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COLORADO DEPARTMENT OF HEALTH, Water Quality Control Division Rationale - Page 7. Permit No. CO-0036684. VI. TERMS AND CONDITIONS OF PERMIT a) Purpose of WET Testing: Section 6.9.7 of the Regulations for the State Discharge Permit System (6.1.0), passed by the Water Quality Control Commission (WQCC), has established the use of WET testing as a method for identifying and controlling toxic discharges from wastewater treatment facilities. WET testing is being utilized as a means to ensure that there are no discharges "in amounts, concentrations or combinations which are harmful to the beneficial uses or toxic to humans, animals, plants, or aquatic life" as required by Section 3.1.11 (1)(d) of the Basic Standards and Methodologies. Chemical analysis of effluent has provided only a partial evaluation of the potential impact a discharge could have on the receiving stream. Also, chemical analysis cannot evaluate the synergistic or antagonistic effect of compounds. There are also compounds for which an accurate or reproducable method of chemical analysis has not yet been developed, as well as compounds which are just beginning to be evaluated for toxic effects. WET testing will provide a more comprehensive means of evaluating the toxicity of a discharge than could otherwise currently be accomplished. b) Instream Waste Concentration (IWC): As a condition of the permit, t e permittee wi 1 e required to conduct routine monitoring for acute toxicity. Acute toxicity occurs when there is a statistically significant difference in the mortality observed, for Ceriodaphnia sp. (water flea) and fathead minnows, between the control and any effluent concentration. Should acute toxicity be detected in any effluent concentration less than or equal to the Instream Waste Concentration (IWC) or should a species mortality in any dilution of effluent (including 100X effluent) exceed 50X, the permittee is required to conduct a series of timely tests to identify and ultimately eliminate or treat the toxicant. The IWC Ss determined using the following equation: IWC = [Facility Flow (FF)/(Stream Low Flow (annual) + FF)] R 100X Using the facility design flow of 0.46 MGD and the acute stream low flow (lE3) of 0.45 MGD, the IWC for the permittee is therefore 50X, which represents a wastewater concentration of SOX effluent to SOX dilution water. c) Chronic Biomonitoring: The determination as to whether or not an individual faci ity must conduct chronic WET testing is dependent upon the ratio of the chronic stream low flow (30E3) to the design flow of the wastewater treatment plant. If the ratio is less than 19:1, chronic WET testing is included as a peewit requirement.