355 
The numbers of POTWs affected strongly influences the volumes of pollutants 
removed and the total cost of treatment. Where the application of the option 
does not depend on water quality conditions, then all 1839 of the POTWs are 
affected. If requirements only apply where there are water quality problems, 
then the number of POTWs affected is reduced to the model estimate of 846. 
These estimates are based on the aoumbers of exceedances as discussed in more 
detail in Chapter 3 and later in this section. 
All of the uniform national programs have the largest impacts on reducing 
the volume of pollutants discharged into waterbodies. This is primarily because 
the requirements apply to more POTWs than do the other options. Option 2 
(categorical pretreatment standards for metal finishers only) has a relatively 
significant impact on the reduction of toxic metal discharges as well as an 
reducing toxic organic discharges. The uniform, national programs also signi- 
ficantly improve (on a percentage basis) the quality of the POTW effluent 
discharge and the quality of the sludge. 
While the options significantly affect the volume of the pass-through 
of toxic pollutants, the importance of these reductions depends on the resulting 
impacts to water quality. In the following subsection, the immediate impacts 
to water quality have been analyzed using exeedences as an indicator. However, 
there can be important water quality impacts even where there are no immediate 
exceedances because the reduction in pollutant discharge can lower ambient 
pollutant levels, peeniiieacin . the attainment of water quality objectives 
downstream. In addition, exceedances are thresholds, and (as discussed later 
in this chapter) there can be benefits associated with reducing pollution 
even where there are no exceedances or where exceedances persist in spite of 
controls. 
4.4.3 Effectiveness In Reducing Water Quality Violations 
As discussed in Chapter 2, an exceedance is a rough indicator of the 
possibility that there may be a water quality violation for a pollutant. The 
