That is my signature on my comments here on the site. When Dave saw it, he emailed me to find out if it was true. Once I replied in the affirmative, he asked if I could write a little about where your gooshy brown babies go after you push the magic chrome lever on your Al Bundy Special Ferguson. So here goes.
I do in fact work at a poop plant: the Everett (Washington) Water Pollution Control Facility (EWPCF). Here's an aerial view from Google Maps [1].
The big highway to the west is Interstate 5, just north of the Snohomish River Bridge. The plant was first built in 1960 and has been expanded several times since then. Prior to the plant being built, sewage from the city was discharged directly into the Snohomish River and Port Gardner Bay. We are currently under construction for the latest improvement/expansion.
The aeration pond on the south end is 30 acres, the oxidation pond (the big one) is 135 acres, and the polishing pond on the north end is 27 acres. Total volume in all three ponds is about 1325 acre-feet (431,817,500 gallons), and the permitted treatment capacity for the pond system is 15.3 MGD (million gallons per day). My office is in the trailer at the SW corner of the oxidation pond.
If you zoom in to the four round tanks in the lower left corner of the first picture, you can see the other treatment train, the Trickling Filter, Solids Contact (TFSC) system:
The TFSC system is much more economical in land use, but costs more to operate and has fixed flow rates (8 or 16 MGD); so we need the lagoons for surge volume. Sometimes during storms we can peak at over 100 million gallons per day influent flow; without the surge volume we'd have to send untreated wastewater to the river, and no one wants that.
Let's follow your dookie through the system. We'll call him Sam. Sam's first stop is the screw pumps at the headworks, where he's lifted up because almost all the flow from here on is by gravity.
Under those covers are four big (~7 foot diameter) Archimedes screws [2] that lift the raw sewage up to the bar screens.
The four bar screens are designed to take out the big pieces (larger than about 3/4"). This stuff is dropped onto a conveyor that drops it into a bin where dump trucks take it away to a landfill. The stuff can include hunks of wood, balls of grease, more trash and paper than you can imagine...
...and even the occasional critter.
After the bar screens, Sam flows to the grit chambers, where the heavy sand and grit is separated and goes into the hopper with the screenings. You don't want to see the inside of the hopper.
All this equipment used to be open to the sky, but we have covered it all to reduce the odor problems. From here on out, though, there are no covers. The pipes you see above the covers take the stinky air to a compost bed, where it is deodorized. From the grit chambers, the flow goes to the aeration cells, where mixers provide oxygen to the bacteria that do the real work. By providing the bugs with oxygen, they break down the organic material in Sam and his friends into compounds that are much less polluting.
When they have done their job, they die and sink to the bottom, where we dredge them up every couple of years, compost them, and send them off to a tree farm to grow two-by-fours.
While operating a poop plant is dirty and sometimes difficult work, the only reason that human operators are there is to keep the bugs happy, well-fed, and provided with plenty of oxygen. Well, except for the bugs that die with oxygen -- then the humans' job is to keep oxygen away.
By the way -- I am not now, nor have I ever been, a treatment plant operator; I deal with Sam and his friends far upstream in the collection system. But that's another story. Here are a couple of shots of the aeration cells:
As you can see in the first one, the local seagulls treat us like a buffet. After the aeration cells, the partially-treated water (in which Sam has long ago disintegrated into his component parts) flows under the road to the facultative oxidation pond -- the big lagoon. Facultative means that the bacteria living here can metabolize either aerobically [3] or anaerobically [4]. The water isn't mixed, so the top layers stay aerobic because of wind action and the bottom stays anaerobic; so the waste gets further broken down.
In this pond, most of the solids that haven't settled out in the aeration cells drop out; we'll dredge it someday, though it hasn't been dredged since it was built.
Not everything gets caught by the bar screens -- for slender things, it depends on the orientation of the object. For example, this critter managed to make it through:
And, for some reason, Poopettes seem to like to flush these:
We literally have tens of thousands of these beach whistles lying in the rip-rap around the lagoons. And tens of thousands more get screened out of the composted biosolids when we dredge the lagoons. Ladies, these aren't biodegradable and belong in the trashcan, not the toilet. The basics of what should get flushed distills down to this: if you haven't eaten it, or used it to wipe off something you've eaten, it goes in the trash. That also applies to the device that these applicators are designed to insert. Wrap 'em with a wad of Charmin if you are embarrassed by them, but please, please, please don't flush 'em.
After the oxidation pond, the waste flows to the polishing pond, where the smallest solids settle. From there it gets chlorinated to kill pathogens, dechlorinated, and then gets discharged to the Snohomish River, which is about where the wind line is in this picture.
That's the simple side. The TFSC is a more complex system. Currently the TFSC system takes its influent from the first aeration cell through a small bar screen and is pumped to the top of the trickling filters.
Up here, it goes through a set of distributor arms that go around using reaction, just like a garden sprinkler.
From here the wastewater trickles about twenty feet down through corrugated plastic media that has a heavy bacterial mat growing on it. The bacteria colony remains aerobic because air flows up through the media while the wastewater flows down. From the trickling filters, it flows to the aeration basins (the "solids contact" part of TFSC), where it mixes with return activated sludge prior to being aerated. This is where the majority of the bacterial action takes place -- in a conventional activated sludge plant, there are no trickling filters, and all the bugs' work is done here.
From the aeration basins, the wastewater flows to the secondary clarifiers, where the sludge settles to the bottom and the clean water flows out through the troughs called launders. The sludge is scraped off the bottom and part of it is pumped back to the head of the aeration basins (that's the return activated sludge -- so called because the bugs in it are hungry and ready to eat) while the rest is sent to the aeration lagoon to join with the other sludge that's already there awaiting dredging and composting before becoming tree food.
Here's another view of the launder that shows how clean the water is:
From here, the water is chlorinated and then pumped across town...
..to the Kimberly-Clark toilet paper plant (full-circle sorta thing), where it is used as non-contact cooling water before joining with their wastewater and being discharged to Port Gardner Bay -- about where the sailboat is in this photo, around three hundred feet deep. The steam rising in the background is the Kimberly-Clark plant.
That's the quick tour of the poop plant where I work. I've been doing my job for about twenty-five years now, fifteen with Everett; and I really do enjoy my work. In all that time my offices have all been at poop plants. And while there are occasional olfactory assaults, as we say in the business, "Smells like money to me."
