200 Mr. Harry Grimshaw 07i 



with their oxygen to the sewage instead of robbing it of its 

 oxygen. This is a very important property, as it is 

 acknowledged on all hands that oxidation of sewage means 

 purification, and it is a significant fact, as will be seen later, 

 that in the presence of oxygen those organisms (bacteria) 

 which, so to speak, burn up and destroy the sewage matters 

 are enabled to flourish, whilst without oxygen, or with a 

 limited supply, the germs of putrefaction and decay are 

 encouraged in their growth ; giving in the one case inno- 

 cuous products, such as carbonic acid gas and water, and in 

 the other carbonaceous and sulphurous gases, which are so 

 dangerous and offensive. The germs of life flourish in the 

 presence of oxygen, those of disease and death in its absence. 



The ordinary persulphate Fe2(So4)3 and perchloride 

 FcaCle have been used experimentally at different times, 

 the largest trials recently having been carried out at Salford, 

 where the perchloride of iron was used. 



In the majority of cases it is necessary to use a certain 

 amount of lime in treating sewage with these salts, but not 

 nearly so much as is the case with sulphate of alumina on 

 " copperas." The precipitation takes place at once, and the 

 rate of subsidence is good, the sulphur compounds are 

 absorbed, and a fair purification effected without filtration. 

 A most exhaustive and accurate comparison of the action 

 of lime, alum, copperas, and persulphate of iron upon sewage 

 was carried out in 1890 by the Massachusetts State Board 

 of Health, whose report is a model research on the sewage 

 question. This investigation into the action of chemical 

 precipitants resulted in the following general conclusions : — 



" The lime process has little to recommend it. Precipita- 

 tion by copperas has produced some satisfactory results. 

 Ferric sulphate and alum are to be preferred to either of the 

 before-mentioned compounds, while chief preference is to 

 be given to ferric sulphate as a precipitant. 



"With 1,000 lbs. of alum per million gallons, costing 45 



