Rheology and Dewaterability of Municipal Sewage Sludge by mikeholy

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									           Rheology and Dewaterability of Municipal Sewage Sludge
           K. Klinksieg*, U. Moshage**, N. Dichtl*


           * Institute of Sanitary and Environmental Engineering, Technical University of Braunschweig,
           Pockelsstrasse 2a, 38106 Braunschweig, Germany
           (E-mail: K.Klinksieg@tu-bs.de, N.Dichtl@tu-bs.de)
           ** Dahlem Consulting Engineers, Bonsiepen 7, 45136 Essen, Germany
           (E-mail: U.Moshage@dahlem-ingenieure.de)


           Abstract: Sewage sludges belong to the class of non-Newtonian fluids and show shear thinning behavior with a yield
           point as well as thixotropic behavior. Rheological properties depend on the total solids content, the temperature, the
           pH-value, chemical conditioning and particle size. For sewage sludges these rheological properties are not only
           important for the design of pumping and transporting facilities, but also for sedimentation and dewatering. A
           correlation between rheological parameters and the sedimentation behavior of sewage sludges on the one hand and
           the total solids content achieved by mechanical dewatering on the other hand has been investigated. A correlation
           between rheological parameters and the full scale dewatering results was determined. The regression analysis shows
           a potential function with a coefficient of determination of R?=0.89. The deviation of the full-scale dewatering and the
           rheological parameters amounts to a maximum ± 2.3%. The identified correlation was validated by comparing the
           rheological parameters with laboratory dewatering results such as the dewatering results after centrifugation at 1,000
           g and 48,000 g and with the characteristic dewatering value TS(A).
           Keywords: dewatering; rheology; sedimentation; sewage sludge



INTRODUCTION
Rheology is concerned with the flow and deformation behavior of fluids and solids. Although first investigations
on rheological behavior of sewage sludge have already been conducted in the thirties of the last century (Merkel,
1934; Hatfield, 1938), at present rheological parameters receive no consideration for characterization of sludge
properties usually. Publications about the utilization of rheological characteristics of sewage sludge for
practical applications refer mainly to the hydraulic calculation of sludge conveyor lines (Annen, 1961; Proff and
Lohmann, 1997; Slatter, 1999) and to the optimization of polymer dosage (Campbell and Crescuolo, 1989;
Christensen et. al., 1993; Dentel and Abu- Orf, 1995; Papavasilopoulos, 1997; Abu- Orf and Dentel, 1997, The
influence of rheological characteristics on settling behavior of sewage sludges and the solids content
accomplishable during dewatering have not been examined yet, although the importance of the viscosity for
settling behavior already has been considered in Stokes’ law.
Sewage sludges belong to the class of non-Newtonian fluids and show shear thinning behavior with a yield
point as well as thixotropic behavior.
In Figure 1 the flow behavior of a Newtonian fluid and a sludge is compared. Contrary to Newtonian fluids, the
shear stress of sludges does not increase linearly when increasing the shear rate linearly. This is due to
reduction of thixotropic structures and to the increase of molecular orientation. This is also the reason why the
shear stress is lower when the shear rate again is reduced. After a recovery period, the original structure in
the sludge is developed again.




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Figure 1 Flow behavior of Newtonian fluids and sludges
Rheological properties depend on the total solids content, the temperature, the pH-value, chemical conditioning
and particle size. For sewage sludges these rheological properties are not only important for the design of
pumping and transporting facilities, but presumably also for sedimentation and dewatering. A correlation
between rheological parameters and the sedimentation behavior of sewage sludges on the one hand and the
total solids content achieved by mechanical dewatering on the other hand has not yet been thoroughly
investigated. Based on those facts, this research work focuses on this correlation between rheological
properties and the dewatering results of municipal sewage sludges achieved in full-scale dewatering plants.
If the dewatering results can be predicted by the determination of rheological parameters, the assessment of
the dewatering capability of mechanical dewatering devices for specific sludges will be possible. Furthermore
it would be possible to improve the dewatering results, if the rheological properties of a sewage sludge could
be influenced.
And if dewatering of sludge is understood as an extensive sedimentation or thickening, then a correlation
between the rheological characteristics and the behavior of sedimentation or thickening must exist.
MATERIALS AND METHODS
The rheological characteristics of sewage sludge were measured with the rotation rheometer Rheostress 1
from ThermoFisher Scientific. As measuring system the coaxial cylinder system Z34 DIN with a gap of 1.44 mm
was used, as seen in Figure 1.




Figure 2 Geometry of the coaxial cylinder system Z34 DIN
Before measuring rheological characteristics, a sieving took place according to ISO 3310/1 with a mesh size
from 1.0 mm and an adjustment of the TS content via centrifugation and following dilution to a solid content
of 5% TS. Sludges are exposed to different shear rates during waste water and sludge treatment, which cannot


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always be determined exactly. Therefore a measuring program is advisable, where the sludge sample is exposed
first to a rising, thereupon to a constant and finally to a falling shear rate. Accordingly rotation measurements
were accomplished. The shear rate was increased from 0 to 500 s-1 over 180 seconds (1), then kept constant
over 60 seconds with 500 s-1 (2) and finally reduced from 500 to 0 s-1 over 180 seconds (3). The measuring
program and the collected data are shown in Figure 3.




Figure 3 Measuring program and collected data
Since the shear stress values of the upward ramp (1) could be affected by shear stress of the sample prior to
the measurement, only the flow curves of the downward ramp (3) were considered during the following
investigations.
For the characterization of dewatering properties all determined sludges were centrifuged in an ultra centrifuge
of the type OTD combi (company Sorvall) at 18,000 rpm over 30 minutes. This number of revolutions
corresponds to an acceleration of approx. 48,000 g at the bottom of the sample cup. After centrifugation, the
supernatant was decanted and the remaining sediment solid mass was taken out of the sample cup for the
determination of the TS. The TS determined afterwards in this work is named dewatering characteristic value
TS (48,000g). In parallel a centrifugation of the sludges was carried out in a beaker centrifuge (Labofuge,
company Heraeus) at 1,000 g for 30 minutes. The following determination of the TS of the sludge cake results
in the laboratory dewatering characteristic value TS (1,000g).
Also thermo-gravimetric lab tests were used for determination of the dewaterability of sludge. At the Technical
University of Braunschweig this method was adjusted and calibrated, so that a direct statement can be made
concerning the maximum total solids content in the sludge cake after mechanical dewatering in centrifuges
(Kopp and Dichtl, 2000).
RESULTS AND DISCUSSION
In Figure 4 the shear stress and the full-scale dewatering results of 14 digested sludges are shown. A
correlation between rheological parameters and the full-scale dewatering results was determined. The
regression analysis shows a potential function with a coefficient of determination of R?=0.89. The deviation
of the full-scale dewatering and the rheological parameters amount to a maximum ± 2.3%.
The high coefficient of determination and the small absolute deviation of the full-scale dewatering result of
the regression function confirm the dependency of the dewatering result on the viscosity of the sludge
determined with a fixed shear rate. The small number of available samples, however, does not allow to derive
a secured prognosis of the dewatering result in full-scale decanters from the determined results.




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Figure 4 Correlation between the shear stress and the full-scale dewatering results
Industrial decantation centrifuges differ in the form of the cylinder, the cone length, geometry of the discharge
snail and the rotational speed. Since these factors also have an influence on the dewatering result, a
comparison of dewatering results achieved with different aggregates is limited. Therefore, in the following the
rheological characteristics are related to laboratory dewatering values. These values have the advantage that
all examined sludge samples are exposed to identical conditions and that obtained dewatering results for
specific sludges can be compared directly.




Figure 5 Correlation between the shear stress and the total solids content after centrifugation at 1,000g and
48,000g over 30 minutes
The identified correlation was validated by comparing the rheological parameters with laboratory dewatering
results such as the dewatering results after centrifugation at 1,000 g and 48,000 g (Figure 5) and with the
characteristic dewatering value TS(A) (Figure 6). The well-established parameter TS(A) is based on a thermo-
gravimetric measurement of the free water content in sewage sludge, since only the free water fraction can
be separated during mechanical dewatering (Kopp and Dichtl, 2000).




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Figure 6 Correlation between the shear stress and the TS(A) measured by thermo-gravimetric lab tests
In Figure 5 as well as in Figure 6, correlations between lab dewatering results and rheological measurement
are shown. The three regression analyses (Figures 5 and 6) show potential functions with the coefficient of
determination between R?=0.81 and R?=0,84. Thus an estimation of the sludge dewaterability can be made
using rheological properties.
According to the findings, the prediction of the dewatering result of municipal sewage sludges can be
accomplished by measuring rheological parameters like shear stress at defined shear rates.
Investigations were executed to determine the dependency of the temperature on the dewaterability. The
temperature was varied from 5°C to 55°C in laboratory experiments and dewatering results improved at rising
temperatures. It has to be pointed out that the highest content of total solids after dewatering was reached
at a temperature of 55°C. These results of the laboratory experiments could also be observed during full-scale
experiments. Better dewatering results were obtained for digested sludges with higher temperatures due to a
change from mesophilic to thermophilic digestion in full scale digesters as shown in Figure 7.
Before the temperature change, the digested sludge had a temperature of approx. 25 °C during dewatering
in the decanter. A solid content was reached on the average of 23.5% in the discharge of the aggregate. After
the temperature increase, the solid content increased to 25% TS at a dewatering temperature of approx. 40
°C. This improvement of the dewatering result cannot be determined by the so far known procedures for
predicting dewatering results. The thermo-gravimetric determination of the free water content after Kopp and
Dichtl (2000) is conducted at a constant ambient temperature of 35°C. The laboratory tests using
centrifugation for the prediction of the dewatering result were operated at ambient temperature. Contrary to
these tests, rheological investigations can be conducted under various defined temperatures.
The shear stress of the digested sludge which is reduced with increasing temperature causes a rising of the
theoretical full-scale dewatering result, if the prediction function is applied. The prediction function results
in a calculated TS of 24.8% after dewatering with a dewatering temperature of 40 °C resulting from
thermophilic digestion. This value is nearly identical to the existing full-scale dewatering result of 25.0% TS.
In this case the characteristic value TS(A) determined in parallel was far below the solid content found in the
full-scale decanter discharge.
This example of the thermopilically digested sludge shows that the influence of heating on the dewatering
result can be defined by rheological measurement. Furthermore, the obtained results can be used to evaluate
a possible cost optimization.



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Figure 7 Dependency of temperature on the shear stress and the expected dewatering result
If the dewatering of sludge is understood as an extensive sedimentation or thickening, then a correlation
between the rheological characteristics and the behavior of sedimentation or thickening must exist. Laboratory
sedimentation and thickening experiments showed a dependency between viscosity measurements at defined
shear rates and sludge level during sedimentation after a certain period of time. It is the objective of the
actual research, to extend the above described approach for the determination of the dewaterability to the
settling behavior of sludge by using its rheological properties. In Figure 8 first results are shown. The
dependency between the sludge volume index of eight activated sludges and the shear stress at a shear rate
of 500 s-1 is described. Contrary to the investigations concerning the dewatering of sewage sludge, the data
base is still small, but a dependency already becomes apparent. With rising SVI also the shear stress increases.
Further investigations will follow, in order to validate the dependency between the sedimentation behavior of
sewage sludges and their rheological characteristics.




Figure 8 Dependency of SVI on the rheological parameter shear stress



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CONCLUSION
A correlation between the full-scale dewatering results in decanters and the rheological properties at a shear rate of
500 s-1 was determined. The regression analysis shows a potential function with a coefficient of determination of
R?=0.89. The deviation of the full-scale dewatering results from the rheological parameters amounts to a maximum
of ± 2.3%. This correlation could be validated by a supplementing comparison of the flow behavior to the characteristic
dewatering value TS(A) and laboratory dewatering values. In relation to other determination methods for characteristic
values for estimation of dewatering results, a rheological measurement offers the advantage that the temperature
prevailing at full-scale dewatering can be simulated by maintaining a given temperature in the sludge sample during
the measurement. Furthermore, the obtained results can be used to evaluate a possible cost optimization.
Also a dependency between the sludge volume index of activated sludges and the shear stress at a shear rate
of 500 s-1 could be determined. It is the objective of the actual research, to extend the knowledge regarding
the dependency between the rheological characteristics and the behavior of sedimentation or thickening.
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