Chemical Engineering September 2014 - 51

Cover Story
Feature Report
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
O
perators and engineers in the
chemical process industries
(CPI) who are responsible for
the daily operations associated
with sludge dewatering and
drying equipment are undoubtedly
aware that wastewater-sludge
drying is not an easy task. A large
number of the problems that arise
during these operations are due to
the sticky phase of sludge, which
makes the drying of sludge as
much an art as a science. Although
insight into the sticky behavior
of sludge during drying processes
is critical for successfully drying
sludge in practice, the sticky phase
of sludge is not a common topic in
the standard engineering curriculum
at universities and colleges,
whereas topics such as the theoretical
design of dryers typically are.
Also, most research papers and reference
works on sludge drying tend
to emphasize the different types
of sludge dewatering and drying
technologies [1,2], rather than the
physical property changes that the
sludge undergoes while being reduced
in volume due to water removal.
The sludge's sticky character
is typically mentioned in just a
few lines, without elucidating the
reasons behind its existence. Reference
3, (a paper with the appropriate
title " Can You Handle Sticky
Cakes?, " ) concludes that " difficulties
in solid-liquid separations
usually arise because conventional
design optimizes only the unit operations
and takes no account of
the nature of the thickened product
or possible handling problems "
[3]. This assessment applies well to
the case of sludge dewatering and
drying. Providing more insight into
the peculiar sticky phenomenon
of sludge is the aim of this article.
Further, an overview of applied
strategies to tackle the operational
issues related to the sludge's " glueyness "
will be provided.
Growing mountains of sludge
Increasingly stringent environmental
regulations over time have resulted
in enhanced industrial and
domestic wastewater treatment
(see, for example, Chem. Eng. Oct.
2005 and Oct. 2013 issues [4,5]). The
improved wastewater treatment
has resulted, in turn, in increased
volumes of waste activated sludge.
In a recent review paper on the application
of wastewater-sludge drying,
some data are presented on the
quantities of generated municipal
sewage sludge, illustrating these
Figure 1. A rather simple laboratory
protocol can be used to map the sticky
phase of waste sludge (see Ref. 7)
increasing " mountains " of sludge.
In China, more than 9 million tons
of dry solid sludge were produced
in 2009. In the European Union
and the United States, this number
amounted to about 12 and 8 million
tons in 2010, respectively [6].
To lower the sludge volumes (in
order to decrease costs for further
downstream processing, including
transport, storage and incineration
costs), mechanical dewatering
and thermal drying of sludge are
important onsite unit operations in
CPI wastewater treatment plants
(WWTPs). Sludge management in
industrial sludge-handling installations
is, and will continue to be, a
challenge, and sticky phase of sludge
is an issue common to all.
The sticky phase of sludge
When sludge is partially dewatered,
it behaves as a sticky, paste-like
substance. In that state, it literally
glues onto the surface of the
dewatering and drying equipment.
In order to gain insight into the
stickiness of sludge, the authors of
this article developed a laboratory
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