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Pre-Selection of Flocculants
Using a Separation Analyzer
Guest article by T. Sobisch, LUM GmbH
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The operation of high performance
dewatering machines such as high pressure filter presses and
decanters relies heavily on the use of flocculants for sludge
conditioning. Due to the varying composition of wastewater sludges
pre-selection of promising flocculants in front of field tests is
a frequent task. To this end, a lot of different laboratory
methods have been developed, which are in part very sophisticated
or on the other end may be very simple but strongly based on
individual judgment and experience.
 The
focus of our company is on separation stability of disperse
systems. We develop methods and equipment for control and solution
of related practical problems. The aim of our ongoing work is to
offer a solution based on our separation analyzer LUMiFuge 114.
Principle of Measurement and
Application
The separation analyzer (Fig. 1) is
an analytical centrifuge with an integrated opto-electronic sensor
system which allows to trace the local and temporal changes of
light transmission during rotation.
Up to eight samples can be
investigated simultaneously. A centrifugal acceleration in the
range of 12 to 1200 x g is used to speed up the separation
process, thereby reducing the time necessary for investigation.
The measuring scheme is depicted below.
Fig. 2: Measuring scheme of
separation analyzer LUMiFuge 114
In preset time intervals the local
transmission is determined over the entire sample length
simultaneously whereby transmission profiles are generated. The
transmission profiles change during centrifugation according to
the progress of separation. The separation process can be depicted
as time course of the relative position of the boundary between
supernatant and sediment or of the transmission averaged over a
chosen part of sample length. The essential new feature is that
the overall separation process can be traced. Therefore the
dynamic behavior of the sludge under centrifugal acceleration can
be investigated.
What do we see if we investigate
sludge samples with or without flocculants?
Fig. 3: Dewatering behavior of a
sludge during centrifugation without and with flocculant: time
course of centrate height (measured as distance to the center of
rotation), first 900 s at 1200 x g (compression) then 900 s at 12
x g (dilatation)
Figure 3 shows the typical behavior
of sludge during a compression - dilatation cycle. The change in
sludge volume is depicted as time course of the radial position of
the phase boundary corresponding to the increasing height of the
centrate. First the samples where compacted at high centrifugal
acceleration afterwards the acceleration has been reduced. Due to
flocculant addition a faster and higher compaction is obtained.
From the picture it is evident that common centrifugation tests
predict to low values for sludge dewaterability because the
sediment volume increases again after switching-off the
centrifuge.
Two parameters were derived from
these kind of measurements to evaluate the performance of
flocculants. First the relative sludge volume per total solids in
the compressed state, second to characterize the dewaterability of
the sludge cake during the first seconds of centrifugation the
ratio of the initial sludge volume reduction in relation to the
residual value.
General Procedure
Of course, evaluation of flocculant
performance is not quite so simple. Not only the dynamic behavior
of the sludge under centrifugal acceleration has to be addressed
adequate modeling of shear forces acting on the conditioned sludge
is also a key requirement. This can only be managed by adapting
the sample pre-treatment, so that similar changes take place in
the sludge samples as in practice.
Our laboratory procedure consisted
of the following steps: Preparation of stock solutions of
flocculants in tap water, according to the recommendations of
suppliers and field applications. The sludge samples were first
mixed with the flocculant solution by moderate stirring (600 rpm
for 1 minute) followed by additional shearing with a high speed
Ultra Turrax homogenizer (8000 rpm for 90 seconds) to model the
shear forces inside the decanter. The step of additional shearing
was modified later on in that a baffled stirred reactor was used
instead (2000 rpm for 1 minute). The dewatering behavior of the
samples stirred moderately and of the samples additionally sheared
were compared for different flocculants and flocculant
concentrations using the separation analyzer at a centrifugal
acceleration of 1200 x g.
Four cationic flocculants of the
Floclair series (Münzing Chemie GmbH, Heilbronn) were studied in
more detail. The products compared were all of the emulsion
polyacrylamid-type with a degree of charge of 65 %. They differed
in molar mass and structure as follows.
- Floclair DK-65S with medium
molar mass and a high degree of branching.
- DK-65H with high molar mass and
a lower degree of branching.
- DK-65 and DK-65N both linear
with medium and low molar mass.
Case Study I: Flocculant
Selection - Anaerobic Sludge Digested in a Sludge Reactor
As an example the following
comprises the results of flocculant testing for dewatering of an
anaerobic sludge (total solids 4 %, ignition loss 36 %), which has
been digested in a sludge reactor.
Fig. 4: Comparison of the
performance of different flocculants without and with additional
shearing - residual sludge volume
In relation to the residual values
it can be seen that for all flocculants the additional shearing
reduces dewaterability - the residual sludge volume is increased.
For higher flocculant concentrations the two curves approach each
other. The linear products exhibit a significant better
performance than the branched flocculants. The medium molar mass
DK-65 produced the best results, in this case the lowest values
for the sludge volume could be obtained.
Figure 5 shows the comparison of
the flocculant performance in relation to the initial
dewaterability (during the first 30 s of centrifugation). In case
of this sludge the same trend could be observed as in relation to
the residual total solids of the sludge cake, so flocculant
Floclair DK-65 resulted in the fastest compression of the sludge
cake even at a concentration of 100 ppm.
Fig. 5: Comparison of the
performance of different flocculants without and with additional
shearing - initial dewaterability
However, during dewatering of this
sludge with a decanter under field conditions it was experienced
that DK-65 and DK-65H as well are suitable flocculants, but DK-65N
and DK-65S are not applicable. Further laboratory investigations
revealed that dewatering results substantially deteriorated for
DK-65N if sludge samples are strained mechanically prior to
flocculant addition (stirring 5 minutes at 600 rpm). On the other
hand with DK-65 and DK-65H only minor changes of performance were
observed, after pretreatment of sludge samples similar results
were obtained for the initial dewaterability. Therefore, it may be
assumed that differences between laboratory and field results are
due to the shearing of the sludge during transportation by pumps.
For this reason changes of sludge structure prior to flocculant
addition should be considered as an important factor in relation
to flocculant performance.
Case Study II: Effect of
Mechanical Preconditioning - Anaerobic Sludge Digested in an Old
Sludge Basin
The laboratory procedure could not
only be used for comparison of different flocculants for a special
sludge to be dewatered, but also for the investigation of the
influence of other parameter like sludge aging or mechanical
stresses during pumping.
Figure 6 demonstrates the effect of
mechanical conditioning on two anaerobic sludges from an old
sludge basin (S1 - total solids 6 %, ignition loss 41 % - was
obtained in autumn, S7 - total solids 5 %, ignition loss 39 % -
was obtained in spring).
They were continuously pumped with
a peristaltic pump in the laboratory and at different times the
sludge dewatering behavior was investigated for different
flocculant concentrations. The total solids content of the sludge
cake is depicted as function of treatment time. Superfloc E4208 is
a linear product used on-site for dewatering of S7, SD2065 is
branched.
Fig. 6: Effect of mechanical
preconditioning (pumping) on sludge dewatering comparison of
linear and branched flocculant (Superfloc E4208 and SD2065) for
anaerobic sludge samples from an old sludge basin (S7 with 6 month
longer storage time)
As it was observed on-site, for
sludge S7 dewaterability increased with prolonged mechanical
conditioning (reduction in flocculant demand during the working
week by 30 %!).
A quite different behavior was
found for S1. For the linear product dewaterability more and more
decreased with pumping, however, for the branched product
dewaterability improved after going through a minimum. So, after
15 days of pumping this product got more efficient than the linear
one. There are at least two major counteracting effects, which
influence the dewaterability. First, loose agglomerates are broken
up, which would have not a sufficiently high mechanical stability
- during flocculation more homogeneous flocs will then be formed.
Second, mechanical stresses will shift the particle size
distribution to lower values. The results show that the actual
effect of mechanical preconditioning can only be deduced by
experiment either in the field or in laboratory.
Relation Between Experimental
Results and Dewatering Performance
The parallel behavior in assessment
of flocculant performance based on residual total solids and
initial dewaterability (see Fig. 4 and 5) is not found in every
case. During experiments carried out on a number of sludges and
flocculants it turned out there is a trend that based on residual
total solids the performance of polyelectrolytes with lower molar
mass or high degree of charge is often overvalued. Moreover, it
was found that in many cases residual total solids gave the same
ranking as the widely used capillary suction time.
These correlations are demonstrated
by the following picture (Figure 7).
The relative statistical ranking
for results based on capillary suction time, residual total solids
and initial dewaterability are compared. A value of 100 would mean
that the flocculant in question was the best for all sludges.
It can be seen that for DK-65N with
lower molar mass CST and residual solids gave a substantial higher
ranking than the initial dewaterability. On the other hand DK-65S
with a high degree of branching exhibits a significant lower
ranking by the CST-method.
Based on the general procedure
presented a screening procedure has been worked out. For each
flocculant only one concentration appropriate for field conditions
was tested. Thereby, per measurement with the separation analyzer
8 flocculants can be compared. Promising flocculants should
produce high total solids and a high initial dewaterability as
well.
Fig. 7: Relative statistical
ranking of flocculant performance for a range of different sludges
- based on results obtained by CST-Method and Lumifuge
Conclusions
The separation analyzer Lumifuge
114 is an efficient tool for the characterization of sludge
samples and flocculant optimization. The development of
appropriate methods for sample pre-treatment is essential for
getting relevant results for sludge dewatering under practical
conditions.
Flocculant performance could be
estimated by evaluation of stability against intensive shearing,
total solids obtained and initial dewaterability of the sludge
cake. Results were in good agreement with field results.
The beneficial or negative
influence of mechanical pre-conditioning of sludges before
flocculant addition could only be deduced by experiment. The
separation analyzer can be used to this end for model
investigations.
A screening procedure has been
developed. Efficient flocculants should produce high residual
total solids and good initial compressibility as well.
Lab-scale investigations deliver
more reliable results if the dynamic behavior of the sludge under
centrifugal acceleration is also investigated. The separation analyzer
Lumifuge 114 can provide results about the compression behavior of
sludges in the range between 10 and 100 s. So far no other method
or device is known which can deliver such results.
For more information contact our
author:
- Mr. T. Sobisch
LUM GmbH
Rudower Chaussee 29 (OWZ)
D-12489 Berlin
Germany
Web site: http://www.lum-gmbh.de
- E-mail: info@lum-gmbh.de
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