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The sludge digestion plants at the Northern Outfall Works worked satisfactorily
and some 550,000 cu.ft. of the resultant gas per day containing about 70 per cent. methane
was used as a source of power at the works.
Regular sampling and examination was undertaken from all sections of the plant
and its ancillaries and the scientists and engineers were constantly in touch to consider
what modification in plant or its operation could be made to improve efficiency. Good
progress has been made on the new sedimentation tanks which will be in operation in
1955, and will further reduce the B.O.D. loading on river.
In addition to plant control work to advise the Chief Engineer, many research
problems were examined and brief details of a few examples are as follows :
(a) Tests were made at a 12 foot depth of water on the efficiency of aeration of
rectangular flow diffusers, 4 in. and 7 in. dome diffusers. It was found that at normal
operating pressures the 7 in. dome was the most efficient but at slightly greater
pressures the flat tile gave the best results.
(b) Experiments with synthetic detergents showed that they retarded the aeration
of water by diffusers, even in low concentrations of 1 to 10 parts per million. There
is a possibility therefore that they may retard the self-purification of a river by
reducing the oxygen uptake from the air and may also reduce the efficiency of the
activated sludge process.
(c) Work on de-watering of sludge at temperatures about 35°C. showed that
with the Northern Outfall sludges the separation of water was unreliable and the
cost not justified.
(d) Experiments made of outflowing sludge from the digestion plant showed
that the extension of the primary tank would not be a satisfactory financial proposition.
(e) Mixtures of gas liquor and sewage were purified in a laboratory plant with
activated sludge to establish the ratio of the relative difficulty of the oxidation of
sewage and the particular gas effluent received. This work is continuing and the
results will be of financial importance and have a bearing on river pollution considerations.
(J) Research has continued on the estimation of sulphide in river water, especially
when small amounts of both oxygen and sulphide are present together, as has been
shown to occur.
Trade wastes
The London County Council General Powers Act, 1953, which came into force on
1st April, 1954, enables the Council to exercise a closer control over the discharge of
trade wastes into the London sewerage system. Industrial discharges have increased in
volume and complexity over the past few years in view of the many new processes
using a wider range of chemicals than formerly. The analysis of samples frequently
presents difficulty to ensure that
(i) the matter is neither, alone or in combination with the contents of the sewers,
dangerous, a cause of a nuisance or injurious to the health of persons entering the
sewer
(ii) the matter will not injure the sewer, interfere with the free flow of the
contents of the sewer or be likely to be prejudicial to the treatment and disposal by
the Council
(iii) the material does not contravene any act, order or agreement in force.
Cases have been examined where a discharge of synthetic resin adhesive completely
blocked a local pipe sewer and also where a manufacturer wished to discharge large
quantities of the powerful sterilising agent phenyl mercuric acetate. These examples
show the diversity of the problems presented.
Safety
Regular examination of inflammable gas detector lamps of the expanding metal spiral
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