NATURAL PURIFICATION OF WATER 63 



sedimentation are most manifest when the flow of water is rapid 

 enough to prevent the accumulation at any point of the products of 

 bacterial multiplication, but not so rapid as to interfere with a 

 comparatively rapid action of gravity.* 



In the case of a tidal stream the conditions are different, as 

 recently pointed out by Foulerton.f In such rivers the disease- 

 producing bacteria are deposited not only on the bed of the stream, 

 but also on the mud, or sludge, on the banks, and are uncovered by 

 water at low tide. It now requires only the agency of a fly, feeding 

 first on the organic matter in the sewage-contaminated mud and then 

 on some human food, milk for instance, to convey the bacillus of 

 typhoid fever from the river to some human being. An additional 

 way by which a bacillus of this kind may survive after it has been 

 discharged into a river is by its being deposited on the bed of the 

 stream where there are shell-fish layings. It has been proved that 

 the typhoid bacillus can survive for a considerable time in the liquor 

 contained in the shell of the oyster or the mussel, and in this way it 

 may escape destruction by finding itself once more inside the con- 

 sumer of the shell-fish. Therefore, in the case of sewage discharged 

 into a tidal river, owing to lack of dilution and sedimentation, it is a 

 menace to the inhabitants on the banks in one or both of these ways. 

 The exact degree of danger depends first upon the extent to which 

 the sewage is purified before its discharge into the stream, and 

 secondly upon the distance from the source of pollution at which con- 

 tamination of the water by special sewage bacteria is still appreciable. 



This principle of sedimentation operates upon all bacteria, which 

 are often carried down on gross particulate matter. The number 

 of B. coli is reduced quite appreciably by storage of water (Clark 

 and Gage). Many species remain in the mud, sand, or other deposit 

 at the bottom of the stream or reservoir. The parasitic organisms 

 die on account of the unfavourable environment. 



(g) Oxidation. — Many experiments and observations have been 

 made to prove that large quantities of oxygen are used up daily in 

 oxygenating the Thames water. Oxygenated water will come up 

 with the tide and down with the fresh water from above London. 

 There will also be oxygen absorption going on upon the surface of 

 the water, and from these three sources enough oxygen is obtained 

 to oxidise impurities and produce what is really an " effluent." In 

 many smaller streams the opportunity for oxidation is afibrded by 

 weirs and falls. 



Probably all these factors play a part in the self -purification of 

 rivers, but we may take it that oxidation, dilution, and sedimentation 

 are three of the principal agencies. The test of purification is in the 



* Jour, of Stat. Med., 1901, p. 518. 



t Report on Pollution of Tidal Ouse, 1903, p. 11. 



