MODERN ASPECTS OF WATER PURIFICATION 325 



Darwin, and Wallace, were its first Uniformitarians. Largely through the 

 influence of Lyell, it was Uniformitarianism that won the day in Geology, and 

 Catastrophism is now extinct. Possibly the day is at hand when a similar 

 fate will overtake Biological Mutation, and I venture to predict to the reader 

 that when that happens, its successor will be the Theory of Precession now 

 under its eyes. 



MODERN ASPECTS OF WATER PURIPICATION" (R. J. S. 



McDowall, M.B., D.Sc.)- 



The problem of water supply has been of vital importance and interest to 

 the people of all time, and even to-day is one to which some of the most 

 modern discoveries are applied. 



The ancients simply sought an ample source of supply and conveyed it to 

 the desired place by aqueducts, often of great length. Fortunately, too, they 

 possessed a natural immunity to many of the diseases prevalent now in more 

 civilised communities. Gradually, however, the ever-increasing population 

 necessitated larger supplies, and the higher races lost their immunity, with 

 the result that the provision of a water supply good both in quality and 

 quantity became a most important question wherever people congregated 

 together. Thus the utilisation of sources of doubtful purity became necessary 

 as did also the introduction of water purification systems. 



During recent years, the outlook on such questions has been considerably 

 modified as the result of the great success which has attended the purification 

 of water by chlorine, especially during the war, and there seems every reason 

 to believe that the chlorination method will in course of time largely replace 

 the methods at present in use. 



Hitherto the piece de resistance in water purification has been sedi- 

 mentation and slow sand filtration, together or separately according to 

 requirements. Simple storage or sedimentation has been found to cause the 

 destruction of some percentage of the organisms and, if the water is reasonably 

 good at first, this may be considered sufficient ; but usually the water is passed 

 from the storage reservoirs to slow sand filters. The essential part of the 

 latter is fine sand supported on coarser sand and rubble, but the most 

 of the purification takes place at the surface of the sand, where a scum 

 forms which is the most efficient part of the filtering medium. Unfortunately, 

 as the scum becomes thicker, the flow through the filter becomes less 

 until it is insufficient for requirements, and, although the filter may be working 

 excellently from the bacteriological point of view, the scum has to be removed. 

 This is often a very laborious and expensive process. The cost of running 

 such a plant and the initial cost of installation, which may include the purchase 

 of land, is very great, and it is but natural that any other methods which 

 reduce the expense without sacrifice of efficiency should receive a due con- 

 sideration. Until the war, however, little attention was paid to the question, 

 at least in this country, most authorities being satisfied at having arrived at 

 a reliable method of purification. But, as we shall see, the war has changed 

 the outlook, and now the chlorination introduced by the late Sir G. Sims 

 Woodhead looks like supplanting the older method in spite of considerable 

 opposition. 



This method depends on the fact that free chlorine in very dilute solution, 

 0-5-1 per million, will kill all pathogenic organisms in water provided there 

 is no other organic material present which will take up the chlorine. There 

 are several ways of introducing the chlorine, but that usually adopted is that 

 in which use is made of bleaching powder — which, if of proper strength, should 

 contain a third of its weight of chlorine, which is liberated when the salt is 

 dissolved. A concentrated bleaching powder solution of known strength is 

 allowed to run into the water system at a convenient point where thorough 



