Answer Geek: After You Flush

<br> -- Q U E S T I O N: OK, so you've answered the question about power systems and power grids. How about sewer systems? All the water that goes down the drain has to go somewhere for cleaning. How does it get there and how is it cleaned for human consumption?

— Maxine R.

A N S W E R: It's a dirty question, but somebody has to answer it. After all, of the many technological innovations, engineering marvels, and sheer feats of logistics, manpower, and capital investment that make modern human society workable, what is the most important? Transportation, communication, energy, modern agriculture are all vital stuff, I grant you.

But what about our ability to deal with waste water and trash we generate? But without our extensive networks of sewage pipes and massive sewage treatment plants, we might well drown in a stinking sea of our own filth. So a good argument could be made that a good sewer system is really the foundation of civilization.

Yucky Water

So, Maxine, not to be indelicate, but the real question you're getting at here is, "what happens after you flush?" For the sake of simplicity and brevity, we're going to limit the scope of the answer to urban sewage systems for residential users. That leaves out the septic tanks used by people who live in the rural areas, as well as industrial systems used by big manufacturers.

On the other hand, we will cover the basic workings of the 16,000 or so waste water systems that handle the 100 gallons a day of yucky water that the average American citizen flushes, pours, and rinses down the drain from toilets, sinks, showers, dishwashers, and washing machines each and every day.

OK. You flush. Where does it go? Into a pipe that leads out of your house and ties into the local sewer system. The pipe leaving my house is four inches in diameter and it is connected to a six-inch pipe that also serves two of my neighbors. That pipe leads out towards the street where it connects to a larger pipe that serves the neighborhood.