42 



BULLETIN 57, U. S. DEPARTMENT OF AGRICULTURE. 



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 2^ 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. 



