Water is very important to us. Factories and plants need water for water pollutionindustrial（工业的） uses and large pieces of farmland need it for irrigation（灌溉）. Without water to drink, people die in a short time. Today most water sources are so dirty that people must purify（净化） water before drinking. Water becomes dirty in many ways: industrial pollution is one of them. With the development（发展） of industry, plants and factories pour tons of industrial wastes（废弃物，垃圾） into rivers every day. The rivers have becomeseriously（严重地） polluted, and the water is becomingunfit（不适合的） for drinking or irrigation. The same thing has also happened to our seas and oceans. So, the problem of water pollution is almost worldwide. Scientists of many countries have done a lot of work to stop pollution. The polluted water in some places has become clean and drinkable（适于饮用的） again. Perhaps one day the people in all towns and cities will be drinking clean water. That day, we believe, is not very far off