The “standards” that can be applied to wastewater discharges, or any discharge regulated under a National Pollutant Discharge Elimination System (NPDES) permit in a number of different ways, and can be applied differently to different types of discharges. Three common types of standards which are often confusing to permittees are 1) water quality standards, 2) effluent limits, and 3) permit limits. Here we provide a brief description to highlight some of the differences between these three types of standards and how they might interact.
Water Quality Standards
Water quality standards (also known as ambient water quality criteria) are specific standards set by States and apply to the quality of surface waters – lakes, rivers, streams, etc. Ideally, these standards reflect the highest concentration of a chemical that can be present in a given water body that will still allow it to meet its designated uses. These water quality standards usually apply to designated uses which are assigned to each water body.
Effluent limits (sometimes called categorical standards, or categorical effluent limits) are standards that apply to the quality of wastewater discharges from a specific “category” of industry-type. These limits apply to all dischargers within that category, no matter where they discharge to. These limits will typically apply to "end-of-pipe". Effluent limits can apply to wastewater discharges going to a sanitary sewer as well as to surface waters.
Permit limits are specific standards that apply to a given permittee and show up in their NPDES permit. They can reflect Effluent Limits that might apply to that permittee, if they are a categorical discharger. The permit limits will also reflect limits on specific chemicals that are needed to meet the water quality standards associated with the receiving water(sometimes refered to as "water quality-based effluent limits" or WQBEL). This does not mean that the discharger will be allowed a discharge that will begin the receiving water up to its Water Quality Standard. State Antidegradation Policies may require limits on specific chemicals that are well below their Water Quality Standards in the receiving water.
[Read more about Antidegradation Policies]
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