THE SEWAGE DISPOSAL PROBLEM 193 



description of these and other forms, the reader is referred to 

 the various technical books. There is no doubt that by means 

 of sprinklers and percolating filters the absorption processes are 

 much more favoured than in contact beds. Aeration is more 

 thorough, and thereby the vital activity of micro-organisms and 

 the higher forms of animal life is encouraged. On the other 

 hand, there is some risk of unpleasant smell arising from the 

 beds, and in the warmer months of the year flies swarm upon 

 them and may become a serious source of trouble and nuisance 

 in the neighbourhood, especially if there happen to be dwellings 

 near the works. 



General Conclusions in regard to Filter Beds 



The Royal Commission, in its Fifth Report (1908), formulated 

 some interesting conclusions in regard to filter beds as a result 

 of prolonged investigations. 



" Within ordinary limits the depth of a contact bed makes 

 practically no difference to its efficiency per cube yard, and it 

 is generally inadvisable to construct contact beds of greater 

 depth than 6 feet or less than 2 ft. 6 in." 



" For practical purposes, and assuming good distribution, 

 the same purification will be obtained from a given quantity of 

 coarse material, whether it is arranged in the form of a deep 

 or a shallow percolating filter, if the volume of sewage liquor 

 treated per cube yard be the same in each case." 



"In the case of percolating filters of fine material, if the 

 liquid to be purified were free from suspended and colloidal 

 solids, and if thorough aeration could be maintained, the same 

 statement made in regard to coarse filters might also hold 

 good of fine bacterial filters. But in practice these conditions 

 can scarcely be maintained with large rates of flow, and 

 therefore probably the greatest efficiency can be got out of a 

 given quantity of fine material when arranged as a shallow 

 filter." 



" Taking into account the gradual loss of capacity of contact 

 beds, a cubic yard of material arranged in the form of a per- 

 colating filter will generally treat about twice as much tank 

 liquor as a cubic yard of material in a contact bed." 



It is generally desirable to remove from the sewage, by a 

 preliminary process, a considerable proportion of the grit and 



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