The Secret Journey of a Flush

The story

You press the handle, the water swirls, and everything disappears with a whoosh. But have you ever wondered where it all actually goes? Because here is the first secret. There is no such place as away.

Under your home, a pipe leads to a bigger pipe, which joins a bigger one still, until your flush is travelling through a hidden city of tunnels beneath the streets. Engineers lay most of these pipes on a gentle slope, so the water mostly slides downhill by gravity alone, with pumps lending a push wherever the land refuses to cooperate. Beneath some cities, the main tunnels are so wide you could drive a bus through them.

The journey ends at a place with a very polite name, the water reclamation plant. There, the clean-up begins like a series of sieves. First, metal screens catch the things that should never have been flushed, the wet wipes and toys and odd surprises. Then the water rests in wide tanks, where sand and grit sink to the bottom, and grease floats to the top to be skimmed away.

Now comes the real magic, and it is not a machine at all. The dirty water flows into huge bubbling pools that are alive, holding trillions upon trillions of hungry bacteria. To them, much of what your body got rid of is a feast. They eat it, and as they eat, the water clears. In a modern treatment plant, much of the hardest cleaning is done not by machines or chemicals, but by this army of creatures too small to see, working day and night, helped by the other cleaning steps around them. When they finish, the water settles once more, gets a final polish and a disinfecting, and slips back into rivers and seas, clean enough to hand back to nature. Not every place in the world has plants like these yet.

And in Singapore, the story goes one astonishing step further. There, used water travels through deep tunnels to special plants, where it is pushed through filters so unbelievably fine that almost nothing but water itself can squeeze through, then treated with beams of light. The result is called NEWater, and it is so pure that the factories building computer chips insist on it, because ordinary tap water is not clean enough for their delicate machines. In dry spells, some of it even tops up the reservoirs. Yesterday's flush, made cleaner than new.

So the toilet is not really an ending. It is a doorway onto one of the greatest loops on Earth. The water in your flush has been oceans and clouds and rain and rivers, over and over, for billions of years. Dinosaurs drank it. Earth's water just keeps travelling its great cycle, borrowed, cleaned, and passed along, again and again.

Which leaves one small favour to ask of you. The tiny cleaners are mighty, but they cannot handle everything, and some things should never start the journey at all. Only pee, poo and toilet paper belong in the bowl. The toilet is a door, not a dustbin.