ARTIFICIAL METHODS OF SEWAGE DISPOSAL. 251 



under control, and that only a few of the sewage-bacteria are really 

 dangerous. If the organic matter were allowed to be broken up by the 

 sewage bacteria and the sewage then set free without further treatment, 

 there would be serious danger owing to the possibility of the multipli- 

 cation of those sewage-bacteria that are pathogenic. But before being 

 set free the sewage is brought under conditions which give the patho- 

 genic or potentially-pathogenic bacteria very little chance of multiplying 

 to any great extent, so that if the organic matter be used up without 

 any undue multiplication of the latter organisms, the end for which 

 these processes were devised, has been achieved. To obtain an insight 

 into the working of the various kinds, it is necessary to understand 

 that there are essentially two phases in the work of decomposition, one 

 in which the anaerobic bacteria and one in which the aerobic bacteria 

 have full scope for their activity either successively or simultaneously. 

 For the anaerobic bacteria break down solid organic matter into liquefi- 

 able simpler substances which, as explained above, can be readily 

 attacked by aerobic bacteria in the presence of oxygen. 



The first of the biological processes in this country was the "cultiva- 

 tion tank" invented by Mr. Scott Moncrieff in 1891. It consists of a 

 chamber filled with large stones (preferably flints), and carried by a 

 grating. The crude sewage is made to rise slowly through the chamber. 

 As it passes upwards, a large portion of the suspended matter is caught 

 on the stones, the object of the latter being to afford a resting place for 

 the anaerobic bacteria which are certain to collect on them. These 

 bacteria attack the solid sewage thus caught, the result being that the 

 solid matter is liquefied. After passing through the stones the sewage 

 matter is brought into contact with highly oxygenated water, and 

 is then passed through a "nitrification channel" where the organic 

 matter is attacked by aerobic bacteria. The cultivation tank is a 

 special contrivance for favouring the multiplication of the anaerobic 

 bacteria, whilst the nitrification channel does the same for the 

 aerobic bacteria. 



Next came in point of date, Mr. Cameron's septic tank. By a septic 

 tank is meant one in which putrefaction is taking place. It consists 

 essentially of a chamber through which sewage is allowed to flow con- 

 tinuously, the inlets and outlets being preferably submerged, so as not 

 to interfere with the scum which forms on the surface. It diff'ers from 

 the cultivation tank in that no material, e.g. stones, is placed inside to 

 furnish surfaces for the liquefying bacteria to cling to ; otherwise it is 

 identical in that the work of disruption is effected by the anaerobic 

 bacteria contained in the sewage itself. The efficiency of the septic 



