For the above reasons and others beyond the scope of this paper, I 

 strongly disapprove of such pollution-control measures or requirements. 

 They tend to discourage the selection of sites for industrial plants 

 where effects on the environment will be minimized, because they do not 

 permit reduction of waste disposal costs by choosing the more favorable 

 locations. They also discourage the conservation of water by industry, 

 because more frugal use of water that results in great reductions in 

 volume of effluents, usually results also in some increases of the con- 

 centrations of toxicants in the effluents. 



Instead of arbitrarily limiting the concentrations of toxicants in ef- 

 fluents for protection of aquatic life outside the mixing zones, it is 

 surely more reasonable to limit the amounts of these harmful substances 

 discharged per unit of time. These amounts can decrease while the toxi- 

 cant concentration increase, if the volume of the effluents discharged 

 per unit of time is simultaneously reduced. The measured toxicity of a 

 wastewater is a function of the concentration of the toxicant or mixture 

 of toxicants present. If all poisons were equally toxic, it would be a 

 measure of their total concentration whenever several are present. The 

 toxicity of any solution expressed in toxicity units (t.u.), sometimes 

 called the "toxicity concentration". This is equal to the reciprocal of 

 its median lethal concentration (LC50) expressed as the decimal fraction 

 by volume (percent by volume : 1U0). For example, if the LC5Q is 0.2 (or 

 20 percent) by volume, the toxicity is 1/0.2 or 5.0 t.u. An application 

 factor prescribed for an effluent is equal numerically to its permissible 

 concentration at the boundary of the mixing zone expressed in appropriate 

 (corresponding) toxicity units. The expression of toxicity levels in 

 such units has been shown by experiments to be useful in the estimation 

 or prediction (by summation) of the toxicities of various mixtures of 

 poisons whose individual toxicities have been determined (Brown, 1968; 

 Warren, 1971, p. 210). 



An approach to the regulatory problems that has recently been gaining 

 favor in the United States and Canada is to express the output of toxi- 

 cants from each important source as a value to which the name "toxicity 

 emission rate" (T.E.R.) has been given (California State Water Resources 

 Control Board, 1972). This value can be computed by dividing the rate of 

 flow of the effluent by the determined median lethal concentration ex- 

 pressed as a decimal function by volume. For example if the determined 

 96-hour median tolerance limit or LC50 of an effluent is 0.20 (20 percent) 

 by volume, and its flow rate is 2.0 m3/min.: 



3 3 



T.E.R. - - IU 96_ nr . |f • min. IU mTnT^' 



bU 



where t.u. represents toxicity units based on results of 96-hour tests. 



Uniform requirements that limit the T.E.R. per unit of industrial pro- 

 duction of a given kind (for example, per ton of cellulose pulp produced 

 per day by a sulfate-process pulp and paper mill) would be much more rea- 

 sonable than are those that simply limit the toxicity of the effluents 



