NC0045993_Other Agency Documents_20240308

AMMONIA TOXICITY POLICY

In 1990 the EPA decreed that each delegated State develop controls on ammonia as a toxicant (after years of considering it to be primarily an oxygen-consuming waste). North Carolina
uses two instream criteria adopted from EPA research:

Summer instream criteria = 1.0 mg/L
Winter instream criteria = 1.8 mg/L

(Note: These are called “criteria” because standards have not been formally adopted for non-trout classified waters in North Carolina.)

In order to evaluate whether the discharge has the potential to cause instream toxicity, the allowable effluent concentrations of ammonia (summer and winter) are calculated with the
mass balance equation (using the IWC Excel Spreadsheet in Appendix A). Both summer 7Q10 and winter 7Q10 are used in the equation. Ammonia is the only toxicant that is limited differently
for summer and winter seasons. An upstream concentration of 0.22 mg/L is assumed. If the allowable concentration is greater than 35 mg/L, no limit should be imposed. If the allowable
concentration is less than 35 mg/L, potential toxicity may be occurring. If there was already an existing ammonia limit based on oxygen-consumption, the more stringent ammonia limit
should be placed in the permit.

If the mass balance calculation indicates the need for ammonia limits, it is Division policy to set limits no more stringent than 2 mg/L (summer) and 4 mg/L (winter) for small discharges
(those less than 1.0 MGD). This is based on the Division's determination of BAT (best-available technology) for these types of facilities. Therefore, even if the calculation for a
winter ammonia limit yields a 3.5 mg/L result, a 4 mg/L limit would be given.

For any new or expanded flow, an ammonia limit should be given. However, for existing flows where potential toxicity may be occurring, the permit holder should be given the option to
have the ammonia limit(s) placed in their permit or to have a quarterly chronic, WET test, pass/fail at the instream waste concentration using Ceriodaphnia as the test organism. Many
times the facility will need a compliance schedule to be placed in the permit in order for renovations to be made at the site. Such schedules should be coordinated with the regional
office and the permit holder but should not exceed three years.