502 AGRICULTURAL ENGINEERING 



not sweep away fast enough. If there is much grease in the 

 sewage it is likely to become hardened over the surface of the 

 walls, making them water-tight. To overcome this difficulty 

 common lye has been used to cut the grease, with good success. 

 All cesspools should be arranged with a manhole, which will 

 permit the settlings or solid matter which collects in the 

 bottom to be removed at regular intervals, perhaps once 

 a year. 



Many cesspools that have been in use for years are entirely 

 satisfactory as far as observations go. The success of these 

 is undoubtedly due to the purifying bacterial action which 

 the sewage undergoes in the tank. At best, however, the 

 cesspool is a dangerous means of disposing of sewage, and 

 new installations should be of more improved design. Often 

 the contamination of the water supply is effected at an un- 

 dreamed-of distance, resulting in typhoid fever, dysentery, 

 and other complaints. 



Principles of Sewage" Disposal. The principle involved 

 in the purification of sewage in the modern disposal plant, 

 regardless of whether it be for city or private use, is largely 

 that of destroying the suspended matter in the water by 

 bacterial action. Outside of this, some results are brought 

 about by settling, thus caring for a part of the suspended 

 material. 



When the sewage from a farmhouse, consisting of the wash 

 water from kitchen and dairy and the discharge from plumb- 

 ing fixtures, is drained into a dark reservoir and not disturbed 

 for a time, rapid bacterial action takes place. The bacteria 

 which work in a tank of this sort do not need light or air to 

 live. The action is simply this: the bacteria feed upon the 

 organic matter of the sewage and thereby partially destroy it; 

 in addition, a part of this solid matter, or sludge, as it is called, 

 is liquified. 



