Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
~2.~ l
- 1 - O.Z. 0050/034784
Dewateri~g of sewa e sludges on filter presses
The present invention relates to a process for
dewatering, on a filter press, sewage sludges of which
the sludge solids contain less than 70% of organic matter,
and whose dewatering characteristics have been improved
by addi-tion of an organic flocculant.
In sewage plants o~ industrial companies, and in
central iarge-scale municipal sewage pl~nts, in-to which
substantial amounts o~ in~ustrial effluent are le~, 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 o~ the
sludge so as not to pollute the environment is to ash it
completely Combustion 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 concentrated to a very high
degree. The specific water content (kg of water/kg of
sludge solids) of the sludges to be combusted should be
very low.
In con~entional processes, the sludges are
dewatered on decanters or perforated-
belt presses, using organic flocculants. In such cases
a subsequent energy-consuming drying process must be used
so as to concentrate the dewatered sewage sludges further
and allow them to be combusted economically.
Even in the most widely used dewatering process,
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namely one-step dewatering on a chamber filter press,
using iron salts and lime as dewatering assistants 7 and
employing inert additives, for example ash, sand or rock
powder, the filter cak~ producecl are not per se suitable
for combustion. Substantial amounts of additional
fuels, such as coal and/or oil~ must be introduced into
them. Furthermore, the coatings and crusts caused by
the lime interfere with the combustion equipment.
Using a different dewatering process (German Laid-
Cpen Application DOS 2,920,350), more thorough mechanical
dewatering of sewage sludges on chamber filter 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 filtration 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 su~ficient mechanical
stability for the process. m e sewage sludge can,
without su~fering damage, be conveyed, pumped and filtered
in presses under pressures of up to 15 bar. For sewage
sludges of which the solids contain less than 65% of
organic matter, the sludge solids contents achieved are
so high that the calorific value of the filter cake is of
an order which permits self-sustaining combustion
In the cas~? 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 self-sustaining combustion. In this
:~ 3. ':i 2 ,3 9 ~
3 --
process it is sometimes necessary to reduce the thickness
of the layers handled in the filter chambers, so as to
achieve particularly high sludge solids contents.
It is an object of the present invention to pro-
vide a dewatering process which does not suffer from the
above disadvantages and in which the requisite concen-
tration of the sludge solids to above 10% is achievable
by simpler and more expedient measures. Accordingly,
it is also an object of the invention to produce stable
sludge floc so that the pressure filtration can be effected
directly on a filter press and the prior gravitational
filtration can be dispensed with. It is also an object
of the invention to produce filter cakes which are capable
of self-sustaining combustion. It is a further ob]ec-t
of the invention to provide a dewatering process which
gives a filter cake whose entire calorific content can,
when the cake is combusted, in place of primary energy
sources, in a power station, be converted to electricity
and steam.
The present inven-tion provides a process for
dewatering sewage sludges of which the sludge solids contain
less than 70% of organic matter, and whose dewatering
characteristics have been improved by adding organic flocculan-ts,
on a filter press, wherein the sewage sludges are concentrated
by adding an organic floculant during thickening by gravity
sedimentation from 0.5-5.0% by weight of sludge solids to
6-14% by weight of sludge solids, and especially 8-12% by weight
of sludge solidsJthe sewagesludges are mixed with a suitable
additive such as finely divided coal or ash or mixtures thereof
in an amount of from 0.5 to 1.5 parts by weight per part by
weight of sludge solids, and the sludges are treated, before,
during or after admixture of the additives, with a further
'.1;
_ 4 _
2-8 kg of organic flocculant/tone o~ sludye solids and
are then dewatered on -the filter press.
I~ a particular embodiment of the invention, the
sewage sludges are treated with additional amounts of
organic flocculant before admixture o~ the additive, in
particular b~ mixing the sludge with the flocclllant solu~
tion in a stirred ~essel for 1-3 minutes at a stirrer
speed of 5-50 r.p.m.or by bringing the sewage sludges into
contact with the flocculant solution in -the space of less
lo -than 1 minute in static tube mixers which have spirals
in their interior.
According to a further embodimen-t of the invention,
the treatment of the sewage sludge with a further amount o~
organic flocculant can also be effected simultaneously
with the admixture o~ the additives in a mixer in the
course of 1-5 minutes at a stirrer speed of 5-50 r.p.mO
The organic flocculants used for the process
according to the invention are flocculant mixtures which
consist of a flocculant which is 30-40~o cationically
modified and a flocculant which is 70-90% cationically
modified, these two flocculants being present in the ratio
~rom 1:3 to 3;1, preferably 1:1.
For the purposes of the in~ention9 sewage sludges
include primary sludges which are obtained by sedimentation
in the preliminary clarification stage o~ a sewage plant9
mixed sludges which are a mixture of activated sludges
(surplus sludges), from biological sewage plants, with
primary sludges, digested sludges and mineralized sludges
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which are ~ormed, for example, by extended aeration at
ambient temperature. m ese sludges can be of municipal,
industrial or mixed origin.
These sewage sludges arise with solids contents
of 0.5-5.0%. The organic to-tal solids in the sludge
solids as a rule account for less than 70%. Such sewage
sludges can be concentrated by organic flocculants to
6-14% and especially 8-12% by weight of solids by gravity
sedimentation in the thickeners of a sewage plant.
Accordingly, not only is sedimentation speeded up, but the
degree of thickening is also substantially increased.
Commercial types of organic flocculants can be
used. These are water-soluble, macromolecular compounds
which are obtained by polymeriza~io~ 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. These
flocculants can differ from one another in respect of
their different electrical charge (ie. they may be cationic~
anionic or electrically neutral) and in respect of their
degree of polymerization.
A suitable type and amount of flocculant is chosen
by conventional methods on the laboratory scale.
Anionic or cationic flocculants can be used for
~he thickening step They are employed in amounts of
from 0.5-2.0 kg of active ingredient/tonne of sludge
solids.
The prior selection of the organic flocculant ~o
be employed for extensive improvement of the dewatering
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- 6 ~ O.Z. 0050/034784
characteristics of the sludge mixtures according to the
invention is made in accordance with the methods des-
cribed in German Laid-Open Application DOS 2,837,017
In accordance with the object of the invention,
the flocculation reaction should produce sludge ~loc
ha~ing a particularly high mech,~nical stability.
We have found, surprisingly, that mixtures of
organic flocculants which consist of one flocculant with
30-40% cationic modifica-tion and one flocculan-t with
70-90% cationic modification give sludge floc which is
mechanically particularly stable. The two flocculants
can be mixed in the ratio of from 1:3 to 3:1, but pre-
ferably of 1:1. The requisite amounts of flocculant
are 2-8 kg of active ingredient/tonn~ of sludge solids.
They depend on the nature of the particular sludge and on
the solids concentration.
m e flocculants are employed as OoO5~0~2% strength
aqueous solutions The dilute flocculant solutions are
prepared from the solid and fluid commercial products by
conventional methods in commercial equipment.
m e flocculant solutions are fed directly into
the sludge line downstream of the sludge pump. It can
be advantageous to use such aids as, for example, a cone
mixer, or a static mixer, of small cross-section, for
introducing the solutionO
In the process according to the invention,
addi-tives, such as finely divided coal and ash, are intro-
duced into the thickened sewage sludges.
Suitable finely divided coals are coal fines which
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are obtained on separating the fines from waste rock by
flotation and subsequen-t dewa-tering. Coal slurries
resulting from wet dressing processes are also suitable.
Further, sieve fractions of power-station coal or of green
coke from refineries, the fract:ions having a particle size
of up to 1 mm, can be used.
Ashes of various origins can also be used as
additives. It is particularly economical to use ashes
which result from the combustion of garbage or from the
combustion of the sewage sludges themselves. The ashes
should be substantially lime-free and should have a pH of
from 6 to at most 8. ~n ash having a pH of above 8 has
an adverse effect on the subsequent flocculation.
m e 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 filter cakes suitable for use as a fuel for
powPr stations are desired, ie. if the filter cakes are
to have a particularly high calorific value, whilst the
specific water content is to be very low, the addition of
ash is dispensed with and only finely divided coal is
added. Ratios of sludge solids:coal solids of from
1:1.2 to 1;1.5 have proved particularly advantageous.
m e additives are homogeneously dispersed in the
sewage sludge by means of mixers, which can be operated
batch~ise or continuously. m e residence time in the
mixers is from 1 to 5 mi~utes, and the stirrer speeds are
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- ~ _ O.Z. 0050/034784
~rom 5 to 50 r.p.m.
Because of the particularly high mechanical
stability of the sludge floc achieved when using the
flocculant mixtures according to the invention~ the treat-
ment of the sewage sludges with further amounts of organic
flocculants can be carried out even before introducing the
additives. We have found, surprisingly, that using
this procedure the highest degree of dewatering of the
sludge mixtures upon pressure filtration in the ~ilter
press is achieved.
The pretreatment of the thickened sewage sludges
~ith the aqueous 0,05-0.2% strength flocculant solutions
can be carried out in a stirred vessel through which the
material flows upward (a dynamic flocculating reactor)
in 1-3 minutes at stirrer speeds of 5-50 r.p.m. The
f~oc ripening time and the most advantageous energy input
can be determined by the method described in German Laid-
Open Application DOS 2,920,434.
mis pretreatment can be carried out equally
successfully in static tube mixers which possess spirals
in their interior. Using these, the floc ripening times
are less than 1 minute, because of the special hydraulic
conditions prevailing. As a rule~ a plurality of mixer
elemen-ts are arranged in series. This mixing zone
(constituting a static flocculating reactor) forms a part
o~ the sludge line.
In another embodiment, the treatment of the sewage
sludges with aclditional amounts o~ organic ~locculants can
be effected si~lultaneously with the introduction of the
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- ~ - O.Z. 0050/03~734
addi-tives in a mixer, in 1-5 minutes at stirrer speeds of
5-50 r p.m. In this case, -the mixer also serves as a
flocculating reactor. The floc ripening time required
for the flocculation, and the necessary energy input, can
be achieved with this method~ With some sludge mix
tures, this type of pretreatment results in e~ually
advantageous dewatering effects upon pressure filtration
on a filter press as those achieved with the method des-
cribed above.
The flocculated sludge mixtures which contain the
additives are collected in an intermediate vessel, which
serves as a buffer vessel, and are conveyed from there
into the filter presses.
Suitable filter presses are chamber filter presses,
plate-and-frame filter presses &nd membrans filter presses.
If the additives are used in amounts of 0.5-1.5 parts by
weight per part by weight of sludge solids, relatively
thick layers of up to 40 mm can be pressed. mis is an
advantage of the process according to the invention, since
it allows relatively thick layers to be pressed when
using organic flocculants as dewatering assistants,
whereas previously this required the use of iron salts
and lime.
Pressing is carried out for 90-180 minutes under
pressures of up -to 15 bar. If the measures provided
by the invention are employed, -the press cakes satis-
factorily release from the filter cloths, so that they
can be ejected automatically.
m e so:lids contents achieved in the press cakes
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- 10 - O.Z. 0050/034784
are surprisingly high:
In the method in which finely divided coal is
added to the sewage sludges in a ratio of sludge solids:
coal solids of from 1:1.2 to 1:1.5, solids contents of the
press cake of 55-65% are achievable. The specific
water contents of the press cakes are of the order of
1.4-1.6 kg per kg of sludge solids. The calorific
values are from 9~200 to 14,600 kJ/kg (2,200-3,500 kcal/kg).
Because of the high calorific value and the low specific
water contents, these press cakes can be used as power
station fuel.
m e particular advantages of this method are that,
first, the finely di~ided coal which is initially used as
a filter aid can be fully utilized as a primary energy
source, and, secondly~ the entire calorific content of
the organic solids of the sewage sludges can be utilized
to generate electricity and steam. mls method is of
particular importance for large municipal sewage plants,
since municipal sewage sludges are particularly energy-
rich and provide ad~antageous preconditions for combustion.
In the method in which finely divided coal and ashin ratios of sludge solids:coal solids:ash of from
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.
m e specific water contents of the press cakes are of
the order of from 1.~ to 1.8 kg/kg of sludge solids.
l`he calorific values are 4,600-6,300 kJ/kg (1,100-
1,500 kcal/kg). These press cakes can be used for self-
sustaining combustion in fluidized bed furnaces or multi-
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~ O.Z. 0050/034784story furnaces.
This method allows cheap disposal of 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 of this embodiment of the process
according to the invention over the prior art processes
is that the more effective mechanical dewateri~g directly
produces filter cakes which are capable of self sustaining
combustion. ~ccordingly, energy-intensive drying of
the dewatered sewage sludges, or the addition of relatively
large amounts of finely divided coal and/or fuel oil, are
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 sludges are 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~edO
The examples which follow illustrate the invention.
Percentages are by weight.
In Examples 1-5, a mixed sludge from a mechanical-
biological sewage plant was used; it consisted of 80 percent
o~ activated sludge and 20 percent of primary
sludge, wlth 65% organic matter in the slu~ge solids.
The mixed sludge was concentrated in a rotary
thickener by adding a 0.2,~ strength aqueous solution of
3 g ~.
- 12 _ O,Z, 0050J034784
a 40% anionically modi~ied flocculant, in an amount of
1 kg of active ingredient/tonne of sludge solids,
The solids content of the thickened mixed sludge was 9.5%.
EXAMPLE 1
Flocculation
A 0.2%strength aqueous solution of a ~locculant mix-
tureconsisting o~ equal par-ts of a 40% cationicallymodified
organic flocculant and a 90% cationically modified
organic flocculant was dispersed in the mixed sludge
referred to above, by means of a cone mixer fitted into
the sludge line. The total amount of organic flocculan~s
added was 6,6 kg of active ingredient/tonne of sludge
solids.
m e mixed sludge containing the flocculant mixture
was then stirred for 2 minutes in a cylindrical vessel with
vertical flow. The screw-type stirrer used ran at
20 r.p.m.
Conditionin~
A 50% strength aqueous suspension of coal sludge
and sewage sludge ash was then added to the flocculated
sludge in a double~shaft mixer in the course of 4 minutes,
at a stirrer speed of 20 r.p,m,, the amount added corres-
ponding to a ratio o~ sludge solids:coal solids:ash of1:0.3:0.6. The calorific value of the solids in the
coal sludge employed was 23,000 kJ/kg (59500kcal/kg).
The total solids content of this sludge mixture was 12.6%.
Pressure_filtration
The flocculated and conditioned sludge was intro-
duced into a collecting vessel and conveyed from there,
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~ ~3 - O.Z. 0050/03~78~
by means 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. The
press time was 150 minutes. The press cakes automati
cally detached from the filter cloths.
Data of the press cakes
Solids content: 58.5%
Specific water content: 1.35 kg of water/kg of sludge
solids
Calorific value Hu: 5,605 kJ/kg (1,341 kcal/kg)
The press cakes can be subjected to sel~-sus-tainir~
combustion in a fluidized bed furnace.
EXAMPLE 2
The procedure described in Example 1 was followed,
except that the 0.2% strength aqueous flocculant solution
was introduced simultaneously with admixture of the
additives in the double-shaft mixer, under the same con-
ditions as before.
After pressure filtration on a chamber filter
press, the press cakes were easily ejectable.
Data of the ~ress cakes
Solids content: 56.2%
20 Specific water content: 1.47 kg of wa-ter/kg of sludge
solids
Calorific value Hu: 5,321 kJ/kg (1,173 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.
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EXAMPLE 3
Flocculation
,
A 0.1% strength aqueous solution cf a flocculant
mixture consisting of one part o~ a 40% cationically
modified organic flocculant and 2 parts of a 90% cationi-
cally modified organic flocculant was dispersed in the
mixed sludge by means of a cone mixer. me total amoun-t
of organic flocculants added was 5.8 kg of active
ingredientttonne oP 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 zone was
30 seconds.
Condi-tionin~
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 of
sludge solids to coal solids was 1:1.3. The finely
divided coal used was a flotation concentrate which had
a calorific value of 31,000 kJ/kg (7,400 kcal/kg).
m e total solids content of this sludge mixture
was 13~o.
Pressure filtration
Pressure filtration was carried out as described
in Example 1. The press time was 150 minutes, The
press cakes were dry and detached satisfactorily ~rom the
filter cio r hs.
Data of the press cakes
Solids con-tentO 61,8%
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Specific water content: 1.42 kg of water/kg of sludge
. solids
Calorific value Hu: 13,800 kJ/kg (3,300 kcal/kg)
The calorific value was of the same order as that
of a good-quality lignite (HU 2,500-~,500 kcal/kg).
m e press cakes were combustible in a power station boiler,
in order to generate secondary energy (electricity and
steam).
LXAMPLE 4
The procedure described in Example ~ was followed,
but the 0.1% strength aqueous flocculant solution was added
simultaneously wi-th admixture of the finely divided coal
in the double-shaft mixer, under otherwise identical con-
ditionsO
A~ter pressure filtration on a chamber filter press,
it was possible to eject the press cakes automatically
Solids content: 59.2~
Specific water content: 1.58 kg of water/kg of sludge
solids
Calorific value Hu~ 13,108 kJ/kg (3,136 kcal/kg)
These press cakes, again, were combustible in a
power station.
EXAMPLE 5
Flocculation
-
A 0.1% strength aqueous solution of a flocculant
mixture, which consisted of equal parts of an organic
30% cationically modified flocculant and an organic 90%
cationically modified flocculant was dispersed in the
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mixed sludge by means of a cone mixer. The total
amount of organic flocculants added was 5.5 kg of active
ingredients/tonne of sludge solids. The sludge con-
taining t~e flocculant mixture was then treated, with
stirring, in a cylindrical vessel with vertical ~low.
The screw stirrer ran at 15 r.p.m.
Conditioning
A 50% strength aqueous suspension of coal sludge,
as used in Example 1, was added to the flocculated sludge
in the course of ~ 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 1:1.4.
m e total solids content of the sludge mixture was
13~0%.
Pressure filtration
Pressure filtration was carried out as described
in Example 1. The press time was 160 minutes. It
was possible to eject the press cakes automatically.
Dcata of the press cakes
Solids content: 60.4%
Specific water content: 1.57 kg of water/kg of sludge
solids
Calorific value Hu: 10~809 kJ/kg (2,586 kcal/kg)
To establish the most advantageous poin-t of
addition of the organic flocculant when carrying out the
dewatering according to the invention as a laboratory
experiment, a mixed sludge ~hich in origin, composition
and thickening conditionscorresponded to the mixed sludge
used in Examples 1 to 5 was employed on ~ive successive
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days. The solids content of the thickened mixed
sludge samples was 8.9-9.2% and the organic content in
the sludge solids was 64~2-65.8'~.
The sludge pretreatment was carried out by 3
methods in each ca~e:
Method A
500 ml of mixed sludge are introduced into a tall
1 liter beaker. A 0.2% strength aqueous flocculant
solution is then added in the course of 10 seconds whilst
stirring at 50 r.p.m. by means of a blade stirrer.
The flocculated sludge is then treated ~or a further
120 seconds at the stated stirrer speed.
m e stirrer speed is then increased to 200 r.p.m.
and the relevant additive,(finely di~ided coal, ash) are
added in the stated amounts in the course of 30 seconds.
m e 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 stirring the mixture
at 200 r.p.mO The total mixing time is ~ minutes.
The stirrer speed is then ~ropped to 50 r.p.m.
and the flocculant solution is added in the course of
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10 seconds. m e flocculated sludge mixture is there-
after treated for a further 120 seconds at the stated
stirrer speed,
m e reason for increasing the stirrer speeds, when
admixing the additivesl compared to the practical experi-
ments in Examples 1 to 5, is to achieve a comparable energy
input.
The sludge samples pretreated by Methods A, B and
C were immediately subjected to pressure filtration on a
laboratory press. m ey 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 divided coal (a coal
sludge in the form of a 50~ strength aqueous suspension)
and sewage sludge ash were added to the mixed sludge
samples. The ratio of sludge solids:coal solids:ash
was 1:0.3:0.6 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 flocculant added was 6.5 g of active
ingredient/kg of sludge solids.
The following degrees of dewatering (solids con~
tents of the press cakes) were found after pressure fil-
tration:
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- 19 - O.Z. 0050/034784
Mean ~alue (x) Standard deviation (s)
Method A 5508% 0 5
Method B 55 0% 1.6
Method C 52.8% 2.9
COMPARATIVE EX~MP1E 2
In this series of experi:men-ts, a ~lotation concen-
trate ~as added to the mixed sludge. The ratio of
sludge solids to coal solids was 1:1.3.
The flocculant solution used was a 0.2% strength
aqueous solution o~ a ~locculant mixture which consists
of one part of an organic 40% cationically modified
flocculant and 2 parts of an organic 90% cationically
modified fIocculant. The total amount of organic
flocculants added was 5.8 g of active ingredient/kg of
sludge solids.
After pressure filtration, the following solids
contents of the press cakes were found:
Mean value (x) Standard deviation (s)
Method A 58.1% 1.1
Method B 57.5% 2.5
Method C 56.2% 2.8
Comparative Examples 1 and 2 show that with
addition, according to the invention, of the flocculants
before or during admixture of the additives, the most
advantageous dewatering effects, with the smallest range o~
fluctuation, are achieved.
EXAMPLES 6 AND 7
(Use of green coke and of power station coal)
m e mixed sludge used was -the same as-that employed
.
3~
- 20 - 0 . Z . 0050/034784
in Comparative Examplesl and 2. The solids conten-t of
the thickened sludge was 8.9~ and the organic content of
the sludge solids was 65.3%.
The sludge pretreatment with the organic flocculants
and the additives was carried ou-t according -to Method A.
The pressure filtration was carried out as described in
Comparative EXperiments 1 and 2.
The flocculant solution employed was a 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. The total amount of organic flocculants
added was 6.o g o~ active ingredient/kg o~ sludge solids.
The additives used were sieve fractions~ o~
particle size less than 1 mm, either of green coke from
a refinery ~Example 6) or of power station coal (Example
7). m e ratio o~ sludge solids to coal solids was
1:1.3.
m e calorific values of the solids in these
materials were as ~ollows:
Green coke: Hu: 35,100 kJ/kg (85400 kcal/kg)
Power station coal: Hu: 29,700 kJ/kg (7,100 kcal/kg)
Data of the ~ress_cakes
Example 6 Example 7
~green coke) (power station coal)
Solids content (%): 59~6 58.8
Specific water content 1.56 1.61
(kg of water/kg of sludge
solids)
Calorific walue Hu~ (kJ/kg) 14,638 12,590
(kcal/kg) 3,502 3,012
7~3~
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~ xamples 6 and 7 show that green coke and power
station coal can be employed in ~he same way as finely
divided coal in the process according to the invention.
COMPARATIVE EX~MPLE ~
In carrying out Example 1, the flocculated and
conditioned sludge being conveyed from a collecting vessel
into a chamber filter press by means of a diaphra~m 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 subjec-ted 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 comParison, a flocculant mixture not according
to the invention was e~ployed, consisting of one part of
~n organic 70% cationically modified flocculant and 2 par-ts
o~ an organic 90% cationically modi~ied flocculant.
In other respects, the procedure described in Example 1
was followed.
Sampling and testing on the laboratory press were
carried out under comparable conditions to those stated
above.
Solids content of the press cakes (labora ~
Sampling position: Upstream Downstream
of pump of pump
Example 1 54.8% 54.1%
~using a flocculant mixture
according to the invention)
Comparative Example 3 53.Z% 43.8%
using a flocculant mixture not
according to the invention)
X3~ ~
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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 according
to the invention, -to withstand mechanical stress~
