They're like a water purification plant that helps clean the drinking water for a city.

肾就像净水厂一样，用于净化一座城市的饮用水。

Table salt is often made by drilling into underground seabeds, pumping out the saltwater, and then refining it in purification plants.

食盐通常是通过钻入地下海床，抽出盐水，然后在净化厂中精制而成。