54、Taking away a city's rubbish is a big job. Every day trucks come into a city to collect it. Most rubbish is made up of things we can't eat or use. If we kept these things, we would soon have a mountain of rubbish.
In some cities the rubbish is collected and taken outside the city to a dump. Often the city dump is placed where the ground is low or there is a big hole. The kitchen rubbish is broken into small pieces and sent into the sewage（污水，排水）system. The sewage system takes away the used water from toilets, bathtubs and other places.
To keep mice and flies away, some earth is used to cover the newly dumped rubbish. Later, grass may be planted on the rubbish-filled land. Finally, a house or a school may be built there, and then you'd never know that this had once been an old rubbish dump.
In other cities the rubbish is burnt in special places. The fire burns everything except the metal.Sometimes the metal can be used again in factories where things are made of metal. The food parts of rubbish are put in special piles where they slowly change into something called humus（腐殖质）, which looks like black earth. It is rich with the kinds of things that feed plants and help make them grow.