194 SEWAGE AND ITS PURIFICATION 



or effluents ; in fact, English experience with the land plants 

 grown on sewage farms has pronounced them to be safe. 

 Wurtz and Bourges^ grew cress, radishes, and lettuce in earth 

 watered with various pathogenic cultures, and found the bacilli 

 on the stalks of the plant at a height of even a foot above the 

 ground. Potatoes infected with anthrax and planted were 

 allowed to grow : the bacillus was recovered from the stalks as 

 long as loi days afterwards. It is noted that out-of-door con- 

 ditions, such as the cleansing effect of rain, and the bactericidal 

 action of sunlight, are different from those in the laboratory. 

 But in heavy storms mud may be splashed to a great distance, 

 and so contaminate the leaves or fruit. A French Commission 

 was appointed in 1902 on the subject of possible dangers from 

 raw vegetables and fruit grown on sewage farms, and recom- 

 mended to the Comite d'Hygiene Publique that vegetables and 

 fruit intended to be consumed in the raw state should not be 

 allowed upon land fed with sewage. 



Many attempts have been made to use ozone, either in admix- 

 ture with air, to be passed through or over the sewage, or to be 

 generated electrolytically in the sewage itself. Hagen (1881) 

 ozonized air by the silent discharge, passed it through sewage, 

 then ozonized it again, absorbing the carbonic acid by lime, so 

 making the process continuous. Marmier and Abraham^ have 

 used ozone for sterilizing the water-supply at Lille, and state 

 that it removes nitrates and organic matter, and all germs except 

 B. subtilis. The cost of the plant is given at ;f 500 for sterilizing 

 5,000 cubic metres per day. 



Thermal Methods. 



To raise the entire volume of sewage to a heat sufficient to 

 sterilize it would be obviously impossible in practice; in addition, 

 besides the odours evolved, it would leave a liquid which, on 

 fresh inoculation with microbes from air, water, or earth, would 

 become as foul as before. 



The Liernur process is a combination of conservancy, pneu- 

 matic removal, and disinfection. At Amsterdam in 1871 a trial 

 was made on a small quarter of 15,000 inhabitants for the 

 conveying of the fgecal matter and closet water, excluding the 

 household slops. The system was applied at Leyden, Riga, 

 and other places, and afterwards carried out more completely 



^ Archives de Mcdedne Expcrimentale, July, 1901. 



2 Comptes Rendiis, 1899, cxxviii., 1034 ; Revue d'Hygiene, 1899, 321, 540. 



