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34
As mentioned in a previous annual report, in an endeavour to improve the
condition of the water of the River Thames a programme of additions to the existing
sewage treatment works has been agreed by the Council. At the end of the year the
completion of the new detritus pits at the Northern Outfall Works was approaching,
and progress had been made there on the construction of the new sedimentation tanks.
Progress continued, too, with the construction of the "old reservoir" sludge digestion
plant.
The other works approved in principle by the Council comprise additional 60
million gallons per day diffused air activated sludge plants at both the Northern and
the Southern Outfall Works and, also at both works, sludge digestion plants to deal
with the whole of the sludge produced. None of these had been started at the end
of the year owing to the fact that they had not been licensed for the supply of materials
by the Ministry of Housing and Local Government.
In view of the delay in the construction of these works and the bad condition of
the water of the River Thames in the previous summer, consideration was given to
any temporary palliative measures which could be adopted to prevent a still further
deterioration in the state of the river. It appeared that the chlorination of some of
the sewage effluent was the only practicable course and it was decided to chlorinate
that part of the primary sedimentation effluent from the Northern Outfall Works
which was not subsequently submitted to secondary treatment. The first necessary
action was to divert the waste gas effluent liquor to flow through the existing activated
sludge plant in order to prevent it preferentially utilising the chlorine which was to be
added to the rest of the flow. The average chlorine demand of the latter was then determined,
it being about 40 parts per million for a flow of about 140 million gallons
per day. Treating this to about one-third of its demand thus required about 50 tons of
chlorine per week. This full degree of treatment was not attained until the latter
part of the summer.
Natural oxidation by the dissolved oxygen in the river water is the final process
in the purification of the effluents from the sewage treatment works. In this process,
the dissolved oxygen in the water is used up and, in order to prevent the water, after
admixture with the sewage effluent, becoming putrescent, the rate of this using-up
of dissolved oxygen must be less than the rate of reabsorption of oxygen from the air
at the surface or air/water interface, so that there is at all times some oxygen present
in the water.
Hence the most important single test to indicate the condition of a river water
is the dissolved oxygen content; similarly, the B.O.D. (biochemical oxygen demand)
of an effluent is of significance as indicating the "oxygen load" imposed on the river
by that effluent. It follows that, for the river to be maintained in a satisfactory
condition, sewage must be subjected to a sufficient amount of treatment to ensure
that the oxygen demand of the effluent will be of such magnitude that it will not
exhaust the dissolved oxygen in the receiving water.
The chemical state of the water of the river is thus of great concern to the
Council and a weekly examination of it is therefore made at 25 points between
Teddington Weir and the sludge dumping area in the estuary. In addition, the water
off the two outfall works is sampled daily at both high and low tide.
An important factor in determining the pollution load with which the water of a
river will deal satisfactorily is the fresh water flow; in the case of the River Thames,
this is the amount coming over Teddington Weir after the Metropolitan Water Board
has, above that point, abstracted the quantity required for water supply. The
statutory minimum flow is 170 million gallons per day, but the appropriate Minister
can, by Order, permit the reduction of this to 50 million gallons per day. The summer
of 1951, particularly the latter part of it, was wet and the rainfall maintained the
flow at a higher level than that of 1950 and of the dry year of 1949. In spite of this
fact and the chlorination of part of the effluent as already described, the condition of
the water of the river, particularly in the area between London Bridge and Gravesend,