Table Of ContentCFCeooavvteeurrr eSS ttRooerrpyyort Part 2
Challenges of Drying
Sticky Wastewater Sludge
In wastewater-sludge drying
and dewatering operations,
many issues arise from the
sticky properties of the sludge.
Here are some insights to address them
Bart Peeters, Monsanto Europe
Raf Dewil and Ilse Smets,
KU Leuven BioTeC
Operators and engineers in the erence 3, (a paper with the appro- Figure 1. A rather simple laboratory
chemical process industries priate title “Can You Handle Sticky protocol can be used to map the sticky
phase of waste sludge (see Ref. 7)
(CPI) who are responsible for Cakes?,”) concludes that “difficul-
the daily operations associ- ties in solid-liquid separations
ated with sludge dewatering and usually arise because conventional increasing “mountains” of sludge.
drying equipment are undoubt- design optimizes only the unit op- In China, more than 9 million tons
edly aware that wastewater-sludge erations and takes no account of of dry solid sludge were produced
drying is not an easy task. A large the nature of the thickened prod- in 2009. In the European Union
number of the problems that arise uct or possible handling problems” and the United States, this number
during these operations are due to [3]. This assessment applies well to amounted to about 12 and 8 million
the sticky phase of sludge, which the case of sludge dewatering and tons in 2010, respectively [6].
makes the drying of sludge as drying. Providing more insight into To lower the sludge volumes (in
much an art as a science. Although the peculiar sticky phenomenon order to decrease costs for further
insight into the sticky behavior of sludge is the aim of this article. downstream processing, including
of sludge during drying processes Further, an overview of applied transport, storage and incinera-
is critical for successfully drying strategies to tackle the operational tion costs), mechanical dewatering
sludge in practice, the sticky phase issues related to the sludge’s “glu- and thermal drying of sludge are
of sludge is not a common topic in eyness” will be provided. important onsite unit operations in
the standard engineering curricu- CPI wastewater treatment plants
lum at universities and colleges, Growing mountains of sludge (WWTPs). Sludge management in
whereas topics such as the theoret- Increasingly stringent environmen- industrial sludge-handling instal-
ical design of dryers typically are. tal regulations over time have re- lations is, and will continue to be, a
Also, most research papers and ref- sulted in enhanced industrial and challenge, and sticky phase of sludge
erence works on sludge drying tend domestic wastewater treatment is an issue common to all.
to emphasize the different types (see, for example, Chem. Eng. Oct.
of sludge dewatering and drying 2005 and Oct. 2013 issues [4,5]). The The sticky phase of sludge
technologies [1,2], rather than the improved wastewater treatment When sludge is partially dewatered,
physical property changes that the has resulted, in turn, in increased it behaves as a sticky, paste-like
sludge undergoes while being re- volumes of waste activated sludge. substance. In that state, it liter-
duced in volume due to water re- In a recent review paper on the ap- ally glues onto the surface of the
moval. The sludge’s sticky charac- plication of wastewater-sludge dry- dewatering and drying equipment.
ter is typically mentioned in just a ing, some data are presented on the In order to gain insight into the
few lines, without elucidating the quantities of generated municipal stickiness of sludge, the authors of
reasons behind its existence. Ref- sewage sludge, illustrating these this article developed a laboratory
ChemiCal engineering www.Chemengonline.Com September 2014 51
Sticky phase of Sludges 1 and 2
2,000
Cover Story
1,750
Sludge 5
(30% org.)
protocol that allows its Sludge 2
(47% org.)
vtaisilusa olinz athtieo np.r otFoocro l, sdeee- s (Pa) 1,500
Ref. 7. In short, a rather s
e
simple laboratory device str
(Figure 1) measures the ar 1,250
he Sludge 1
shear stress necessary S
(49% org.)
for letting pre-consol-
idated sludge (with a 1,000
certain composition)
start moving over a steel
Sludge 3
surface. A higher shear 750 Sludge 4
(35% org.)
stress requirement in- (35% org.)
dicates that the sludge
is stickier [7,8]. By plot-
500
ting the shear stress as %DS 0003400740500205101309060000800000790000000000040
1.4.7.9.1.3.5.6.8.0.2.4.6.7.8.0.1.3.5.6.8.9.1.2.5.6.7.9.1.3.5.6.8.1.3.5.7.0.2.5.8.1.4.7.0.3.6.8.0.
a function of the sludge 111112222223333333444444555556666777788899990
1
dry-solid percentage
(%DS), one obtains a Figure 2. The sticky behavior of sludges depends on dryness and organic content (org.). There
map of the sticky behav- are three repeats of the shear stress test at every %DS, and bars are the 95% confidence interval
(lines are to guide the eye). Data from Sludges 1 and 5 are adapted from Refs. 7 and 10, respectively
ior. In Figure 1, a sludge
sample is shown after it
was tested, along with the cylinder do better to take the plunge and 1. At low sludge dryness (high water
where this sludge sample was con- map the sticky phase for the sludge content), the wet sludge does not
tained during the shear test. in their own facilities. behave as a sticky substance be-
The mapping of the stickiness is The reason why the sticky phase cause at that stage, it is a biopo-
shown in Figure 2 for five different occurs at higher dryness for sludge lymer solution with low concen-
sludges from the Monsanto WWTP with a lower organic content will be- tration. The sludge makes loose
in Antwerp, Belgium. The organic come clear after we have explained contact with the surfaces of the de-
contents vary from 49% for sludge the (proposed) mechanism for the watering and drying equipment (to
1 to 30% for Sludge 5 (the organic sticky phenomenon of sludge [10]. be compared with aquaplaning)
fraction was analyzed according to An understanding of why sludge 2. When the sludge’s dryness in-
the standard methods [9]). The data should behave in such a sticky man- creases (and water content de-
depicted in Figure 2 suggest that the ner during the course of drying be- creases), the biopolymer solution
sludge’s sticky character depends on gins with a look at activated sludge becomes more and more concen-
both the sludge’s dryness and its or- as a matrix of long biopolymers trated — and becomes more and
ganic content. Considering Sludges (such as extracellular polymeric more sticky — until it reaches
1 and 2, samples behave most sticky substances — EPS) wherein the mi- the dryness region, wherein it be-
in the dryness range from 25–40% croorganisms are entrapped [11,12]. haves with most stickiness (this is
DS. Therefore, this dryness region In other words, in the context of its sludge-specific sticky phase)
is called the sticky phase of slud- explaining the physical property 3. The quite abrupt decrease in
ges 1 and 2 [7]. Likewise, the sticky changes of drying sludge, we think of sludge stickiness at higher dry-
phase of Sludges 3 and 4 is in the sludge as a kind of “biopolymer ma- ness, just beyond the sticky phase,
dryness region from 35–60%DS. For trix” whereby the biopolymers glue is attributed to the cavities that
Sludge 5, the sticky phase reaches together the microorganisms of the develop at the contact surface be-
from about 45–60% DS. sludge. The central role of the EPS tween the sludge and dryer equip-
These data support the concept in the sludge floc structure was also ment. These cavities are a result
that the sticky phase appears in highlighted in Chemical Engineering of the further concentration of
a particular dryness region (often [13]. During dewatering and drying, the biopolymer solution to such
cited in literature sources), but that the gluing biopolymers become more a critical extent that the biopoly-
the exact location of the sticky re- and more concentrated and a sticky mers will not spread out any more
gion depends on the characteristics mixture develops. Keeping in mind on the dryer wall to the same ex-
of the sludge, as exemplified by the the concept of a biopolymer solution, tent that they did at a somewhat
different curves in Figure 2. the stickiness curves for each of the lower dryness. At this point, the
Some room for error is thus very five individual sludges represented adhesiveness of the drying sludge
much justified in the interpretation in Figure 2 (which, in general, can decreases spectacularly with only
of the data provided in literature be qualitatively summarized by the small increases in dryness.
sources. Moreover, instead of using curve depicted in Figure 3) is ex- Figure 3 presents a summarizing
this information, engineers might plained as follows [10]: scheme of the sludge consistency
52 ChemiCal engineering www.Chemengonline.Com September 2014
Changes in physical consistency of sludge
ss) Liquid Sticky Dry-dust the drying stage, by either increas-
ne ing or decreasing the %DS, and the
e
hear stress ss – adhesiv ttc1hro. i Anigr cgwdee eprltiels -de e xsbaptynal ab tilhniinseehn dbeo idavo bampotoeivlvtyehem: odear ep mmprpaoltaorcyihxs
Se
n back-mixing of finally dried mate-
ki
c rial into the raw (mechanically de-
sti
( % DS watered) sludge feed stream of the
Dewatering / drying dryer. By doing so, the average %DS
in the blended feed to the dryer is
Figure 3. The changes in physical consistency of waste sludge during the course increased beyond the sticky phase
of dewatering and drying are summarized here of the sludge [2, 8, 22]. As a result,
the sludge mixture becomes crum-
during its course of dewatering and before the sludge will exhibit th e bly before being introduced into
drying. For a conceptual represen- same viscoplastic behavior com- the dryer, and it becomes easier to
tation of the sludge sticky behavior, pared to sludge with a higher EPS handle. For Sludge 1 (in Figure 2),
see Ref. 10. concentration. The latter sludge will an increase of the solids dryness
This brings us back to the ques- already attain its maximum sticki- to 45% DS would suffice, as illus-
tion raised above — that is, why does ness at lower dryness. trated in the upper part of Figure
sludge with a lower organic content 4, whereas for sludge 5, back-
have its sticky phase at higher dry- Sticky-phase strategies mixing of dried material would be
ness levels? Lower organic content The negative effects of the sticky needed to achieve about 65% DS in
of the sludge (as a result of a tempo- phase on dewatering and drying the feed to the dryer
rarily higher amount of precipitated installations include, but are not 2. A less established technique in-
CaCO salts in the sludge floc be- limited to, higher torque require- volves combined mechanical dew-
3
cause of changing wastewater com- ments in both decanter centrifuges atering and flash-drying systems
position, for example [14]) implies a [10, 15, 16] and sludge paddle dry- [20, 21]. By lowering the sludge
lower EPS content per unit mass of ers [17–19], and sludge buildup on %DS after the mechanically de-
sludge. Again keeping in mind the dryer surfaces [20, 21], potentially watering stage (that is, at the
above described biopolymer matrix causing equipment damage. beginning of the thermal drying
of sludge, the lower EPS concentra- To prevent these issues, strategies stage), one postpones the timing
tion at the start will require a more applied in industry are threefold. and place in the flash dryer where
extended dewatering or drying of Two strategies aim at controlling the sludge goes through its sticky
the sludge (or lower water content) the sludge %DS at the beginning of phase. By applying this strategy
References Washington, DC: American Public Health tion.& Separation 35, pp. 883–887, 1998.
Association, 2005.
1. Isaacs, M., Heywood, N., Blake, N., Alder- 17. Komline-Sanderson, Komline-Sanderson
man, N., Getting a Grip on Sludge, Chem. 10. Peeters B., Dewil R. and others, Addition of Paddle Dryer. Drying Technology for Bio-
Eng., October, pp. 80–90, 1995. polyaluminiumchloride (PACl) to waste acti- solids, Sludges and By-products (http://
vated sludge to mitigate the negative effects www.komline.com/downloads/brochures/KS-
2. Chen, G., Yue, P.L., Mujumdar, A.S., Sludge of its sticky phase in dewatering-drying op- SDB_080714.pdf), 2008.
dewatering and drying, Drying Technology erations, Water Research 47, pp. 3,600–3,609,
20, pp. 883–916, 2002. 2013. 18. Arlabosse, P., Chavez, S., Lecomte, D.,
Method for thermal design of paddle dryers:
3. Carleton, A.J., Heywood, N.I., Can you han- 11. Flemming, H.C., The perfect slime, Colloids application to municipal sewage sludge, Dry-
dle sticky cakes?, Filtration & Separation 20, and Surfaces B: Biointerfaces 86, pp. 251– ing Technology 22, pp. 2,375–2,393, 2004.
pp. 357–360, 1983. 259, 2011.
19. Ferrasse J.H., Arlabosse, P., Lecomte, D.,
4. An, L., Biological Wastewater Treatment: Se- 12. Sheng, G.-P., Yu, H.-Q., Li, X.-Y., Extracellu- Heat, momentum, and mass transfer mea-
lecting the Process, Chem. Eng., October, pp. lar polymeric substances (EPS) of microbial surements in indirect agitated sludge dryers,
38–43, 2013. aggregates in biological wastewater treat- Drying Technology 20, pp. 749–769, 2002.
5. Schultz, T.E., Biological Wastewater Treat- ment systems: a review. Biotechnology Ad- 20. Peeters B., Dewil R., Smets I.Y., Improved
ment, Chem. Eng. October, pp. 44–50, 2005. vances 28, pp. 882–894, 2010. process control of an industrial sludge-dryer
6. Bennamoun, L., Arlabosse, P., Léonard, A., 13. Peeters, B., Herman, S., Monitor Cations installation through binary logistic regres-
Review on fundamental aspect of application in CPI Wastewater for Better Performance, sion modeling of the fouling issues, J. Process
of drying process to wastewater sludge, Re- Chem. Eng. May, pp. 56–62, 2007. Control 22, pp. 1,387–1,396, 2012.
newable and Sustainable Energy Reviews 28, 14. Peeters B., Dewil R. and others, Quantifica- 21. Peeters, B., Mechanical dewatering and ther-
pp. 29–43, 2013. tion of the exchangeable calcium in activated mal drying of sludge in a single apparatus.
7. Peeters, B., Dewil, R. and others, Using a sludge flocs and its implication to sludge set- Drying Technology 28, pp. 454–459, 2010.
shear test-based lab protocol to map the tleability. Separation and Purification Tech- 22. Léonard, A., Meneses, E., Le Trong, E.,
sticky phase of activated sludge, Environ- nology 83, pp. 1–8, 2011. Salmon, T., Marchot, P., Toye, D., Crine, M.
mental Engineering Science 28, pp. 81–85, 15. Peeters B., Dewil R. and others, Avoiding Influence of back mixing on the convective
2011. sludge stickiness through addition of poly- drying of residual sludges in fixed bed, Water
8. Strand, A., Alsaker, J., Process and means aluminiumchloride (PACl). In: Proceed- Research 42, pp. 2,671–2,677, 2008.
for drying of sticky materials, U.S. Pat- ings of the 11th World Filtration Congress 23. Li, H., Zou, S., Li, C. Liming pretreatment
ent 7,562,465 B2 (www.freepatentsonline. (WFC11), Graz, Austria (April 16-20, 2012), reduces sludge build-up on the dryer wall
com/7562465.pdf), 2009. paper P368, 2012. during thermal drying. Drying Technology
9. APHA-AWWA-WEF, Standard Methods for 16. Leung, W.W.-F., Torque requirement for high- 30, pp. 1,563–1,569, 2012.
the Examination of Water and Wastewater. solids centrifugal sludge dewatering, Filtra-
ChemiCal engineering www.Chemengonline.Com September 2014 53
CCoovveerr SSttoorryy
Authors
1) increase and control solids dryness at the beginning of the drying stage by
Bart Peeters is a manufac-
adding (recycling) final dried product to the dewatered cake, to increase solids
turing technologist at Mon-
dryness beyond sticky phase before entrance in dryer santo Europe N.V. (Haven
627, Scheldelaan 460, 2040
Antwerp, Belgium; Phone:
+32 3 568 5762; Email: bart.
[email protected]),
Mechanical Thermal where he has been working
dewatering drying since 1998. He first served
as a process improvement
engineer at Eastman’s PVB
%DS 2 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 polymer manufacturing
plant on site, till 2004. Since then, he has been
Sludge Cake STICKY Final dried working at the environmental department of
feed PHASE product the company, with particular responsibilities in
improving the wastewater treatment plant and
the combined sludge centrifuge-dryer installa-
tion. Being a Master Black Belt he is also the Six
2) lower and control solids dryness at the beginning of the drying stage by lowering
Sigma program leader at the Monsanto Antwerp
the cake dryness after mechanical dewatering to postpone and, hence, avoid site. While working at Monsanto’s WWTP, he
sticky issues at the beginning of the (flash) dryer obtained his Ph.D. in Engineering from the KU
Leuven (Belgium) in 2011 on the research topic
“Effect of activated sludge composition on its de-
waterability and sticky phase”. Prior to that, he
Mechanical Thermal received his M.S.Ch.E. degree in 1998 from the
KU Leuven, plus his M.Bio.Ch.E. degree in 1996
dewatering drying from the university college De Nayer. He is the
author of 20 papers in scientific journals, techni-
cal magazines and international conferences.
%DS 2 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100
Sludge Cake STICKY Final dried Raf Dewil holds a M.S. Ch.E.
feed PHASE product (KU Leuven, 2003) and a PhD
in Bioscience Engineering
(University of Antwerp, 2006).
3) reduce sludge's intrinsic stickiness by adding PACl additive Currently, he is an assistant
professor with the Chemical
Engineering Department of
KU Leuven, where he heads
the Process and Environ-
Mechanical Thermal mental Technology Labora-
dewatering drying tory. Within the Faculty of
Engineering Technology of
the same university, he chairs the Sustainable
%DS 2 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 Chemical Process Technology Cluster. His main
research domains include (i) (bio)chemical con-
Sludge Cake STICKY Final dried version methods for the valorization of biomass
feed PHASE product and organic wastes (currently main focus on
anaerobic digestion), and (ii) the integration of
physicochemical treatment methods in waste-
Figure 4. Practical control strategies to tackle sticky issues during the course of water treatment (such as, advanced oxidation
sludge drying processes, ultrasound, microwave). He has au-
thored or co-authored over 60 publications in
peer-reviewed international journals and about
(depicted in the middle of Figure is explained by the bound hydra- 90 presentations at international conferences.
4), the drying sludge behaves as a tion water associated with the su- He is an associate editor with the Journal of
Environmental Management, a member of the
sticky paste only at a less critical per-aluminium structures of PACl editorial board of several international journals
place in the dryer, such that no op- solutions, attached to the exterior and a regular invited speaker at international
conferences.
erational issues are encountered of the sludge flocs upon dosing
3. Finally, an additive can be intro- of PACl to sludge. These shields Ilse Smets holds a Master
duced into the raw sludge feed of hydration water act as a type in BioSciences Engineering
and a Ph.D. in Engineering:
to mitigate, or even almost com- of lubrication (aquaplaning) and Chemical Technology, both
pletely eliminate the stickiness of guide the underlying sticky biopo- of the KU Leuven (Belgium).
She is currently associate pro-
the sludge (conceptually presented lymers through the dryness range, fessor at the Department of
Chemical Engineering of that
at the lower part of Figure 4). This where they otherwise would cause
same university. She heads a
can be accomplished by adding the sticky issues of partially dried team of about eight doctoral
researchers and focuses on
polyaluminium chloride (PACl) — sludge [10]. Also lime addition pre- monitoring, modeling and
a technique thoroughly tested and treatment of sludge would reduce optimization of biological wastewater-treatment
systems ranging from classic activated sludge
validated by the authors [10, 15, the sludge buildup on dryer walls, systems to membrane bioreactor (MBR) sys-
20]. The addition of rather small based on recent laboratory results tems, with a specific focus on the bioflocculation
aspects of activated sludge. Her teaching activi-
amounts of PACl to the raw sludge of Li and others [23]. ties also lie predominantly in this field. She is
a board member of B-IWA, the Belgian branch
feed of a combined centrifuge-dryer
of the International Water Association and is
system (only 10–20 L on a volumet- Acknowledgments the chair of the Bioprocesses and Biosystems
Technical Committee of IFAC, the International
ric sludge flow of 7 m³) has become The authors gratefully acknowledge Federation in Automation and Control. Her ac-
common practice at the Monsanto the mapping of the sludges’ sticky tive contributions to the field of wastewater
treatment, and bioprocesses in general, are fur-
WWTP in Antwerp for more than phase by Luc Vernimmen from the ther witnessed by several review duties, many
four years. The beneficial effect of Monsanto Laboratory. ■ memberships of International Programming
Committees and the co-organization of several
PACl conditioning of waste sludge Edited by Scott Jenkins conferences and workshops.
54 ChemiCal engineering www.Chemengonline.Com September 2014
