Phosphorus “Reasonable Potential”

Summary (updated: January 5, 2015)

Brown & Caldwell’s June 2014 “Review of USEPA Methods for Setting Water Quality-Based Effluent Limits for Nutrients

Click to Download (PDF)

Prepared for National Association of Clean Water Agencies, Washington D.C.

Executive Summary

Nutrient control is one of the chief regulatory and economic issues facing wastewater utilities today.
USEPA’s national strategy on nutrients has focused on the derivation of numeric nutrient criteria (NNC),
and the use of those criteria to develop water quality-based effluent limits (WQBELs) in a manner
analogous to what has been historically performed for toxic substances. For example, USEPA has not
developed separate permitting guidance for nutrients, but has largely relied on the calculation methods
of the Technical Support Document for Water Quality-Based Toxics Control (TSD). There has been a great
deal of variability in the degree to which regulatory agencies have modified these methods to account for
the differences between nutrients and toxics, and in many settings, no modification has been performed.

This report presents a technical review of federal guidance for deriving WQBELs for nutrients in National
Pollutant Discharge Elimination System (NPDES) permits. The report was commissioned by the National
Association of Clean Water Agencies (NACWA) as an independent review of which elements of federal
toxics-based permitting methods are valid or invalid for nutrients, or would require modifications to be
valid for nutrients. This review demonstrates that due to fundamental differences in how nutrients and
toxics affect receiving waters, the use of toxics-based methods is often inappropriate. The review also
seeks to identify potential alternatives where warranted. NACWA intends this review to facilitate ongoing
discussions with agencies and stakeholders on regulatory solutions for nutrients.


On March 21, 2014, the Association hosted a Working Group meeting at Maine Municipal Association (Augusta) to hear representatives from both Maine DEP and USEPA describe the approach for developing a numeric criterion for phosphorus that incorporates the flexibility to reflect the response of the receiving water. This process will rely on data that include: the existing phosphorus concentrations in the receiving water; the 7Q10 (i.e., low flow condition) of the receiving water; the dilution factor of the receiving water; and the facility’s permitted maximum discharge.

Six Maine facilities were invited to attend this working session to keep the discussion manageable and due to space limitations.

Current Status

The Association is presently working on comments that reflect the general concerns we have for our members.  Due to the number of facilities that will be impacted, the Association’s comments will not be able to reflect any facility-specific conditions or unique situations.


Documents Provided by Maine DEP and USEPA for March 21, 2014 Meeting




Maine DEP Guidance Documents

Cost Estimates

Phosphorus Analytical Costs (updated March 31, 2014)

Lab #1 (Southern Maine): $30.00 per sample

Note: Detection limit: .22 mg/l; composite sample for facility w/ flow less than 0.25 MGD

MEWEA Commenters

Annaleis Hafford (Olver Associates)
Phyllis Arnold Rand (Greater Augusta Utility District)
Aubrey Strause (Verdant Water)- email Aubrey
Bill Taylor (Pierce Atwood)
Assistance/Review: Travis Peaslee or Mac Richardson (LAWPCA); Scott Firmin (Portland Water District)