6 MASS. EXPERIMENT STATION BULLETIN 446 



In the spring of 1944 the tanks were pumped out, the new receiving chamber 

 with the tip-bucket was built, and operation was started as soon as the dormitory 

 was occupied which was in the latter part of September. The installation re- 

 quired attention only about twice each day to record the amount of flow as indi- 

 cated by the counter on the tip-bucket and to remove toilet paper that accumu- 

 lated in the manhole on the dam that diverted the sewage to the line serving the 

 tahks. 



At the end of four weeks, the accumulation of scum in the north and middle 

 tanks (8 and 12 hours retention respectively) was about 2 inches; in the south 

 tank (24 hours retention), about 1 inch. By December 1, about two and one- 

 half months after operation was started, so much scum had accumulated in the 

 north tank that the outlet was plugged, and the middle tank was nearly in the 

 same state. The south tank had only about 2J^ inches of scum. The outlet 

 pipes had vertical "T" arms extending 5 inches below the outlet level, which will 

 give an idea of the accumulation of scum in the north and middle tanks. An 

 examination of- the scum in these tanks revealed that it contained considerable 

 quantities of undigested solid matter of the sewage. 



Cold weather dictated the closing of the tanks for a time because cleaning was 

 impracticable. In late March, 1945, the scum was removed and operation was 

 resumed. At the end of April the cadets left the dormitory, so no further results 

 were obtained. Some examinations of effluents were made and are reported later. 



After the second closing of the tanks the contents were removed and the 

 horizontal "T" arms of the inlet and outlet pipes in the north and middle tanks 

 were exchanged for longer arms that reached 12 inches below the levels of the 

 horizontal inlet and outlet pipes. The tanks were put in operation again in late 

 September, 1945. The scum began to form promptly in all three tanks, and in 

 six weeks was about 6 inches thick in the north tank, 5 inches in the middle 

 tank, and about IJ^ inches in the south tank. By December 26, the thickness 

 of the scum was 13 inches in the north tank, 11 inches in the middle tank, and 

 about 2J^ inches in the south tank. Again, examination of the scum in the 

 north and middle tanks revealed the presence of substantial amounts of un- 

 digested solid matter of the sewage, while there was practically none in the scum 

 in the south tank. 



This portion of the investigation gave results in favor of the conventional 

 24-hour retention period recommended for and commonly employed in septic- 

 tank operation. The rapid accumulation of scum with its content of undigested 

 solid matter in the north and middle tanks indicated that the rapid flow of sewage 

 prevented efficient functioning in them as compared with the south tank which 

 functioned in the usual manner of septic tanks, and the thickness of the scum in 

 the north and middle tanks was all out of proportion to the capacity of the tanks. 



DISPOSAL FIELD 



The disposal field was laid out in anticipation of the volume of flow from the 

 tanks. The south, or normal-flow, tank was provided with a lateral 50 feet long; 

 the middle tank with two laterals 50 feet long each; and the north tank with 

 two laterals 75 feet long each. Thus, the ratio of the lengths of the laterals was 

 the inverse of the sewage-retention periods of the tanks. These lines are shown 

 in diagram I. The soil of the field was light silty loam. The disposal lines were 

 laid in trenches excavated to a depth of 20 inches. The width was 18 inches at 

 the bottom and 22 inches at the top. The top of the drain tile was 11 inches 

 below the surface of the ground. The trenches were filled with gravel to one 

 inch above the tiles. The remainder of the fill was the soil taken from the trenches. 



