Section 352.550  Establishing Whole Effluent Toxicity Conditions

a)         If the PEQ derived from Section 352.530 is less than or equal to the PEL calculated in Section 352.540, it will be concluded that there is no reasonable potential to exceed.  Under such circumstances a permit limit will not be set unless otherwise justified under one or more provisions of Section 352.430.

b)         If the PEQ is greater than the PEL, and more than 10 facility specific data values were used in deriving the PEQ, either a whole effluent toxicity limit will be incorporated into the permit or the causative toxic substances will be limited consistent with Subpart D of this Part.

c)         If 10 or fewer data values were used in deriving the PEQ,  the Agency will calculate an alternative PEQ, using the method specified in Section 352.421(b).  If the alternative PEQ is greater than the PEL, appropriate limits will be incorporated into the permit, as in the situation where more than 10 data values are available.  If the alternative PEQ is less than or equal to the PEL, the Agency will either establish appropriate limits in the permit or incorporate a monitoring requirement and reopener clause to reassess the potential to exceed within a specified time schedule, not to exceed one year. In determining which of these options to use in any individual application, the Agency shall consider the operational and economic impacts on the permittee and the effect, if any, deferral of a final decision would have on an ultimate compliance schedule if a permit limit were subsequently determined to be necessary.

d)         It is the preference of the Agency to limit the individual toxic substances producing the toxicity whenever they can be identified.  Therefore whole effluent toxicity limits will not be imposed whenever the toxicity can be resolved by regulating individual substances. If, however, a WET limit is necessary, the limit will be set at the PEL calculated pursuant to Section 352.540.  If compliance cannot be achieved upon permit issuance, the permit may also include requirements for a toxicity reduction evaluation program, interim discharge limits and a compliance schedule.