42 
BULLETIN 57, U. S. DEPARTMENT OE AGBICTJLTTJBE. 
The sand filter usually is a bed of sand 3 to 4 feet thick which is 
fine on top and gradually increases in size to coarse gravel at the 
bottom. The sewage from the tank is distributed over the filter by 
means of tile laid loose- jointed over the surface in much the same 
manner as in the ground- 
surface distribution sys- 
tem. The filter should be 
sufficiently porous and 
there should be sufficient 
natural or artificial under- 
drainage to allow every 
dose of sewage to sink 
away rapidly. Sewage 
should not stand on the 
surface of the filter for 
any length of time, as this 
will soon destroy its puri- 
fying properties. About 
45 square feet of filter 
should be provided for each 
person served by the sewer. 
The area should be divided 
into from three to five 
beds so that each bed may 
be allowed to rest occa- 
sionally. Figure 35 shows 
a plan and partial section 
of a sand filter for a family 
of eight persons. 
In constructing a filter 
a sufficient area should be 
leveled off and small earth 
embankments be made 18 
inches to 2 feet high to in- 
close the beds. The depth 
of the filtering material 
will depend largely on the 
porosity of the subsurface 
and the means of underdrainage, but it is well to have it not less 
than 2h feet; 3 to 5 feet is better, but the depth should not exceed 
6 feet. A good plan is to allow a minimum of 1 cubic yard of filter- 
ing material for every 50 gallons of sewage flow. 
Fig. 34.— Cross section of single tile sewage disposal system, 
showing second tile below for underdrainage. 
