounds/day
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 100 mgd
 1.14 mgd (or 2.05 mgd with on-site storage) 1.40 mgd
Key:
-- = not applicable or does not apply
IPS = Influent Pump Station
lb = pounds
mgd = million gallons per day
mg/L = milligrams/liter

Permit information

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

NPDES permit 6/1/2024 - 5/31/2029

NPDES permit 8/1/2015-7/31/2020

(Application for NPDES permit renewal was submitted in 2019. Permit administratively extended until renewed.)

Reclaimed water permit: application for reclaimed water permit renewal was submitted in 2019. Permit administratively extended until renewed.

NPDES permit 3/1/2018-2/20/2023

(Application for NPDES permit renewal was submitted in 2022. Permit administratively extended until renewed.)

Reclaimed water permit: application for reclaimed water renewal permit was submitted in 2023. Permit expected to be administratively extended in prior to expiration in April until renewed.

NPDES permit 3/1/2017-2/28/2022

(Application for NPDES permit renewal was submitted in 2021. Permit administratively extended until renewed.)

NPDES and reclaimed water permit (combined) 11/1/2021-10/31/2026
Puget Sound Nutrient General Permit: South Plant, West Point, Brightwater, and Vashon: coverage issued 4/1/22; expiration 12/31/26
Key:
BOD = biochemical oxygen demand
NPDES = National Pollutant Discharge Elimination System
TSS = total suspended solids

Resource recovery

  System West Point South Brightwater Vashon Carnation
Electricity produced (2023)

8.82 million kWh

 8.355 million kWh 0.465 million kWh -- -- --
Methane gas sold (2023)
(Both West Point & Brightwater also use digester gas to power plant equipment, such as boilers and digesters)		 2.911 million therms -- 2.911 million therms -- -- --
Loop biosolids produced (2023) 124,195 wet tons (28,776 dry tons) 42,120 wet tons (11,796 dry tons, average percent solids: 28.0) 66,544 wet tons (13,769 dry tons, average percent solids: 20.7)

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

15,530 wet tons (3,211 dry tons, average percent solids: 20.7)

397,872 gallons (33 dry tons, average percent solids: 2)

Untreated solids hauled to South Plant for further treatment.

997,842 gallons (42 dry tons, average percent solids: 1)

Untreated solids hauled to South Plant for further treatment.
Carbon sequestration (2023)
(From Loop biosolids land application)		 34,644 MtCO2e
 16,437 MtCO2e 13,725 MtCO2e 4,481  MtCO2e
 -- --
Reclaimed/recycled water (filtered) used at treatment plants (2023) 783.4 mg/yr 187 mg/yr

Note: Additional lower quality (non-Class A) water is also used for in-plant processes.		 69.4 mg/yr

Note: Additional lower quality (non-Class A) water is also used for in-plant processes.		 522 mg/yr (as C3 - water, membrane effluent)
 -- 5 mg/yr (assumed)
Reclaimed/recycled water (Class A) distributed offsite (2023) 39.6 mg/yr -- 4.1 mg/yr
(offsite use for irrigation and street cleaning)

0 mg/yr

Note: No reclaimed water was distributed to customer sites due to pipe failures throughout the conveyance system.

 -- 35.5 mg/yr
(distributed to Chinook Bend Natural Area Wetland Enhancement Site)
Key:
-- = not applicable or does not apply
kWh = kilowatt-hour
mg/yr = million gallons per year
MtCO2e = metric tons of carbon dioxide equivalents

Financial information

Wholesale sewer rate $55.11 per month (beginning 1/1/2024)
Sewage treatment capacity charge $74.23 per residential customer equivalent (RCE) per month (beginning 1/1/2024)
Biennial budget (2023-2024) Operating: $391.8 million
Capital: $984 million (including a disappropriation of $65 million of completed/legacy projects)
Number of employees (as of Dec. 2023) About 719 full-time equivalents (FTEs); 39 term-limited temporaries (TLTs) and 36 short-term temporaries (STTs), which includes 18 high school and college interns.
WTD expenditures for capital facilities

View projects under construction		 $3.3 billion (2023–2028)

Industrial pretreatment

Industrial pretreatment program
Industrial/commercial facilities with permits (aka Significant Industrial Users) 99
Industrial/commercial facilities with letters and discharge authorizations 503
Approximate number of compliance samples taken (except BOD and TSS)  1,619
Inspections conducted  233
Key:
BOD = biochemical oxygen demand
TSS = total suspended solids

Combined sewer overflow and rainfall

Combined sewer overflows (CSOs)
CSO control program
Baseline volume (1980–83) 2.3 billion gallons per year
Current volume (Jan.–Dec. 2023) 644 million gallons per year
Number of King County CSOs 38 (view CSO status map)
Total rainfall (measured at SeaTac) Total rainfall (2024): 34.97 inches
Long-term average annual rainfall: 38.29 inches (1945–2016)

 

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