2C>6 



MICROBIOLOGY OF WATER AND SEWAGE. 



Mean of Monthly Examinations for the Year. 



Microorganisms per c.c. 



At source After storage After filtration 



The action of the filter bed is due to the mechanical obstruction of 

 impurities, to oxidation of the organic matter, and to nitrification due 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 filtering action, 

 as the following figures show: 



Bacterial Content of 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 . 1880 



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 before 

 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 purification 

 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 remains for 

 six to twenty-four hours. The aluminium sulphate is decomposed by the 

 lime in the water and forms insoluble aluminium hydrate; and the sul- 

 phuric acid combines with the lime. The hydrate of aluminium is pre- 



