256
( 14 )
It is therefore evident that the existing works in the Pokfoolum and Taitam valleys are sufficient to secure an average daily supply of 2,700,000 gallons.
109. Conduits.-The Pokfoolum conduit, as previously stated, is capable of conveying to the Pokfoolum filter beds 2,000,000 gallons per diem, and the Taitam conduit is capable of discharging at the Albany filter beds, 7,000,000 gallons per diem, quantities greatly in excess of the average daily supply obtainable from the respective catchment areas and other areas in their immediate vicinity."
FILTRATION OF WATER.
110. Filtration of Water.-Having dealt with the yield of the Catchment Areas, and the capacity of the Reservoirs and Conduits, it is necessary now to consider the subject of the filtration of water previous to its being delivered into the distribution system.
On reference to the report of Mr. E. W. LUCAS, Acting Government Analyst, accompanying my annual report for the year 1891, it will be found that, during heavy rains, the water derived from the catchment areas is very turbid, and contains as much as 10 grains per gallon of finely divided suspended matter.
111. Works have been completed both at Pokfoolum and Taitam to avoid the immediate filtration of water in this condition by the construction of bye-passes intercepting the streams at the heads of the reservoirs which run comparatively clear long before the matter in suspension in the water in the reservoir has settled. These works therefore admit of the clear water being intercepted direct from the streams and dispense with the necessity of drawing very turbid water from the reservoirs; thus to a large extent relieving the filter beds.
112. The filtration of water in connection with public supplies has received a good deal of attention of late, and the supposition that the action of sand filters properly constructed and used is confined or1 almost confined to the holding back of comparatively the grosser substances suspended in the water appears in the light of recent investigation to be erroneous, and that this mechanical action is of far less importance in the purification of water than the vital action which depends on the activities of the gelatinous layer of living matter gradually deposited on the surface of the filtering medium.
"Thus the longer a filter has been in use the more efficient it becomes, provided, of course, that the surface layer has not acquired such density as to interfere with the passage of the water; and, consequently, the recommendation which was commonly given in former times that a filter bed should be cleansed as often as possible, appears to have been a mistake; cleansing, by which the efficient superficial mem- brane is removed, should only be carried out when the filter has become unduly blocked." (Report of Royal Commission on Metropolitan Water Supply 1893.)
It is therefore a matter of importance that, after the cleansing of a filter, time should be given for the formation of the gelatinous layer, and this can be done by allowing the water to stand on the filter bed for some time before filtration is actually commenced. The rate of filtration should be uniform and not too rapid, care being taken not to disturb the gelatinous membrane.
As a general rule, it may be taken that the sand composing a filter should never be less than 1'.3" in thickness, and the rate of filtration should not exceed 500 gallons per square yard of filter area per 24 hours.
113. The Pokfoolum filter beds are four in number having a combined area of 1,310 square, yards, and the Albany filter beds are six in number having a combined area of 3,245 square yards, giving a total area of 4,555 square yards; from this, however, must be deducted about 10 per cent, to admit of their being periodically cleansed without interruption of the water supply, giving a net available area of, say, 4,000 square yards. Taking 600 gallons per square yard per 24 hours as the maximum rate of filtration admissible, we get 2,400,000 gallons per diem as the quantity of water these filters are capable of filtering.
The existing filters are therefore insufficient for the effectual filtration of the whole of the water obtainable from the existing works in the Pokfoolum and Taitam valleys.
DISTRIBUTION OF WATER WITHIN THE CITY AND HILL DISTRICTS.
114. On the completion of the works now in progress, viz., the construction of service reservoirs at Kennedy Town and Wanchai, the laying of new mains along Wyndham Street, Aberdeen Street, Staunton Street, Wing Wah Lane and a portion of Queen's Road West, the project referred to in paragraph No. 52 will have been completed.
115. It is, however, necessary to lay new mains in Caine, Robinson and Bonham Roads, the existing mains which were laid many years ago being inadequate to meet the requirements of the districts.
116. Further, the condition of No. 2 Tank constructed in 1860 is very unsatisfactory, only a few feet of water can be put into it without involving a serious loss by leakage.
This tank is very conveniently situated immediately above the central portion of the Chinese quarter of the City and is at sufficiently low a level to admit of being supplied by the exhaust from the Garden and Arbuthnot Road motors.
117. The cost of laying the above mains and making good No. 2 Tank is estimated at $39,000, and on the completion of these works the distribution works will be found ample to meet all reason-, able requirements within the present built area of the City for some years to come.