Test organisms are placed in glass or plastic bottles
containing the water to be tested and left there for 2-4 days. Bottles
are set up for pure effluent, several dilutions of that effluent and
a control water (where no more than 10% of the organisms should die).
The percentage of organisms surviving exposure in each bottle is determined
and the toxicity of the effluent is expressed as the effluent dilution
which is lethal to 50% (LC50) of the population. An acute toxicity unit
is defined as 100/LC50. The State of Michigan limits discharges to 1
acute toxic unit. If the effluent contains more than that, the treatment
plant must add new processes to reduce the toxicity of their discharge.