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bacilli may serve for this purpose in man, and because there is no near prospect that this information
can be obtained.
Storage and filtration.
But while the Royal Commission appears to be disposed to attach more value than I do to the
conditions I have mentioned as affording safety to the London population, it evidently regards as
essential to this safety " that there is adequate storage and that the water be efficiently filtered before
delivery to the cousumer." Adequate storage is not absolutely defined by the Commission, beyond that
it is to mean that the storage is sufficient to obviate the necessity of taking in turbid storm water and
to allow of due settlement.
" Efficient filtration" is still less defined by the Commission, and if we are to assume, as I
think we must from the evidence given before the Commission and the statements made in the report,
that filtration of this kind is to be regarded as essential to the safety of the London population, no
uncertainty ought to attach to the meaning of this expression.
The Commission thus refers to the action of filter-beds : " The action of a filter-bed appears to
be partly mechanical, partly vital, but the mechanical action which is confined, or almost confined, to
the holding back of the comparatively grosser substances suspended in the water, and which was
supposed until recently to be the only operation in a filter, is now held to be of far less importance
than the vital action which depends on the activities of the gelatinous layer of living matter gradually
deposited on its surface. A new filter composed of perfectly purified sand has no effect in producing
either chemical or bacteriological purification, but in course of use a layer charged with living microbes
is deposited upoh the surface, and it is by these organisms which constantly increase in number, and
also penetrate the sand to a slight distance, that both the nitrification of organic matter, and the
arrest of other microbes is effected. 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 membrane is removed, should only be carried out when the filter has
become unduly blocked."
During the present year, Koch, writing on water filtration and cholera,* says "In the process of
filtration the important point is first that a proper layer of slime should be formed, and that it should
not be disturbed during the process of filtration, and that when by further continuous deposit it becomes
too thick, and therefore to impermeable to water, it should be removed." Further he says that " it
has also been discovered that in the gradual wearing out of the sand layer it should never be allowed
to get below a certain thickness, about 30 centimetres, and that a certain speed, about 100 millimetres
in the hour, must be allowed for the movement of the water through the sand layer to obtain the most
perfect purification."
Koch's definition of efficient filtration may be gathered from the following. " If a filter works
satisfactorily in every respect, there will be found, as experience shows, less than 100 germs capable of
developing in one cubic centimetre of filtered water; and this is independent of the amount of bacteria
contained in the natural water, whether such amount should be 100,000 or only a few hundred in a
centimetre. But the slightest disturbance in the process of filtration, for instance, the quickening of
the rate of filtration to over 100 millimetres, disturbances of the surface, mud covering, &c., lead as a
consequence to an immediate increase of germs in the filtered water," and again " water must be
allowed to rest at least four-and-twenty hours over the sand in order to enable the filtering mud layer
to be formed."
In another part of this paper he says—
" But I consider it to be very probable that, according to the investigations of Fraenkel and
Piefke, if the pace of filtration is 100 millimetres the bacteria of cholera cannot be completely kept
back by the sand filters."
Beyond this, Koch has shown how the freezing of the sand layer during cleansing of the filter
and the removal of ice from the surface of the filter, as well as the growth of microscopic plants in
periods of the summer, interfere with the process of filtration.
Although the advantage of slow filtration compared with rapid filtration has been for some time
understood, there has been no such precise definition of the conditions required for efficient filtration.
It is obvious that to comply with the requirements of Koch, there is needed not only an extensive
provision of filter beds, but knowledge and skill as to their employment and the constant application of
biological tests as to their efficiency. The Local Government Board, on 31st August, 1892, having
regard to the prevalence of cholera on the continent and its possible spread to this country, addressed
letters to the various companies supplied from the Thames and Lea, calling attention to the special
necessity for efficient filtration, and recommending, inter alia, that the rate of filtration should be as
slow as possible consistent with the required supply of water. It is evident that in future there should
be no need for any qualification such as that introduced into the recommendation ; that the rate of
filtration should be governed solely by the consideration of efficiency, and that the conditions of filtration
should be strictly regulated so as to secure uninterruptedly a result which should be precisely defined.
Evidence of experience.
Considering now the evidence of experience, it must be fully recognised that the death rate of the
London populatiou from typhoid fever is low, lower indeed than that of many other towns receiving a
water supply not exposed to excremental pollution, and this is equally true, whether the death rate of
London as a whole be considered or whether the rates of those parts supplied from different sources be
considered separately. This of course does not absolutely negative the possibility that water has caused
typhoid fever in London, but it is, so far as it goes, favourable to the London water companies.
* Water filtration and cholera by Professor Koch, The Practitioner, Vol. 41, No. II.