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The old works are to be retained in use, with the new works for
trade waste and domestic sewage treatment located on each side.
Pumping is necessary throughout, the lifts ranging between 18 to
34 feet. At the old works, screened sewage is received into a
sump and lifted. The first stage of treatment is in cone bottom
detritus tanks, proceeding into " downward " and " upward
flow chambers, and thence into four rectangular sedimentation
tanks, 100 ft. long by 30 ft. wide and 71/2 ft. deep. After sedimentation
the effluent either runs off or is retained in rectangular tidal
tanks until the head allows of discharge against the tide level.
Penstocks and flap valves prevent backflow from the river. Sludge
from the tanks gravitates to a sludge well, whence two 100 gallon
Shone ejectors propel it through pipes for disposal on land by
ploughing in.
The extensions for treatment of domestic sewage have been
designed to deal with an estimated flow of 3 m.g.p.d. and a prospective
flow of 41/2 m.g.p.d. After passing through an Airdrie type
mechanical screen of 1/2 inch bar spacing, the sewage is lifted and
conducted into a circular sedimentation tank of the Pruss type
providing 3 hours detention. Following this, the sewage is to be
aerated in Simplex tanks providing 4 hours detention plus 71/2 per
cent, of activated sludge. The purification being partial, provision
is made for the re-aeration of the activated sludge in diffused air
ridge and furrow tanks. These provide 14 hours detention and a
25 per cent, concentration of sludge. The activated sludge is settled
out in cone bottom tanks which provide 3 hours detention and the
effluent run off to a pumping station from where it is discharged.
The domestic sewage has a strength on oxygen absorbed of 14 parts
per 100,000. After sedimentation the figure is 7, whilst after
aeration, it becomes about 2. This shows a purification of nearly
86 per cent, on raw sewage and 71 per cent, on sedimented liquor.
It is proposed to treat the sludge in a Dorr Digester with a
capacity of 1.33 cubic foot per inhabitant. If the sludge is sufficiently
digested, it will be dried in lagoons ; if imperfectly digested,
lxtrtions of the lagoons will be converted into secondary digestion
tanks where the digestion process may be completed.
For treatment of trade waste, the 42 inch trade sewer discharges
into the old screen pit and pump well. It is proposed to
allow tor sedimentation in Dorr Clarifiers for a volume of 4 m.g.p.d.
with a detention of 3 hours. It will possibly later be necessary to
'nstal percolating filters in addition. The effluent will run to a
common collecting chamber for pumping to the effluent culvert.
The sludge is to be treated on ordinary underdrained sludge beds.
1 he estimated cost of the entire scheme, which will provide
for treatment of a flow of 81/2 m.g.p.d. is £268,593.