Chemical Engineering September 2016 - 53

Disintegrated floc structure (Ca2+
bridges exchanged by Na+). The
filaments can still provide clear
supernatant as they act as a filter
Conventional
activated sludge floc
Filament
Conventional bacteria
EPS (Extracellular
polymeric substances)
Enmeshed CaCO3 solids
FIGURE 4. This schematic represents
sludge that has filaments protruding from
the conventional flocs. For illustrative
purpose, a disintegrated floc is shown in
the upper right corner; it has disintegrated
due to a loss of the bridging calcium ions;
the filaments, however, do play an important
role as a filter, preventing the " wash
out " of the small solids that are produced
when flocs disintegrate
Na+
Ca2+
Sodium ions
Calcium ions
Filament
of jelly-like EPS. These biopolymers are
well recognized in the microscopic photographs
shown in Figure 2, appearing
as hazy, fluffy-looking " clouds. " The microorganisms
produce larger amounts
of these EPS when there is insufficient
P and N available. Specifically, when the
organisms are not able to produce sufficient
P- and N-containing cell material
from the breakdown of the organic components
in the wastewater, they will, instead,
start producing more extracellular
polysaccharides.
-0.6
-0.65
-0.7
20.4% DS
19.3% DS
-0.85
-0.8
15.7% DS
14.3% DS
-0.85
0.2
0.4
log Ps [Ps in kPa] (logarithm of compaction pressure)
0.6
0.8
1
1.2
1.3(Ps = 20 kPa)
1.4
1.6
1.8
1.6 (Ps = 40 kPa)
2
Impacting settleability
The settling ability of a sludge is measured
by means of the sludge-volume
index (SVI), which expresses the volume
(in mL) that 1 g of activated sludge (referring
to dry solid matter) occupies after
30 min of settling. The SVI is determined
using an easily performed sedimentation
test in the laboratory, in accordance with
the Dutch norm NEN 6624 [4].
Using this test method, the mixedliquor
sample taken from the biodegradation
basin is initially diluted with final
clarified water, to obtain a settled volume
in the range from 200 to 300 mL/L.
The SVI of 1,230 sludge samples (covering
a 12-year period, from 2003 until
2015) is shown in Figure 3, as a function
of the sludge's inorganic fraction. This
inorganic fraction is another important
sludge-quality parameter, determined as
the complement of the sludge's volatile
(organic) fraction, which is determined
according to APHA Standard Meth ods
[5]. The data in Figure 3 are further stratified
according to the type of sludge -
that is, conventional sludge flocs (data
points are shown in black color; N =
914) and sludge with filaments (data
points are shown in red; N = 316).
From Figure 3 it can be discerned that
the inorganic fraction has a major influence
on the SVI. For instance, a higher
inorganic fraction produced a heavier
floc, and this led to better sludge-settling
capabilities, which is represented
by a lower SVI value (Note: The WWTP
discussed here handles a stream that is
characterized by varying and sometimes
high levels of calcium [6]).
-0.9
FIGURE 5. As a result of differences in their structure, conventional activated sludge flocs (data in
black) and sludge with filaments and excessive biopolymers (data in red) show variable compaction
curves (In this case, both sludge samples have a 45% inorganic fraction; DS refers to dry substance)
54
So-called " ideal sludge flocs " (the
data points that are shown in black in
Figure 3) are characterized by a maximum
SVI of about 120 mL/g [2], which
was attained at a low inorganics fraction
of 15%. On the other side of the inorganics
spectrum (at an inorganics rate
of 90%), the SVI is as low as 20 mL/g.
As discussed earlier, the presence of
filaments has a clear detrimental impact
on sludge settling. At a high inorganic
fraction (for instance, 70%), the negative
impact of the filaments remains limited.
In the WWTP discussed here, the average
SVI increases from 25 to 50 mL/g.
This still yields a sludge with very good
settling qualities.
However, when the heavy inorganic
fraction decreases as a result of
less well-precipitated solids enmeshCHEMICAL
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SEPTEMBER 2016
log (%DS) (logarithm of sludge cake dry substance)

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Chemical Engineering September 2016

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