290 VETERINARY BACTERIOLOGY 



Sedimentation is probably the most potent factor in freeing 

 a contaminated water from bacteria. The bacteria themselves 

 have a slightly greater specific gravity than water, and tend to go 

 to the bottom under the influence of gravity. This occurs more 

 rapidly when other and larger solid particles are in suspension; 

 flocculation and more rapid sedimentation then frequently occur. 

 Advantage is taken of this fact in the artificial purification of 

 water, and coagulants of different kinds are added, which carry 

 down the bacteria, together with other suspended material. 

 Diminution of food-supply with consequent disappearance of many 

 bacteria is likewise important. Probably light destroys some 

 organisms, and others are ingested by protozoa. Some species 

 do not develop in the presence of certain other forms, that is, 

 they exhibit antibiosis. Water-plants, alga, and natural obstruc- 

 tions of all kinds to water-flow exert a filtering action. Changes 

 in temperature may inhibit the growth and even destroy some bac- 

 teria. A contaminated stream is constantly diluted by the influx 

 of ground- water and of tributaries. 



Purification of Drinking-water. For domestic purposes, water 

 may be effectually purified by heating to the boiling-point for a 

 few minutes. All the pathogenic bacteria are quite effectually 

 eliminated by this method. Berkefeld porcelain filters, if properly 

 constructed, are also efficient. They must be cleaned and sterili- 

 ized at short intervals, otherwise the organisms will grow through 

 the pores of the filter, and the water passing through will be as 

 contaminated as the original supply. Where a city-supply must 

 be purified, it is commonly pumped into reservoirs and allowed 

 to settle, with or without the addition of coagulants. It is then 

 passed through filters of various types, usually sand. Passage 

 through a properly constructed filter of this type has been shown 

 to be exceptionally efficient. Such a system requires careful 

 supervision. An efficient system will remove over 99 per cent, 

 of the bacteria originally present. Some city supplies are pumped 

 under pressure through sand-filters. This does not seem to be as 

 efficient a means of ridding the water of bacteria as the other, as a 

 filter, in any case, to retain its highest efficiency, must remain for 

 some time undisturbed, and filters of the latter type require fre- 

 quent cleaning and washing. The installation of filtration plants 



