25. So we have to clean it up! But how? Well, fortunately, there IS an efficient sewage treatment procedure.

26. Rubbish such as rags, tins and bits of wood is screened out.

27. Sand and grit is removed by allowing it to sink to the bottom of these channels.

28. The solid particles goes into tanks called 'clarifiers' where smaller particles settle to the bottom as sludge.

29. Bacteria, algae and worms living in the filters use the air between the stones and break down the sewage.

30. Some sewage is flowed into circular beds with rotating arms...

31. ... or into rectangular tanks where the scraper moves backwards and forwards. Activated sludge and raw sewage are sometimes mixed together in a tank. This is called the activated sludge process.