40 JOURNAL OF ECONOMIC ENTOMOLOGY [Vol. 12 



Assuming that the nitration power of the bed is a proper index for 

 its efficiency in sewage purification (this is the index usually so ac- 

 cepted by Sanitary Engineers), the following table will serve to show 

 that the efficiency of the bed was in no wise diminished by the sub- 

 mergences of the season. 



Effect on Nitrification in Plainfield Filters 

 Part Per Mill N. as Nitrate 



Number of Days before Flooding Dates Number of Days after Flooding 



7 6 5 4 3 2 1 Flood 12 3 4 5 6 7 



7.0 10.0 12 6/14 11.4 Eff. of quarter flooded only 



. 11 8/9 11. 10. 8.0 



13.0 9.0 13 8/24 13.0 



13.0 8/31 13.0 



13. 13 9/17 13.0 



13 9/24 13.0 13.0 13.0 



6.1 10/14 10.0 11.0 10.0 



6.6 6.6 6.6 10/31 6.6 6.6 6.6 



This submergence was repeated on August 24, August 31, Septem- 

 ber 17, September 24, October 14 and October 31. 



With the exception of the periods from August 24 to August 31 and 

 September 17 to September 24, the time elapsing was two weeks or 

 greater. The result of the practice of allowing two weeks to elapse 

 before the submergence is repeated appears in the escape of a consider- 

 able number of flies. A repetition of the treatment at the end of one 

 week cuts the fly pest entirely out. The two-week period gives enough 

 time for the flies to reach the adult stage and to reinfest the filter im- 

 mediately after the water is withdrawn. Furthermore, it is possible 

 although not actually determined that the egg is able to resist the sub- 

 mergence. 



It is unmistakeably indicated that the filter should be submerged 

 once each 10 days until the flies are reduced. Let it be understood, 

 however, that at no time following August 24 did the fly assume the 

 proportion of a pest, although the interval between submergence with 

 two exceptions was sufficient for the insect to pass through its life cycle. 



The work against the fly will be opened during the coming season by 

 submergence of the bed in the spring before the flies emerge. This 

 submergence will be followed by treatments at 10-day intervals until 

 the fly has been so reduced as not to require such frequency. 



The greatest limiting factor in the application of this method of con- 

 troling the Sprinkling Sewage Filter Fy to the various sprinkling sewage 

 filter beds of the country is the fact that many engineers have con- 

 structed these beds without retaining walls. In such cases the writer 

 would recommend the construction of wooden walls backed by an 

 earthen fill or the construction of concrete retaining walls, in such a 



Plate 2. General views of sprinkling filter illustrating submergence. A. — Filter 

 inactive, note apparatus for closing and opening the bulkhead gate; B — filter ap- 

 proaching complete submergence, sprayers discharging; C — Filter completely sub- 

 merged. 



