276 SEWAGE AND ITS PURIFICATION 



1. The suspended matter must be previously removed by 

 sedimentation ; 



2. If any passes over it must be retained as far as possible on 

 the surface of the bed ; 



3. The surface of the bed must be raked over from time to 

 time. 



4. Periodical intervals of rest must be allowed. 



Professor Boyce found in 1900 : in the effluent from the first 

 contact bed 331,700 bacteria and 1,420 coli per c.c. ; from final 

 (second) contact 115,100 bacteria and 329 coH. The Ship 

 Canal contained some 6,000 coli per c.c. above the sewage 

 outfall. 



The closed tank and single contact gave an effluent which 

 " generally resisted putrefaction in the incubator test, in conse- 

 quence of its containing a comparatively high proportion of 

 nitrate." The effluent from the open tank was passed through 

 the beds C and D, and by the " double contact " a better result 

 was naturally obtained. *' With four fillings per day, every 

 sample was non-putrescible, and well within the limit of im- 

 purity." In other instances where the Mersey and Irwell Joint 

 Committee's standard (i grn. per gal. of O absorbed in four 

 hours, and o"i grain albuminoid NH3) was infringed, it was 

 shown to be due to trade refuse, '' which, being non-putrescible, 

 does not cause nuisance in the Ship Canal." " The object of 

 purification is primarily the production of an effluent free from 

 putrescibility, and not one in which the chemical ingredients, 

 are below some necessarily more or less arbitrary standard." 

 (See Chapter III., p. 56 ; Chapter V., p. 130.) 



The report confirmed previous observations by Adeney {Proc. 

 R. Soc. Dublin, September, 1895), myself (/. San. Inst., vol. xviii.,. 

 parti., 1896), Fowler (summer of 1896), Scott-Moncrieff (patent 

 4994 of 1898), and others, as to the utility of mixing a nitrated 

 effluent from a " second contact " bed with that from a first, 

 whereby a liquid was obtained which satisfied the incubator 

 test, so that " only one-fifth of the filter acreage need be at a 

 low level," and the area of the second contact beds could be 

 considerably reduced. I had also before suggested the intro- 

 duction of a portion of the nitrated effluent into the septic tank 

 itself, as the denitrification change which takes place on mixing 

 is due to the reaction between the nitrates and the organic 

 matter, and could be induced earlier in the process as soon as 

 the organic matter is in a soluble reacting condition, while it is 



