Responses of Coagulant Type, Dosage and Process Conditions to Phosphate Removal Efficiency from Anaerobic Sludge

Abstract

Phosphorus, a crucial component of life, may cause eutrophication if it is discharged untreated into the aquatic ecosystem. Phosphate (PO43-) may exist at an elevated level in anaerobic digestion (AD) effluents and can lead to the clogging of pipes by forming struvite crystals. This study was conducted to assess the responses of coagulant type, dosage and process conditions to phosphate removal efficiency from anaerobic sludge. The experiments were performed in two steps. First, a sensitivity test was conducted to compare five coagulant types (alum, poly-aluminum chloride (PAC), FeCl2, FeCl3 and PAC + FeCl3) at standard coagulation conditions. The results showed that PAC would be the best coagulant among the tested, while a combination of PAC and FeCl3 may be beneficial under circumstances. Second, an optimization study was performed for PAC using response surface methodology employing central composite design. Among the three independent variables (coagulant dosage, slow mixing duration and agitation speed), the dosage was the sole significant variable for phosphate removal efficiency, while the other two had limited effects. A future study to optimize the rapid mixing conditions would give additional insights into the process. The results of this study may be useful to design a process to counteract phosphate discharges from AD plants, as well as to reduce the risks of pipe clogging and maintenance problems due to crystalline struvite formation in the later stage of AD.

Full text

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Citation: Kim, D.W.; Yu, S.I.; Im, K.;
Shin, J.; Shin, S.G. Responses of
Coagulant Type, Dosage and Process
Conditions to Phosphate Removal
Efficiency from Anaerobic Sludge.
Int. J. Environ. Res. Public Health 2022,
19, 1693. https://doi.org/10.3390/
ijerph19031693
Academic Editor: Paul B.
Tchounwou
Received: 10 January 2022
Accepted: 30 January 2022
Published: 1 February 2022
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4.0/).
International Journal of
Environmental Research
and Public Health
Article
Responses of Coagulant Type, Dosage and Process Conditions
to Phosphate Removal Efficiency from Anaerobic Sludge
Dae Wook Kim, Sung Il Yu, Kyuyong Im, Juhee Shin and Seung Gu Shin *
Department of Energy Engineering, Future Convergence Technology Research Institute, Gyeongsang National
University, Jinju-daero, Jinju 501, Korea; wd6780@gmail.com (D.W.K.); tjd3427@naver.com (S.I.Y.);
jayoou03098@naver.com (K.I.); shinjh@gnu.ac.kr (J.S.)
*Correspondence: sgshin@gnu.ac.kr
Abstract:
Phosphorus, a crucial component of life, may cause eutrophication if it is discharged
untreated into the aquatic ecosystem. Phosphate (PO
43-
) may exist at an elevated level in anaerobic
digestion (AD) effluents and can lead to the clogging of pipes by forming struvite crystals. This study
was conducted to assess the responses of coagulant type, dosage and process conditions to phosphate
removal efficiency from anaerobic sludge. The experiments were performed in two steps. First,
a sensitivity test was conducted to compare five coagulant types (alum, poly-aluminum chloride
(PAC), FeCl
2
, FeCl
3
and PAC + FeCl
3
) at standard coagulation conditions. The results showed that
PAC would be the best coagulant among the tested, while a combination of PAC and FeCl
3
may
be beneficial under circumstances. Second, an optimization study was performed for PAC using
response surface methodology employing central composite design. Among the three independent
variables (coagulant dosage, slow mixing duration and agitation speed), the dosage was the sole
significant variable for phosphate removal efficiency, while the other two had limited effects. A future
study to optimize the rapid mixing conditions would give additional insights into the process. The
results of this study may be useful to design a process to counteract phosphate discharges from AD
plants, as well as to reduce the risks of pipe clogging and maintenance problems due to crystalline
struvite formation in the later stage of AD.
Keywords:
struvite; coagulation; response surface methodology; central composite design; optimiza-
tion
1. Introduction
As a component of nucleic acids and many other biomolecules, phosphorus (P) is
an essential element for living organisms [
1
]. Thus, phosphorus is regarded as one of the
macro-nutrients that are required for the growth of microorganisms in biological waste
and wastewater treatment processes, such as anaerobic digestion (AD) [
2
]. Conversely, an
excessive level of phosphorus in the water bodies, rooted from point and non-point sources,
may lead to eutrophication and serious algal blooms in the aquatic environment [
3
]. To
prevent this, phosphorus level in discharges is strictly regulated in most countries.
AD involves strict and facultative anaerobic microbes to degrade organic materials
under reducing environments. Although AD can successfully convert organic carbon into
biogas, a combustible, methane-rich fuel, the biochemical pathways of AD do not show
significant effects on phosphorus removal [
4
]. Instead, organic phosphorus in the feedstock
is largely decomposed into a more available form, such as phosphate (PO
43-
), and remains
in the digestate. Therefore, a direct discharge of the digestate has the potential to raise
aforementioned environmental issues to the aquatic ecosystem.
Struvite is a phosphate mineral with the formula of NH
4
MgPO
4·
6H
2
O [
5
]. Crystalline
struvite is often formed in AD plants and causes problems such as pipe clogging [
6
]. Once
clogged, the pipes need extensive maintenance efforts to remove struvite crystals. Thus,
Int. J. Environ. Res. Public Health 2022,19, 1693. https://doi.org/10.3390/ijerph19031693 https://www.mdpi.com/journal/ijerph

Int. J. Environ. Res. Public Health 2022,19, 1693 2 of 9
preventing its formation has been considered as a precautionary alternative [
7
]. Removing
one of the precursors (i.e., ammonium, magnesium and phosphate) from anaerobic sludge
can limit struvite precipitation in the plants [8].
Coagulation-flocculation is a widely practiced treatment method to remove solids
and some ionic species from wastewater discharges [
9
]. Coagulation of phosphate in
the anaerobic sludge can be accomplished by establishing a coagulation process after the
digestion stage. The effective coagulants may include Al-based ones, such as alum and
poly-aluminum chloride (PAC), and Fe-based ones, such as FeCl
2
and FeCl
3
[
10
]. While
the former (Al-based) is known to have excellent cohesiveness, the latter (Fe-based) has a
wide range of usable pH and is less affected by low temperature. In addition, the dosage of
coagulant, the agitation strength and the reaction time should affect the phosphate removal
efficiency. Selection of optimal coagulant and process conditions can lead to an efficient
phosphate removal from anaerobic effluents and control of the formation of struvite [11].
This study was designed to collect information to establish the coagulation process for
phosphate removal from AD sludge. The experiments were conducted in two stages. First,
four different compounds (alum, PAC, FeCl
2
and FeCl
3
) and one combination of them (PAC
with FeCl
3
) were compared as coagulant. Second, two best-performing coagulants (PAC
and FeCl
3
) were further evaluated for optimization by their working conditions: dosage,
agitation speed and reaction time. Response surface methodology (RSM) using a central
composite design (CCD) was employed for the optimization [12].
2. Materials and Methods
2.1. Sludge and Coagulants
Anaerobic sludge was collected from a domestic wastewater treatment plant (Jinju,
South Korea). The characteristics of the sludge, including the total phosphorus and
phosphate-P (PO
43-
-P) concentrations, are shown in Table 1. The coagulants used in
this study were alum (Al
2
(SO
4
)
3·
18H
2
O), PAC (Al
2
(OH)
5·
Cl), FeCl
2
(FeCl
2·
4H
2
O) and
FeCl
3
. The coagulants were prepared as stock solutions (300 g/L; 170 g/L for PAC) before
use.
Table 1. Characteristics of the anaerobic sludge samples (n= 4).
Parameter Average Standard Deviation Minimum Maximum
pH 7.5 0.2 7.2 7.7
Conductivity (µS/cm) 6.0 0.7 5.2 6.9
Total solids (mg/L) 21,855 2252 20,360 25,085
Volatile solids (mg/L) 15,281 1576 14,115 17,535
Total phosphorus (mg/L) 989 186 789 1197
PO43—P (mg/L) 169.2 10.8 153.4 177.9
Total nitrogen (mg/L) 2388 414 1963 2745
NH4+-N (mg/L) 634 108 480 730
Mg (mg/L) 7.1 0.4 6.6 7.6
2.2. Coagulant Sensitivity Experiment
The sensitivity of coagulation according to coagulant type was tested using a jar-tester
(C-JT-1, Changshin Science, South Korea). Anaerobic sludge (300 mL) was put in a beaker
(1 L) and kept at 30
◦
C during the reaction. Five doses (1, 2, 3 and 5 molar ratios of either
Al or Fe to phosphate) were tested using alum, PAC, FeCl
2
and FeCl
3
as the individual
coagulant. In addition, a mixture of PAC and FeCl
3
(PAC + FeCl
3
) was also tested; in this
case, a dose of nratio meant that both Al and Fe were added at nmolar ratio to phosphate,
summing up to 2n total metal ions. The total reaction time was set as 18 h, where 10 min
of initial rapid mixing was followed by 17 h 50 min of slow mixing. The agitation speed
for the rapid mixing was fixed at 150 rpm, while two different agitation speeds (20 and
50 rpm) were tested for the slow mixing period. The pH was checked after the coagulant
was added and the rapid mixing (10 min) was completed; if the pH was below 7.0, it was

Int. J. Environ. Res. Public Health 2022,19, 1693 3 of 9
corrected to 7.0 by adding NaOH (5 M). After completion of the reaction, the remaining
phosphate concentration was measured by spectrophotometry using HS-PO
4
(P)-H and
HS-PO4(P)-L kits (Humas, South Korea).
The remaining phosphate concentration profiles according to the dosage were evalu-
ated using an exponential decay model (Equation (1)):
y=y0+aexp(−bx) (1)
where yis the phosphate concentration, y
0
is the persisting phosphate concentration, ais
the removable phosphate concentration, bis the decay constant and xis the dose.
In addition, potential interaction of the two coagulants in the combination trial (PAC +
FeCl
3
) was assessed by comparing the phosphate removal efficiencies of the three trials
(PAC, FeCl
3
and PAC + FeCl
3
). First-order (liner), 1.5th-order (interaction) and second-
order (quadratic) models were developed to fit the results of the combination trial to the
results of the mono-coagulant trials (Equations (2)–(4)):
RP+F =β0+βPRP+βFRF(2)
RP+F =β0+βPRP+βFRF+βPFRPRF(3)
RP+F =β0+βPRP+βFRF+βPFRPRF+βPP RP2+βFFRF2(4)
where R
P+F
is the phosphate removal efficiency of the combination trial (PAC + FeCl
3
), R
P
and R
F
are, respectively, the phosphate removal efficiency of the PAC and FeCl
3
trials and
β
’s are the coefficients. The modeling procedure was conducted using R software package
(R Core Team, Austria).
2.3. Optimization Using RSM
Following the sensitivity experiment, PAC and FeCl
3
were further studied for optimiza-
tion using RSM. Jar tests were performed as Section 2.2 with the following modifications.
Three independent variables were investigated for their responses to phosphate removal
efficiency (Table 2for PAC and Table S1 for FeCl
3
): coagulant dosage (“Dose”, X
1
), slow
mixing duration (“Time”, X
2
) and agitation speed during the slow mixing (“RPM”, X
3
). The
range of the variables was determined following the sensitivity test and some additional
preliminary experiments (data not shown): 1 to 3 for X1, 20 to 180 min for X2and 20 to 70
rpm for X
3
. A CCD was developed using three levels (
−
1, 0, +1) of each variable: Dose (1,
2, 3), Time (20, 100, 180 min) and RPM (20, 45, 70 rpm). The face-centered design contained
18 trials: eight factorial points, six axial points and four center points (quadruplicate).
A quadratic model was derived to describe the responses of the independent variables
(Dose, Time and RPM) with the residual phosphate-P concentration ([PO
4
-P]) (Equation (5)):
Ym=β0+
n
∑
i=1
βiXi+
n
∑
i=1
βii X2
i+
n−1
∑
i=1
n
∑
j=i+1
βij XiXj(5)
where Y
m
is the response variable (i.e., [PO
4
-P]), and X
i
and X
j
are the independent variables.
The
β0
,
βi
,
βii
and
βij
are, respectively, the constant coefficient, the linear coefficients, the
quadratic coefficients and the interaction coefficients.
The statistical significance of the model was expressed as the coefficient of determi-
nation R
2
. To derive the interaction between the independent variables and the response
variable, an analysis of variance (ANOVA) was conducted. Statistical significance was
confirmed by the F-test and the model term was evaluated as a p-value with a 95% confi-
dence level. In addition, the optimal point was searched and visualized by a contour plot.
Minitab software (version 17) was used for experimental design and statistical analysis.

Int. J. Environ. Res. Public Health 2022,19, 1693 4 of 9
Table 2. The experimental design and results for the optimization study using PAC.
Run Number
(Randomized)
CCD Condition Residual [PO4−P] (mg/L)
Dose (X1)Time (X2)
(min)
RPM (X3)
(rpm) Actual Predicted
1 2 100 45 1.500 1.538
2 1 180 70 25.650 26.632
3 3 180 70 1.180 −0.554
4 3 20 70 0.370 2.271
5 2 100 45 1.275 1.583
6 1 20 70 15.800 15.948
7 3 20 20 0.520 −0.286
8 2 100 45 1.775 1.538
9 3 180 20 0.865 0.894
10 1 180 20 34.250 32.525
11 2 100 45 1.250 1.538
12 1 20 20 15.925 17.836
13 1 100 45 25.475 24.424
14 2 180 45 1.440 4.153
15 2 100 70 0.900 −0.132
16 3 100 45 0.895 1.770
17 2 20 45 1.110 −1.779
18 2 100 20 0.680 1.536
3. Results and Discussion
3.1. Sensitivity Test
The five coagulant trials (alum, PAC, FeCl
2
, FeCl
3
and PAC + FeCl
3
) were compared
for their phosphate removal efficiency from the anaerobic sludge (Figure 1). Overall, the
phosphate removal efficiency elevated as the coagulant dosage increased. This tendency
was confirmed by the high coefficient of determination (R
2
> 0.99) for the exponential decay
models (Table 3). Total phosphorus was removed between 9% and 19% compared to the
initial value (1197 mg/L).
The stoichiometric requirements for complete removal of phosphate (PO
43−
) are 1.0 for
Al
3+
or Fe
3+
and 1.5 for Fe
2+
. However, an elevated dose of 1.5–2.5 has been suggested as a
practical guideline for efficient coagulation using Al3+ [13]. Similarly, removal efficiencies
of ~90% or higher were observed at a coagulant dosage of 3 in this study (Figure 1), and
nearly complete phosphate removal was achieved for all trials at a dosage of 5.
Among the individual coagulants (alum, PAC, FeCl
2
and FeCl
3
), PAC showed the
most efficient phosphate removal, according to coagulant dosage (Figure 1). The phosphate
removal efficiencies for PAC were 77–80% or 98% at doses of 1 or 2, respectively, while
the counterpart efficiencies were 47–55% (dose 1) or 73–87% (dose 2) for alum, FeCl
2
and
FeCl
3
. Likewise, the exponential decay constants (b; Equation (1)) of PAC was twice as
high as those of the other chemicals at both 50 and 20 rpm agitation speeds (Table 3). The
higher coagulation performance of PAC over the other chemicals may be attributed to
its polymeric characteristics [
14
]. In addition, PAC has been gaining more attention as a
coagulant in the water industry for its “bridging” ability and lesser sludge production [
15
].
The co-treatment using PAC and FeCl
3
certainly improved the phosphate removal
compared to the mono-treatments, with 90–95% efficiency for dose 1 and 99% for dose 2
(Figure 1). However, dose “1” of PAC + FeCl
3
in this study means dose 1 of PAC plus dose
1 of FeCl
3
; therefore, compensation for the double dosage is necessary for comparison. The
compensated exponential decay constants for PAC + FeCl
3
were 1.819 at 50 rpm or 1.173
at 20 rpm, which were above (1.650; 50 rpm) or below (1.522; 20 rpm) the decay constants
for PAC only (Table 3). A similar recipe, poly-aluminum ferric chloride (PAFC), has been
reported as an efficient coagulant for organic and particulate matters in wastewater [
16
].
However, insufficient mixing at 20 rpm might have limited the coagulation performance of
the combinatory compounds in this study.

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Int. J. Environ. Res. Public Health 2022, 19, x FOR PEER REVIEW 5 of 9
compensated exponential decay constants for PAC + FeCl3 were 1.819 at 50 rpm or 1.173
at 20 rpm, which were above (1.650; 50 rpm) or below (1.522; 20 rpm) the decay constants
for PAC only (Table 3). A similar recipe, poly-aluminum ferric chloride (PAFC), has been
reported as an efficient coagulant for organic and particulate matters in wastewater [16].
However, insufficient mixing at 20 rpm might have limited the coagulation performance
of the combinatory compounds in this study.
Figure 1. The residual [PO4-P] profiles and model predictions according to the coagulant type. Ag-
itation speed of (a) 50 rpm or (b) 20 rpm was applied for the slow mixing (17 h 50 min) after the
initial rapid mixing (150 rpm) of 10 min.
Table 3. The exponential decay constants (b) and the coefficients of determination (R2) derived from
the sensitivity test.
Coagulant 50 rpm 20 rpm
b R2 b R2
Alum 0.687 0.991 0.679 0.994
PAC 1.650 0.999 1.522 0.999
FeCl2 0.805 0.993 0.825 0.999
FeCl3 0.765 0.994 0.621 0.997
PAC + FeCl
3
3.639
(1.8
19
)
*
0.999
2.346 (1.173)
*
0.999
* The decay constant was halved because the total dosage was double for PAC + FeCl3.
To speculate if any interaction existed between PAC and FeCl3 on the coagulation of
phosphate, three models (linear, interaction and quadratic) were compared to fit phos-
phate removal efficiency data between the co- and mono-treatments. Among the three
models, the interaction model showed the best fitting (R2 = 0.977), followed by the linear
(R2 = 0.953) and the quadratic (R2 < 0.8; data not shown) (Figure 2). The interaction model
had the following form (Equation (6)):
RP+F = 0.1349 + 0.9361RP + 0.4824RF − 0.5808RPRF (6)
where RP+F is the phosphate removal efficiency of the combination trial (PAC + FeCl3), and
RP and RF are the phosphate removal efficiency of the PAC and FeCl3 trials, respectively.
These results imply that PAC (coefficient of 0.9361) was about twice more influential than
FeCl3 (coefficient of 0.4824) to the overall phosphate removal efficiency when combined.
Interestingly, the coefficient for the interaction term was negative (−0.5808), possibly due
to the negative apparent interaction of the two coagulants (1.173 < 1.522) for the 20 rpm
trials (Table 3).
Figure 1.
The residual [PO
4
-P] profiles and model predictions according to the coagulant type.
Agitation speed of (
a
) 50 rpm or (
b
) 20 rpm was applied for the slow mixing (17 h 50 min) after the
initial rapid mixing (150 rpm) of 10 min.
Table 3.
The exponential decay constants (b) and the coefficients of determination (R
2
) derived from
the sensitivity test.
Coagulant 50 rpm 20 rpm
bR2bR2
Alum 0.687 0.991 0.679 0.994
PAC 1.650 0.999 1.522 0.999
FeCl20.805 0.993 0.825 0.999
FeCl30.765 0.994 0.621 0.997
PAC + FeCl33.639 (1.819) * 0.999 2.346 (1.173) * 0.999
* The decay constant was halved because the total dosage was double for PAC + FeCl3.
To speculate if any interaction existed between PAC and FeCl
3
on the coagulation of
phosphate, three models (linear, interaction and quadratic) were compared to fit phosphate
removal efficiency data between the co- and mono-treatments. Among the three models,
the interaction model showed the best fitting (R
2
= 0.977), followed by the linear (R
2
=
0.953) and the quadratic (R
2
< 0.8; data not shown) (Figure 2). The interaction model had
the following form (Equation (6)):
RP+F = 0.1349 + 0.9361RP+ 0.4824RF−0.5808RPRF(6)
where R
P+F
is the phosphate removal efficiency of the combination trial (PAC + FeCl
3
), and
R
P
and R
F
are the phosphate removal efficiency of the PAC and FeCl
3
trials, respectively.
These results imply that PAC (coefficient of 0.9361) was about twice more influential than
FeCl
3
(coefficient of 0.4824) to the overall phosphate removal efficiency when combined.
Interestingly, the coefficient for the interaction term was negative (
−
0.5808), possibly due
to the negative apparent interaction of the two coagulants (1.173 < 1.522) for the 20 rpm
trials (Table 3).

Int. J. Environ. Res. Public Health 2022,19, 1693 6 of 9
Int. J. Environ. Res. Public Health 2022, 19, x FOR PEER REVIEW 6 of 9
Figure 2. Three-dimensional plots of the (a) linear and (b) interaction models for the phosphate
removal efficiency using coagulant combination (PAC + FeCl3) to individual coagulants (PAC or
FeCl3).
3.2. Responses of Dosage, Reaction Time and Agitation Speed
The sensitivity experiment showed that PAC and FeCl3 were the best coagulant, in
terms of the dosage-to-removal efficiency, to precipitate phosphate from anaerobic
sludge. To explore the optimum conditions for phosphate coagulation, an RSM with face-
centered CCD was employed. Three independent variables were selected for investiga-
tion: coagulant dosage (“Dose”, X1), slow mixing duration (“Time”, X2) and agitation
speed during the slow mixing (“RPM” (revolution per minute), X3). The range of the var-
iables was determined following the sensitivity test and some additional preliminary ex-
periments (data not shown): 1 to 3 for X1, 20 to 180 min for X2 and 20 to 70 rpm for X3. In
addition, a separate preliminary experiment was conducted to compare the rapid mixing
duration of 1, 2, 3, 5, 10 and 30 min, and 10 min was concluded as the optimum time.
Eighteen conditions, including the quadruple trials at the center point, were assessed
for their phosphate removal efficiencies (Tables 2 and S1). While the RSM analysis was
not able to produce a suitable model for FeCl3 due to lack-of-fit, the analysis has success-
fully derived a quadratic model to estimate the residual phosphate concentration for PAC
(Equation (7)):
Ym = 58.26 − 55.34X1 + 0.1550X2 + 0.048X3 + 11.56X12 − 0.000055X22 −
0.00134X32 − 0.04222X1X2 + 0.0445X1X3 − 0.000501X2X3 (7)
where Ym is the residual [PO4-P], X1 is the coagulant dosage, X2 is the slow mixing duration
and X3 is the agitation speed during the slow mixing. This model fit well with the actual
data (R2 = 0.916, p = 0.007, Table 4) with no significant lack-of-fit (p > 0.05) and a good
agreement between the two datasets (Table 2, Figure 3). Thus, this model (Equation (7))
can be regarded as an adequate estimation of the responses of the independent variables
to the dependent variable within the defined region.
The significance of the model terms was verified using ANOVA (Table 4). Out of the
three, only one linear term, D (X1), could be regarded significant (p < 0.05). This result
agrees with the contour patterns where Dose is the major driver of the residual [PO4-P]
(Figure 4a,b). Conversely, the slope of the contour according to Time (X2) and RPM (X3)
variations was only mild (Figure 4c). Except for Dose × Dose, the quadratic and the inter-
action terms were not statistically significant (p > 0.05), and no clear pattern of interaction
was observable from the RSM plots (Figure 4).
Overall, Dose was the sole parameter that significantly shaped the phosphate re-
moval efficiency from the anaerobic sludge using PAC. This is in accordance with previ-
ous studies where the amount of added coagulant was considered as an important factor
Figure 2.
Three-dimensional plots of the (
a
) linear and (
b
) interaction models for the phosphate re-
moval efficiency using coagulant combination (PAC + FeCl
3
) to individual coagulants (PAC or FeCl
3
).
3.2. Responses of Dosage, Reaction Time and Agitation Speed
The sensitivity experiment showed that PAC and FeCl
3
were the best coagulant,
in terms of the dosage-to-removal efficiency, to precipitate phosphate from anaerobic
sludge. To explore the optimum conditions for phosphate coagulation, an RSM with face-
centered CCD was employed. Three independent variables were selected for investigation:
coagulant dosage (“Dose”, X
1
), slow mixing duration (“Time”, X
2
) and agitation speed
during the slow mixing (“RPM” (revolution per minute), X
3
). The range of the variables
was determined following the sensitivity test and some additional preliminary experiments
(data not shown): 1 to 3 for X
1
, 20 to 180 min for X
2
and 20 to 70 rpm for X
3
. In addition, a
separate preliminary experiment was conducted to compare the rapid mixing duration of
1, 2, 3, 5, 10 and 30 min, and 10 min was concluded as the optimum time.
Eighteen conditions, including the quadruple trials at the center point, were assessed
for their phosphate removal efficiencies (Table 2and Table S1). While the RSM analysis
was not able to produce a suitable model for FeCl
3
due to lack-of-fit, the analysis has
successfully derived a quadratic model to estimate the residual phosphate concentration
for PAC (Equation (7)):
lYm= 58.26 −55.34X1+ 0.1550X2+ 0.048X3+ 11.56X12−0.000055X22−
0.00134X32−0.04222X1X2+ 0.0445X1X3−0.000501X2X3
(7)
where Y
m
is the residual [PO
4
-P], X
1
is the coagulant dosage, X
2
is the slow mixing duration
and X
3
is the agitation speed during the slow mixing. This model fit well with the actual
data (R
2
= 0.916, p= 0.007, Table 4) with no significant lack-of-fit (p> 0.05) and a good
agreement between the two datasets (Table 2, Figure 3). Thus, this model (Equation (7)) can
be regarded as an adequate estimation of the responses of the independent variables to the
dependent variable within the defined region.
The significance of the model terms was verified using ANOVA (Table 4). Out of
the three, only one linear term, D (X
1
), could be regarded significant (p< 0.05). This
result agrees with the contour patterns where Dose is the major driver of the residual
[PO
4
-P] (Figure 4a,b). Conversely, the slope of the contour according to Time (X
2
) and
RPM (X
3
) variations was only mild (Figure 4c). Except for Dose
×
Dose, the quadratic and
the interaction terms were not statistically significant (p> 0.05), and no clear pattern of
interaction was observable from the RSM plots (Figure 4).

Int. J. Environ. Res. Public Health 2022,19, 1693 7 of 9
Table 4. The ANOVA results of the quadratic model for PAC derived from RSM.
Term Degree of Freedom F-Value p-Value
Model 10 7.60 0.007
Dose (X1) 1 9.81 0.017
Time (X2) 1 3.71 0.095
RPM (X3) 1 0.72 0.426
Dose ×Dose (X12)1 8.90 0.020
Time ×Time (X22)1 0.25 0.632
RPM ×RPM (X32)1 0.25 0.630
Dose ×Time (X1×X2) 1 2.54 0.155
Dose ×RPM (X1×X3) 1 4.50 0.072
Time ×RPM (X2×X3) 1 4.66 0.068
Lack-of-fit 4 4.62 0.120
Int. J. Environ. Res. Public Health 2022, 19, x FOR PEER REVIEW 7 of 9
[10,17]. As the coagulation performance tends to saturate when a coagulant dose increases
[18], optimization is crucial for the economic feasibility of the process. Depending on the
coagulant type and target [PO4-P], a dose of 2–5 can be suggested from the results of this
study. Because the effects of the slow mixing regime (i.e., the duration and the agitation
speed) were unclear in this study, optimizing the rapid mixing conditions and minimizing
the slow mixing step could be tested in future studies. However, as shown in the case of
PAC + FeCl3 (Table 3), slow mixing conditions could affect the coagulation performance
of a combinatory chemical. The effects of pH on the coagulation between PAC and phos-
phorus have been shown two-fold: improved coagulation at acidic conditions (3.0–5.5),
on the one hand, and elevated Al(OH)3(s) level at neutral range on the other hand [19].
Thus, it could be inferred that the neutral pH (~7.0) applied in the coagulation process in
this study may have facilitated the coagulation process. One of the limitations of this study
is that only one type of sludge (from a domestic wastewater treatment plant) was tested.
Indeed, our preliminary experiments with two other sludge types (from a food waste di-
gestion plant and a combined food waste and sewage sludge digestion plant) showed
comparable phosphate removal performance (data not shown); a future study for various
sludge types would be valuable. Finally, it should be noted that the [PO4-P] in the anaer-
obic sludge is generally higher than the total phosphorus levels in typical sewage (<10
mg/L) [20]; therefore, a further optimization study would be necessary for sewage [21].
Table 4. The ANOVA results of the quadratic model for PAC derived from RSM.
Term Degree of freedom F-value p-value
Model 10 7.60 0.007
Dose (X1) 1 9.81 0.017
Time (X2) 1 3.71 0.095
RPM (X3) 1 0.72 0.426
Dose × Dose (X12) 1 8.90 0.020
Time × Time (X22) 1 0.25 0.632
RPM × RPM (X32) 1 0.25 0.630
Dose × Time (X1×X2) 1 2.54 0.155
Dose × RPM (X1×X3) 1 4.50 0.072
Time × RPM (X
2
×X
3
)
1
4.66
0.068
Lack-of-fit 4 4.62 0.120
Figure 3. The actual and predicted [PO4-P] profiles after the coagulation process using PAC.
Figure 3. The actual and predicted [PO4-P] profiles after the coagulation process using PAC.
Int. J. Environ. Res. Public Health 2022, 19, x FOR PEER REVIEW 8 of 9
Figure 4. Contour plots of the responses of the residual [PO4-P] according to PAC dose (Dose), slow
mixing time (Time) and slow mixing speed (RPM). Cross-sections are shown for (a) Dose × Time,
(b) Dose × RPM or (c) Time × RPM.
4. Conclusions
The phosphate removal from AD sludge using coagulation was assessed in two steps.
The sensitivity test compared five coagulant types to conclude that PAC (or combined
PAC and FeCl3) would be the most efficient coagulant. The optimization study produced
a suitable quadratic model for PAC but not for FeCl3. The dose of PAC was the significant
variable for phosphate removal efficiency, while the effects of the duration and agitation
speed for the slow mixing were limited. Following the results of this study, some future
study directions could be suggested: (1) using different anaerobic sludge types, (2) opti-
mizing rapid mixing conditions and (3) exploring different variable ranges for other co-
agulant types.
Supplementary Materials: The following supporting information can be downloaded at:
www.mdpi.com/xxx/s1, Table S1: The experimental design and results for the optimization study
using FeCl3.
Author Contributions: Conceptualization, S.G.S.; methodology, D.W.K. and J.S.; formal analysis,
D.W.K.; investigation, D.W.K., S.I.Y. and K.I.; writing—original draft preparation, D.W.K.; writ-
ing—review and editing, all authors; supervision, S.G.S.; funding acquisition, S.G.S. All authors
have read and agreed to the published version of the manuscript.
Funding: This work was supported by Gyeongsang National University Grant in 2020–2021.
Conflicts of Interest: The authors declare no conflict of interest.
References
1. Madigan, M.T.; Martinko, J.M.; Parker, J. Brock Biology of Microorganisms, 9th ed.; Prentice Hall: Upper Saddle River, NJ, USA,
2000; pp. 29–47.
2. Daneshgar, S.; Callegari, A.; Capodaglio, A.G.; Vaccari, D. The Potential Phosphorus Crisis: Resource Conservation and Possible
Escape Technologies: A Review. Resources 2018, 7, 37.
3. Smith, V.H. Eutrophication of freshwater and coastal marine ecosystems a global problem. Environ. Sci. Pollut. Res. 2003, 10,
126–139.
4. Lin, H.; Gan, J.; Rajendran, A.; Reis, C.E.R.; Hu, B. Phosphorus removal and recovery from digestate after biogas production. In
Biofuels-Status and Perspective; IntechOpen: London, UK, 2015.
5. Gong, W.; Li, Y.; Luo, L.; Luo, X.; Cheng, X.; Liang, H. Application of struvite-MAP crystallization reactor for treating cattle
manure anaerobic digested slurry: Nitrogen and phosphorus recovery and crystal fertilizer efficiency in plant trials. Int. J. En-
viron. Res. Public Health 2018, 15, 1397.
6. Fattah, K.P. Assessing Struvite Formation Potential at Wastewater Treatment Plants. Int. J. Environ. Sci. Dev. 2012, 3, 548–552.
7. Shu, L.; Schneider, P.; Jegatheesan, V.; Johnson, J. An economic evaluation of phosphorus recovery as struvite from digester
supernatant. Bioresour. Technol. 2006, 97, 2211–2216.
8. Doyle, J.D.; Parsons, S.A. Struvite formation, control and recovery. Water Res. 2002, 36, 3925–3940.
Figure 4.
Contour plots of the responses of the residual [PO
4
-P] according to PAC dose (Dose), slow
mixing time (Time) and slow mixing speed (RPM). Cross-sections are shown for (
a
) Dose
×
Time, (
b
)
Dose ×RPM or (c) Time ×RPM.
Overall, Dose was the sole parameter that significantly shaped the phosphate removal
efficiency from the anaerobic sludge using PAC. This is in accordance with previous studies
where the amount of added coagulant was considered as an important factor [
10
,
17
].
As the coagulation performance tends to saturate when a coagulant dose increases [
18
],
optimization is crucial for the economic feasibility of the process. Depending on the
coagulant type and target [PO
4
-P], a dose of 2–5 can be suggested from the results of this
study. Because the effects of the slow mixing regime (i.e., the duration and the agitation
speed) were unclear in this study, optimizing the rapid mixing conditions and minimizing

Int. J. Environ. Res. Public Health 2022,19, 1693 8 of 9
the slow mixing step could be tested in future studies. However, as shown in the case of
PAC + FeCl
3
(Table 3), slow mixing conditions could affect the coagulation performance of a
combinatory chemical. The effects of pH on the coagulation between PAC and phosphorus
have been shown two-fold: improved coagulation at acidic conditions (3.0–5.5), on the
one hand, and elevated Al(OH)
3(s)
level at neutral range on the other hand [
19
]. Thus, it
could be inferred that the neutral pH (~7.0) applied in the coagulation process in this study
may have facilitated the coagulation process. One of the limitations of this study is that
only one type of sludge (from a domestic wastewater treatment plant) was tested. Indeed,
our preliminary experiments with two other sludge types (from a food waste digestion
plant and a combined food waste and sewage sludge digestion plant) showed comparable
phosphate removal performance (data not shown); a future study for various sludge types
would be valuable. Finally, it should be noted that the [PO
4
-P] in the anaerobic sludge
is generally higher than the total phosphorus levels in typical sewage (<10 mg/L) [
20
];
therefore, a further optimization study would be necessary for sewage [21].
4. Conclusions
The phosphate removal from AD sludge using coagulation was assessed in two steps.
The sensitivity test compared five coagulant types to conclude that PAC (or combined PAC
and FeCl
3
) would be the most efficient coagulant. The optimization study produced a
suitable quadratic model for PAC but not for FeCl
3
. The dose of PAC was the significant
variable for phosphate removal efficiency, while the effects of the duration and agitation
speed for the slow mixing were limited. Following the results of this study, some future
study directions could be suggested: (1) using different anaerobic sludge types, (2) optimiz-
ing rapid mixing conditions and (3) exploring different variable ranges for other coagulant
types.
Supplementary Materials:
The following supporting information can be downloaded at: https:
//www.mdpi.com/article/10.3390/ijerph19031693/s1, Table S1: The experimental design and results
for the optimization study using FeCl3.
Author Contributions:
Conceptualization, S.G.S.; methodology, D.W.K. and J.S.; formal analysis,
D.W.K.; investigation, D.W.K., S.I.Y. and K.I.; writing—original draft preparation, D.W.K.; writing—
review and editing, all authors; supervision, S.G.S.; funding acquisition, S.G.S. All authors have read
and agreed to the published version of the manuscript.
Funding: This work was supported by Gyeongsang National University Grant in 2020–2021.
Conflicts of Interest: The authors declare no conflict of interest.
References
1.
Madigan, M.T.; Martinko, J.M.; Parker, J. Brock Biology of Microorganisms, 9th ed.; Prentice Hall: Upper Saddle River, NJ, USA,
2000; pp. 29–47.
2.
Daneshgar, S.; Callegari, A.; Capodaglio, A.G.; Vaccari, D. The Potential Phosphorus Crisis: Resource Conservation and Possible
Escape Technologies: A Review. Resources 2018,7, 37. [CrossRef]
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Smith, V.H. Eutrophication of freshwater and coastal marine ecosystems a global problem. Environ. Sci. Pollut. Res.
2003
,10,
126–139. [CrossRef] [PubMed]
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Lin, H.; Gan, J.; Rajendran, A.; Reis, C.E.R.; Hu, B. Phosphorus removal and recovery from digestate after biogas production. In
Biofuels-Status and Perspective; IntechOpen: London, UK, 2015.
5.
Gong, W.; Li, Y.; Luo, L.; Luo, X.; Cheng, X.; Liang, H. Application of struvite-MAP crystallization reactor for treating cattle
manure anaerobic digested slurry: Nitrogen and phosphorus recovery and crystal fertilizer efficiency in plant trials. Int. J. Environ.
Res. Public Health 2018,15, 1397. [CrossRef] [PubMed]
6.
Fattah, K.P. Assessing Struvite Formation Potential at Wastewater Treatment Plants. Int. J. Environ. Sci. Dev.
2012
,3, 548–552.
[CrossRef]
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Shu, L.; Schneider, P.; Jegatheesan, V.; Johnson, J. An economic evaluation of phosphorus recovery as struvite from digester
supernatant. Bioresour. Technol. 2006,97, 2211–2216. [CrossRef] [PubMed]
8. Doyle, J.D.; Parsons, S.A. Struvite formation, control and recovery. Water Res. 2002,36, 3925–3940. [CrossRef]

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Park, H.; Lim, S.-i.; Lee, H.; Woo, D.-S. Water blending effects on coagulation-flocculation using aluminum sulfate (alum),
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