Revealing the Underlying Mechanisms of How Initial pH Affects Waste

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Revealing the Underlying Mechanisms of How Initial pH Affects Waste Activated Sludge Solubilization and Dewaterability in Freezing and Thawing Process Xuran Liu, Qiuxiang Xu, Dongbo Wang, Qi Yang, Yanxin Wu, Jingnan Yang, Jilai Gong, Jun Ye, Yifu Li, Qilin Wang, Yiwen Liu, Bing-Jie Ni, Guangming Zeng, and Xiaoming Li ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b04427 • Publication Date (Web): 05 Oct 2018 Downloaded from http://pubs.acs.org on October 7, 2018

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ACS Sustainable Chemistry & Engineering

Revealing the Underlying Mechanisms of How Initial pH Affects Waste Activated Sludge Solubilization and Dewaterability in Freezing and Thawing Process Xuran Liu,1,†,‡ Qiuxiang Xu,1,†,‡ Dongbo Wang,*,†,‡ Qi Yang,†,‡ Yanxin Wu,†,‡ Jingnan Yang,†,‡ Jilai Gong,†,‡ Jun Ye,§ Yifu Li,†,‡ Qilin Wang,∥ Yiwen Liu,∥ Bing-Jie Ni,∥ Guangming Zeng,†,‡ Xiaoming Li†,‡ †College

‡Key

of Environmental Science and Engineering, Hunan University, Changsha 410082, P.R. China

Laboratory of Environmental Biology and Pollution Control (Hunan University), Ministry of Education,

Changsha 410082, P.R. China §Hunan

Qing Zhi Yuan Environmental Protection Technology Co., Ltd, Changsha 410004, P.R. China

∥ Centre

for Technology in Water and Wastewater, School of Civil and Environmental Engineering, University of

Technology Sydney, Sydney, NSW 2007, Australia 1Xuran

Liu ([email protected]) and Qiuxiang Xu ([email protected]) contributed equally to the

work. Corresponding author: Dongbo Wang Tel: +86 731 88823967; Fax: +86 731 88822829 E-mail: [email protected] (D. Wang)

1

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ABSTRACT:

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In this study, the effect of initial pH ranging from 3 to 11 on waste activated sludge (WAS)

solubilization and dewaterability in freezing and thawing (F/T) process was investigated.

Experimental results

showed that alkaline conditions enhanced the solubilization of sludge in F/T treatment whereas acidic conditions improved the dewaterability of sludge.

Optimum solubilization with organic substances being 189.7 mg COD/g

VSS occurred at initial pH 10, which was 12.9 times higher and more biodegradable than the control.

Optimal

dewaterability performance was achieved at initial pH 4, with capillary suction time and specific resistance to filterability reduction being reached to 85.4% and 87.8%, respectively.

It was found that F/T treatment at initial

alkaline condition also obtained good dewatering performance, and F/T treatment at initial acidic condition acquired fine solubilization too.

Mechanism explorations exhibited that the OH- and freezing had synergetic

effects on the degradation of extracellular polymeric substances (EPS) to enhance sludge solubilization, and OHcan be concentrated to a much higher level in the liquid-like boundary region upon freezing, which further strengthened this effects.

The H+ and freezing also showed synergetic effects on the protonation of functional

groups of EPS and flocculation of the colloidal sludge to improve sludge dewaterability.

This study clearly

reveals the role and mechanisms of initial pH on F/T process aiming at solubilization and dewaterability of sludge, and might provide supports for the application of F/T-based strategy in field situations in the future. Keywords: Waste activated sludge, freezing, solubilization, dewaterability, pH

INTRODUCTION Activated sludge processes are widely applied in wastewater treatment plants (WWTPs), a large volume of waste activated sludge (WAS) is therefore generated as a coproduct.1,2 environment-friendly treatment and disposal are needed.

To avoid secondary pollution, effective and

The dewatering of WAS is one effective strategy for 2


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the reduction of transportation and disposal costs.3,4

To date, many conditioning approaches such as thermal

treatment, freezing, ultrasonic pretreatment, microwave irradiation and chemical oxidation have been proposed for WAS dewatering.5-8

Among them, freezing and thawing (F/T) process has been proven to be effective, with

its environment-friendly and economical characters in moderate to cold climates.9,10

The formed intracellular or

extracellular ice crystals caused by low temperature transform the loose structure of sludge into denser and compacted form.

And meanwhile, small individual sludge flocs are further compressed into larger form, thus

leading to the release of bound water.11,12 Most of the reported studies in terms of F/T treatment concentrated on the impacts of freezing temperature or speed on the sludge dewatering characters.

For example, Lee and Hsu reported that freezing at speed of

11.1µm/s not only reduced the bound-water to 50% but also remarkably decreased the resistance to dry or filtration.10

Hung found that when the sludge was “unidirectional” freezing at freezing speed 2.5µm/s and

40µm/s, capillary suction time was respectively reduced by 71.8% and 64.0%, and zone settling velocity was respectively improved to 560 and 0µm/s.13

These excellent investigations clearly revealed the underlying

mechanism of how freezing speed affect the sludge dewatering characters of F/T treatment.

Apart from the

freezing temperature and speed, the electrolyte and dissolved organic material in sludge were also demonstrated to affect sludge dewaterability.

Vesilind et al.9 reported that freezing with the addition of 2% w/w sodium chloride

did not make a significant improvement of sludge filterability, excluding the double-layer compression effect in F/T treatment.

Discrepantly, Kawasaki et al.14 found that the addition of 2% w/w sodium chloride in freezing

process destroyed the sludge settleability and water content of dewatering cake, and this conclusion was further confirmed by Chu et al.15, which revealed that the presence of sodium chloride retarded the flocs gross migration,

3



thereby reducing the corresponding critical freezing speed.

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Despite these significant advances, the effect of

initial pH of F/T process on WAS dewaterability has not been studied before. Apart from the dewatering performance, the characteristics of dewatering liquor have recently attracted more and more attention. polysaccharides).

It is known that WAS contains large proportions of organics (e.g., proteins and

During the dewatering process, parts of particulate organics in WAS would be released into

the dewatering liquid, which needs to be introduced to the influent of WWTPs to reduce environmental pollution.16,17

In many WWTPs, carbon source in the influent is not sufficient.18-20

To achieve satisfactory

nutrient removal efficiency, commercial carbon sources supplement is needed, which thereby increases the operational costs of WWTPs.16

As an alternative, supplementing wastewater organics with carbon sources

recovered from sludge solubilization have recently gained growing interests.

Fox example, it was found that

both the biochemical oxygen demand (BOD) and chemical oxygen demand (COD) in the dewatering liquor increased by about 300% ~ 800% after -8 ℃ freezing for 36 h and 25 ℃ thawing for 10 h probably due to the release of extracellular and intracellular organics.13,21-22 solubilization of sludge in freezing.

Recently, Sun et al.16 examined the effect of nitrite on

They found that the addition of 400 mg/L nitrite in freezing process

increased the releasing of organic substances and meanwhile improved the freed organics’ biodegradability, which hence furnished more carbon source for recovery.

The findings reported unfold a novelty horizon on the

enhancement of WAS solubilization for recovery of carbon resource using the F/T treatment.23-26

Although

some efforts were devoted to exploring sludge solubilization during F/T treatment, the effect of initial pH of F/T process on sludge solubilization have not been conducted so far. This study therefore aims at exploring the effect of initial pH on WAS solubilization and dewaterability via 4


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freezing and thawing.

To clarify the effect of initial pH from 3 to 11 of F/T process on WAS solubilization, the

concentrations and characteristics of released organic substances after treatment were first compared.

Then, the

dewatering performance of treated WAS were elucidated, characterized by capillary suction time (CST), specific resistance to filterability (SRF), and water content of dewatered cake (Wc).

In addition, the mixtures

physicochemical properties such as zeta potential, pH, particle size and settleability after different initial pH of F/T treatment were also examined.

Based on the results obtained, the role of initial pH during F/T treatment of

sludge and its implications were comprehensively discussed.

This work reveals for the first time the impact of

initial pH of F/T process on sludge solubilization and dewaterability, and the findings reported here provide supports for the application of freezing and thawing-based strategy in field situations in the future, especially in the areas with cold climate.

MATERIALS AND METHODS WAS Source.

The WAS employed in this study were gained from a WWTP in Changsha, China.

collected sludge was concentrated by settling at 4 °C for 24 h before being used.

Characteristics of the

concentrated WAS are displayed in Table 1. Table 1 Characteristics of the concentrated waste activated sludge. Parameters

Unit

pH

Mean value a 6.8 ± 0.1

TSS (total suspended solids)

g/L

27.3 ± 0.2

VSS (volatile suspended solids)

g/L

19.4 ± 0.1

TCOD (total chemical oxygen demand)

g/L

28.5 ± 0.2

SCOD (soluble chemical oxygen demand)

g/L

0.16 ± 0.01

Total protein

g COD/L

13.8 ± 0.1

5


The


a

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Total carbohydrate

g COD /L

2.0 ± 0.1

NH4+-N (ammonium nitrogen )

mg/L

42.6 ± 2.4

PO43--P (orthophosphate–phosphorus)

mg/L

8.5 ± 1.2

Data are shown as arithmetic mean of three replicates ± standard deviation.

Experimental Procedure.

Batch experiments of F/T treatment in the different initial pH were carried out

using serum vials with a liquid volume of 150 mL.

Nine vials first received 100 mL concentrated sludge, and

then adjusted the initial pH of sludge to 3.0, 4.0, 5.0, 6.0, 7.0, 8.0, 9.0, 10.0 and 11.0, respectively, by slowly adding 2.0 M hydrochloric acid or 2.0 M sodium hydroxide.

Afterward, the sludge was frozen at -5 ℃ for 24 h

and thawed for 25 ℃ for 5 h, because the treatment conditions are generally accepted as a suitable strategy for sludge solubilization and dewaterability.13,16

By contrast, batch experiments in the different initial pH (same as

above) were conducted, and then the sludge was only incubated for 29 h at 25 ℃ ambient conditions, with the initial pH 7.0 without freezing as the control. EPS Extraction and Analysis.

EPS of WAS was extracted through a heat-extraction method, and the detailed

procedures are provided in Supporting Information.27,28

Three-dimensional excitation-emission matrix (EEM)

spectra of the soluble-EPS (S-EPS) was obtained by FP-6500, JASCO.

The EEM spectra were measured with a

scanning emission (Em) spectra range from 200 nm to 550 nm at 5-nm increments and a scanning excitation (Ex) wavelength range from 200 nm to 500 nm at 5-nm increments. with both Ex and Em slit widths maintaining at 10 nm.

The scanning speed was set at 12000 nm/min

In addition, EEM spectra of S-EPS were quantitatively

analyzed by the Fluorescence Regional Integration (FRI) technique.

Organics shown in EEM spectra were

assigned to tyrosine-like proteins, tryptophan-like proteins, fulvic acid-like substances, soluble microbial by-product and humic acid-like substances.

Among these organics, tyrosine-like proteins and soluble microbial

6


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by-product are biodegradable for microorganisms, while tryptophan-like proteins, fulvic acid-like substances and humic acid-like substances are non-biodegradable.19,20 Sludge Flocs Characterization.

The measurements of sludge volume index at 30 min (SV30) after different

treatments were applied for assessing their settleability.

Zeta potential and size distribution of sludge flocs were

respectively determined using Brookhaven ZetaPALS and Malvern Mastersizer 2000.

The sludge micrographs

were detected using biological microscope (LEO 1430 VP, U.S.A.), and the relevant pretreatment method was reported previously.7,16 Sludge Dewaterability Tests. sludge.

The 304B CST equipment from Triton was used for determining CST of treated

Measurements of SRF of sludge after different treatment were conducted using the vacuum filtrations

method reported in previous studies, and the detailed processes and calculation methods are provided in Supporting Information. Other Item Analysis.

Water content (WC) of dewatering cake was detected according to Standard Methods.29 TSS, VSS, COD (or TCOD), NH4+-N, PO43--P, BOD of samples were detected according

to Standard Methods.29

Phenol-sulfuric method was used for the measurement of carbohydrates, and

Lowry-Folin method was chose for the determination of proteins.30-32

Statistical Analysis.

All the experiments were performed in triplicate. The results were statistically

significant when the probability was less than 0.05 (i.e., p < 0.05).

Pearson correlation coefficients (R) for linear

regression between CST (SRF/WC) and EPS compositions were conducted by SPSS 18.0.

RESULTS AND DISCUSSION Effect of Initial pH on Sludge Solubilization via Freezing and Thawing Process.

Figure 1 presents

the sludge solubilization with varying initial pH (3.0-11.0) after 24 h freezing and 5 h thawing treatment.

7


It was


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obvious that F/T treatment resulted in enormous increasing of SCOD, indicating the release of organics from particulate phase to soluble phase.

In the control test (29 h storage without freezing at initial pH 7.0), SCOD in

total-EPS only reached 169.5 mg/g VSS. F/T treatment at initial pH 7.0.

In comparison, SCOD in total-EPS increased to 301.7 mg/g VSS after

It was reported that the EPS and cell wall/membrane of sludge might be

destroyed during F/T treatment.11,33 Moreover, concentrations of SCOD in different initial pH of F/T process were quite different.

The SCOD in S-EPS after F/T treatment at initial alkaline pH (9.0, 10.0 and 11.0) was

significantly higher than those at acidic pH (3.0, 4.0 and 5.0) or neural pH (6.0, 7.0 and 8.0) or in the control without pH adjustment.

For example, the released SCOD in S-EPS raised to 189.7 mg/g VSS after F/T

treatment at initial pH 10.0, which was 12.9 times than that in control, and 2.3 times than that in initial pH 7.0. Previous studies also reported that the dissolved organics in S-EPS increased to 29.5 mg/g VSS after 24 h freezing treatment (-5 ℃ in the initial pH of 6.5),16 and the SCOD increased by about 3-8 times after -8 ℃ freezing for 36 h without pH adjustment,13 which are consistent with the observations in this study.

Apparently, alkaline

condition during F/T process resulted in higher solubilization of substrates from sludge EPS or cells.

In

addition, the SCOD in S-EPS of WAS without freezing at different initial pH (3.0-11.0) after 29 h incubation is presented in Figure S1, which shows a similar trend with Figure 1A and a strongly consistent with previous studies.34,35

When the initial pH increased from 10.0 to 11.0, the SCOD increased significantly from 46.5 to 68.3

mg/g VSS.

However, during the F/T process, further increase of initial pH (from 10.0 to 11.0) did not cause

significant increase of SCOD in S-EPS (p < 0.05), the reasons might be that the hydrolysis rate of WAS was restricted by the low temperature and mass transfer resistance.4,13 little difference with S-EPS.

The tendency observed in LB-EPS showed

With a rising of initial pH values from 3.0 to 11.0, SCOD in LB-EPS increased

persistently from 36.4 to 103.5 mg/g VSS.

In addition, it was found that TB-EPS content remained almost 8


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unchanged at different initial pH (p > 0.05).

Figure 1. Effect of initial pH on SCOD (A), protein (B), carbohydrate (C) and VSS reduction (D) in F/T process. Error bars represent standard errors of triplicate tests. The proteins and carbohydrates are generally considered to be the main components of WAS.36,37 work, their contents account for 48.4% and 7.0% of the basis of sludge TCOD, respectively. analyses of soluble proteins and carbohydrates in EPS were conducted (Figure 1B and 1C).

In this

Thus, further

It can be found that

their influence on solubilization of proteins and carbohydrates showed little difference with the SCOD in EPS. For example, the contents of proteins and carbohydrates in S-EPS after F/T treatment in initial alkaline pH were also more than those in acidic pH or neural pH or in the control.

On one hand, the freezing process destroyed

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Page 10 of 27

the EPS of sludge, and on the other hand, alkaline pH induced the dissociation of acidic groups in EPS and repulsions between the negatively charged EPS, thus the proteins and carbohydrates were largely released into liquid phase.38

The above results were also supported by the VSS reduction of sludge after F/T treatment.

VSS

reductions after different initial pH of F/T treatment were similar to those of SCOD, protein and polysaccharide, and the VSS reduction increased from 5.8% to 13.5% with the initial pH increasing from 7.0 to 10.0 (Figure 1D). All the results indicated that the F/T treatment at any initial pH values significantly improved solubilization of sludge, and the initial alkaline pH further enhanced this degree, which resulted in more releasing of soluble organics and was beneficial for sequential recycling.

Fluorescence EEM Spectra and FRI Technique Assessment of Dissolved Organics Material (DOM).

Since the organics released in the dewatering liquid need to be introduced to the influent of WWTPs

and are supposed to be utilized as carbon source for biological nitrogen and phosphorus removal, their biodegradability after different initial pH of F/T treatment are therefore needed to be evaluated.

The FRI

-supported EEM technique were applied here, and the components location of “biodegradable” region and “non-biodegradable” region are shown in Fig. 2 and Fig. S2.

Results showed that the percentage of fluorescence

intensity of Region-I (PIV,n) and Region-IV (PV,n) was 62.1% and 34.6% at control test without freezing at initial pH 7.0, 67.8% and 25.1% at initial pH 3.0 test with F/T, 72.5% and 15.1% at initial pH 7.0 test with F/T, and 78.3% and 11.8% at initial pH 10.0 test with F/T.

Clearly, the F/T treatment at initial alkaline condition resulted

in more release of biodegradable substances and less release of non-biodegradable substances into dewatering liquid.

Previous studies demonstrated that F/T treatment could change various functional groups of organics

appearing in WAS, meanwhile, the alkaline pH could enhance the solubilization and hydrolysis of organics from WAS.13,39

In addition, comparing with control, initial pH 3.0 and initial pH 7.0, initial pH 10.0 caused higher 10


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fluorescence intensity of Region IV, indicating the vast solubilization and hydrolysis of complex organics, which confirmed that alkaline condition via F/T process resulted in a higher releasing of particulate organic matters from WAS.

Figure 2. The variations of EEM spectra of DOM after F/T treatment with initial pH (A) Control, (B) pH 3.0, (C) pH 7.0, (D) pH 10.0 and their related BOD (E), NH4+-N, PO43—P (F).

All samples were diluted by 80 times for

EEM spectra, B/C means BOD/COD, and error bars represent standard errors of triplicate tests. 11



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The above conclusions can be further confirmed by the BOD/COD of released organic matters (Figure 2E). It can be observed that BOD/COD of supernate after F/T treatment at initial pH 10.0 reached 0.75, much larger than those of control, pH 3.0 and pH 7.0, which was respectively 0.41, 0.58 and 0.64.

And as shown in Figure

S2, the BOD obtained in DOM was in the sequence of pH 10.0 > pH 7.0 > pH 3.0 > control.

Meanwhile, the

release of NH4+-N and PO43--P at any initial pH values was low (BOD/NH4+-N = 15~33; BOD/PO43--P = 60~150, shown in Figure 2F).2,40

These phenomena jointly illustrated the F/T treatment at initial alkaline condition

caused more soluble organics from sludge in dewatering liquor and meanwhile improved their biodegradable, which well supported the utilization as carbon source for biological nitrogen and phosphorus removal.

Physicochemical Characteristics of Pretreated Sludge.

To explore the influence of initial pH

(acidic/neutral/alkaline) of F/T process on sludge physicochemical properties, zeta potential, particle size, volume index of sludge flocs were respectively measured and summarized in Figure 3.

As shown in Figure 3, after F/T

treatment of WAS, the zeta potential and volume index decreased greatly, while the median particle size increased largely, which has also been observed by other researchers.

However, apart from the sole F/T treatment, the

initial pH values of sludge via F/T process also did significant impact on these characteristics.

For example, the

volume index (30 min) of sludge increased from 29.4% to 54.9%, the zeta potential and median particle size decreased from -0.9 mv and 297.3 µm to -3.2 mv and 223.5 µm, respectively, with the initial pH increasing from 3.0 to 10.0.

The reason for these trends was probably due to that the protonation of functional groups of EPS

reduces the surface charge as well as denatures the protein’s tertiary structure at acidic condition, while these negatively charged sludge particles are electrostatically repulsive, limiting the destabilization and flocculation of sludge particles at alkaline condition.41,42

Although the F/T treatment alone showed good performance on sludge

flocs aggregation, the acidic condition further enhanced this degree. 12


All the above observation provided

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supports for the sludge dewaterability, which would be explored in the following analyses.

Figure 3. Effects of initial pH on zeta potential, median particle size and volume index of WAS via F/T process. Error bars represent standard errors of triplicate tests. Table 2 outlines the pH of sludge after treatment with or without F/T process at different initial pH values. It could be found that the final pH of sludge reduced at alkaline condition (pH 7.0 versus 11.0) but increased at acidic condition (pH 3.0 versus 6.0), which has also been observed in previous study.43

However, during the F/T

process, the pH drop or rise was higher than that from the sludge without F/T treatment.

This could be attributed

to the higher degree of solubilization, hydrolysis or acidification of sludge, thereby large amount of protein, fatty acid and ammonia produced during F/T treatment.

It was reported that protein is an amphoteric substance and

own a good buffer capability, thus high concentration of protein could result in better stability of pH of liquor.34,35 In this study, the changes of pH listed in Table 2 showed good agreement with the concentration of soluble protein displayed in Figure 1B, which further confirmed the conclusion in previous study.5,35

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Table 2. pH changes in the end of pretreatment at different initial pH values. Raw sludge

Freezing pretreated sludge

Initial pH

Final pH

Initial pH

Final pH

3.0

3.78 ± 0.11

3.0

4.23 ± 0.12

4.0

4.64 ± 0.12

4.0

5.01 ± 0.09

5.0

5.56 ± 0.08

5.0

5.78 ± 0.07

6.0

5.63 ± 0.09

6.0

6.12 ± 0.08

Control (7.0)

6.81 ± 0.05

7.0

6.86 ± 0.06

8.0

7.58 ± 0.07

8.0

7.15 ± 0.10

9.0

8.26 ± 0.05

9.0

7.49 ± 0.06

10.0

9.03 ± 0.09

10.0

8.38 ± 0.09

11.0

9.85 ± 0.010

11.0

9.53 ± 0.08

Impacts of Initial pH of Freezing and Thawing Process on Sludge Dewaterability. Impacts of initial pH (from 3.0 to 11.0) of freezing and thawing process on sludge dewaterability were explored.

Figure 4A

illustrates that sludge CST decreased from 86.5s to 17.1s after F/T treatment, which thereby achieved 80.23 % reduction efficiency.

It was reported that freezing could markedly compress small individual sludge floc into

larger form, which thereby enhanced sludge dewatering characteristics.13,44

The CST values increased slowly

from 13.8s to 33.9s when pH in beginning of treatment rose from 3.0 to 11.0, showing a good correlation (CST = 20.86 – 3.37*pH + 0.41* pH2, R2 = 0.978), and reached the maximum reduction of 84.51 % at pH 4.0.

The

above results agreed with the zeta potential and particle size changes after F/T treatment with different initial pH demonstrated in Figure 3.

Similar observation of dewatering performance was also observed, with an increasing

of initial pH of F/T treatment, the water residual in dewatered cake increased linearly (WC = 70.39 + 1.09*pH, R2 = 0.971).

However, the values of SRF showed a little difference, with its values varied slightly as initial pH

boosted from 3.0 to 7.0, and increased obviously from 8.4 * 1012 m/kg to 14.8 * 1012 m/kg when pH was ranged from 8.0 to 11.0 (SRF = 11.31 – 1.97*pH + 0.21* pH2, R2 = 0.968). 14


This different trend could be attributed to

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the strong acidic pH (from 3.0 to 5.0), which values has been proven to accelerate the disintegration of sludge floc and increase the release of dissolved organic matter, thereby deteriorated the SRF of sludge.5,7,34

In summary,

the initial acid condition could further improve the dewaterability, while the initial alkaline condition could deteriorate the dewaterability during the F/T process.

Figure 4. Impacts of initial pH on sludge dewaterability via F/T process. (A) The corresponding CST at different pH level. (B) The corresponding SRF and WC at different pH level. measurements and lines represent model fit.

Symbols represent experimental

Error bars represent standard errors of triplicate tests.

Linear correlation between pretreated sludge CST (SRF and Wc) values and S-EPS, LB-EPS, TB-EPS were evaluated and displayed in Figure S3, with the detailed data listed in Table 3.

It can be easily found that CST

(SRF and Wc) values showed positive correlation with S-EPS, LB-EPS, while negative correlation with TB-EPS. CST and EPS concentration (especially LB-EPS), SRF and S-EPS (or LB-EPS), Wc and LB-EPS (or TB-EPS) exhibited remarkable correlations (R > 0.74, р < 0.01), SRF and TB-EPS, Wc and S-EPS displayed moderate correlations between SRF and TB EPS, Wc and S-EPS (0.44 < R < 0.68, р < 0.05).

Further investigations of

Pearson correlation coefficients for linear regression between CST (SRF/Wc) and protein/carbohydrate in EPS were carried out and showed in Table S1.

It could be observed that the correlation between CST (SRF/Wc) and 15



Page 16 of 27

protein or carbohydrate were similar to the correlation between CST (SRF/Wc) and their corresponding EPS (e.g., protein in S-EPS versus S-EPS), this could be attributed to the largely proportion of protein and carbohydrate to EPS of sludge (65-78 % in this study) and were in accordance with others findings.8

As a whole, comparing

with S-EPS and TB-EPS, the correlations between LB-EPS and dewaterability (CST, SRF and Wc) were higher. Previous study reported that LB-EPS could smuggle a great deal of bound water own to its multihole and dispersive structure, demonstrating that LB-EPS has a far greater impact on the CST values of WAS compared to S-EPS and TB-EPS.45-47

Compared to initial acidic or neutral pH, the initial alkaline pH during F/T process

caused more soluble organics shifting in LB-EPS, which thereby resulted to a slight deterioration in sludge dewaterability (see Figure 1 and Figure 4). Table 3. Pearson correlation coefficients (R) for linear regression between CST (SRF/Wc) and EPS compositions. Dewaterability

S-EPS

LB-EPS

TB-EPS

CST

0.81**

0.91**

0.79**

SRF

0.81**

0.87**

0.68*

WC

0.44*

0.95**

0.74**

** Correlation is significant at the 0.01 level (2-tailed).

*Correlation is significant at the 0.05 level.

The data

in bold highlights the relatively higher pearson correlation coefficients.

Scanning Electron Microscopy (SEM).

The morphology and structure of sludge flocs after F/T treatment

with different initial pH could be presented by SEM micrographs.

As shown in Figure 5, the sludge in control

displayed an irregular and copulative structure with not big floc.

After freezing and thawing treatment, porous

and copulative structure of sludge flocs were broken down, and exhibited a newly compact structure with a larger particle size (compared Figure 5C with Figure 5A), which supported the increased particle size observed in Figure 3.

Meanwhile, the initial pH 3.0 via F/T treated sludge exhibited further improvement in flocs compaction 16


Page 17 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


degree and particle size, making the water more easily been removed.

However, after initial pH 10.0 F/T

treatment, the flocs structure became loosely and the flocs size showed a slight decreasing compared with the neutral conditions (shown in Figure 5C), which provided greater adhesion surface area for interstitial and vicinal water thereby deteriorated the dewatering performance.

These observations showed strong supporting for the

results of physicochemical characteristics and dewaterability of sludge in Figure 3 and Figure 4.

Figure 5. SEM of sludge after freezing and thawing treatment with initial pH (A) Control, (B) pH 3.0, (C) pH 7.0, (D) pH 10.0.

Role of Initial pH during Freezing and Thawing Treatment of Sludge and Its Implications.

It has

been demonstrated that the alkaline conditions could largely accelerate the WAS solubilization but severely deteriorated the WAS filterability, while the acid conditions could enhance the WAS dewaterability partly but obtained little organics released for recovery.48-50

However, in this study, different results were gained via the 17



freezing and thawing process.

Page 18 of 27

It was found that the F/T treatment at any initial pH values significantly improved

solubilization of sludge, and the initial alkaline condition further enhanced this degree and meanwhile increased the biodegradable of released organic matters, which well supported the utilization as carbon source for biological nitrogen and phosphorus removal.

Meanwhile, the sludge dewatering performance increased markedly with F/T

treatment, and the acidic condition via F/T process exhibited further improvement while the alkaline condition showed a slight deterioration.

In fact, the pH can affect sludge characteristics in several ways.

During

freezing, the intracellular or extracellular ice crystals formed, which caused the disruption of EPS and cells of sludge.

The alkaline condition induced the dissociation of acidic groups in broken-EPS and electrostatic

repulsion between the negatively charged EPS, thus the proteins and carbohydrates were largely released into liquid phase.

Meanwhile, the OH- can be concentrated to a much higher level in the liquid-like boundary region

upon freezing, thus plays a more effective role on solubilization of sludge compared to liquid phase (see Figure 1 and Figure S1).

Although the large amount of organics released, the formed of intracellular or extracellular ice

crystals during freezing process would also transformed the loose structure of sludge into denser and compacted form and meanwhile further compressed the small individual sludge floc into larger form, thus leading to the release of bound water.

While at acid condition, protonation of functional groups of broken-EPS reduces the

surface charge as well as denatures the protein’s tertiary structure, thus leading to destabilization and flocculation of the colloidal sludge, as evidenced by the increasement of sludge median particle size (from 274.1 μm to 297.3 μm) when the initial pH of F/T process dropped (from 7.0 to 3.0) (see Figure 3), and the sludge dewaterability are further improved. The findings reported here are supposed to provide supports for the implication of freezing and thawing-based strategy in situations in the future, especially in the areas with cold climate. 18


The acidic condition

Page 19 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


is encouraged when sludge dewatering performance is valued, and the alkaline condition is advocated when recovering the biodegradable nutrients from sludge is highlighted.

Table S2 shows the comparison of acidic and

alkaline F/T treatment with other sludge treatment methods in term of solubilization and dewaterability.

It can

be found that the F/T at initial pH 10.0 treatment obtained the highest organics solubilization (189.7 mg COD/g VSS) compared with the integration of acidification and ultrasound treatment (< 80 mg COD/g VSS), Fe(II)-activated peroxymonosulfate oxidation treatment (negative value) and F/T + nitrite pretreatment (130 mg COD/g VSS), and the F/T at initial pH 4.0 treatment obtained higher CST reduction (85.4 %) compared with the integration of acidification and ultrasound treatment (55.2 %), Fe(II)-activated peroxymonosulfate oxidation treatment (90.0 %) and F/T + nitrite treatment (63.6 %), indicating that good solubilization or dewatering performance are alternative for WWTPs by regulating the initial pH of F/T process.50-55

However, it should be

emphasized that this work just clarified the underlying mechanism of how initial pH affect the F/T treatment performance in terms of sludge solubilization or dewatering, through batch-scale experiments only.

Optimum

condition (such as freezing temperature and time) at different initial pH and the relevant solubilization or dewatering performance has not been achieved.

In fact, the climate conditions differ from regions to regions and

from countries to countries, which is an important factor for “initial pH regulating of F/T process” implications. Therefore, technical performance and economic analysis of the findings needs in-depth assessment in real-world conditions in the future.

CONCLUSION In this work, effect of initial pH of F/T process on sludge solubilization and dewaterability was revealed for the first time.

The main conclusions are: (1) F/T treatment at any initial pH values significantly improved 19



Page 20 of 27

solubilization of sludge, and the initial alkaline pH could further enhance this degree, which thereby provided more soluble organics for subsequent recycling. (2) F/T treatment at initial alkaline condition resulted in more releasing of biodegradable substances and less releasing of non-biodegradable substances into dewatering liquid. (3) Sludge dewatering performance increased markedly with F/T treatment, and the acidic condition during F/T process exhibited further improvement while the alkaline condition showed a slight deterioration. (4) Optimum solubilization with organic substances being 189.7 mg COD/g VSS occurred at initial pH 10 of F/T, and optimal dewaterability performance (CST reduction being 85.4 %) was achieved at initial pH 4 of F/T.

(5) By regulating

the initial pH of F/T process, good solubilization or dewatering performance can be alternative for WWTPs. ACKNOWLEDGMENTS This study was financially supported by the project of NSFC (51508178, 51779089, and 51521006), the Natural Science Funds of Hunan Province for Distinguished Young Scholar (2018JJ1002), Huxiang youth talent plan (2017RS3022), and Science and Technology Major Project of Hunan Province (2018SK1010). ASSOCIATED CONTENT Supporting Information: This file contains analytical methods, Table S1-S2 and Figure S1-S3. REFERENCES (1) Zhang, C.; Qin, Y.; Xu, Q.; Liu, X.; Liu, Y.; Ni, B-J.; Yang, Q.; Wang, D.; Li, X.; Wang, Q. Free Ammonia-Based Pretreatment Promotes Short-Chain Fatty Acid Production from Waste Activated Sludge. ACS Sustainable Chem. Eng. 2018, 6 (7), 9120–9129, DOI 10.1021/acssuschemeng.8b01452. (2) Feng, L.; Luo, J.; Chen, Y. Dilemma of sewage sludge treatment and disposal in china. Environ. Sci. Technol. 2015, 49(8), 4781-4782, DOI 10.1021/acs.est.5b01455.

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Abstract Art

Synopsis: Initial pH Ranging from 3 to 11 Affects Waste Activated Sludge Solubilization (For Recovery) and Dewaterability in Freezing/Thawing Process

27


Revealing the Underlying Mechanisms of How Initial pH Affects Waste

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