• Alyssa Barton

What’s That Smell? CSO’s and Why We Still Have Them

Updated: Apr 23, 2019


I recently wrote about how cigarette butts are a pollutant - but do you know how our storm sewer systems work?

Cigarette butts can be washed down stormdrains and out into local streams and rivers. This situation happens when you have what’s called an MS4 – a Municipal Separate Storm Sewer System. With MS4’s, you have sewer lines (pipes in a system that carry folks toilet water from homes and businesses to a treatment plant), separate from storm sewer lines (pipes in a system that carry rainwater aka “stormwater” from the storm drain to a stream, river, or what have you). The stormwater from the storm sewer pipes flows out to waterbodies in what’s called a Sanitary Sewer Overflow, or SSO.

Sometimes, though, you have a system with combined sewer and storm sewer lines. This is where the water from storm sewer pipes is carried to the sewer system and treated with our sewage at a wastewater treatment plant. This is great, since stormwater is the number 1 source of toxic pollution to Puget Sound.

Rainwater flowing over developed surfaces can pick up oil, grease, and heavy metals from cars and tires, chemicals from man-made surfaces and structures, and litter and other pollutants off streets. Treating stormwater at our wastewater treatment plants is ideal because it helps remove debris and chemicals from the water before it dumps into our local waterbodies. However: the combined sewer system is not so great when it rains so much that the quantity of stormwater plus wastewater from our homes and businesses exceeds the system’s capacity.

Here’s a great illustrative photo from the interwebs - Washington’s Department of Ecology has an image just like this on file.


The above diagram shows how, when there’s a storm event and you have a combined sewer system, you can get a rush of too much stormwater flooding into the system and so the combined lines back up. (See the upper righthand square). This situation results in a lovely, pungent, sludgey brew of raw sewage (approximately 10%) mixed with stormwater (approximately 90%) polluting our waters during what’s called a “combined sewer overflow,” or “CSO” event.

This is similar to what happened when the West Point treatment facility at Discovery Park in Seattle experienced a bypass in February of 2017. https://www.seattletimes.com/seattle-news/environment/king-county-fined-361000-over-west-point-treatment-plant-failure/


Here’s a photo of the mix of stormwater and poo water from the bypass at West Point, taken on February 16th, 2017, by the Seattle Times (Copyright: Seattle Times). Notice how, unlike in the diagram previously shared, the water is, in fact, visibly poopy.

West Point was an extreme example of a treatment plant unable to handle inputs of stormwater and sewage water – though it resulted from several system malfunctions. Gnarly, huh?

But CSO’s happen all the time.

King County offers a fun real time map that shows all the combined sewer outfalls around Seattle, and reports when a CSO event is happening near you! https://www.kingcounty.gov/services/environment/wastewater/cso-status.aspx

Why is this happening!!? How is it that cities and counties are allowed to knowingly operate waste treatment systems that sometimes don’t function, shooting millions of gallons of sewage and stormwater into our waters? Doesn’t the Clean Water Act protect against this kind of thing? All good questions you might ask.

Well, as to the why: lots of cities around the nation are on combined sewer systems – one EPA webpage says there are 772 municipalities on combined systems nationwide. About 20% of King County’s service area is served by a combined sewer system – all of these are in Seattle. We just didn’t know any better when we first built our cities, apparently. Perhaps the land planners didn’t account for explosive population growth. The population of Seattle and the Puget Sound region is rapidly growing – per worldpopulationgrowth.com, Seattle has grown from just 1,150 people in 1870 to an estimated population of 659,000 in 2014, and per most sources, severe storm events are getting worse nationwide as well. More people + more water = more CSO’s. Combined Sewer systems just weren’t designed to handle the excessive loads they sometimes now face.

As for the “how are we allowing this” question: well, starting in the 1990’s, the EPA tried to start doing something about CSO’s nationwide under the Clean Water Act. The Agency started working with municipalities to reduce CSO’s, and in Washington State, EPA sued King County under the Clean Water Act for our pollution. Rather than immediately stopping the pollution, in 2013 the parties entered into what’s called a consent decree, or settlement agreement, that says that King County will reduce CSO events to a limit of 1 max per year per CSO outfall by December 31, 2030. By 2030! That’s a long ways a way, and that’s still one huge smelly nasty event per year that’s allowed per outfall! There are 87 CSO outfalls in Seattle alone.

But that’s what EPA says is required. And, that will get us a whole lot closer to no CSO’s than where we’re currently at. One thing you should know regarding the Clean Water Act: it actually doesn’t prevent pollution. It allows it. I’ll get into this a lot more later, but in short, it’s a major flaw, and that’s why EPA decided that 1 CSO per outfall per year is “good enough” of a standard.

In order to prevent more CSO’s from happening, municipalities can separate out their sewer and storm sewer lines (which isn’t ideal, because then stormwater won’t get treated!), build extra treatment facilities, or build storage tanks to hold excessive stormwater during storm events until such time as it can be treated. Green infrastructure is another option.


Image of green stormwater infrastructure in West Seattle,

courtesy of King County Wastewater Treatment Division’s website.

GSI is an approach to water management that protects, restores, or mimics the natural water cycle. Some types of green infrastructure include raingardens, rain barrels, streamside buffers, planting native trees and vegetation, and permeable pavement – these help slow the flow, store, and/or treat stormwater, and can help prevent CSOs by reducing stormwater inputs into combined sewer systems. Capisce?

If I didn’t mention it before, raw sewage contains bacteria and pathogens that can make people sick, pollute our waters, and results in swimming beach and shellfish bed closures. It would be really great if we could prevent both sewage and stormwater from flowing untreated into our waterbodies, since both can cause a lot of harm to people, fish and wildlife. So what can you do?

If you’re in Seattle, check out this map: http://www.seattle.gov/util/cs/groups/public/@spu/@usm/documents/webcontent/02_008214.pdf. If you live in a green area, with “partially separated sewers,” this means your stormwater might get treated, but likely not. If you live in a yellow zone this means that your stormwater is more likely to get treated - but there’s also a chance that during a heavy rain event, a CSO could happen!

The takeaway? Don’t dump litter on the ground because it could end up in a stream or river. Don’t wash your car in your driveway – just take it to a carwash to be safe. Carwashes are required to collect and treat the dirty carwash water that contains toxic chemicals. And if you really care about making the world a healthier, cleaner place to live, you can look into installing a rain garden or rain barrel on your property – check out programs like Rainwise that offer rebates and credits for these projects – and ask your public officials to prioritize stopping ALL CSO events, as soon as possible.

Together we can work to make sure that raw sewage doesn’t get dumped into Puget Sound – or your local waters, wherever they are - untreated.

#waterpollution #pollution #nature #sustainability #cleanwater #water #environment #stormwater #gogreen #pollutionfree #nopollution #solutiontopollution #sewage #npdes #waterquality #combinedseweroverflow #cleanwateract

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©2019 by Alyssa Barton