Purification.—At a recent meeting of the Institution of Civil Engineers, Prof. Frankland read a paper dealing with the question of water purification, in which he remarked that the earliest attempts to purify water dealt simply with the removal of visible suspended particles; but later, chemists have turned their attention to the matters present in solution in water. Since the advance of the germ theory of disease, and the known fact that living organisms were the cause of some, and probably of all, zymotic diseases, the demand for a test which should recognise the absence or presence of micro-organisms in water had become imperative. It was, however, only during the last few years that any such test had been set forth, and this was owing to Dr. Koch, of Berlin. By this means the only great step which had been made since the last Rivers Pollution Commission had been achieved. It had been supposed that most filtering materials offered little or no barrier to micro-organisms; but it was now known that many substances had this power to a greater or less degree. It had also been found that, in order to continue their efficiency, frequent renewal of the filtering material was necessary.
Vegetable carbon employed in the form of charcoal or coke was found to occupy a high place as a biological filter, although previously, owing to its chemical inactivity, it had been disregarded. Being an inexpensive material, and easily renewed, it was destined to be of great service in the purification of water. Experiments were also made by the agitation of water with solid particles. It was found that very porous substances, like coke, animal and vegetable charcoal, were highly efficient in removing organised matter from water when the latter came in contact with them in this manner. Also, it was found that the well-known precipitation process, introduced by Dr. Clark, for softening water with lime, had a most marked effect in removing micro-organisms from water. In the case of water softened by this process, it was found that a reduction of 98 per cent. in the number of micro-organisms was effected, the chemical improvement being comparatively insignificant.
Water which had been subjected to an exhaustive process of natural filtration had been found to be almost free from micro-organisms. Thus, the deep-well water obtained from the chalk near London contained as few as eight organisms per cubic centimetre, whereas samples of river water from the Thames, Lea, and Wey had been known to contain as many thousands.
The same well-known authority on water published the following statements in the Nineteenth Century. He described the subject of domestic filtration as one which, in a town with a water supply like that of London, possesses peculiar interest, and is of no little importance. Most people imagine that by once going to the expense of a filter they have secured for themselves a safeguard which will endure throughout all time without further trouble. No mistake could be greater, for without preserving constant watchfulness, and bestowing great care upon domestic filtration, it is probable that the process will not only entirely fail to purify the water, but will actually render it more impure than before. For the accumulation of putrescent organic matter upon and within the filtering material furnishes a favourable nest for the development of minute worms and other disgusting organisms, which not unfrequently pervade the filtered water; whilst the proportion of organic matter in the effluent water is often considerably greater than that present before filtration.
Of the substances in general use for the household filtration of water, spongy iron and animal charcoal take the first place. Both these substances possess the property of removing a very large proportion of the organic matter present in water. They both, in the first instance, possess this purifying power to about an equal extent; but whereas the animal charcoal very soon loses its power, the spongy iron retains its efficacy unimpaired for a much longer time. Indeed, in spongy iron we possess the most valuable of all known materials for filtration, inasmuch as, besides removing such a large proportion of organic matter from water, it has been found to be absolutely fatal to bacterial life, and thus acts as an invaluable safeguard against the propagation of disease through drinking-water.
It is satisfactory to learn that in countries where the results of scientific research more rapidly receive practical application than is unfortunately the case amongst us, spongy iron is actually being employed on the large scale for filtration where only a very impure source of water supply is procurable. This refers to the recent introduction of spongy-iron filter beds at the Antwerp waterworks. It would be very desirable that such filter beds should be adopted by the London water companies until they shall abandon the present impure source of supply.
Animal charcoal, on the other hand, far from being fatal to the lower forms of life, is highly favourable to their development and growth; in fact, in the water drawn from a charcoal filter which has not been renewed sufficiently often, myriads of minute worms may frequently be found.
Thus spongy iron enables those who can afford the expense to obtain pure drinking-water even from an impure source; but this should not deter those interested in the public health from using their influence to obtain a water supply which requires no domestic filtration, and shall be equally bright and healthful for both rich and poor.
In a publication by Prof. Koch (Med. Wochenschrift, 1885, No. 37) on the scope of the bacteriological examination of water, it is asserted that a large proportion of micro-organisms proves that the water has received putrescent admixtures, charged with micro-organisms, impure affluxes, &c., which may convey, along with many harmless micro-organisms, also pathogenous kinds, i.e. infectious matters. Further, that as far as present observations extend, the number of micro-organisms in good waters ranges from 10 to 150 germs capable of development per c.c. As soon as the number of germs decidedly exceeds this number the water may be suspected of having received affluents. If the number reaches or exceeds 1000 per c.c., such water should not be admitted for drinking, at least in time of a cholera epidemic.
Dr. Link has lately examined a great number of the Dantzig well-waters chemically and bacterioscopically. The results obtained agree, however, very ill with the above opinions of Koch. On the contrary, it appears very plainly that regular relations between the chemical results and those of the bacterioscopic examination do not obtain. Many well-waters, chemically good and not directly or indirectly accessible to animal pollutions, often contained considerable numbers of microbia, whilst other waters, chemically bad and evidently contaminated by the influx of sewage, showed very small numbers of bacteria undergoing development. If we further consider that, by far the majority, indeed, as a rule the totality of the bacteria contained in well-water, are indubitably of a harmless nature, and that when a pollution of the water by pathogenous germs has actually occurred, such germs will not in general find the conditions necessary for their increase, especially a temperature approximating to that of the body and a sufficient concentration of nutritive matter, but that they will rather perish from the overgrowth of the other bacteria inhabiting the water, we shall see that a judgment on the quality of water—according to the results of a bacterioscopic examination extending merely to a determination of the number of germs capable of development—must lead to inaccurate conclusions, which contradict the results of chemical analysis.