PARK SANITATION 863 



filter to receive the effluent from the septic tank. These troughs are arranged 

 in a "crow foot" pattern (Plate 342). The width of the troughs and the 

 number of lateral troughs vary according to the amount of sewage to be 

 applied, and the size of the filter. The width of the main trough is greater 

 at the effluent end and decreases beyond each pair of side troughs. The 

 sides of the troughs are from three to five inches in height. Holes are bored 

 in the sides to permit the ready egress of the sewage which flows through 

 the holes as well as over the top of the sides. 



Septic tank effluent is applied to an intermittent filter by means of a 

 siphon chamber in the same manner as described for subsurface irrigation. 

 A number of small septic tanks may empty into one siphon chamber located 

 near the filter bed. If more than one filter bed is used a diversion chamber 

 should be installed (Plates 337 and 339). A filter bed is dosed from one 

 to three times each twenty-four hours, and the siphon chamber must be 

 adjusted accordingly. The sewage overflows onto the surface of the filter 

 bed, through holes in the sides and over the walls of the troughs, to a depth 



; . s A N D : . ::...' 



UNDER DRAIN 

 PLATE No. 341 

 CROSS SECTION OF INTERMITTENT FILTER 



of about three inches or less, and passes down through the filter where it 

 undergoes nitrification and is rendered more stable. 



Occasionally, a filter will clog and filtration will be interfered with by 

 the matting of the surface of the sand. When this happens the surface of 

 the filter should be thoroughly raked or even spaded. If the service area is 

 operated during the winter in climates where freezing is apt to occur, the 

 surface of the filter should be ploughed into furrows. On a furrowed filter 

 the sewage is applied underneath the ice which forms over the furrows. 



It is better to have several small filters than one large one, so that 

 there will be a longer resting period. Thus, approximately three thousand 

 square feet of filter surface would be required for the effluent from septic 

 tanks in which the sewage for fifty persons is treated. A rectangular filter 

 bed forty by seventy-five feet would provide the necessary surface, but more 

 efficient service will be obtained from two beds thirty by fifty feet each or 

 three beds twenty by fifty each. The small rectangular bed also has the 

 advantage that the underdrains can be placed one on each side, and it 

 is also much easier to secure an even distribution of the sewage on the 



