370 VETEEINAEY HYGIENE 



treated and purified before it is allowed to enter, but there 

 are still places where the difficulties of treatment are so 

 great that the old system of pollution exists. 



Disposal of Selvage. — One of the greatest problems in 

 sanitary science is the disposal of sewage ; from a veterinary 

 standpoint the question is only of general interest, as the 

 matter lies in other hands. As the case stands at present 

 the sewage must either be purified and then allowed to pass 

 into rivers, or utilized on the land. 



The latter sounds a simj)le and feasible system, but in 

 practice it is found that the manurial value of sewage, owing 

 to its enormous dilution, is very small ; sewage farms are 

 generally receptacles for large volumes of very impure 

 water, which by passing through the earth undergoes a 

 rough process of filtration, the effluent passing into the 

 nearest stream. 



The value of a sewage farm unless under the most careful 

 management, is from an agricultural point of view very 

 disappointing. It generally grows a rank rye grass which 

 obtains a local sale, and so helps to cover the cost; but 

 the soil unless under careful management soon gets 

 ' sewage sick.' 



Attempts have been made to purify sewage by precipita- 

 tion with various chemical re-agents, by electrolytic de- 

 composition, and in other ways, but this still leaves the 

 deposit or sludge to be dealt with. This latter is frequently 

 dried, pressed into cakes, got rid of for such manurial value 

 as it possesses, which is very small, or burnt in a cinerator. 

 The sludge is always the difficulty in precipitation 

 processes. 



Another method of disposing of sewage is the biological, 

 viz., by means of micro-organisms. There are several 

 processes in use, but the general principle involved is 

 the collection of the sewage in tanks, where it is first 

 exposed to the action of its own organisms, which in the 

 absence of oxygen render the organic matter soluble 

 with the production of ammonia and carbonic acid. After 

 this it is attacked in a second tank by another set of 

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