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