This section is from the book "A Treatise On Beverages or The Complete Practical Bottler", by Charles Herman Sulz. Also available from Amazon: A Treatise On Beverages.
"By the addition of a small amount of alum to water, it can be filtered through ordinary paper without difficulty, and yields a brilliantly clear nitrate, in which there is no trace of suspended matter. In our experiments, a solution of alum was added to the water, the whole well mixed by stirring or shaking, and then filtered after standing from one to fifteen minutes. So far as we are able to determine, the coagulative and preci-pitative action of the alum is immediate upon thorough mixture, and hence, it is not necessary toallow the mixture to stand before filtration, but it can be filtered immediately after mixing. To determine the amount of alum necessary to precipitate this water, alum was added in decreasing amounts to samples of water, which were then filtered through paper. In this way we found that the minimum limit was about 0.02 gramme of alum to one litre (1.2 grains to one gallon). Beyond that point the action of the alum began to be doubtful, and the water, although clarified by filtration, was not wholly clear. To be sure .of complete clarification, we took double this amount - 0.04 gramme to one litre (2.3 grains to one gallon) - as a standard calculated to give certain results. This amount can be doubled or trebled without fear of any harmful results, but there is no use of adding any more alum than is sufficient to do the work. The determination of the amount of solids removed from the water by the clarification with alum had not yet been finished. We consider it, then, as established that, by the addition of two grains of alum to the gallon of water, or half an ounce to the hundred gallons, water can be rendered capable of immediate clarification by filtration. The clear water obtained by filtration, after adding this amount of alum, contains no appreciable amount of free alum, and, in fact, in the majority of cases, ordinary tests fail to reveal its presence.
"The mixing of the water with the alum previous to the filtration should be done in a separate receptacle. The only requisite here is that the vessel in which the mixing is done must be clean. A pail, jug, can, 6 or any other vessel will do. It is well to have the pail or can marked on the inside with scratches so as to be able without difficulty to judge how much water there is in it, since the amount of alum should be added in about the right proportions. The eye gets very accurate in judging the volume after a little practice, but it is better and just as easy to be accurate. A clean tin can of two or four gallons capacity is a good size, and, if possible, should not be used for any other purpose than for the drinking water. It should be kept scrupulously clean, and after each use should be washed out and dried. It can be graduated by pouring into it a gallon of water, and marking with a file or other sharp point a scratch just at the level of the water. Then another gallon is poured in and its level also is marked. In this way a graduation is easily made which is sufficiently accurate for all the purposes here intended. The necessary amount of the alum solution is added to the water, the whole well mixed by stirring, and then poured into the filter. Here, again, one or two points should be observed. The mixing is best done with a long-handled spoon. A very practical stirrer is a small cake-turner, for by means of its flat end a most thorough mixing can be effected. This mixer should not be used for any other purpose than to mix the water. Experience shows that if the vessels used for mixing or holding the water are not kept perfectly clean, the water may acquire a taste, and this will be laid to the process instead of to lack of care. To facilitate the pouring into the filter, it is well to have the can provided with a mouth or spout.
"The solution of alum is made as follows: Dissolve half an ounce of alum in a cup of boiling water, and when it is all dissolved, pour into a quart measure and fill to a quart with cold water. (This solution should be kept in a bottle labeled 'Alum'). Fifty-four drops of this solution contains 2.3 grains of alum, which is the amount to be added to one gallon of water. A teaspoon, scant full, will be about the right amount to add to every gallon of water to be filtered. No harm would be done if by mistake two teaspoonfuls are added. A more satisfactory method will be to procure a small measuring glass. One fluid drachm will be the right amount. It will be found, without doubt, that the amount required for some waters will be even less than that suggested above. We would suggest, therefore, that those who use this method of clarification determine for themselves by experiment how little of the solution is required to make the water they use run through the filter perfectly bright and clear".
Fig. 9 represents an apparatus invented by Dr. T. C. Higgins of New Brunswick, N. J., for use in applying the alum process for purification of water for drinking and other purposes. Its use simplifies the process very much, and overcomes the difficulties which arise in the use of this or any similar process of purifying water by precipitation, viz., the avoidance of the flocculent precipitate which is separated from the water.
In the alum process particularly this precipitation begins immediately upon the introduction of the alum, and the precipitate is so gelatinous and flocculent that it requires from 24 to 48 hours for complete clarification (if it be not desired to filter it out), so that all the precipitate is at rest on the bottom of the vessel. Then the clear water from above the sediment must be drawn off with care, or currents will be formed which will carry the precipitate through with the clear water.
The cut represents a tank or barrel filled with water to which the alum solution has been added in proper proportion. Figure a represents a round vessel containing some simple filtering media, sponge, cotton or any similar substance. To this is attached a flexible tube leading to the faucet. 6 is a float which keeps Ihe filter just below the surface of the water d d. At all times, upon opening the faucet, a downward current is established, which tends, in addition to the natural gravity, to carry the precipitation downward, as shown by the heavy lines e, in the cut. The bulk of it is therefore kept away from the delivery tube. What little may be floating will be completely arrested in the floating filter. The letters c c represent foot projection,that keeps the filter in position when it approaches the bottom, also prevents the flexible pipe from kinking and impeding the flow.
Fig. 9. - Higgins' Alum Solution Float.
By this arrangement water may be drawn off in a few hours, as the water is constantly drawn from the surface from which the precipitate settles first, and a saving of time of from 24 to 43 hours is made. In using large quantities of water it will be found that the mechanical action of the bulk of precipitate hastens the process and requires less alum.
In the Western country, where bottlers are more or less dependent upon the ordinary water-courses for their aqueous supply, the "alum" method of purifying water should be given a trial. Complaints are frequent that many of the filters now in use have not given the satisfaction expected, but if taken in conjunction with the suggestions given above, would probably lead to better results.