PARK SANITATION 857 



decomposition and liquefaction. It should be placed as close as practicable 

 to the source of the sewage, but in a locality where the odors generated in 

 the tank will not become obnoxious. Small installations are best placed 

 below the surface of the ground, although this is not necessary for the satis- 

 factory operation of the tank. For the construction details of a septic tank 

 see Plates 332, 333 and 334. The size and shape of the septic tank may 

 vary according to the circumstances, and expert advice regarding their 

 construction can usually be obtained from local or state health authorities. 

 The tank proper should have a capacity of five cubic feet per person, but 

 it is not advisable to use a tank containing less than seventy-five available 

 cubic feet. Such a tank will handle the sewage for fifteen persons or from 

 seven hundred and fifty to one thousand gallons of sewage per day. The 

 siphon chamber may be an integral part of the tank or it may be built as 

 a separate unit so that it will receive the effluent from several tanks. The 

 latter arrangement is feasible when a large subsurface irrigation field or 

 intermittent filter is used for the disposal of the effluent. All septic tanks 

 must be properly ventilated and have a manhole opening into the tank. 



A certain proportion of the solids in the sewage settle out and remain 

 in the tank as sludge. Ordinarily, the sludge must be removed when it 

 fills one-third of the tank, which usually occurs at intervals of from six 

 months to two or three years. To clean the tank the overlying liquid is 

 pumped out, after which the sludge is removed with a scoop or by pump- 

 ing and carted away and buried at some convenient place where it will not 

 contaminate a water supply. During hot-dry weather the sludge can be 

 spread in a thin layer on the ground and dried until it can be handled with 

 a spade. A scum will form upon the surface of liquid in the septic tank and 

 this should be occasionally broken up with a stick or pole and allowed to 

 settle to the bottom. Scum should not be permitted to accumulate to a 

 depth of more than six inches. As septic action is dependent upon the 

 growth of bacteria in the sludge, it may be delayed for a time when a new 

 tank, or one that has been idle for some time, is placed in service. During 

 this time the tank acts only as a settling basin, but, even so, a considerable 

 percentage of the solids is removed and subsequent treatment of the 

 sewage rendered much simpler. 



The effluent from a septic tank is putrescible and contains many bac- 

 teria, and must be disposed of in such a manner that it will not create a 

 nuisance or constitute a danger to the health of man. This may be accom- 

 plished in one of several ways, either by subsurface irrigation, filtration, or 

 dilution in a nearby body of water. 



Subsurface irrigation (absorption fields, distribution fields). Where a 

 loose porous soil is available, subsurface irrigation in the form of an absorp- 



