XII 



POLITICAL LIFE 305 



and the rivers on that water-shed are now much more 

 available than formerly for manufacturing purposes. 



To treat successfully sewage pollution from towns is a 

 much more difficult matter. In this case it has been neces- 

 sary to insist on processes of purification being adopted 

 which will render the sewage non-putrescent, and for 

 this purpose some form of filtration must be adopted in 

 addition to subsidence and chemical precipitation. 



The development of the system of filtration has 

 been gradual, and in 1895 at tne request of the 

 Manchester Corporation I undertook an investiga- 

 tion of the question as to the comparative value of 

 land as against artificial filtration, the result being 

 entirely in favour of the latter. This conclusion is 

 clear from the fact that under favourable conditions 

 land is capable of dealing with not more than 20,000 

 gallons of clarified sewage per acre per diem, whereas 

 an equal area of artificial filters will deal with 250,000 

 gallons in the same time. The successful working of 

 the filters which I designed for Manchester, and 

 which have been in action ever since, attracted the 

 attention of other sanitary authorities, and they were 

 largely adopted. On the Mersey and Irwell water- 

 shed in 1898 the area of filter-beds in use was 6*5 

 acres, whereas in 1904 there were 105 acres in use. 

 It must not be supposed that these filters are used for 

 removing the grosser solid impurities ; this is accom- 

 plished by subsidence, with or without chemical 

 precipitation. The essential value of the filter-bed is 

 to get rid of the putrescible material known to chemists 

 as soluble organic matter. This purification is entirely 

 effected by micro-organisms, which serve as Nature's 

 scavengers, and transform a foul liquid into one which 

 is non-putrescent, and may therefore be allowed to 

 flow into any stream not used for drinking purposes. 



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