Chemical Engineering September 2014 - 53

Changes in physical consistency of sludge
Liquid
Sticky
Dry-dust
% DS
Dewatering / drying
Figure 3. The changes in physical consistency of waste sludge during the course
of dewatering and drying are summarized here
during its course of dewatering and
drying. For a conceptual representation
of the sludge sticky behavior,
see Ref. 10.
This brings us back to the question
raised above - that is, why does
sludge with a lower organic content
have its sticky phase at higher dryness
levels? Lower organic content
of the sludge (as a result of a temporarily
higher amount of precipitated
CaCO3 salts in the sludge floc because
of changing wastewater composition,
for example [14]) implies a
lower EPS content per unit mass of
sludge. Again keeping in mind the
above described biopolymer matrix
of sludge, the lower EPS concentration
at the start will require a more
extended dewatering or drying of
the sludge (or lower water content)
References
1. Isaacs, M., Heywood, N., Blake, N., Alderman,
N., Getting a Grip on Sludge, Chem.
Eng., October, pp. 80-90, 1995.
2. Chen, G., Yue, P.L., Mujumdar, A.S., Sludge
dewatering and drying, Drying Technology
20, pp. 883-916, 2002.
3. Carleton, A.J., Heywood, N.I., Can you handle
sticky cakes?, Filtration & Separation 20,
pp. 357-360, 1983.
4. An, L., Biological Wastewater Treatment: Selecting
the Process, Chem. Eng., October, pp.
38-43, 2013.
5. Schultz, T.E., Biological Wastewater Treatment,
Chem. Eng. October, pp. 44-50, 2005.
6. Bennamoun, L., Arlabosse, P., Léonard, A.,
Review on fundamental aspect of application
of drying process to wastewater sludge, Renewable
and Sustainable Energy Reviews 28,
pp. 29-43, 2013.
7. Peeters, B., Dewil, R. and others, Using a
shear test-based lab protocol to map the
sticky phase of activated sludge, Environmental
Engineering Science 28, pp. 81-85,
2011.
8. Strand, A., Alsaker, J., Process and means
for drying of sticky materials, U.S. Patent
7,562,465 B2 (www.freepatentsonline.
com/7562465.pdf), 2009.
9. APHA-AWWA-WEF, Standard Methods for
the Examination of Water and Wastewater.
before the sludge will exhibit the
same viscoplastic behavior compared
to sludge with a higher EPS
concentration. The latter sludge will
already attain its maximum stickiness
at lower dryness.
Sticky-phase strategies
The negative effects of the sticky
phase on dewatering and drying
installations include, but are not
limited to, higher torque requirements
in both decanter centrifuges
[10, 15, 16] and sludge paddle dryers
[17-19], and sludge buildup on
dryer surfaces [20, 21], potentially
causing equipment damage.
To prevent these issues, strategies
applied in industry are threefold.
Two strategies aim at controlling
the sludge %DS at the beginning of
Washington, DC: American Public Health
Association, 2005.
10. Peeters B., Dewil R. and others, Addition of
polyaluminiumchloride (PACl) to waste activated
sludge to mitigate the negative effects
of its sticky phase in dewatering-drying operations,
Water Research 47, pp. 3,600-3,609,
2013.
11. Flemming, H.C., The perfect slime, Colloids
and Surfaces B: Biointerfaces 86, pp. 251-
259, 2011.
12. Sheng, G.-P., Yu, H.-Q., Li, X.-Y., Extracellular
polymeric substances (EPS) of microbial
aggregates in biological wastewater treatment
systems: a review. Biotechnology Advances
28, pp. 882-894, 2010.
13. Peeters, B., Herman, S., Monitor Cations
in CPI Wastewater for Better Performance,
Chem. Eng. May, pp. 56-62, 2007.
14. Peeters B., Dewil R. and others, Quantification
of the exchangeable calcium in activated
sludge flocs and its implication to sludge settleability.
Separation and Purification Technology
83, pp. 1-8, 2011.
15. Peeters B., Dewil R. and others, Avoiding
sludge stickiness through addition of polyaluminiumchloride
(PACl). In: Proceedings
of the 11th World Filtration Congress
(WFC11), Graz, Austria (April 16-20, 2012),
paper P368, 2012.
16. Leung, W.W.-F., Torque requirement for highsolids
centrifugal sludge dewatering, Filtrathe
drying stage, by either increasing
or decreasing the %DS, and the
third is an innovative approach
triggered by the biopolymer matrix
concept explained above:
1. A well-established method employs
back-mixing of finally dried material
into the raw (mechanically dewatered)
sludge feed stream of the
dryer. By doing so, the average %DS
in the blended feed to the dryer is
increased beyond the sticky phase
of the sludge [2, 8, 22]. As a result,
the sludge mixture becomes crumbly
before being introduced into
the dryer, and it becomes easier to
handle. For Sludge 1 (in Figure 2),
an increase of the solids dryness
to 45% DS would suffice, as illustrated
in the upper part of Figure
4, whereas for sludge 5, backmixing
of dried material would be
needed to achieve about 65% DS in
the feed to the dryer
2. A less established technique involves
combined mechanical dewatering
and flash-drying systems
[20, 21]. By lowering the sludge
%DS after the mechanically dewatering
stage (that is, at the
beginning of the thermal drying
stage), one postpones the timing
and place in the flash dryer where
the sludge goes through its sticky
phase. By applying this strategy
tion.& Separation 35, pp. 883-887, 1998.
17. Komline-Sanderson, Komline-Sanderson
Paddle Dryer. Drying Technology for Biosolids,
Sludges and By-products (http://
www.komline.com/downloads/brochures/KSSDB_080714.pdf),
2008.
18. Arlabosse, P., Chavez, S., Lecomte, D.,
Method for thermal design of paddle dryers:
application to municipal sewage sludge, Drying
Technology 22, pp. 2,375-2,393, 2004.
19. Ferrasse J.H., Arlabosse, P., Lecomte, D.,
Heat, momentum, and mass transfer measurements
in indirect agitated sludge dryers,
Drying Technology 20, pp. 749-769, 2002.
20. Peeters B., Dewil R., Smets I.Y., Improved
process control of an industrial sludge-dryer
installation through binary logistic regression
modeling of the fouling issues, J. Process
Control 22, pp. 1,387-1,396, 2012.
21. Peeters, B., Mechanical dewatering and thermal
drying of sludge in a single apparatus.
Drying Technology 28, pp. 454-459, 2010.
22. Léonard, A., Meneses, E., Le Trong, E.,
Salmon, T., Marchot, P., Toye, D., Crine, M.
Influence of back mixing on the convective
drying of residual sludges in fixed bed, Water
Research 42, pp. 2,671-2,677, 2008.
23. Li, H., Zou, S., Li, C. Liming pretreatment
reduces sludge build-up on the dryer wall
during thermal drying. Drying Technology
30, pp. 1,563-1,569, 2012.
ChemiCal engineering www.Chemengonline.Com September 2014 53
Shear stress
(stickiness - adhesiveness)
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