Where the flushes go

Swirl.ly and San Francisco’s sewer system


The average American flushes about 6 times a day.

Push down the handle, flush the toilet and it’s gone. Each flush is sent to the public sewer system and — somewhat magically — transferred through a network of pipes under city streets to the treatment facility. Once there, it’s processed and shipped out as fertilizer for non-food products, inorganic waste, or cleaned and piped out into the ocean, bay, or river.

It’s totally cool in a not-at-all-sexy sort of way, like lots of city jobs. Ever since being a Code for America fellow, I’ve had a passion for this sort of infrastructure and it’s free tours. They have a specificity, grittiness, and heart that can’t be topped.

That’s me, at the Oceanside Wastewater Treatment Facility. Photo by @jeremywmoore

I went on to create my own sewage course in San Francisco, attending urban watershed information sessions (with free lunch), playing the Urban Watershed Award Winning Game, taking free tours of the Oceanside and Southeast Treatment Plants, and visiting the SF Water building, a downtown building which, among many awesomely sustainable things, treats it’s sewage onsite.


My husband, Jeremy Moore, and I went on to build an app about SF Sewage treatment called Swirl.ly. More on that later.

First, I’ll recount some of my high-level nerdings here, so that you can join me in appreciation of sewage treatment infrastructure.

First off, some vocabulary.

A watershed (or a hydrographic basin) is an area of land that shares a drainage point. This determines the direction rain washes away, the direction of creeks and rivers flow, and even as I found out, the direction your plumbing and sewage goes. You’re in one, all the time. Here’s the San Francisco, broken down by watershed.


Every toilet, faucet, gutter, and drain is connected by a network of almost 1,000 miles of sewer pipes are beneath the City’s roadways.


Since we have a Combined Sewer System, the rainwater, street runoff and sewage all share pipes and make their way towards the water treatment facility.

photo by Mike Koozmin of The S.F. Examiner

There’s a lot of stuff worth knowing here. For example: San Francisco treats 40 billion gallons a year. It’s hard to picture, but enough to fill up Candlestick Park every six days.

If the sewer pipes were laid end-to-end it would span the distance between Portland and San Diego. With most over 100 years old and older, SF has to replace 15 miles annually just to keep up.

I was able to find a pipe layer, and a historical waterways layer. Here’s a map with the pipes in white overlaying the historic waterways in blue.

Data from Oakland Museum displayed with Mapbox. mapbox.com/about/maps

Upon inspection you can spot areas like The Wiggle: a one-mile, zig-zagging bicycle route that minimizes hilly inclines for bicycle riders that generally follows the historical route of the paved-over Sans Souci Valley creek bed. This natural creek has been replaced by pipes following underneath roadways on a grid system. Between the steep incline, the sharp corners, the vast amount of impermeable concrete there is a lot of rainfall and sewage to move in a significantly more constricted way, which tends to cause flooding and overflow after heavy rains.

Enter Swirl.ly: an app to map the flushes.

So when you flush the toilet, where it goes depends on your watershed. Swirl.ly uses your location to maps it to the treatment facility, and gives you some more information about your flushes journey.

Swirl.ly showing a flush from my office to the Southeast Treatment Plant.
“Here at Oceanside, we remove between 96 percent and 97 percent of all waste. We’re quite proud of that.” — plant scientist

Once the sewage is treated, it’s sent out with a 4.5 mile pipe into the Pacific Ocean. The site at the end of the pipe is monitored for impact regularly, and the Oceanside Water Treatment facility has gotten numerous awards for protecting the environment.

The Pacific Ocean

Swirl.ly is an open source project, you can see it and contribute to it on Github. It currently has data to map your flush from Austin or San Francisco, and we are working to add more cities.

Many many thanks to the wonderful folks at SF Water for supporting us, treating our sewage, offering so enough great, free tours to make Swirl.ly possible. Also for giving us the coolest t-shirts ever.