Journal of Water Resource and Protection, 2013, 5, 1-4 Published Online July 2013 (
Sludge Density Prediction in a Wastewater Chemical
Coagulation Process
Margarita Teutli-León1, María Elena Pérez-López2
1Engineering Department, Autonomous University of Puebla (BUAP), Puebla, Mexico
2Interdisciplinary Research Center for the Local Integral Research, National Polytechnical Institute at Durango
(IPN-CIIDIR-DGO), Durango, Mexico
Received April 25, 2013; revised May 27, 2013; accepted June 30, 2013
Copyright © 2013 Margarita Teutli-León, María Elena Pérez-López. This is an open access article distributed under the Creative
Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original
work is properly cited.
This paper reports an approach to estimate the sludge density in a physicochemical treatment of municipal wastewater,
experiments considered 4 coagulants (aluminum sulfate SAl, iron sulfate SFe, aluminum polychloride PAX, iron poly-
chloride PIX), and 2 flocculant products (cationic CP and anionic AP polymers). Experimental approach is based on
running a set of jar tests at different coagulant concentrations. After the stirring and resting times took place, pH and
conductivity were registered finding that SAl and SFe either with or without polymers are the coagulants producing the
higher pH drop. Conductivity measures also establish two kind of data since higher conductivity (about 2000 μS·cm1)
was observed for SAl, and PIX, PIX + CP, PIX + AP; otherwise a conductivity about 1300 μS·cm1 was observed for
SAl + PC, SFe and PAX alone and with CP or AP. Settleable solids (SST) determined with an Imhoff cone were similar
for sulfates and polychlorides, but dry sludge (DS) clearly set up two groups the one with higher sludge content corre-
sponds to sulfates group. The quotient of DS divided by the SST provided an estimation of the apparent sludge density,
in this way it was observed that higher densities were obtained for sludge from sulfates at lower coagulant concentrations;
also sludge from SFe was heavier than the one from SAl. Otherwise, polychlorides produced a lighter sludge in respect to
the one obtained with sulfates, and between them the PIX coagulant provided a heavier sludge than the PAX coagulant.
Keywords: Coagulant; Flocculant; Wastewater Treatment; Sludge Density
1. Introduction
Usually wastewater treatment plants (WWTP) are re-
ferred in three levels: primary, secondary and tertiary
treatment. In Latin America, primary wastewater treat-
ment has been implemented for several communities, but
it has been observed that the organic load is high enough
so additional steps are required in order to process the
wastewater, applied alternatives are either a secondary
treatment (biological) or the named Advanced Primary
Treatment (APT), the last one is a physicochemical
process which allows simultaneous precipitation of sev-
eral contaminants, since simultaneously use both coagu-
lant and flocculant for enhancing floc formation and set-
tling. According to Vesilind [1] an estimation of associ-
ated elimination yields correspond to 60% - 90% in set-
tleable solids (SST), 25% - 40% in biochemical oxygen
demand (BOD), 30% - 60% in chemical oxygen demand
(COD), 70% - 90% in phosphorus (P), and 80% - 90% in
Although an APT process use the same infrastructure
than a primary treatment, the fact that municipal waste-
water exhibit seasonal variations in its chemical compo-
sition leads to the need of running several jar tests in or-
der to find the right reagent dosage, which so far define
the amount of sludge produced and its handling proce-
dures. Usually to increase phosphorus removal, the che-
mical requirements impact increasing 15% the amount of
produced sludge since chemical addition may be in ex-
cess of stoichiometric requirements [2]. Another problem
derived from chemical addition is that sludge dewatering
becomes lowered by a raise in salinity, it has been proven
that both organic polyelectrolytes and alum provide an
improvement on dewaterability [3]. According to Kal-
deris [4] the most important factors in sludge manage-
ment costs are: 1) physical and chemical characteristics;
2) quantity; 3) legal framework; 4) potential valorization
and re-use; and 5) land availability and cost. Therefore, it
is important that WWTP operators have the capacity to
opyright © 2013 SciRes. JWARP
predict in a simple way the amount of sludge produced
from simple routine procedures such as determining SST,
and dry sludge.
This work is an approach to correlate SST and dry
sludge amount in order to predict sludge density based on
applied coagulant concentration.
2. Methodology
2.1. Reagents
Samples of municipal wastewater were obtained from a
WWTP located at the southwest in the city of Puebla.
This WWTP applies an APT process using aluminum
sulfate as coagulant. By this reason aluminum sulfate is
considered as one reagent. In jar tests applied chemical
products comprise the coagulants: aluminum sulfate
(SAl), iron sulfate (SFe), aluminum polychloride (PAX)
and iron polychloride (PIX); also, applied flocculants
were cationic and anionic polyacrylamide (CP, AP). All
of these chemicals were commercial products provided
by Kemira Company.
2.2. Instruments
The jar tests were run in a Kemwater flocculator using
the following coagulant concentrations: 40, 60, 80, 100,
120, 140 part per million (ppm) of each coagulant SAl,
SFe, PAX, PIX. Flocculants (CP, AP) were added in a
dosage of 1 ppm. During experimentation a follow up of
chemical parameters was done using portable meters
CONDUCTRONIC brand for pH and conductivity; oth-
erwise, color, turbidity, and COD were measured with
the appropriate routine in a HACH DR 2500 spectro-
photometer. Settleable solids (SST) were measured with
an Imhoff cone, and sludge drying was done in a Shell
Lab stove.
3. Results and Discussion
It is well known that SAl use produces a pH abatement
then it is required to raise the pH before applying the co-
agulant. Experimental pH data show that SAl and SFe
produced a pH drop of about one unit while the polychlo-
ride PAX and PIX drop pH in about 0.5 units.
Figure 1 presented results for the observed conductiv-
ity values for all coagulant and flocculant combinations.
As it can be observed there are two groups one of high
conductivity formed by SAl, PIX, PIX + CP, PIX + AP;
and one of lower conductivity formed by the Sal + CP,
SFe, SFe + CP, SFe + AP, PAX, PAX + CP, PAX + AP.
Determined settleable solids amount (SST) are presented
in Figures 2 and 3. Figure 2 corresponds to sulfates with
and without cationic polymer (CP). As it can be observed
inclusion of the CP produces a drop in the amount of
SST for SAl at all concentrations; otherwise, SFe is only
Figure 1. Conductivity results as function of coagulant
Figure 2. Settleable solids results for SAl and SFe as
function of coagulant concentration.
slightly affected at 80 and 100 ppm having a lowering in
SST, while all other concentrations provide the same
amount of SST with or without CP.
Figure 3 presented obtained results for aluminum and
iron polychlorides. As can be observed the PAX SST
decrease with the addition of either CP or AP; while the
PIX SST amount decrease slightly only with the AP at 80
and 120 ppm coagulant concentration.
Figure 4 presented results for the amount of Dry Slu-
dge (DS) obtained from each jar test, as it can be ob-
served there are two groups the first belongs to the sul-
fates SAl and SFe in the range of 0.2 to 0.35 g·l1, and
the second one in the range of 0.02 to 0.12 g·l1 corre-
sponds to the polychlorides PAX and PIX, with and
without polymers. Data for SAl exhibit an irregular point
at 80 ppm of coagulant, but once the CP is added values
become more regular reducing the sludge amount to the
range between 0.2 and 0.25 g1, interval which is lower
Copyright © 2013 SciRes. JWARP
Figure 3. Settleable solids for PAX and PIX as function of
coagulant concentration.
Figure 4. Dry sludge produced at jar tests as function of
coagulant concentration.
than the one for SFe which register similar amounts with
and without CP. The polychloride PAX produced similar
sludge quantities, in which the presence or absence of
polymer does not make a great difference. The PIX poly-
chloride produced similar amounts of sludge except at
the 80 ppm coagulant in presence of AP, which produced
the lower amount of sludge.
Considering the data from SST (cm3·l1) and sludge
mass (g·l1) it was estimated the sludge density in g cm3,
results are presented in Figure 5 for the sulfate group,
and in Figure 6 for the polychlorides group.
As it can be observed in Figure 5 the higher sludge
density was obtained with the lower coagulant concentra-
tion, use of Sal + CP enable to raise sludge density in
about 50%, while at higher concentration sludge density
increase is not higher to 30%. Otherwise, SFe exhibit
similar amounts of sludge with or without CP at any co-
Figure 5. Estimated sludge density from jar tests using SAl
and SFe coagulants.
Figure 6. Estimated sludge density from jar tests using PAX
and PIX coagulants.
agulant concentration, except for the 100 ppm in which
density increases due to the CP presence.
In Figure 6 it is shown the data for the polychloride
coagulants. As it can be observed the sludge density is
minimal for the PAX, and addition of CP or AP does not
make a great difference in sludge density, obtained val-
ues are not greater than 0.005 g·cm3 for most coagulant
concentrations. Otherwise, the PIX coagulant exhibit
higher density than PAX, and for this coagulant the
polymer makes a difference since PIX + CP is the com-
bination which provides the lower density at concen-
trations of 40 and 60 ppm; but for 80 and 100 ppm these
concentrations produced a higher sludge density.
4. Conclusions
From conductivity measures it can be affirmed that SAl
Copyright © 2013 SciRes. JWARP
Copyright © 2013 SciRes. JWARP
and PIX are the coagulants producing a higher salinity
condition which should affect sludge dewaterability.
Obtained dry sludge amount is higher when sulfates
are used as coagulants either with or without flocculant
presence. Also it is observed that Sal + CP produced
lower SST volumes than SAl, and so far higher density
sludge was obtained. Otherwise, in respect to SAl, SFe
produced a lower conductivity condition, and lower SST
volumes but calculated sludge density resulted greater
than the one obtained for SAl.
From polychlorides data, it can be seem that PAX ex-
hibited an almost uniform conductivity for all coagulant
concentrations, and produced higher SST volumes than
PIX; also dry sludge was less than the one obtained with
PIX; in consequence, calculated sludge density for PAX
was lower than the one obtained with PIX.
In general, estimated sludge densities were higher at
low coagulant dosages. Also, the higher sludge density
corresponds to the sulfates group, since observed range is
between 0.01 - 0.045 g·cm3; the medium range belongs
to PIX since sludge density is between 0.01 and 0.025
g·cm3; finally, the lower density belongs to PAX whose
values are in the range of 0.002 and 0.007 g·cm3.
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Water Environment Federation US, 2003.
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Sludge Production and Processing,” Water and Environ-
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on Mechanical Dewatering of Sludge with and without
Chemical Conditioning,” Environmental Science & Tech-
nology, Vol. 35, No. 23, 2001, pp. 4691-4696.
[4] D. Kalderis, M. Aivalioti and E. Gidarakos, “Options for
Sustainable Sewage Sludge Management in Small Waste-
water Treatment Plant on Islands: The Case of Crete,”
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