Note: Descriptions are shown in the official language in which they were submitted.
This invention concerns a method and apparatus
for thickening sludge particularly, though by no means
exclusively, activated sludge.
~ox many years activated sludge has been thickened
by passage through a settling tank such that the residence
time in the tank is of the order of 24 hours. This technique,
besides requiring much time and large equipment, has the
disadvantage that the thickened sludge will have a solids
concentration of only 1.5 to 2.0% by weight.
With a view to obtaining higher levels of
solids in the concentrated sludge, various techniques
have been developed utilising sophisticated equipment
such as flotation apparatus where suspended solids contained
within the sludge are carried to the surface by small bubbles
of gas produced by electrolytic action or out of solution.
Such flotation apparatus is effective in re-
moving water from the sludge so that the concentrated
sludge will have 5% solids by weight or thereabouts. This
technique has a number of disadvantages in that the equip-
ment used is costly to purchase and maintain and that the
processing costs are high.
The applicant has conducted an extensive re-
search programme with a view to finding a method of
thickening sludge to have a concentration of the order
of 5% by weight of solids with simple equipment and in
an economic manner.
The present invention is based upon the discovery
that if the sludge is dosed with a cationic polyelectrolyte
and then handled very gently it will become relatively
free draining.
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.~ ~
According to one aspect of the present invention,
an apparatus for thickening sludge comprises, an endless :~
band of mesh material which in one section of the apparatus
has a generally horizontal run extending between spaced guide
membersand which is supported between the guide members on
a structure that permits li~uid to drain freely through the
band, means for introducing sludge to be treated into the
apparatus, means for dosing said sludge to be treated with
a cationic polyelectrolyte, a tank for receiving and hold-
ing the dosed sludge for a sufficient period of time to
allow development of a floc structure, means for supplying
sludge from the tank to the upper surface of said band at
the start of said horizontal run. in such a manner that the
floc structure is retained comprising a weir constituting
. 15 the outlet for said tank and a chute of shallow gradient
extending from the weir to said start of the horizontal run
whereby to deposit the sludge on the band without sub~ :
stantial forward velocity, means for driving said band so
that the sludge is conveyed through such run without des-
troying the floc structure of the sludge, and means for
removing thickened sludge from the surface of the band
after said horizontal run.
The invention will be fur-ther apparent
from the following description with reference to
the figures of the accompanying drawing which show,
by way o~ example only, one form of apparatus
4J~
embodying the invention and for carrying out.the
method thereof.
Of the drawing:-
; .
Fig. 1 shows a diagrammatic vertlcal
cross-section through the apparatus;
and Fig. 2 shows a cross-section on the
line II-II of' Fig. 1.
Ref`erring now to the drawing, it will
be seen that the apparatus is.essentially comprised : ::
by an endless band 10 of mesh material which is : ~ :~
supported and guided by rollers 11, 12 and 13, one :
of which is connected to a motor (not shown) for
the purposes of driving the band in the direction
of the arrows shown.
The band 10 extends generally horizontally .. ~:
between the roIlers 11 and 12. The horizontal run
~of the band 10 between the rollers 11 and 12 is
supported by a plurality of transversely extending
longitudinally spaced parallel bars 14. Each of
the bars 14 mounts an upstanding wedge-shaped
member 15 at each of its opposite ends~ thus
.
EF/1515
f'orming a trough-like support for the band 10.
The distance between opposite members 15 on the
bars 14 is less than the width of the band 10,
; whereby the latter is constrained to assume a :~
trough-like configuration during passage through
the horiz.ontal run between the rollers 11 and 12.
It will be understood that whllst the : ;
band 10 is adequately supported over the whole 9f
its length in the horizontal run extending between
the rollers 11 and 12, the arran~ement i5 nonethe-
less such as to of.fer the minimum of resistance to
the free drainage of liquid through the band.
Sludge to be thickened is in~roduced to :
the apparatus through a pipe 16. The sludge is
:~ 15 dosed with a cationic polyelectrolyte as it passes ~.
: .t'nrough the pipe 16, the polyelectrolyte ~eing
injected into the pipe 16 ~hrough a duct 17. The
dosed sludge is passed into a small holding tank
-~ ~ 18 which ensures sufficient residence time to
enable a floc~struc'ure to develop as a result of
the addition of the polyelectrolyte before the
sl:udge leaves the tank 18 by overflow at a weir 19. .
After passage over the weir 19 the sludge flows
:: : ~ downwardl~ over a chute 30:of extremely shallow
.
gradient to be deposlted on the upper~sur~ace of
: LF/1515
"O~
'
the band lO without substantial forward velocity
relative to the band at the start of the
horizontal run extending between khe rollers ll
and 12. As the sludge is conveyed over the
horizontal run between the rollers 11 and 12 :
liquid drains there~rom through the band lO.
A scraper blade 20 is provided to
remove the bulk of thickened sludge from the ~:
surface of the band lO as it passes between the
rollerS 12 and 13. The sludge which ;s removed
by the blade 20 falls into a collecting hopper 21
j from which it can be removed through an outlet 22. ~ :
Guide plates 23 and 24 are provlded to~
collect liquid which passes through the-band 10 :
between the guide rollers ll and 12 and direct it
into a single stream extending over the width o~
the band lO for flow therethrough as the band
lO passes between the rollers 13 and ll~ thus to
wash residual traces of sludge from the band.
The liquid flowing through the band between the
rollers 13 and ll is coIlected on a guide plate
25 which directs it into a gutter 26 from which
the liquid overflows into a drain 27. A plpe 28
containing a valve 29 extends betweerl the gutter
EF/15l5 - 6 -
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26 and the hopper 21 whereby a portion of the.liquid
can be returned to the thickened sludge i~ it should ~.
be too dry for the intended following process.
It is.important that no sucJion or
~ vibration or other techni~ue commonly regarded a5 ~;
an aid to filtration be applied to the horizontaI
~run o~ the band lO between the rollers ll and 12
as such rapidly destroys the floc structure and
prevents free drainage. It is also important
that the method of' passing the dosed sludge to
the band does not subject the sludge to any
violent movement as this again would destroy
the floc structure.:
Itiwil.l be appreciated that it is not
.
intended to limit the invention to the above
:
- example only, many variations, such as might
readily occur to one skilled in the art, being
possible without departing ~rom the scope thereo~
: :: :
hus~ for example~ a vibrator devioe may ~ :
be~provided to operate on the band lO between the
rollers 12 and 13 for the purposes Qf displacing
: thickened sludge therefrom in place of ~he scraper
; blade 20. It is important, ho~ever~ to ensure
that the arrangement lS such ~that no significant;
vibration is imparted to the band between the
rollers ll and 12.
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