382 Report S.A.A. Advancement of Science. 



Percolating filters, or trickling beds, are those in which the 

 effluent is showered or sprayed on to the surface of the filtering 

 medium, either continuously or intermittently, and allowed to slowly 

 percolate through ; these beds are constructed on a concrete floor, and 

 the filtrate flows off this and is collected in a channel, and then flows 

 away. 



The distribution is done by revolving or rotary sprinklers, fixed 

 sprinklers, or Stoddart's trays. The latter is continuous, while the 

 former are either intermittent or continuous. My experience is that 

 it does not effect the result very much, as long as the filter can at 

 times be given a rest, say for two or three days. It is essential that 

 as much air as possible be conducted to the interior of the bed, and 

 for this purpose air drains are laid on the floors, which permit of 

 the air to penetrate into the body of the material, which is aided 

 by the downward flow of water ; the exterior walls of this filter need 

 not be built up solid, as in the case of contact beds, but can be built 

 up to suit the natural slope of the filtering material used. This is, 

 of course, a great saving in cost. 



The depth of these filters should be at least six feet. This fall 

 is not always obtainable, and they are sometimes constructed only 

 three feet deep : but the results cannot be expected to be so good, 

 and contact beds would be better for anything under four feet. 



These beds are worked up to 1,000,000 gallons per acre per 

 diem, but the sewage would require to be weak, and 500,000 gallons 

 is the usual maximum for any town sewage. 



The filtering material in these beds is considerably coarser than 

 that used in contact beds, being up to 2in. and 3in. cubes. The usual 

 material is clinker, but stone or coal is often used, and it is 

 considered by many that the smoofher surface ?ives the better results, 

 for in clinker the effluent collects in the crevices, which becomes 

 stagnant ; capillarv attraction also helps to keep the material water- 

 logged, which is highly undesirable, but the most important point, as 

 in contact beds, is to obtain material that will not disintegrate. 



The advantages of Contact Beds over Continuous Filters are : — 



(i) The distribution need not be so carefully considered. 



(2) Much less fall is lost, which is often a most important 

 factor. 



The disadvantages are : — 



(i) That watertight tanks must be constructed. 



(2) Oxidation is limited, as the amount of air is onlv equal to 

 the volume of sewage. 



(3) The beds are far more liable to clogging. 



(4) The capacity of beds must be much larger for the volume 

 of effluent treated. 



The advantages of percolating filters are : — 



(i) Percolating filters cost less, as no retaining walls of any 

 kind are necessarv. 



