PURIFICATION OF WATER FOR DRINKING PURPOSES 205 



hours, 15,130 microbes in a c.c, whilst the treated 

 water was found after a similar period of rest to be 

 sterile. In another similar experiment the untreated 

 and treated waters gave respectively 2,380 and 8 mi- 

 crobes in a c.c. 



Effect of chemical precipitation on the bacterial con- 

 tents of sewage. — For the most complete account of the 

 removal of micro-organisms from sewage by the use 

 of various precipitants, we must again turn to the 

 masterly work emanating from the State Board of 

 Health of Massachusetts.^ In these investigations the 

 sewage was allowed to run into a tank, where it was 

 thoroughly stirred up, and from it a series of barrels 

 were filled, to each of which the particular chemicals 

 under examination were then added as desired. One 

 barrel was always left to settle without the addition of 

 any chemicals, to serve for comparison. The barrels 

 were 30 inches high and held about 50 gallons each. 

 After the sewage in each barrel had been thoroughlj?" 

 mixed with the chemicals added, it was allowed to- 

 settle, and a sample of the effluent above the sludge 

 was then drawn from a tap situated about 10 inches 

 from the bottom. The time of settling allowed in the 

 following experiments was one hour. 



Bacterial 'purification of Sewage with different amounts of Lime 



Number of 

 bacteria in 1 c.c.. 



Original sewage 196,000 



Ditto after settling 1 hour without additions .... 123,200 



Effluent with 600 lbs. of lime per 1 million gallons of sewage . 95,000 



800 „ „ „ „ 65,000 



1,000 „ „ „ „ 50,000 



1,500 „ „ „ „ 12,180 



2,000 „ „ „ „ 3,150 



^ Experimental Investigations by the State Board of Health of Mas- 

 sachusetts, Part II. p. 737, 1888-1890. 



