August 1, 1902.] 



SCIENCE. 



163 



out limit ; consequently it was thoiight that 

 selvage could be applied continuoiisly to 

 cultivated land, and, if vegetation was not 

 drowned out, not only would perfect puri- 

 fication take place, but an immense profit 

 could be derived from making use of the 

 polluting substances for plant food. So 

 firmly were these ideas, especially the ma- 

 nurial value of sewage, implanted in the 

 English mind, that the most marvelous un- 

 dertakings were planned, and to-day at 

 Barking one sees a tunnel which was 

 started to carry the sewage of London one 

 hundred miles into the interior, with the 

 idea that the. tunnel should be tapped at 

 certain intervals and the sewage sold, at I 

 know not what price per thousand gallons, 

 to fertilize the land. This is hardly less 

 startling than the idea of recovering the 

 gold that is in sea water. 



It is now well known that plant life can- 

 not assimilate the organic matter that oc- 

 curs in sewage unless it is first decomposed 

 by bacteria, and sewage farming has re- 

 sulted in failure except in those few eases 

 where the land was of an exceptional char- 

 acter and of very large area, compared to 

 the population to be served, as one acre to 

 fifty persons. The Paris sewage farms, of 

 which we have all heard so much, are far 

 from being a success. 



Chemical Precipitation. — The chemical 

 treatment of sewage consists in the adding 

 of certain chemicals, usually lime and iron 

 ' sulphate, to crude sewage, allowing the 

 sewage to run into large open tanks, 

 through which it flows with such slow ve- 

 locity that the substances which have been 

 thrown down by the chemicals subside, 

 leaving the supernatant liquid clear and 

 free from all suspended matter, but con- 

 taining practically all of the soluble putre- 

 fying substances that were in the sewage. 



Chemical treatment of sewage is costly, 

 averaging between forty and fifty cents per 



year per head of the population, and the 

 amount of organic matter removed is only 

 about 55 per cent, of the total organic mat- 

 ter in the sewage. The effluent will putrefy, 

 and consequently cannot be emptied into 

 a stream where the dry weather flow is less 

 than ten times the volume of the sewage. 

 The precipitate thrown down is difficult 

 to treat, and if it cannot be carried out to 

 sea and dumped, as is the case of London 

 and Manchester, it must be pressed, and 

 then either carted away to some unoccupied 

 and valueless land, or be burnt. Large 

 plants still exist, however, for treating sew- 

 age chemically, the largest of all being the 

 London disposal works, where 183 million 

 gallons are treated chemically each day. 

 The plant at Worcester, Massachusetts, the 

 largest in this country, treats about fifteen 

 million gallons per day. A few pictures of 

 various parts of this plant may show you 

 that treating chemically fifteen million gal- 

 lons of sewage a day is a serious under- 

 taking. 



Sewage farming and chemical treatment 

 are now considered as methods of the past, 

 and all the modern methods of treatment 

 are the so-called bacterial methods. 



Intermittent Filtration. — The first of 

 these methoas was intermittent filtration. 

 As early as 1865 Dr. Alexander Miiller, of 

 Berlin, showed that by passing sewage in- 

 termittently through sand, the obnoxious 

 substances were removed, but the explana- 

 tion of why the sewa-ge could be thus puri- 

 fied is of much later date. It is now known 

 that fresh sewage contains, in immense 

 members, bacteria which live on dead 

 organic matter and which cause its 

 decomposition. These bacteria can be 

 roughly divided into two great groups, each 

 containing numerous species. These groups 

 are called the anaerobic and the aerobic 

 groups. The anaerobic group embraces all 

 those species that live, grow and multiply 

 out of contact with air and light ; the aero- 



