Chemical Engineering September 2014 - 52

Sticky phase of Sludges 1 and 2
2,000
Cover Story
1,750
protocol that allows its
visualization. For details
on the protocol, see
Ref. 7. In short, a rather
simple laboratory device
(Figure 1) measures the
shear stress necessary
for letting pre-consolidated
sludge (with a
certain composition)
start moving over a steel
surface. A higher shear
stress requirement indicates
that the sludge
is stickier [7,8]. By plotting
the shear stress as
a function of the sludge
dry-solid percentage
(%DS), one obtains a
map of the sticky behavior.
In Figure 1, a sludge
sample is shown after it
was tested, along with the cylinder
where this sludge sample was contained
during the shear test.
The mapping of the stickiness is
1,500
1,250
Sludge 1
(49% org.)
1,000
750
Sludge 3
500
%DS
Figure 2. The sticky behavior of sludges depends on dryness and organic content (org.). There
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
do better to take the plunge and
map the sticky phase for the sludge
in their own facilities.
The reason why the sticky phase
shown in Figure 2 for five different
sludges from the Monsanto WWTP
in Antwerp, Belgium. The organic
contents vary from 49% for sludge
1 to 30% for Sludge 5 (the organic
fraction was analyzed according to
the standard methods [9]). The data
depicted in Figure 2 suggest that the
sludge's sticky character depends on
both the sludge's dryness and its organic
content. Considering Sludges
1 and 2, samples behave most sticky
in the dryness range from 25-40%
DS. Therefore, this dryness region
is called the sticky phase of sludges
1 and 2 [7]. Likewise, the sticky
phase of Sludges 3 and 4 is in the
dryness region from 35-60%DS. For
Sludge 5, the sticky phase reaches
from about 45-60% DS.
These data support the concept
that the sticky phase appears in
a particular dryness region (often
cited in literature sources), but that
the exact location of the sticky region
depends on the characteristics
of the sludge, as exemplified by the
different curves in Figure 2.
Some room for error is thus very
much justified in the interpretation
of the data provided in literature
sources. Moreover, instead of using
this information, engineers might
occurs at higher dryness for sludge
with a lower organic content will become
clear after we have explained
the (proposed) mechanism for the
sticky phenomenon of sludge [10].
An understanding of why sludge
should behave in such a sticky manner
during the course of drying begins
with a look at activated sludge
as a matrix of long biopolymers
(such as extracellular polymeric
substances - EPS) wherein the microorganisms
are entrapped [11,12].
In other words, in the context of
explaining the physical property
changes of drying sludge, we think of
sludge as a kind of " biopolymer matrix "
whereby the biopolymers glue
together the microorganisms of the
sludge. The central role of the EPS
in the sludge floc structure was also
highlighted in Chemical Engineering
[13]. During dewatering and drying,
the gluing biopolymers become more
and more concentrated and a sticky
mixture develops. Keeping in mind
the concept of a biopolymer solution,
the stickiness curves for each of the
five individual sludges represented
in Figure 2 (which, in general, can
be qualitatively summarized by the
curve depicted in Figure 3) is explained
as follows [10]:
52 ChemiCal engineering www.Chemengonline.Com September 2014
1. At low sludge dryness (high water
content), the wet sludge does not
behave as a sticky substance because
at that stage, it is a biopolymer
solution with low concentration.
The sludge makes loose
contact with the surfaces of the dewatering
and drying equipment (to
be compared with aquaplaning)
2. When the sludge's dryness increases
(and water content decreases),
the biopolymer solution
becomes more and more concentrated
- and becomes more and
more sticky - until it reaches
the dryness region, wherein it behaves
with most stickiness (this is
its sludge-specific sticky phase)
3. The quite abrupt decrease in
sludge stickiness at higher dryness,
just beyond the sticky phase,
is attributed to the cavities that
develop at the contact surface between
the sludge and dryer equipment.
These cavities are a result
of the further concentration of
the biopolymer solution to such
a critical extent that the biopolymers
will not spread out any more
on the dryer wall to the same extent
that they did at a somewhat
lower dryness. At this point, the
adhesiveness of the drying sludge
decreases spectacularly with only
small increases in dryness.
Figure 3 presents a summarizing
scheme of the sludge consistency
Sludge 2
(47% org.)
Sludge 5
(30% org.)
(35% org.) Sludge 4
(35% org.)
Shear stress (Pa)
1.0
4.0
7.0
9.3
11.4
13.0
15.0
16.7
18.4
20.0
22.5
24.0
26.0
27.2
28.0
30.5
31.1
33.0
35.1
36.3
38.0
39.9
41.0
42.6
45.0
46.0
47.0
49.0
51.8
53.0
55.0
56.0
58.0
61.0
63.7
65.9
67.0
70.0
72.0
75.0
78.0
81.0
84.0
87.0
90.0
93.0
96.0
98.4
100.0

http://www.Chemengonline.Com

Chemical Engineering September 2014

Table of Contents for the Digital Edition of Chemical Engineering September 2014