Water is necessary for drinking, industries and agriculture. All natural water bodies have a certain level of pollutants such as organics, nitrogen, phosphorus, and metals. The level of these pollutants determines if the water is suitable for certain usage or not. These levels are given in the water quality standards. Accordingly, there are standards for drinking water quality and different standards for the usage of water for a certain industry or for the usage of water to irrigate certain crops. Also, there are standards for the usage of water for recreational activities. On the other hand, there are discharge standards that determine the allowable level of pollutants in the discharge water from different wastewater treatment plants to different water bodies.
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