Wastewater Treatment Plant

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City staff operates and maintains the City's Wastewater Treatment Plant and manages the City's pretreatment program.  The plant is a regional facility that treats flows from the cities of Edmonds, Mountlake Terrace, and Lynnwood; King County; Olympic View Water and Sewer District; and Ronald Wastewater District.

On average the Edmonds Wastewater Treatment Plant treats 5,600,000 gallons per day.


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