34 '^^^ Purification of Sewage by Bacteria. 



for the refuse of the animal world ; and, if we were content to 

 live the life of the primitive man, we might safely throw upon 

 the soil the duty of dealing with our leavings. But it does not 

 follow that this is the natural way to deal with town sewage. 

 It is not a natural state of things to concentrate the excrement 

 of several thousand people upon a few acres of land. It is still 

 less a natural proceeding to swamp this land daily with several 

 thousands of tons of dirty water in addition. It is because we 

 do these things that nature rebels, and our sewage farms turn 

 out failures. The successful purification of sewage by means of 

 land, day in and day out throughout the year, demands far 

 larger areas than are generally available. And, what is not less 

 important, the task requires skilled management of a high 

 order, which it is hard to find, and still harder to induce a 

 sewage committee to pay for. We are therefore thrown back 

 on what are called " artificial '' processes of sewage purification. 



For many years attempts have been made to solve the 

 difficulty by means of strainers, followed by chemical precipita- 

 tion. This process, when properly carried out, does undoubtedly 

 remove from sewage the larger proportion of the suspended 

 impurities, that is to say, of the solid matter visible to the eye, 

 as well as part of the dissolved polluting matter. But the 

 suspended matters are not got rid of by precipitation : they are 

 merely thrown down as sludge. Great expectations were at 

 one time entertained of the manurial value of se^vage sludge ; 

 but here again the hopes formed have been doomed to 

 disappointment ; and in most cases the cost of disposing of this 

 embarrassing substance adds very largely to the expense of 

 dealing with the sewage. 



The capability of filters, when properly handled, to deal with 

 the liquid portion of sewage has long been recognised. It 

 remained to find an effective and inexpensive means of preparing 

 sewage for filtration by freeing it from its suspended solids, and 

 of grappling with the sludge difficulty. Among others whose 

 duties brought them face to face with this problem was the 

 City Surveyor of Exeter, Mr. Donald Cameron. After many 



