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In view of the extreme importance of storage of the water this state of affairs is un- fortunate. It is only necessary to refer to the very valuable work of Dr. Houston, Director of Water Examination, Metropolitan Water Board (Report for month of December, 1910) on this subject to see how much a water loses in safety for the want of adequate storage.
After speaking about filtration Houston writes (p. 28) "Storage, on the other hand, rests on a different basis altogether, it relies not on purely mechanical proportional effects, but on the absolute extinction of the life of all the microbes of epidemic water-borne diseases. It is based on biological not merely mechanical principles, and it aims, with the aid of time and unfavourable surroundings, at effecting its object with a certainty inapplicable to any known filtration process.'
A further advantage of stored water is pointed out that when a new filter bed is started, taking several days to form a proper filtering layer, the filtered water cannot always be run to waste till the bed is working properly. If stored water is used this would not matter; on the other hand with unstored water, if contaminated, the results might be serious.
Houston also considers "it would be safer to use a coarse sand and filter rapidly when dealing with adequately stored water than to use a find sand and filter slowly when dealing with an inadequately stored water
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From his experiments on the vitality of the Typhoid Bacillus (First Research Report) Houston states in practically all the 18 experiments, 99.9% of the Typhoid Bacilli could not be recovered after one week "-" it is obvious that the advantages accruing from even a few days storage may be so material tha there is no justification for the use for filtra- tion purposes of raw unstored river water".
It is obvious of course that Hongkong does not derive its water supply from such a suspicious source as London does and that therefore storage is not such an essential condition. But if safety is to be aimed at, it appears that the fullest use should be made of all facilities for storing the water before filtration. It is especially undesirable that in the rainy season the water should be run direct on to the filter beds if this can be avoided.
In conclusion the chief advantages enumerated by Houston that are obtained by storage may be quoted here :-
"(1) Storage reduces the number of bacteria of all sorts.
(2) Storage reduces the number of bacteria capable of growing on agar at blood heat.
* (3) Storage reduces the number of bacteria capable of growing in a bile-salt medium at blood heat,
chiefly excremental bacteria.
"(4) Storage reduces the number of coli-like microbes.
*(5) Storage reduces the number of typical B. coli.
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(6) Storage alters certain bacteriological river water raties; for example, it reduces the number of typical B. coli to a proportionately greater extent than it reduces the number of bacteria of all sorts.
(7) Storage, if sufficiently prolonged, devitalises the microbes of water-borne disease (e.g., the typhoid
bacillus and the Cholera vibrio).
(8) Storage reduces the amount of suspended matter.
(9) Storage reduces the amount of colour.
(10) Storage reduces the amount of ammouiacal nitrogen.
(11) Storage reduces the amount of oxygen absorbed from permanganate.
' (12) Storage usually reduces the hardness and may reduce (or alter the quality of) the albuminoid
nitrogen,
(13) Storage alters certain chemical river water raties; for example, the colour results improve more
than the results yielded by the permanganate test.
(14) Storage has a marked "levelling" effect on the totality of water delivered to the filter beds.
(15) Storage tends generally to lengthen the life of the filters. (Only under exceptional conditions
is the converse true)
(16) An adequately stored water is to be regarded as a 'safe' water, and the safety change' which
has occurred in a stored water can be recognised by appropriate tests.
(17) The use of stored water enables a constant check to be maintained on the safety of London's
water autecedent to, and irrespective of, filtration.
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