248 SEWAGE AND ITS PURIFICATION 



it is obvious that a certain preparation is necessary before a 

 sewage liquid can take advantage of an excess of oxygen. Con- 

 sequently in all systems professing to depend entirely on 

 oxidation we notice that some preliminary treatment, whether 

 natural or artificial, has occurred, and the solids have been 

 avoided by screening, straining, sedimentation, or precipitation, 

 before the continuous and free aeration has been useful in the 

 third stage. 



Dr. Geo. Reid^ advocates for continuous or " percolating " 

 filters a much finer medium, sifted to ^ inch grain, on the 

 ground that he thus obtains the largest surface for bacterial 

 growth which is compatible with free aeration, and that the 

 bed can be made much shallower and therefore less costly. 

 Such a filter, however, would be liable to rapid clogging unless 

 it was fed with a liquid containing less suspended matter than 

 ordinary septic effluents. 



1 " Nitrification of Sewage by Shallow Filters of Fine Particles," British Assoc, 

 York, 1906. 



