Improving the Dewaterability of Sewage Sludge Using Rice Husk and

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Improving the dewaterability of sewage sludge using rice husks and Fe2+-sodium persulfate oxidation Qiao Xiong, Min Zhou, Hong Yang, Mengjia Liu, Teng Wang, Yiqie Dong, and Haobo Hou ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b03227 • Publication Date (Web): 17 Nov 2017 Downloaded from http://pubs.acs.org on November 20, 2017

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ACS Sustainable Chemistry & Engineering

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Improving the dewaterability of sewage sludge using

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rice husks and Fe2+-sodium persulfate oxidation

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Qiao Xionga, Min Zhoua,b, Hong Yangc, Mengjia Liua, Teng Wanga,c, Yiqie Donga, Haobo Houa,b* a

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School of Resource and Environment Science, Luojiashan street, Wuchang district, Wuhan

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University, Wuhan 430072, P.R. China b

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Hubei Environmental Remediation Material Engineering Technology Research Center,

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Luojiashan street, Wuchang District, Wuhan 430072, P.R. China c

CSIC Environment Engineering Co. Ltd., Zhongshan Road, Wuchang District, Wuhan 430072,

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P.R. China

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E-mail addresses: [email protected] (Q. Xiong), [email protected] (M. Zhou),

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[email protected] (Y. Hong), [email protected] (T. Wang), [email protected] (M.

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Liu), [email protected] (Y. Dong), [email protected] (H. Hou)

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Abstract

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Physical conditioners or skeleton builders are usually used to improve the dewaterability of

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sewage sludge. In this study, low-cost rice husk (RH) was evaluated as an alternative skeleton

ACS Paragon Plus Environment

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builder and combined with Fe2+ and sodium persulfate (SPS) for sewage sludge conditioning.

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The results show that the sewage sludge conditioned with RH and Fe2+/SPS showed good

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dewaterability, and the capillary suction time (CST) was reduced by 92.8 % under optimal

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conditions. Using response surface methodology (RSM), the optimal composite conditioner

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concentrations were 151.5 mg/g dry solid (DS) of SPS, 46 mg/g DS of Fe2+ and 333 mg/g DS of

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RH. After conditioning with Fe2+/SPS, some extracellular polymeric substances (EPS) were

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destroyed, which resulted in the dissolved protein and polysaccharide amounts in the filtrate

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increasing. The water content analysis indicated the bound water was converted to free water

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because of the EPS degradation. The particle size analysis showed that the sludge flocs became

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smaller. After the addition of RH, the sludge formed a stratified, porous structure that improved

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the sludge compressibility and provided outflow passages for free water, which enhanced the

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sludge dewaterability. These results indicated that combining persulfate oxidation with RH

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conditioning is a promising strategy to improve sludge dewaterability.

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KEYWORDS Skeleton builder; Sewage sludge; Dewaterability; Persulfate oxidation;

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Compressibility

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INTRODUCTION

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With the increase in sewage treatment plants in China, the amount of sewage sludge will

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increase to 60 million tons per year (the water content of sewage sludge is 80 %) by the year

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2020.1 The sewage sludge byproduct will cause serious environmental pollution without proper

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treatment and disposal. In addition, in sludge treatment, usage, and disposal, dewatering of

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sewage sludge is a vital step to reduce the volume of the sludge.2 The volume of the sludge can


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be reduced to one twentieth of the original sludge volume if the moisture content is reduced from

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98 % to 60 %. The physical-chemical parameters of the sludge, such as the floc structure, surface

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charge, bound water content and hydrophobicity, can influence the sludge dewaterability.3 Based

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on these factors, researchers have proposed new technologies to achieve low moisture contents in

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sewage sludge to fulfill more stringent disposal regulations.4 Two common approaches are to

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reduce the bound water in sludge and to enhance the sludge compressibility.5

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Extracellular polymeric substances (EPS) are the main constituents of sludge flocs and are

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recognized as the key to sludge dewaterability because their degradation can release bound

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water.6 Activated persulfate oxidation, an advanced oxidation process (AOP), has been

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demonstrated to be a promising advanced sludge treatment method to degrade EPS, and it has the

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advantages of high solubility, strong oxidizing potential, wide operative pH range and slow

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consumption rate.7 Persulfate can generate a strong and non-selective oxidant sulfate radical

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(SO4•−, E0 = 2.6 V) that is activated by heat,8 UV,9 and transition metal ions.10 Ferrous iron

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(Fe2+) is commonly used to activate persulfate. The following equations show the reactions

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between persulfate and Fe2+: 11

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+ 2 → + ·– + (1)

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·– + → +

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(2)

Excess Fe2+ will consume the generated sulfate radicals via Eq. (2) and lead to an ineffective oxidizing ability. Therefore, the amount of Fe2+ must be strictly controlled.

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To increase the sludge compressibility, physical conditioners, including fly ash and lime,12

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lignite,13 wood chips and wheat dregs14 and gypsum,15 are used as skeleton builders. Rice or

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paddy is cultivated in more than 75 counties throughout the world, and the rice husk (RH), which

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is the outer cover of the rice grain, is present in large quantities and considered solid agricultural

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waste.16 RH disposal poses a serious environmental problem. Due to the differences in the rice


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type, crop year, climate and geographical conditions, the chemical compositions of RH vary.17 In

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general, RH contains large amounts of amorphous silica.18 With the increase in public awareness

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of sustainable development, utilization of renewable resources is increasing.19 RH has received

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attention from many researchers as a type of biomass.20-22 Shi et al.23 investigated the effect of

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RH on the hydrothermal treatment of sewage sludge, and the results indicated that the addition of

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RH can reduce the concentration of heavy metals. Wu et al.24 used RH biochar modified with

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FeCl3 (MRB-Fe) to enhance sludge dewaterability, and they discovered that MRB-Fe maintained

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a certain skeleton structure in the sludge cake, which allowed moisture to easily pass through.

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Thus, RH has potential as an economic skeleton builder for sludge dewatering.

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In this study, RH was utilized as a novel skeleton builder in combination with Fe2+/SPS

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oxidation as a novel technique to improve sludge dewaterability. The schematic of this study is

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shown in Fig. 1. The objectives of this study were: (1) to optimize the concentrations of SPS,

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Fe2+ and RH to achieve the highest CST reduction efficiency; (2) to evaluate the dewaterability

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of the conditioned sludge using RH as a novel skeleton builder; (3) to investigate the dewatering

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process of the composite conditioner in terms of the EPS, bound water content, particle size,

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specific surface area, zeta potential and microstructure of the sludge.


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Fig. 1. Schematic of the study.

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METHODS

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Materials

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The raw sludge (RS) used in this study was a mixture of sludge from the primary and

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secondary sedimentation tanks of the Luobuzui Wastewater Treatment Plant, Wuhan, China.

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Municipal wastewater (315,000 m3/d) is treated in this plant with a daily sludge (80 % water

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content) production of 60~70 t/d. RS samples were collected in polypropylene containers and

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stored at 4 °C in a refrigerator. Before the experiments, the RS was taken out of the refrigerator


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and placed in environmental conditions until the temperature reached room temperature. The

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characteristics of the RS were tested according to standard methods (US EPA1995). The main

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characteristics of the RS are shown in Table 1. The RH was collected from Wuhan, Hubei. First,

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the RH was dried in an oven at 105 °C for 5 h to ensure complete dehydration. Then, the RH was

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ground using a ball mill (XQM-4L) for 10 min at 3500 rpm. Finally, the RH was sieved through

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a 0.5 mm sieve, washed with deionized water, and dried at 105 °C for 5 h prior to use in the

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following experiments. The inorganic elemental compositions (Si, Al, Fe, K and Na) of the RS

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and RH were analyzed using an X-ray fluorescence (XRF) analyzer (S4 Pioneer, Bruker AXS).

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The major inorganic oxide contents in the RS and RH are shown in Table 2.

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Sodium persulfate (SPS) (Na2S2O8, purity>99.9 wt.%) and ferrous sulfate (FeSO4·7H2O,

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purity>99.9 wt.%) were analytical reagent grade (Sinopharm Chemical Reagent, China) and

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were used without further purification. The SPS and Fe2+ solutions were freshly prepared

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immediately prior to the experiments. Deionized water was used for all the experiments.

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Table 1. The main characteristics of the RS. Parameter Value

Moisture (%) 80.2±2.22

pH 7.0±0.82

Organic content CST (s) (%) 45.3±0.89 165±2.08

103 104

Table 2. Analysis of the main oxides (%) in RS and RH. Sample SiO2

MgO

Fe2O3 Na2O

CaO

Al2O3 K2O

P2O5

Cl

SO3

RS

48.83 2.15

7.57

0.78

4.38

23.47

2.46

6.25

0.10

2.61

RH

73.50 1.00

0.47

0.34

1.30

13.22

5.58

1.79

1.06

1.65

105 106

Sludge conditioning


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The RS was transferred to 500 mL beakers and stirred for 30 min to ensure the sludge particles

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were evenly mixed with the water. Then the RS was conditioned using the following procedure:

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SPS addition → 150 rpm stirring for 5 min → addition of Fe2+ → 150 rpm stirring for 5 min →

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addition of RH → 150 rpm stirring for 10 min.

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After the conditioning, the conditioned sludge was centrifuged for 20 min and filtered through

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a 0.45 µm membrane filter, and the supernatant was collected for use. The sludge cakes were

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dried at 105 °C for 24 h and used for the mechanism investigation. The experimental setup of the

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sludge conditioning system is shown in Fig. 2.

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CST and settling property

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The CST (capillary suction time) and settling property were used to evaluate the sludge

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dewatering performance. The CST was measured using a 304 M CST instrument (Triton, UK).

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The CST reduction efficiency (Y) was calculated as follows: Y

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CSTb (s) is the initial CST of the RS, and CSTa is the CST of the sludge after the conditioning.

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The settling property was determined via the following procedure: 100 mL of the raw and

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conditioned sludge was transferred to a measuring cylinder, and the interface position was

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recorded every 10 min for 90 min.

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Fig. 2. Schematic diagram of the experimental setup.


– , where

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RSM design

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A Box–Behnken design25 was used to optimize the concentrations of SPS, Fe2+ and RH. Table

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3 shows the ranges and levels of these three constituents, which were defined using preliminary

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tests. The CST reduction efficiency was considered the response. Seventeen runs were required

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for a complete set of the experimental design, as shown in Table 4, and the experimental results

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were analyzed using the Design Expert 8 software. The criteria for the factors and the response

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set for the optimization are presented in Table 5. The goal of the optimization was to maximize

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the CST reduction efficiency.

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Table 3. Ranges and levels of the factors in the optimization experiments using the Box-

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Behnken design. Coded variable level Independent variable

Symbol

Low

High

-1

1

SPS concentration (mg/g DS)

X1

100

200

Fe2+ concentration (mg/g DS)

X2

20

60

RH concentration (mg/g DS)

X3

200

500

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Table 4. Seventeen Box–Behnken design experiments.

Experimental design X1 Experiment number SPS concentration (mg/g DS)

X2

1

0

-1

Response X3

Fe2+ RH concentration concentration (mg/g DS) (mg/g DS) 1


CST reduction efficiency (%)

55.7

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2

0

0

0

65.2

3

0

1

1

55.0

4

0

0

0

41.0

5

-1

1

0

81.0

6

1

0

1

55.0

7

0

-1

1

50.0

8

0

0

0

85.0

9

1

1

0

90.5

10

1

0

-1

75.0

11

0

-1

-1

85.0

12

-1

-1

0

88.0

13

0

1

-1

90.0

14

0

0

0

82.0

15

1

-1

0

85.0

16

-1

0

-1

85.0

17

0

0

0

84.3

137 138

Table 5. Criteria for the factors and the responses for the optimization. Variable

Goal

Lower limit

Upper limit

Importance

X1, SPS concentration (mg/g DS)

In range

100

200

3

X2, Fe2+ concentration (mg/g DS)

In range

20

60

3

X3, RH concentration (mg/g DS)

In range

200

500

3

CST reduction efficiency (%)

Maximize

–

–

3

139 140

Conditioning mechanism investigation


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A set of experiments with different formulations was conducted (Table 6) to elucidate the roles

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of each constituent and the influence of the temperature. The concentrations of the constituents

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were obtained from the RSM optimization study. RS was used as the control. As shown in Table

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6, the sludges conditioned with RH at 25 °C; SPS and Fe2+ at 25 °C; SPS, Fe2+ and RH at 25 °C;

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SPS, Fe2+ and RH at 52 °C; and SPS, Fe2+ and RH at 80 °C are labeled RH25, SF25, SFR25,

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SFR52 and SFR80, respectively. The RS and conditioned sludges were analyzed for the CST

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reduction efficiency, settling property, EPS content, bound water content, particle size, specific

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surface area, zeta potential and microstructure.

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EPS extraction and analysis

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To evaluate the influence of EPS on the sludge dewaterability, soluble EPS (S-EPS) and bound

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EPS (B-EPS) were extracted using a modified heat method.26 First, the conditioned sludge was

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centrifuged in a 50 mL tube at 4000 rpm for 5 min. The liquid supernatant was collected, and the

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sludge cake was used for the B-EPS extraction. Test samples were passed through a 0.45 µm

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membrane before analysis. All the S-EPS and B-EPS were analyzed for protein (PN) and

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polysaccharide (PS) contents. The PN content was analyzed using the modified Lowry method

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with bovine serum albumin.27 The PS content was analyzed using the anthrone method with

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glucose as the standard.28

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Particle size and specific surface area

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The particle sizes of the RS and the conditioned sludges were measured using a BT-9300ST laser particle size analyzer (Bettersize, China). The specific surface areas and pore structures of the samples were measured using a nitrogen

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adsorption apparatus (BELSORPmini, BEL, Japan).

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Bound water content measurements


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Thermogravimetric (TG) and differential scanning calorimetry (DSC) were performed to

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determine the amount of total and bound water in the sludge cakes. Briefly, the RS and the

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conditioned sludges were centrifuged at 4000 rpm for 10 min. Centrifuged sludge cakes were

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obtained and used for the following analysis. A thermal analyzer (STA449c/3/G, NETZSCH,

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Germany) was used to record the TG thermographs of the sludge cake. Pure N2 was used as the

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carrier gas. The sludge weight loss as the temperature increased from 30 to 150 °C was regarded

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as the total amount of water (Wt).5 The bound water content was determined using a Q2000 DSC

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analyzer (TA, USA). DSC curves were obtained by fast cooling the samples from room

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temperature to –20 °C and then reheating them to 10 °C at a rate of 2 °C/min. The amount of

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free water (Wf) was calculated using the following formula: Wf = Q/∆H, where Q is the heat

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absorbed during the melting process, and H is the melting enthalpy of free water (∆H = 333.3

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J/g). The bound water (Wb) was calculated by Wb = Wt – Wf.29-30

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Microstructural analysis

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The morphology of the sludge cake samples was observed by field emission scanning

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electronic microscopy (FESEM, Sigma ZEISS).

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Table 6. Different conditioning procedures for sludge.

Symbol

Conditioners

Temperature (oC)

Concentration (mg/g DS) SPS Fe2+ RH

Conditioning procedures

RS

None

25

0

0

0

150 rpm/30 min

RH25

RH

25

0

0

333

150 rpm/30 min→RH→150 rpm/15 min

SF25

SPS/Fe

25

152

46

0

150 rpm/30 min→SPS→150 rpm/5 min→Fe2+ solutions→150 rpm/5 min

SFR25

SPS/Fe2+/RH

25

152

46

333

150 rpm/30 min→SPS→150 rpm/5 min→Fe2+ solutions→150 rpm/5 min→RH→150 rpm/10

2+


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min SFR52

SFR80

2+

SPS/Fe /RH

2+

SPS/Fe /RH

52

152

80

152

46

46

333

150 rpm/30 min→SPS→150 rpm/5 min→Fe2+ solutions→150 rpm/5 min→RH→150 rpm/10 min

333

150 rpm/30 min→SPS→150 rpm/5 min→Fe2+ solutions→150 rpm/5 min→RH→150 rpm/10 min

180 181

RESULTS AND DISCUSSION

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RSM optimization results

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Experimental results were evaluated using the Box-Behnken design to yield approximate

184

functions for the dependent variable, the CST reduction efficiency (Y). The following fitting

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polynomial Eq. (3) was obtained from the data fitting.

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Y

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91.6 + 2.25 + 15.88 – 1.88 + 0.75 – 1.25 +

188

5.00 – 21.8 – 20.55 – 16.05

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where X1, X2, and X3 are the coded values of the concentrations of SPS, Fe2+ and RH,

190

respectively.

(3)

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Table 7 illustrates the variance regression model analysis for the CST reduction efficiency. The

192

model F value is 99.64, and the value of “Prob > F” is less than 0.0001, which indicates that the

193

model is significant. In addition, the “lack of fit F value” is 3.28, and the lack of fit is not

194

significant relative to the pure error. A non-significant lack of fit indicates the model is good.

195

Fig. 3 shows that the R2 for the experimental and predicted values is 0.9923. Thus, the model can

196

reliably describe the behavior of the composite conditioner for sludge dewaterability. The value

197

of “Prob > F” is less than 0.0500, which indicates the model terms are significant. Therefore, X1,

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X2X3, X12, X22, and X32 are the significant model terms in this case.


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The goal of the optimization was to determine the concentrations of SPS, Fe2+ and RH to

200

achieve the highest CST reduction efficiency. To achieve this goal, the optimal coded values of

201

the factors were X1 = 0.176, X2 = 0.371, and X3 = 0.035. Accordingly, the concentrations of SPS,

202

Fe2+ and RH were 151.5, 46, and 333 mg/g DS, respectively.

203

To validate the accuracy of the model, three validation experiments were carried out under the

204

optimal conditions. The CST reduction efficiency results were 92.5, 93.2 and 95.5 %,

205

respectively, which showed the reliability of the model.

206

Fig. 4 shows the three-dimensional response surfaces generated using Design Expert. The third

207

factor was maintained at the zero level when the interactions of the other two variables were

208

discussed. Fig. 4a shows that the CST reduction efficiency significantly increased when the RH

209

and SPS concentrations increased in the ranges of 125-175 mg/g DS and 275-425 mg/g DS,

210

respectively. Higher concentrations of RH and SPS had a negative impact on the sludge

211

dewatering performance. Fig. 4b demonstrates that at a low Fe2+ concentration, an increase in the

212

RH concentration had a slight effect on the CST reduction efficiency. At a high Fe2+

213

concentration, the CST reduction efficiency sharply increased as the RH concentration increased

214

within the range of 200-400 mg/g DS. Fig. 4c shows that the Fe2+ and SPS concentrations have a

215

strong synergistic effect on the CST reduction. An increase in the SPS concentration can

216

improve the CST reduction efficiency in a certain range, and beyond that range, a lower CST

217

reduction was obtained.

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Table 7. Analysis of the variance regression model for the CST reduction efficiency Source

Sum of df Squares

Mean Square

F Value

p-value Prob > F

Model

7616.31

9

846.26

99.64

SFR80. In contrast, the bound water

259

content is in the reverse order. The SFR80 conditioned sludge can reduce the bound water in RS

260

from 0.145 to 0.037 g/g. The results agree with the sludge dewatering performance, as shown in

261

Fig. 5, which indicated that the improvement in the sludge dewatering performance was

262

accompanied by a reduction in the bound water in the sludge.

263

Table 8. Water composition of the RS and the conditioned sludges. RS Total (g/g)

RH25

SF25

SFR25

SFR52

SFR80

0.746

0.733

0.683

0.638

0.637

0.635

0.630

0.623

0.634

0.591

0.600

water 0.145

0.116

0.110

0.049

0.047

0.037

water 0.780

Free water (g/g) Bound (g/g) 264

265 266

Fig. 7. TG curves of the RS and the conditioned sludges.


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267 268

Fig. 8. DSC thermograms of the RS and the conditioned sludges.

269

Effect of the composite conditioner on the zeta potential

270

The zeta potentials of the RS and the conditioned sludges were analyzed to determine the

271

colloidal stability, which plays an important role in sludge dewatering. The evolution of the

272

surface charge in terms of the zeta potential is shown in Fig. 9. The sludge was originally

273

negatively charged with a –21.00 mV zeta potential at 25 °C. The use of RH causes a small

274

increase in the negative charge (–23.7 mV), which implied that RH alone was not an effective

275

conditioner for sludge dewatering. After conditioning with SF25, the zeta potential sharply

276

increased from –21.00 mV to –5.23 mV at 25 °C. In addition, after adding RH as a skeleton in

277

the SF conditioner, the zeta potential increased from –5.23 mV to –2.59 mV at 25 °C, –1.001

278

mV at 52 °C and –0.373 mV at 80 °C. The results indicated that the adoption of SFR could

279

increase the zeta potential by neutralizing the negative surface charges, and a higher temperature

280

can increase the impact. A decrease in the sludge negative charge corresponds to a reduction in

281

repulsions and improves the sludge dewaterability.32 After conditioning with SFR, the zeta


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282

potential was nearly 0 mV, which favors sludge dewatering. The results present a consistent

283

evolution profile for the sludge dewatering performance, as shown in Fig. 5.

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284 285

Fig. 9. Zeta potential of the RS and the conditioned sludges.

286

Effect of the composite conditioner on the particle size

287

The particle size distributions of the RS and the different conditioned sludges were determined

288

to investigate the particles, and the results are shown in Fig. 10. The particle size of the sludge

289

conditioned by RH (39.13 µm) was larger than that of the RS (32.8 µm). After conditioning with

290

SPS and Fe2+ (SF) at 25 °C, the change in the average median particle size of the sludge can be

291

neglected. However, after conditioning with SPS, Fe2+ and RH (SFR) at 25 °C, 52 °C, and 80 °C,

292

the average particle sizes of the sludge decreased to 28.76 µm, 29.67 µm and 26.88 µm,

293

respectively. This may be due to the combination of the RH and Fe2+/SPS oxidation degrading

294

the organics in the sludge flocs into smaller molecules via the highly reactive sulfate radical and

295

the destruction of the EPS structure and breakage of the dense sludge flocs into smaller particles,


21

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296

which can result in a better sludge dewaterability and agrees with the zeta potential results in Fig.

297

9. In addition, the particle size of the sludge did not change substantially with the different

298

treatment temperatures, which implied that the dewaterability was stable for the three different

299

mild temperatures. Thus, SFR is an effective and alternative composite conditioner for sludge

300

dewatering.

301 302

Fig. 10. Particle size distributions of the RS and the conditioned sludges.

303

Effect of the composite conditioner on the specific surface area

304

As shown in Table 9, the specific surface areas of the RS and RH25 were relatively large,

305

12.59 and 9.51 m2/g, respectively. The specific surface areas of the SF25, SFR25, SFR52, and

306

SFR80 conditioned sludges were comparatively smaller, 3.93, 2.26, 2.01 and 2.69 m2/g,

307

respectively. A relatively large specific surface area has been reported to be associated with a

308

large amount of water, which is hard to dewater.29 The lower specific surface area of the sludge

309

flocs was attributed to a decrease in the associated water and an improvement in the sludge


22


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310

dewatering. Considering the sludge dewatering performance results in Fig. 5, the sludge with a

311

smaller specific surface area shows a better sludge dewatering performance. In addition, Table 9

312

shows that the pore volume also decreased, which may due to a reduction in the interlayer water

313

in the sludge flocs. The average pore size of the sludge increased from 19.25 nm to 34.52 nm

314

after conditioning with SPS, Fe2+ and RH (SFR25). The large pores in the sludge can provide

315

channels for water flow and improve the permeability of the sludge. These textural parameters of

316

the sludge cakes are consistent with the above analysis.

317

Table 9. Textural parameters of the RS and the conditioned sludges. Sample BET surface area (m2/g) Pore volume (cm3/g) Average pore size (nm) RS

12.59

0.061

19.25

RH25

9.51

0.048

20.01

SF25

3.93

0.032

32.42

SFR25

2.26

0.021

34.52

SFR52

2.01

0.018

36.62

SFR80

2.69

0.023

33.69

318 319

Effect of the composite conditioner on the microstructure of the sludge

320

Fig. 11 shows the FESEM images of the RS and the conditioned sludges. As shown in Fig.

321

11(a), the RS structure is plate-like and not porous, and many irregular particles can be observed

322

on the plate. The RH25 image shows only a few differences, which can be neglected. After

323

conditioning with SF25, the plate was clear, and a porous structure was achieved. Fig. 11(d), (e)

324

and (f) show that the SFR structure is a stratified structure, and channels and pores exist between

325

the lamellae for free water to pass through. In addition, Fig. 11(f) shows more agglomeration of


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326

the sludge flocs at 80 °C, leading to a high-definition, porous structure, which favors sludge

327

dewaterability.33 The SEM results were supported by the particle size analysis.

328 329

Fig. 11. FESEM images of the RS and the conditioned sludges: (a) RS, (b) RH25, (c) SF25, (d)

330

SFR25, (e) SFR52, and (f) SFR80.

331

Based on these results and analyses, a mechanism was proposed. The generated persulfate

332

sulfate radical was activated by Fe2+, which led to EPS degradation and the release of bound

333

water, as shown in Fig. 6 and Table 8. The large sludge floc particles were degraded into smaller

334

particles, as shown in Fig. 10. The RH addition enhanced the sludge compressibility and allowed

335

the free water to more easily flow, as illustrated in Fig. 5 and Fig. 11, which enhanced the sludge

336

dewaterability.

337 338

CONCLUSIONS

339

The joint application of Fe2+/SPS combined with RH as a skeleton builder was successfully

340

demonstrated to be effective for sewage sludge dewatering, which indicated RH is a suitable,


24


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341

alternative skeleton builder. After the RSM optimization, the optimal concentrations of the

342

composite conditioner were 151.5 mg/g DS of SPS, 46 mg/g DS of Fe2+ and 333 mg/g DS of

343

RH, and under these conditions, a CST reduction efficiency of 92.8 % was achieved. The

344

mechanism investigation indicated that the high dewaterability of the conditioned sludge can be

345

mainly ascribed to the synergistic effect of Fe2+/SPS and the RH. First, the EPS degraded into

346

dissolved organics (polysaccharides and proteins), which resulted in the conversion of bound

347

water into free water. Second, the addition of the RH created a stratified, porous structure in the

348

sludge, and this structure contains outflow passages for free water. Based on the results of the

349

present investigation, the Fe2+/SPS-RH composite conditioner is a promising candidate for

350

improved sludge dewaterability.

351 352

AUTHOR INFORMATION

353

Corresponding Author

354

*Corresponding author Tel.: +86 18771025991

355

Notes

356

The authors declare no competing financial interest.

357

ACKNOWLEDGMENTS

358

This work was financially supported by the major scientific and technological innovation subject

359

in the Hubei province, China (No. 2016ACA162).

360 361

REFERENCES


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For Tables of Contents Use Only

449 450

Schematic diagram of the research and the utilization of RH for sludge dewatering is in favor of

451

sustainable development.

452 453


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Improving the Dewaterability of Sewage Sludge Using Rice Husk and

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