195 



ture begins to rise in early spring, and then the filter begins to "un- 

 load" and the effluent becomes laden with large quantities of heavy 

 earthy suspended matter and the sludge in the sprinkling filter be- 

 comes rapidly reduced. The significant thing in this connection is 

 the fact that this unloading period is coincident with the maximum 

 abundance of the worms, which decrease somewhat in number soon 

 after the greater part of the unloading has occurred. Whether or 

 not the worms are responsible for the unloading remains to be proven, 

 but the circumstantial evidence indicates that they are at least partly 

 responsible for it. Cognizance must, however, be taken of the fact 

 that other organisms also are abundant in the sprinkling filter at this 

 time, and it is possible that the unloading is the result of the combined 

 mechanical action of a number of associated organisms. 



Relation to Piitrescibility. — Before going into the discussion of 

 the experiments which were made in this connection it is necessary 

 to make clear the meaning of certain important terms which are in 

 constant use. in sewage investigations. 



In order to explain what is meant by the term piitrescibility and 

 to indicate its precise application in sewage disposal work it is nec- 

 essary to explain in considerable detail certain chemical and physical 

 conditions which exist in ordinary sewage. Phelps ('09, p. 75) 

 gives a very clear account of the application of this term in the fol- 

 lowing rather lengthy quotation : — 



"Putrescibility, as applied to organic matter in general, implies 

 the ability of that matter to undergo offensive putrefactive decompo- 

 sition Such decomposition is always anaerobic and it is 



usually accompanied by the evolution of offensive odors. These two 

 phenomena have, therefore, formed the basis of most putrescibility 

 tests. Some criteria of putrefaction which have been employed are : 

 (i) Development of offensive odors; (2) fonnation of black sedi- 

 ment; (3) reduction in the amount of dissolved oxygen; (4) loss of 

 all dissolved oxygen; (5) loss of all available oxygen, including that 

 of nitrates and nitrites; and (6) increase in the oxygen-consumed 

 figure. Some of these tests are based on partial reduction of the 

 available oxygen in the effluent ; others depend on the complete reduc- 

 tion of the available oxygen and subsequent anaerobic fermentation. 

 The tests most commonly employed belong to the latter group, de- 

 pending on the production of odor or of hydrogen sulphide, blacken- 

 ing of the liquid, or reduction of organic dyes. The test which depends 

 on an increase in the oxygen-consumed figure during incubation is 

 also in that class, because anaerobic fermentation alone renders or- 

 ganic matter more readily oxidizable. 



