164 



SCIENCE. 



[N. S. Vol. XVI. No. 396. 



bic, those species that live, grow and mul- 

 tiply only in contact with air. Each group 

 plays its own special part in the destruc- 

 tion of the effete matter contained in house- 

 hold waste. The anaerobic bacteria act 

 first. They disintegrate the solid animal 

 and vegetable matters, liquefy them, and 

 hring them into solution. The aerobic bac- 

 teria act iipon the disintegrated and lique- 

 fied compounds and, by a process of oxida- 

 tion, change them into harmless gases or 

 mineral substances. 



For the destruction of dead organic mat- 

 ter both groups of bacteria are necessary; 

 the anaerobic to disintegrate and liquefy 

 the complex organic substances, the aero- 

 bic to change those simplified and liquefied 

 compounds into harmless products. 



By passing sewage intermittently 

 through sand, conditions favorable to the 

 growth, retention and action of bacteria 

 are brought about, and the obnoxious sub- 

 stances are destroyed by the aid of these 

 microscopic organisms. 



The credit of showing that sewage could 

 be purified on a practical scale by intermit- 

 tent filtration through sand is due to the 

 Massachusetts State Board of Health. 



Their experiments, published in 1890, 

 showed that all that was necessary to de- 

 stroy organic matter in sewage was to pro- 

 vide conditions favorable to the action of 

 bacteria. These conditions they found 

 were fulfilled by providing suitable mate- 

 rial on which the microorganisms could be 

 retained; surrounding these microorgan- 

 isms at certain intervals with air, and pro- 

 viding periods during which they could 

 rest. A suitable material was sand, from 

 four to five feet in depth, and the surround- 

 ing the bacteria with air at definite inter- 

 vals, and allowing periods of rest, was ac- 

 complished by underdraining the sand and 

 by allowing the sewage to flow on the sand 

 only sis hours out of each twenty-four. 

 The combination of these conditions gave 



us the process called intermittent filtra- 

 tion. 



Plants built to answer the above condi- 

 tions consist of a number of sand beds, each 

 of about one acre superficial area, care- 

 fully leveled, underdrained and divided 

 from each other by dikes varying from 

 three to six feet in height. Crude sew- 

 age, after passing through a grit cham- 

 ber and screens to remove road washings 

 and large floating substances, as rags and 

 sticks, is run successively upon the various 

 beds, none of the beds receiving sewage for 

 a longer period than six hours out of each 

 twenty-four, for if the beds received the 

 sewage continuously there would be abso- 

 lutely no air present at any time in the bed, 

 and air, as we have seen, is necessary for 

 the life of the aerobic bacteria. By apply- 

 ing the sewage only six hours out of the 

 twenty-four, as the liquid drains out of the 

 bed, air enters to take its place, and the 

 conditions favorable to the action of both 

 anaerobes and aerobes are maintained. 



By this process from 50 to 75,000 gal- 

 lons of domestic sewage, sewage not con- 

 taining a large proportion of manufac- 

 turing waste, can be purified each day on 

 one acre of sand bed area, so that the pol- 

 luting substances are to such an extent re- 

 moved that the liquid, as it runs away from 

 the bed, is clear, bright, almost odorless, and 

 can be emptied into a very small stream 

 without fear of causing trouble. 



Intermittent filtration is undoubtedly 

 the best method that is known for the pu- 

 rification of sewage of cities and towns 

 which have in their neighborhood compara- 

 tivel3^ large areas of sandy soil, but is not 

 applicable for cities or towms which would 

 be obliged to construct beds with sand 

 brought from any distance. This point was 

 quickly perceived in England, where sand 

 is not of common occurrence, and the bac- 

 terial sewage work in England started with 

 the problem, Can the amoimt of land re- 



