38 BACTERIOLOGICAL EXAMINATION OF WATER. 



throughout the filter. Sand as coarse as 0*48 mm. requires a 

 longer period of operation to yield normal results. 



The question as to whether a filter should be used continu- 

 ously or intermittently is considered by the Massachusetts 

 Board to depend on the relation of the dissolved oxygen to the 

 organic matter in the water. Some waters are saturated with 

 oxygen and contain only a small quantity of organic matter ; 

 these may be filtered continuously with satisfactory results. 

 Successful filtration by the ordinary continuous method is out of 

 the question when the water contains much organic matter and 

 very little or no free oxygen. Whether the beds are worked 

 continuously or intermittently it is acknowledged that an 

 effluent under satisfactory conditions will not contain more than 

 100 bacteria per c.c. At the Altona works the filtered water 

 usually showed counts under 30 per c.c. ; 50 to 70 per c.c. were 

 rarely found. The London water companies have not been so 

 successful, those companies which derive their water from the 

 Thames and Lea being hampered by insufficient storage. The 

 impurity of the water in the rivers is so largely increased during 

 periods of flood as to render it a matter of vital importance that 

 there shall be sufficient provision of subsidence reservoirs to 

 enable the necessary daily supplies being taken while keeping 

 the intakes closed. A certain amount of suspended organic 

 material is required to assist in forming the gelatinous film in 

 the filter bed, but an excess soon blocks up the bed and necessi- 

 tates frequent cleaning and consequent disturbance of the body 

 of the filter. It was at one time thought that double filtration 

 might improve river supplies ; but in the process of double 

 filtration the organic matter will be almost entirely arrested on 

 the first filter leaving only a small amount of material to form 

 the film in the second filter, and as a result the partially purified 

 water from the first bed passes through the second bed with only 

 a very slight improvement. The exact conditions which will 

 give the best results must be worked out more or less experi- 

 mentally for each supply. It is also of the first importance 

 that there shall be means of making a daily bacteriological 

 examination of the filtered water from each bed. Filtered water 

 containing more than 100 organisms per c.c. must not be passed 

 into the reservoir for pure water ; the beds must be so 



