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NPDES - Water Quality Parameters and Effluent Limits
NPDES permits regulate a number of water quality parameters. Since every facility is different, each individual NPDES permit is different, too. The permit processor, together with aquatic biologists and stream modelers, decides which parameters require limits. Sometimes a limit is not necessary, but further monitoring is required to gain more information about the quality of the facility's effluent.
The Water Resources Division must derive an appropriate limit for each parameter of concern. When a limit is derived, two factors must be taken into account: water quality and available treatment technology.
The State of Michigan's Part 4 Rules (of Part 31, Water Resources Protection, of Act 451 of 1994) specify water quality standards which shall be met in all waters of the state. The rules require that all designated uses of the receiving water be protected. Designated uses include: agriculture, navigation, industrial water supply, public water supply at the point of water intake, warmwater or coldwater fish and other indigenous aquatic life and wildlife, partial body contact recreation, and total body contact recreation from May 1 to October 31.
Federal regulations specify industry-specific standards which are achievable using available treatment technology. Using the Code of Federal Regulations, the permit processor derives treatment technology based effluent limits. The permit processor must then compare the water quality based effluent limit to the treatment technology based effluent limit for each parameter. The more stringent limit is used as the effluent limit in the NPDES permit.