Effect of Chemical Conditioning of Sewage Sludge on Dewatering

Performance and Structural Components

Tian Wan

1,*

, Chuanrui Li

, Xiaokang Ju

, Xingxing Yan

, Min Wang

, Jiehui Ren

, and Wen Cheng

State Key Laboratory of Eco-hydraulics in Northwest Arid Region of China, Xi’an University of Technology, No.5, South

Jinhua Road, Xian, Shaanxi, 710048, P.R. China

The third reclaimed water plant of Xi'an water purification Co., Ltd, Xi 'an, China

Keywords:

Excess sludge, dewatering performance, Extracellular polymer, fractal dimension

Abstract: Chemical conditioning is a commonly used sewage sludge pretreatment method of dewatering in wastewater

treatment plants. As a result of a large number of chemicals used, the cost of sewage treatment plants increased,

and negative environmental impact might be happened. Therefore, it is necessary to illustrate the dewatering

performance and structural components during chemical conditioning, in order to control the dosage of

chemicals. In this paper, the excess sludge was treated by cationic polyacrylamide (CPAM). The physical and

chemical properties of excess sludge were analyzed. The moisture content of sludge filter cake, the change of

moisture distribution, the specific resistance of filtration, and capillary suction time (CST) of sludge were

used as dewaterability parameters. The Zeta potential and particle size of sludge were measured, and the

change of fractal dimension was analyzed. The changes of protein and polysaccharide content in extracellular

polymeric substances (EPS) during conditioning were explored. Results showed that with the optimal CPAM

dosage of 0.225 g/L, the moisture content of sludge cake reached 70.19 %, the specific resistance of sludge

was 0.15×1012 m/kg, and CST was 6.45s. After conditioning, the Zeta negative electricity decreased, the

particle size increased, the fractal dimension increased, and the contents of protein and polysaccharide in EPS

increased. Tight EPS (TB-EPS) was released to loosen EPS (LB-EPS) and dissolved EPS (S-EPS), indicating

that cationic polyacrylamide (CPAM) can effectively improve the dewatering performance of sludge.

1 INTRODUCTION

With the rapid development of urbanization and

industrialization, the total amount of sewage

discharge and the output of municipal sludge in China

have increased significantly. The disposal and

management of sewage sludge is urgent. High

moisture content in excess sludge causes a huge

volume, which is the mainbottleneck problem for

effective disposal (Zhou et al., 2014). Dewatering

was known as the most useful approach to improve

the treatment and reuse of excess sludge.

Conditioning is commonly used as pretreatment

method to increase the sludge cake volume (Zhen et

al., 2018). Among various conditioning methods,

chemical coagulants such as ferric and aluminum

salts and bioflocculant are the most commonly

method to improve sludge dewatering performance in

wastewater treatment plant, due to the easy operation

(Guo et al., 2021). However, large number of

chemical coagulants have great negative impact on

disposal equipment, and huge adding dosage also

increases the operation cost of WWTP.

Compared with inorganic coagulants, organic

polymer coagulants have the advantages of small

consumption, safe treatment, simple operation, weak

acidity or weak alkalinity in water and small

corrosiveness in sludge conditioning. Cationic

polyacrylamide (CPAM) is a typical and common

organic polymer coagulant. CPAM conditioning

greatly improved the dewatering performance of

sludge (Wu et al., 2021). Guo et al (2020). found that

CPAM was a chain polymer with a large number of

positive charge groups, which is opposite to the

charged property of surplus sludge and can play its

electric neutralization role. Moreover, CPAM has

high characteristic viscosity and strong adsorption

and bridging effect on sludge particles.

EPS plays an important role in the water capacity

of sludge, and a large amount of combined water in

sludge is in EPS. (Guo et al., 2021). CPAM had a good

promoting effect on proteins in EPS (Bi et al., 2015).

440

Wan, T., Li, C., Ju, X., Yan, X., Wang, M., Ren, J. and Cheng, W.

Effect of Chemical Conditioning of Sewage Sludge on Dewatering Performance and Structural Components.

In Proceedings of the 7th International Conference on Water Resource and Environment (WRE 2021), pages 440-448

ISBN: 978-989-758-560-9; ISSN: 1755-1315

As an organic colloidal substance, CPAM was

adsorbed on the outside of cells. The surface of

activated sludge EPS was negatively charged, and

CPAM combined with the negative functional groups

on proteins to promote the secretion of proteins by

cells.

Although a lot of research has been done on

sludge dewatering and conditioning, it is difficult to

further improve the sludge dewatering performance

due to the lack of understanding of the structural

component changes in sludge conditioning process.

Therefore, it is necessary to illustrate the changes

mechanisms of components and properties of sludge

flocs, such as surface charge, particle size distribution

fractal division.

Based on this, CAPM is used as conditioner to

adjust the excess sludge to improve the sludge

dewatering performance. The relationship between

sludge structure and dewatering performance was

studied by measuring the specific resistance of

sludge, moisture content of sludge cake, capillary

suction time and the change of sludge water transfer

law and fractal dimension during conditioning. At the

same time, in order to further explore the mechanism

of CPAM on improving the dewatering performance

of excess sludge, Zeta potential and EPS component

changes in excess sludge were measured and

analyzed. The implementation of this study will

contribute to the improvement of sludge dewatering

and thus the management of wastewater treatment

plants.

2 MATERIALS AND METHODS

2.1 Sources and Basic Properties of

Experimental Sludge Samples

The sludge used in the experiment was taken from the

surplus sludge of the second sedimentation tank of

Xi'an No. 4 sewage treatment plant and stored in a

refrigerator at 4℃after being retrieved. Table 1 shows

the basic physical and chemical properties of the raw

sludge in this experiment. At least three parallel

samples were taken for each characteristic index, and

it was found that there was no significant change in

the characteristics of sludge sampled three times

during the experiment.

Table 1: Basic physicochemical properties of raw sludge.

moisture content

organic

matter

Specific resistance of filtration

×10

m/kg

CST

Zeta potential of supernatant

Redox potential

L1 96.12 36.34 0.45 23.25 -18.93 -280.0

L2 96.07 36.30 0.49 23.21 -18.71 -280.3

L3 96.15 36.52 0.53 23.74 -19.14 -280.2

The scanning electron microscope was used to

observe the raw sludge, as shown in Figure 1. The

internal structure of the raw sludge is composed of

granular and dense particles, and the surface is

relatively smooth, complete and basically non-

porous.

SEM can qualitatively describe the sludge floc

structure. The effect of CPAM conditioning on the

surface structure of sludge flocs was analyzed.

Scanning electron microscopy (SEM) was used to

observe the raw sludge and CPAM treated sludge. As

shown in Figure 1, the internal structure of the raw

sludge is composed of dense granular particles with

smooth surface and complete block structure. After

CPAM conditioning, the internal pore structure of

sludge particles is more abundant, and it is easier for

water to flow out, so the filtration performance is the

best.

2.2 Main Instruments and Reagents

Six-axis mixer (JJ-4, Changzhou Zhiborui Instrument

Manufacturing Co, LTD.); Uv-visible

spectrophotometer (DR5000, Hach Company);

Electronic Balance (CP213, Ohaus Instrument Co,

LTD.); Specific resistance of filtration measuring

device (TG-250, Shanghai Tongguang Science and

Education Instrument Co, LTD.); Zeta potential

Analyzer (ZS90, Malvern); Fluorescence

spectrophotometer (F-7000, Hitachi); Laser particle

size analyzer (BT-9300S, Dandong Baxter

Instrument Co, LTD.).

Cationic polyacrylamide (CPAM), sodium

hydroxide (NaOH), anthrone (C14H10O),

concentrated sulfuric acid (H2SO4), Folin-phenol

reagent, glucose and potassium dichromate

(K2Cr2O7) were analytically pure.

Effect of Chemical Conditioning of Sewage Sludge on Dewatering Performance and Structural Components

441

(a) Raw sludge 200 μm

(b) CPAM treated sludge 50μm

Figure 1: Caption to Scanning electron microscope (SEM)

images of raw sludge.

2.3 Experimental Methods

Six 250 mL conical bottles were added to 100 mL of

surplus sludge, and 0.0375, 0.075, 0.15, 0.225, 0.3,

0.375 g/L of 0.3% CPAM solution were added,

respectively, stirring at 300 r/min for 5 min. After

conditioning, the sludge dewatering index was

determined; The composition and distribution of

sludge EPS were analyzed. At least three groups of

parallel samples were set up in each experiment.

2.4 Analysis Method of Experimental

Indexes

In this experiment, 12 million CPAM was configured

into 0.3% solution for determination, and the brinell

funnel method was used to determine specific

resistance of filtration. The moisture content of

sludge was determined by gravimetric method

(Klomklao et al., 2017); The particle size distribution

of sludge was determined by laser particle size

analyzer (Zeng et al., 2019).

Modified Folin-Lowry method and anthrone-

sulfuric acid method were used to determine the

concentration changes of proteins and

polysaccharides in EPS before and after sludge

conditioning. Extraction of EPS from Sludge by

NaOH method (Sun et al., 2017), and the residual

cells and other substances in the extract were

removed by 0.45μm filter membrane.

A fluorescence spectrophotometer (F-7000,

Hitachi) was used to determine the three-dimensional

fluorescence spectrum. The parameters were set as

the excitation wavelength (Ex) was 240 ~ 550 nm, the

emission wavelength (Em) was 260 ~ 600 nm, the

step length was 4 nm, and the slit width was 3 nm.

3 RESULTS AND DISCUSSION

3.1 Effect of Cationic Polyacrylamide

Conditioning on Sludge Dewatering

Performance

The influence results of moisture content and

moisture distribution of sludge cake under different

dosage of CPAM are shown in Figure 2. As can be

seen from Figure 2, with the increase of CPAM

dosage, the moisture content of mud cake decreased

first and then increased. When the dosage of CPAM

was 0.225 g/L, the moisture content of mud cake

reached the lowest value of 70.19%, and the reduction

rate was 26.99%. With the increase of CPAM dosage,

the proportion of free water in sludge firstly increased

and then decreased, and reached the maximum value

of 80.08% when CPAM dosage was 0.225 g/L. The

change trend of bound water was just opposite to the

change of free moisture content, which reached the

minimum value of 19.92% when CPAM dosage was

0.225 g/L. The reason for this phenomenon may be

that sludge particles become unstable under the action

of electrical neutralization and aggregate into flocs,

which gradually form larger flocs through the action

of long chain bridge and net capture of flocculant, and

then settle under the drive of gravity and external

force(Wang et al., 2014).The low dosage of

flocculant leads to the weak effect of electric

neutralization, which is insufficient to make most of

the suspended sludge particles accumulate and settle.

Excessive flocculant will lead to electrostatic effect

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442

and certain space between particles, which can restore

the stability of sludge particles and fail to achieve the

effect of flocculation and dehydration.

(a) Sludge moisture distribution

(b) Sludge moisture content

Figure 2: Caption to Effect of different CPAM dosage.

The changes of specific resistance and CST under

different CPAM dosage were shown in Figure 3. With

the increase of CPAM dosage, the specific resistance

of sludge first increased, then gradually decreased to

the minimum value and remained basically

unchanged. When the CPAM dosage was 0.225 g/L,

the specific resistance reached the minimum value of

0.15×10

m/kg, and the reduction rate was 66.67 %.

With the increase of CPAM dosage, CST gradually

decreased to the minimum value and remained

basically unchanged. When the CPAM dosage was

0.225 g/L, CST reached the minimum value of 6.45 s,

and the reduction rate was 72.26 %. The sludge

dewatering performance was significantly improved.

(a) Specific resistance of filtration

(b) Capillary suction time

Figure 3: Caption to Effect of different CPAM dosage.

3.2 Analysis of Sludge Zeta Potential,

Particle Size and Fractal Dimension

The change of Zeta potential of sludge after CPAM

conditioning was shown in Figure 4(a). With the

increase of CPAM dosage, the Zeta potential

increased first and then decreased, and reached the

maximum value of -12.07 mV when CPAM dosage

was 0.225 g/L. The Zeta potential determines the

flocculation and sedimentation performance of

colloidal particles, and the decrease Zeta potential

negative electricity indicates that the dewatering

performance of sludge has been improved (Hiroyuki,

2019).

It can be seen from Figure 4(b) that with the

increase of CPAM dosage, the repulsion between

sludge flocs decreases, and the particle size of sludge

particles increases, which strengthens the adsorption

bridging effect of sludge flocs (Dai et al., 2018) .The

fine sludge particles after conditioning firstly

destabilise and aggregate through the effective

Proportion of water distribution/

（）

Polyacrylamide dosage/ g/L

Free water Bound water

Moisture content of mud cake/

（）Polyacrylamide dosage/ g/L

0.10

0.15

0.20

0.25

（

SRF/ ×10

-1

）

（

Polyacrylamide dosage/ g/L）

CST/s

（）

Pol

acr

lamide dosa

Effect of Chemical Conditioning of Sewage Sludge on Dewatering Performance and Structural Components

443

electroneutralization of CPAM, and then form large

and dense sludge flocs under the action of net capture

and sweep between long-chain polymer molecules

with large specific surface area. The large and dense

floc structure determines its excellent settlement

performance.

The cumulative yield under the sieve under

different CPAM dosages is shown in Table 2. The

grain size-cumulative yield under the sieve is drawn

in the log-log coordinate system, and its slope value

B is obtained after fitting. The fractal dimension D of

sludge can be calculated according to the formula D

= 3-b, as shown in Table 3. Fractal dimension is a

measure of fractal irregularity (Li et al., 2020). The

higher the value of D, the more compact the flocs

(Tang et al., 2020). As can be seen from Table 3, with

the increase of CPAM dosage, the fractal dimension

D also gradually increases, indicating that the sludge

dewatering performance becomes better after CPAM

conditioning

(a) zeta potential (

)

article size of sludge

Figure 4: Caption to Effect of different CPAM dosage.

Table 2: Distribution of cumulative yield under screen at different CPAM dosages.

Particle size

/μm

Cumulative yield under screen/%

CPAM dosin

uantit

0.0375 0.075 0.15 0.225 0.3 0.375

5 0 0.09 0.13 0.34 0.55 1.96

10 0.28 0.68 0.76 1.39 2.09 4.69

30 1.96 4.77 4.24 7.56 12.57 25.13

50 4.57 8.16 8.44 15.99 25.91 47.57

70 8.33 15.33 15.07 27.13 40.15 66.06

100 15.66 20.76 26.64 42.99 56.93 84.18

300 71.22 71.87 81.54 90.58 92.97 98.04

500 88.33 88.49 94.24 96.83 96.95 98.04

1000 100 99.96 99.87 99.66 99.45 98.04

2000 100 100 100 100 100 100

Table 3: Fractal dimension of sludge after CPAM conditioning.

Fractal dimension

CPAM dosin

uantit

0.0375 0.075 0.15 0.225 0.3 0.375

1.14 1.13 1.11 0.94 0.84 0.63

D 1.86 1.87 1.89 2.06 2.16 2.37

-20

-18

-16

-14

-12

-10

（）Pol

acr

lamide dosa

Zeta potential/mV

10 100 1000

e percentage

Particle size/

0.0375 g/L

0.0750 g/L

0.1500 g/L

0.2250 g/L

0.3000 g/L

0.3750 g/L

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444

3.3 Effect of Cationic Polyacrylamide

Conditioning on the Structure

Characterization of EPS

3.3.1 Effect of Conditioning Process on EPS

Component and Content Change

(a) Distribution of protein when adding CPAM

(b) Distribution of polysaccharide when adding CPAM

Figure 5: Caption to effect of different CPAM dosage.

EPS is a highly hydrated biopolymer, which plays an

important role in the stability of biological

flocculation and the separation of solid and water

(Salama et al., 2016). Proteins and polysaccharides

account for about 70% ~ 80% of the total EPS (Zhang

et al., 2021) .The analysis of the changes in protein

and polysaccharide contents of EPS layers in sludge

helps to clarify the mechanism of sludge dewatering

performance. The changes of protein and

polysaccharide contents in different sludge

components under different CPAM dosages are

shown in Figure 5.

Figure 5(a) shows the changes of protein content

in different components of EPS. With the addition of

CPAM, protein content concentration in total EPS

increases. When the dosage of CPAM was increased

from 0.0375 g/L to 0.15 g/L, the protein content in

total EPS decreased gradually. When the dosage of

CPAM was 0.15 g/L to 0.3 g/L, the protein content in

total EPS increased first and then decreased, and

reached the maximum value at 0.225 g/L. The change

trend of protein content in TB-EPS was consistent

with that in total EPS, and reached the maximum

when CPAM dosage was 0.225 g/L. The protein

content of S-EPS decreased slightly with the increase

of CPAM dosage. The change trend of protein in LB-

EPS was just opposite to that in S-EPS, which

gradually increased with the increase of CPAM

dosage. This indicates that the addition of CPAM

transfers proteins in S-EPS layer to LB-EPS layer and

TB-EPS layer, which changes the distribution pattern

of proteins in sludge stratification, thus achieving the

purpose of sludge particle flocculation and improving

the dewatering performance of sludge (Tripathy et al.,

2001).

It can be found from Figure 5(b) that when the

dosage of CPAM increased from 0.0375 g/L to 0.15

g/L, the polysaccharide content in total EPS

decreased slowly. When the dosage of CPAM was

0.15 g/L to 0.3 g/L, the polysaccharide content in total

EPS increased first and then decreased, and the

polysaccharide content reached the maximum at

0.225 g/L. The change trend of polysaccharides in

LB-EPS was exactly opposite to that in S-EPS, and

gradually increased with the increase of CPAM

dosage. The change trend of polysaccharide content

in TB-EPS was consistent with that in total EPS, and

reached the maximum when the dosage of CPAM was

0.225 g/L. The results showed that after CPAM was

added, the distribution pattern of polysaccharides in

sludge stratification was changed by compressing the

double layer and the principle of electrical

neutralization. The polysaccharides in S-EPS were

transferred to LB-EPS and TB-EPS, so that the sludge

particles were flocculated (Zhang et al., 2017).

100

200

300

Protein content/

（

）

mg/L

Pol

acr

lamide dosa

（）/

S-EPS LB-EPS

TB-EPS Total EPS

120

The polysaccharide content/

（）

mg/L

Polyacrylamide dosage

（）

/g/L

S-EPS LB-EPS

TB-EPS Total EPS

Effect of Chemical Conditioning of Sewage Sludge on Dewatering Performance and Structural Components

445

(a) Protein-like substance (b) Humic acid-like

Figure 6: Effect of cationic polyacrylamide dosage on the distribution of maximum fluorescence intensity of different

components: (a) Soluble microbial by-product-like; (b) Tyrosine protein; (c) Humic acid-like; (d)Humic acid-like.

3.3.2 Analysis of EPS Fluorescence

Characteristics

Fluorescence spectroscopy is a sensitive method for

the analysis of organic matter, which has been widely

used in the analysis of organic matter in sludge EPS.

To study the changes of fluorescent organic matter in

sludge dewatering process treated by CPAM

conditioning, three-dimensional fluorescence

spectroscopy analysis of raw sludge and sludge after

conditioning was carried out (Zhu et al., 2012).

Figure 6 is the distribution map of the maximum

fluorescence intensity of EPS components under

different CPAM dosage. From Figure 6(a), with the

addition of CPAM, the protein-like substances in EPS

gradually decreased, Zhang et al (2016) pointed out

that the decrease of protein-like content is helpful for

sludge dewatering. It can be seen from Figure 6(b)

that with the addition of CPAM, the content of marine

humic substances in LB-EPS and S-EPS changed

little, and the content in TB-EPS decreased gradually.

It can be seen from Figure 6 (c) that the addition of

CPAM had little effect on the fulvic-like acid content,

and the fulvic-like acid content in EPS was basically

unchanged. It can be seen from Figure 6(d) that the

total amount of humic acids in sludge after CPAM

conditioning increased, and the content of TB-EPS

increased significantly. The content of LB-EPS

decreased slightly, while the content of S-EPS

remained basically unchanged. The increase in

humic-like substance content indicates that organic

substances in sludge form humic acid through

polymerization, and charged substances are

condensed into polymers, so that the charge on the

surface of sludge is low, which is conducive to sludge

dewatering. The increase in humic-like substance

content in TB-EPS may be due to the release of

substances wrapped by EPS in cell phase into EPS.

0.0375 0.075 0.15 0.225 0.3 0.375

Maximum fluorescence intensity/ (×10

）

Polyacrylamide dosage/(g/L)

S-EPS

LB-EPS

TB-EPS

0.0375 0.075 0.15 0.225 0.3 0.375

Maximum fluorescence intensity/ (×10

）

Polyacrylamide dosage/(g/L)

S-EPS

LB-EPS

TB-EPS

0.0375 0.075 0.15 0.225 0.3 0.375

Maximum fluorescence intensity/ (×10

）

Polyacrylamide dosage/(g/L)

S-EPS

LB-EPS

TB-EPS

0.0375 0.075 0.15 0.225 0.3 0.375

Maximum fluorescence intensity/

(

×10

）

Polyacrylamide dosage/(g/L)

S-EPS

LB-EPS

TB-EPS

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4 CONCLUSION

(1) The optimal dosage of CPAM was 0.225 g/L for

the dewatering performance of excess sludge. Under

these conditions, the moisture content of sludge filter

cake was 70.19%, the specific resistance of sludge

was 0.15×10

m/kg, and the CST was 6.45s.

(2) After the conditioning of cationic

polyacrylamide (CPAM), the sludge particle size

increased, the fractal dimension increased, the sludge

Zeta potential increased from -18.93 mV to -

12.07mV, the electronegativity decreased.

(3) After CPAM conditioning of the surplus

sludge, through the action of electrical neutralization

and dehydration, colloidal particles are destabilized to

form large flocs, which changes the distribution of

proteins and polysaccharides in each layer of sludge,

so that intracellular substances and TB-EPS are

released into LB-EPS and S-EPS, so as to improve the

performance of sludge dehydration.

(4) After conditioning, the protein-like

concentration of S-EPS in surplus sludge EPS

decreased, while the humic acid-like concentration of

TB-EPS increased.

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