A NOTE CONCERNING MECHANICAL FILTRATION.
Having been intimately acquainted with the extended experiments conducted under order of the United States court at the Elmira filtering plant during June, 1897, it occurs to me that sundry observations made during the test might be of interest to this association. For the information of those unarquainted with the type of mechanical filter used at Elmira let it be said that each filter (of which there are eighteen in the battery) consists of a wooden tub thirteen feet six inches in diameter and fourteen feet high, and open at the top. The bed of filtering sand three feet six inches deep, together with the washing machinery,occupies the upper portion of the tank, leaving below an empty chamber six feet high, through which the raw water must pass on its way towards the sand.
To discover just what this chamber would do in the way of relieving the filter proper of a portion of its work was the reason for instituting the test. It would be proper here to state that the dose of alum had been added to the water before the chamber in question was reached, and also to observe that the water entered the chamber in a tangential direction, near the circumference of its circular bottom, and flowed away through a pipe in the centre of its roof. Such an arrangement naturally caused the water to take up a movement of rotation, more or less coincident with the surface of a cone, whose base was the floor of the chamber and whose apex was ha orifice in the centre of the roof. Timbers supporting the sand layer above naturally interfered with the uniformity of this motion. It was not denied that coarse and heavy material, such as sand, would be retained by the chamber; but there were those who held that flocculent aluminum hydrate, with its entangled or“ coagu’ated “material, was much too light to settle to any material extent under the conditions obtained. As a result of the seven days’ trial run, with an average of 10,731 gallons of water passing through each filter per hour, a number of in teresting observations were secured, showing that the chambers in question certainly fulfilled a useful function, and did relieve the filters to a material degree. Upon opening the chambers at the end of the seventh day, a large quantity of aluminum hydrate was, in every case, found lying upon the floor in forms like drifted snow, and varying in depth from almost nothing near the circumference to fourteen inches and more towards the centre of the bottom. It was impossible to calculate the volume of these drifts; but it was plainly equivalent to many cubic feet. It was proposed to determine the purification value of this apparently large precipitation by catching it in a filter-press, and subsequently weighing it,after drying. This was accordingly done with the sediment from one of the tanks, with the result that the large volume of coagulated material made a very indifferent showing when translated into weight. It was,and still is, however,my strong opinion that volume, not weight, is the important factor to be considered when dealing with this precipitate. It must be admitted that the material forming the sediment banks in the Elmira tanks was very light; but it was heavy enough to fall, or else it would not have been there; and, provided it be heavy enough to fall, it is certainly true that the greater its bulk for a given weight, the better will be its efficiency, for it will cover so much more space while falling, and thereby increase the chances of entangling suspended impurities during its formation and descent. Rain drops falling through the air carry down impurities, of course; but, if transformed into flaky snow, their purifying action is much increased, although their specific gravity is lessened. It is for these reasons that I think the relative efficiency of the chamber below the sand bed should be measured by the volume of the retained precipitate rather than by its weight.
* Paper read at the convention of the American Water Works association at Buffalo, N. Y., June, 1808.
The photographs accompanying this “ Note” scarcely show the other feature of these aluminum hydrate deposits, which to me was of considerable interest.namely, the numerous balllike aggregations occurring throughout the mass. It would seem that the hydrate had in many places been rolled together and built up after the same manner that a boy accomplishes the gradual enlargement of a ball of snow. This formation I believe to have resulted from the rotary motion given the water by the special device for its introduction already alluded to. ’ Motion such as that is,in my opinion,decidedly favorable to the formation of large flocks in a liquid in which aluminum hydrate is already formed. Certainly it is a mistake to suppose, as has been so often maintained, that the most rapid “coagulation ” and precipitation of the hydrate is to be secured by permitting the liquid containing it to remain in a state of complete rest. The photographs shown herewith illustrate that gentle rotary motion, far from being objectionable, is a distinct advantage. The two jars here shown differ from each other but in the fact that the left hand one had its contenis gently rotated, while the content of the companion was maintained at complete rest. In each the same quantity of hydrate had previously been formed. The photographs show how progressively advantageous was the rotary motion for the formation and deposit of large flocks of the hydrate; nor is this to be wondered at when one considers that the movement in question furnished opportunity for more frequent collision, with resulting adhesion of the growing flocks. As already stated, the evidence of such action on the large scale in the Elmira tanks was very pronounced.
With reference to the bacteria efficiency of these tanks, it was interesting to observe that an average of 42.6 per cent, of all the same germs present in the raw water had been removed by the time sand was reached.
Finally, a word wdth regard to the claim so often advanced, that a closed pressure filter, operating without a chamber between it and the point of introduction *f the alum, does not act as a surface strainer, but that the deeper portions of tbe sand bed have their parts to play in the work of purification, as well as the surface layer. This claim I found to be perfectly true. By tapping such a filter at points both above and below the sand surface, I wasenabled to secure samples of water from various depths in the sand body. Analytical results showed that the alum was not entirely decomposed, and, consequently, the chemical equation was not fully satisfied until considerable depths in the sand bed had been reached. This was to have been expected,in view of the fact that time is an element in all chemical reactions, and a pressure filter, running under the conditions named, allows but small time to elapse after the addition of the alum until the sand is reached. As a corollary to this proposition, the tanks we have been discussing must also serve as reaction chambers, permitting of complete decomposition of the alum, and such I found them to be upon further investigation.