Wastewater Treatment Plant
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The Edmonds Wastewater Treatment Plant (WWTP) is a regional facility.

The original Primary Treatment plant was built in 1957. The plant was upgraded two times by 1967 to handle the increased flows. Partnerships with Mountlake Terrace and Ronald Sewer District were established in 1959 and with Olympic View Water and Sewer District in the late 1960’s.

From 1986 to 1991 the treatment plant was expanded to incorporate secondary treatment. The $35 million construction cost was funded by Federal grants (50%) and by the four (4) partners (50%): MLT, OV, and RSD (together constitute 49%), and the City of Edmonds (51%). The original 30 year agreement stated that while the plant is owned and operated by the City of Edmonds, the other 3 partners own their purchased capacity and they pay their portion of the annual operating and capital expenses. The original agreement focused on construction of the new facility. A new thirty (30) year Agreement that is in the final stages of approval focusses on new regulatory requirements, joint use areas, and supporting the partnership that will enable The City of Edmonds to keep treatment costs low for its citizens.

In 1988 the City entered into an agreement with King County (Metro). This agreement contained the following provisions:

o   Sewage from Metro’s Richmond Beach service area would be treated at Edmonds. Metro would abandon the Richmond Beach Treatment Plant (due to regulatory issues and not wanting to add secondary treatment) and turn it into a pumping station.

o   Metro would build a pump station and sewer line(s) to facilitate a flow swap to West Point from Edmonds East, which alleviated the need of MLT to upgrade their pump station.

o   The agreement detailed the construction project, flow exchange, and payment methodologies.

The original agreement was amended in 1993, 2000, and 2012. The agreement expires in 2036.

Regulatory Compliance

The WWTP must comply with all local, State, and Federal Laws. The operating performance is directed and regulated by:

o   WA State Department of Ecology who issues the NPDES permit.

o   Puget Sound Clean Air Agency who issued the construction permit for the incinerator and oversees air emissions and Title V Permit.

o   Environmental Protection Agency who regulates the incinerator emissions.

Each agency’s permit requires various monthly, semi-annually, annually, and “as needed” reports.

Design and operating information

o   Treatment Process            Activated Sludge

o   Rated Capacity                 11.8 Million Gallons per Day (MGD)

o   Average Flow                   Winter 6.5 MGD        Summer 4 MGD


Staff: Currently there are16 FTE’s authorized. The majority of staff require specialized training, knowledge, experience, and certifications. Based on need, the plant is staffed 14 to 24 hours/day, 365 days/year.

The plant maintains an accredited laboratory where the chemical, physical, and bacteriological processes that treat the wastewater are analyzed. The analysis of wastewater is necessary for the plant to demonstrate compliance with its discharge permit. The laboratory is staffed five days a week by a Laboratory Technician. Certified Operators provide laboratory support on the weekends.

The treated wastewater (effluent) from the plant either flows by gravity or may be pumped (depending on tidal influence) to the two (2) outfall pipes. The outfall pipes are located approximately 1300 feet offshore in Puget Sound at an average depth of 70 feet. Ocean current modeling demonstrates that this location provides excellent mixing of the treated wastewater within Puget Sound. Treatment plant staff monitor effluent quality to ensure compliance with all regulatory requirements and to ensure that the waters of Puget Sound are protected.

The solids that are removed during the treatment process are burnt in an incinerator and the resulting ash is disposed of in a landfill. The incineration process has come under increased scrutiny by the EPA which has implemented more stringent regulatory requirements for continued use of incinerators. Because of these more stringent requirements, plant staff is working diligently to find an environmentally sound alternative to incineration. 

The Lab Analyst is responsible for all chemical, physical and bacteriological analysis of wastewater samples necessary to meet the discharge permit and process control requirements.  The analyst assembles results, maintains lab records and manages data using a computer and implements a quality control program for all lab methods and techniques.

In addition, the lab analyst is the lead person at the plant for implementing the Hazardous Communication Act (Right to Know).  MSDS sheets are maintained for the plant as well as for the lab.  Training on hazardous chemicals, lab safety and procedures is conducted by the lab analyst as well.

Fluidized Bed Incineration

The bio-solids being fed to the incinerator are 18% to 22% solids.  The bio-solids are pumped with a two piston concrete pump at the rate of 6 to 9 gpm into the fluidized sand bed of the incinerator.  The sand bed is made up of approximately 16,000 lbs of green lightening sand.  Diesel is used as an auxiliary fuel in order to maintain a bed temperature of 1400 degrees Fahrenheit.  The air supply is preheated to 1200 degrees Fahrenheit by a heat exchanger in order to reduce the auxiliary fuel consumption.  The pre-heated air is forced up through the bed of hot sand and causes the sand bed to be fluidized.  When the sand bed is fluidized it acts like boiling water.  1000 gallons of bio-solids can be burnt and it will only generate two quarts of ash that will need to be disposed of.

The addition of 50 gpm of water at the inlet to the Venturi is used to encapsulate the ash particles of the exhaust gases and drop the ash and water out for ultimate disposal.  The Venturi is used to create a pressure drop of 29" of water (1.03 psi).  The exhaust gases are then forced through a 4 stage tray cooler that has 200 gpm of water flowing in the opposite direction of the exhaust gases.  Each stage of the tray cooler has a plate with small holes in it that the air must flow through.  The water runs across the top of each plate and the exhaust gases bubble through the water.  The exhaust gases are reheated with a natural gas fired burner in order to eliminate the vapor plume for aesthetics.

Effluent Pumping Station

The effluent pump station at the Edmonds Wastewater Treatment Plant houses five effluent pumps, two variable frequency drives (VFD), three constant speed drives, discharge control valve, motor control center, effluent sampler, and water feature pump.  The effluent pumps have been designed to accommodate the peak rate of 21.5 million gallons per day.

Effluent from the Edmonds Wastewater Treatment Plant may discharge by gravity at elevation 9.5 and below or by pumping above elevation 9.5.  Discharge of effluent is through a 48-inch pipe which eventually reduces to a 36-inch pipe prior to entering Puget Sound.  At Olympic Beach the discharge pipe divides into two separate 36-inch outfall lines, each furnished with 36-inch butterfly valve to control direction of effluent flow.  The final 160-feet of each outfall is a diffuser section with 8-inch diffuser parts spaced every 30-feet.


The City of Edmonds most recent accomplishment was receiving Department of Ecologys' Plant of the Year Award for O & M excellence in Region 10.  The City's application concentrated on cost effective operation coupled with highly automated facility and tightly integrated operations between operators, mechanics, and electricians.

The design of the Edmonds Wastewater Treatment Plant won a national engineering excellence award in 1991 by the American Consulting Engineers Council for the secondary treatment expansion.  The plant upgrade was designed by HDR, Inc.

The City received a congratulatory letter from the Department of Ecology following the design and installation of a dechlorination system performed by plant staff.  Staff design of the dechlorination process, consisting of sodium bisulfite feed and ORP control, was installed at a cost of only $40,000 to the City.  Department of Ecology's letter to the City states this cost was extremely low for a plant with an 11.8 million gallon per day capacity.


Treated effluent is chlorinated for disinfection purposes and then dechlorinated before being discharged from the Edmonds Wastewater Treatment Plant (EWWTP) to Puget Sound.

Disinfection - The primary purpose of chlorination at the EWWTP is disinfection (removal of disease causing micro-organisms pathogens).  Chlorination may also serve other purposes such as:

  • Odor reduction
  • Aid in grease removal
  • BOD reduction
  • Nitrate reduction
  • Control bulking activated sludge
  • Algae control

Two factors are extremely important in disinfection:  1)  Time of contact and 2) Concentration of disinfecting agents.

Dechlorination - The purpose of dechlorination at the Edmonds Wastewater Treatment Plant is to remove the chlorine residual in the effluent caused by the disinfection process.  This is necessary in order to meet the chlorine residual permit requirement and also reduce the toxic effects of chlorine in the secondary treated effluent.

Contact Public Works

Public Works & Utilities Administration
Operations & Maintenance Center
7110 210th St SW
Edmonds, WA  98026

Phone: (425) 771-0235
Fax: (425) 744-6057

Public Hours:
8:00am to 4:30pm M-F
Closed Weekends and Holidays

For Emergencies Call 911

Wastewater Treatment Plant
200 2nd Ave S
Edmonds, WA 98020

Phone: (425) 771-0237
Fax: (425) 771-0255