SEWAGE DISPOSAL 345 
retain all sediment and floating material, since the dis- 
charges permit the withdrawal of the liquid from near the 
middle of the tank only. This is one of the main features 
of the tank. All inorganic matter entering the tank will 
gradually settle and, of course, remain in it. Some of 
the organic matter tends to settle during the first 24 hours, 
aS rae after which it comes to the 
i ape = =9surface to be gradually 
wasted away by the action 
of bacteria. This wasting 
away is naturally very 
slow, and since the slowly 
gathering organic matter 
— rr 
Te4r. 
nearly all remains in the 
A B Cc 
ake washes first section of the tank, 
= = . 
this must be large enough 
Fig. 74.—Cross Section of Septic : i: : 
Tank. to provide for a consider- 
able accumulation of it. 
The tank should be built air tight, except in two places. 
At the right is an air inlet, consisting of a goose-neck 
pipe, which renders the vent at the top more effective. 
This vent consists of a long shaft extending beyond the 
top of the dairy, thus carrying off the foul gases caused 
by the decomposition of the material within. One-inch 
gas pipe, properly fastened, will serve as a satisfactory 
vent. 
In order to afford communication of sections A and C 
with the vent, the two partitions should not be built 
quite as high as the tank. There should be at least one 
inch space between the top of the partitions and the cover. 
A 14-inch gas pipe should be laid over the tank 
through which the water from the cooler and vats may 
be discharged directly into the drain. This water 
