The farms are said to yield small profits over the expenses of working them, consequently contribute scarcely anything toward the cost of pumping the sewage.
The great reason why this method of disposal has been possible and profitable in a degree at Berlin is that this large and beautiful city lies in the midst of an extensive sandy area, the greater part of which in the immediate neighborhood is quite sterile and therefore comparatively thinly populated. Better conditions for such sewage farms could not exist.
Statistics compiled by Chas. S. Swan, M. Am. Soc. C. E. in his paper, Notes on European Practice in Sewage Disposal, in Jour. Assn. of Eng. Soc. Vol. 7 No. 7 shows the volume of sewage per acre per day (yearly average) to vary from 2155 to 29450 gallons, and the average depth per annum varies from 2.4 to 32.8 feet.
The nineteenth annual report of the Massachusetts State Board of Health 1888 states that on an ordinary farm in Massachusetts 2500 gallons per acre per day are as much as could be applied to any valuable grass crop. This would require such a large area for large cities that irrigation could not be depended upon for preventing the pollution of streams.
It is evident from the above report that the irrigation system can be used to advantage only where large tracts of low land can be obtained at a low cost, and where help and plenty of cheap labor can be obtained.
Intermittent Downward Filtration
Filtration means the concentration of sewage on an area of especially chosen porous ground, or on an especially prepared bed as small as will absorb and purify it. The intermittency of application is a necessary requirement even in suitably constituted soils wherever complete success is arrived at. The reason for this is that free oxygen is indispensable to nitrification and the nitrifying organism.
This method has been in operation only during the last thirty years, and the correct theory of its action has been found out within the last ten years.
Dr. Frankland, in the first report of Rivers Pollution Commission of England, shows that not only a chemical action but also a biolytic action takes place with the assistance of minerals and the oxygen of the air. This biolytic action is known as nitrification, and consists in the oxidation of the nitrogen of ammonia and its ultimate conversion into nitric acid. This change takes place in two stages, each characterized by a distinct organism. The office of one of these organisms is to convert ammonia into nitrite, while the other converts nitrites into nitrate.
The bacteria are present or quickly develop in the sewage which may be considered a nutrient medium for them by reason of containing a large amount of their natural nitrogenous food.