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acres for a designed capacity of 2.535 M.G.D. and this had to be formed at two levels with expensive retaining walls. This difficulty of finding suitable sites was no doubt the cause of the system of separate small filters spaced out along the conduit rather than one. single concentrated filtration plant. This, however, is now possible by the use of rapid gravity filters which have an output of 15,625 galls/sq. yd. per 24 hours with the result that all the Shing Mun water is filtered at one plant capable of treating 20 M.G.D., and now the Tytam Scheme is to be similarly centralised with the erection of an 11 M.G.D. plant at Eastern which is now under construction. This plant will occupy less than 0.3 acres as compared with a total of about 7.5 aces in the three plants it is to replace.
The use of pressure filters has been tried at only one small plant which supplies Stanley Peninsula. It has not been very satisfactory in service and as extensions are required it is to be replaced with rapid gravity filters.
All the rapid gravity filters in use are of the well known Paterson type, with air and water scour, chemical treatment consisting of alum and lime dosing to create a floc, with chlorination after filtration.
Table No. 5 gives details of the various filter plants from which it will be seen that the present working capacity is only 37.0 M.G.D.. As the summer peak demand has already reached 40.5 M.G.D. the need for more filtration plant is obvious.
The result is that it is very possible that restrictions will have to be introduced in Summer, not this time from a shortage of water but from inability to filter enough. Next Summer is likely to be critical in this respect, as the new Eastern Rapid Gravity Filters will not be ready in time. This year it was only possible because the peak demand did not exceed 40 M.G.D. for more than two days running, so that the extra could be made up by overloading the filters at the expense of efficiency, and by drawing on the Service Reservoirs. Alternatively of course a larger service reservoir capacity would enable higher isolated daily peak loads to be handled.
After filtration all water is chlorinated. Paterson chloronomes are installed at most of the filter beds, which provide a controlled amount of liquid chlorine at the outlet from the filters, but unfortun- ately supplies of chlorine gas are so poor due to shortage of the chlorine cylinders in England and most of the Waterworks' own cylinders having been lost during the war, that much of the chlorinat- ing is at present being done by adding a solution of chloride of lime from stand-by drip feed tanks, not a very satisfactory method.
However, the quality of the water has been maintained at a satisfactory level. The output from the filters is regularly tested and
So
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