162 



SCIENCE. 



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



population, household waste, the washings 

 of the streets, and the refuse of every 

 branch of industry. The total amount of 

 this refuse matter varies, but on the aver- 

 age the sewage of a city is pure water con- 

 taining one tenth of one per cent, of city 

 waste ; seven pounds in 1,000 gallons. In 

 addition to this refuse matter, sewage con- 

 tains in immense numbers, approximately 

 150 millions to the liquid ounce, those mi- 

 croscopic organisms called bacteria, and in 

 the perfect treatment of sewage the bac- 

 teria as well as the refuse matter must be 

 removed. 



The perfect treatment of sewage, the re- 

 moval of the microorganisms as well as the 

 polluting substances, i. e., changing sewage 

 back again into a water supply, is possible, 

 but at the present time not practicable on 

 account of cost, and all that so far has been 

 attempted has been to remove the polluting 

 substances of city waste, so that after treat- 

 ment the sewage will not be offensive to the 

 sense of either sight or smell and when 

 emptied into a stream can cause no offense. 



The earliest method of sewage treatment, 

 which, in fact, was not treatment but 

 merely disposal, was to carry the waste 

 products of a community into the ocean or 

 into the nearest stream. This method, 

 known as dilution, is allowable for cities 

 situated on the sea, or on rivers whose flow 

 is very large as compared to the sewage, 

 100 to 1, and the unfiltered water of which 

 is not used as a water supply. 



Very few cities are so fortunately sit- 

 uated as to make use of this method, and for 

 most cities the treatment or purification of 

 sewage must be considered as imperative 

 as the obtaining of pure water. How can 

 this be accomplished? How can the pol- 

 luting substances be best removed so that 

 the sewage after treatment can, without 

 causing offense, be emptied into a small 

 stream? This is one of the great sanitary 

 problems of the daj^, and I shall try to 



give you this evening an outline of the six 

 principal processes by which the necessary 

 purification is to a greater or less degree 

 accomplished. 



These methods in historical order are : 

 Sewage farming, chemical precipitation, in- 

 tennittent filtration, contact bed treatment, 

 septic tank treatment, continuous filtration. 



Sewage farming is applying the sewage 

 to cultivated land. Chemical precipitation 

 is the adding of certain chemicals to the 

 sewage to remove or throw down the pol- 

 luting substances. The four remaining 

 methods, intermittent filtration, contact 

 bed treatment, septic tank treatment and 

 continuous filtration, the so-called modern 

 methods, are all based on the fact that the 

 microorganisms or bacteria always present 

 in sewage will, vmder proper conditions, 

 destroy all the obnoxious substances con- 

 tained in sewa-ge. 



The earliest method of sewage disposal, 

 as I have already said, was to run the 

 waste products of the community into the 

 nearest river or stream. In a sparsely set- 

 tled region this caused little complaint; 

 but as the district became inhabited and 

 the towns and villages upon the stream in- 

 creased in size, it became apparent that 

 this method, though removing the filth 

 from one's own door, would endanger the 

 health and comfort of a large part of the 

 conununity. It was then that the question 

 of sewage treatment became the subject of 

 scientific inquiry. Without going into any 

 detail as to the early work done upon the 

 subject, I will only state that it resulted in 

 the general adoption of either sewage farm- 

 ing or chemical precipitation as the best 

 means of destroying the polluting sub- 

 stances contained in sewage. 



Sewage Farming. — This method was 

 based on the idea that plant life was capa- 

 ble, in itself, of decomposing the complex 

 matters contained in sewage, and that its 

 capacity to do this work was almost with- 



