404 Irrigation and Drainage 



CLIMATIC CONDITIONS FAVORABLE TO SEWAGE 

 IRRIGATION 



Since the fermentive processes which convert organic matter 

 either into nitric acid, which is the nitrogen supply for most 

 cultivated crops, or into free nitrogen gas can take place rap- 

 idly only under temperatures above 50 F., it follows that sewage 

 irrigation is best suited to warm climates, where crops may 

 be grown the year round, and where the fermentive processes 

 will be least checked by frosts. In tropical and semi-tropical 

 climates, therefore, sewage disposal by surface irrigation may 

 best be practiced when other needful conditions are also favor- 

 able. 



In cold climates, like those of the northern United States 

 and Canada, where the ground is frozen during five months or 

 more of each year, it is plain that only about one-half of the 

 sewage water can be used in crop production, and that during 

 only about one -half of the year can there be much oxidation 

 and denitrification of organic matter. Under these conditions, 

 therefore, if water is applied to land one-half of it must be 

 filtered by the soil without the concurrent purification which 

 results from fermentation, and this being true, there can be 

 only so much of purification as naturally results from the 

 physical filtration and such chemical fixation as the soil may be 

 capable of accomplishing. 



It is true that the purification of sewage resulting from 

 filtration through soil is very considerable, so that if isolated 

 lands of sufficient area are selected for this purpose, the organic 

 impurities reaching the ground water will be greatly reduced. 

 It is also true that in cold climates fields to which no sewage 

 has been applied during the warm season may be reserved 

 specially for the reception of it during the winter. These soils 

 would, therefore, be comparatively dry and capable of receiving 

 6 to 12 inches of water and of retaining it by capillarity 

 until warm weather could subject it to organic purification, 

 and when crops could also be made to utilize the nitrates 

 developed and other fertilizers brought by the water. 



