82 BACTERIOLOGY FOR NURSES 



Ultraviolet Rays. Exposure of a clear water to ultraviolet 

 rays has a speedy effect on bacteria. Most of the vegetative forms 

 are killed in from ten to twenty seconds ; a longer time, however, 

 is necessary if the water is turbid. After preliminary rough fil- 

 tration to remove coarse particles the water supply of Marseilles 

 passes a quartz tube mercury arc lamp three times. It is claimed 

 that between 98 and 99 per cent of the bacteria present are de- 

 stroyed during the process. 



Sewage Purification. The role which bacteria play in the 

 disintegration and consequent purification of sewage is an impor- 

 tant one, even though the details of the process are somewhat 

 obscure. 



The treatment of sewage may consist of one, two, or all of the 

 following processes: (1) screening or removing the larger sub- 

 stances that might injure filters, etc. Screens may vary all the 

 way from gratings of iron bars to fine ones of wire cloth. The 

 material screened is pressed and burned under a boiler or buried 

 in land and the effluent passed into (2) a sedimentation tank. 

 Several different types of tanks have been devised for the purpose, 

 in all of which the underlying principle is the disintegration by 

 bacteria of the organic material present. From the sedimentation 

 tank the sewage is carried to (3) a filter bed, usually composed of 

 some form of porous material such as coke or brick, through which 

 it percolates to underdrains below. The final process (4) is the 

 removal or destruction of bacteria in the effluent. Chlorinated 

 lime is one of the best substances for the purpose; about fifty 

 pounds per million gallons for good effluents will destroy from 

 95 to 99 per cent of the bacteria. 



Methods of bacteriological examination to determine the effi- 

 ciency of a purification plant are the same as those described for 

 the examination of water except that smaller quantities are used. 

 In a sewage effluent as in water the absence of the colon bacillus 

 is regarded as an indication of its harmlessness. 



