« EelmineJätka »
THE EFFECT OF THE AERATION OF NATURAL WATERS.
It is a very common belief that water deprived of air will deteriorate in quality and become unfit for use, and that the only way to maintain its purity is to keep it freely exposed to the air. The mountain stream which breaks over rocks and stones is thought to be a good illustration of the intimate connection between the aeration of water and its purity.
On the other hand, it is not generally known that the waters of many deep wells which we prize for their high purity, their clearness, coolness and good taste, contain no air, and have been preserved in this condition in the earth, for aught we know, for centuries. There is clearly a need of a clarification of our ideas on this subject.
Pure water, preserved from contact with organic matter, remains unchanged indefinitely. If exposed to the air it dissolves a definite amount of the gases of which the air is composed, nitrogen, oxygen and carbonic acid, — the amount dissolved being dependent upon temperature and pressure. If the pressure is removed, or if the water is boiled, the gases escape, to be reabsorbed when the water is exposed to the air again at ordinary temperatures; but in no sense does the air exert any preservative action on the water or tend to keep it “ fresh ” or “ sweet."
The condition of affairs is, however, entirely changed when water contains organic matter which is capable of undergoing decomposition. The familiar process of the change of organic into mineral matter by decomposition is one of oxidation, and the necessity of the presence of air to carry on this change is well understood. The breaking up of organic matter when oxygen is not present is one of putrefaction, the products of which are usually very offensive.
The analogy between the comparatively slow process of oxidation in nature and the destruction of organic matter by combustion has seemed to justify the inference that we can hasten the former as we can the latter by increasing the supply of oxygen. Acting on this assumption, it is not uncommon in waterworks practice to aerate the water by causing it to flow over a series of steps, or by forcing it into the air as å fountain, or by pumping air under pressure into the distribution system. Whatever the method employed for aerating the water, the idea behind it is that the water will be thereby purified by oxidation to a degree beyond that which would take place if the water were exposed to the air on the surface only.
During the past two years numerous experiments have been made in the laboratory of the State Board of Health to test this theory of accelerated oxidation, and all the experiments have given negative results. The question, let it be clearly understood, is not one of supplying oxygen to an impure water, like sewage, which contains no oxygen, but this : Will an impure water, which contains at all times more or less free oxygen in solution, be purified more rapidly by oxidation if the amount of oxygen is increased by spraying the water or by pumping air into it; can the natural process of oxidation be hastened by these means? It is to this question that the experiments give a negative answer. If we look for the cause of this failure to hasten oxidation by increasing the amount of oxygen we find that it rests in the inherent nature of the process, — a process which is only remotely analogous to the chemical process of combustion. In combustion we have the direct chemical combination of carbon and hydrogen with oxygen, and, by varying the supply of oxygen, we can at will make the combustion slow or rapid. The case is entirely different in the oxidation of organic matter in nature. Here we have to do with the living activity of bacteria, which, in some way not fully understood, causes first the carbon and hydrogen of the organic matter and then the nitrogen to combine with oxygen. This process can only be hastened by increasing the number of bacteria, or by providing more favorable conditions for their activity. Thus we know that the temperature at which bacteria are most active differs with different species, but we have no evidence that, provided some free oxygen is present, the activity of the bacteria of decomposition is in the least affected by its amount. Here the analogy of bacterial oxidation and combustion ceases.
The first series of experiments was made to ascertain whether there was any change in the nitrogen compounds in waters under different conditions of aeration, namely :
1. By exposing water contained in bottles to the air of the room. 2. By drawing a current of air through the water by means of an
aspirator. 3. By shaking the water with air in a bottle, in a shaking machine driven
by an electric motor, the air being renewed from time to time by
removing the stopper from the bottle. 4. By exposing the water to air under a pressure of sixty to seventy-five
pounds to the square inch in soda-water siphons.
RESULTS OF AERATION OF WATER BY A CURRENT OF AIR DRAWN THROUGH
THE WATER IN A FLASK BY MEANS OF AN ASPIRATOR,* BY AIR UNDER PRESSURE, AND BY SHAKING THE WATER WITH AIR IN A BOTTLE.
First Experiment with Cochituate Water.
[Parts per 100,000.)
* The air used for aspiration was taken from outside the building. The air of laboratories where gas is burned contains enough nitrogen in the form of nitrites to vitiate an experiment of this character.
It was found that there was a very small amount of free ammonia taken up by pure water from the air in the cases of prolonged aeration.