Sewage Purification 405 



To handle the sewage in this manner, it would be needful 

 to bring it to the fields in underground conduits, and to have 

 the lands laid out for flooding in checks of suitable size, sur- 

 rounded by barriers of the desired height, but- the great diffi- 

 culty to be met is the amount of land needful for such a 

 system. Allowing 50 gallons of sewage per day per person, 

 a city of 30,000 would require 828 acres to receive the sewage 

 during 180 days if each check were to be flooded to a depth 

 of 12 inches. 



THE PROCESS OF SEWAGE PURIFICATION BY IRRI- 

 GATION OR INTERMITTENT FILTRATION 



The extremely careful and extended investigations con- 

 ducted by the State Board of Health at Lawrence, Mass., begun 

 in 1888 and still in progress, have shown that the purifying 

 of sewage as it passes slowly over the surface of sand grains 

 freely exposed to contained air, is the result of bacterial growth, 

 and that 'when these germs are not present the sewage comes 

 through the filter as impure as it went in so far as its dangerous 

 nitrogen compounds are concerned. But if it is allowed to 

 pass through slowly enough in the presence of an abundance 

 of air, the water emerges with so nearly all the nitrogen com- 

 pounds converted into nitrates that it is as free from them 

 as the purest spring water. 



The essential condition is that an inch or two of water 

 shall be spread out over the surface of the soil grains in 

 enough of the upper soil, where free oxygen may gain access 

 to the colonies of niter-forming germs which multiply there 

 and feed upon the organic nitrogen in the water, if only 

 there is an abundance of free oxygen to meet their other 

 needs. When a new quantity of water is added to the soil, 

 the purified layer is swept downward by the new supply, 

 which at the same time drags in after it a fresh supply of 

 air, and thus the work goes on. 



If the sewage water is added too rapidly, before the germs 



