If you’re like most people, you probably assume that when you flush your toilet sanitation wizards simply cast the scourgify charm as your expulsions exit the main pipe coming from your house, thereby eliminating it with little effort or expense. While that would be an easy solution, it turns out for primarily historic reasons, a different, much more complicated system, was put in place to make sure your home isn’t constantly backed up with sewage and that the hundreds of billions of gallons of waste water we produce daily can be reasonably safely put back into general circulation on our little space craft named Earth.

So how does this wonder of modern technology work? First, a quick caveat that the exact mechanisms can vary from public sewer system to sewer system and, of course, in more rural areas many use septic systems, which we’ll get into in a bit.

But in the general case when talking mass systems, once your excretions exit your front and rear valves into the canine water bowl, and then are flushed, this all goes into the pipes in your home where it ultimately joins with other wastewater. Eventually, this is all trapped when it encounters the Princess of Friendship, Twilight Sparkle, and her friends that your daughter flushed. When this happens, you call a plumber who will do his or her thing to ensure Twilight, Pinkie Pie, and Rarity never see Equestria again as they are expunged from the pipes of your domicile and out into larger unground pipes.

Once out, this all, in turn, mixes with your neighbors’ expulsions, waste water, and their kids’ toys, meaning there’s always potential that you and your neighbor’s poop sometimes touches. Once in the wider system, this slurry may also potentially be mixed in with water from rain and the like that goes down storm drains.

From there, it enters ever larger pipes on its way towards a wastewater treatment plant with many connections along the way. For a reference of the scale here, the city of Boston is estimated to have just under 60,000 miles (100,000 km) of sewer pipes all feeding into their wastewater treatment plants.

Noteworthy here is that the piping systems have to be carefully designed to account for elevation changes and the like and to minimize the chances of clogging at a pivotal low elevation point. Should they not, one who has a home at lower elevation than most could well see their toilets and sinks absolutely exploding with My Little Ponies, particularly in a well populated area.

On this note, some systems do require pumping stations at certain low points, but, regardless, the wastewater treatment plants are usually put at lower elevations than the region they are serving.

Of course, clogs along the way from home to wastewater processing plant can, and do, happen, given random debris can get into the systems via storm drains and the like and, the bigger problem, even adults flush an awful lot of things they shouldn’t.

On that note, if you are flushing literally anything but toilet paper or your expulsions down the toilet, your wastewater treatment plant operators think you’re kind of worse than the douche you flushed. This includes flushing so-called “flushable wet wipes” which are technically flushable, but so is a kitten. Doesn’t mean you should wipe your butt with one and flush it. (Seriously, just get a toilet seat add-on bidet- $20-$50, ten minutes to install, pays for itself in savings on TP or wet wipes extremely rapidly, and is just vastly more sanitary.)

Or as Deputy Commissioner for the Bureau of Wastewater Treatment in New York, Pam Elardo, states,

Even if it says ‘flushable’ on the box, if it’s not toilet paper, it should not be flushed. So what happens is all those baby wipes, and facial wipes, and Clorox bleach wipes and whatever makeup stuff that people flush – tampons, condoms, everything – it comes to the plant. We have to screen out that debris before we put it into the treatment plant. We do our best to screen it out, and we spend over $7 million a year hauling off just stuff that gets stuck in our screen. Even with the screens, a lot of (those) rags, and baby wipes, and facial stuff gets through the screens and ends up clogging pipes… When it clogs pipes it’s really bad… you’ve got raw sewage that can’t flow and you have to have people in there getting inside the pumping mechanism to retrieve the wipes and all the garbage people throw in there. If I didn’t have the staff or the expertise or the people that stay on top of it, we’d be backing up sewage into people’s homes all the time or overflowing sewage into the receiving waters so it’s something that we constantly have to put up with.

And just as an idea of the volume of water they deal with, she notes New York City wastewater treatment plants process anywhere from 1.3 to 3 BILLION gallons of wastewater PER DAY.

So, seriously, stop it.

In any event, once the sewage gets to its ultimate destination of the wastewater processing plant, as Commissioner Elardo alluded to, the first thing that needs done is to screen off tampons, wet wipes, condoms, diapers, Pennywise, sticks, etc.- anything that can’t be properly broken down or would clog the main system.

This is accomplished by a series of screens, with these points periodically then being cleaned out, whether via automated scrapers or, in smaller facilities, sometimes manually done. The trapped objects are then taken to their final destination of landfills or incinerators.

For reference here, the city of Tampa, Florida notes on their page trying to get people to stop flushing anything but toilet paper and human expulsions, that they transport about 5,000 cubic yards, or about 350-400 dump truck loads, per year of such screened off garbage that shouldn’t have been flushed.

Or to sum up for them- the workers are tired of dealing with your crap and want simply to only have to deal with your shit.

As you might guess from this, much like many other systems we depend on for our daily lives, should workers at these plants stop working even just for a day or two, society would go to crap quite quickly. So take the time to thank a sanitation worker today.

In any event, after the initial screening, the remaining solid and liquid matter will usually enter something you can more or less think of like a really large garbage disposal. The purposes of this is to grind and stir everything remaining sufficiently so that what you have left is a nice deep brown sludgy liquid- similar looking to hot chocolate, but, you know, not smelling as good.

Unless of course that’s your thing. We’re not here to judge. You do you.

Now, at this point there will still be some solids that can’t be easily broken down, and so the next step is for this mixture to go into a grit chamber where these solids, like sand, rocks, wedding rings, your hopes and dreams, and the like are allowed to sink to the bottom and will once again be removed regularly and transported to a landfill.

Important to note here is that because things need slowed down at this point, there are often large basins to potentially hold excess influx of wastewater if the system itself can’t keep up with what is coming in at a given moment, which particularly may be the case during abnormally heavy rainfall or if there is a clog that develops in the processing system.

In the next stage, any remaining solids, usually organic in nature at this point, are allowed to settle to the bottom or, for faster removal, some system use pressurized air to inject bubbles into the slurry. This has the dual benefit of, first, causing much of the remaining solids to float to the surface where they can be quickly skimmed off and, second, helps to promote the growth of the microbes that at this stage are responsible for breaking things down in the slurry.

The skimmed off solids are often then transported to a thickener system and then to a digester area where various microbes and enzymes are used to break down the sludge.  Later, once sufficiently processed, this matter will be transported to an area where it will be dried out and used for things like fertilizer. Noteworthy here is the gasses given off in these steps are also sometimes harvested for bio-fuel production, sometimes even used to generate power to run the plant itself, or more.

As noted by the aforementioned New York Commissioner Pam Elardo.”[We] don’t necessarily use all our gas and we’re getting this extra gas, National Grid came along. We created a program where they’re going to build a scrubber, which is basically a method to clean the gas and strip out the water and other impurities from the gas. Then they’re going to put that gas directly into their regional pipeline, which then will be going to people’s homes so they can cook their dinner, and then use their toilets, and then put the waste in our system. Then we’ll create more gas from that.”

The circle of life.

Going back to the remaining liquids, after most all the solids are removed at this stage, as ever what happens varies a bit in specifics from system to system, but at a high level, the liquid is generally placed somewhere where microbes continue the processing. In the simplest form of processing, this may even be in a special pond or lagoon, which may also be seeded with things like certain planktonic life, like daphnia, to help the microbes process things.

In higher throughput systems, however, things tend to need to happen a lot faster, so in these cases, other methods are used to speed the process up. For example, aluminum chloride is often used to remove excess phosphorous, certain bacteria are used to process nitrogen, etc.

On that note, once everything is sufficiently processed by our single celled friends, they are then given one of the most important of life lessons when the water and microbes are transported to a tank or system designed to kill said microbes once they are no longer useful to society. This may come in the form of killing them with UV light, chlorinating the water, ozonation, etc.

Speaking of ozonation, while relatively expensive compared to other methods of sanitizing, this also has a huge benefit of being shown to break down many pharmaceuticals and other chemicals that find their way into the drains and the like, which is an ever increasing problem.

After all of this processing, it’s also quite common for one last filtration system to be used, running the liquid through activated carbon and sand filters, similar to many water purifiers you might use at home.

Finally, the surprisingly clean water is released back into the environment via rivers, into the ocean, and sometimes even piped in to another system to be used for things like irrigation water in particularly dry regions. For example, approximately half of all agricultural water in Israel comes from its wastewater plants.

As for Septic Systems, these are essentially just miniaturized versions of all this. While there are a variety of design possibilities, in general the wastewater and solids go into a tank where it may be ground up and pumped into a secondary tank, or may simply just stay in the first tank; whatever the exact design, the solids ultimately mostly settle to the bottom and the liquids are pumped or gravity fed off to pipes where the waste water is released back into the ground. At that point, soil and microbes finish the cleaning process.

As for the solids that sit in the tank, the organic variety are slowly broken down over time, with the microbes in your gut generally providing what’s needed to make that happen. Of course, over time they may still build up sufficiently to need someone to come pump out the tank. But in a well designed system where people are only putting toilet paper, wastewater, and human expulsions in, this may take many years before a pumping is needed.

If you liked this article, you might also enjoy our new popular podcast, The BrainFood Show (iTunes, Spotify, Google Play Music, Feed), as well as:

• What Did People Use for Wiping Before Toilet Paper?
• Why is Toilet Paper Always White?
• Why Don’t Americans Use Bidets?
• Did People in the Middle Ages Really Throw Fecal Matter Out of Their Windows?
• The Great Stink of 1858