198 



were made on the following grades of sewage : ( i ) raw sewage, 

 (2) septic-tank effluent, (3) settling-tank effluent, and (4) sprinkling- 

 filter effluent. The suspended niiatter in sewage is of two kinds, 

 namely, the settling and the non-settling suspended matter. The 

 former is of such a nature that it can be removed by filtration through 

 ordinary filter paper, or will be deposited when sewage is stored ; but 

 the latter must be removed by chemical precipitation, by biologic 

 treatment, or by the use of special filtering devices. The non-settling 

 suspended matter is colloidal in nature and is known as the pseudo- 

 colloidal content of sewage. Each of the four above-mentioned efflu- 

 ents were tested in three ways: (i) by using the raw material, (2) 

 by using the sewage after its passage through ordinary filter-paper, 

 which removed the settling suspended matter, and (3) by using the 

 material from which the settling suspended matter had been removed 

 by filtration and the pseudo-colloidal matter had been removed by 

 filtering the liquid through a Gooch crucible connected with a filter 

 pump. Since investigation has shown (Lederer, '12b) "that the finely 

 divided slowly settling suspended matter and the pseudo-colloidal mat- 

 ter not capable of settling make up the greater part of the putrescibil- 

 ity," the tests were made in a way to permit a study of the effect of 

 the worms on the liquid when one or both of the above-mentioned 

 substances are present. The sampling bottles of the Sewage Testing 

 Station were used in the tests. These bottles have a capacity of 128 

 cc. All of the glassware, such as pipettes, sampling bottles, etc., was 

 sterilized before using. The bottles were filled with the various 

 grades of sewage and then worms were transferred to each. Vig- 

 orous worms fresh from the sprinkling filter were used in every case. 

 Before they were put into the test bottle they were carefully cleaned 

 by transferring them from one to another of a series of vessels con- 

 taining pure water, in order to prevent extraneous material from 

 entering with them. They were then counted out in lots of 100, and 

 after removing all excess water each lot was weighed on a fine ana- 

 lytical balance, and those lots which weighed approximately the same 

 were selected for the tests. Each lot was placed in a separate bottle 

 which was corked in such a way that nO' air bubbles were enclosed. 

 For each individual test a check experiment was carried on, sintilar 

 in all respects except that no worms were used. Thus a single series 

 involved twenty-four tests. 



Determinations of putrescibility involve the use of delicate indi- 

 cators which aid in the accurate detection of the beginning of an- 



