BACTERIA IN LONDON WATER 41 



examine the returns to recognise the marked reduction in the number 

 of bacteria, in some cases amounting to 98 per cent., brought about 

 by filtration. In respect to the latter, the effect of season, some note 

 is required. It will be seen that during 1902 the figures are fairly 

 uniform throughout the year, showing, on the whole, a rise in winter 

 and spring, and a fall during summer. But in 1903 the returns show 

 wide variation which calls for explanation, which is as follows : 



The water supply in December 1902, on the whole, maintained an equal microbic 

 purity to that of November. This exceptional condition of the supply, the com- 

 paratively small number of bacteria, for the winter months is no doubt due to the 

 absence of floods in the Thames Valley, and to the unusually mild character of the 

 season. "If the large deficit in the rainfall is made up," wrote Crookes and 

 Dewar, * * no doubt there will be in the near future a period when the filtration of 

 the London waters will require more than usual care. As the general filter-beds 

 are, however, in good working order, we believe these difficulties, should they 

 arise, will be overcome satisfactorily." 



The standard of general organic purity during 1902, as defined by chemical 

 methods, was maintained. As regards the month of December, the Thames-derived 

 Companies showed decided differences among themselves, which, as the supply 

 comes from the same source, were essentially connected with differences in storage 

 capacity and variations in the structure of the filter-beds, although the latter is of 

 less importance in removing soluble organic matter. 



Crookes and Dewar add: "The longer our experience of the bacteriological 

 method, as applied to the analysis of the filtered supply, and the wider its application, 

 the more we are convinced of its primary importance as a safeguard to the public. 

 It enables us to define in a much more delicate way than is possible by chemical 

 analysis what is an efficiently filtered water, and thereby enables the chemist to 

 warn the engineer the moment any one of his filters show signs of defective 

 working. Whether the supply as regards the organic matter in solution varies 

 more or less according to the season of the year, is of relatively small moment 

 as compared with the knowledge that the microbic impurity is reduced to a 

 minimum. " 



That was the position at the end of 1902. But at the turn of the year, owing to 

 the great increase in the rainfall, the microbes in the unfiltered Thames water rose 

 from about 6000 to 13,000, that is, the bacteriological impurity about doubled, 

 whereas the unfiltered New River water underwent little or no alteration. The 

 result of this increase was that the filters of the Thames-derived Companies, which 

 were not working at their best, furnished a larger number of samples from the filter 

 wells, showing an increase in the number of bacteria which was the inevitable result 

 of an increased rainfall. 



Things remained thus until April, when in comparison with the month of March 

 the bacteriological quality showed considerable improvement, a result which might 

 have been anticipated from the advent of summer, and the improved natural 

 conditions associated with vegetable growth ; a state of things which generally 

 improves the quality of the water obtained from such collecting areas as the valleys 

 of the Thames and Lea. But in June, when the number of bacteria ought to have 

 been low, as ordinarily there would be a small rainfall, an exceptional condition of 

 things arose. The total excess of rainfall amounted to 49 '9 per cent, on the thirty 

 years' average, so that during the month of June actually 22 -5 per cent, or an 

 amount approaching one-half of the previous excess, of rain fell in the valley of the 

 Thames. Such an amount of rain is altogether exceptional in twenty- two years' 

 experience. The result was that the proportion of vegetable matter in solution, 

 and therefore the colour of the water, were both quite exceptional for the summer 

 months. Nevertheless, the general filtration was adequately and effectively 

 performed, as is shown by the bacteriological results. Similar conditions prevailed 

 in August and October. The exceptional rainfall, which amounted to sixty per 



