18 AMERICAN MUSEUM (HIDE LEAFLETS 



is almost sure to follow. The land clogs and becomes "sewage sick," a 

 local nuisance is created and more or less unpuriried sewage must he dis- 

 charged into the nearest watercourse. Where local conditions and adminis- 

 trative efficiency are less favorable than at Berlin the economic advantage 

 disappears. The most recent studies of the British Royal Commission 

 indicate that cropping of irrigated land scarcely pays for itself — still less 

 contributes toward the cost of sewage treatment. In the arid regions of 

 the western part of the United States where every drop of water, as such, 

 is precious and where the manurial value of sewage is rcenforccd by its 

 water value, sewage farming becomes really profitable. In many parts of 

 California and Colorado and other western states irrigation is clearly indi- 

 cated as the best method of sewage treatment. Elsewhere, its application 

 is more than problematical. The idea of converting the wastes of a city 

 into walnut groves and fields of waving corn is an attractive one. The 

 engineer, however, always wants to know the cost; and here, as in other 

 modes of sewage utilization, it is poor policy to recover valuable elements 

 that cost more to recover than their intrinsic value. 



INTERMITTENT FILTRATION THROUGH SANB 



The real art of sewage disposal began only when the crude process of 

 broad irrigation was freed from the seductive bope of agricultural gain and 

 developed intensively and scientifically as a means for sewage disposal pure 

 and simple. Mainly through the experiments of the Massachusetts State 

 Hoard of Health at Lawrence, it was shown that the essential process in 

 sewage purification, cither by dilution or broad irrigation, was an oxidation 

 of organic compounds by the nitrifying bacteria, and that this process could 

 be carried out much more efficiently by carefully controlling the conditions 

 surrounding it. For a filter bed or substratum for the support of the growth 

 of nitrifying bacteria, a fairly porous sand should he used, and the sewage 

 should lie applied in regulated intermittent doses with rests between for the 

 supply of oxygen necessary to the process. By such means the rate of 

 filtration can he raised from .">,()()() to 10,000 gallons per acre per day ( for 



broad irrigation) to 50,000 or 100,000 gallons. An intermittent filter 

 of half an acre in area would therefore care for the sewage of five hundred 



persons while five acres of broad irrigation area would be needed for a 

 similar population. 



The ((instruction of intermittent filters in regions like the northeastern 

 part of the United States is extremely simple. This part of the country is 



