Figure 1

Generalized dose-response relationship suitable for estimating no-observed-effect concentration (NOEC), lowest-observed-effect concentration (LOEC), maximum-allowable toxicant concentration (MATC, equal to mean of NOEC and LOEC values), and LC50 and EC50 values (concentrations needed to kill 50 percent of the test organisms, or reduce the response variable by 50 percent, respectively).

In effluent testing, organisms are reared in various dilutions of effluent for a specified period of time with specified food, temperature, and light conditions. The ability of the organisms to survive, grow, and (in some cases) reproduce is measured and compared with the responses of organisms reared in a negative control (i.e., water known to be of good biological quality). The highest effluent concentration that causes no adverse effect is referred to as the no-observed-effect concentration (NOEC). The next-higher tested concentration, which shows the first statistically detectable effect of the effluent on the organisms, is referred to as the lowest-observed-effect concentration (LOEC). For regulatory purposes, effluent testing is used to establish a reliable estimate of an effluent's NOEC, LOEC, or LC50 (concentration of effluent that is lethal to half of the test organisms in a specified period of time) (Figure 1). The statistical procedures for estimating these concentrations are well defined (Figure 2). The NOECs can be



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