AGRICULTURAL ENGINEERING. 



1033 



to treat more than 30,000 to 60.000 gals, per acre per 24 hours at a given time (750 to 

 1,500 persons per acre); or more than 10,000 to 20,000 gals, per acre per 24 hours, 

 calculated on the total irrigable area (250 to 500 persons per acre). Further, that 

 soil not well suited for purification purposes, worked as a surface irrigation or as a 

 combined surface irrigation and filtration farm, should not be called upon to treat 

 more than 5,000 to 10,000 gals, per acre per 24 hours at a given time (125 to 250 per- 

 sons per acre) ; or more than 1,000 to 2,000 gals, per acre per 24 hours, calculated en 

 the total irrigable area (25 to 50 persons per acre). It is doubtful if the very worst 

 kinds of soil are capable of dealing quite satisfactorily even with this relatively small 

 volume of sewage. The population per acre is calculated on 40 gals, of sewage per 

 head per day. It is here assumed that the sewage is of medium strength, and is 

 mechanically settled before going on to the land." 



The commission further conclude regarding the management of sewage farms that 

 " the effectual purification of sewage, even with suitable land, can only be accom- 

 plished when the farming operations are relegated to the background and the pro- 

 duction of a good effluent considered of primary importance. On the other hand, 

 the manager knows that the crops will probably form an important item in his 

 receipts at the end of the year, and he not unnaturally wishes it to appear that the 

 farm is being worked economically under his supervision. Hence there is a temp- 

 tation to grow remunerative crops, e. g., cereals, that can not be sewaged (at all 

 events for the greater part of th year), or to refrain from the further sewaging of 

 crops which may he damaged thereby; meanwhile the land which is under sewage 

 must needs yield, owing to the lack of rest, increasingly unsatisfactory effluents. 

 There may of course be some farms where the large area at command in proportion 

 to the volume of sewage to be ' treated' renders the growing of grain crops justifia- 

 ble, but these are exceptions to the general rule. Land is usually too expensive in 

 the immediate vicinity of towns to allow of this, and the tendency is to take too little 

 rather than too much land for a sewage farm. . . . 



''We are unable to recommend the abandonment of farming operations even in 

 connection with filtration sewage farms, because if intelligently pursued they make 

 for profit wuth increased efficiency of the land. The farming operations, however, 

 should always be under the control of the authorities responsible for the proper 

 working of the farm, and the manager should receive written and explicit directions 

 to regard the crops as of secondary importance to the uniform and satisfactory puri- 

 fication of the sewage." 



From a large number of analyses of the sewages used on the different farms and of 

 the effluents obtained, the following averages are calculated: 



Average comjxjsidon of sewages and effluents. 

 [Parts per 100,000 by weight.] 



a From permanganate at 26.7° C. (80° F.) in four hours. 



The results show a percentage of chemical purification varying from 84 to 98 per' 

 cent calculated on albuminoid nitrogen, and from 81 to 99 per cent based on oxygen 

 absorbed. 



From the mechanical analyses of the soils and subsoils and a study of their specific 

 gravity, porosity, and water-holding capacity, the conclusion is drawn "that it is 

 possible to say from the mechanical analysis of a soil whether or not it would be 

 suitable for sewage purification by filtration, and the same thing holds in a lesser 

 degree as regards suitability for surface irrigation. Further, they tend to indicate 

 that the maximum number of particles per gram of soil, which is allowable if the 



