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Appendix.

THE PURIFICATION OF DRINKING WATER BY ALUM.

Messrs. J. A. WANKLYN and E. T. CHAPMAN in their treatise on Water Analysis (1879, p. 121) refer, on the authority of the late Admiral SHERARD OSBORNE, to the use by the Chinese of alum for the purification of the water of the river Peiho at Tientsin; and many European travellers in China are familiar with the rough and ready method the natives adopt in out-of-the-way places for securing a supply of clear water by this agency.

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2. To Europeans generally the method is perhaps not so familiar as that known as the Clark process," chiefly because the necessity for the employment of alum is not so frequent as in cases where lime would be the more preferable reagent. Waters highly charged with carbon dioxide and containing a large amount of chalk in solution may be readily freed from the excess of chalk by the addition of lime; but for the purification of turbid water containing clayey matter in suspension there is no process which gives such satisfactory results as the alum treatment. Broadly speaking the lime or "Clark process is applicable to clear water containing an excess of inorganic matter in solution; the alum method to waters containing but little inorganic matter in actual solution but a large amount in suspension. The water collected on the island of Hongkong on the hill-slopes between Victoria Peak aud Mount Kellet and stored in the Pokfulam reservoir affords a good illus- tration of the latter type. In the report of the analyst to the Government of this Colony for the year 1882 Mr. MCCALLUM gave the results of the monthly analyses of this water. The average quantity of total solid matter in solution in 1882 was 3-4 grains per gallon, 4-7 being the highest and 3.1 the lowest readings. The water was always more or less turbid.

3. About six years ago I made some experiments with a view to devising a scheme for treating this water with a precipitant as it was impossible with ordinary filters to clarify the water. It was found that alum in the proportion of 3 grains to the imperial gallon gave highly satisfactory results. After a few hours the water could be filtered perfectly clear and bright.

4. In June last year when the Pokfulam water was unusually turbid after the terrific rain-storms of the preceding month, I made analyses before and after the alum treatment. In the subjoined schedule will be found the results expressed in grains per imperial gallon. The figures are very instructive. Water containing 19-6 grains of total solid matter in solution and suspension yielded after treatment with alum and filtration only 2-8 grains per gallon. It will further be noted that the albuminoid ammonia figure and the quantity of oxygen required to oxidize the organic matter are reduced by one half. The appearance of water so purified leaves nothing to be desired. Such a water combines the organic purity of water from deep wells in chalk formations with the freedom from inorganic matter displayed by the best upland surface waters.

5. In the annual report of the State Geologist of New Jersey for 1884 the application of the alum process for the purification of the new Brunswick city water was dealt with at some length by Professors PETER F. AUSTEN and FRANCIS A. WILBER. The Chemical News of May 22nd, 1885, (Vol. 51, p. 241) contained a reprint of this section of the report. The authors found that "by the addition of 2 grains of alum to the gallon water can be clarified by standing and that neither taste nor physiological properties will be imparted to it by this treatment. By increasing the amount of alum the time required for the separation and settling can be diminished, and vice versa, by diminish- ing the amount of alum added, a greater time will be required for the clarification." They also found that 1-2 grains to the gallon was practically the smallest amount that could be employed so as to cause a separation and settling of the suspended matter. They further proved on subjecting the deposit to ultimate analysis that a large amount of organic matter was precipitated along with the silica, alumnia, &c.

6. My own observations in the treatment of the Pokfulam water with alum are in the main iden- tical with those of Professors AUSTEN and WILBER in the case of the New Jersey water. In practice it has been found useful to employ 3 grains of alum to the gallon. Within an hour of the addition of the salt the water can be readily filtered perfectly clear; complete subsidence of the suspended matter can be effected with this quantity in two or three days according to the state of the water. When the water is excessively turbid the porportion of alum should be slightly increased.

7. For the Pharmacy and wards of the Hongkong Civil Hospital the water is treated in a tank of the capacity of 400 gallons. The supply-tap communicates with a flexible india-rubber tube which is attached to a float so as to allow the orifice to remain about 2 inches below the surface. In this way by drawing off the upper layer, instead of waiting for the whole of the suspended matter in the tank to subside, the water is available for use in a much shorter time.

I have never found it necessary to use alum in a larger proportion than 1 oz. to 100 gallons (4 grains to 1 gallon). On the direct testing of water so purified the presence of alum could not be detected with the reagents in ordinary use.