254 



OUTLINES OF BACTEEIOLOGY. 



beds, but unlike the method of working the contact beds, it flows 

 through the filter without being brought to rest. The filter is con- 

 sequently not filled and emptied alternately. Continuous filtration may 

 be accomplished either by the streaming filter in which the liquid is 

 made to flow out at the same rate as it flows in, or by some form of 

 trickling filter in which the liquid is showered or sprayed into the 

 filter instead of flowing in. The majority of trickling filters are fed by 

 what are known as rotary sprinklers (Fig. 134), consisting of pipes 



Fig. 134.^Trickling filter with rotary spriukler. (After Lafai'.) 



pivoted in the centre of the filter and having perforations on the opposite 

 sides of the two arms, from which the effluent is showered on to 

 the bed, the reaction of the liquid against the side of the pipe being 

 utilised to rotate the sprinkler. As the spindle revolves, the points of 

 impact of the jets are continually changed, thus tracing a series of 

 circles in the filter. 



In all these filters adequate oxidation is secured by making the sides 

 or walls as open and porous as possible. 



§ 7. EFFECT OF BIOLOGICAL TREATMENT ON PATHOGENIC 



ORGANISMS. 



All the biological methods that we have mentioned are successful in 

 that they are able to alter sewage to such an extent that the eflBuent 

 contains no putrefiable matter. But the effluent is not free from 

 bacteria, and it is important to know the nature of these bacteria. It 



