Sessional_Paper_1892 — Page 129

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A REPORT ON THE “WHITISH APPEARANCE" OF THE TAITAM

AND POKFULAM WATERS.

1. PRELIMINARY:-The hills forming the watersheds of Taitam and Pokfulam consist chiefly of Granite and Syenite. Both are highly felspathic rocks, more or less weathered, disintegration having occurred in some places to such an extent as to form clayey beds of laterite with boulders of unaltered rock lying in them.

2. The water draining from the slopes at these hills constitutes the chief supply of the city, and although in some respects it answers to the description of an "upland surface water," yet it must be taken into consideration that several of the more important streamlets are fed by springs which exist even in seasons of extreme drought,

3. Except during and immediately after heavy rains the water in these streams is clear, containing less than 5 grains of total solids per gallon, but during and after rainstorms, the waters are rendered turbid from the presence of suspended matter, derived chiefly from the disintegrated granite, to which the reddish colour is due. This undesirable feature is much aggravated wherever the hill sides have been cut into by road making or building operations. Recently this has been partially remedied by turfing over the exposed surfaces, but I doubt if it is now possible, to avoid the deteriorating effect, due to the exposed surfaces of roads. I suggest that no further excavations of any kind should be permitted within the drainage areas without the strictest conditions being enforced, and as far as possible all exposed surfaces turfed.

4. FILTRATION :-The turbid water that comes down during heavy rains is extremely difficult to filter. It frequently contains as much as 10 grains of finely divided suspended matter in each gallon. At Pokfulam the immediate filtering of water in such a condition is obviated very simply. After a rainstorm the muddy water is allowed to flow directly into the reservoir, where it is immediately discharged by the scouring-out pipe, or undergoes a partial subsidence before being drawn off. soon however as the principal stream has cleared itself, it is intercepted, and conducted by pipes laid along the bank of the reservoir into the conduit direct. The results of such an arrangement are very apparent. The filtering beds receiving comparatively clear water are not clogged up so quickly, and do not require such frequent cleaning. They are also better able to exercise their purifying and oxidising functions. This is a most important point and should not be overlooked, for if a filter bed has had an almost impermeable layer of clayey material deposited on its surface, it is evident that greater pressure must be brought to bear to force the water through if the normal rate of filtration is to be maintained. This tends to drive fine particles for some distance into the filtering medium, blocking up the interstices, and preventing the efficient working of the filter.

5. Could the same system be carried out at Taitam, comparatively clear water being led directly to the conduit, a great improvement would be effected. May the 19th last is an instance when after an exceedingly heavy rainfall following a long period of dry weather, the water issuing from the Pokfulam Conduit, contained less than 3 grains per gallon of total solids, while that delivered from Taitam contained 13 grains per gallon. It will be observed that the difference in total solids between these two is 10 grains, chiefly due to matter in suspension, then, supposing two million gallons pass through the beds daily, over one ton per day of suspended matter has to be removed, and this ton of material remains as a gelatinous scum on the surface of the filters.

6. The sand in use at either works does not seem to be very suitable. Being collected from the sea shore, it contains a great deal of broken shell, which I do not think can be so good a filtering medium as a sand consisting chiefly of quartz grains, broken down sandstone, &c.

7. It is owing to the composition of the sand that the water is somewhat harder after filtration than it was before. This is no doubt caused by a little Bicarbonate of lime being taken into solution from the action of the Carbonic Acid in the water on the broken shell contained in the sand. This increase in hardness in no way impairs the dietetic value of the water, but is rather an advantage as the water is naturally so very soft.

8. STORAGE: The storage of the water after filtration is a matter of great importance, especially in the immediate vicinity of towns. The principal companies in England store all their filtered water in covered reservoirs. In a tropical climate the advantages to be derived from following a similar plau would appear to be manifold. At the Pokfulam Filtering Station the filtered water is so stored in a covered tank, where being protected from the direct rays of the sun and all external influences, it is preserved cool and clean, and suffers little if any deterioration.

9. At the Albany on the contrary, the service tank is open, and in an exposed situation, consequently the water gets heated, loses a part of its dissolved gases, and rapidly. becomes flat and vapid. Besides this the surface of the water in the tank never appears to be clean, always containing more or less floating debris, which under the existing conditions cannot possibly be kept out, and probably accounts for the slightly larger amount of Ammonia in excess of the Pokfulam supply.