268 MICROBIOLOGY OF WATER AND SEWAGE 



Mean of Monthly Examinations for the Year 



to the living bacteria in the scum which forms on the top of the layer of 

 sand. Of these, the last is the most important; for until this gelatinous 

 layer forms, the filter does not act properly — in fact, it has little filter- 

 ing action, as the following figures show: 



Bacterial Content or Water Before and After Cleaning the Sand Filter 



Before cleaning, i.e., before removing the scum layer. . . 42 per c.c. 



One day after cleaning 18S0 



Two days after cleaning 752 



Three days after cleaning 208 



Four days after cleaning 156 



Five days after cleaning 102 



Six days after cleaning 84 



Thus provision must be made to permit the scum or film to form be- 

 fore the filtered water is used for domestic purposes. 



The rate of filtration must be regulated; for if the water is allowed to 

 exceed a certain rate (101.6 mm. or 4 inches per hour), inefficiency 

 follows. 



Coagulating Basins and Filtration. — This method of purifica- 

 tion consists in adding a coagulant, such as basic sulphate of aluminum, 

 by means of a mechanical device which regulates the quantity, as the 

 water is pumped into the coagulating basins or reservoirs, where it re- 

 mains for six to twenty-four hours. The aluminum sulphate is decom- 

 posed by the limp in the water and forms insoluble aluminum hydrate; 

 and the sulphuric acid combines with the lime. The hydrate of alumi- 

 num is precipitated in large flocculent masses, entangUng all particles 

 of soil or organic matter; and these, being deposited on the surface of the 



