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tideway appears to be deteriorating. At the end of July when the weather was very
hot the condition of the water of the river made it necessary to treat the effluent with
ferrous sulphate. The weather conditions during August obviated the need for the
continuance of the treatment, which accordingly ceased on 22nd August.
Biological
treatment of
effluent.
Good progress is being made with the construction of the additional sewage
purification plant at the Northern Outfall, Beckton, which, it is hoped, will be
put into use during the coming summer.
This plant, designed on the experience gained during 10 years working of an
experimental plant at Crossness, provides for the treatment of settled sewage in
a series of aeration channels of novel type, followed by separation of the purified
liquid from sludge, and reactivation of sludge by air blowing through diffuser tiles.
The settled sewage will be taken from two of the existing precipitation channels,
which for this purpose have been divided about three quarters of the way along
their length by the construction across them of a weir chamber, and treated, with
the addition of free humus, in a series of eleven two-tier channels, each 300 feet
long. Circulation will be provided by motor-driven Hartley paddles fixed at each
end of each channel. In passage from inlet to outlet of a channel, the liquid
will flow six times from an upper to a lower, or from a lower to an upper compartment,
a roof, as it were, of fresh air being kept in the lower compartments. The capacity
of the aeration channels is 198,000 cubic feet.
The mixture of sludge and liquid will flow for separation into sedimentation
tanks, which are of two types:—
(a) Three tanks of total capacity of 99,000 cubic feet, 29 feet deep,
and each consisting of 4 inverted pyramids, arranged for central feed, peripheral
discharge of effluent, and gravitational discharge of sludge from the bottom.
(b) Three Dorr tanks of total capacity 97,000 cubic feet, maximum depth
10½ feet, in which the flow is through practically square tanks, with the
effluent discharge over a weir along the top of the side opposite to the inlet,
and the sludge induced to flow by a slowly rotating apparatus down the sloping
bottom of the tank to the centre, from whence it is discharged by gravitation.
The effluent from both sets of these tanks will be discharged through a
culvert passing under the existing precipitation channels, and mixed with the
ordinary effluent before discharge into the river. The humus, after discharge
from the settling tanks, will be pumped into 4 humus reactivation channels,
formed in the posterior portions of the existing Nos. 1 and 2 precipitation
channels beyond the newly formed weir chamber. These channels are each
about 290 feet long, and have a total capacity of 166,000 cubic feet. In each
channel there will be 3 rows of air diffuser tiles, each row being placed in the
bottom of a ridge running the length of the channel.
Sludge digestion.—Work on sludge digestion on a laboratory scale has been
carried out, mainly with a view to ascertaining the effect of temperature and of
stirring on the period of digestion, and the volume, composition and calorific value
of the gas evolved. Work at present in progress at a digestion temperature of
55°C has given variable results, but generally indicates a marked reduction in the
digestion period, without such an increased gas production as would make good the
additional heat requirements.
The uncertainty of making deductions of value on the large scale from these
experiments led to sanction for the construction of a large experimental sludge
digestion unit. This will consist of a deep circular, concrete tank, of about 30,000
gallons capacity, roofed in, and provided with hot-water pipes for heating, and
a stirring mechanism. Arrangements are made for sampling, and for temperature
observations, at varying depths at the sides and in the centre, as well as accurate
measurements of the influent and effluent sludge. Between the gas collector to the
gas holder, provision is made for condensers, flame trap, meters and calorimeter.