根据以下内容回答题：
The water we drink and use is running short in the world .we all have to learn how to stop wasting our limited(有限的) water. One of the steps we should take is to find ways of reusing it . Experiments have already been done in this field.
Today in most large cities ,fresh water is used only once ,then it runs into waste system . But it is possible to pipe the used water to purifying factory. There it can be filtered and treated with chemicals so that it can be used again ,just as it were fresh from a spring(喷泉).
But even if every large purified（净纯） and reused its water, we still would not have enough. Then we could turn to the ocean. All we’d have to do to make use of the seawater on earth is to get rid of (去掉)the salt. This process(过程)is called desalinization ，(脱盐)and it is already in use in many parts of the world.