PURIFICATION OF WATER FOR DRINKING PURPOSES 211 



absence of organisms in any pond or stream where 

 food is present is a hopeless task. It thus happens 

 that, while the organisms are the real cause of the 

 trouble, their removal from sewage is often of less im- 

 portance than the removal of the matter in the sewage 

 on which they feed. The proportion of organic matter 

 removed does not necessarily represent the proportion 

 of food for organisms removed, for some kinds of or- 

 ganic matter are no more suitable food for bacteria 

 than is saw-dust for horses. An effluent from a sewage 

 filter, where nitrification is complete, containing 2 per 

 cent, of the total organic matter of the sewage, will not 

 serve as food for bacteria, because it has been worked 

 over already by bacteria in the filter, and nearly every- 

 thing available has been removed. If, on the other 

 hand, sewage is mixed with fifty times its volume of 

 pure water, so that it contains the same amount of 

 organic matter as the effluent, the bacteria will increase 

 enormously for a few days. From this point of view, 

 the effluent is many times purer than is indicated by 

 the ratio of its organic matter to that of the sewage. 



' With sewage precipitation the case is entirely differ- 

 ent, for here there is no bacterial action. There is, 

 however, some reason to think that the organic matter 

 left is not so good a food, and therefore not so dan- 

 gerous as that removed. Sewage settled alone will 

 keep turbid with organisms, and in a day or two masses 

 of zoogloea (dead or resting bacteria) separate from it. 

 Sewage precipitated by either copperas, ferric sulphate, 

 or alum in suitable quantities, has repeatedly remained 

 so clear that the bottom of the barrels could be dis- 

 tinctly seen through more than two feet of liquid for 

 one or two weeks. . In these cases no flakes of zoogloea 

 so characteristic of untreated sewage have been seen, 

 and the odour is much less than that of sewage alone. 



p 2 



