DETERIORATION OF WATER IN RESERVOIRS

AND CONDUITS,

ITS CAUSES AND MODES OF PREVENTION.

BY CHARLES B. BRUSH, C.E., HOBOKEN.

.I.

The subject is a very broad one, and all the phases of it are by no means understood at the present time.

Certain things, however, have been definitely ascertained.

It is a fact that the quality of each water-supply varies at certain seasons, under certain conditions and at different periods. Generally, water is in its best condition in fall and spring, and at its worst condition in midsummer and in midwinter.

In hot, dry, dog-days weather, many streams go through what is known as the purging process. The water becomes dead and lifeless, and if allowed to remain at rest for any considerable length of time, a formation known as algæ appears on the surface. Sometimes these algæ are of a green color, taking the appearance of ferns in the water, and sometimes the formation has the appearance of green paint, which can be skimmed off from the surface. At other times the formation

is brownish in color and collects in patches, with brown shreds or stems extending some distance down into the water. Sometimes these algæ give the water a distinct taste and smell, more or less offensive. All formations of algæ, however, are destroyed and disappear as soon as the water is put in motion, either by heavy winds or by running the water off rapidly, especially over a rough and irregular surface. If allowed to remain, the water cures itself, the algæ disappearing after a few weeks and leaving the water again in its normal condition. These algae always appear more quickly on water that has been filtered, either naturally or artificially. Waters that are obtained from subterranean sources, when stored in reservoirs, rapidly deteriorate. It may be stated as a general principle that water is delivered in its.

best condition when taken from a running stream and delivered directly to consumers without coming to rest during its passage.

The popular test of the purity of the water-supply is a pleasant taste and an absence of color.

Distilled water is the purest we can obtain, yet it is flat, tasteless and insipid. The worst sewer contamination and other forms of pollution often occur without discoloring the water at all. As a matter of fact, water that is discolored by sediment is often that water in its best condition, because the sediment is due to the fact that an immense and abnormal volume of water is flowing off from the watersheds, and any pollution that may be in the water is so diluted as to be incapable of harm.

Still, the demand for clear water is so pronounced that it will have to be supplied to consumers. This necessitates reservoirs, with their attending evils. Of course, reservoirs are also required for many other reasons. On small drainage areas water must often be impounded during the wet season, to be delivered during the dry season. Again, it is necessary to store waters at proper elevations so that a supply in times of conflagration and other heavy drafts, may be always at hand, without calling upon the pumps to do an unusual amount of work at certain hours of the day, and very little at other hours.

If, however, the storage of water from any source in reservoirs is greater than twenty to thirty days' supply, the deterioration of the water becomes more or less marked. It is rare that water is stored in distributing reservoirs in our large cities more than ten days' supply, and frequently it amounts to a supply of less than five days.

This deterioration is practically due to stagnation, and stagnation begins to manifest itself as soon as the oxygen in solution of the water becomes less than three-tenths of one per cent. The normal quantity of oxygen in solution in good water is about six-tenths of one per cent., but it runs as high, at times, as nine-tenths of one per

cent.

The best means of preventing stagnation in reservoirs that are known to-day consist in keeping the water, if possible, in motion, but where that is impracticable, as is generally the case, I know of no better means than of forcing air into the bottom of the reservoir and keeping the water aërated. The same applies to conduits. In the seasons when the water deteriorates the conduits should be frequently blown off, and on long force mains I have found it very desirable to

pump air into the mains and let the water and air mix freely until they are both delivered in the reservoir, taking care, of course, to see that all summits are kept free from harmful accumulations of air by opening air cocks at these points as often as may be necessary.

The offensive taste and smell that accompany algae are usually those of cucumbers, dead fish, &c. The popular notion is that the deterioration of water is due to animal or vegetable life growing in the reservoir or on the banks of the stream. These are not the causes of the unfortunate condition of the water, but rather the results or effects of that condition.

A reservoir should be treated like an aquarium. There should be a proper balance of animal and vegetable life in order to keep the water pure and wholesome. I have obtained excellent results by stocking my reservoirs with German carp, silver and gold-fish, and, after these have grown large enough, by adding black bass. These fish are scavengers respectively of vegetable and animal matter in the

water.

In 1884 we had trouble with algæ in our reservoir at Hoboken. We found that the water while in motion was pure and sweet, but after being allowed to remain at rest it became stagnant and algæ appeared on the surface. The reservoir was shut off, and after two or three weeks the water purged itself and became all right again. In the meantime, the water was pumped past the reservoir directly into the city, and no unpleasant taste or smell was noticed.

In Hackensack the supply was formerly distributed from a reservoir the capacity of which was equal to about thirty days' consumption. Complaints had been made during the hot, dry weather of this algæ difficulty, and it was cured by simply abandoning the reservoir and allowing the water to pass through a tank holding about five thousand gallons, the supply of which was regulated by an automatic float.

In Greenwood Cemetery the water was taken from a permeable stratum about forty feet below the surface of the ground. The water as it was delivered into the reservoir was bright and clear generally, and its appearance was entirely unobjectionable. In the hot, dry weather, however, brown algae appeared on the surface of the water in patches of about twelve to eighteen inches in diameter, with long fibers reaching to the bottom of the reservoir. I found that by forcing the air into the bottom of the reservoir I could entirely break up this algæ formation, the only question being the amount of air required.

In New Rochelle the greatest care is taken to prevent all forms of pollution to the water-supply; the capacity of the reservoir, however, is about three hundred times the daily draft. At times the offensive taste and smell from water drawn from the lake were quite unpleasant. I found by wasting the water through the bottom, by introducing fountains throughout the town and allowing the water to waste through the mains at the extreme end of the town, that the quality of the supply was greatly improved. Subsequently I introduced air into the delivery main in such a way as to have the air rise through the main into the gate-house at the reservoir, thus forcing all the water delivered through the mains to become fairly charged before it reached the consumers. Since then no further difficulty has been experienced.

DETERIORATION OF WATER IN RESERVOIRS,

ITS CAUSES AND PREVENTION.

BY GEORGE W. RAFTER, MEMBER AM. SOC. CIVIL ENGINEERS.

II.

The subject introduced to your notice by the former paper is one of importance and interest, and it is an encouraging sign of the times that the circumstances affecting the quality of a water-supply are becoming more and more matters for scientific inquiry.

The causes which produce deterioration, sudden or otherwise, of quality in water, either stored in reservoirs or flowing in conduits, have existed from the beginning of time, and, probably, all impounded waters have, on occasion, been more or less affected by them, but it is only since the great multiplication of public water-supplies during the last thirty or forty years, that the troubles herein referred to have been made the subject of any considerable amount of study and investigation, and, even at the present time, while we may say that relatively considerable advances have been made, we are still obliged to conclude that detailed knowledge of the matter is, after all, still in its infancy.

We have, nevertheless, a number of cases which have been rather carefully studied, and in some of which the cause has been fairly determined, while in others, although studied, the reason for the trouble remains uncertain. The following, without being in any sense a complete statement of such cases, may still be taken as embodying a number which are typical.

The water of Lake Cochituate was introduced into Boston in 1848, and in June, 1851, the first recorded trouble with the Boston water occurred. The consumers complained that the water as drawn from the services had a bad taste, and a thorough flushing of the distribution mains throughout the city seems to have been an efficient