7 to 21 days whereas the rate constant is an order of magnitude higher at 0.127 day-.

The effect of the much higher rate constant for the STAs is dramatic as shown by

comparing Figure 4-20 for the EAASR and Figure 4-21 for the STAs. An assumed rate

constant of k equals to 0.127 day-1 causes the concentration to reach the assumed

background level in 50 days over the entire range of assumed inflow concentrations.

Figure 4-22 shows the same information with the x axis rescaled to a maximum residence

time of 50 days instead of 150 days.

 The results shown in Figure 4-22 suggest that using residence times in the 7 to 21

day range provide a relatively good level of performance. For a residence time of 15

days, the associated outflow concentrations and % control are shown in Table 4-16. In

this case, the percent control ranges from a low of 60% for an initial concentration of

0.025 mg/L to a high of 76% if the initial concentration is 0.150 mg/L, though all are

approaching the background concentration.

 These results indicate the following key points about the EAASR/STA system:

* The STAs are much more effective water quality controls than the EAASR in that
 they achieve significant removals in 7 to 21 days of residence time as dramatically
 illustrated by comparing Figures 4-25 and 4-27.

* The EAASR can provide significant water quality improvements if the residence
 times exceed 25 days and the initial concentrations are relatively high.

* For the same area, the residence times in the STAs can be expected to be
 significantly less than for the EAASR since they are operated at much shallower
 depths.

* The initial concentration has a significant effect on performance especially if
 performance is measured as percent pollutant removal.