Ask the Engineer: Where Does Our Wastewater Go?


In the last Ask the Engineer article we discussed where our drinking water comes from. Today we’re going to take a look at where our water goes after we use it. Everything that goes down the toilet, the shower, the sink or any drains. Where does it all go?

sewer_lateral_diagram[1]All of these sources of wastewater in each of our homes and businesses combine into a pipe that exits our property. This is often referred to as a side sewer or sewer lateral, and is generally 4-8″ in diameter. Each of these side sewers dumps into the sewer mains in our streets which are generally 8″ and larger in diameter. You have all no doubt seen manhole covers in the streets that have “SEWER” written on them. These are cylindrical concrete structures, generally at least 4-feet in diameter and as deep as 20 feet, that have sewer mains coming in and out of them.


I’ll use the PG version, but we in the public works industry know the phrase “crap rolls downhill” as more than just a metaphor for inheriting your bosses problems, it’s how sewer systems work. We design sewer systems to take advantage of gravity as much as possible by allowing all of the wastewater in the pipes to flow downhill. If the topography of the area can accommodate it, it will continue to flow by gravity to point where it will be treated. If not, it will need to be pumped to where it needs to go. Sewer mains that have pumped wastewater in them are called force mains. The pumps are “forcing” the wastewater to go uphill.

The City of Edmonds has three pump stations, 168 miles of sewer mains, 3,300 manholes and about 9,300 connections from homes and businesses.

The original sewer system in Edmonds was constructed prior to 1920 and as a combined storm and sanitary sewer system, discharging untreated wastes in the the Puget Sound. During the 1950’s efforts were made to separate the storm system from the sanitary sewer and in 1957 the Wastewater Treatment Plant was constructed near 2nd and Dayton. Currently the majority of the wastewater is treated at the Edmonds Wastewater Plant while wastewater from the north part of the City flows to the Lynnwood Wastewater Treatment Plant.

There are five steps that happen at the Edmonds Wastewater Treatment Plant to clean the wastewater:

wwtpHeadworks Screening

When the wastewater enters the facility it goes through a 1/4″ screen to remove all large objects like cans, rags, sticks, rocks, plastic packets etc. carried in the wastewater stream.

Primary Clarifiers

These are tanks that are used to settle sludge while grease and oils rise to the surface and are skimmed off.

Aeration Basin for Biological Treatment

This process involves air or oxygen being introduced into the wastewater combined with organisms to develop a biological floc composed of bacteria and protozoa which reduces the organic content of the wastewater. In other words, this process helps all the tiny particles clump together so that they settle out and can be removed.

Secondary Clarifiers

Additional tanks that are used to settle out sludge and solids produced during the biological treatment.

Chlorine Disinfection

The primary purpose of chlorination at the EWWTP is disinfection (removal of disease causing pathogens).

After the wastewater is treated, it is discharged through a 48-inch pipe which eventually reduces to a 36-inch pipe prior to entering the Puget Sound. At Olympic Beach the pipe divides into two separate 36-inch outfall lines that discharge the treated wastewater about 800-feet offshore.


Dustin DeKoekkoek, P.E. is a civil engineer with RH2 and designs public infrastructure projects all over the Pacific Northwest. Have a question about the topic covered here or for a future “Ask the Engineer” column? Email Dustin at [email protected] or leave a comment below.  You can also connect with Dustin on LinkedIn here.

9 Replies to “Ask the Engineer: Where Does Our Wastewater Go?”

  1. Here’s a question I received via email:
    “Can you tell me if the chlorine has a chance to “off gas” from the treated water prior to that water entering the sound? Gotta have an impact on pH, yes?”

    Yes, the treated wastewater goes through a dechlorination process before being discharged 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 but more importantly to reduce the toxic effects of chlorine in the treated effluent.


    1. Chris, great question. All of the water is taken out of the solids removed from the wastewater and they are incinerated on-site. The ash produced in the process is transferred to a local landfill and the air emissions from the incinerator are regulated by the Puget Sound Clean Air Agency.


  2. Dustin,

    So, what goes into the Sounds – is sterilized.

    You didn’t mention, and I’ve heard – drugs, including parmaceuticals and heavy metals – do flow into the sound, and that caffeine is now found in some marine life.

    What’s the story?



  3. The odor on Dayton street from the plant is awful when walking around that area. Can this be remedied, or what is causing this odor to settle outside of the plant? I am assuming that the people that live there have gotten used to it and probably don’t notice it, but it is stinky!


  4. We are still wondering about the odor from the facility……This morning we smelled that same odor very strong on 5th and Dayton. …….still wondering about this as we could smell it on the street on 5th


    1. Hi Tere — I forwarded your questions/concerns to Public Works Director Phil Williams, and here’s what he said:
      “There will always be some impacts when a wastewater treatment plant is located in the middle of a city’s downtown. This is particularly so when it is located so close to residential land uses. The City works very hard to keep a very low profile at the facility including managing odors. All of the primary treatment units are covered and the air captured goes through carbon absorption treatment. As a result the most noticeable and objectionable odors are well controlled. That is not to say that perfection can be achieved. There will always be occasions when some odor can be detected. That said we get very few complaints on an annual basis considering our location.”


  5. Thank you Phil. Maybe we notice it so much because we are there frequently. It certainly is an attractive facade there!


  6. My wife and I live on Dayton St. less than 4 blocks east of the waste water treatment plant. In the 10 years we’ve lived here the wind has never carried any bad odor our way. Additionally, we walk by that facility almost every day, rain or shine, and have never detected any unpleasant odor. We never walk on the 2nd Ave. side of it, so perhaps that’s where the bad smell is being noticed.


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