Basic Course: Supplemental Topics (NPDES Permit 16)

Characterizing the Effluent & Receiving Water: Establish Allowable Dilution/Mixing

Where dilution/mixing is allowed, often a determination can be made about whether rapid and complete mixing of the effluent will occur in the receiving water, such that all applicable criteria for the pollutant of concern will be met almost immediately at the discharge location in the receiving water. This type of mixing would be expected for so-called effluent-dominated streams or where mixing is mechanically induced using a diffuser.

For situations where dilution is not immediate, the State/Tribe’s water quality standards may allow for mixing zones. A mixing zone is a limited area near the discharge point where the pollutant criteria can be exceeded until more complete mixing of the effluent is attained, as long as toxic conditions are prevented and the designated use of the water body as a whole is not impaired as a result.

Key Point. Where water quality standards do not allow for consideration of dilution/mixing, water quality criteria must be attained at the effluent’s point of discharge (end of pipe).

Learn More. Situations in which the permit writer might assume that water quality criteria should be attained at the point of discharge, even though consideration of dilution/mixing has been included in the water quality standards. Proceed to the Learn More Topic. » (Note: This link launches a pop-up window.)