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32
producing well over half a million cubic feet of sludge gas per day. This effected a
considerable economy of other fuel when used for power production on the works.
In the programme of extensions is also included a major sludge digestion plant to deal
with the whole of the primary sedimentation sludge produced, and when this is complete
some years hence the works should be practically self-supporting from the power
production aspect.
At the Southern Outfall Works the programme of new works includes a diffused
air activated sludge plant to treat 60 million gallons per day of primary sedimentation
effluent—from the total flow of about 100 million gallons per day—and a sludge digestion
plant to deal with all the primary sedimentation sludge, which in this case also
should make the works practically self-supporting from the power production aspect.
Research continued on the new process discovered by officers of the Chemical
Branch for dewatering sludge by slow freezing with the addition of chemicals. The
pilot plant to deal with 1 ton per day was utilised and the filtration problem was studied.
In co-operation with a firm of refrigerating engineers and officers of the Chief Engineer's
Department a design study was made to assess the economic aspects of the process. In
view of its high value as a fertiliser, special attention was given to the question of the
recovery of activated sludge by this process. Sludges from other sources outside the
Council's service were examined and, in this respect, one from an Atomic Energy
Research Establishment should be specially mentioned since this technique was
adjudged to have solved a very difficult problem.
Research work was also done on many other matters during the year, for instance
on the operation of a new proprietary type of heater which was tried out in the pilot
sludge digestion plant with promising results.
The inauguration of the 'old reservoir' sludge digestion plant entailed much work,
for it was essential, from the safety aspect, to ensure that the air purging had been
satisfactorily accomplished.
The contents of synthetic detergents in sewage and at various stages of the treatment
process were determined in connection with the proceedings of a Government Committee
dealing with this matter.
Work was also done on the relative efficiencies of the standard type of plate air
diffuser in an activated sludge plant and the new type of dome diffuser. Certain potential
disadvantages of the latter became apparent though it has advantages from the aspect of
simplicity of maintenance.
In view of the high nitrogen content of activated sludge (6 to 7 per cent. on a dry
matter basis) research was initiated during the year—and was still continuing at the end
of it—on the availability of this nitrogen. It appeared that it was relatively very high,
being of the same order as that of dried blood, which is generally regarded as the greatest
of the ordinary fertilisers in general use. It must be remembered that it is mostly of
protein origin being derived from protozoa, etc.
Throughout the year weekly conferences continued to be held at County Hall between
the Divisional Engineer (Main Drainage) and the Chemist-in-Chief, and their respective
senior officers to consider the current analytical results and all aspects of the sewage
treatment problem, and to decide on any variation of the operational practice which
seemed desirable. These meetings were of the utmost value and helped greatly to
maintain the cordial spirit of co-operation which exists in this joint service. At both
Outfall Works, too, the collaboration continued on a most friendly basis between the
officers of the two departments concerned.
Some 14 premises outside the County which are maintained by the Council have
individual small sewage treatment plants. The effluents from these were examined
periodically in order to ensure that efficient purification had been achieved.