THE BACTERIOLOGY OF SEWAGE 355 



at hand to show the superiority of filter-beds, it is to be 

 expected that their use will be recommended in preference. 

 The advantages of filter-beds (particularly such as are 

 automatically filled and discharged) over land are very 

 great : 



1. Far less space is required. 



2. There is no fear of the filters becoming foul, and 



consequently the danger of causing a nuisance is 

 diminished. 



3. The purification effected is probably greater, and is 



unquestionably more uniform. 



4. The labour required is very slight ; and the non- 



dependence on the skill or judgment of an attendant 

 renders the filters much more reliable than a farm. 



5. Where filters are used the purification of the sewage is 



the sole object in view, so cannot be sacrificed to 

 exigencies of cropping, as in the case of land. 



In order that bacterial filters may continue to exert 

 their oxidising and nitrifying effect, it is essential that 

 they should be supplied with air, either artificially, by 

 causing air to enter the filter-bed as in Ducat's system 

 and Lowcock's system, or by working duplicate series of 

 filter-beds so as to allow time for aeration, as in the Sutton 

 filters, the septic tank process, and in Scott-Moncrieff's 

 filter. 



Sufficient analytical data have not yet been collected to 

 enable the relative merits of these processes to be compared 

 but evidence on the results of all of them will probably be 

 available before long. 



The action of such beds is to rapidly decrease the figures 

 representing the ' albuminoid ammonia ' and ' oxygen 

 absorbed,' while the formation of nitrates in considerable 

 quantities is also invariably to be expected. The presence 

 of nitrates should indeed be insisted on in an effluent as 



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