DISTRIBUTION AND DISTRIBUTORS 287 



as to disturb the filter, (2) for proper aeration the bed should be 

 run out completely after each discharge, (3) the opening of the 

 discharge valve should be gradual, so as to prevent any initial 

 rush of effluent, such as I have frequently found to render the 

 first runnings from these filters turbid and inferior. 



B. In the Continuous System, it is of extreme importance 

 that the liquids should be' distributed uniformly over the material. 

 The cost of so doing is only a small fraction of the total cost 

 of the material of a bed, and by attention to distribution the 

 amount of material required for dealing efficiently with a given 

 sew^age may be reduced considerably. 



The problem of spreading a liquid issuing from a narrov^ 

 channel evenly over a broad area is not a simple one. In 

 upward filtration it is easy; the liquid rising from the bottom 

 naturally distributes itself throughout the filter. But when 

 the introduction occurs from the top, there are considerable 

 mechanical difficulties. Where sand filtration is used, it is 

 chiefly necessary to protect the sand from disturbance by a 

 coarser heavy layer of flint or stones, to run the liquid on the 

 top, and trust to the evenness of the fine layer for equal dis- 

 tribution. The deficiency of aeration, and blocking of the beds, 

 are faults of this method when applied to sewage (see 

 Chapter IX.). 



With aerating filters of open material, flushing the liquids, 

 however rapidly, from penstocks at the sides or in the middle, 

 leads to the formation of channels, and only a local use of the 

 filtering mass ; therefore many arrangements for spreading the 

 fluid more equally have been devised. Networks of split pipes 

 or iron or wooden troughs are not satisfactory. Perforated 

 pipes occasion trouble, through blocking by solid matters, there- 

 fore the ends have to be made with openings, so that the tubes 

 can be brushed through at intervals : the corrosion of the iron 

 by the chlorides and nitrates in the liquid also blocks up the 

 holes. In some cases the tubes have been made of gun-metal, 

 but this also is liable to corrosion, particularly along the lines 

 where it may have been joined or soldered. These difficulties, 

 however, have not prevented many of these contrivances from 

 being made to work well. 



I. Stationary Sprayers. 



{a) Mr. Stoddart, of Bristol, introduced a distributor depend- 

 ing on the dropping of the sewage from vertical points and not 



