Completing the Cycle: What Happens to Water After We Use It?

Once water has been used, it must somehow re-enter the freshwater supply. Some of the water is evaporated (e.g., if it is used to generate steam for industry, or if we drink the water and then sweat). The evaporated water eventually collects in clouds and returns to the earth via precipitation. However, most of the water that we use remains in the liquid state, and is returned to the freshwater supply directly (as runoff) or via wastewater treatment facilities.

Wastewater from municipalities (i.e., household and business use), industries, and agriculture typically follow different pathways in their return to the freshwater supply, although municipal and industrial wastewater may sometimes be combined. Wastewater from all of these sources may contain contaminants that are harmful both to humans and to the environment. Hence, water-treatment facilities (sewage facilities) must be employed to remove these harmful products before the water is returned to the freshwater supply in the environment.

Muncipal wastewater may contain bacteria-harboring fecal material, as well as small amounts of grease, suspended solids, and chemicals from household use. The water passes through a wastewater-treatment facility before it is then disposed of into rivers, lakes, or estuaries, where any remaining contaminants will be diluted. Other methods of disposal, including irrigation and evaporation, are used more rarely. The wastewater-treatment facilities use many of the same techniques described above for treating water to deliver to consumers. The degree to which the water must be treated before it is returned to the environment depends on the quality of the wastewater, as well as how the water into which the wastewater is disposed will be used. For instance, wastewater that is emptied into lakes that are used for recreational swimming and fishing must be treated particularly carefully.

Industries typically do not actually consume much water (the water is used for applications, such as cooling and processing, that do not significantly diminish the amount of water), so the amount of industrial wastewater is generally very large. Industrial wastewater may contain many contaminants, such as toxic metals, organic chemicals, and radioactive materials. Before it can be returned to the water supply, this wastewater must be carefully evaluated and treated, using many of the same techniques for water treatment described above. Then, the industrial wastewater, like municipal wastewater, is released into rivers, lakes, or estuaries.

Agricultural wastewater often contains soil sediment, as well as potentially harmful materials such as pesticides, fertilizers (which may cause an overgrowth of algae), and animal wastes (which may harbor disease-causing organisms). Unfortunately, the wastewater from agriculture often drains directly into streams, rivers, and lakes. Hence, a great deal of concern has arisen about the types of pesticides and fertilizers that are used.