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Enhanced short-chain fatty acids from waste activated sludge by heat-CaO2 advanced thermal hydrolysis pretreatment: parameters optimization, mechanisms and implications Xuran Liu, Qiuxiang Xu, Dongbo Wang, Qi Yang, Yanxin Wu, Jingnan Yang, Yiwen Liu, Qilin Wang, Bing-Jie Ni, Xiaoming Li, Hailong Li, and Guojing Yang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b05799 • Publication Date (Web): 07 Jan 2019 Downloaded from http://pubs.acs.org on January 8, 2019

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ACS Sustainable Chemistry & Engineering

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Enhanced short-chain fatty acids from waste activated sludge by heat-CaO2

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advanced thermal hydrolysis pretreatment: parameters optimization,

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mechanisms and implications

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Xuran Liua,b, Qiuxiang Xua,b, Dongbo Wanga,b*, Qi Yanga,b, Yanxin Wua,b, Jingnan Yanga,b, Yiwen Liuc, Qilin

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Wangc, Bing-Jie Nic, Xiaoming Lia,b, Hailong Lid, Guojing Yanga,b,e

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aCollege

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bKey

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Changsha 410082, P.R. China

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cCentre

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

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

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

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of Technology Sydney, Sydney, NSW 2007, Australia

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d School

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e College

of Energy Science and Engineering, Central South University, Changsha 410083, PR China of Biological and Environmental Sciences, Zhejiang Wanli University, Ningbo 315100, PR China

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First author: Xuran Liu ([email protected])

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Corresponding author: Dongbo Wang

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Tel: +86 731 88823967; fax: +86 731 88822829

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E-mail: [email protected] (D. Wang).

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ACS Paragon Plus Environment

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ABSTRACT:

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enhancing fermentative short-chain fatty acids (SCFAs) production from waste activated sludge (WAS).

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Various pretreatment conditions including heating temperatures, CaO2 doses and times were optimized.

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Simulate and experimental results showed that the optimal pretreatment conditions were temperature of

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67.4 °C, CaO2 of 0.12 g/g VSS and time of 19 h, under which the maximum SCFAs yield reached to 336.5 mg

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COD/g VSS after 5 days of fermentation, with the percentage of acetic acid accounted for 70.1%.

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Mechanism investigations exhibited that CaO2 and heat pretreatment caused positive synergy on sludge

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solubilization and SCFAs production.

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alone, the heat-CaO2 pretreatment not only facilitated the organics released from WAS but also increased the

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proportion of biodegradable organic matters, which thereby providing more organics for subsequent SCFAs

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production.

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acid-forming enzymes while inhibited the coenzymes of methanogens during fermentation process.

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addition, heat-CaO2 pretreatment and subsequent fermentation worked well in removal of refractory organic

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pollutants and pathogens contained in WAS.

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used as an effective method for both valuable carbon source recovery and refractory pollutants removal in the

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WAS treatment process.

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Keywords: Waste activated sludge; Short-chain fatty acids; heat-CaO2 pretreatment; Anaerobic fermentation;

In the present work, heat-CaO2 advanced thermal hydrolysis pretreatment was applied for

Compared with the control, heat pretreatment and CaO2 addition

It was found that the heat-CaO2 pretreatment improved the activities of both hydrolytic and In

Further analysis indicated that heat-CaO2 pretreatment can be

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2


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INTRODUCTION

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Waste activated sludge is the byproduct of biological process in sewage treatment, and its output is large and

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gradually increasing, which has been an extremely important environmental issue confronted by wastewater

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treatment plants (WWTPs) contemporarily.1,2

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can cause secondary pollution and bring about significant environmental risk to the ecosystem.3

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comprises a substantial content of organic matters, i.e., proteins, carbohydrates and lipids, making it being

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valuable sources for material recycling instead of a simple landfill treatment.4,5

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fermentation has attracted much attention for its simultaneous sludge stabilization and SCFAs recovery, which

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can be produced in situ and was used as an attractive alternative external carbon source for biological

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denitrification and phosphorus removal in WWTPs.6-8

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Meanwhile, the inappropriately treated and disposed of WAS WAS

Recently, anaerobic

Anaerobic digestion comprises several successive biochemical processes, e.g., solubilization, hydrolysis,

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acidification and methanogenesis.6,9

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and hydrolysis processes are slow, which limits the release of organic substances for subsequent

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acidification.10

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process.8

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need to be accelerated, and the SCFAs consumers should be restricted.

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and pathogens concentrated in WAS (e.g., preservatives, pharmaceutical and fragrance) are hard to be

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biodegraded during conventional anaerobic process, which might bring dangers to ecological environment and

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human health in ultimate disposal of WAS.11-13

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fermentation of sludge, and simultaneously eliminate the recalcitrant pollutants, a method based on CaO2 was

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put forward and implemented.

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Owing to complex and rigid structure of sludge flocs, the solubilization

Meanwhile, the generated SCFAs are readily consumed as substances in methanogenesis

Thus, to obtain higher SCFAs production, the process of solubilization, hydrolysis, acidification Besides, the recalcitrant pollutants

To improve the SCFAs production from anaerobic

Calcium peroxide (CaO2), one of the solid peroxides, has been proven to largely enhance SCFAs 3


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production when it was introduced to sludge fermenters.14-16

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H2O2, and Ca(OH)2, which made great contributions to the increasing rate of solubilization and hydrolysis

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process, the restriction of SCFAs consumers activities and the removal of endocrine-disrupting compounds

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during anaerobic fermentation.14-16

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sludge fermentation increased from 73 to 284 mg chemical oxygen demand (COD) of per gram volatile

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suspended solids (VSS) with the addition of CaO2 from 0 to 0.2 g/g VSS, and the corresponding optimal

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fermentation time decreased from 11 to 6 d.14

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gram of total suspended solids (TSS), at least half of the endocrine-disrupting compounds would be removed,

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and the sludge solubilization could meanwhile be significantly promoted.15

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show a brand-new vision towards the addition of CaO2 and provide a charming approach for sludge

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treatment.17-19

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dosages inevitably increase the operation costs.

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50% VSS reduction of sludge still needed 20 d fermentation time, largely increasing the requirement of

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fermenter volume.14,18

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application.

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of methanogens (initial pH value only 9.1), which would lower the concentrations of SCFAs accumulation.14

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Therefore, any progress that could reduce the required quantity of CaO2, promote the SCFAs production and

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further enhanced the VSS reduction at a shorter fermentation time, is supposed to be valuable for economic

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and environmental benefit.

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In the water phase, it would decompose to O2,

For example, Li reported that the maximum SCFAs yield from anaerobic

With the introduction of 0.34 g or more CaO2 to sludge per

These distinguished explorations

Nevertheless, the dosages of CaO2 reported are at high levels, and such levels of CaO2 And even at such high dosage of CaO2 (i.e., 0.2 g/g VSS),

These disadvantages lower its value-in-use partly and hamper the scale-up

Furthermore, the addition of CaO2 alone (e.g., 0.2 g/g VSS) could not fully restrain the activity

Thermal pretreatment, at high or low temperature, has been one of the most-studied pretreatment

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approaches and successfully applied in the fields of science and industry, especially in WWTPs.20

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pretreatment of sludge would be conducive to the disruption of sludge flocs, solubilization of intracellular or 4


Thermal

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extracellular organic matters, inactivation of pathogens and removal of exogenous pollutants, which is beneficial to sludge hydrolysis step, increment of biogas production and reduction of the retention time.21-23 Recently, the combination of oxidants (e.g., peroxide, ozone and persulfate) and thermal treatment (i.e. advanced thermal hydrolysis) has attracted increasing attention.

For example, Cacho Rivero et al. reported

that the combination of thermal and hydrogen peroxide (2.0 g H2O2/g VSS and 90 °C) showed synergistic effect on increasing the VSS destruction, methane production, and chemical oxygen demand removal of sludge.24

Wang et al. explored the pretreatment of WAS using CaO2 combination with microwave

irradiation for the enhancement of biomethane yield, finding their positive synergies on sludge solubilization and biodegradation.19

Abelleira et al. compared the behavior of advanced thermal hydrolysis pretreatment

and thermal hydrolysis pretreatment on solubilization and subsequent methane production of sludge, finding that the introduction of H2O2 to thermal hydrolysis pretreatment largely decreased the requirement of operation conditions, such as the temperatures, times and pressures (advanced thermal hydrolysis: 115 °C, oxygensupplied/oxygenstoichiometric = 0.2, 5 min, 1 bar versus thermal hydrolysis: 170 °C, 30 min, 8 bar).25,26 With the synergistic effect of thermal and oxidant pretreatment, better treatment efficiency, lower addition of hydrogen peroxide or ozone and lower operation conditions could be achieved.

In addition, compared with

high temperature which needs lots of energy supplies, tough conditions, and may induce the generation of refractory or inhibitory intermediates, low temperature of thermal pretreatment (< 100 °C) can be a viable alternative to avoid such limitations because the oxidative solubilization of sludge particles by H2O2 or O3 can alleviate the need for high-temperature heating.22,27

According with the literatures demonstrated above, we

therefore conjectured that the combination of low temperature thermal and CaO2 pretreatment could be a solution for reducing the required quantity of CaO2, facilitating the degradation efficiency of organic solids, promoting SCFAs yield and removing the persistent organic pollutants and pathogens. 5


Thermal energy is

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readily available in a WWTP configured with digesters/fermenters and cogeneration.

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The heat-CaO2

advanced thermal hydrolysis pretreatment which has never been investigated, if turned out to be effective, could easily be implemented. It should be noted that the effects of heat-CaO2 advanced thermal hydrolysis pretreatment largely depends on the temperature, CaO2 dose, and time of treatment, which requires comprehensive investigations and analyses.

Since the doses of oxidant added for solubilization and hydrolysis of sludge were encouraged

as mild as possible, the lower doses of CaO2 (0.05 to 0.15 g/g VSS) were chosen.14,16,19 temperatures of 40, 60 and 80 °C were selected.

Operation

Too long pretreatment time will increase the energy inputs

and the volume of pretreatment tanks, therefore, shorter retention times (i.e., 6, 15 and 24 h) were elected in this study based on the previous literatures.

Firstly, the pretreatment conditions including heating

temperatures, CaO2 doses and times were optimized according to experimental design for maximal SCFAs production.

Under the optimization conditions of heat-CaO2 pretreatment, the underlying mechanisms were

explored by comparing the organics released, the proportion of biodegradable organic matter and the specific activities of enzymes involved in anaerobic fermentation with the blank, heat and CaO2 alone pretreatment. The removal of refractory organic pollutants and pathogens contained in WAS were also assessed.

And

finally, the implications of this heat-CaO2 advanced thermal hydrolysis pretreatment are comprehensively analyzed.

For all we know, this is the first work revealing the effects and optimization of heat-CaO2

advanced thermal hydrolysis pretreatment on sludge solubilization and subsequent fermentative SCFAs production.

The findings obtained in this work are supposed to make a sound contribution to the field of

CaO2-based or heat-based sludge anaerobic fermentation and help engineers to explore more advanced methods for both emerging pollutants removing and nutrient recycling from WAS.

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MATERIALS AND METHODS WAS Source.

The WAS for batch experiments was obtained from the secondary clarifier of a WWTP in

Changsha, China. further using.

The collected sludge was first filtrated by a sifter (1.0 mm), and then stored at 3 °C for

Its major characteristics are displayed in Table S1.

Experimental Plan: Design of Experiments.

Response surface methodology (RSM) was used to

investigate the effect of three independent factors (i.e., temperatures, CaO2 dose and times) among heat-CaO2 pretreatment on fermentative SCFAs production of WAS and the optimum conditions based on SCFAs production.

A three-level, three-variable Box-Behnken (BBD) with Design Expert 10 software (Stat-Ease,

Inc., USA) was employed to guide the experiments, with the variable level normalized as -1, 0, and 1 based on the formula (Eq. (1)) below: 𝑥𝑖 =

𝐻𝑖 + 𝐿𝑜 2

𝐻𝑖 ― 𝐿𝑜

+ 𝑋𝑖

(1)

2

where Hi was the un-coded high level and Lo was the un-coded low level of the variable.28

The coded

independent factors employed in the RSM are exhibited in Table 1. Table 1 Experimental schedule based on a matrix of RSM with BBD and experimental results. Run

Experimental Factors Temperatur

CaO2

e (°C)

(g/g VSS)

1

80

2

order

Coded Variables

Responses SCFAs yield

Time (h)

X1

X2

X3

0.1

24

1

0

1

304.2

60

0.05

6

0

-1

-1

198.4

3

60

0.1

15

0

0

0

311.8

4

40

0.1

24

-1

0

1

197.1

5

60

0.1

15

0

0

0

299.4

6

40

0.15

15

-1

1

0

231.2

7


(mg COD/g VSS)

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7

60

0.1

15

0

0

0

315.7

8

60

0.1

15

0

0

0

305.4

9

80

0.1

6

1

0

-1

266.5

10

40

0.05

15

-1

-1

0

133.9

11

40

0.1

6

-1

0

-1

173.2

12

60

0.15

6

0

1

-1

297.8

13

80

0.05

15

1

-1

0

284.4

14

60

0.15

24

0

1

1

319.7

15

60

0.05

24

0

-1

1

273.0

16

80

0.15

15

1

1

0

285.5

17

60

0.1

15

0

0

0

315.2

Heat-CaO2 Pretreatment and Subsequent Anaerobic Fermentation Tests. seventeen identical serum bottles with a 1.0-L working volume.

Batch tests were conducted in

All the bottles first received 600 mL WAS.

Then, dosed with preset amounts of CaO2 (i.e., 0.05, 0.10, or 0.15 mg/g VSS).

And then, the bottles were

heated at 40, 60 and 80 °C with different pretreatment time 6, 15 and 24 h (Table 1).

Temperature was

controlled by an electronic temperature controller, and all bottles were mixed (200 rpm) using magnetic stirrers.

Once, the desired treatment time was reached, the bottles were speedily cooled to a 35 ± 1 °C.

After pretreatment with different time and cooled to 35 ± 1 °C, nitrogen gas was purged to all the bottles for 6 min.

After being sealed, the bottles were fixed and agitated at 200 rpm at 35 ± 1 °C.

The SCFAs

production and composition were monitored on a daily basis. Verification of the Optimal Results and Investigation of the Underlying Mechanisms.

Based on the

results of Section “Heat-CaO2 Pretreatment and Subsequent Anaerobic Fermentation Tests”, the optimal 8


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conditions for SCFAs yield from WAS was achieved with temperatures of 67.41 °C, 18.97 h and 0.12 g CaO2/g VSS addition.

A verification and investigation experiment was processed in 5 duplicate serum

bottles with a 1.0-L working volume.

All the bottles first received 600 mL WAS.

The bottle without any

treatment was set as the control, and other four bottles was respectively treated with sole heat (67.4 °C), sole CaO2 (0.12 g/g VSS), the heat-CaO2 (0.12 g CaO2/g VSS under 67.41 °C) for 19 h and 19 h heat (67.4 °C) pretreatment followed by CaO2 (0.12 g/g VSS) addition.

After pretreatment, all the bottles were speedily

cooled to a 35 ± 1 °C, then were operated the same as described in Section 2.3.

Besides, gas samples were

collected at preset times on a daily basis. Analytical Methods.

COD, TSS, VSS, NH4+-N and PO43--P were measured based on Standard Methods.29

Proteins and carbohydrates were respectively measured via Lowry-Folin and phenol-sulfuric methods.30,31 The pH and oxidation reduction potential (ORP) were respectively determined by pH and ORP probes (Leici PHSJ-6L, China).

Extracellular polymeric substances (EPS) of sludge were extracted through

heat-extraction methods, and the detailed procedures are provided in the Supporting Information.

The

composition and concentration of SCFAs was analyzed by Agilent 6890N GC equipped with flame ionization detector, and the procedure was detailed in Supporting Information.

The biogas composition was detected

using gas-chromatograph (GC112A, China) configured with a thermal conductivity detector based on the methods reported in our previous literatures, and the detailed volumes were modulated to standard conditions.9,32,33

The species and total detection frequency of selected organic contaminants were analyzed

by GC-MS, and the detailed procedure is shown in Supporting Information.

Quantify of Fecal Coliform in

sludge is according to the methods reported in previous publications (detailed in Supporting Information), with its unit being log MPN/g TSS, and the MPN means most probable number.34,35

A three-dimensional

excitation-emission matrix (EEM) fluorescence spectrometer (Hitachi F-4600 FL, Japan) was applied to 9


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Page 10 of 32

analyze the spectral character of dissolved organic matter (Supporting Information).

Meanwhile, the

fluorescence regional integral (FRI) technique was applied to evaluate the distribution of biodegradable (tyrosine-like (I) and soluble microbial by-product (IV)) and non-biodegradable (tryptophan-like (II), fulvic acid-like (III) and humic acid-like (V)) matters (see Supporting Information).36,37

Additionally, the detailed

approaches for the analysis of lactate dehydrogenase (LDH) release, protease, acetate kinase (AK), and F420 activities are introduced in Supporting Information. Statistical Analysis.

All measurements were conducted in triplicate.

An analysis of variance with SPSS

22.0 software (SPSS Inc., Chicago, IL) was employed to evaluate the significance of results, and p < 0.05 was considered to be statistically significant.

Results and Discussion Determination of Factors of Pretreatment Affecting SCFAs Production.

The SCFAs yields at

experimental conditions described in Table 1 were used to estimate values for the coefficients of the quadratic polynomial model according to RSM.

For the SCFAs yield (Eq. (2)) the model equation is:

Y (SCFAs) = 304.60 + 50.50X1 + 30.88X2 + 19.93X3 - 24.25X1X2 + 3.75X1X3 - 13.00X2X3 - 57.05X12 - 19.30X22 - 17.80X32

(2)

where X1, X2, and X3 represent the values (in coded units) of temperature, CaO2 dose, and time, respectively. Analysis of variance (ANOVA) in Table 2 showed that the model is statistically good (probability for the regression is significant at 95% → p = 0.000) and there is no lack of fit (probability for lack of fit is not significant at 95% → p = 0.184).

Figure S1 showed that the predicted results are near to the observed results

(R2 = 0.9692), suggesting that the developed model successfully describes the correlation between factors and the maximum SCFAs yield.

And all three factors (temperature, CaO2 dose, and time) involved in heat-CaO2

pretreatment are found to have significant effects on SCFAs yield. 10


In addition, as shown in Table 2,

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interaction, polynomial, and quadratic effects were also found to be significant in the implemented models (p < 0.05), except the interaction of X1X3 (i.e. temperature and time, p = 0.4321> 0.05), the reasons of these results would be explored in the following analyses. Table 2 Analysis of variance (ANOVA) for the SCFAs yield. Source

Sum of Squares

Df

Mean Square

F-Value

P-value

Significance

Model

51810.58

9

5756.731

71.07702

< 0.0001

**

X1-Temperature

20402

1

20402

251.8988

< 0.0001

**

X2-CaO2

7381.125

1

7381.125

91.13304

< 0.0001

**

X3-Time

3081.125

1

3081.125

38.04193

0.0005

**

X1X2

2352.25

1

2352.25

29.04268

0.0010

**

X1X3

56.25

1

56.25

0.694506

0.4321

X2X3

676

1

676

8.346415

0.0233

*

X1

2

13704.01

1

13704.01

169.2002

< 0.0001

**

X22

1568.379

1

1568.379

19.36441

0.0032

**

X3

1334.063

1

1334.063

16.47137

0.0048

**

Residual

566.95

7

80.99286

Lack of Fit

377.75

3

125.9167

2.662086

0.1839

Pure Error

189.2

4

47.3

Cor Total

52377.53

16

R-Squared

0.9692

Adj R-Squared

0.9553

Adeq Precision

26.292

Pred R-Squared

0.8690

2

Response Surface Plots and Optimization of Pretreatment Conditions.

Based on the above results, an

optimization of pretreatment conditions was performed by Design Expert (8.0) to maximize the SCFAs of the treated sludge, and the recommended optimal conditions are temperatures of 67.41 °C, time of 18.97 h and 0.12 g CaO2/g VSS.

The maximum SCFAs at optimum operating condition were predicted to be 328.96 mg

COD/g VSS by the software.

11


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Figure 1. Two-dimensinal contour plots and three-dimensional response plots for SCFAs yield: (A) As function of X1 (Temperature) and X2 (CaO2 dose), for a constant pretreatment time (18.97 h); (B) As function 12


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of X1 (Temperature) and X3 (Time), for a constant CaO2 (0.12 g/g VSS); (C) As function of X2 (CaO2 dose) and X3 (Time), for a constant temperature (67.41 °C). Two-dimensinal contour plots and three-dimensional response plots for SCFAs yield are shown in Figure 1A, B and C by keeping one factor constant at the optimal level and changing the other two factors. 1A shows the interaction between temperature and CaO2 dose at constant pretreatment time 18.97 h.

Figure The

SCFAs yield first increased when the temperature changed from 40 to 57.5 °C and then decreased slightly when the temperature increased from 77 to 80 °C in the CaO2 dose range from 0.05 to 0.15 g/g VSS.

The

maximum SCFAs yield distributes in the region of temperature: 57.5 to 77 °C and CaO2 dose: 0.08 to 0.15 g/g VSS.

Figure 1B represents the interaction between pretreatment time and temperature at constant CaO2 0.12

g/g VSS.

As the temperature increase, the SCFAs yield first increased and then decreased slightly, the same

trend was found in the effect of reaction time. 12 to 24 h and temperature: 59 to 76 °C. temperature of 67.41 °C.

The maximum SCFAs yield distributes in the region of time:

Figure 1C displays the effect of CaO2 dose and time at constant

Except the CaO2 dose hihger than 0.14 g/g VSS, increasing both the CaO2 dose

and reaction time enhanced ultimate SCFAs yield.

Previous studies reported that thermal pretreatments at

50-90 °C could solubilize organic substances in sludge solids for utilizing as fermentation or digestion substrates,21,38 and addition of CaO2 could also promote hydrolysis and acidification process and meanwhile inhibit the methane producing process.14,16

Nevertheless, higher pretreatment temperatures or times or

dosages of CaO2 would suppress or even kill the acidogenic microorganisms, lower their combined effects and increase the operational costs, which might go against the SCFAs production.14,23,27

Our previous study

found that the temperatures over 70 °C inhibited the hydrolytic and acidogenic enzymes of microorganisms.21 Li et al. reported that the SCFAs production from fermenters declined with the addition of CaO2 from 0.2 to 0.3 g/g VSS, and the reason was attributed to the strong alkaline condition that inhibited the activity of 13


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Page 14 of 32

acidogenic bacteria in fermenter which received higher dose of CaO2 (i.e., > 0.2 g/g VSS).14 The SCFAs Production of Optimized Conditions.

The SCFAs yield from sludge anaerobic fermentation

of sludge under several pretreatments cases was displayed in Figure 2A.

It can be found that the SCFAs

yield in these fermenters first increased gradually with the fermentation time from 0 to 6 days, and then decreased after 6-8 days of fermentation.

Compared with the control, both the heat and CaO2 pretreatment

significantly promoted the SCFAs yield, with the maximal SCFAs yield being 232.7 and 208.6 mg COD/g VSS at 6 and 4 days fermenation, respectively.

However, when CaO2 addition and heat pretreatment were

combined (i.e., heat-CaO2), the SCFAs yield further increased.

For instance, the maximal SCFAs yield in

the heat-CaO2 pretreated fermenter was 336.5 mg COD/g VSS, which was 6.84-times, 1.45-times and 1.61-times higher than the control, sole heat pretreated fermenter and sole CaO2 pretreated fermenter, respectively.

Furthermore, only 5 days were needed to reach maximal SCFAs yield in the heat-CaO2

pretreated fermenter, which was shorter than that of sole heat (6 d) and control (7 d).

Clearly, the heat-CaO2

pretreatment largly enhanced the SCFAs production, and meanwhile shortened the fermenation time.

Figure

2 also shows the SCFAs yield of 19 h heat (67.4 °C) pretreatment followed by CaO2 (0.12 g/g VSS) addition in the fermenter.

It was observed that the corresponding maxmal SCFAs yield reached to 288.4 mg COD/g

VSS at fermenation time of days 8, which was much less than that from the heat-CaO2 pretreated fermenter, demonstrating that the effect of combined heat and CaO2 pretreatment was synergetic rather than simply superimposed.

14


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Figure2.

The total SCFAs production from WAS anaerobic fermentation, with WAS being pretreated by

sole heat (67.4 °C), sole CaO2 (0.12 g/g VSS), 19 h heat (67.4 °C) followed by CaO2 (0.12 g/g VSS) (i.e., heat→CaO2), and combination of heat with CaO2 (67.4 °C + 0.12 g CaO2/g VSS) for 19 h (A). percentage of individual SCFA at day 5 (B).

And the

Error bars represent standard deviations of triplicate tests.

Figure 2B shows the percentages of individual SCFA after 5 days of fermentation.

From Figure 2B,

heat and CaO2 pretreatment visibly affected the fractions of six individual acids, and for all the tests, the acetic and propionate acid were the dominant SCFAs (65.8%-81.5% in total).

For instance, the percentage of

acetic acid was 38.4% in the control while this value increased to 45.5% in the heat pretreated fermenter, 55.2% in the CaO2 pretreated fermenter, and 70.1% in the heat-CaO2 pretreated fermenter, respectively.

And

meanwhile, the percentages of propionic, iso-butyric, iso-valeric decreased largly in these pretreated fermenters, especially in the heat-CaO2 pretreated fermenter.

These data jointly demonstrated that the

heat-CaO2 pretreatment not only significantly promoted the SCFAs yield from sludge anaerobic fermentation but also improved the acetic acid percentage of the producded SCFAs, which were preferred for WWTPs to use as carbon source in biological nutrients removal systems.39 In addition, the percentage of individual SCFA during the fermentation process (not just day 5) are shown in Figure S2.

From the Figure S2, it can be easily found that the percentage of individual SCFA, 15



Page 16 of 32

especially the acetic acid, propionic and iso-valeric acid, varied largely with the pretreatment methods and the fermentation times.

For instance, after 1 days of fermentation, the percentage of acetic acid was 29.6% in

the control and 48.8% in the heat-CaO2 pretreated fermenter, the propionic acid was respectively 23.8% and 21.4%; and after 5 days of fermentation, the percentage of acetic acid was 38.4% in the control and 70.1% in the heat-CaO2 pretreated fermenter, the propionic acid was respectively 24.2% and 11.4%.

Both the heat and

CaO2 pretreatment improved the percentage of acetic acid, and the combined pretreatment further ehnhanced this degree.

Li et al., found that the addition of CaO2 enhanced the acetic acid yield during sludge

fermentation, the percentage of acetic acid increased largely while the propionic acid decreased with the fermentation time, and the microbial effect was considered the main reason.14

Zhang et al., reported that the

percentage of acetic acid tended to increase while the propionic acid tended o decrease under higher temperture conditions (~55 °C).6

It was assumed here that the joint effect of heat and CaO2 attributed to the

improved percentage of acetic acid in combined pretreatment samples, ant the reasons might be attributed to the pretreatment changed the composition of substrates, the activities of microbial and the environment for SCFAs producing, which would be further investigated in the following. Underlying Mechanisms of How Heat-CaO2 Pretreatment Enhances SCFAs Production: Effect of Heat-CaO2 Pretreatment on WAS Disintegration.

As shown in Figure 3A, the COD of soluble EPS

(S-EPS, i.e., dissolved organic matter), loosely bound EPS (LB-EPS), and tightly bound EPS (TB-EPS) varied largely after pretreatment with sole heat, sole CaO2, and heat-CaO2.

In the control test, the concentration of

COD in S-EPS and LB-EPS was merely 12.2 and 33.8 mg COD/L, respectively.

For CaO2 pretreatment,

COD in S-EPS and LB-EPS increased to 94.5 and 96.7 mg COD/L, respectively.

Heat pretreatment shows

more effective in release of COD in S-EPS compared with CaO2 pretreatment, with the COD in S-EPS and LB-EPS rose by 146.0 and 64.2 mg COD/L, respectively.

The combined pretreatment performed better than

16


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either sole heat or sole CaO2 pretreatment, the COD in S-EPS and LB-EPS increased to 224.2 and 141.7 mg COD/L, which was 18.4 and 4.2 fold higher than the control, respectively.

The LDH, a cell membrane

integrity marker, was also detected to assess the lysis of sludge cells under different pretreatments

16, 40.

It

was found in Figure 3B that both the heat and CaO2 pretreatment increased the lysis of sludge cells, and their combination further enhanced this degree, which shows good supports for the varions of COD in Figure 3A. It was reported previously that both the heat and CaO2 pretreatment could enhance the disintegartion of sludge flocs.2,21,16

Liu et al. found that the soluble COD would increase to 5600 mg/L after 24 h heat pretreatment at

70 °C, and Wang et al. verified that the addition of 0.25 g CaO2/g VSS would induce 4500 mg/L increasing of soluble COD after 48 h fermentation.21,16

This study further confirmed these conclusions.

importantly, the heat and CaO2 pretreatment showed combined effect on sludge disintegration.

And more Previous

studies reported that H2O2 and Ca(OH)2 would be released from CaO2, and the decomposition products, such as OH-, ·OH, H2O2, would co-exist in sludge system, which have been proved to affect the disintegration of sludge.21,26,27

The joint effect of heat, OH-, ·OH, H2O2 further enhanced the disintegration.

Figure 3C and

Figure 3D displays the variations of pH and ORP values with the pretreatment and subsequent fermentation. It can be seen that with the addition of CaO2, both the pH and ORP values increased sharply in the first 12h and then decreased gradually.

Interestingly, the heat-CaO2 pretreatment obtained higher pH (i.e., alakaine)

and higher ORP (i.e., oxidizing capacity) compared with that of control, sole heat, and sole CaO2 during the pretretment time (i.e., first 19h).

For example, the pH and ORP values were respectively 9.4 and 147 mv at

time of 12 h in the heat-CaO2 pretreatment reactors, while 6.8 and -105 mv, 6.8 and -128 mv, 9.2 and 103 mv in the control, sole heat, and sole CaO2 pretreatment reactors.

Previous studies demonstrated that heat and

alkaline treatment showed synergistic action in destruction of sludge walls, and the reactive oxygen of ·OH and ·O2− released from H2O2 and CaO2 could cause the damage of sludge cell walls, thereby accelerating the 17


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Page 18 of 32

disintegartion of sludge matrices and releasing of soluble organics for SCFAs production.1,41,42

Figure3. The COD of S-EPS, LB-EPS, TB-EPS (A) and the release of LDH (B) after pretreatment with sole heat (67.4 °C), sole CaO2 (0.12 g/g VSS), and combination of heat with CaO2 (67.4 °C + 0.12 g CaO2/g VSS) for 19 h.

Variations of pH (C) and ORP values (D) with the pretreatment and subsequent fermentation time.

Error bars represent standard deviations of triplicate tests. Underlying Mechanisms of How Heat-CaO2 Pretreatment Enhances SCFAs Production: Heat-CaO2 Pretreatment Affects the Biodegradability of the Organics Released.

The results shown above

demonstrate that the heat, CaO2 and heat-CaO2 pretreatment largely facilitated the release of organic matters from sludge flocs.

However, both the biodegradable matters (e.g., carbohydrate, protein) and

non-biodegradable matters (e.g., humic matters, refractory materials) are usually contained in sludge, good 18


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solubilization of sludge for subsequent fermentation does not mean good or high quality and quantity intermediate or final products.36,43

Therefore, the EEM was used to reveal the character changes of dissolved

organic matters after different pretreatments, with the FRI methods as assist. of EEM of dissolved organic matters.

Figure 4 exhibits the changes

It can be seen that the Pi,n of Region IV and Region V were

respectively 63.4% and 36.2% in control test, and after pretreatments of sole heat, sole CaO2 and heat-CaO2, their values increased to 69.7%, 73.5%, 82.6% (Region IV) and decreased to 30.0%, 26.1%, 15.9% (Region V), respectively.

Previous studies reported that the organic matter located in Region IV can be classified into

soluble microbial byproduct-like substance, which is supposed to be biodegradable, while the matter located in Region V can be divided into humic acid-like substance and are considered as non-biodegradable.16,21 Clearly, heat-CaO2 pretreatment liberated more biodegradable matters and decayed humic acid-like matters into liquid supernatant, which are beneficial to efficient bio-utilization by hydrolysis and acidogenesis microorganisms.

Meanwhile, heat-CaO2 pretreatment exhibited the highest fluorescence intensities in

Region IV in comparison with the control test, sole heat and sole CaO2 pretreatment, indicating a larger solubilization of complex organic matters, which further confirmed that heat-CaO2 resulted in more substrates releasing than sole heat or CaO2 pretreatment reported in Figure 3.

Moreover, a new peak located at the

Ex/Em of about 280/310 nm was observed in heat-CaO2 pretreatment supernant, suggesting that the combination pretreatment caused changes in the chemical structure of the organic matters in sludge.

The

appearances above corporately reflected the heat-CaO2 pretreatment facilitated the release of accostable substances from the intractable sludge matrixes and promoted the entire biodegradability of these substrates, which was one reason for the improvement SCFAs yield shown in Figure2A.

19


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Page 20 of 32

Figure4. The variations of EEM spectra in dissolved organic matter after pretreatment of (A) Control, (B) heat, (C) CaO2, and (D) heat + CaO2. to V).

Pi,n represents the percent fluorescence response of Region “i” (from I

All samples were diluted by 60 times.

Figure S3 displays the VSS reduction after 5 days of fermentation. 20


In control, the VSS reduction ratio

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only reached to 8.2%, while the values in sole heat, sole CaO2 and heat-CaO2 pretreated fermenters being 24.7%, 32.4% and 44.6%, respectively, which further supported the improved biodegradability of sludge shown in Figure 4 and enhanced SCFAs yield in Figure 2A. Effect of Heat-CaO2 Pretreatment on the Methane and Hydrogen Production during Anaerobic Fermentation.

Next to the bacteria performing hydrolytic, acidogenesis, and acetogenesis reactions, the

methanogens would convert intermediary degradation products (e.g., SCFAs, hydrogen) into methane.7,44 Thus, the cumulative methane and hydrogen production during anaerobic fermentation of sludge after different pretreatments were explored.

Methanogens (e.g., SCFAs consumers) are sensitive to harsh

environments, as shown in Figure5A, the heat, CaO2 and their combination pretreatments in this study inhibited the methane production, with the combination pretreatment performing most severe.

For example,

after 5 days of fermentation, the cumulative methane productions of the control, sole heat, and sole CaO2 pretreated sludge were 5.3, 1.5, 1.6 mL/g VSS, respectively, while the heat-CaO2 pretreated sludge were 0.4 mL/g VSS.

The reasons for above observations might be attributed to the higher pH (i.e., pH 8.7 to 9.4 at

first two days fermentation) and higher ORP (i.e., ORP 147 to -129 mv at first two days fermentation) shown in Figure 3C and Figure 3C, and these facts indicated that the heat-CaO2 pretreatment significantly suppressed the consumption of SCFAs (especially acetic acid) for methane production (p < 0.05), which was another reason for the improved SCFAs production shown in Figure 2A.

21


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Page 22 of 32

Figure5. The profiles of cumulative methane and hydrogen production during anaerobic fermentation of pretreated sludge.

Error bars represent standard errors of triplicate tests.

Figure 5B shows the hydrogen production of these reactors.

It can be observed that these pretreatments

largely accelerated the production of hydrogen, with the maximal yield being to 14.5 mL/g VSS of heat-CaO2 pretreated sludge after 7 days of fermentation.

This hydrogen yield is equivalent to that reported in our

recent study, e.g., 0.25 g/g VSS CaO2 pretreatment (10.9 mL/g VSS).1

The heat-CaO2 pretreatment not only

remarkly lowered the addition of CaO2 but also further improved the production of hydrogen.

It should be

noted that the hydrogen produced could be utilized for generation of thermal energy using cogeneration to sludge heat pretreatment.45,46 Relative Activities of Key Enzymes during Anaerobic Fermentation. of anaerobic fermentation of WAS for SCFAs production.

Figure 6A illustrates the pathway

It is known that anaerobic fermentation

comprises several successive biochemical processes, that is, solubilization, hydrolysis, acidification and methanogenesis, which could be reflected via determining the activity of enzyme involved in each process.7,33 To clarify the performance of these bioprocesses of the pretreated sludge during anaerobic fermentation, three representative enzymes, i.e., protease, acetate kinase, coenzyme F420, which are respectively as the response of protein hydrolysis, acetate producing and methane producing steps, were singled out. 22


After 5 days of

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fermentation, the activities of above three enzymes in each fermenter were evaluated, with the results displaying in Figure 6B.

Results showed the activity of protease and acetate kinase were respectively

improved by 122.6% and 120.1%, 115.3% and 112.6%, 141.5% and 136.3% at sole heat, sole CaO2 and heat-CaO2 pretreated fermenters, while the activities of F420 were respectively reduced by 56.4%, 59.2%, and 48.8%, suggesting that the heat-CaO2 pretreatment not only largely improved the hydrolysis and acidification of sludge but also remarkably suppressed the methanogenesis, in accordance with the promotion of solubilization and acidification, as well as the inhibition of methane production of the sludge (see Figure 2A, Figure 3A and Figure 5A). Waste Activated Sludge Soluble Protein (Carbohydrates) α-glucosidase Protease

Amino Acids (Monosaccharide) BK BK

Other short chain fatty acids (e.g. propionic)

(A) (A)

AK

Acetic acid

H2＆CO2

F420

CH4

Figure6. Schematic diagram of anaerobic fermentation of WAS and key enzymes involved in SCFAs production (A); Relative activities of key enzymes involved in SCFAs production (B). acetate kinase, BK represents butyrate kinase.

AK represents


Effect of Heat-CaO2 Pretreatment on the Removal of Refractory Organic Contaminants and Fecal Coliform during Anaerobic Fermentation.

As the main product of sewage treatment, WAS not only

involves some organic substances (e.g., proteins, carbohydrates), but also concentrates all sorts of contaminants and pathogens exist in sewages.

The presence or residual of contaminants and pathogens in

sludge might bring risks to the environment, thus, it is meaningful to reveal the effect of heat-CaO2 23


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pretreatment on the removal of pollutants and pathogens contained in sludge.

Page 24 of 32

In this study, 14 representative

organic contaminants and pathogen indicator Fecal Coliform were selected and measured, with the total detection frequency being 95 in 6 duplicate samples and 4.8 log MPN/g TSS in raw sludge (Table S2 and Figure 7).

It should be noted that these selected organic contaminants could be classified as preservatives

(e.g., benzoate, sorbic acid), pharmaceutical (e.g., acetaminophen, carbamazepine, diazepam, and ibuprofen), cosmetics and fragrance (e.g., phenethyl alcohol, cyclopentadecanone), and are hard to be biodegraded during conventional anaerobic process.14,36

Figure 7 shows the effect of heat, CaO2 and heat-CaO2 pretreatment on

the removal of refractory organic contaminants and MPN of fecal coliform after 5 days of fermentation.

In

the control fermenter (19 h storage at 25 °C without any pretreatment), these selected organic contaminants were still detected and the total detection frequency was 86, the MPN of Fecal Coliform increased to 7.3 log MPN/g TSS.

When the sludge was respectively pretreated by sole heat, sole CaO2 and heat-CaO2, the

organic compound species was respectively reduced to 14, 11 and 6, the total detection frequency was respectively lowered to 70, 43 and 11, and the MPN of fecal coliform was respectively reduced to 3.4, 2.4 and 1.1 log MPN/g TSS.

It was reported previously that H2O2 and Ca(OH)2 would be released from CaO2, and

the decomposition products, such as OH-, ·OH, H2O2, would co-exist in sludge system,1 which have been proved to affect the degradation of the selected refractory organic pollutants and the inactivation of the pathogens.

Moreover, the combined action of heat would further enhance their effects.

The above facts

suggested that although sole CaO2 pretreatment worked well in removal of pollutants and pathogens contained in sludge, the heat-CaO2 pretreatment further improved its effectiveness and efficiency, which are beneficial to further disposal of sludge.

24


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Figure7. Effect of heat, CaO2 and heat + CaO2 pretreatment on the removal of refractory organic contaminants and fecal coliform after 5 days of fermentation. organic compounds in 6 duplicate samples. representative organic contaminants.

“Frequency” indicates the occurrence of

Organic compound species means the residual of 14


25



Overall Understanding and Implications.

Page 26 of 32

Heat-CaO2 advanced thermal hydrolysis pretreatment was

applied for enhancing fermentative SCFAs production from WAS in the present work.

Various pretreatment

conditions including heating temperatures, CaO2 doses and times were optimized via RSM with a Box-Behnken design.

Through regression analysis, it was found that the maximum SCFAs yield was well

fitted by a quadratic polynomial equation (R2 = 0.97), and was significantly influenced by all three factors. The desirable pretreatment conditions found were 67.4 °C, 19 h and 0.12 g CaO2/g VSS addition, under which the maximum SCFAs yield reached to 336.5 mg COD/g VSS at day 5 in confirmation test, with the percentage of acetic acid accounted for 70.1% (Figure 2).

Mechanism investigations exhibited that CaO2

combined with heat pretreatment might reveal positive synergies on sludge solubilization and SCFAs production.

Compared with the control, sole heat and sole CaO2, the heat-CaO2 pretreatment not only

accelerated the release of organics from sludge matrixes but also improved the entire biodegradability of these organics, which thereby provide more organics for subsequent SCFAs production (Figure 3 and Figure 4). Results showed that heat-CaO2 pretreatment enhanced hydrolyzing and acid-producing enzymes activities but inhibited the coenzymes of methanogens during fermentation process (Figure 5 and Figure 6).

In addition,

heat-CaO2 pretreatment and subsequent fermentation worked very well in removal of refractory organic pollutants and pathogens contained in WAS (Figure 7).

For all we know, this is first work applying

heat-CaO2 advanced thermal hydrolysis pretreatment for the enhancement of fermentative SCFAs yield from sludge meanwhile revealing underlying mechanisms. A concept of “nutrient removal-energy recovery” of the heat-CaO2 pretreatment technology applied in a WWTP was put forward as shown in Figure S4.

WAS from a thickener is first pretreated with heat-CaO2 for

19 h, and then is transferred to a fermenting unit for SCFA production and hydrogen production simultaneously, with the sludge retention time being controlled at 5 days. 26


After the pretreatment and

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fermentation, large amounts of nitrogen and phosphorus are liberated into SCFAs-rich liquids, which can be efficiently recovered using struvite precipitation.7,30

The thermal energy for heating of pretreatment reactor

can be obtained from the cogeneration system using hydrogen produced in fermenting unit.

However, it was

observed in batch tests that the cumulative methane production of the heat-CaO2 pretreated sludge were 0.4 mL/g VSS and the SCFAs production flattened off after 5 days of fermentation (Figure 2 and Figure 5), thus a continuous regulation might be useful for SCFAs production in continuous mode, which would be considered in the following work.

Besides, when the heat-CaO2 pretreatment applying in the real WWTPs, some issues

or studies, such as more detailed running or control parameters, methods for the recovery of released nitrogen and phosphorus in the fermentation liquid, and approaches for the collection and utilization of produced hydrogen, need to be considered or carried out in the future.

Conclusion.

Heat-CaO2 advanced thermal hydrolysis pretreatment was first applied for enhancing

fermentative SCFAs production from WAS in the present work.

The pretreatment parameters including

heating temperatures, CaO2 doses and times were optimized for maximum SCFAs yield via RSM with Box-Behnken designs and the optimum condition was verified.

The main conclusions are: (1) Desirable

pretreatment conditions for maximum SCFAs yield were 67.4 °C, CaO2 of 0.12 g/g VSS and 19 h, under which the maximum SCFAs yield reached to 336.5 mg COD/g VSS after 5 days of fermentation, with the percentage of acetic acid accounted for 70.1%.

(2) Heat-CaO2 pretreatment not only accelerated the release

of organics from sludge matrixes but also improved the entire biodegradability of these organics, which thereby provide more organics for subsequent SCFAs production.

(3) Heat-CaO2 pretreatment enhanced

hydrolytic and acid-producing enzymes activities, while inhibited the coenzymes of methanogens during fermentation process.

(4) Heat-CaO2 pretreatment effectively removed the refractory organic pollutants and

inactivated the pathogens. 27


ACS Sustainable Chemistry & Engineering 1 2 3 4 462 5 6 7 463 8 9 464 10 11 12 465 13 14 466 15 16 17 467 18 19 20 468 21 22 469 23 24 470 25 471 26 27 472 28 473 29 474 30 31 475 32 476 33 477 34 35 478 36 479 37 480 38 481 39 40 482 41 483 42 484 43 44 485 45 486 46 487 47 48 488 49 489 50 490 51 491 52 53 492 54 493 55 494 56 57 495 58 496 59 497 60

Page 28 of 32

Acknowledgment 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), and Huxiang youth talent plan of Hunan Province (2017RS3022), and Science and Technology Major Project of Hunan Province (2018SK1010). Supporting Information Available This file contains analytical methods, Table S1-S2 and Figures S1 –S4. References (1) Wang, D.; Zhang, D.; Xu, Q.; Liu, Y.; Wang, Q.; Ni, B.-J.; Yang, Q.; Li, X.; Yang, F. Calcium peroxide promotes hydrogen production from dark fermentation of waste activated sludge. Chem. Eng. J. 2019, 355, 22-32, DOI 10.1016/j.cej.2018.07.192. (2) Li, Y.; Yuan, X.; He, J.; Wang, D.; Wang, H.; Wu, Z.; Jiang, L.; Mo, D.; Yang, G.; Guan, R.; Zeng, G. Recyclable zero-valent iron activating peroxymonosulfate synchronously combined with thermal treatment enhances sludge dewaterability by altering physicochemical and biological properties. Bioresour. Technol. 2018, 262, 294-301, DOI 10.1016/j.biortech.2018.04.050. (3) Appels, L.; Baeyens, J.; Degrève, J.; Dewil, R. Principles and potential of the anaerobic digestion of waste-activated sludge. Prog Energ Combust. 2008, 34, 755-781, DOI 10.1016/j.pecs.2008.06.002. (4) Wang, Y.; Wang, D.; Liu, Y.; Wang, Q.; Chen, F.; Yang, Q.; Li, X.; Zeng, G.; Li, H. Triclocarban enhances short-chain fatty acids production from anaerobic fermentation of waste activated sludge. Water Res. 2017, 127, 150-161. DOI 10.1016/j.watres.2017.09.062. (5) Hreiz, R.; Latifi, M. A.; Roche, N. Optimal design and operation of activated sludge processes: State-of-the-art. Chem. Eng. J. 2015, 281, 900-920, DOI 10.1016/j.cej.2015.06.125. (6) Zhang, P.; Chen, Y.; & Zhou, Q. Waste activated sludge hydrolysis and short-chain fatty acids accumulation under mesophilic and thermophilic conditions: effect of ph. Water Res. 2009, 43, 3735-3742, DOI 10.1016/j.watres.2009.05.036. (7) Zhao, J.; Wang, D.; Li, X.; Yang, Q.; Chen, H.; Zhong, Y.; Zeng, G. Free nitrous acid serving as a pretreatment method for alkaline fermentation to enhance short-chain fatty acid production from waste activated sludge. Water Res. 2015, 78, 111-120, DOI 10.1016/j.watres.2015.04.012. (8) Yuan, H.; Chen, Y.; Zhang, H.; Jiang, S.; Zhou, Q.; Gu, G. Improved bioproduction of short-chain fatty acids (SCFAs) from excess sludge under alkaline conditions. Environ. Sci. Technol. 2006, 40, 2025-2029, DOI 10.1021/es052252b. (9) Wang, D.; Zhao, J.; Zeng, G.; Chen, Y.; Bond, P. L.; Li, X. How does poly (hydroxyalkanoate) affect methane production from the anaerobic digestion of waste-activated sludge? Environ. Sci. Technol. 2015, 49, 12253-12262, DOI 10.1021/acs.est.5b03112. (10) Xu, Q.; Liu, X.; Zhao, J.; Wang, D.; Wang, Q.; Li, X.; Yang, Q.; Zeng, G. Feasibility of enhancing short-chain fatty acids production from sludge anaerobic fermentation at free nitrous acid pretreatment: Role 28


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Abstract Art

Heat-CaO2 pretreatment can be used as an effective method for valuable carbon source recovery and pollutants removal in WAS treatment

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Enhanced Short-Chain Fatty Acids from Waste ... - ACS Publications

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