230 Microbes and You 



become covered with a thick zoogloeal material, the filtration speed 

 is reduced to an uneconomical level. By treating water with a co- 

 agulating chemical first, it is possible to establish an eflFective 

 Schmutzdecke more quickly than is true of slow sand filtration. But 

 clogging of the filters also takes place more rapidly. To com- 

 pensate for this, water flow is reversed through the filter bed with 

 the aid of mechanical devices. Such rapid filters can treat up to 

 one hundred and twenty-five million gallons of water per acre of 

 filter surface per day. 



Rapid sand filters are almost as effective bacteriologically as 

 slow sand filters and they occupy a good deal less space, but be- 

 cause of the added mechanical devices, rapid filters are more 

 expensive to operate than are slow sand filters. No matter which 

 system is employed, however, chlorination is a wise practice as an 

 added precaution. 



As one looks over the progress that has resulted from the com- 

 bined efforts of engineers, chemists, and biologists in providing an 

 abundant, safe water supply to our centers of dense population, 

 it is truly a remarkable achievement in the history of preventive 

 medicine. 

 '^ SWIMMING POOLS 



V Persons engaged in performing clinical bacteriological analyses 

 can usually tell when late spring or early summer has arrived on 

 the basis of the number of cultures submitted from persons suffering 

 from ear infections. The call of the "ole swimmin' hole" prompts 

 individuals to bathe in ponds which may at times be grossly 

 polluted. Rats commonly live on the banks of such bodies of 

 water, and they may spread pathogenic organisms. According to 

 careful surveys, between 50 and 90% of wild rats harbor Leptospira, 

 which may cause fatal jaundice, and these bacteria are excreted 

 in the urine of rats. Weil's disease, which has a fatality rate of 

 about 25%, is caused by Leptospira icterohemorrhagiae. 



Sinus trouble for some persons may be traced to swimming 

 holes that were grossly polluted, and fungus infections are fre- 

 quently suspected of having a similar origin. 



Artificial pools constructed indoors and outdoors can be the 



