SEPTIC TANK STUDIES 3 



below a dormitory from which sewage was supplied by tapping a manhole in 

 the main that served the building. 



The installation consisted of three chambers, each of 2' x 3' x 4' dimensions 

 with a capacity of 179.5 gallons. In effect there were three separate tanks of 

 identical size constructed in one block. Sewage was diverted from the manhole 

 by means of a dam that permitted approximately he desired quantity to flow 

 and enter a rece'ving chamber from which it was apportioned to the three tanks 

 in quantities ihat provided three different retention periods: the recommended 

 24 hours in one tank, 12 hours in another, and 8 hours in the third. 



The first receiving chamber proved to have several faults of des'gn and opera- 

 tion and it soon became obvious that cnanges would have to be made. About 

 th's time the State Department of Health engineer entered the Army and the 

 Experiment Station took charge. Under the new manag ment, the late Profes- 

 sor C. I. Gunness, Experiment Station engineer, designed a new receiving chamber 

 which is shown in diagram II. The chamber was divided, by lengthwise par- 

 titions, into two equal compartments, one of which served the north tank (desig- 

 nated as N) and provided a quantity of sewage that would permit a retention 

 period of 8 hours. The other half of the chamber was divided by a cross par- 

 tition into two compartments that had a capacity ratio of 2 to 1. The larger of 

 these served the middle tank (designated as M) and permitted a retention period 

 of 12 hours, and the smaller compartment served the south tank (designated as S) 

 and provided the conventional retention period of 24 hours. Thus the sewage 

 volume flow ratio for the N, M, and S tanks was 3:2:1. 



A tip-bucket extended the length of the receiving chamber with a partition 

 lengthwise through its center which divided the tip-bucket into two sections of 

 equal capacity. These respective sections emptied sewage into the comparable 

 compartments of the receiving chamber. The tip-bucket was hinged at the bot- 

 tom on an iron rod which ran its length, and was so arranged that when one side 

 tipped to empty sewage into the chamber, the other was upright to receive sewage 

 which was fed to the tip-bucket through a trough above it having a slit running 

 lengthwise to distribute the incoming sewage as evenly as possible along the 

 length of the tip-bucket. A cross section of the tip-bucket is shown in diagram II. 



A mechanical counter was attached to the tip-bucket to record the number of 

 tips. Since the capacity of the tanks and of the tip-bucket was known, it was 

 possible to calculate the number of tips required each 24 hours to furnish the 

 desired quantity of sewage to each tank. 



A considerable trial-and-error period was necessary to arrive at the correct 

 diversion of sewage from the manhole. At best, the supply of sewage to the tank 

 was irre^lar. There were several peaks during the day: first, in the morning 

 when the residents of the dormitory were getting up, then at noon when some of 

 them were back at the dormitory for a time, and again in the evening. Another 

 source of discrepancy, during the period of operation of the tanks, was the dif- 

 ference of personnel housed in the dormitory. During the first two seasons (fall 

 and early winter, 1943-44 and 1944-5) Army Air Corps cadets occupied the 

 ixiilding, and the third season (fall and early winter, 1945) it reverted to student 

 use and was occupied by girls. The cadets were not in the dormitory during the 

 morning or afternoon, while during the last season some of the girls were in at 

 all times. There were two other sources of discrepancy: the dormitory was 

 unoccupied from May to late September each year; and the heavy snow and 

 hard freezing of midwinter made operation impracticable from January through 

 March because the receiving chamber and collecting wells were not housed, 

 with the result that control of flow and collection of samples were difficult at all 

 times and frequently impossible. 



