Sewage is the word we often use to talk about the waste that humans create though going to the toilet, washing and anything else where water is used to remove unwanted materials. Another term is wastewater.

Water is very useful to our way of life. It is vital for drinking, of course, but water has other useful properties. More materials dissolve (go into solution) in water than any other liquid. Water is cheap. Water is plentiful. Water is quite dense, so we can flush solid waste away by passing water through pipes. It pushes the solid waste ahead of it, or breaks it up by water pressure and then carries it in suspension.

Now these are all useful properties. But the waste water has to go somewhere, and if not dealt with responsibly it will eventually find its way back into rivers and we may end up drinking it.

Water in rivers is bad to drink simply because there are so many things dissolved in it, but also because human waste contains microbes that we do not want to drink back into our systems or we would get very ill at best, and be killed at worst. So that is why people take the treatment of sewage so seriously.

You can see that treating water that comes down sewerage pipes is quite difficult because it contains bits of solid waste and also materials like microbes in suspension and chemicals in solution.

Remember that huge volumes of water are involved, so we can't just somehow clean it all up by adding chemicals. We wouldn't have enough chemical.

So what do we do? We start by getting rid of the solids by passing water through filters that trap it. That is the easy bit. To get rid of the solids in suspension, we add a small amount of chemical that causes it all to clump together and then settle out. Then we filter it again. We get rid of microbes by using ultra violet light and by passing oxygen through the water, and we add a small amount of chlorine to finish off the job.

When you think that we do this to countless millions of litres of water a day, you can see that dealing with sewage is more complex and important than you might first have thought.