PURIFICATION OF WATER FOR DRINKING PURPOSES 213 



there is a certain definite amount of lime, depending 

 upon the composition of the sewage, which gives a 

 better result than less, and as good or a better result 

 than more. This amount of lime is that which exactly 

 suffices to form normal carbonate with all the carbonic 

 acid of the sewage. It is possible in a few minutes, by 

 simple titration, to determine approximately the amount 

 of uncombined carbonic acid present in sewage, and 

 how much lime will be required to combine with it. 

 It is also possible to determine in a similar way, after 

 mixing, whether enough or too much lime has been 

 added. The amount of lime required by Lawrence 

 sewage averages about 1,600 pounds per million 

 gallons. 



' Ordinary house-sewage is not sufficiently alkaline to 

 precipitate copperas, and a small amount of lime must 

 be added to obtain good results. The quantity of lime 

 required depends both upon the composition of the 

 sewage and the amount of copperas used, and can be 

 calculated from titration of the sewage. Very imper- 

 fect results are obtained with too little lime, and when 

 too much is used the excess is wasted, the result being 

 the same as with a smaller quantity. 



'After mixing the sewage with both copperas and 

 lime, if enough or too much lime has been used the 

 mixture will colour phenolphthalem red, while, if too 

 little has been used, no colour will be produced. This 

 test can conveniently be used by people having no 

 knowledge of chemistry, and affords an easy and very 

 accurate method of applying enough lime, and of avoid- 

 ing a useless excess. 



' Using in each case a suitable amount of lime, the 

 more copperas us.ed the better the result ; but, with 

 more than one-half a ton per million gallons, the im- 

 provement does not compare with the increased cost. 



