NATURAL PURIFICATION OF WATER
We have already noticed that rivers purify themselves as they proceed. There are many excellent examples of this self-purification. The Seine as it runs through Paris becomes highly polluted with every sort of filthy contamination. But twenty or thirty miles below the city it is found to be even purer than above the city before it received the city sewage. In small rivers it is the same, provided the pollution is less in amount. Whilst authorities differ with regard to the mode of self-purification, all agree that in some way rivers receiving crude sewage are able in a marvellous degree to become pure again.
The conditions influencing this phenomenon are as follows:
(a) The Movement of the Water. It is probable, however, that any beneficial result accruing from this cause is due, not to any mechanical factor in the movement, but to the extra surface of water available for oxidation processes.
(b) The Pressure of the Water. It is believed that the volume of water pressing down upon any given area beneath it weakens the vitality of certain microbes. In support of this theory, it is urged that the number of bacteria capable of developing is less the greater the depth from the surface. Yet it must be remembered that mud at the bottom of a river, or at the bottom of the sea, is teeming with living organisms.
(c) Light. We have seen how prejudicial is light to the growth of organisms in culture media. This is so, though to a less extent, in water. Arloing held that sunlight could not pierce a layer of water an inch in thickness and still act inimically on micro-organisms. But Buchner found that the sun's rays could pass through fifteen or twenty inches and yet be bactericidal. This evidence appears contradictory. On the whole, however, authorities agree that the influence of the sun's rays upon water is distinctly bactericidal and causes a marked diminution in the quantity of organisms after acting for some hours. Especially will this be so when the water is spread over a wide area and is therefore shallow and stationary, or moving but slowly.
(d) Vegetation in Water. Pettenkofer, in his observations upon the Iser below Munich, has shown how algæ bring about a marked reduction in the organic matters present in water.
(e) Dilution. There can be no doubt in anyone's mind that the pollutions passing into a flowing river are very soon diluted with the large quantities of comparatively pure water always forthcoming. But this, whilst it would lower the percentage of impurity, cannot remove impurities.
(f) Sedimentation. Whilst Pettenkofer attributes self-purification to oxygenation and vegetation, most authorities are now agreed that it is largely brought about by the subsidence of impure matters, and by their subsequent disintegration at the bottom of the river. Sedimentation obviously is greatest in still waters. Hence lake water contains as a rule very few bacteria. "The improvement in water during subsidence is the more rapid and pronounced the greater the amount of suspended matter initially present" (Frankland). Tils has pointed out that the number of micro-organisms was invariably smaller in the water collected from the reservoir than in that taken from the source supplying the latter. Percy Frankland has demonstrated the same effect of sedimentation by storage as follows:
| No. of Colonies in 1 cc. of water. | |
| 1. Intake from Thames, June 25, 1892 | 1,991 |
| 2. First small storage reservoir | 1,703 |
| 3. Second all st"orage re" | 1,156 |
| 4. Large storage reservoir | 464 |
The large reservoir would of course necessitate a prolonged subsidence, and hence a greater diminution than in the small reservoirs. Many like examples might be cited, but a typical one such as the above will suffice.
(g) Oxidation. Many experiments and observations have been made to prove that large quantities of oxygen are used up daily in oxygenising the Thames water. Oxygenated water will come up with the tide and down with the fresh water from above London. There will also be oxygen absorption going on upon the surface of the water, and from these three sources enough oxygen is obtained to oxidise impurities and produce what is really an effluent. In many smaller streams the opportunity for oxidation is afforded by weirs and falls.
Probably all these factors play a part in the self-purification of rivers, but we may take it that oxidation, dilution, and sedimentation are three of the principal agencies.
We may here digress to refer in passing to the facts obtainable from Sir Edward Frankland's report on Metropolitan water supply in 1894, as they will afford a connecting link between self purification and artificial purification. Judged by the relatively low proportion of carbon to nitrogen, the organic matter present in the water was, as usual, found to be chiefly, if not entirely, of vegetable origin. An immense destruction of bacteria was found to be effected by storage in subsidence reservoirs. The bacterial quality of the water might differ widely from its chemical qualities. These three facts are of primary importance in the interpretation of water reports, and it will be well to bear them in mind. Sir E. Frankland also refers to the physical conditions affecting microbial life in river waters. The importance of changes of temperature, the effect of sunlight, and rate of flow had been referred to in previous reports. Respecting the relative proportion of these factors, he adds:
"The number of microbes in Thames water is determined mainly by the flow of the river, or, in other words, by the rainfall, and but slightly, if at all, by either the presence or absence of sunshine, or a high or low temperature. With regard to the effect of sunshine, the interesting researches of Dr. Marshall Ward leave no doubt that this agent is a powerful germicide, but it is probable that the germicidal effect is greatly diminished, if not entirely prevented, when the solar rays have to pass through a comparatively thin stratum of water before they reach the living organisms."
From which it is clear that evidence favours the effect of sedimentation and dilution. These two factors in conjunction with filtration are, practically speaking, the methods of artificial water purification, with which we are now in a position to deal.