30 
expected from the proposed outfaiil, and an average dilution of 
only about 1/25 (based on radioactivity measurements) occurs 
around the existing outfall, the above value should be corrected 
by a factor of 25/60 giving an average value of about 15,000/m1 
for the proposed outfall. One could also use the MPN‘s of the 
primary treatment effluent per se and correct these for the 
expected initial dilution. Plant data, summarized by the 
Hyperion Engineers, give the following values for the coliform 
content of primary effluent: 259,000/m1, as the geometric mean 
of 257 grab samples taken over a year's period at the same 
clock time for each sample; 561,000/m1 as the geometric mean 
of 24 samples taken over a 24 hour period; 389,000/m1 as the 
geometric mean of a similar series taken over a different 24 
hour period. Applying the factor of 1/60 as the minimum 
expected dilution, the above values would yield coliform 
densities at the boil of 4,300, 9,400, and 6,500/m1, respec= 
tively. Using the boil counts obtained on the tracer cruise, 
calculating back to the initial count of the effluent (see 
Table I) on the basis of radioactivity, and again assuming a 
minimum dilution of 1/60 at the boil for the proposed outfall, 
a boil count of 12,000/m1 is obtained. The highest and lowest 
estimates from these data differ by a factor of four which 
represents a little less than 2 hours disappearance time at the 
rate of 3 hours for 90% disappearance. The highest value, 
15,000/m1 at the boil, is arbitrarily accepted as the average 
1. This is the figure cited by the Hyperion Engineers as resulting 
from Dr. Brooks’ studies on the design of the proposed outfall. 
A larger or smaller value would change the calculations propor- 
tionately. 
