Conditioning of resuspension excess sludge with chemical oxidation technology: The respective performance of filtration and expression stage in compression dewatering

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S1383-5866(19)32519-5 https://doi.org/10.1016/j.seppur.2019.116317 SEPPUR 116317

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Separation and Purification Technology

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17 June 2019 8 November 2019 15 November 2019

Please cite this article as: X. Tan, Y. Chen, Q. Xue, Y. Wan, L. Liu, Conditioning of resuspension excess sludge with chemical oxidation technology: The respective performance of filtration and expression stage in compression dewatering, Separation and Purification Technology (2019), doi: https://doi.org/10.1016/j.seppur. 2019.116317

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(Manuscript） Conditioning of resuspension excess sludge with chemical oxidation technology: The respective performance of filtration and expression stage in compression dewatering

Xun Tan a,b, Yijun Chen a,d,*, Qiang Xue a,c, Yong Wan a,c, Lei Liu a,c

a

State Key Laboratory of Geomechanics and Geotechnical Engineering, Institute of Rock and Soil Mechanics,

Chinese Academy of Sciences, Wuhan, 430071, China b University

of Chinese Academy of Sciences, Beijing, 100000, China

c

Hubei Key Laboratory of Contaminated Clay Science and Engineering, Wuhan, 430071, China

d

Jiangsu Institute of Ecological Soil Co.Ltd., Yixing, 214200, China

*Corresponding author at State Key Laboratory of Geomechanics and Geotechnical Engineering, Institute of Rock and Soil Mechanics, Chinese Academy of Sciences, Wuhan, 430071, China Tel: +8615827274084. E-mail address: [email protected] (Y.J Chen)

1

Abstract： Excess sludge has a high yield, rich in organic matter and high water content. The combinational method from chemical oxidation treatment and secondary mechanical dehydration is a powerful approach to further reduce the moisture content of the sludge. This work mainly studies the physicochemical properties of resuspension excess sludge (RES) with the treatment of chemical oxidation, the samples are processed by the compression filter device in a laboratory scale, and the three-stage Terzaghi-Voigt model was used to characterize the expression stage dewatering of samples. The experimental results indicate that: i) The improvement of the dewaterability of resuspension excess sludge is due to moderate chemical oxidation, which is related to the mode of migration and mineralization of organic substances in the stratified structure of the extracellular polymer substance (EPS). ii) When the protein in supernatant + slime is more mineralized than moved in, the dewaterability of the sample is improved. iii) The change of moisture removal for ternary consolidation was very consistent with the change of SRF in the filtration stage (ρ = 0.95, p<0.01). Based on the results, we discuss how chemical oxidation affects the dewatering performance of filtration and expression stages in the solid-liquid separation. This means that the surface characteristics of the sludge particle and the liquid properties play critical roles in solid-liquid separation mechanism.

Keywords: sludge; Fenton; resuspension; EPS; filtration; consolidation

Abbreviations A

filtration area, m-2

aE

ompressibility coefficient of primary consolidation, Pa-1

B

the ratio of secondary consolidation to the total consolidation, -

CST

capillary suction time, s

Ce

consolidation coefficient based on specific solid volume, (m2/s)

D(10)

the particle diameter when the cumulative volume percentage reaches 10%, μm

D(50)

the particle diameter when the cumulative volume percentage reaches 50%, μm

D(90)

the particle diameter when the cumulative volume percentage reaches 90%, μm

E1

the elastic coefficient of Terzaghi spring element, Pa

EEM

excitation-emission matrix

e

local void ratio,-

F

the ratio of ternary consolidation to the total consolidation, -

LB-EPS

loosely bound extracellular polymeric substances 2

L

cake thickness, m-1

L1

initial cake thickness, m-1

Lf

final cake thickness, m-1

i

number of drainage surfaces, -

m

the ratio of the wet cake mass to dry cake mass, -

n

compressibility index, -

RES

resuspension excess sludge

R

the resistance of the filter medium, m-1

Rc

the resistance of the filter cake, m-1

s

the ratio of the solids mass to the slurry mass, -

Uc

consolidation ratio, -

SS

supernatant + slime

TS

total solids, g/L

Tv

time factor, -

Tv50

theoretical time factor when Uc=50%, -

t

dewatering time, s

TB-EPS

tightly bound extracellular polymeric substances

TOC

total organic carbon

u

local apparent relative velocity of liquid to solid, m/s

VS

volatile solids, g/L

V

cumulative volume of filtrate discharging from the filter during the dewatering time, mL

WSD

the width of size distribution

w

the mass of solids deposited per unit area of filter area, kg/m2

ΔP

applied pressure, Pa

Ps

the local solid compressive pressure, Pa

PN

protein

PS

polysaccharides

α, SRF

the specific filter cake resistance to filtration, m/kg

αav

the harmonic mean of the local resistance values through the depth of the cake, m/kg

α0

empirical coefficient, m/kg

η

creep constant, s-1

θc

consolidation time, s

θc*

the end of consolidation time,

θc50

the consolidation time of Uc=50%, s

μ

viscosity of liquid, Pa•s

ρ

parson’s correlation coefficient, -

ρl

the filtrate density, kg/m3

ρS

true density of soild, kg/m3

ω0

total solid volume in cake per unit filtration area, m

1. Introduction The production of wastewater sludge is large, and the water content of it is high. 3

Dewatering is an effective way to reduce the cost of resource utilization in the process of solid waste management [1]. In China, the belt filter presses and centrifuges are mainly used in the process of sludge dewatering, whereas the water content of dewatered sludge is still as high as 70wt%~85wt% [2, 3]. Most of the research of dewatering focus on the thickened sludge of sedimentation tank that belongs to typical wastewater sludge [4-6]. However, due to dehydration equipment limitations in most of sewage treatment facilities, the urgent demand for sludge disposal is hard to satisfy at present. The hydrothermal treatment is one of the methods to treat dewatered sludge[7, 8], but the need of high temperature and high pressure equipment also limits the feasibility of its industrial scale. In recent years, chemical oxidation technology has become an effective method to pretreat sludge to improve its dewaterability (Table S1). Chemical oxidation technology has a high competitive potential compared with other methods when the cost is controlled[9]. Chemical oxidation of the dewatered excess sludge in combination with the secondary mechanical compression dewatering is a new concept, and the resuspension of dewatered sludge is necessary to make this concept feasible. Because the water content of dewatered sludge can be reduced to about 70wt% after a belt or centrifugal dewatering, and dewatered sludge in a plastic state. Chemical treatment needs to use water to suspend dewatered sludge into a slurry state again for stirring and reaction. It seems counterintuitive to add water before dehydration, but the using of slurry is convenient for the mixing of agents and the uniform reaction, and slurry can be pumped and piped. And the filtrate of Fenton reaction treatment sludge contains Fe2+ and Fe3+ [5], which can be used as water for the resuspension slurry, to realize the tailwater circulation and reduce the amount of reagents and water. Multiple advantages make the concept technically feasible and cost controllable. In hydrothermal treatment, dewatered sludge can be heated and pressurized directly in the reactor without the problem of resuspension [7, 10]. Zhang et al [11] reported Na2SiO3 as conditioner used to deep dewater of dewatered sludge, but no mention of resuspension. Before the lignite dewatering by the mechanical thermal expression, there are reports on the process of adding water [12], which has a precedent to follow. The key features of the Fenton reaction system are [Fe2+], [H2O2] and pH, these parameters determine the overall reaction efficiency [13, 14]. Studies on the degradation of pollutants 4

showed that the optimal pH of Fenton reaction was around 3 [15-17], and good dehydration performance of thickened sludge could still be achieved under neutral pH conditions[5], and such reaction is called Fenton-like reaction [18]. The extracellular polymer substance (EPS) of sludge is a gel material containing a large amount of organic matter (Mainly protein, polysaccharide, humic acid), which is closely related to the dewaterability of sludge[19]. However, the relationship between the distribution of organic matter in EPS and the dewaterability of samples, as well as its changes under chemical oxidation, remains some unclear. Two questions were the focus of this paper. First, some studies indicate that the content of organic substance in slime, filtrate or Souble-EPS is related to the dewaterability of sample [20, 21], but under the treatment of chemical oxidation, the content of organic substance in slime, filtrate or Souble-EPS has been reported to increase [5, 6, 22] as well as decrease [23, 24]. In our work, a series of single factor tests were set with the agent of Fenton and Fenton-Like reaction according to previous studies. We studied the physicochemical properties of resuspension excess sludge (RES) treated by chemical oxidation before and after, and found that the improvement of the dewaterability of RES is due to moderate chemical oxidation, which is related to the mode of migration and mineralization of organic substances in the stratified structure of EPS. Second, SRF and CST theoretically characterize sludge dewaterability just in the filtration stage [19, 25-27], few theoretical studies have explored the dewaterability of expression stage. Pressure filtration dewatering is the typical solid-liquid separation, and the liquid discharge is actually composed of two different physical processes. The filter cake filtration is the first stage of the solid-liquid separation process of the sludge sample, and the further compaction of the filter cake results in the discharge of filtrate that is the second stage [28]. This work, the laboratory scale compression filter device was used to conduct dewatering tests. We explored in detail how chemical oxidation affects filtration and expression stages in the solid-liquid separation, and the three-stage Terzaghi-Voigt model [10, 29-32]was used to characterize the expression stage dewatering of samples. The change of moisture removal for ternary consolidation was very consistent with the change of SRF in the filtration stage (ρ = 0.95, p<0.01). 2. Materials and methods 5

2.1 Resuspension excess sludge The dewatered sludge was generated from the urban sewage treatment plant in Yixing, China. The sewage treatment plant has a capacity of wastewater treatment of 75,000 m3 d-1 and is equipped with an improved AAO process. The fresh black dewatered excess sludge was taken as a sample, it contained cationic polyacrylamide and was dewatered by a belt press, and it appeared a soft state similar to jelly. Samples were transported to the laboratory by keeping in ice inclusion boxes. Our test samples were obtained through the following process. The water content of dewatered excess sludge was 84.53 ± 0.80 wt%. The dewatered excess sludge was resuspended with deionized water to be a slurry with the water content of about 90 wt%. Deionized water was used from a water purifier system (Mili-Q advantage A10, Merck Millipore, Germany). After stirring at a low speed for 10 minutes, the slurry was filtered through a 2 mm bore diameter nylon standard sieve to remove impurities and undistributed blocks. Finally, the water content of the slurry was conditioned with deionized water to the goal value, that was, the original RES in this study was obtained. pH is 7.5 ± 0.1. The water content is 93.14 ± 0.03wt%. TS is 70.8 ± 0.2 g/L. VS is 33.8 ± 0.1 g/L. The samples were packed in polyethylene bottles and kept in medical reefer at 4℃. The experiments were accomplished in two batches within three weeks. 2.2 Chemical oxidation treatment To date, a single factor experiment [9, 33], Box-Behnken design [34, 35], orthogonal experiment [36], and central composite design [5, 37, 38] was used to research Fenton system optimal conditions for promoting dewatering. The effects of [Fe2+]/[H2O2], pH and pressure on the filtration and expression stage of the sample dewatering process were mainly studied. According to the results of the above previous literature, a series of single factor tests were set (Table 1). The proportion of [Fe2+]/[H2O2] was changed with different amounts of Fe2+ when the amount of H2O2 was fixed at 85 mg/g VS [38] . Meanwhile, the persulfate treated samples were tested. After shaking the bottle and making all test samples uniform, 165.00g sample was accurately weighed and put into a 500mL beaker, a Jar Tester (ZR4-6, ZhongRun, China) was used. The chemical oxidation process: Adjust initial pH: 50wt% H2SO4 (60 s, 150 rpm) → FeSO4 7H2O (3 min, 150 rpm) → 30wt% H2O2 or Na₂S₂O₈ (25 min, 100 rpm) → Lime (5 min, 150 rpm). Deionized water was supplemented into the sample by dropper to eliminate the 6

difference in water content of all samples, the difference was caused by the addition of reagents. The reagents were analytical grade. The temperature in the laboratory control at 20-25℃. After treatment, the physicochemical properties of samples were measured. 2.3 Dewatering test method In the oil and gas industry, the filtration apparatus is used to determine the filtration behavior and cake-building characteristics of a drilling fluid. In order to be suitable for the lowpressure dewatering of sludge slurry, the corresponding improvement was made to make the dehydrating cylinder conform to the laboratory pressure filter with piston (Fig. 1), which is recommended by[39]. The maximum capacity of the dehydrating cylinder was 200mL, and the effective filtration diameter was 72mm. A pressure source was provided by an air compressor or a compressed nitrogen cylinder. The testing pressure range of the device was 0~800 kPa, and the accuracy was 10 kPa. The filter medium was composed of a filter paper (Quantitative medium speed 90mm, Double Loop, China), a stainless steel metal mesh (0.1mm aperture) and a stainless steel porous filter plate (2mm aperture and 2mm thick). The quality of filtrate was measured in real time by an electronic balance (0.01g accuracy), data was collected by a computer. With this structure, the characteristics of solid-liquid separation and the whole dewatering process of sludge samples can be tested under low-pressure filtration conditions. 150.00g prepared sample (TSS 70.80 ± 0.2 g/L, water content 93.14±0.03 wt%, volume 145.35mL) was carefully transferred to the dehydration cylinder for the compression dewatering test. 2.4 Filtration and consolidation model analysis 2.4.1 The physical process of solid-liquid separation The essence of pressure filtration dewatering is the physical process of solid-liquid separation [28, 40] that non-soluble solid sludge particles in the suspension are blocked by the filter medium and gradually formed filter cake, and the liquid is discharged under the pressure difference. The sludge sample gradually changes from suspension to solid cake in this process. At the beginning of filtration, solid particles are mainly intercepted by the filter medium, and the solid particles first accumulate on the surface of the medium. The filter cake begins to form. In a very short time, the filter resistance of the cake far exceeds the medium, and the filter cake mainly plays the role of intercepting solid substance. At the same time, the filter cake gradually 7

thickens and the interface between the filter cake and the suspension continues to rise (the upper-end is squeezed by the piston, and only the lower end is drainage surface). Until the suspension completely disappears, the lower end surface of the piston comes into contact with the upper end face of the filter cake. The filter cake filtration is the first stage of the solid-liquid separation process of the sludge sample. The newly formed filter cake still has high water content and a loose structure. Under expression, the sludge particles contact with each other and gradually bear the stress. The further compaction of the filter cake results in the discharge of filtrate, which is the second stage of the solid-liquid separation process. The behavior of water removal in the filtration stage is controlled by the filtration equation, and the behavior of water discharge in the expression stage can be described by consolidation theory. 2.4.2 Filtration stage The basic filtration equation is [28]: 1 𝑑𝑉 𝐴 𝑑𝑡

𝐴∆𝑃

(1)

= 𝜇(𝛼𝑐𝑉 + 𝐴𝑅)

Where: A is the filtration area. t is the filtration time. V is the cumulative volume of filtrate. ΔP is the pressure gradient. μ is the viscosity of filtrate. α is the Specific filter cake Resistance to Filtration (SRF). R is the resistance of the filter medium. And c=ρls/(1-ms). ρl is the filtrate density. s is the ratio of the solids mass to the slurry mass, m is the ratio of the wet cake mass to dry cake mass at end of filtration. Integrate equation (1): 𝑡

𝑉=

𝐾1

2𝑉

(2)

+ 𝐾2

where K1=αμc/A2ΔP, K2=μR/AΔP. Plotting t/V against V may lead to a straight line from which filter cake and medium properties can be evaluated. To represent the influence of filtration pressure on the formation of filter cake, the average specific resistance can be calculated as follows [28, 40]: 𝛼𝑎𝑣 = 𝛼0(1 ― 𝑛)∆𝑃𝑛

(3)

where αav is defined by the harmonic mean of the local resistance values through the depth of the cake. α0 is an empirical coefficient. n is the compressibility index. When n~1 the cake would be regarded as very compressible, n~0.5 suggests a moderately compressible cake, and n<2 suggests that the cake is almost incompressible. 8

2.4.3 Expression stage Chang et al.[31] assumed that the response of the void ratio in the filter cake is attributed to three elements:(1) A spring (Terzaghi element); (2) a dashpot-spring element (Voigt element); and (3) a dashpot. Therefore,

( ) =( ) ( ) ∂𝑒

∂𝜃𝑐

∂𝑒

∂𝑃𝑠

𝑤

(1)

∂𝑒

𝜃𝑐

∂𝜃𝑐

𝑤

+

∂𝑒 (2)

( ) ∂𝜃𝑐

𝑃𝑠

+

∂𝑒 (3)

( ) ∂𝜃𝑐

(4)

𝑃𝑠

where e is the local void ration, θc is the consolidation time, Ps is the local solid compressive pressure. In combination with equation (4), the solution consolidation ratio Uc is expressed as:

{

𝐿1 ― 𝐿

(

𝑈𝑐 = 𝐿1 ― 𝐿𝑓 = (1 ― 𝐵 ― 𝐹) 1 ― exp ―

)} +𝐵{1 ― exp ( ― 𝜂𝜃 )} +𝐹{ }

𝜋2𝑖2𝐶𝑒𝜃𝑐 4𝜔20

𝜃𝑐

𝑐

𝜃𝑐∗

(5)

where 1-B-F, B, and F are the fractions occupied by the primary, secondary, and ternary consolidation. L is the cake thickness. L1 and Lf is the initial and final cake thickness respectively. i is the number of drainage surfaces. Ce is the consolidation coefficient. ω0 is the total solid volume in cake per unit filtration area. η is the creep constant. θc* is the end of consolidation time. When the condition of equation (6) is satisfied, equation (5) becomes equation (7): 𝜋2 𝑖2𝐶𝑒 4

∙

𝜔20

(6)

≫ 𝜂𝜃𝑐

(7)

𝑈𝑐 = (1 ― 𝐹) ―𝐵𝑒𝑥𝑝( ― 𝜂𝜃𝑐)

Equation (6) means that a markedly lesser rate for secondary consolidation than that for primary consolidation. And under an extremely large θc limit, equation (5) becomes equation (8). 𝐹

(8)

𝑈𝑐 = (1 ― 𝐹) + 𝜃 ∗ 𝜃𝑐 𝑐

2.5 Sample physicochemical properties The extraction of EPS was based on ultrasonic treatment and centrifuge separation [21]. 40 g of the sample (the water content of 93.14wt%) was centrifuged at 2000g for 15 min, and the bulk solution was collected as Supernatant + Slime (SS). The bottom sediment was collected and resuspended to original volume by using a buffer solution consisting of Na3PO4, NaH2PO4, NaCl, KCl (The molar ratio was 2:4:9:1). The suspension was then centrifuged at 5000g for 15 min, and the organic substance in the bulk solution comprised the LB-EPS. The sediment was 9

again resuspended with the buffer to original volume, the suspension was then subjected to ultrasound at 20 kHz and 480 W for 10 min and centrifuged at 20000 g for 20 min, organic substance in the bulk solution comprised the TB-EPS. Polytetrafluoroethylene (PTFE) membranes (Labsee, China) with a pore size of 0.45μm were used to remove the particulates in the SS, LB-EPS, and TB-EPS solutions. Protein (PN) was determined by the Bradford method, using bovine serum albumin as the standard. Polysaccharides (PS) was measured by the anthrone method, with glucose anhydrous as the standard. The units of PN and PS in the EPS solutions were mg/L of sludge slurry. The average of three sets of parallel experimental data was calculated. The organic part of the sample’s dry mass was measured by a 550℃ ignition method [41]. pH was measured using a water quality multiparameter analyzer (DZS-706, Leici, China). CST was measured by using a capillary water absorption chronometer (304M, Triton, UK), equipped with an 18 mm diameter funnel [42]. The size of the sludge sample’s particles was measured by a laser diffractometer (Mastersizer 3000, Malvern, UK). The zeta potential of the sample’s filtrate was obtained using a Zeta potentiometer (ZS90, Malvern, UK). 3D-EEM (excitationemission matrix) spectra of EPS solution were measured by a Hitachi F-7000 (Japan). An excitation range of 200-450 nm at 5 nm increments and an emission range of 250-550 nm at 1 nm increment. A scan speed of 2400 nm/min with 5 nm of excitation and emission slit bandwidths, and the voltage of the photomultiplier tube was 700 V. The spectra were plotted using OriginPro 8.0. The EPS samples were diluted with deionized water until concentrations of dissolved organic carbon (DOC) less than 10 mg/L before measured [43]. The DOC of EPS solution were measured by a Shimadzu TOC-L (Japan). pH had significant effect on EEM analysis, so pH of solution was adjusted to be 4 ~ 5 by 1wt% H2SO4 (Guaranteed reagent) prior to analysis in order to remove the interference. 2.6 Data analysis The physicochemical properties tests and dewatering tests of the samples were repeated more than 3 times, and the data were presented as Mean ± Standard Deviation. Pearson’s correlation coefficient (ρ) was used to evaluate the linear correlation between the two parameters. Pearson’s correlation coefficient is between -1 and +1, the minus sign denotes a negative correlation, and the plus sign denotes a positive correlation. A relationship between 10

two parameters was considered a significant linear correlation when (ρ>0.95, p<0.01) and a nearly linear correlation when (0.80<ρ<0.90, p<0.01), and there was no correlation between two parameters when (ρ<0.80, p>0.05). Data analysis was carried out using the software SPSS version 20.0. 3 Results and discussion 3.1 Sludge physicochemical properties 3.1.1 EPS components In Fig. 2(a), the content of PN in Supernatant + Slime (SS) and TB-EPS of chemical oxidized samples was significantly reduced compared with that of RES samples. In Fig. 2(b), the content of PS in SS of oxidized samples was significantly increased compared with that of RES. And the content of PS in TB-EPS of oxidized samples was slightly reduced compared with RES. That meant the oxidation effects promoted more part of PS stripping from the particles, and oxidation effects also led to the migration of PS from TB-EPS to SS and LB-EPS. Noted that the distribution of PN and PS in sludge stratified structure varies greatly among different types of sludge samples in previous literature (Table S1). Because the source of samples accounts for these distinctions of EPS components. RES was obtained from the dewatered sludge with deionized water resuspension, the organic substance distributions in RES structure was may not only relate to the package tightness of EPS，but also the organic substance solubility. By considering the EPS extract procedure, centrifugal forces made PN and PS detach from sludge particles, and PN and PS dissolved into water and transferred from the core to the SS. The content of PN and PS in the LB-EPS was very low, due to the short resuspension time of the buffer solution to the sediments. After ultrasonic treatment, relatively more PN and PS were stripped from the sludge particles. PN and PS were then dissolved and transferred to TB-EPS. A large amount of organic substance still fixed on RES sediment after ultrasonic extraction, which is inconsistent with the characteristic of activated sludge which most of the organic substance in the EPS [44]. The changes in organic matter before and after chemical oxidation of different samples are not consistent (Fig. S1). When the indicator of dewaterability becomes well (before / after >1), for most of the thickened sludge and the mixed sludge, PN and PS in SS or LB-EPS increase after oxidation, PN and PS in TB-EPS mostly decrease after oxidation, but for the 11

types other than thickened sludge, PN in SS and TB-EPS decrease and PS in TB-EPS decrease after oxidation. Therefore, the opposite changes of PN in SS meant that there may be different mechanisms for various types of sludge samples under chemical oxidation treatment. A preliminary explanation was that, for the sludge of undisturbed sedimentation tank, oxidation treatment promoted the disintegration of EPS or the destruction of some cells. As a result, the amount of organic substance which released to SS was higher than the amount of organic substance mineralized in SS. For sludge treated by pretreatment methods (including suspension), organic substance had been partially transferred from the inside to the outside in the initial treatment, further oxidation promoted the mineralization of organic substance in SS. That meant the mineralization might be more pronounced than the migration of PN and PS in SS under oxidation treatment. The pH affects the morphology of ions in the reaction system [45]. Under neutral or high pH conditions, Fe2+ is quickly consumed and converted to Fe3+, this process produces precipitation. While under low pH conditions, Fe2+ can continuously participate in catalyzing H2O2 reaction to produce highly oxidizing •OH. Therefore, the Fenton reaction under acidic conditions is more oxidative. Moreover, Fenton reaction mainly has the chemical coagulation effects when the ratio of [Fe2+] to [H2O2] at high, and Fenton reaction mainly has the chemical oxidation effects when the ratio of the two reagents is reversed [13]. But the measurement results showed the fact that reactions under neutral condition resulted in lower content of PN in SS than that of reactions under acidic. This meant that the migration might be more pronounced than the mineralization of PN and PS under higher oxidation. Lu et al. [46] also reported that acidification leads to the disintegration of sludge floc to release more organic matter into slime. On the other hand, the coagulation effects of Fe3+ was also related. The coagulation effects enhanced the degree of the binding between organic substance and EPS or sludge particles[5, 24]. The changes in total organic carbon (TOC) of the EPS solution was consistent with the changes in total concentrations of PN and PS (The units of PN and PS were the concentrations in EPS solution) before and after chemical oxidation (Fig S2). The difference between the concentrations of TOC and the total concentrations of PN and PS represented non-protein and non-polysaccharide organic substances, which may be humic. Table 2 presents the results of 12

the 3D-EEM fluorescence spectrum peak of three EPS fractions obtained under various treatments and Table S3 presents the 3D-EEM fluorescence spectra. The 3D-EEM fluorescence spectrum could be divided into four major regions: Aromatic PN at λex/em = 230 nm/340 nm (Peak A), Tryptophan-like PN at λex/em = 280 nm/335nm (Peak B), Fulvic acid at λex/em = 240 nm/420 nm (Peak C), and Humic substances at λex/em = 350 nm/440 nm (Peaks D) and 270 nm/450 nm (Peak E) [47]. The measures showed that Aromatic PN, Tryptophan-like PN, and Fulvic acid were the main substance in the EPS solution of RES. In addition, the fluorescence intensity of SS, LB-EPS, and TB-EPS solution of the chemical oxidized samples was significantly reduced compared with that of RES. And the fluorescence intensity of the EPS solution of the Fenton treatment sample was lower than that of the Persulfate treatment sample. The concentrations of organic in solution has a positive relationship with fluorescence intensity when the DOC concentrations of the solution is low [43, 48]. Therefore, the reduction of protein organics in the EPS solution improved the dewaterability of sludge samples after chemical oxidation treatment. But the pH of the solution had a significant effect on fluorescence intensity (See Table 2 EPS solution pH = 4.50 and 8.44 of RES), so the difference of the fluorescence intensity of EPS solution between Fenton and Fenton-Like treatment sample was not obvious. It is acknowledged EPS is closely related to sludge dewaterability [19]. The results may indicate that the dewatering performances of RES under chemical oxidation treatment have a solid-liquid separation mechanism different from that of thickened sludge samples. 3.1.2 Particle size distributions The volume lower diameter, volume median diameter and volume upper diameter (D(10), D(50), D(90)), the average value in 6 tests, was (8.8 ± 0.1 μm, 45.0 ± 0.5 μm, 225.0 ± 4.2 μm). And the width of size distribution (WSD) (D(90) / D(10)) was 25.6. Yu et al. [5] reported the distributions of thickened sludge from the secondary sedimentation tanks were (16.0μm, 47.2μm, 100.1μm) and the WSD was 6.3. In comparison with Yu’s data, D(50) is similar and WSD is larger, which means that RES has a wider distribution of different size particles. However, the broad particle size distribution of the sludge is detrimental to its dewatering capacity [49]. After the oxidation conditioning, three kinds of particle size of the sample were reduced slightly (Fig. 3). Theoretically, because of the formation of more Fe3+ in Fenton-Like reaction than Fenton reaction and persulfate reaction, the former should have better coagulation 13

effects compared with the latter. But there was no significant distinction among the different oxidation samples. Moreover, the particle size of the sample was further reduced after adding lime in the Fenton reaction. However, the relationship between the sludge particle size change and sludge dewatering capacity was contradictory. In traditional theory, the particle size of solid particles in the slurry was larger, the slurry solid-liquid separation effect could be better[49]. Because the fine particles in the slurry are easy to block the filter medium, and it tends to form a high compressibility filter cake with the low permeability, which causes drainage obstruction. To explain the improvement of sludge dewatering capacity but the decrease of particle size after oxidation treatment, Liu et al.[50] reported that dispersed small particles are more suitable for high pressure dewatering compared to aggregated large flocs. But Fenton treated samples also showed significant improvement in dehydration capacity under low pressure (less than 1MPa) filtration (Fig 6(a)). The reasonable explanation may be that the influence of organic matter content in sludge EPS on dewatering capacity is higher than that of sludge particle size. Another possible reason is the disturbance of the sludge particle size test procedures. Agglomerations among sludge flocs caused by Fe3+ were unstable in the reaction system. And pure water was used as the dispersant in the dispersion tank of the particle size analyzer. The mixing and dispersion of the sample flocs in the dispersion tank damaged the unstable agglomeration in the Fenton reaction system, failing to obtain the real “equivalent particle size” value of the flocs. Empirically, the mixing rate of the dispersion tank should be set to the appropriate speed. But in any case, using a laser particle size analyzer to measure the particle size of the treated sludge will always be subject to some degree of interference, especially to characterize the coagulation or flocculation effects in the suspension. 3.1.3 Zeta potential Among all samples, the negative value of zeta potential decreased after oxidation (Fig. 4). Two factors contributed to the numerical decrease of zeta potential. First, Fe2+/ Fe3+ with a higher positive charge squeezed into the sliding surface to neutralize the negative charge on the surface of sludge particles. Second, a large number of negatively charged proteins wrapped on the surface of sludge particles were oxidized and degraded. The results are consistent with previous studies [24, 51]. Meanwhile, samples of Fenton reaction under acidic conditions had 14

the lowest negative value with zeta potential. A large number of H+ in the liquid were also involved in neutralizing the surface charge of the sludge particles. The numerical reduction of zeta potential indicated the compression of the double electric layer around the sludge particles, which reduced the repulsive force between the particles, and the particles tended to agglomeration and precipitation. 3.2 Filtration stage and expression stage dewatering behaviors 3.2.1 Transition point and end point of data analysis A complete dewatering process consists of two stages: the filtration stage and the compression stage (Fig. 5), the overall dewatering characteristics of these sludge samples markedly differed. Yech proposed a method for locating the transition point between the two stages. A certain amount of gas was retained in the dehydration chamber and this was then forced out through the cake once the piston touched the cake surface [31]. In this study, the suspension sample was poured into the dehydrating cylinder from the bottom of the cylinder, and after the sealing, the dehydrating cylinder was inverted and installed on the air injection valve. Naturally, a small amount of air was left between the piston and the suspension. At the end of the filtration stage, a small amount of gas was discharged first which caused a slight delay in starting the compression stage, which can be well observed from the data, as indicated by the vertical arrow in Fig 5. Shirato et al. [52] proposed a graphical method for locating transition points. We adopted the more intuitive first method. The time of the end point controlled the water content of the final filter cake. It was also important to analyze the end point of the dewatering process. The time of the end point controlled the water content of the final filter cake. Most of the previous studies set the same dewatering time for all samples to calculate the dehydration efficiency, and tests were carried out until the drainage was balanced. Because not only the characteristics of sludge before and after treatment were significantly different, but also the dewatering process was different under different pressures, and setting the same dewatering time for all samples cannot fully reflect their properties. The filtrate output efficiency should be considered as the identification of end point of data analysis. The filtrate output efficiency presented a downward trend with time (Fig. S3). If the filtrate mass in the 5-minute segment was reduced to 0.20g or the filtrate mass in the 10-minute segment was reduced to 0.40g, the dehydration was considered to reach equilibrium, 15

with all types of samples were applicable considered. And this point time was taken as the end of data analysis. As illustrated by the horizontal arrow in Fig. 5(a). For sample, at the end point time t=26438s of RES dewatering test analysis at 300kPa, and the total filtrate volume V= 115.44mL. The analysis of dewatering behavior will be some degree of uncertain if the transition point and end point of the dewatering process were not determined by uniform condition. And according to the acceptance of filtrate discharge efficiency, compared with the fixed dehydration time as the end point, it was more suitable for the physical process of solid-liquid separation. 3.2.2 Extent and efficiency of dewatering in filtration and expression stage Chemical oxidation and pressure can improve the total filtrate discharge and total dewatering efficiency (the ratio of the total filtrate discharge mass to the total dehydration time). In Table 3, for the same type of sample, when the pressure was raised from 80kPa to 700kPa, the percentage increase in total filtrate mass from high to low was：RES (7.01%) > Fenton (6.62%) > Fenton + CaO (5.84%) > Persulfate (5.83%). (Noted that the sample was treated with Fenton + CaO, and part of the water in the sample reacted with CaO to generate crystallized water.) Another hand, the total filtrate mass of the Fenton treated sample was increased by 4.73% (80kPa), 4.67% (300kPa) and 4.35% (700kPa) compared with that of RES samples. And in Table 3, the most total filtrate mass from the Fenton treated samples. Noted that the filtrate mass and the dewatering efficiency have different performances in the filtration and expression stage (Table 3). (Total filtrate mass (mtf) = Filtration filtrate mass (mff) + Expression filtrate mass (mef)). For the same type of samples, when the pressure was raised from 80kPa to 700kPa, the percentage of mef to mtf increased. Such as, the size of percentage change was from highest to lowest: Fenton (9.8 percentage points) > RES (9.1 percentage points) > Persulfate (5.9 percentage points) > Fenton + CaO (2.4 percentage points). Moreover, the mff of Fenton treated sample was decreased by-0.48% (80kPa), 3.88% (300kPa) and 1.41% (700kPa) compared with the mff of RES sample. Also noted that the promoting effect of chemical oxidation on the filtration efficiency (the ratio of the filtrate mass in filtration stage to the filtration time) greater than the expression efficiency (the ratio of the filtrate mass in the expression stage to the expression time). The filtration efficiency of Persulfate, Fenton and 16

Fenton + CaO treated sample at 700kPa was 6.10-fold, 7.85-fold and 9.30-fold higher than that of RES sample, and the expression efficiency of those was 2.56-fold, 2.94-fold and 1.30-fold higher than that of RES sample, respectively. Furthermore, for the same type of sample, the effect of pressure on the filtration efficiency was slightly, but the effect of pressure on the expression efficiency was little. Such as the filtration efficiency of RES, Persulfate, Fenton, and Fenton + CaO sample at 700kPa was 1.32-fold, 1.23-fold, 1.33-fold and 1.11-fold higher than those at 80kPa, and the expression efficiency of those at 700kPa was 1.00-fold, 1.02-fold, 1.20-fold and 0.94-fold higher than those at 80kPa, respectively. The results showed that the pressure rise caused the filter cake to compress in the filtration stage, resulting in the filtration filtrate mass decrease. However, the pressure rise in the compression stage can force the water in the filter cake to further discharge. Pressure and chemical oxidation can improve total dewatering efficiency mostly by improving filtration efficiency. Dewatering performance of samples with different Fenton reaction conditions at 300kPa is presented in Table 4. The mff of Fenton reaction samples was generally higher than mff of Fenton-Like reaction samples. But for the mef, the results were reversed. Also, the molar ratio of Fe2+ and H2O2 had an influence on mtf, i.e. Under different pH conditions, mtf reached its peak value at different [Fe2+] / [H2O2]. Another hand, the dewatering efficiency of Fenton-Like reaction samples was higher than that of Fenton reaction samples both in the filtration and expression stages. Under neutral conditions, the filtration efficiency reached its peak at [Fe2+] / [H2O2] = 0.79; but under acidic conditions, the filtration efficiency had a small change. Besides, the expression efficiency was less affected by the molar ratio of Fe2+ and H2O2. Consequently, the dewatering test results of oxidized RES samples were contrary to the oxidized thickened sludge samples [37, 53, 54]. Fenton reaction has a stronger oxidation capacity than Fenton-Like reaction, while the formerly treated RES samples were inferior to the latter treated RES samples by the in terms of filtrate discharge and efficiency. In particular, reactions under acidic conditions can increase the filtration filtrate mass, but the expression filtrate mass was lower than that of reactions under neutral conditions. Such results may indicate again that dewatering performance of RES under chemical oxidation treatment different from that of other types of sludge samples. 17

3.2.3 Specific resistance in filtration stage In the filtration stage, as the thickness of filter cake increased, the resistance of the filter cake increased with time. The filtration resistance was proportional to the mass of dry solids deposited per unit filtration area, and SRF reflected the ratio of these two, which was the slope. Another indicator related to dewatering performance during the filtration phase is CST (Fig. 6(c)). For highly compressible sludge samples, SRF of filter cake was sensitive to pressure, and different SRF was usually obtained under different pressure (Fig. 6(a)). Obviously, the SRF of chemical oxidized sludge samples was significantly smaller than that of RES samples. Meanwhile, compressibility index n reflected the sensitivity of filter cake to pressure. The compressibility of filter cake was may not necessarily strongly correlated with SRF. Under the filtration pressure of 300kPa, the SRF of samples which were treated by different Fenton reaction conditions are shown in Fig. 6(b). The results showed that the change of SRF to [Fe2+] / [H2O2] in the filtration stage was just opposite to the change of the filtration efficiency. But the influence of pH on dehydration indicators of oxidized RES samples was contrary to that conclusion from thickened sludge [37, 53, 54]. The relations between the value of SRF and EPS compositions will be discussed in the 3.3 section. 3.2.4 Rheological model of expression stage (1) Consolidation curve The performances of sample expression tests were presented in the form of consolidation curves (Fig. 7). The expression stage was following the filtration stage. The expression time θc was calculated from zero to the time of end point. Chang et al.[31] illustrated that the experimental points deviate from the two-stage Terzaghi-Voigt model theoretical straight line in the final stage of expression biosludges, which as illustrated by the arrows in the ln(1-Uc)-θc diagrams in our study (Fig. 7(c)(d)). Ternary consolidation exists in the biosludges dewatering test, but not exists in the inorganic slurry dewatering test. Neglecting the ternary consolidation would incur an error in estimating the model parameters[30]. To avoid overly complex, this thesis temporarily only focuses on the rationality of the consolidation curves and the control parameters (1-B-F, B, F) which were concluded by fitting the shape of the consolidation curves. In the diagram of Uc - θc, ln(1-Uc) - θc, and ln(1-Uc-F) - θc, the RES sample curve was on the right side of the sample curve treated by chemical oxidation, from left to right: Fenton + 18

CaO, Fenton, Persulfate, and RES. This implied that the chemical oxidation treatment decreased the sample’s resistance in expression dewatering, and the chemical oxidation sample had less time to reach the equilibrium condition (filtrate mass in the 5-minute segment was reduced to 0.20g). When the pressure was raised from 80kPa to 700kPa, the curves of the chemical oxidation sample slightly shift to the right, and the curves of the RES sample significantly shift to the right, that meant, RES samples had a longer time to reach the equilibrium condition. (Because the degree of consolidation Uc was calculated according to equation (5)). Notably, the final express filtrate mass was different at various pressures when the equilibrium condition was reached, i.e., there were different Lf values for each test sample. The slight increase in pressure made the filter cake need a longer consolidation time to discharge more filtrate, resulting in the curves shift. Such data could reasonably reflect the phenomenon of filter cake consolidation under low pressure (less than 1MPa). In previous studies [10, 31], the lifting pressure (higher than 2MPa) would shift the curve to the left, indicating that under the action of high pressure, the filter cake not only discharges more filtrate but also shortens the filter cake final equilibrium time. Importantly, the variation of curve morphology will lead to the variation of fitting parameters. (2) Control parameters The purpose of the fitting test consolidation curves was to obtain a model that can be used to predict the expression dewatering process. Although the models were fitted according to the morphological characteristics of the consolidation curves, the components of the rheological model had real physical significance and the models were not fitted by simple empirical formulas. In equation (5), the unknown parameters were Ce, ω0, F, θc*, B and η. After obtaining the test consolidation curves (Fig. 7), the consolidation coefficient Ce was firstly calculated. Shirato et al.[52] first introduced the one-dimensional consolidation issue in soil mechanics into the consolidation of sludge cake and deduced the relationship between Ce and Terzaghi consolidation coefficient Cv. In the two-stage Terzaghi-Voight model and three-stage TerzaghiVoight model, Ce is both defined by [31, 55]: 1

(9)

𝐶𝑒 = 𝜇𝜌𝑠𝛼𝑎𝑣𝑎𝐸

Where ρs is the true density of solid particles; αav is the average specific resistance of filter cake 19

during consolidation; aE is the compression coefficient of primary consolidation, be defined as

aE=(1+e)/E1; e is the local void ratio of filter cake; E1 is the elastic coefficient of Terzaghi spring element. According to the definition of equation (9), it is not convenient to calculate Ce. In the theory of saturated soil seepage and consolidation of soil mechanics[56, 57], Semiempirical Casagrande plot method is widely used, i.e. the method of square roots of time or time logarithm is used to fit the test curve with the theoretical curve to determine the Ce value. In this paper, Ce was calculated by square roots of time method in accordance with Shirato et al. [52]: When the initial excess hydrostatic pressure is uniformly distributed, the theoretical Terzaghi one-dimensional consolidation equation is [55, 56]: 𝐿1 ― 𝐿

𝑈𝑐 = 𝐿1 ― 𝐿𝑓

∞ =1―∑

8

𝑁 = 1(2𝑁 ― 1)2𝜋2

×𝑒

―

(2𝑁 ― 1)2𝜋2 𝑇𝑣 4

(10)

Where N is an integer and Tv is the time factor:

Tv=Ceθc/ω02.

(11)

When Uc=50%, the theoretical time factor can be obtained as Tv50=0.196 through equation (10). The consolidation time θc50 of Uc =50% was found in Fig. 7(a)(b). The consolidation coefficient Ce can be obtained through equation (11). (Fig. S4). A point that was deviated from the straight line in the ln(1-Uc)-θc diagram (Fig. 7(c)(d)) was the starting of the ternary consolidation stage. By fitting the approximate line stage of the ternary consolidation stage in the Uc-θc diagram (Fig. 7(a)(b)) and using equation (8), the parameter F and θc* were obtained. Then by fitting the linear stage in the middle part of the ln(1-Uc-F)-θc diagram (Fig. 7(e)(f)) and substituting F into equation (7), the parameter B and η were got. The results of the parameter are shown in Fig. 8. Thus, parameters 1-B-F, B and F represent respectively the fractions of moisture removed by primary consolidation, secondary consolidation and ternary consolidation [31]. At 700kPa, the moisture removal for primary, secondary and ternary consolidation (1-BF, B, F) for Fenton + CaO, Fenton, Persulfate and RES were respectively (0.04, 0.81, 0.16), (0.06, 0.74, 0.20), (0.00, 0.75, 0.25) and (0.05, 0.43, 0.52) (Fig. 8 (a)). RES sample to Fenton + CaO treated sample, the percentage of moisture removal for secondary consolidation and ternary consolidation was significantly changed from 43% to 81% and 52% to 16%. And the relative moisture removal of primary consolidation had no obvious regular change. For RES 20

samples at 80kPa, 300kPa and 700kPa, the values (1-B-F, B, F) were respectively (0.01, 0.30, 0.69), (0.05, 0.38, 0.57) and (0.05, 0.43, 0.52). The percentage of moisture removal for secondary consolidation and ternary consolidation changed from 30% to 43% and 69% to 52%. For Fenton oxidation samples, the values (1-B-F, B, F) at three pressures were respectively (0.00, 0.74, 0.26), (0.05, 0.72, 0.23) and (0.06, 0.74, 0.20). The percentage of moisture removal for ternary consolidation decreased from 26% to 20%. The results showed that both chemical oxidation and small pressure rise would reduce the F value. It discreetly for a notion that favorable conditions for promoting sample’s expressing performance will reduce the moisture removal for ternary consolidation. Such phenomenon was reflected again in the samples consolidation results under various Fenton reaction conditions (Fig. 8(b)). The percentage of moisture removal for ternary consolidation of Fenton-Like reaction samples (17%~26%) was generally lower than that of Fenton reaction samples (28%~37%). Other hand, under neutral condition, F first decreased and then increased with the [Fe2+] / [H2O2] increase, and when [Fe2+] / [H2O2] = 0.79, F reached the lowest value of 17%. Under the acidic condition, F first increased and then decreased, and when [Fe2+] / [H2O2] =0.96, F value reached the peak value of 37%. Interestingly, the change of moisture removal for ternary consolidation was very consistent with the change of SRF in the filtration stage. 3.3 Properties correlation analysis The Pearson correlation analysis of parameters is presented in Table S2. SRF had a significant positive linear correlation with the content of PN in SS (ρ=0.99, p<0.01). The results indicated that for RES samples the content of PN in SS was the main factor affecting SRF, which is consistent with previous studies [20, 21]. Meanwhile, there was no correlation between the filtration filtrate mass mff with SRF (ρ =0.35, p=0.28) or the content of PN in SS (ρ =0.31, p=0.35). The expression filtrate mass mef had a nearly negative linear correlation with SRF (ρ =-0.81, p<0.01) and the content of PN in SS (ρ = -0.84, p<0.01). SRF and CST theoretically characterize sludge dewaterability well just in the filtration stage. So no parameter can be associated with total filtrate mass mtf in a strong linear relation because the mtf in sludge solidliquid separation also depends on the external pressure and dehydration time. Morphological control parameters were used to characterize the expression of dewatering 21

behaviors. The new discovery was that the B value had a significant negative linear correlation with the content of PN in SS (ρ=-0.98, p<0.01) and a negative linear correlation with the content of PN in TB-EPS (ρ=-0.91, p<0.01). On the contrary, the F value had a positive linear correlation with the content of PN in SS (ρ=0.94, p<0.01) and a nearly positive linear correlation with the content of PN in TB-EPS (ρ=0.85, p<0.01). The expression efficiency had a significant positive linear correlation with the consolidation coefficient Ce (ρ =0.96, p<0.01). Interestingly, the SRF had a significant negative linear correlation with B value (ρ = -0.98, p<0.01) and a significant positive linear correlation with F value (ρ = 0.95, p<0.01). Based on this, the interrelationship between the filtration and the consolidation could be built. This means that the control parameters could be the theoretical indicators of expression dewatering. 3.4 Discussion of dewatering mechanism The change of SRF reflected that the change of filtration resistance of filter cake when the solids content of slurry and filtration pressure was consistent. More generally, when the solid and liquid moves relatively, the filtration resistance is equal to the flow resistance when the liquid moves relative to the solid in the filtration stage. Flow resistance comes from two aspects: One is the surface friction associated with viscosity. The second is the shape drag associated with geometric obstacles. The former causes the friction between the solid particles and the liquid surface, and the friction between the different velocity laminar flow, which forms the flow resistance or pressure loss. Assuming the laminar flow state, and the shape resistance can be ignored. Except for the surface properties of moving sludge particles, the properties of the liquid were equally important. For RES samples before and after chemical oxidation, the content of PN in SS was almost linearly related to SRF, and the water of SS was also mainly removed by mechanical compression in solid-liquid separation. PN is a macromolecular organic compound that hates water, and the dissolution of PN will increase the viscosity of the liquid, thus the flow resistance on the surface of the liquid and sludge particles was increased in relative motion. The chemical oxidation can degrade the PN in liquid significantly and reduce SRF (Fig. 9(a)(b)). But traditional studies suggested that wastewater sludge oxidation promoted the decomposition of EPS to release water wrapped by flocs [58, 59], which suitable for activated sludge or thickened sludge, but seems unreasonable for RES samples. Because as long as the expression 22

time of filter cake was long enough, the water content of cake without oxidation treatment can also be reduced to a lower extent. The significant difference before and after the oxidation treatment of the RES sample was the great improvement of dewatering efficiency rather than the change in the amount of filtrate discharge. More importantly, experiments indicated that the advantage of reducing SRF was not to be obtained by stronger chemical oxidation (Fig. 9(d)). Because sludge particles are not a single stable substance with a fixed shape, and sludge particles are multi-component complex flocs formed by EPS covering insoluble inorganic particles, bacterial cells, and other impurities. When oxidation exceeded a certain level, it will further destroy and disintegrate the so-called sludge particles, which caused more PN to be released into the liquid, this PN was originally encapsulated inside the sludge particles (Fig.9(c)). As a result, the filtration dewatering performance of RES samples will be deteriorated when the migration of organic substances be more pronounced than the mineralization of it in SS. Besides, flocculation was also important for lowering the content of PN in the liquid, it was the opposite of the effect of promoting the disintegration of sludge particles, and flocculation can encapsulate PN into the flocs to form large sludge particles. As far as we know, the ternary consolidation generally exists in biosludges dewatering, while the dewatering of the silt, kaolin, and clay ends in the secondary consolidation [30, 31]. The filter cake formed by the former has extremely higher compression than that of the latter. The change of SRF of the sample filtration stage was consistent with the change of the filtrate removal of ternary consolidation or was opposite to the change of the filtrate removal of primary added secondary consolidation, which indicated that they were almost affected by the same mechanism. The liquid and the surface properties of sludge particles play an important role in this mechanism. When SRF or CST, an indicator of dewaterability in the filtration stage, is high, the properties of filtrate and particle surface are unfavorable for filtration in the system, the moisture removal of primary added secondary consolidation is hindered, and the moisture removal of ternary consolidation are increased. Of course, more in-depth research is needed to fully understand this complex mechanism, due to the limitations of experiments in this study. 4 Conclusion This study reports for the first time the physicochemical properties of resuspension excess sludge. Pressure and chemical oxidation can both improve total filtrate discharge and total 23

dewatering efficiency. The improvement of the dewaterability of resuspension excess sludge is due to moderate chemical oxidation, which is related to the mode of migration and mineralization of organic substances in the stratified structure of EPS, when the amount of PN in SS is more mineralized than moved in, the dewaterability of the sample is improved. Meanwhile, the content of PN in SS is negatively linearly correlated with the moisture removal of secondary consolidation (ρ = -0.98, p<0.01) and positively linearly correlated with the moisture removal of ternary consolidation (ρ = 0.94, p<0.01). The parameters in three-stage Terzaghi-Voigt model could be the theoretical indicators of expression dewatering, and the change of moisture removal for ternary consolidation was very consistent with the change of SRF in the filtration stage (ρ = 0.95, p<0.01). Acknowledgments We thank Jiangsu Institute of Zoneco Soil Co.Ltd for support; P. Wang, M.L Wei, W. Wei, and S.H Zhang for technical assistance; Y. Liu, X.J Wang, and W.J Wang for test support, L.G Jiang, Y. Xu, and G.F Wang for help with laboratory coordination. C.G Song for help with field sampling work. This work was support by the grants from the National Natural Science Foundation of China (51625903, 51609241, 51827814), the National Natural Science Foundation of China/Hong Kong Research Grants Council Joint Research Scheme (51861165104), the Yixing “Taodu Talents” Project (CXTD201601C), the Youth Innovation Promotion Association CAS(2017376).

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Air injection

Dehydrating cylinder

Piston

Pressure reducing valve

Computer

sludge

Pressure source

Filter medium

（b）

（a） Fig. 1.

Schematic of the improved filter device: (a) Testing state; (b) The profile structure of the dehydrating cylinder

28

260 240

TB-EPS LB-EPS Supernatant + Slime

PN (mg/L)

220 200

30 20 10 0

l l l O te ic ic ic 1) 2) utra neutra neutra 3-acid 9-acid 4-acid n+Ca rsulfa batch batch e n o Pe ES ( ES ( .43 -0.79 -1.14 S-0.4 S-0.7 S-1.1 Fent R R S-0 S S

Sample

(a)

300

TB-EPS LB-EPS Supernatant + Slime

PS (mg/L)

250

200

150

100

50

0

1) 2) aO ate dic dic dic ral ral ral eut -neut -neut 3-aci 9-aci 4-aci ton+C ersulf (batch (batch n 4 7 1 P S S .43 -0.79 -1.14 S-0. S-0. S-1. Fen RE RE S-0 S S

Sample

（b） Fig. 2. 29

The organic substance in various chemical oxidation treatment sample: (a) The distribution of PN in sample stratified structure; (b) The distribution of PS in sample stratified structure

S-0.43-neutral S-0.79-neutral S-1.14-neutral S-0.43-acidic S-0.79-acidic S-1.14-acidic

Volume density (%)

4

264

D (90) (μm)

5

3

D (90)

240 216

2

192 1

46.2 1

10

100

Particle size (μm)

42.9

1000

39.6

（a）

9.0

D (10) (μm)

volume density (%)

36.3

Fenton Fenton+CaO Persulfate RES

4

D(50)

D(50) (μm)

0

3

2

D (10)

8.4 7.8 7.2

1

al cidic cidic cidic CaO lfate RES al al nal utr utr utr igi u -ne 9-ne 4-ne .43-a .79-a .14-a nton+ Pers Or 3 .1 S-0 S-0 S-1 Fe .7 .4 S-0 S-0 S-1

0 1

10

100

Particle size (μm)

1000

Sample

(c)

(b)

Fig. 3. Particle size distribution of (a) Fenton samples under different reaction conditions and 30

(b) samples was treated by different methods; (c) Changes of particle size of D (10), D (50) and D (90) -20

Zeta potential (mV)

-16

-12

-8

-4

0

ral utral utral acidic acidic acidic +CaO ulfate tch 1) tch 2) eut e e s 3-n .79-n .14-n -0.43- -0.79- -1.14- enton Per ES (ba ES (ba 4 . F S S S R R S-0 S-0 S-1

Sample

Fig. 4. Sample zeta potential under different conditioning conditions 350

RES 80 KPa RES 300KPa RES 700KPa

300

t / V (s/mL)

250 200 150 100 50 Expression

Filtration

0 0

20

40

60

V (mL) （a）

31

80

100

120

140

Fenton 80 kPa Fenton 300 kPa Fenton 500 kPa Fenton 700 kPa

80

t / V (s/mL)

60

Scale in

40

20

0

Expression

Filtration

60

40

20

0

V (mL)

80

140

120

100

（b） 160

S-0.43-neutral S-0.43-acidic S-0.79-neutral S-0.79-acidic S-1.14-neutral S-1.14-acidic

140 120

Scale in

t / V (s/mL)

100 80 60 40 20 Expression

Filtration

0 0

20

40

60

V (mL)

80

100

120

140

（c） Fig. 5. Dewatering performances: (a) The complete dewatering process of RES samples with different pressures dewater; (b) pH =7.5, [Fe2+] /[ H2O2]=0.79, Fenton reaction treated sample dewatering process with different pressure dewater; (c) Fenton reaction treated sample of different condition parameters dewatering process under 300kPa. (The 32

vertical arrows indicate the transition points and the horizontal arrows indicate the end points of the analysis).

Fenton Fenton+CaO Persulfate RES(batch 1)

Fenton neutral 300kPa Fenton acidic 300kPa

5.0E+11

n=0.90

4.5E+11

Specific resistance (m/kg)

Specific resistance (m/kg)

1E+13

1E+12 n=0.96 n=0.89 n=0.99 1E+11

4.0E+11 3.5E+11 3.0E+11 2.5E+11 2.0E+11 1.5E+11

1E+10 60

80 100

200

400

600

0.3

800 1000

Filtration pressure (kPa)

（a）

0.4

0.5

0.6

0.7

0.8

[Fe2+] / [H2O2]

0.9

1.0

1.1

1.2

（b）

160 140

CST (s)

120 100 80 60 40 20 0

l l l e c c utra neutra neutra -acidic -acidi -acidi n+CaO rsulfat tch 1） tch 2） -ne Pe S (ba S (ba .43 -0.79 -1.14 -0.43 S-0.79 S-1.14 Fento S E S-0 S S R RE

Sample

(c)

Fig. 6. The results of dewatering tests in the filtration stage: (a) SRF of samples under different pressures dewater; (b) SRF of samples with different chemical oxidation under 300kPa; (c) CST of samples with different chemical oxidation.

33

θc=5500

θc=5200

1.0

θc=12000

θc=6300

0.8

Uc

0.6

0.4

Fenton 700kPa Fenton+CaO 700kPa Persulfate 700kPa RES 700kPa

0.2

0.0 0

5000

10000

θc (s)

15000

20000

(a) θc=4800

1.0

θc=4400

0.8

θc=5500

θc=10400

θc=8000

θc=12000

Uc

0.6

0.4 Fenton 80kPa Fenton 300kPa Fenton 700kPa RES 80kPa RES 300kPa RES 700kPa

0.2

0.0 0

5000

10000

θc (s)

(b)

34

15000

20000

0 -2

θc=6300 θc=12000

θc=5500

-4

ln (1-Uc)

θc=5200

-6 -8

-10 Fenton 700kPa Fenton+CaO 700kPa Persulfate 700kPa RES 700kPa

-12 -14 0

5000

10000

θc (s)

15000

20000

（c） 0 θc=12000

-2

θc=5500 θc=4400 θc=4800

ln(1-Uc)

-4

θc=8000 θc=10400

-6 -8 Fenton 80kPa Fenton 300kPa Fenton 700kPa RES 80kPa RES 300kPa RES 700kPa

-10 -12 -14 0

5000

10000

θc (s)

（d）

35

15000

20000

0 -1

ln(1-Uc-F)

-2 -3 -4 -5 -6

Fenton 700kPa Fenton+CaO 700kPa Persulfate 700kPa 700kPa RES

-7 -8 0

1000

2000

3000

θc (s)

4000

5000

（e） 0

ln (1-Uc-F)

-2

-4

-6

Fenton 80kPa Fenton 300kPa Fenton 700kPa RES 80kPa RES 300kPa RES 700kPa

-8

0

1000

2000

3000

θc (s)

4000

5000

（f） Fig. 7. The Uc versus θc, ln(1-Uc) versus θc, and ln(1-Uc-F) versus θc performances of sample tests in expression stage: (a)(c)(e) Samples of different chemical oxidation treatments under 700kPa; (b)(d)(f) Fenton treated and RES samples under different pressures. (Arrows in (a)(b) indicate the starts of the final phase of consolidation, which were used to estimate the parameters F and θc*, and the dashed lines are regression lines based on equation (8). Arrows in (c)(d) indicate the test data deviating from the straight line. The straight lines in (e)(f) are the regression line based on equation (7) from the intermediate period of test data, which were used to estimate the parameters B and η*).

36

1.0

F B 1-B-F

0.17 0.15 0.16

0.20 0.26 0.23

0.31 0.31

0.8

0.25

0.57

0.52

0.69

Value

0.6

0.4

0.72

0.74

0.76 0.77 0.81

0.74

0.68 0.69

0.75

0.38

0.2

0.0

0.43

0.30

0.06 0.00 0.05 Pa Pa Pa 80k 300k 700k

Fenton

0.08 0.08

0.04

Pa Pa Pa 80k 300k 700k

Fenton+CaO

0.00 0.00 0.00 Pa Pa Pa 0 8 k 300k 700k

Persulfate

Sample

0.01 0.05 0.05 Pa Pa Pa 80k 300k 700k

RES

（a） 1.0 0.17 0.26

0.25

0.22

0.21

0.28

0.8

0.32

0.34

0.68

0.64

0.00

0.01

0.38

0.31

F B 1-F-B

Value

0.6 0.74

0.4

0.70

0.72

0.70

0.74 0.72

0.62

0.69

0.2

0.0

0.04

0.05

0.09

0.05

0.06

0.00

0.00

0.00

tral tral tral tral tral dic idic idic idic idic neu 4-neu 9-neu 6-neu 4-neu 3-aci 64-ac 79-ac 96-ac 14-ac 3 4 . . . . 4 6 7 9 1 . . . . . . 0 0 0 1 0 0 0 0 0 1 S S S S SSSSSS-

Sample

（b） Fig. 8. The results of control parameters in the three-stage Terzaghi-Voight model: (a) Samples of different chemical oxidation treatment; (b) Fenton treated samples with different reaction conditions under 300kPa

37

Fig. 9. The improvement of the dewaterability of resuspension excess sludge thanks to moderate chemical oxidation, which is related to the migration and mineralization of protein in SS: (a) RES sample before chemical oxidation, sludge particles are multi-component complex flocs; (b) Under moderate chemical oxidation, PN in SS is significantly degraded; (c) After strong chemical oxidation, the migration be more pronounced than the mineralization of PN; (d) The relationship between the content of PN in SS and SRF, oxidation intensity is a critical aspect affecting separation system.

38

Table 1 Experiments design Batch

Sample Fenton

pH 7.5

Pressure (kPa) 80, 300, 500, 700

Fenton + CaO

7.5

80, 300, 500, 700

Persulfate

7.5

80, 300, 500, 700

RES

7.5

80, 300, 500, 700

1

2O2]=0.79

[Fe2+]/[H

2O2]=0.79

CaO=100 mg/g VS S2082-=230 mg/g VS Fe2+=84 mg/g VS None

300

[Fe2+]/[H

300

[Fe2+]/[H

300

[Fe2+]/[H

S-0.96-acidic

300

[Fe2+]/[H2O2]=0.96

S-1.14-acidic

300

[Fe2+]/[H2O2]=1.14

S-0.43-neutral

300

[Fe2+]/[H2O2]=0.43

S-0.64-neutral

300

[Fe2+]/[H2O2]=0.64

300

[Fe2+]/[H2O2]=0.79

S-0.96-neutral

300

[Fe2+]/[H2O2]=0.96

S-1.14-neutral

300

[Fe2+]/[H2O2]=1.14

300

None

S-0.43-acidic S-0.64-acidic S-0.79-acidic

2

Reagent [Fe2+]/[H

S-0.79-neutral

RES

3.1

7.5

7.5

39

2O2]=0.43 2O2]=0.64 2O2]=0.79

Table 2 3D-EEM fluorescence spectrum peaks of EPS solution Supernatant+Slime S-0.64-neutral

S-0.64-acidic

Persulfate

(solution

(solution

(solution

pH=5.05)

pH=4.25)

pH=4.60)

Peak A

3504

3086

Peak B

3552

Peak C

Deionized RES (solution

RES (solution

pH=4.50)

pH=8.44)

3259

4466

10510

3

3256

3511

4221

8200

7

1772

2355

1886

2036

3498

-3

Peak D

746

1195

556

827

1061

1

Peak E

1144

2502

1224

1537

2279

0

LB-EPS Peak A

603

510

804

2267

2024

Peak B

535

592

820

1274

1082.4

Peak C

373

446

435

877

842.2

Peak D

136

220

153

279

281

Peak E

256

444

320

566

518

TB-EPS Peak A

12846

12641

19283

188419

112750

Peak B

13346

10370

20456

196429

91200

Peak C

2813

4723

3451

20977

9520

Peak D

1151

2527

1449

9516

3474

Peak E

2646

5628

3831

18855

8625

40

water

Table 3 Filtrate discharge and dewatering efficiency under different pressure RES Pressure (kPa)

Persulfate

Fenton

Fenton + CaO

mtf

mff

mef

mtf

mff

mef

mtf

mff

mef

mtf

mff

mef

(g)

(g)

(g)

(g)

(g)

(g)

(g)

(g)

(g)

(g)

(g)

(g)

80

111.03

89.69

21.34

113.81

85.61

28.20

116.28

90.12

26.16

115.5

81.20

34.30

300

115.44

89.37

26.07

117.13

84.56

32.57

120.83

85.91

34.92

117.66

86.25

31.41

700

118.81

85.18

33.63

120.44

83.41

37.03

123.98

83.98

40.00

122.25

82.98

39.27

F (g/s)

E (g/s)

F (g/s)

E (g/s)

F (g/s)

E (g/s)

F (g/s)

E (g/s)

80

0.00684

0.0018

0.04473

0.0045

0.0532

0.0044

0.0757

0.0064

300

0.00781

0.0017

0.04682

0.0048

0.0664

0.0052

0.0699

0.0068

700

0.00901

0.0018

0.05495

0.0046

0.0707

0.0053

0.0838

0.0060

（mtf: Total filtrate mass，mff: filtration filtrate mass，mef: expression filtrate mass，the water content of all samples was precisely controlled to 93.14%） F: Filtrate removal efficiency in filtration, E: Filtrate removal efficiency in expression)

41

Table 4 Filtrate discharge and dewatering efficiency under different Fenton parameters under 300kPa Fenton Filtrate removal

Filtrate removal

efficiency in

efficiency in

filtration （g/s）

expression (g/s)

33.86 ± 1.39

0.0428

0.0045

84.96 ± 0.72

37.25 ± 0.71

0.0500

0.0043

123.29 ± 0.02

86.92 ± 0.65

36.37 ± 0.65

0.0654

0.0047

S-0.96-neutral

122.07 ± 0.10

88.13 ± 0.10

33.94 ± 0.07

0.0496

0.0040

S-1.14-neutral

122.55 ± 0.08

88.25 ± 0.23

34.30 ± 0.31

0.0545

0.0043

S-0.43-acidic

121.74 ± 0.20

91.26 ± 0.33

30.48 ± 0.53

0.0345

0.0037

S-0.64-acidic

122.38 ± 0.08

91.78 ± 0.47

30.60 ± 0.40

0.0286

0.0034

S-0.79-acidic

121.31 ± 0.06

91.57 ± 0.05

29.74 ± 0.10

0.0261

0.0031

S-0.96-acidic

121.34 ± 0.06

92.08 ± 1.12

29.26 ± 1.06

0.0247

0.0031

S-1.14-acidic

121.73 ± 0.08

92.85 ± 0.74

28.87 ± 0.67

0.0276

0.0031

Sample

mtf

mff

mef

(g)

(g)

(g)

S-0.43-neutral

121.55 ± 0.09

87.69 ± 1.45

S-0.64-neutral

122.21 ± 0.06

S-0.79-neutral

Highlights



After chemical oxidation of resuspension sludge, the content of PN in Supernatant + Slime and TB-EPS decreased.



The improvement of the dewaterability of resuspension sludge is due to moderate chemical oxidation.



The dewaterability of resuspension sludge is improved when the organic substances in supernatant + slime are more mineralized than moved in.



The content of PN in Supernatant + Slime is positively linearly correlated with the moisture removal of ternary consolidation.



The change of moisture removal for ternary consolidation was very consistent with the change of SRF in the filtration stage.

42

Conflict of Interest Dear Editor:

No conflict of interest exists in the submission of this manuscript, and the manuscript is approved by all authors for publication. I would like to declare on behalf of my co-authors that the work described was original research that has not been published previously, and not under consideration for publication elsewhere, in whole or in part. All the authors listed have approved the manuscript that is enclosed.

Yijun Chen, Ph. D Research assistant Corresponding author E-mail address: [email protected]

43