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removal of solids, the effluent is of appreciably poorer quality than that from the new
plant. The very substantial improvement in the primary sedimentation effluent since the
installation of the new tanks has been continued, a slight increase in the primary effluent
suspended solids load reflected the incidence of increased flows of slightly stronger sewage.
The percentage removal of suspended solids was similar to that in the previous year and
there was an appreciable increase in the percentage of the biochemical oxygen demand
load which was removed.
Secondary treatment—The paddle-type activated sludge plant continued to operate
throughout the year. The influent to this plant was supplied from the old sedimentation
channels and was of much greater suspended solids content and strength than in the
previous three years; this caused some deterioration in the quality of the effluent from the
paddle plant, although the total removal of pollution load was greatly increased. For most
of the year the activated sludge solids concentration in the mixed liquor was appreciably
greater than the average and this increase in sludge solids was probably largely responsible
for the increased removal of polluting material from the influent.
At the beginning of the year two units of the diffused air plant, which had been brought
into service in September, 1959, were in operation. Other units were added at short intervals
and by the middle of February the whole installation of six units was in use. Few difficulties
were encountered in operating this plant and the results over the whole year were very
satisfactory. The activated sludge was generally of very satisfactory settling characteristics,
of good colour and smell, and contained good numbers of representative activated sludge
organisms.
Sludge digestion plant—The new sludge digestion plant was in full operation by the
second quarter of the year; each of the 16 primary digestion tanks treating approximately
200 tons of mixed crude and activated sludge daily. The starting up of the plant occasioned
little difficulty, partly because the use of stored seeding sludge on a large scale and the
daily addition of only small amounts of fresh sludge avoided the occurrence of an acid
phase of digestion. In the early weeks of operation gas production was unusually high in
relation to the charges as a result of gas evolution from the seeding sludge, but a short
time after all the primary digestion tanks had been in service under normal operating
conditions the gas production stabilised at about 2.5 million cubic feet per day but occasionally
reached nearly 3 million cubic feet. This performance, except during periods of temporary
reduction of charges, was well maintained during the year.
The results from this plant differed appreciably from those of the earlier plants; the
total solids content of the ingoing sludge was appreciably less and the organic content
considerably greater, because of the increased proportion of activated sludge in the daily
charges, and the organic content of the digested sludge was correspondingly greater. The
new digestion plant appeared to be very efficient; after conditions had stabilised the volume
of gas produced per pound of organic matter digested was about 25 per cent, greater than
in the original pilot plant and over twice as great as in the old reservoir plant, although
the carbon dioxide content of the sludge gas showed a slight increase.
Regular daily gas safety tests were carried out at all potentially hazardous points at the
sludge digestion plant and in the new power house. These tests, for which a portable
detecting device was used, gave early warning of small leaks and enabled them to be
corrected before they became dangerous. Special investigations on the sludge digestion
plant and the power house, connected with problems of safety, corrosion, foaming and on
the composition of the gas used in the turbines, were undertaken as required.
Effluent liquors from the North Thames Gas Board—The daily examination of effluent
liquors discharged into sewers from the Beckton Products Works of the North Thames
Gas Board was continued.
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