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 Lynnwood has six pump stations, 104 miles of sewer mains, and about 4,300 acres of area served.

Most of the wastewater from Lynnwood is collected and treated at the Lynnwood Wastewater Treatment Plant. Parts of the City of Lynnwood receives sewer service from the Alderwood Water District and a small portion is served by the City of Mountlake Terrace. Both deliver the City of Lynnwood wastewater to King County Metro for treatment and disposal. The Lynnwood Wastewater Treatment Plant is located along the shore of Browns Bay on the Puget Sound and also accepts some flows from Edmonds and Mountlake Terrace.

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

WWTP[1]Headworks Screening

When the wastewater enters the facility it goes through a 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 plant is disinfection (removal of disease causing pathogens).

After the wastewater is treated, it is discharged through a 36-inch diameter 240 foot long x 130 foot deep diffuser that disperses the flow through multiple 3 to 4 inch ports into Browns Bay of Puget Sound.


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.

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