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 Mountlake Terrace has three pump stations, 69 miles of sewer mains, 1,500 manholes and about 5,500 connections from homes and businesses.

Approximately 80% of the wastewater from Mountlake Terrace is collected and pumped to the City of Edmonds for treatment. The remaining 20% flows through Brier and Lake Forest Park and is treated by King County Metro at the West Point Treatment Plant near Discovery Park in Seattle.

The wastewater that goes to the City of Edmonds is treated at the City’s treatment plant near 2nd and Dayton.

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 or leave a comment below.  You can also connect with Dustin on LinkedIn here.
  1. I think I’ll cancel my reservation at Anthony’s. Somehow the view won’t be the same ever again.

    Seriously, this is a great series of articles. Thanks for taking the time to write them.

  2. I know some homes in the greater area are on septic tanks. Are there homes in MLT that are on septic tanks or is everyone hooked up to the wastewater main?

    1. Great question, Bret. There are some septic tanks still in use in the City. Most of the the sewer system was built in the late 50’s/early 60’s so some of the older developed lots still use septic tanks but the number is less than 30. Any new development is required to connect to the City’s sewer system.

  3. I really enjoy these clearly written and informative pieces. It’s very helpful to know how things work. Thanks, Dustin.

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