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Effect of liming on the convective drying of urban residual sludges.

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ASIA-PACIFIC JOURNAL OF CHEMICAL ENGINEERING
Asia-Pac. J. Chem. Eng. 2010; 5: 909–914
Published online 19 January 2010 in Wiley Online Library
(wileyonlinelibrary.com) DOI:10.1002/apj.421
Research Article
Effect of liming on the convective drying of urban residual
sludges
Y. Huron,1 T. Salmon,1 M. Crine,1 G. Blandin2 and A. Léonard1 *
1
2
Laboratory of Chemical Engineering, University of Liège, FNRS, Belgium
Lhoist Research and Development, Nivelles, Belgium
Received 13 April 2009; Revised 21 November 2009; Accepted 26 November 2009
ABSTRACT: Sludge liming is largely used in the wastewater industry. This operation, at first realized to stabilize
sludge, has also proved to enhance dewatering and to improve the resulting cakes texture. Nevertheless, almost nothing
is known about the impact of lime addition on sludge drying behavior. As drying can be considered as a necessary
step after mechanical dewatering, it is crucial to evaluate the influence of the sludge pre-treatments on their drying
kinetics. This study shows that both pre- and post-liming exert a positive effect on sludge convective drying behavior.
The negative impact of sludge destructuring by pumping is also established.  2010 Curtin University of Technology
and John Wiley & Sons, Ltd.
KEYWORDS: liming; sludge; convective drying; cohesion; adhesiveness
INTRODUCTION
The addition of alkaline chemicals, mostly lime, as a
stabilization method is widely used within wastewater
treatment plants (WWTPs). Liming raises the pH of
the sludge. At a pH of 12 or higher, pathogens and
microorganisms can be either inactivated or destroyed
provided that adequate mixing and sufficiently long contact times are realized. Moreover, during lime hydration
reaction, its addition contributes to increased sludge
dryness. Sludge liming can be performed before or after
the dewatering step. For pre-liming, lime is combined
with other conditioners in order to enhance dewatering
while for post-liming, lime is mixed with the sludge
cake after mechanical dewatering. Even though the texturing effect of liming is well-known,[1,2] especially for
landspreading purposes, its impact on a subsequent drying operation has never been deeply investigated up to
now. However, drying is and will remain a critical and
necessary pre-treatment after mechanical dewatering.[1]
By decreasing the water content below 5% on a humid
basis, it reduces the mass and volume of waste and, consequently, the cost for storage, handling, and transport.
The removal of water to such a low level transforms
the sludge into an acceptable combustible. Furthermore,
the dried sludge is a pathogen-free, stabilized material provided the temperature is sufficiently high. For
*Correspondence to: A. Léonard, Laboratory of Chemical Engineering, University of Liège, FNRS, Belgium.
E-mail: A.Leonard@.ulg.ac.be
 2010 Curtin University of Technology and John Wiley & Sons, Ltd.
limed sludge, drying allows to obtain a stable, granular,
and marketable product to be used as an agricultural
soil amendment. Concerning sludge drying, it should
be stated that this technique cannot be implemented,
for economic reasons, in small and medium WWTPs.
Nevertheless the number of sludge dryers progressively
increases, both in large WWTPs and in centralized
sludge treatment facilities. According to a real lack
of knowledge, the present study aims at studying the
influence of both pre- and post-liming on the convective drying kinetics of sludge. Moreover, the influence
of sludge destructuring by a passage through a cavity
pump is also considered in the present study.
MATERIALS AND METHODS
Sludge samples
The drying study has been performed on dewatered
samples produced either in wastewater treatment plants
(WWTPs) or at the laboratory. For sludge obtained
in real conditions, pre- and/or post-limed samples
were taken in addition to nonlimed dewatered sludge
(Set 1, 2, 4). For Set 4, raw and pre-limed sludge
samples were also collected after their passage through
a cavity pump used to convey the dewatered product
to the containers. At the lab-scale, post-limed samples
were produced in order to evaluate the impact of the
liming level on the drying kinetics (Set 3). Principal
characteristics of the sludge samples are listed in
910
Y. HURON ET AL.
Asia-Pacific Journal of Chemical Engineering
Table 1. Sludge samples and their characteristics.
Type of sludge
Samples collected
Theoretical
lime dose
(% CaO/DS)
1
Anaerobically digested
2
Physico-chemical
3
Biological
4
Biological
Raw dewatered sludge
Pre-limed dewatered sludge
Post-limed dewatered sludge
Raw dewatered sludge
Post-limed dewatered sludge
Raw dewatered sludge
Post-limed dewatered sludge
Post-limed dewatered sludge
Post-limed dewatered sludge
Post-limed dewatered sludge
Post-limed dewatered sludge
Raw dewatered sludge
Pre-limed dewatered sludge
Pre-limed dewatered sludge
Pre-limed dewatered sludge
Destructured raw sludge
Destructured pre-limed dewatered sludge
–
21
21
–
25
–
10
20
30
40
50
–
15
25
35
–
25
Set no.
Table 1. The ‘dry solids’ (DS) and ‘volatile solids’
(VS) contents were determined according to Standard
Methods.[3] The theoretical lime addition is related to
the amount of dry solids. This theoretical CaO content is
the value which was supposed to be reached according
to the realized lime addition. The values of DS and
VS are supposed to increase and decrease, respectively,
with increasing levels of liming. At the lab-scale (Set
3), lime addition has been easily controlled by weighing
the lime, the dry solids content having been determined
previously. At an industrial scale (Set 4), the control
of the relative flow rate of liquid sludge and lime milk
was more complicated. This explains why, for Set 4,
almost no difference was obtained in terms of DS and
VS contents, for the three pre-defined theoretical lime
doses. This reflects the technical difficulties to produce
small quantities of pre-limed samples at increasing
dosages within wastewater treatment plants. From the
measured values of DS, actual lime addition of 19,
13, and 25% are obtained instead of 15, 25, and 35%,
respectively.
Convective dryer
Before drying, sludge samples were extruded manually
through a circular die of 12 mm. The drying experiments were carried out in a discontinuous pilot scale
dryer reproducing most of the operating conditions prevailing in a full-scale continuous belt dryer. A detailed
description has been previously published.[4] Air at
130 ◦ C crosses a bed of 12 mm diameter extrudates
lying on a perforated grid, with a superficial velocity
initially fixed at 1 m/s. No addition of water vapor is
 2010 Curtin University of Technology and John Wiley & Sons, Ltd.
Actual
lime dose
(% CaO/DS)
DS
(%)
VS
(%DS)
–
22
19
–
29
–
9
22
27
38
45
–
19
13
25
–
17.3
32.0
39.1
38.2
27.4
35.4
24.5
26.6
29.8
31.2
33.8
35.6
16.6
19.8
18.7
20.7
15.6
18.3
41.3
26.0
27.6
41.7
34.6
43.4
40.0
35.1
33.4
27.8
23.1
61.1
39.7
41.0
41.8
57.0
41.5
performed, so that air absolute humidity corresponds
to the one prevailing in the surroundings of the dryer.
Depending on the sludge, about 0.6–0.8 kg of fresh
sludge was used for each trial. The mass was recorded
every 30 sec in order to determine the drying kinetics.
RESULTS
Figure 1 shows the mass decrease for samples of Set 1.
A simple examination of the slope of the three curves
during the first 20 min of drying clearly shows that the
loss of water is more rapid in the case of pre- and postlimed sludge. As expected, the final mass of the two
limed samples is higher than the raw one. A way to
study the drying kinetics consists in representing the
drying rate in function of the water content on a dry
basis. The so-called Krischer’s curves[5] are shown in
Figure 2. This kind of shape is classically encountered
when drying sludges in a convective system.[6] Three
zones can be observed: first, a pre-heating zone during
which the solid temperature and the drying rate increase,
this latter reaching progressively its maximum value,
secondly a narrow plateau, and finally a long decreasing
rate zone, during which drying proceeds up to completion. It can be clearly observed that both pre- and
post-liming accelerate the drying process: for a given
water content, the drying rate is higher when lime is
added. Moreover, the results show that this positive
effect is even more marked in the case of pre-liming.
Drying tests carried out with the second set of sludge,
for which only raw and pre-limed samples were available at the industrial site, also indicated an increase of
the mean drying rate when pre-liming was performed.
Asia-Pac. J. Chem. Eng. 2010; 5: 909–914
DOI: 10.1002/apj
Asia-Pacific Journal of Chemical Engineering
EFFECT OF LIMING ON SLUDGE CONVECTIVE DRYING
produces an increase of the specific evaporation capacity of about 18 and 15%, while an increase of about
52% is observed with pre-liming. It must be noticed
that, as liming leads to a reduction of the initial water
content, shorter drying times were expected, due to a
decrease of the amount of water to be eliminated. However, as showed in Figure 2, at a same water content,
the drying rate becomes higher throughout the drying
process when liming is performed. There is thus a clear
enhancement of the drying process, meaning that the
drying time is shorter, and the evaporation capacity is
higher, than the one which would have been obtained in
the case of a sole decrease of the initial water content,
without liming, e.g. by means of a higher mechanical
dewatering efficiency.
In order to see how the lime dosage influences the
drying behavior, drying experiments were carried out
on samples with increasing levels of post- (Set 3) and
pre-liming (Set 4). Results obtained with post-limed
samples are depicted in Figure 3. They indicate that
there exists a threshold from which lime addition has
a positive impact on sludge drying. Indeed, for lime
addition up to 20%, the drying kinetics remains the
same as for raw sludge. From a dosage of 30%, an
acceleration of the drying process is clearly observed on
the curves by an increase of the drying rate. The values
of the drying times and specific evaporation capacities
are presented in Table 3. For additions between 30
and 50%, a marked decrease of the drying time,
together with an increase of the evaporation capacity
is observed.
As mentioned previously, it has been difficult to
obtain samples with different levels of pre-liming on
the industrial site. Even though the operating conditions
related to lime addition were adjusted to produce, in
theory, increasing pre-liming levels (15, 25, and 35%),
the three samples of Set 4 have an initial dryness
corresponding to an addition of 19, 13, and 25%
CaO related to the dry solids. This can be related
to difficulties in controlling the respective flow rates
of liquid sludge and lime milk or to fluctuations
of the liquid sludge dry solids content. The results
presented in Table 3 confirm that the drying behavior
of the three pre-limed samples are not significantly
different. However, they indicate that an addition of
700
Raw sludge
600
Pre-limed sludge
Mass (g)
500
Post-limed sludge
400
300
200
100
0
0
20
40
60
Time (min)
80
100
120
Figure 1. Evolution of mass vs time – Set 1.
30
Drying rate (g/min)
25
Raw sludge
20
Pre-limed sludge
15
Post-limed sludge
10
5
0
0.0
0.5
1.0
1.5
2.0
2.5
Water content (kg water/kg DS)
Figure 2. Krischer’s curves – Set 1.
Table 2 shows, for the two first set of sludges, the
drying time required to achieve 90% of dry solids content and the corresponding specific evaporation capacity. This latter is important for process extrapolation as
it refers to the amount of water removed by surface
unit of the belt dryer and by hour. As a benefit to lime
addition, the residence time within the dryer decreases
significantly and the specific evaporation increases. This
means that a liming operation will lead to a decrease
of the energy consumption necessary to dry the sludge.
For samples of Set 1 and Set 2, respectively, post-liming
Table 2. Influence of post and pre-liming on the drying time required to reach 90% DS and the corresponding
specific evaporation capacity.
Set no.
1
2
Sludge treatment
Actual lime dose
(% CaO/DS)
Drying time
(90% DS) (min)
Specific evaporation
capacity (kg/m2 h)
Raw dewatered sludge
Pre-limed dewatered sludge
Post-limed dewatered sludge
Raw dewatered sludge
Post-limed dewatered sludge
–
22
19
–
29
48
27.5
36
57.5
40
24.3
37.0
28.8
34.9
40.2
 2010 Curtin University of Technology and John Wiley & Sons, Ltd.
Asia-Pac. J. Chem. Eng. 2010; 5: 909–914
DOI: 10.1002/apj
911
Y. HURON ET AL.
Asia-Pacific Journal of Chemical Engineering
20
10
Raw sludge
Post-limed sludge - 10%
Post-limed sludge - 20%
Post-limed sludge - 30%
Post-limed sludge - 40%
Post-limed sludge - 50%
5
0
0.0
0.5
1.0
1.5
2.0
2.5
Drying rate (g/min)
15
Drying rate (g/min)
912
3.0
Raw sludge After pumping
15
Pre-limed sludge After pumping
10
Raw sludge
5
Pre-limed sludge
0
Water content (kg water/kg DS)
0
Figure 3. Krischer’s curves – Influence of lime dosage on
the drying kinetics.
about 20–25% CaO is sufficient to enhance the drying
rate, as indicated by the important decrease of the drying
time in comparison with the raw sludge.
On the field, it is well-known that the passage of the
sludge through a pump will change its mechanical properties and lead to its destructuring. Dryer manufacturers
are also aware of the negative impact of pumping operation on sludge drying kinetics. In some cases, they
are forced to recycle dried solids at the dryer feeding in order to recover an acceptable initial sludge
texture[10] . However, no scientific study can be found
dealing with that subject. Figure 4 clearly confirms what
is observed industrially. A strong decrease of the drying rate is obtained when the sludge is destructured
due to pumping. For raw sludge, the maximum drying rate is lowered down to 5 g/min. Even though a
decrease of the drying rate also occurs with pre-limed
samples, it can be observed that the liming operation counterbalances partially the negative effect due
to destructuring.
Results presented in Table 4 indicate clearly that,
after passage through the pump, the drying time is
multiplied by two, for both raw and pre-limed sludge
samples. However, due to the texturing effect of the
2
4
Water content (kg water/kg DS)
6
Figure 4. Krischer’s curves – Influence of a pumping
operation on the drying kinetics.
liming step, the specific evaporation capacity remains
quite acceptable for destructured pre-limed sludge.
DISCUSSION
The results obtained in this study show that sludge
liming exerts a positive effect on the resulting drying
kinetics. Concerning the dosage, it seems that there
exists a threshold from which lime addition improves
sludge drying, especially in the case of post-liming.
These observations are in agreement with previous
research dealing with backmixing[2] which showed that
there exists a threshold value of sludge rigidity, below
which the bed of extrudates collapses and above which
the bed undergoes expansion. The texturing effect
of the liming operation leads to an increase of the
sludge extrudates rigidity. This gives a sludge bed with
improved permeability and enhanced exchange area for
heat and mass transfer, leading to an increase of the
drying rate.
Figure 5 shows the evolution of the elastic modulus
vs frequency for the samples of Set 1, determined
Table 3. Influence of the lime dosage on the drying time required to reach 90% DS and the corresponding specific
evaporation capacity.
Set no.
3
4
Sludge treatment
Actual lime dose
(% CaO/DS)
Drying time (90% DS)
(min)
Specific evaporation
capacity (kg/m2 h)
Raw dewatered sludge
Pre-limed dewatered sludge
Pre-limed dewatered sludge
Pre-limed dewatered sludge
Pre-limed dewatered sludge
Pre-limed dewatered sludge
Raw dewatered sludge
Pre-limed dewatered sludge
Pre-limed dewatered sludge
Pre-limed dewatered sludge
–
9
22
27
38
45
–
19
13
25
78
91
89.5
59
54.5
65
92.5
53.5
58
53.5
22.5
18.6
18.2
26.7
27.7
22.4
21.4
34.8
32.6
34.7
 2010 Curtin University of Technology and John Wiley & Sons, Ltd.
Asia-Pac. J. Chem. Eng. 2010; 5: 909–914
DOI: 10.1002/apj
Asia-Pacific Journal of Chemical Engineering
EFFECT OF LIMING ON SLUDGE CONVECTIVE DRYING
Table 4. Influence of sludge destructuring on the drying time required to reach 90% DS and the corresponding
specific evaporation capacity.
Set no.
4
Sludge treatment
Actual lime dose
(% CaO/DS)
Drying time
(90% DS) (min)
Specific evaporation
capacity (kg/m2 h)
Raw dewatered sludge
Destructured raw sludge
Pre-limed dewatered sludge
Destructured pre-limed dewatered sludge
–
–
13
17.3
92.5
200.5
58
115.5
21.4
9.9
32.6
16.5
1E+07
2
Destructuring
Adhesiveness (N)
Elastic Modulus (Pa)
1E+06
1E+05
1E+04
1E+03
Raw sludge
1E+02
Post-limed sludge
1E+01
1E+00
1E-01
Pre-limed sludgeAfter pumping
Raw slugdge After pumping
Pre-limed sludge
1.5
Destructuring
1
Liming
Raw sludge
0.5
Pre-limed sludge
0
1E+00
1E+01
Frequency (Hz)
1E+02
1
1.5
2
2.5
Cohesion (kPa)
3
Figure 5. Influence of pre and post-liming on sludge elastic
modulus – Set 1.
Figure 6. Influence of pumping on the cohesion and the
adhesiveness of raw and pre-limed sludges.
by small amplitude oscillatory measurements. Details
about the measurement methodology can be found in
Ref. [7]. An improvement of the rheological properties,
i.e. an increase of the elastic modulus, is observed when
both pre- or post-liming is performed. The same trend
was found for the viscous modulus. This improvement
of sludge stiffness can be, at least partly, attributed to
the higher solid content resulting from liming. Indeed,
the change of the solid phase composition can also have
an impact on the rheological properties.[8,9]
Even though rheological measurements help in understanding the positive effect of liming on the drying
kinetics, they fail to explain why higher drying rates
are obtained with pre-limed sludges, cf. Figure 2 and
Table 1. Indeed, similar elastic moduli are obtained
for both pre- and post-limed samples as indicated in
Figure 5. This could mean that the internal water transport is affected by the liming operation, but in a different
way according on whether lime addition is carried out
before or after dewatering. Another possibility lies in
the inadequacy of the rheological properties characterization technique.
Rheological measurements were also performed on
the destructured samples used to evaluate the impact
on pumping on the drying kinetics. Results showed an
increase of the elastic modulus after pumping, which
is in contradiction with a deterioration of the drying
behavior. As the destructuring samples look stickier
than the initial ones, the poor drying behavior could
come from a lowering of the water diffusion inside
the sludge. As small amplitude oscillatory measurements are probably not the best tool to evaluate sludge
mechanical properties, cone penetrometer testing was
used to have an idea of the cohesion[2] and the adhesiveness of the samples. The cohesion is obtained from the
maximal load recorded when a metallic cone (35 mm
height, 30◦ angle) is forced into a sludge sample placed
in a cylindrical container (85 mm diameter, 45 mm
height). The tests were realized at 25 ◦ C, with a penetration depth of 30 mm. The adhesiveness corresponds
to the maximal load recorded when removing the cone
from the sludge, after the penetration step. Results presented on Figure. 6 can be interpreted in two ways.
First, the texturing effect of liming is confirmed by an
increase of cohesion. Secondly, the destructuring due
to pumping lead to a decrease of the cohesion together
with an increase of the adhesiveness. This reflects the
visual stickiness character of the destructured sludge as
illustrated by Figure 7.
Penetrometry results allows to interpret the observed
drying behaviors: an increase of the cohesion will accelerate the drying process by improving the extrudate
bed permeability whereas an increase of the adhesiveness will slow down the internal transport of water.
Nevertheless, results allow to indicate that a high cohesion can partly counterbalance a high adhesiveness.
Indeed, destructured pre-limed dewatered sludges dry
more rapidly than destructured raw sludge, but they
 2010 Curtin University of Technology and John Wiley & Sons, Ltd.
Asia-Pac. J. Chem. Eng. 2010; 5: 909–914
DOI: 10.1002/apj
913
914
Y. HURON ET AL.
Asia-Pacific Journal of Chemical Engineering
(a)
(b)
Raw dewatered sludge (a) before and (b) after passage
through a cavity pump. This figure is available in colour online at
www.apjChemEng.com.
Figure 7.
present a higher adhesiveness together with a higher
cohesion. At the light of these results, some other tests
were performed on post- and pre-limed samples produced from the same raw sludge. Higher cohesion values were obtained for pre-limed samples, which is in
agreement with the drying results obtained with sludge
of Set 1. This is quite logical as post-liming required
the use of a cavity pump. This also confirms that cohesion is a more representative property than elastic and
viscous moduli to interpret sludge drying behavior on
the basis of a mechanical characterization.
the cost of liming with the decrease of the dryer energy
consumption.
Acknowledgements
A. Léonard acknowledges the Belgian Fonds de la
Recherche Scientifique (FNRS-FRS) for a Research
Associate position. The authors also thank Sevar Anlagentechnkik GmBh for providing the convective pilot
dryer and the technician of Lhoist R&D for realizing
the penetrometer tests.
CONCLUSIONS
This study shows, for the first time, the positive effect of
lime addition on the urban residual sludge convective
drying kinetics. An increase of the mean drying rate,
and consequently, a decrease of the required drying time
were observed, whatever the sludge origin. Moreover,
better results were obtained with pre-limed than with
post-limed sludges. This study also reports the negative
impact of sludge destructuring owing to pumping.
The resulting decrease of cohesion and increase of
adhesiveness reduce the extrudate bed permeability
and lower the internal water transport, respectively.
This combination of effects impairs the sludge drying
behavior.
Future study will be realized in order to determine
the water internal diffusion coefficient and confirm
its relation with sludge adhesiveness. Moreover, an
economic study will be carried out in order to compare
 2010 Curtin University of Technology and John Wiley & Sons, Ltd.
REFERENCES
[1] G. Padox, G.M. Remy. In Proceedings of the 9th European
Biosolids and Biowastes Conference, Wakefield, UK, 2004.
[2] A. Hil, A.E. Judenne, M. Remy. In Proceedings of the 10th
European Biosolids and Biowaste Conference, November
2005 , 2005; Wakefield, UK Session 18, Paper 59.
[3] ASAE. ASAE Standard, D245.5, St. Joseph, Michigan, 1996.
[4] A. Leonard, E. Meneses, E. Le Trong, T. Salmon, P. Marchot,
D. Toye, M. Crine. Water Res., 2008; 42, 2671–2677.
[5] I.C. Kemp, B.C. Fyrh, S. Laurent, M.A. Roques, C.E. Groenewold, E. Tsotsas, A. Sereno, C. Bonazzi, J.J. Bimbenet,
J.J. Kind. Dry. Technol., 2001; 19, 15–34.
[6] A. Léonard, S. Blacher, P. Marchot, M. Crine. Dry. Technol.,
2002; 20, 1053–1069.
[7] A. Léonard, P. Vandevenne, T. Salmon, P. Marchot, M. Crine.
Environ. Technol., 2005; 25, 1051–1058.
[8] J.C. Baudez. PhD thesis, Ecole Nationale du Génie Rural, des
Eaux et Forêts, Paris, France (in French), 2001.
[9] P.S. Monteiro. Water Sci. Technol., 1997; 36, 61–67.
[10] G. Kreuzer, Personal Communication, 2005.
Asia-Pac. J. Chem. Eng. 2010; 5: 909–914
DOI: 10.1002/apj
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