Note: Descriptions are shown in the official language in which they were submitted.
7~
Dewaterinq of sewaqe sludges on filter presses
The present inven-tion rela-tes to a process for
dewatering sewage sludges by mixing -these homogeneously with
additives, such as finely divided coal or ash or mixtures
thereof, adding organic flocculants, removing the greater
part of the water, in a first dewatering step, by gravity
filtration without applying a pressure differential and then
dewatering more thoroughly in a second dewatering step, by
pressure filtration on filter presses.
In sewage plants of industrial companies, and in
central large-scale municipal sewage plan-ts, in-to which
substantial amounts of industrial effluent are led, it is
becoming increasingly necessary to dispose of the resulting
sewage sludge by combustion. Sewage sludges of such origin
can contain constituents which prevent the sludge from being
dumped. A reliable way of disposing of the sludge so as not
to pollute the environmen-t is to ash it comple-tely. Combus-
tion of the sewage sludges is also desirable because of the
great increase in transport costs and because of shortage of
space on most dumps.
Economical combustion is only feasible if the
sludge.solids are beforehand concentra-ted -to a very high
degree. The specific water con-tent (kg of water/kg of
sludge solids) of the sludges to be combusted should be
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very low.
In con~entional processes, the sludges are
dewatered on decanters or perforate~-
belt presses, using organic ~locculants. In such cases
a subsequent energy-consuming d~ing process must be used
so as to concentrate the dewatered sewage sludges further
and allow them to be combusted e~conomically
E~en in the most widely used dewatering process,
namely one-step dewatering on a chamber ~ilter press,
using iron salts and lime as dewatering assistants, and
employing inert additi~es, ~or example ash, sand or rock
powder, the filter cake3produced are not per se suitable
for combustion. Substantial amounts of additional
fuels, such as coal andlor oil, must be introduced into
them. Furthermore, the coatings and crusts caused by
the lime interfere with the combustion equipmen-t.
Using a ~ifferent dewatering process ~German Laid-
Open Application DOS 2,920,350), more thorough mechanical
dewatering of sewage sludges on chamber ~ilter presses,
using organic flocculants, is attainable. The sewage
sludges are pretreated with organic flocculants under
defined conditions, freed from the greater part of the
sludge liquor by gravity ~iltration - and then sub-
jected to pressure filtration in a chamber filter press.
The prior concentration of the sludge solids to 10-20%
results in the sludge floc having sufficient mechanical
stability for the processO m e sewage sludge can,
without suffering damage, be con~eyed, pumped and filtered
in presses under pressures of up to 15 bar~ For sewage
7 ~ 3~ ~
~ 3 - O.Z. 0050/034785
sludges of which the solids contain less.than 65% of
organic matter, the sludge solids contents achieved are
so high that the caloriflc value of the filter cake is of
an order which permits self-sust;aining combustion.
In the case of sewage sludges having a higher content of
organic matter in the solids, it; is necessary to add a
small amount of coal sludge in order to obtain a filter
cake which permits sel~-sustaining combustion m e
coal slurry is added to the sewage sludge before the
organic flocculant is added In this process it is
sometimes necessary to reduce the thickness of the layers
handled in the filter chambers, so as to achieve particu-
larly high sludge solids contents.
It is an object of the present invention to provide
a dewatering process ~hich does not suffer from the above
disadYantages. In particular, it is an object of the
invention to avoid having to reduce the thic~ness of layers
handled in -the filter chambers and to improve and
extend the dewatering process according to German Laid-~pen
Application DOS 2,920,350.
It is a further object of the invention to provide
a dewatering process which gives a filter cake whose entire
calorific content can be converted to electricity and
steam when the cake is c~mbusted in place of primary
energy sources in a power station .
We have found that this object is achieved,
according to the invention, if the additives, such as
finely divided coal or ash or mixtures thereof, are added
in an amount of from 0.5 to 1.5 parts by weight per part
~ ~ 7 ~ 3~ ~
- 4 - O.Z. 0050/034785
by weight of sludge solids and ~rom 1 to 7 kg of
organic flocculant/tonne of sludge solids is added to
the sewage sludges before or during admixture o~ the
additives.
In a particular embodiment of the invention, the
sewage sludge is treated with the organic flocculants
before admixture of the additives, in particular by mixing
the sewag2 sludge with thefloeculant solutionin astirred
~essel for ~roml to 3 minutes at astirrer speedof from5 to50
r.p.m.or bybringingthe sewage into contac-twiththe ~loccu-
lant solution in the space of less t~an l ~inute in static
tube mixers which have in-ternal spirals.
According to a further embodiment of the invention,
the treatment o~ the sewage sludge with the organic floccu-
lant can also be effected simultaneously with the admixture
of the additives in a mixer in the course of ~roml to 5
minutes at stirrer speeds o~ 5-50 r.p.m.
The organic flocculants used for the process
according to the in~rention are flocculant mixtures which
consist o~ a 30-40% ~ationically modified ~locculant
and a 70-9G% cationically modified ~flocculant,
~these two flocculants being present in the ratio
~rom 1:3 to 3:19 preferably 1:1.
For the purposes of the invention, sewage sludges
include primary sludges which are obtained by sedimentation
in the preliminary clari~ication stage of a sewage plant,
activated sludges (surplus sludges) from biological sewage
plants, mixed sludges which are a mixture of acti~rated
sludges with primary sludges, ~ld digested sludges and
- 5 ~ O.Z. 0050/0347~5
mineralized sludges which are formed, for example, by
extended aeration at ambient temperature. These sludges
can be of municipal, industrial or mixed origin.
The solids contents of -these sewage sludges can
vary within wide limits. In general, however, sludges
which have been concentrated by gravity sedimentation in
thickeners are employed. In that case, the sludge
solids contents are, for example, of the order of 2-4% by
weight for activated sludges~ 4-10% by weight in the case
of digested sludges and 5-12% by weight in the case of
primary sludges.
Commeroial types o~ organic flocculants can be
used. These are water-soluble, macromolecular compounds
which are obt-ained by poiymerization or copolymeri-
zation of acrylamide, acrylic acid and/or its salts~or
acrylate or methacrylate esters which have been specially
modified by a choice of their alcohol component. m ese
flocculants can differ ~rom one another in respect of
their different electrical charge (ie. they may be cationic,
anionic or electrically neutral~ and in respect o~ ~heir
degree of polymeri~ation~
A suitable type and amount o~ flocculant is chosen
by conventional methods on the laboratory scale.
m e prior selection of the organic flocculant to
be employed for extensive improvement of the dewatering
characteristics of the sludge mixtures according to the
i~vention is made in accordance with the method~ described
in German Laid-Open Application DOS 2,837,01~.
It is desirable that the flocculation reaction
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- 6 - o.Z. 0050/03~785
should produce sludge floc having a particularly high
mechanical stability.
We have found, surprisingly, that mixtures of
organic flocculants which consist of one 30-40% cationi-
cally modified flocculant and one 70-90% catlonically
modified flocculant give sludge floc l~hich is
mechanically particularly stable The two flocculants
can be mixed in the ratio of from 1:3 to 3:1, but pre-
~erably of 1:1.
o me requisite amounts of flocculant are 1-7 kg
of active ingredient/tonne of sludge solids. They depend
on the nature of -the particular sludge and on the solids
concentration. The flocculants are employed as 0.05-0.2%
strength aqueous solutions. m e dilute flocculant
solutions are prepared from the solid and fluid co~mercial
products by con~en~ional methods in commercial equipment
m e flocculant solutions are fed directly into
the sludge line downstream of the sludge pump. It can
be advan-tageous to use such aids as, for example t a cone
mixer, or a static mixer, of small cross-section, for
introducing the solution.
In the process according to the invention,
- additives, such as finely divided coal and ash, are intro-
duced into the sewage sludges which have been thickened by
gravity sedimentation.
Suitable finely divided coals are coal fines which
are obtained on separating the fines from waste rock by
flotation and subsequent dewatering. Coal slurries
resulting from wet dressing processes are also suitable.
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Further, sievs fractions of power-station coal or of green
co~e from refineries, the fractions having a particle size
of up to 1 mm, can be used.
Ashes of various origins can also be used as
additives. It is particular:Ly economical to use ashes
which result from the combustion of the sewage
sludges themselves. ~e ashes should be substantially
lime-free and should have a pH of from 6 to at most 8.
An ash having a pH of above 8 has an adverse effect on
lo the subsequent flocculation.
The amounts of the additives employed depend on
the particular objective. If filter cakes capable of
self-sustaining combustion are to be produced, the ratio
of sludge solids:coal solids:ash is from 1:0.2:0.~ to
1:0.5:1Ø
If ~ilter cakes suitable for use as a fuel for
power stations are desired, ie. if the filter cakes are
to have a particularly high calorific value, whilst ~he
specific water content is to be very low, the addition of
ash is dispensed with and only finely divided coal is
added. Ratios o~ sludge solids-coal solids of from
1:1 to 1;1.5 have proved particularly advantageous.
m additives are homogeneously dispersed in -the
sewage sludge by means of mixers, which can be operated
batchwise or continuously. The residence time in the
mixers is from 1 to 5 ~.inutes, and the stirrer speeds are
from 5 to 50 r.p.m.
Because of the particularly high mechanical
stability of the sludge floc achieved ~hen using the
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_ ~ _ O.Z, 0050/03~785
~locculant mixtures according to the invention, the treat~
ment of the sewage sludges with the organic
flocculants can be carried out even before introduci~g the
additives. We have found, surprisingly~ that using
this procedure the highest degree of dewatering of the
sludge mixtures upon pressure fi.ltration in the ~ilter
press is achieved.
1m e pretreatment of the thickened sewage sludge
with the aqueous 0.05-0.~% strength flocculant solutions
can be carried out in a stirred vessel through which the
material flows upwards (a dynamic flocculating reactor)
in 1-3 minutes at stirrer speeds of 5-50 r.p.m. The
floc ripening time and the most advantageous energy input
can be determined by the method described in German
Patent Application P 29 20 434Ø
This pre-treatment can be carried out equally
successfully in static tube mixers ~hich have internal
spirals. Using these, -~he floc ripeni~g times
are less than 1 minute, because o~ the special hydraulic
conditions prevailing. As a rule, a plurality of mixer
elements are arranged in series. mis mixing zone
(constituting a static flocculating reactor~ forms a part
of the sludge line~
In another embodiment, the treatment of the sewage
sludges with additional amounts of organic flocculants can
be effected simultaneously with the introduction of the
additives in a mixer9 in 1-5 minutes at stirrer speeds o~
5-50 r.p,m. In this case, the mixer also serves as a
flocculating reactor. I~e floc ripening time required
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for the flocculation, and the necessary energy input, can
be achieved with this method. With some sludge mixtures,
this type of pretreatment results in dewatering e~fects
as advantageous as those achieved with the method des~
cribed above.
After the sludge pretreatment described above,
the flocculated sludge mixtures which contain the additives
are fed to the first dewatering apparatus, and subjected
to gravity filtration. Continuously operated cylindrical
screening drums have proved to be particularly sui-table. In-
side each ofthese drumsthere is a cylindrical cage,with the
filter cloth stretched over the peripheral surface of the
cage. The filter cloth may consist of metal wire or
of synthetic fibers, eg. polypropylene fibers or polyester
fibers. The screening drum may additionally have
internal fitments which continuously create a new sludge
surface, so that the sludge is turned thoroughly as it
passes through the drum. Such internal fitments may
be baf~le plates, weirs or hollow spirals. The residence
time, in the screening drum, of the sewage sludge to be
dewatered is determined by the length of the apparatus,
the throughput, and the degree to which the apparatus is
filled. The drums run at speeds o~ from 1 to 20 r.p~m.
The residence time in this dewatering stage is from l to
10 minutes, especially from 2 to 5 minutes.
This gravity filtration results in sludge solids
contents of from 12 to 24j~6 by weight, and as much as from
~0 to 80% of the water is removed.
The pre-dewatered sludge mixtures are c-ollected
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- 10 - O.Z. 0050/034785
in an intermediate vessel, which ac-ts as a buff0r vessel,
and are conveyed from there into the filter presses
Suitable filter presses are chamber filter presses,
plate-and-frame filter presses and membrane filter pressesO
If the additives are used in amounts o~ 0.5-1.5 parts by
weight per part by weight of sludge solids, relatively
thick layers o~ up to 40 mm can be pressed. This is an
advantage of the process according to the invention, since
it allows relatively thick layers to be pressed when
using organic flocculants a$ dewatering assistants~
whereas previously this required the use of iron salts
and lime.
The press times, which are from 60 to 90 minutes 5
are more advantageous than in single-stage dewatering on
filter presses. Because of the preliminary dewatering
by gravity filtration, fil-ter press capacity is subsequen-tly
saved. The ~ilter presses operate under pressures of up
to 15 bar. If the measures provided by the invention
are employed, the press cakes detach satisfactorily from
the filter cloths, so that they can be ejected automati-
cally.
The solids contentsachieved in the press cakes
are surprisingly high:
In the me-thod in which finely divided coal is
added to the sewage sludge in a ratio of sludge solids:
coal solids of from 1:1 to 1:1.5, solids contents of the
press cakes of55-65% are achievable. The specific
water contents of the press cakes are of the order of
13 -1.6 kg per kg of sludge solids. The calorific
3~3~
- ~1
values are from 9,200 to 149600 kJ/kg (27200-3,500 kcal/kg).
Because of the high calorific values andthe low specific
water contents, these press cakes can be used as power
station fuel.
m e particular advantages o~ this method are that,
first7 the ~inely divided coal ~rhich wasinitially used as
a filter aid can be fully utiliz:ed as a primary energy
source, and, secondly, the entire calorific conten-t o~
t~e organic solids of the sewage sludges can be utilized
to generate electricity and steam. This method is of
particular importance for large municipal sewage plants,
since municipal sewage sludges are particularly energy-
rich and provide advantageous preconditions for combustion.
In the method in which finely dîvided coal and ash
in ratios of sludge solids:coal solids:ash o~ ~rom
1:0.2:0.3 to 1:0.5:1.0 are added to the sewage sludges,
press cakes with solids contents of 50--60% can be obtained.
The specific water contents of the press cakes are of
the order of from 1~2 to 1.8 kg/kg of sludge solids.
The calorific values are 4,600-6,300 kJ/kg (l,lO0-
1,500 kcal/kg). These press cakes can be used for self-
sustaining combustion in fluidized bed furnaces or multi-
story furnaces.
rhis method allows cheap disposal o~ the sewage
sludges with-minimum energy consumption, in cases where
combustion in a power station, so as to generate energy,
is not possible.
The advantage o~ this embodiment of the process
according to the invention over the prior art processes
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is that the more thorough mechanical dewatering directly
produces filter cakes which are capable of self-sustaining
combustion. Accordingly, energy-intensive drying of
the dewatered sewage sludges 7 or the addition of relati~ely
large amounts of finely divided coal andjor fuel oil, is
no longer necessary. Thus, the process according to
the invention produces a useful primary energy carrier
from the sewage sludges, or saves primary energy in dis-
posal of the sludges. Where the sludge` is- combusted
in fluidized bed furnaces, greater operating safety is
achieved as a result of the absence of lime. Further-
more, because of the great reduction in volume, the
capacity of the dewatering equipment and combustion
equipment is better utili~ed.
m e E~amples which follow illustrate the invention.
Percentages are by weight.
In Examples 1-4, a mixed sludge from a mechanical-
biological sewage plant was used, it consisted of 80
percent o~ activated sludge and 20 percent o~ primary
sludge, with 65% organic matter in the sludge solids.
The mixed sludge was concentrated in a rotary
thickener by gravity sedimentation.
The solids content of the thickened mixed sludge
was 4.5%.
EXAMPLE 1
Flocculation
A 0.2% strength aqueous solution ofa flocculant mix-
ture consisting of equal parts of 40~Ocationicallymodified
organic flocculant and a 90% cationically modified
7 ~
- 13 - -Z~ 0050/0347~5
flocc~lant w~s dispersed in the mixed sludge
referred to above 9 by means of a cone mixer fitted into
the sludge line. The total amo~nt of organic flocculants
added was 6.4 kg of active ingredient/tonne of sludge
soli~s.
The mixed sludge containing the flocculant mlxture
was then stirred for 2 minutes in a cylindrical ~essel with
vertical flow. m e screw-type stirrer used ran a-t
20 r.p.m.
Conditionin~
A 50% strength aqueous suspension of coal sludge
and sewage sludge ash was then added to the ~locculated
sludge in a double-shaft mixer in the course of 3 minutes,
at a stirrer speed of 20 r.p.m., the amount added corres-
ponding to a ratio of sludge solids:coal solids:ash o~
1:0.3:0.6. The calorific value ~ of the solids in the
coal sludge employed was 23,000 kJ/kg (59500 kcal/kg).
m e total solids content of this sludge mixture was 7.1%.
Gra~itY filtration
m e flocculated and conditioned sludge was pre-
dewatered to a solids content of 19.2% by gravity filtration
for 3.5 minutes in a horizontal screening drum rotating at
8 r.p.m.
Pressure ~iltration
The pre-dewatered sludge was introduced into a
collecting vessel and conveyed from there,bymeans of
diaphragm pumps, into a chamber filter press with a plate
spacing of 33 mm. Pressure filtration was effected up
to a final pressure of 15 bar. m e press time was 90
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- 14 - O~Z. 3050/034785
minutes. m e press cakes automatically detached from the
filter cloths.
Data of the press cakes
Solids content: 60.3%
Specific water content: 1.25 kg of water/kg of sludge
solids
Calorific value Hu: 5~856 kJ/kg (1,401 kcal/kg)
m e press cakes were-capable of sel~-sustaining
combustion in a fluidized bed furnace.
E~AMPLE 2
The procedure described in Example 1 was followed,
except that the 0 2% strength aquèous flocculant solution
was introduced simultaneously with admixture of the
additives in the double-shaft mixer, under the same con-
ditîons as before.
A~ter gra~ity filtration in the screening drum~
the solids content was 18.9%.
After pressure filtration on a chamber filter
press, the press cakes were automatically eJectable.
Data of the press cakes
Solids content: 59.1%
Specific water content: 1.3~ kg of water/kg o~ sludge
solids
Calorific value Hu: ~,6~ kJjkg (1,~62 kcal/kg)
The press cakes were combustible in a fluidized
bed furnace, without addition of fuel. This example
shows that the object of the invention is achieved~even
by the simplified procedure, on flocGulation and con-
ditioning.
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15 - o.Z. 0050/03~785
EXAMPLE 3
Flocc lat on
A 0.1% strength aqueous solution of a flocculan-t
mixture consisting of one part of 40% cationically
modi~ied organic flocculant and 2 parts of 90% cationi-
caIly modified org~nic flocculant was dispersed in the
mixed sludge by means o~ a cone mixer, The total amoun-t
of organic flocculants added was 5.5 kg of active
ingredient/tonne of sludge solids. The sludge con-
taining the flocculant mixture then flowed through a mixing
zone consisting of a series of static tube mixers (from
Kenics). The residence time in this mixing 7one was
30 seconds.
Conditioning
Finely divided coal was then added to the floccu-
lated sludge in a double-shaft mixer in the course of 3
minutes, at a stirrer speed of 25 r.p.m. The ratio o~
sludge solids to coal solids was 1:1. 3. me finely
divided coal used was a flotation concentrate which had
a calorific value ~ of 31,000 kJ/kg (7,400 kcal/kg).
The total solids content of this sludge mixture
was 7.9%,
Gravity filtration
m e pretreated sludge was pre-dewatered to a
solids content of 22.5% by gravity filtration for 3
minutes in a horizontal screening drum rotating a-t
10 r.p.m.
Pressure filtration
Pressure filtration was carried out as described
16 ~ o.Z. Oo~o/03~785
in Example 1. m e press time was 70 minutes~ The
press cakes were dry and detached satisfactorily from the
filter cloths.
Data of the press cakes
Solids content: 62.5%
Specific water content: 1.38 kg of water/kg of sludge
solids
Calorific value Hu: 13,982 kJ/kg (37345 kcal/kg)
The cAlorific value was of the same order as that
of a good-quality lignite (Hu: 27500_3,500 kcal/kg).
m e press cakes were combustible in a power station boiler9
in order to generate secondary energy (electricity and
steam).
. EXAMPLE 4
The procedure described in Example ~ was followed,
but the 0.1% strength aqueous flocculant solution was added
simultaneously with admixture of the finely divided coal
in the double-shaft mixer, under otherwise identical con-
ditions.
m e solids content after pre-dewatering on the
perforated drum was 18.8%.
After pressure filtration on a chamber filter press,
it was possible to eject the press cakes automatically.
Data of the press cakes
Solids content: 61.2%
Specific water content: 1.46 kg of water/kg of sludge
solids
Calorific ~alue : 13,644 kJ/kg (3,264 kcal/kg)
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~ 17 - o.Z~ 0~50/034785
These press cakes 9 again, were combustible in a
power station~
EXA~IE 5
In this example, a primary sludge ob-tained from
the pre-clarifying stage of a sewage plant was usedO
It was concentrated to 6.5% solids content by gravity
sedimentation for 24 hours in a ro-tary thickener.
The sludge solids contained 51.5% of organic matter.
The calorific value HU of the sludge solids content (?of
the solids) of this primary sludge was 12,960 kJ/kg
(3,100 kcal/kg).
Flocculation
A 0.1% strength aqueous solution of a flocculant
mixture, which consisted of 2parts of an or~a~ic 30% cat
ionicallymodified flocculant and lpart ofan organic90% cationi-
cally modified flocculant was dispersed in the primary
sludge by means of a cone mixerO The -total amount of
organic flocculants added was 2.5 kg of ac-tive ingredient/
tonne of sludge solids.
m e sludge containing the flocculant mixture was
then treated for 2 minutes, with stirring, in a cylindrical
vessel with vertical flow. The screw stirrer ran at
30 r,p.m.
Con itionin~
Finely divided coal (flotation concentrate), as
used in Example 3 9 was next added to the flocculated sludge
in the course of 4 minutes in a double-shaft mixer, at a
stirrer speed of 25 r.p.m. The amount added was chosen
to give a ratio of sludge solids:coal solids of 1:1.5.
- 18 - O.Z. 0050/034785
The total solids conten-t of the sludge mixture
was 12.9%.
Cr~vi-~ iltr~ti~
The pretreated sludge was pre-dewatered -to a solids
content o~ 20.8% by gravity filtration for 3 minutes in a
horizontal screening drum rotating at 10 r.p.m.
Pressure filtration
~ ~ r
Pressure ~iltration was carried out as described
in Example 1. A~ter a press time of 75 minutes, it was
possible to eject the press cakes automatically.
Data of the press cakes
Solids content: 63.0%
10 Specific water content: 1.47 kg of water/kg of sludge
solids
Calorific value Hu: 14,053 kJ/kg (3,362 kcal/kg)
m e press cakes can be used as power station fuel.
To establish, by
laboratory experiment, the most advantageous point o~
~ddition o~ the organic flocculant for the dewatering
process according to the invention, mixed sludge samples
which in origin, composition and thickening conditions
correspondedto themixedsludgè used in Examples 1 to 4
were employed on five successive days. m e solids
content o~ the thickened mixed sludge samples was
4.~-4.8% and the organic content in the sludge solids was
64.8-66.7%.
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19 - o.Z~ 0050/034785
The sludge pretreatment was carried out by 3
methods in each case:
Method A
500 ml of mixed sludge ,Rre introduced into a tall
1 liter beaker. A 0.2% strength aqueous flocculant
solution is then added in the course of 10 seconds ~ilst
stirring at 50 r.p.m. by means of a blade stirrer.
The floccula-ted sludge is then treated for a further
120 seconds at the stated stirrer speed.
The stirrer speed is then increased to 200 r.p.m.
and the relevant additi~es(finely divided coal, ash) are
added in the stated amounts in the course of 30 seconds.
The total mixing time is 3 minutes.
Method B
500 ml of mixed sludge are introduced into a
1 liter beaker. The additives and the flocculant
solution are then added successively in the course of
30 seconds, whilst stirring at a speed of 200 r.p.m.
The total mixing time is 3 minutes.
Method C
500 ml of mixed sludge are introduced into a
1 liter beaker. The additives are added successively
in the course of 30 seconds, whilst stirri~g the mixture
at 200 r~p.m. The total mixing time is 3 minutes.
The stirrer speed is then dropped to 50 r.p.m.
and the flocculant solution is added in the course of
10 seconds. The flocculated sludge mixture is there-
after treated for a further 120 seconds at the stated
stirrer speed.
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- 20 - O.Z. 0050/034785
The reason for increasing the stirrer speeds, when
admixing the additives, compared to the practical experi-
ments in Examples 1 to 4, is to achieve a comparable energy
input.
m~ sludge samples pretreated by Methods A9 B and
C were subjected, in a first de~atering step, to gravi-ty
filtration for 30 minutes and then, in a second dewatering
step, to pressure filtration on a laboratory press. They
were dewatered for 1 minute under increasing pressure, and
then for 2 minutes under a constant pressure of 10 bar.
COMPARATIVE EXAMPLE 1
In these experiments, finely di~ided coal (a ooal
sludge in the ~orm of a 50% strength aqueous suspension)
and sewage sludge ash were added to the mixed sludge
samples. m e ratio of sludge solids:coal solids:ash
was 1:0.~:0,6 in each case.
- The flocculant solution used was a 0.2% strength
aqueous solution of a mixture of equal parts of an
organic 40% cationically modified flocculant and an
organic 90% cationically modified flocculant. The total
amount of organic flocculants added ~as 6.o g of active
ingredient/kg of sludge solids.
The following degrees of dewaterir~ (solids con
tents of the press cakes) were found after gravit~ filtra-
tion and after pressure filtration:
.7'Z~3~`
- 21 - 0. Z . 0050/034785
Gravity filtration Pressure filtration
x ~ x s
Method A16.7% 0.9 57.6% o ~
Method B16.7% 0.9 57.1% 1.4
Method C15.8% 1.254.2~o 2.5
~x = mean value, s = standard deviation)
COMPARATIVE EXAMPLE 2
In this series of experiments, a flotation concen-
trate was added to the mixed sludge. The ratio of
sludge solids to coal solids was 1:1.3.
- The flocculan-t solution used was a 0.2% strength
aqueous solution of a flocculant mixture which consists
of one part of an organic 40% cationically modified
flocculant and 2 parts o~ an organic 90,~ cationically
modified flocculant. The total amount of organic
flocculants added was 5 5 g of active ingredient/kg of
sludge solids.
The following solids contents of the press cakes
were found after gravity filtration and after pressure
filtration:
Gravi-ty ~iltration Pressure filtration
x s x s
Method A 17.8% 0.5 60.3% 1.4
Method B .7.6% 0.6 59.5% 2.1
Method C 16.2% 1.3 58.1~ 2.6
(x = mean value, s = standard deviation)
Comparat;ive Examples 1 and 2 show that with
addition, accorcLing to the invention, of the flocculants
before or during admixture of the additives, the most
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advantageous dewatering e~fects, with the smallest range of
fluctuation, are achieved.
EX~MPLES 6 AND 7
(Use of green coke and of power station coal)
The mixed sludge used was the same as thatemployed
in Comparative Examples 1 and 2. Th~ solids content o~
the thickened sludge was 4.6~ an.d the organic content of
the sludge solids was 65.oo~.
The sludge pretreatment with the organic ~locculants
and the additives was carried out according to Method A.
m e gravity ~iltration and pressure filtration werecarried
out as described in Comparative Experiments 1 and 2.
m e flocculant solutionemployed wasa 0.2%strength
aqueous solution of a flocculant mixture consisting of
equal parts of an organic 40~ cationically modified
flocculant and an organic 90% cationically modified
flocculant. m e total amount o~ organic flocculants
added was 5.5 g o~ active ingredient/kg of sludge solids.
The additives used were sieve fractions, of
particle size less than 1 mm, either of green coke from
a refine.ry (Example 6) or of power station coal (Example
7~. m e ratio of sludge solids to coal solids was
l:1.3.
The calorific values of the solids in these
materials were as follows:
Green coke: Hu: 35,100 kJ/kg (8,400 kcal/kg)
Power station coal: Hu: 29~700 kJ/kg (7,l00 kcal/kg)
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Data of the press cakes
Example 6 Example 7
(green coke) (power station coal~
Solids content (%)
after gravity filtration17.2 16.8
after pressure filtration 62.8 60 5
Speci~ic water content1.36 1.50
(kg of water/kg of sludge
solids)
Calorific values Hu: ~
~kJ/kg) 15,550 13,033
(kcal/kg) 3,720 3,118
Examples 6 and 7 show that green coke and power
station coal can be employed in the same way as finely
divided coal in the process according to the invention~
CQMPARATIVE EXAMPLE 3
In carrying out Example 1, the ~locculated, con-
ditioned and pre-dewatered sludge being conveyed from a
collecting vessel into a chamber filter press by means of
a diaphragm pump, a sample of this sludge was -taken upstream
of the pump and another sample downstream of the pump.
The samples were taken when the press pressure had reached
15 bar.
m e samples were then subjected to pressure filtra
tion on a laboratory press. They were dewatered for 1
minute under increasing pressure and for 2 minutes under
a constant pressure of 10 bar.
For co~parison, a flocculant mixture not according
to the invention was e~ployed~ consisting of one part of
an organic 70% cationically modified flocculant and 2 parts
of an organic 90% cationically modified flocculant.
In other respects, the procedure described in Example 1
was ~ollowed.
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24 - o.z. 0050/034785
Sampling and testing on the laboratory press were
carried out under comparable conditions -to those stated
above.
Solids content of the_E~ akes (laborator~_pr ss~
Sampling position: Upstream Downstream
of pump of pump
Example 1 57.3% 56.9%
n~ a flocculant mixture
according to the invention)
Comparative Exam~le~3 54.0% ~8.9%
(using a flocculant mixture not
according to the i~vention)
This comparative example shows the great ability
of the sludge floc obtained on using the flocculant mix-
ture according to the invention and the procedure accordir~
to the invention to withstand mechanical stress.
