292 VETERINARY BACTERIOLOGY 



which automatically discharges through one or more siphons when- 

 ever it becomes filled. The sewage passes from here either into con- 

 tact beds or into filter-beds. The former consist of beds of crushed 

 rock, usually with water-tight walls. The sewage may be sprinkled 

 over the surface constantly and allowed to trickle through (the 

 so-called trickling filter), or it may be poured onto the bed in bulk, 

 and held in contact with the crushed stone for a time and then dis- 

 charged. In either case the sewage becomes thoroughly aerated, 

 and the aerobic bacteria rapidly oxidize the organic matter pres- 

 ent. A filter-bed, on the other hand, is constructed of sand under- 

 laid with gravel and stone. The sewage is spread out over the 

 surface and is allowed to seep through. Opportunity for thorough 

 aeration of the sand and gravel is given by the time elapsing be- 

 tween the discharges from the dosing chamber. Frequently 

 several beds are used, and the sewage is discharged first upon one 

 then upon another. The organic material is retained, probably 

 by absorption, and, as the sewage passes through, the bacteria are 

 largely filtered out, and the organic material is, for the most part, 

 quite completely oxidized. The water leaving the drains under 

 these filter-beds is relatively pure, in some cases quite as pure as 

 water from the average shallow well. As has been stated, there 

 are many modifications of the type of disposal plant. It has 

 been adapted to use for the farm-house as well as the city. 



