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54
which has been the case in former years when such examination has been made,
there was a progressive deterioration from Putney to Isleworth, due to the effect
of pollution from sources other than those affecting the lower reaches.
During the last ten years districts in the Thames valley, which had been thinly
populated, have been developed either industrially or as residential suburbs, and,
in consequence, the volume of sewage effluent reaching the river either directly or
via its tributaries has increased, probably without adequate increase of means
of treatment. The effects of such increase would naturally be more evident in a
season like the summer of 1929 than in some other recent summers, and the condition
recently observed is attributable to the combination of the two causes, a greater
volume of pollution and a less volume of good river water from the upper reaches.
It is also obvious that an effect of low fresh water flow is to allow the travel
upwards of water which has passed the Council's outfalls, and is to some extent
charged with effluent from these places; but the point must be made that the
progressive improvement which would be observed on travelling up-stream if this
were the only cause, has not been observed and the facts point to appreciable local
pollution. Not only has the upper tideway been found to be poorly aerated, but
positive evidence, chemical and bacteriological, of pollution has been more marked
in these regions than in some places nearer the Council's outfalls.
River subway
tunnels.
In connection with improvements in ventilation of the river subway tunnels
at Blackwall and Rotherhithe, a systematic examination of the air has been made
over a long period. Determinations of carbon monoxide, of black dust (soot) by
Owen's air filter and of humidity by a recording hygrometer have been made in this
investigation. Some determinations of sulphur dioxide were also made by Mr.
B. A. Ellis, M.A., A.I.C., of the Government Laboratory, by the kind permission
of Sir Robert Robertson, K.B.E.
It appears that conditions in the tunnels are more variable in summer than
in winter owing to the fact that their natural ventilation is greatly affected by the
temperature of the open air. Where this is higher than the temperature of the air
in the tunnel, the cooler polluted air below does not rise through the shafts. Under
these conditions, the humidity of the tunnel air also becomes greater than that of
the outside air. As standards of maximum pollution that should be permitted
under normal traffic conditions, it was recommended that the carbon monoxide
content of the tunnel air should not be allowed to exceed two parts per 10,000,
and the suspended matter six units on Owen's fog shade scale (i.e., 1.92 milligrammes
suspended matter per cubic meter of air). As a result of the series of experiments,
it appeared that, generally speaking, in neither tunnel was the proportion of carbon
monoxide excessive under normal conditions. Rotherhithe was worse than Blackwall,
but only comparatively few samples in either case exceeded the suggested limiting
figure. The results of the Owen's air filter showed that normally, as regards suspended
matter, the worst conditions occur about 10 a.m. on weekdays, and from the hygrometer
figures it appeared that the fogs occurring in the tunnels were "smoke fogs"
and not "water vapour" fogs. The conclusion was also reached that these smoke
fogs might become the limiting factor for safety as soon as or sooner than the carbon
monoxide content, as the suggested maximum suspended matter figure was frequently
exceeded. The additions made to the ventilating plant at Blackwall while
this investigation was in progress considerably improved the atmospheric conditions,
particularly at the south end ; but it appeared that still further improvement
in ventilation was necessary in both tunnels. It was also recommended that
an alternative source of power supply should be provided for the ventilating fans
at each tunnel, for on the occasion of two current failures at Rotherhithe, the tunnel
air was found to be extremely bad.
Biological
treatment of
sewage.
A series of comparative experiments on the treatment in two forms of double
channel plant of the effluent obtained by sedimenting sewage have shown that the