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The plant for the other two processes forming part of the general investigation
namely, treatment of mixtures of sewage and activated sludge by mechanical agitation
and by spraying, has been designed and constructed. It is expected that during
the current year these processes will be in continuous action alongside the plant
already referred to.
In connection with the examination of the water of the river Thames, the
seasonal prevalence of super-saturation with dissolved oxygen of the water of the
estuary of that river has formed the subject of a paper in the Biochemical Journal
by the Medical Officer (General Purposes) and the Chemist, in which the phenomena
and their causes are discussed fully. It is shown that a marked, sometimes very
great, excess of oxygen in solution is observed annually in the Spring. Theoretical
considerations and evidence based on observations, on the prevalence of green
growths in the water and determinations of hydrion concentration, point to the
conclusion that photosynthesis is mainly responsible for the seasonal excesses of
dissolved oxygen. Besides indicating the source of the excess of oxygen, the authors
show, by argument based on the known conditions of heterogeneous equilibrium
and by actual experiments with waters containing upwards of three times the proportion
of oxygen proper to water saturated with atmospheric air, that even with
fairly vigorous agitation, supersaturation persists for some time and, in still
water, for a very long time. These experimental results were consistent with the
observation that supersaturation was most often associated with quiet atmospheric
conditions.
This phenomenon of a seasonal supersaturation by oxygen of the water of the
Thames Estuary was an unexpected discovery, as a result of the systematic examination
which has been carried on for many years in connection with the observations
of the results of sewage effluent entering the Thames. Were no unoxidised matter
to enter the tidal waters of the Thames there would, during the period of seasonal
prevalence, continuously flow through the heart of London water surcharged with
oxygen, brought up from the sea on the flood and from the upper river on the ebb.
In its passage this supersaturated water would, as a result of the agitation
associated with its flow, set free this newly-formed oxygen which, by the mechanical
action of the tides, would be forced into the streets in the neighbourhood of the
river.
It has been found that considerable shoaling has taken place in the Edinburgh
Channel in the estuary, and this shoaling has been attributed to the practice of
depositing dredged material, sludge, etc., in the Black Deep. The Board of Trade,
in informing the Council of this, stated that it had been decided to close the Black
Deep area and, as a temporary measure, to open a new deposit area in the Middle
Deep. A conference was subsequently called by the Board of Trade, at which
the Council, the Admiralty, the Corporation of Trinity House, the Port of London
Authority and the Ministry of Agriculture and Fisheries were represented. The
Medical Officer (General Purposes) attended this conference, at which it was agreed
that the best solution practicable was to move the present area in the Black Deep
about one mile N.E. of its existing position. Action has now been taken on this
decision.
The condition of the river generally during the year showed the lasting effects
of the great drought of 1921. During the winter period, November, 1921-April,
1922 inclusive, a stretch of water observed to be over 32 miles, probably about
35 miles in length, extending up to Waterloo Bridge or higher, contained on the
average less than 50 per cent, of the dissolved oxygen required for saturation. In
the summer period, May to October, 1922, a similar stretch of water extended
nearly as far downwards and certainly much further towards Teddington.*
* It was not until well into 1923 that increased fresh water flow materially shortened, by dilution
and propulsion, this region of greatly reduced aeration and dispersed a smaller region where the average
dissolved oxygen content was 10 per cent. or less of that required for saturation.
River
Thames.