Where Does The Potty Go?

We’re out shopping, and my son announces – as small children are wont to do – that he needs “to go to the potty”. So I’m standing in the men’s room with him as he goes about his business, back to him as he finishes and flushes. And then he asks the title question: “where does the potty go?”

It hits me that I have, at best, an extremely vague idea. But I do my best. “Well, it goes through pipes into a sewer.”

“And then into the river?” he asks, lighting up. We’ve looked at storm water drains before, walking home form daycare.

“No,” I tell him. “It goes to a treatment plant. Like in Curious George.”

“Oh,” he responds. Then another thought strikes him. “How do they clean the pee out?”

Now I’m stumped. Stumped, and slightly grossed out. But curious.

How Does A Toilet Work?


I’ll be honest, here. When I searched for “how does a toilet work”, I never anticipated that I’d find myself on a web page hosted by the Massachusetts Institute of Technology. But they have a straight-forward article from a student titled How A Toilet Works, and it turns out to be all about siphons.  And physics.

Everyone understands that, when you pull the flush handle, water empties from the tank into the bowl. What happens next is based on the “trapway” in that diagram up there – the famous “u-bend”. The water level rises in the bowl, and because of the way water works that also causes the water level to rise in the trapway. This puts you in a position of having greater pressure in the bowl than in the air filling the trapway, and the water moves from the area of greater pressure to the area of lower pressure until the pressure equalizes.

What makes flushing especially interesting has to do with water velocity. See, as described in the Bernoulli Equation, fluids under constant pressure will move faster through narrow channels. That’s why you can spray water faster from a hose when you put your thumb over the opening – the velocity increases as you constrict the channel. Well, the trapway channel is significantly narrower than the mouth of the bowl. So, as water is pumped in, the bowl drains faster than it fills until the bowl is completely drained. Then the siphon effect breaks down and the bowl refills with clean (for “you still don’t want to drink that” values of “clean”) water.

John Harrington and his successors were pretty ingenious, weren’t they?

Once You’ve Flushed, Where Does It Go?

Once you’ve flushed, there are three primary places (in the United States, at least) that the waste water can go: into a septic drain field, into a septic tank, or into a sewer. Which one you use really depends on where you are, what sort of access you have to municipal water and sewer services, and local ordinances.

Septic drain fields are also known as leach fields or leach beds. They consist of “a network of perforated pipes that are laid in underground gravel-filled trenches to dissipate the effluent from a water-based collection and storage/treatment or (semi-)centralized treatment technology.” Basically, your waste water flows out the into the pipes, which should be buried deeply (the article mentions ten feet of soil). Wastewater leaks out of the pipes, with the water being absorbed and percolating downwards and the (I love this phrase) “organic materials” being consumed by bacteria and other microfauna.

The leach bed is a somewhat more sophisticated version of a soak pit, which is pretty much what it sounds like: a pit that receives wastes or wastewater and holds it as it soaks into the surrounding soil. If you see an old-fashioned outhouse, you’ve seen the surface component of a soak pit.

A septic tank is, quite literally, a big tank. YOur waste water empties into that tank, and then is emptied out and disposed of at a later date.

By no stretch of the imagination are these the only means of disposing of waste water. The SSWM (Sustainable Sanitation and Water Management) web site discusses several other techniques and technologies.

How Do You Clean It?

If you’ve got a leach bed or a soak pit or something similar, cleaning the waste water is relatively straight-forward. You let nature handle it, and you don’t dump toxic chemicals in your toilet while you do. However, if you’ve got something like a septic tank or a sewer hookup, then you turn to wastewater and sewage treatment plants. And according to the Wastewater treatment page on the Food and Agriculture Organization of the United Nations website, you’ve pretty much got two options for treatment: conventional treatment and natural biological treatment.

Conventional wastewater treatment is what you probably first think of when you hear about a “sewage plant”. It “consists of a combination of physical, chemical, and biological processes and operations to remove solids, organic matter and, sometimes, nutrients from wastewater,” and it generally goes through four stages with the imaginative names of preliminary, primary, secondary, and tertiary (or advanced).

  • Preliminary treatment removes “coarse solids and other large materials often found in raw wastewater”, generally through screens and filters designed to trap large objects (which, in this context, includes “grit”). water velocity is generally high in preliminary treatment to keep particulate solids from settling.
  • Primary treatment is “the removal of settleable organic and inorganic solids by sedimentation, and the removal of materials that will float (scum) by skimming. The goal is to remove 25% – 50% of the biochemical oxygen demand (the dissolved oxygen in water used by microorganisms to degrade and consume organic compounds), 50% – 70% of the suspended solids, and 65% of the oil and grease in the water. The resulting settled sludge is pumped out intermittently for processing in – wait for it – sludge processing units (generally by allowing it to be eaten by bacteria in an anaerobic process, allowing it to dry before processing further).
    • Once water has gone through primary treatment it is generally considered acceptable for wastewater irrigation, which can be used for all crops not intended for human consumption and some crops that are directly consumed by humans (orchards and vineyards, for instance).
  • Secondary treatment “follows primary treatment and involves the removal of biodegradable dissolved and colloidal organic matter using aerobic biological treatment processes”.  The process seems counter-intuitive after primary treatment, because microorganisms are added back to the water, along with more air, to consume more of the organic matter remaining in the water.  Further settling tanks (or sometimes rotating tanks) are then used to separate out the microorganisms in a process that removes 85% f the biochemical oxygen demand and suspended solids.
  • Tertiary treatment kicks in when there are specific contaminates that are not removed by the primary and secondary treatments.  They are many and varied, and depend on what contaminate or contaminates are to be removed.

Natural biological treatment often follows the same process as conventional treatment, but makes use of natural processes (such as evaporation) or plant life (such as algae or trees) to filter organic contaminates from the water.  The processes are generally slower than conventional treatment, but also tend to cost less to maintain and operate.

So, where does the potty go?  Out into our infrastructure, and eventually into the ecosystem.