Buildings and Yards. 525 



The "complete method" of sewage disposal. — The theory on which 

 the method of complete destruction of injurious sewage depends, is that 

 organic matter, — that part of the sewage which decomposes and becomes 

 offensive, — is in course of time converted into mineral salts through the 

 agency of bacteria. Of these bacteria there are two kinds concerned. 

 One, working in the dark, liquifies all the solid matter, such as paper, 

 banana skins, etc., making the sewage simply a dirty looking liquid ; the 

 other kind acts in the presence of light and air to clarify and oxidize the 

 liquid, making it clear and bright. Both kinds of bacteria are always in 

 the sewage and require only proper surroundings to go at once to work. 

 With this principle in mind, a properly designed treatment will include a 

 closed dark tank of a capacity about equal to the day's flow of sewage, 

 in which tank the required liqui faction may take place. This tank is 

 usually built under ground to keep it warm and the sewage flows con- 

 tinuously in and out. 



The rest of the process may take place either by allowing the effluent 

 from' this tank to flow slowly through artificially prepared beds of sand 

 three feet deep and of sufficient extent so that there is a square yard for 

 every fifty gallons per day, or for 300 gallons a bed of eight feet square. 

 Since the bed would need to have the surface raked off occasionally, it 

 would have to be made in duplicate so that one bed could be always 

 working. The two beds should be underdrained and the outflow led 

 away into any natural drainage. It would be better in a northern climate 

 to build a light wooden roof over these beds, which would themselves 

 be below ground, but this is not necessary, as such an arrangement 

 is often run all winter, as far north as Albany, by running fur- 

 rows through the beds to concentrate the sewage instead of letting 

 it run over the bed in a thin sheet. Instead of building artificial beds, a 

 piece of lower ground 200 to 300 feet from the house, may be used to 

 receive the efifluent of the tank. Over this the sewage would run between 

 the beds in furrows about four feet wide, or even slowly in a thin sheet 

 over the surface of grass land. If the appearance of the sewage is con- 

 sidered objectionable, the flow may be taken into small agricultural drains, 

 laid twenty feet apart on a grade of about four inches to 100 feet, in 

 which case the sewage will leach out between the pipes and be purified 

 in the soil. The bacteria concerned are chiefly in the top soil, so that 

 surface furrows or surface overflows are best; but if the drains are not 

 more than twelve inches below the surface good results will follow, even 

 in the coldest weather. 



