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74
made that the dosage should be increased to ensure a sufficient margin of safety.
As the results of slight deteriorations found in the quality of the water drawn from
service taps, the chief engineer was advised during the year that the tanks or reservoirs
at eight institutions should be cleaned and chlorinated, and after this treatment the
quality of the water again became of the highest class.
Sewage
treatment
The addition of ferric chloride to the sewage has been continued with advantage.
Chlorination in the sewers has been found to improve the condition of the sewage
and is being continued.
Three of the five activated sludge units under construction have been brought
into use with satisfactory results, and it is hoped that still further improvement
will ensue when the others commence work and will tend to restore the balance
between oxygen absorption and re-absorption at the critical times of low flow of
river water and high temperature.
The dry matter in individual cargoes of sewage sludge sent to sea from the
northern outfall varied from 539 per cent, to 7 88 per cent., the average being
6.91 per cent. In regard to the cargoes from the southern outfall, the variations
were from 4.7 to l0.1 per cent., with an average of 7.2 per cent., which are rather
higher than those of last year.
Vehicular
tunnels
The systematic investigations, commenced in 1928, as to the condition of the
air in Blackwall and Rotherhithe tunnels, were continued during 1937, 101 and
104 samples of air, respectively, from these tunnels having been examined for the
carbon monoxide content and 26 from each for black suspended matter. The increase
in motor traffic using the tunnels has continued at the same considerable
rate at which it has been occurring since this work was started. However, on the
whole the atmospheric conditions can be said to have been no worse than in the
previous year. The chief engineer, at present, has in hand works which will effect
a large increase in the ventilating capacity of the plant at Blackwall tunnel.
Atmospheric
pollution
The work on atmospheric pollution, which has been carried on systematically
for some years, has been continued. The methods of examination and number and
location of the observation stations remained as detailed in the report for last year,
and the results of much of the work have, as in the past, been communicated to
the Department of Scientific and Industrial Research ot' His Majesty's Government
for inclusion in its comprehensive annual report on this subject.
Cod-liver oil
In addition to its use in various preparations in the Council's hospitals, cod-liver
oil in conjunction with malt is largely used in the form of " cod-liver oil and malt
meals " for additional feeding of school children. In order that the assay of the
vitamin A content (this being the most important constituent) may be made by the
most accurate method, a quartz spectrograph has been added to the equipment
of the laboratory during the year, and a number of samples examined by means of
it. The results of all the tests have shown the supplies to be of satisfactory quality,
in fact considerably above the minimum standard required by the British pharmacopoeia.
Fire
extinguishers
At the request of the chief officer London Fire Brigade, an investigation was
carried out as to the relative toxicity of the products from the use of fire extinguishers
of the carbon dioxide, carbon tetrachloride, and methyl bromide types.
A " standard " fire was generated in a small room by the ignition of wood wool and
high tension cable. The fire was then extinguished by the use of one of the extinguishers
and samples of the atmosphere in the room at different levels collected at
once, and after an interval, by aspiration through glass gas tubes. The contents of the
tubes were then analysed to determine the proportion of various toxic gases present,
such as carbon monoxide, carbon dioxide, chlorine, bromine, hydrochloric acid,
hydrobromic acid, carbonyl chloride and carbonyl bromide. Taking into consideration
the results of the analyses and information gained from a research into the available
scientific literature on the toxicity of these substances, it was concluded that methyl