SEPTIC TANK STUDIES 13 



Table 5. — Summary of Biochemical Oxygen Demand Tests From 

 Septic Tank Effluents. 



* The number of tests represents the number of collection dates. There are 12 tests instead of 

 13 for the South Tank because the value for one of its tests was so much higher than any of the 

 other values that it was considered a probable error and discarded. The average for this tank 

 was based on the 12 tests. The scores were based on the same 12 tests for each tank: 3 points for 

 the high value. 2 points for the middle value, and 1 point for the low value for each collection date. 



The main point at issue, however, is the quality of the effluents, and in this 

 the south tank would have to be considered superior on the basis of B.O.D. de- 

 terminations. This decision agrees with the amounts of scum that accumulated 

 in the three tanks and with the observation that the scum in the north and middle 

 tanks contained considerable sewage solids which had disintegrated very little. 



Free Ammonia Determinations 



Free ammonia in the effluent results from the bacterial decomposition of the 

 protein portion of the sewage solids. Consequently the amount of ammonia 

 recovered in the tests and expressed as nitrogen would be interpreted as a direct 

 indication of the relative activity of digestion of sewage solids in the tanks. If 

 the total digestive efficiencies of the three tanks were in proportion to the 3:2:1 

 volume ratio of sewage handled by the tanks, then the south tank effluent should 

 have shown much the smallest quantity of ammonia. Reference to table 6, 

 however, shows that the averages for the three tanks were nearly the same and 

 that the score for the south tank was only a little lower than the scores for the 

 other two tanks. This agrees with the results of the B.O.D. determinations and 

 with the presence of undigested sewage solids in the scum in the north and middle 

 tanks. 



Table 6. — Summary of Determinations of Free-Ammonia Nitrogen 

 From Septic Tank Effluents. 



* The 18 tests represent 18 collection dates which include the dates represented by the B. O. D. 

 tests of table 5. There are fewer B. O. D. tests than ammonia tests because facilities were not 

 available for making the B. O. D. test on all of the collection dates. The scores are based on a 

 comparison of values for the three tanks on each collection date: 3 points for the high value, 2 

 points for the middle value, and 1 point for the low value. The fractions are accounted for by the 

 north and middle tanks having the same values on several dates. 



