304 BACTERIOLOGICAL AND ENZYME CHEMISTRY 



outlet of the tanks and the point of final discharge. More- 

 over, even if the total amount of fall be adequate, yet if the 

 ground slopes gradually away, it may be more economical to 

 put down primary and secondary contact beds and so avoid 

 the excavation necessary for deep trickling filters. The 

 amount of solid matter discharged from contact beds, especi- 

 ally from secondary beds of fine material, is not so great as 

 from open percolating filters, and consequently, in such a 

 case, the final provision of humus tanks may be on a less 

 extensive scale. This is sometimes a factor in the choice of 

 method. On the other hand, the material in contact beds 

 generally needs to be removed and washed more often than 

 the medium in trickling filters. 



As already indicated, a concentrated sewage lends itself 

 for various reasons to treatment by contact beds, at any rate 

 as a preliminary step. Probably the most satisfactory method 

 in such a case is preliminary treatment in contact beds, 

 followed by final purification on trickling filters. For weak 

 sewages, it is probable that trickling filters are always to be 

 preferred. The method of preliminary treatment conditions 

 primarily the grade of material to be used on the filter bed. 

 The freer the effluent from suspended or colloidal matter, 

 the finer the grade of material that can be economically used. 

 Where there is little depth available for filter beds, the thorough 

 clarification of the sewage may be desirable, so that fine-grade 

 material can be used, and the lack of depth made up for by 

 the extended surface area of the particles. 



THE INTERPRETATION OF SEWAGE AND WATER ANALYSES 



The methods used in the analysis of sewage and sewage 

 effluents must be looked for in the books specially devoted 

 to the subject. A short space may be usefully given here to 

 the interpretation to be placed on the results of these analyses, 

 when viewed in the light of the information given in the fore- 

 going pages. 



