PURIFICATION OF WATER FOR DRINKING PURPOSES 207 



Series B. Different proportions of Copperas with the corresponding 

 proportion* of Lime 



Number of 

 bacteria in 1 c.c. 



Original sewage . . _ . ' ' . . ; . . . . 322,400 



Ditto after settling 1 hour without additions .'.., .'. , .. ..271,400 



lb. copperas Ib. lime 



Effluent with 100 and 530 l per 1 million gallons of sewage . 20,620 



200 580 . 16,640 



400 630 . 7,370 



700 720 . 9,204 



1,000 800 . 6,972 



2,000 -1,100 . 1,269 



In both the above series of experiments the lime was 

 added to the sewage first, and thoroughly incorporated 

 with it before the addition of the copperas. 



In order to see if a better result could be obtained 

 by mixing the sewage with the copperas first, and then 

 adding the lime, or if as good a result could be obtained 

 by precipitating the copperas with lime, and adding the 

 precipitated ferrous hydrate to the sewage, the follow- 

 ing experiments were made : 



Bacterial purification of Seioage with the same amounts of 

 Copperas and Lime added in different ways 



Number of 

 bacteria in 1 c.c. 

 Original sewage . . . . , . . . . . 507,800 



Ditto after settling 1 hour without additions . . . . 422,400 



Effluent with 500 Ibs. of copperas and 650 Ibs. of lime per million 



gallons of sewage ; copperas added first .... 15,950 



Ditto, lime added first ........ 15,800 



Effluent with 1,000 Ibs. of copperas and 300 Ibs. of lime per 

 million gallons of sewage ; lime and copperas added 



together ... . 9,476 



Effluent with 1,000 Ibs. of copperas and 800 Ibs. of lime per 

 million gallons of sewage; lime and copperas added 

 together . .... 12,576 



Ditto, copperas added first 9,944 



Ditto, lime 10,406 



1 It should be noted that in this sewage 500 Ibs. of lime were required 

 to combine with the free carbonic acid, whilst the additional quantity of 

 lime employed corresponds to the sulphuric acid in the copperas added, 

 and thus serves to decompose the latter. 



