(1) Whole effluent toxicity limits assigned pursuant to WAC 173-205-050(2) are eligible for removal upon permit renewal if:

(a) The permittee has demonstrated compliance with the whole effluent toxicity performance standard associated with that limit for a minimum of three consecutive test years following effluent characterization or for an entire subsequent permit term; and

(b) The permittee has not made any changes within the last three years which would otherwise require additional effluent characterization pursuant to WAC 173-205-060.

(2) The department may condition the nonassignment of a whole effluent toxicity limit for a permittee that has attained a performance standard described in WAC 173-205-050 (2)(a), or subsection (1) of this section on routine monitoring with a rapid screening test.

(a) Before making such condition, the department shall consider the potential for treatment system upsets, control equipment failures, spills, accidental releases to the wastewater system, and any other event which could result in a toxic discharge.

(i) Chemical monitoring may be required to assess increases in effluent toxicity if it:

(A) Can account for the potential sources of toxicity; and

(B) Is associated with water quality-based effluent limits or any other permit mechanism that requires a response to increases in effluent toxicity.

(ii) Rapid screening tests shall be required if the department determines there is the potential for an event at the facility which could result in a toxic discharge that would otherwise go unnoticed.

(b) Rapid screening tests for acute toxicity shall be expected to have a maximum mortality proportion of twenty hundredths in one hundred percent effluent.

(i) The mortality proportion shall be calculated by subtracting the number of test organisms living in one hundred percent effluent at the end of the test from the number of test organisms living in the control and dividing the result by the number of test organisms living in the control.

(ii) The one hundred percent effluent test concentration and the control shall have equal numbers of test organisms.

(c) Rapid screening tests for chronic toxicity shall be expected to have no statistically significant difference in response between the acute critical effluent concentration and the control using the method in Appendix H of EPA/600/4-89/001 or an equivalent method approved by the department. Rapid screening tests for chronic toxicity must meet the chronic statistical power standard.

(d) Whenever a rapid screening test result fails to achieve the standard of (b) or (c) of this subsection, the permittee shall be required to immediately retest with all of the acute or chronic toxicity tests used in the last permit with whole effluent toxicity testing.

(e) The results of the acute or chronic toxicity tests conducted in response to a rapid screening test shall be evaluated by the department to determine the need for new whole effluent toxicity characterization requirements in the next permit or the need for immediate administrative orders to implement the regulatory process which begins in WAC 173-205-090.

(f) All rapid screening tests shall be performed by laboratories accredited by the department in accordance with chapter 173-50 WAC.