190 



which comes to them in two ways : ( i ) direct from the air by con- 

 tact, and (2) from the sewage, which contains dissolved oxygen when 

 it enters the filter bed. 



In all of the sprinkling filters except No. 3 the stones composing 

 the filter bed are of a size which produces a series of continuous and 

 rather spacious interstices. These may be reduced to some extent 

 by the settling material from the sewage but as they never become 

 completely filled they constitute a series of air spaces and thus form 

 a source of oxygen supply for the organisms inhabiting them. That 

 these lumbricillid worms take oxygen directly from the air is shown 

 by the fact that specimens have been kept for several days in vessels 

 containing moist filter paper which served to prevent the undue drying 

 of the worms. This was also demonstrated when at one time an ac- 

 cident to the pumping machinery eliminated for over three days the 

 possibility of oxygen from sewage reaching the worms, the only liquid 

 present in the upper zone being that held by the sludge. 



The sewage, which constitutes the influent of the sprinkling fil- 

 ters, coming as it does from the settling and septic tanks, has a very 

 low dissolved oxygen content, often showing a total absence of dis- 

 solved oxygen, particularly during the hot season. However, this 

 influent passes through a nozzle which breaks the stream up into a 

 spray, throwing it out into the air, and this, in falling, is distributed 

 over the whole of the top surface of the filter bed and between its 

 individual stones. The result of this process is that the liquid be- 

 comes oxygenated to a considerable degree. The spraying also brings 

 about the loss of most of the carbon dioxide. Consequently, regard- 

 less of its source, the sewage which flows over the worms has a con- 

 siderable dissolved oxygen content. The effectiveness of the sprin- 

 kling filter as an oxygenating device is shown by the fact that in filter 

 No. 4 on certain days in September and October, 191 2, when the in- 

 fluent (at the entrance of the nozzle) showed a total absence of dis- 

 solved oxygen, the effluent contained from 11.2 to 14.7 p.p.m. That 

 this oxygen can be utilized by the worms is shown by the fact that 

 when placed in tap water they lived for days, and even weeks, under 

 conditions which prevented their getting oxygen from any other 

 source. 



Resistance to Decreased Oxygen. — It would seem that organisms 

 as active as these worms would require a considerable amount of free 

 oxygen, and that the sprinkling filter environment is such as to afford 

 a generous supply. A number of extended experiments were made 

 with the view of determining the effect of a decreased supply of 

 oxygen. The methods employed in these experiments are as follows : 



