BACTERIOLOGY OF WATER 24! 



filter-bed no longer passes the required quantity of water 

 under the maximum permissible head. The sufficiency of 

 this practice has not been clearly shown. No general rule 

 can be given for the depth to which the top layer must be 

 removed, as it varies with the nature of the water and 

 sarid, temperature, etc. The epochs of scraping and other 

 interference, as by storm water, with the filter-beds, have 

 been frequently observed to synchronise with groups of 

 typhoid cases in the districts served with the water in 

 question. A properly working filter should remove 98 or 

 99 per cent, of the organisms, and a filtered water should 

 not contain B. coli in 100 c.c. 



The sand filters of the Metropolitan Water Board vary 

 in depth from 2 to 4| feet, and the rate of filtration 

 varies from 0-89 to T94 gallons per square foot per hour. 

 Each acre of filtering surface requires to be cleaned on 

 the average seven times during the year. Filtered 

 Thames and Lea waters contain two to three colon bacilli 

 per litre (Houston). 



The Sterilisation of Water. 



The use of bacterial filters is dealt with on p. 239. 

 Water may be efficiently sterilised by heat. In apparatus 

 for this purpose, the hot sterile water passes through 

 tubes or compartments surrounded by the incoming 

 water, with the result that the sterilised water is cooled 

 and at the same time the unsterilised water is heated 

 to such a temperature that comparatively less fuel is 

 required to bring it to the necessary temperature. Water 

 sterilised by heat loses part of its natural salts and part 

 of its dissolved gases, and may be mawkish in flavour. 

 For such an apparatus to be safe it must be fitted with 

 an automatic valve which does not allow water to leave 

 the apparatus until it has been raised to the required 

 temperature. Such an apparatus would be safe, but it 

 cannot be cheap in first cost, and its weight and the cost 

 of fuel are against its popularity except on a small scale. 



The distillation of water is sometimes practised where 

 water contains too great a quantity of salts to be drink- 

 able. The distillate is, of course, sterile if properly collected, 

 but, like all boiled water, it must be aerated to make it 

 agreeable to the palate. 



Bisulphate of sodium (15 grains per pint) is said to 



