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Major-General Scott adds that some of the companies, in addition to subsiding reservoirs and
filters of the ordinary type, have constructed works which enable them to derive a supply of water from the
beds of sand and gravel which extend over the Thames valley in the neighbourhood of the intakes, and
in some cases to utilise these beds as filters. He refers to the fact that the Grand Junction Company
use six acres of filters of a rough character constructed of sand in its natural condition, and
used for preliminary filtration, which is succeeded by filtration through washed sand and gravel; and that
the East London, the Grand Junction, the Southwark and Vauxhall and the Lambeth Companies collect
from the gravel subsoil, water which is pumped through the ordinary filters before filtration. The New
River Company possess 13 wells in the chalk, the water of which is mixed with that drawn from the Lee,
and the whole is filtered.
Experience has increasingly demonstrated the value of bacterial examination of water as a test
of efficiency of filtration, and Dr. Frankland's report gives account of the results obtained by such
examination of the waters supplied by the several companies. In the first four months of the year
the samples were collected at the stand-pipes in London, during the remaining months at the works of
the respective companies immediately after the water left the filters, and before it was pumped into the
distributing mains. The tubes containing water collected at the works were hermetically sealed and
packed in ice. The number of microbe colonies developed from one cubic centimetre of each water
examined during the last eight months of the year ranged between: Chelsea, 3 and 16; West Middlesex,
3 and 32; Southwark, filter No. 1, 4 and 148, filter No. 2, 2 and 292, filter No. 3, 10 and 996, gravel
water, 6 and 168; Grand Junction, 10 and 236; Lambeth, 4 and 138; New River, 3 and 140; East
London, 4 and 134; Kent, 1 and 19. These maxima were, however, exceptional, and the examinations
generally showed a number of microbe colonies more nearly approaching the minima.
Section 4 of the Metropolis Water Act, 1852, provides that "From and after the thirty-first day
of December, 1855, every company shall effectually filter all water supplied by them within the
metropolis for domestic use, before the same shall pass into the pipes for distribution, excepting any
water which may be pumped from wells into a covered reservoir or aqueduct, without exposure to the
atmosphere, and which shall not be afterwards mixed with unfiltered water." It has hitherto been held
that this requirement is complied with if the water is free from suspended matter, and necessarily the
water supplied has not been judged by any bacterial standard. The need for some authoritative definition
of the term " effectually filter " is obvious.
Reference to the table extracted from Major General Scott's report shows the great difference in
the provision made by the several companies for securing the supply to the consumers of effectually
filtered water. The deficiencies of the several companies as regards purification works has been
frequently pointed out by the water examiner. In the absence of any statutory definition of efficient
filtration, the following statement by Professor Koch is of interest—
"(1) The pace of filtration must not exceed 100 mm. in the hour. To make sure of this
each separate filter must be provided with a contrivance by which the movement of water in
the filter can be restricted to a certain pace, and continually regulated so as to keep that pace.
"(2) Each separate filtering basin must, when in use, be bacteriologically investigated once
each day. There should therefore be a contrivance enabling samples of water to be taken
immediately after they have passed the filter.
"(3) Filtered water containing more than 100 germs capable of development in a cubic
centimetre should not be allowed to reach the pure water reservoir. The filter should
therefore so be constructed that insufficiently purified water can be removed without its mixing
with the good filtered water. To these rules I have still to add some observations. In strictness
the two last rules would suffice to remove the danger of infection from filtered water so tar as
is possible to do so in the case of filtration through sand ; but I think it is questionable whether
it is necessary always to insist upon the rule of a daily bacteriological examination of each
separate filter. If waterworks by their good construction and their proper and intelligent
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