Effect of pre-treatments on biological methane potential of dewatered sewage sludge under dry anaerobic digestion

Accepted Manuscript Effect of pre-treatments on biological methane potential of dewatered sewage sludge under dry anaerobic digestion Lu Wenjing, Pan Chao, Arun Lama, Fu Xindi, Ye Rong PII: DOI: Reference:

S1350-4177(18)31268-9 https://doi.org/10.1016/j.ultsonch.2018.11.022 ULTSON 4391

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Ultrasonics Sonochemistry

Received Date: Revised Date: Accepted Date:

21 August 2018 21 November 2018 24 November 2018

Please cite this article as: L. Wenjing, P. Chao, A. Lama, F. Xindi, Y. Rong, Effect of pre-treatments on biological methane potential of dewatered sewage sludge under dry anaerobic digestion, Ultrasonics Sonochemistry (2018), doi: https://doi.org/10.1016/j.ultsonch.2018.11.022

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1

Effect of pre-treatments on biological methane potential of

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dewatered sewage sludge under dry anaerobic digestion

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Lu Wenjing*, Pan Chao, Arun Lama, Fu Xindi, Ye Rong

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School of Environment, Tsinghua University, Beijing 100084, China

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* Corresponding author: Tel/fax: +86 10 62796540. E-mail: [email protected].

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Abstract

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The aim of the study is to enhance hydrolysis of dewatered sewage sludge (moisture

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content 80-83%) to tackle the problem of low biological methane potential (BMP) and low

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efficiency of dry anaerobic digestion. Different pre-treatment i.e. physical (ultrasonication),

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chemical (acid, ozone) and combined (ultrasonication-ozone) methods were investigated and

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evaluated in terms of BMP and biodegradation. Ultrasonic pre-treatment had the best result

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among the single technologies, the BMP increased by 104.7%, while total solid (TS), volatile

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solid (VS) and chemical oxygen demand (COD) reduction were improved by 30.1%, 36.9%

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and 33.9%, respectively, over control. Combined pre-treatment (ultrasonication-ozone)

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showed more significant enhancement than single methods as evidenced by 138.2% higher

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BMP and 53.7%, 63.7% and 57.3% more reduction in TS, VS, COD, respectively, over

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control. The BMP increment positively correlated either with energy input, concentration or

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dose of pre-treatment applied. Among the tested methods, the physical pre-treatments out-

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compete chemical ones. Ultrasonic combined with ozone pre-treatment technology has good

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energy and economic feasibility.

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Keywords: Dry anaerobic digestion; Sewage sludge; Bio-methane potential; Pretreatment.

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1. Introduction

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China has achieved rapid improvement in sanitation through construction of wastewater

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treatment plant (WWTP) in recent years. However, it resulted in drastic increment of sewage

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sludge (SS) production. In 2015, the total sewage sludge production has reached 43 million

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tons (moisture content >80%)[1]. Anaerobic digestion(AD) process has been a well-

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established technology for the treatment of organic fraction of various waste materials [2, 3].

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In general, liquid anaerobic digestion(LAD) requires excess amount of water, which in many

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cases become very problematic for handling the digestate, especially the huge amount of

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waste water effluent. Therefore, dry anaerobic digestion(DAD) gains attestation to new

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advances worldwide. DAD is one of the remarkable methods of bio-stabilization of organics,

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energy recovery with lower production of leachate and feasible handling of digestate which

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can go directly for post-composting process [4]. The energy efficiency of DAD of sewage

36

sludge for methane production is often higher than that of LAD. Guendouz et al. [5] showed

2

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that since the DAD does not contain large amount of water in the substrate (sludge), its reactor

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volume is greatly reduced, consequently, the biogas yield in unit volume of DAD is much

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higher than that of LAD. DAD possesses a number of advantages over LAD of sewage sludge

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like smaller reactor volume, no wastewater discharge and treatment, lower energy

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requirements for heating and lower energy loss [5].

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However, the high content of total solids (TS) makes it difficult to operate DAD at both

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laboratory and industrial scales [6]. It is shown that reactors with total solid content above 20%

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degraded the performance of AD [7]. Fernández et al. [7] suggested that high TS could bring

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about a reduction in substrate degradation rate and hence the reduction in biogas production.

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Karim et al. [8] mentioned that when TS increases in reactors, mixing plays a more important

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role for improving the biogas production. In the process of sewage sludge DAD, limitation

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of mass transfer is also negligible. Compared with LAD, the mass transfer limitation of DAD

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due to low water content will limit the flow of fermented substances to a certain extent, and

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cause accumulating of inhibitor in the DAD system [6]. Application of DAD is limited due

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to longer time required for methanization of waste. Among the different stages of AD,

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hydrolysis is recognized as the rate-limiting step [9]. Different pre-treatments are proposed

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and tested in order to disintegrate sludge and lysis the microbial cells to release both extra

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cellular and intracellular organic compounds, which helps to accelerate the subsequent

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methanization and reduce the treating time [10].

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Acid pre-treatment is thought to be more suitable for lignocellulosic substrates not only

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because it breaks down the lignin but also the hydrolytic microorganisms are capable of

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acclimating to acidic conditions [11]. The effect of acid pre-treatment prior to AD was

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investigated and concluded to enhance the hydrolysis stage and to contribute in pathogen

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reduction [12]. However, no further test for the anaerobic biodegradability of this waste was

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carried out. Ozone oxidation is one of the commonly used advanced oxidation process for

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treating wastewater or sludge from WWTP [13]. When sludge ozonation is carried out, ozone

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decomposes into hydroxyl radicals and reacts with organic fractions, subsequently the

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refractory organic structures are oxidized and transformed into biodegradable low-molecular

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compounds [14]. Weemaes et al.[15] found ozone-treated sludge resulted in a reduction of

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total solid and increased methane production 2.2 times. As compared to other treatment

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methods, ultrasonic treatment is considered to be environmentally safe and economically

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competitive [16]. The principle of ultrasonic treatment is based on the cavitation process to

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disintegrate cell walls [17]. Ultrasonic pre-treatment is believed to disintegrate sludge flocs

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and rupture microbial cell wall which results in the release of soluble substances and hence

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enhances digestion process and biogas production [18]. The study of Wang et al.[18] found

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that high energy intensity causes disintegration of particulate matter resulting in a reduction

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in particle size and increase in soluble matter fraction. Different studies using ultrasonic pre-

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treatment have demonstrated that the method is effective on enhancing biogas production and

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improving sludge reduction in LAD system [19-27].

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Effectiveness of various combined technologies in improving the performance of AD

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has been proved. For example, combination of ultrasonic pre-treatment with alkaline [28]

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and acid pre-treatments [29, 30] has been found to improve sludge disintegration. Ozone has

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also been reported to be effective in enhancing the performance of ultrasonic pre-treatment.

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Xu et al. [31] showed the possibility of combining ultrasound and ozone to enhance

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disintegration of sewage sludge and the methane production. Tian et al.[32] concluded that

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subsequent ozonation complemented ultrasonic pre-treatment in improving biogas

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production and removal of volatile solid, the combined pre-treatment shortened the AD and

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solid retention time from 20 days to 10 days. However, detailed insight in their wide

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application and economic analysis will further prove their effectiveness [33].

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How to increase biogas production is the major concern in terms of environment and

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economy. Although DAD has the big advantage of avoiding waste-water generation and

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treatment, low mass transfer due to low water content in the system is the major impede for

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microbial activities, especially the hydrolysis process. In the meantime, the technology of

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pre-treatment has been extensively studied in LAD system of sludge with water content >

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90%, which provides a great prospect of application in the field of DAD of sludge. Upon the

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problem recognized, this study tends to employ different pre-treatments prior to AD which

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would speed up the hydrolysis step and enhance the subsequent process of DAD. Among the

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many technologies, we have selected the following four conditions: a). clear mechanism of

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influence, b). technical operation is relatively easy, c). energy consumption is low, and d).

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secondary pollution is low. For combined pre-treatment, we choose to study the physical and

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chemical techniques that are most effective in the single treatment test. The efficiency of

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different pre-treatments on DAD, including single (ultrasonication, ozonation, acid) and

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combined (ultrasonication-ozonation) pre-treatment were evaluated which forms a basis for

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deciding the convenient and efficient pre-treatment method to be applied for the better

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performance of dry anaerobic digestion project.

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2. Materials and methods

2.1. Feedstock

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Dewatered sludge used in the study was from a wastewater treatment plant in Kunming

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city, Yunnan Province, which is located in southwest of China. Fresh effluent collected from

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food waste AD reactor was used as inoculum. Both the sludge and inoculum sample were

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dewatered by centrifugation at 15000 rpm for 30 mins to adjust moisture content in the range

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of 70-80% to perform DAD. Characteristics of dewatered sewage sludge and inoculum were

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shown in Table.1.

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Insert Table.1. here

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2.2. Pre-treatment and analytical methods

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2.2.1. Ultrasonication (ULS) pre-treatment

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In order to assure uniform pre-treatment, ULS was performed by ultrasonic cell

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disruption system with a frequency of 21kHz, consisting 6mm probe (SCIENTZ-II D,

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NINGBO SCIENTZ BIOTECHNOLOGY CO., LTD, China). 90g of sludge sample was

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poured into a 200mL glass beaker and placed in the chamber for sonication. The density of

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the dewatered sludge used was 1.17g/mL, and the volume of sewage sludge added to each

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reactor was 105mL. The sludge in the beaker was agitated every 6 minutes during sonication

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for homogenous treatment. A specific energy (SE) of 4.5, 11.5, 22.5, 53.5 kJ/gVS

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corresponding to the input power of 22.6, 57.8, 113.2, 269.1W were applied to the sludge

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sample for 30 minutes and temperature was maintained around 4°C by placing the beaker in

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an ice-water bath.

123

2.2.2. Ozone pre-treatment

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Ozone dose for sludge treatment was set in the range of 0.05 to 0.11 gO3/gVS, i.e.0.05,

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0.07, 0.09, and 0.11gO3/gVS. During the process pure oxygen (99.9%) was used to convert

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oxygen to ozone with high voltage converter (OL8OF/DST, Ozone service, Canada). Ozone

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concentration in the ozone generator effluent was adjusted by changing ozone generator

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power. Ozone was then purged into the reactor through a bubble diffuser (6mm) at a constant

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flow rate of 0.5 L/min.

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2.2.3. Acid pre-treatment

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Acidification of sludge was carried out by adding 34.5% HCl (hydrochloric acid)

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stepwise to the dewatered sludge until the required pH was obtained, and in total six different

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pH level were set (Table.2). Acidified sludge samples were placed at 4°C for 48 hours prior

134

to being neutralized, i.e. adjusted back to the initial pH of the sample (pH = 7.1) by adding

135

3M NaOH.

136

Insert Table.2. here

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2.2.4. Combined pre-treatment

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The choice of combined pre-treatment is based on the results of single pre-treatment,

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which demonstrated ultrasound and ozone pre-treatment were more efficient for dewatered

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sludge. The dewatered sludge sample was subjected to ultrasonication at specific energy (SE)

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of 53.5kJ/gVS for 30 mins. Ozonation was carried out subsequently for combined pre-

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treatment with varying dose, i.e. 0.05, 0.07, 0.09, and 0.11gO3/gVS.

143 144

After the treatments aforementioned, the sludge samples were subjected to DAD for biological methane potential (BMP) evaluation.

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2.2.5 Analytical methods

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Chemical oxygen demand (COD) was determined by oxidation of the organic

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compounds with K2Cr2O7 [34]. Total solid (TS) and volatile solid (VS) were determined by

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gravimetric loss on evaporation at 105oC for 8 hours and then the sample was ignited in

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muffle furnace at 550 oC for 2 hours.

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Sample for pH measurement were prepared by diluting 5g of sample in 50mL of deionized water. pH of the suspension was measured by the pH meter (Mettler Tolledo).

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Total volatile fatty acids (TVFAs) were determined by gas chromatographer (Shimazdu

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2010, Tokyo, Japan). 5g of sludge sample was mixed with 50 mL of KCl (2mol L-1) for

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extraction on a thermostatic shaker (HWY-211, ZhiCheng Shanghai, China) for 12 hours at

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37°C. The supernatant of the sample after centrifugation was then filtered through a 0.45µm

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syringe filter before acidified with 85% phosphoric acid (1:10, volume:volume). Acidified

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supernatant was then injected into the GC(Agilent7890A) for VFAs analysis. In this study,

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we determined VFAs including acetic acid, propionic acid, butyric acid, isobutyric acid,

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valeric acid, isovaleric acid, caproic acid, and isohexanoic acid.

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2.3. Experimental set up for biological methane potential test

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A modified BMP test method, based on the procedure outlined by Owen et al. [35], was

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followed to evaluate potential methane production of the pre-treated sewage sludge. 90 g of

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pre-treated sludge (TS=17.8%, VS=0.94g) and 100g of inoculum (TS=27.0%, VS=0.57g)

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were added to 250 mL serum bottle followed by mixing and closing of the reactor. In order

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to create anaerobic condition rapidly, the headspace of the serum bottle was purged with

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nitrogen for 60 seconds. Bottles were then placed in a thermostat water bath at 37°C for

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cultivation for 15 days. The capillary tubes attached with serum bottle were released from

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the reactor into the gradual cylinder containing 3M NaOH to remove CO2 in the biogas. The

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reactors were stirred every day periodically for mass transfer and homogenization. Control

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trails were set which contained 90g of raw sewage sludge and 100g of inoculum. All

171

treatments were set at triplicates (n=3).

172

Insert Fig. 1. here.

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BMP was recorded every 12 hours in graduated cylinder, which indicated by volume

174

displacement by gas pressure (Fig.1). BMP was calculated by dividing methane volume (mL)

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(minus the basic yield of the inoculation) by dry weight of the sample VSadded to each bottle

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(gVS) as shown in the following equation:

177

𝐵𝑀𝑃 = [𝐶𝐻4 ‒ 𝐶𝐻4（𝑖𝑛𝑜𝑖𝑛𝑜𝑐𝑢𝑙𝑎𝑡𝑖𝑜𝑛）]/(𝑉𝑆 ∗ 𝑡𝑜𝑡𝑎𝑙 𝑚𝑎𝑠𝑠)

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Where, BMP(mL/gVS) is biological methane potential, CH4 (mL) is the methane

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production of sewage sludge and inoculum, CH4（inoculation） (mL)is the methane production

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of 100g of inoculum , VS (%) is volatile solid content of the sewage sludge(wet weight), total

181

mass (g) is wet weight of sewage sludge.

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(1)

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2.4. Data analysis

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Results of BMP were expressed as means ± standard deviations (SD). T-test to determine

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statistical difference between treatments was carried out by comparing the value through one-

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way analysis of variance (ANOVA), followed by Tukey’s multiple-comparison test (SPSS

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IBM statistics 23.0). The mean difference is considered significant at the 0.05 level.

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3. Results

3.1. Effect of pre-treatments on bio-degradation process

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VS, TS and COD were tested as major properties of organic matter and indicator of bio-

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degradation during DAD of sludge. These properties of dewatered sludge sample were

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measured before and after pre-treatment (Table.3). In general, among all the pre-treatments

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methods investigated in this research, as the dose/specific energy increases, the release of

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organic matter increases, resulting in a decrease in VS and COD. While the remaining pre-

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treatments did not show significant changes in the TS, VS and COD of the sludge, the acid-

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treated sludge had TS and VS decreased by 1-2 percentage points. The reason for the result

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may be that the acid treatment causes more hydrolysis of the extracellular polymer in the

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dewatered sewage sludge, and there are more microbial cells to collapse and rupture. On the

198

other hand, VS, TS and COD do not seem to be the significant indicator for the efficiency of

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pre-treatment.

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Insert Table.3. here

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These parameters (VS, TS and COD) were also determined after DAD experiment, and

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their reduction as major indicators of biodegradation for DAD of sludge receiving different

203

pre-treatment were compared (Fig.2). All the pre-treated trails showed improved bio-

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degradation performance in DAD experiment than the control trail. Acid pre-treatment

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showed effect on biodegradation during 15 days of DAD (Fig.2A). Removal efficiency was

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described as follows: TS ranged from 20.3% to 29.5%, VS from 22.5% to 31.5% and COD

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from 22.3% to 28.5%, which were all higher than the control trail. As could be seen in Fig.2B,

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organic solid removal efficiency increased with the increase in the ozone dose from 0.05 to

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0.11 gO3/gVS and hence highest organic matter reduction was observed at ozone dose of 0.11

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gO3/gVS. Removal efficiency was as follows: TS ranged from 17.7% to 28.9%, VS from

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22.5% to 30.6% and COD from 27.8% to 35.9%. Ultrasonic pre-treatment with increasing

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energy input (SE) had pronounced effect on organic matter degradation during 15 days of

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DAD. The removal efficiency of TS, VS, and COD were as follows: TS ranged from 19.1%

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to 30.1%, VS from 22.7% to 36.9% and COD from 26.8% to 33.9% at SE of 4.5, 11.5, 22.5

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and 53.5 kJ/gVS respectively (Fig.2C). Combined pre-treatment was found to improve most

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significantly organic matter removal efficiency (Fig.2D). Removal efficiency was as follows:

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TS ranged from 30.7% to 53.7%, VS from 37.6% to 63.7% and COD from 41.7% to 57.3%.

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The results demonstrate that pre-treatment has improved bio-degradability of dewatered

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sludge, and the combined method has better effect than the single pre-treatments.

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222 223

Insert Fig. 2. here.

3.2. Variation of physico-chemical properties as affected by pre-treatments

pH and TVFA of sludge samples were analysed and compared before and after DAD experiment (Fig.3).

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As shown in Fig.3A, final pH of acid pre-treated sludge ranged from 6.4 to 7.2 and final

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TVFA of pre-treated sludge ranged from 1255.2 to 1832.4mg/L. The final TVFA after DAD

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in the acid pre-treated sludge and the control trail all increased compare to the initial TVFA.

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In ozone pre-treated trails, the final pH after DAD ranged from 6.4 to 7.1 indicating optimum

228

DAD processing condition (Fig.3B). Initial and final TVFA ranged from 1153.8 to 1199.9

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mg/L and 1088.3 to 2175.0 mg/L, respectively. When the concentration was lower than

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0.09gO3/gVS, the final TVFA in DAD system of sludge tended to accumulate. Initial pH of

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the ultrasonic pre-treated sludge ranged from 7.0 to 7.4 (Fig.3C), whereas final pH of the

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digester ranged from 6.4 to 7.3 which is within the favourable range of methanogens[36].

233

Nevertheless, the final TVFA of the pre-treated sample ranged from 988.3 to 2215.2 mg/L.

234

Final pH tended to increase with the increasing of specific energy. This may be because the

235

VFAs in the sludge tend to exist in a short chain form (such as acetic acid) with the increasing

236

of specific energy, which is more biodegradable, so the residual VFAs are less after the DAD

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and the pH is higher. This also explains when the specific energy was higher than 25.5kJ/gVS,

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the final TVFA in DAD system of sludge is lower than the initial sample. Final pH value of

13

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the combined pre-treated sludge ranged from 6.4 to 7.2 and final TVFA of the pre-treated

240

sludge ranged from 732.3 to 1635.9mg/L. Variation in ozone dose also affected pH of the

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DAD system of sludge as evidenced by increasing pH with increasing of ozone dose (Fig.3D),

242

less VFA remained in the trails receiving high dose of ozone. In addition, all the DAD system

243

with pretreated sludge showed higher pH as compared to the control. Highest methane yield

244

was noted from the trails with final pH 7.2. This gives indication of better methaniation

245

condition inside the DAD system with pre-treated sludge. Not surprisingly, DAD system

246

without pre-treatment was found to have lower pH (pH6.5) and higher TVFA (2115.0g/L) as

247

compared to the pre-treated trails. Combined pre-treatment at increasing the ozone dose and

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constant specific energy of ultrasonication helped to increase the buffering capacity of the

249

digester, thus leading to highest digestibility of dewatered sludge.

250

251

Insert Fig. 3. here.

3.3. Effect of pre-treatments on biological methane potential by DAD

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BMP of the dewatered sewage sludge was calculated based on the total methane

253

production on daily basis and at the final day of DAD (15th day). Results showed that pre-

254

treatment had a significant effect on BMP as they were all higher than the control trail (Fig.4).

255

Insert Fig. 4. here.

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Sludge with acid pre-treatment, i.e. at pH5, pH4, pH3 and pH2, produced

257

22.2~28.1mLCH4/gVSadded which were 30.5%~65.3% higher than the control trail (Fig.5A).

14

258

Methane production observed in all the acid pre-treated samples were significantly different

259

from that in control (p<0.05), among which trails with lowest pH (pH 2) during pre-treatment

260

had most significant effect (p=0.011) on disintegration of organic matter and thus increased

261

methane production. Sludge pre-treated with different doses of ozone, i.e. 0.05, 0.07, 0.09

262

and 0.11gO3/gVS resulted in 27.0~32.9mLCH4/gVSadded which were 54.3%~88.0% higher

263

than the control (Fig.5B). The promotion on methane production is positively related to

264

ozone concentration during pre-treatment. Sludge receiving ultrasonic treatment with

265

specific energy produced 28.8~35.0mLCH4/gVSadded which were 68.2%~104.7% higher than

266

the control (Fig.5C). Among the four SE (4.5, 11.5, 22.5, and 53.5kJ/gVS), the highest energy

267

input (53.5kJ/gVS) showed the most significant increment (p=0.0) in methane production,

268

followed by 22.5kJ/gVS (p=0.001), 11.5kJ/gVS (p=0.002) and 4.5kJ/gVS (p=0.005). Sludge

269

pre-treated with ultrasonic at a constant specific energy of 53.5kJ/gVS combined with ozone

270

resulted in a further increasing of BMP than any single pre-treatment. 103.1%~138.2%

271

higher of BMP than the control trail were achieved (Fig.5D). BMP of dewatered sludge

272

subjected to ULS-ozone treatment was found to increase with the increasing dose of ozone.

273

274

Insert Fig. 5. here.

3.4. Energy balance

275

The correlation between COD degradation rate and energy input was analyzed, and the

276

results showed that the two chosen parameters were significantly correlated (Fig.6). For the

15

277

trail with significantly improved biodegradability, of which the TS degradation rate was

278

higher than 30%, the energy consumption calculation was conducted (Fig.7) (see

279

SUPPORTING FILE for calculation details). The energy input-output ratio required for

280

the degradation of per unit TS decreased from 1.41 (with only 53.3kJ/gVS pre-treatment trail)

281

to 0.69 (with 53.3kJ/gVS+0.11gO3/gVS pre-treatment trail), which reflected an energy-

282

saving trend. The energy input-output ratio showed a downward trend with the increase of

283

ozone concentration in the combined pre-treatment technology.

284

Insert Fig. 6. here.

285

Insert Fig. 7. here.

286

4. Discussion

287

In order to elucidate the impact of pre-treatment on DAD of sewage sludge, different

288

pre-treatment methods i.e. physical (ultrasonication), chemical (acid, ozone) and combined

289

(ULS-ozone) treatments were applied, and evaluated in terms of hydrolysis and

290

biodegradation (TS, VS, and COD), the digester performance (pH and TVFA) and BMP. The

291

results demonstrated that hydrolysis which is considered as the rate-limiting step of AD could

292

be speeded up by using suitable pre-treatment methods.

293

Methane production observed in all the acid pre-treated samples was significantly

294

different from that in control. Improved BMP might be due to increased degree of cell lysis,

295

which resulted in the release of soluble organic compounds making it more easily available

16

296

to bacteria [11]. According to Devlin et al.[12], acid pre-treatment might play role in the

297

breakdown of the polymers into monomers or oligomers carrying out partial hydrolysis which

298

in turn will result in increased rate of digestion.

299

Ozonation improved BMP with the increase of ozone dose. Effect of ozonation might

300

have heightened and produced more amount of biodegradable substance that could be easily

301

utilized by the related microorganism leading to the successful operation of DAD [37].

302

Reduction of TS, VS and COD was found to be positively correlated to BMP. When the

303

ozone concentration is low, VFAs accumulates during the DAD process (Fig.3B). It is

304

speculated that some VFAs which are less biodegradable (such as propionic acid or some

305

long chain fatty acid) accumulate in the system, and when the O3 concentration is high

306

enough, it can promote further breakdown of those VFAs.

307

The result of ultrasonic pre-treatment followed by DAD is consistent with other studies

308

[18, 22]. Ultrasonic pre-treatment may enhance disintegration of sludge flocs and caused

309

microbial cells to rupture and subsequently release soluble substances or specific enzymes

310

which ease the digestion process. In addition, studies on the efficacy of ultrasonic pre-

311

treatment on AD have revealed that ultrasonic conditions including input power have an

312

important role in enhancing biodegradability [18, 22]. Organic matters reduction tend to

313

increase with the increase of SE in pre-treated samples which might have attributed to

314

destruction of the higher molecules and the cell lysis of the zoogloea with solubilizing the

315

solid into liquid phase as was reported by Tiehm et al.[17]. In the study, the highest organic

17

316

matter removal efficiency was observed at highest SE (53.5kJ/gVS). As removal rate of

317

organic matters is positively correlated with the specific energy, so it has an effect on

318

anaerobic digestion.

319

Highest BMP of the pretreated sludge over the control trail manifestes that combination

320

of two pre-treatment methods (ultrasonication-ozone) is effective on improving the DAD

321

performance. This could have been possible due to synergetic effect of the two treatments.

322

Anjum et al.[38] have also reported that ozonation and ultrasonication have a synergetic

323

effect on sludge solubilization. Tian et al. [32] reported ultrasonication-ozone pre-treatment

324

to increase methane potential around 30% (from 3.5 to 4.5mL/day) with domestic sludge.

325

Among the different pre-treatment conditions employed in the study, pre-treatment of highest

326

(SE) and highest ozone dose (53.5kJ/gVS + 0.11gO3/gVS) showed the best performance in

327

DAD system which resulted in the highest BMP i.e. 138.2% higher as compared to control.

328

Further, higher VS removal efficiency suggested that more organic matters were digested

329

and converted into biogas.

330

Zhao’s study [39] showed that the ultrasonic effect can optimize the pore structure of

331

the sludge, the moisture permeability and diffusivity will be increased consequently. In

332

addition, both temperature rising caused by ultrasound thermal effect and ultrasonic vibration

333

excitation energy can accelerate vibration migration of liquid molecules, and further enlarge

334

effective diffusivity of moisture in sludge. Mass transfer limitation is one of the key rate-

335

limiting steps in DAD, and ultrasonic technology is one of the effective counter

18

336

measurements in improving the mass transfer of DAD of dewatered sewage sludge, making

337

material exchange and energy flow between metabolites and microorganisms more efficient.

338

Referring to the results obtained, it also could be concluded that the physical pre-

339

treatments out-compete chemical ones. The BMP in DAD system enhanced by different pre-

340

treatments was in the order of Combined (ultrasonication-ozone) > Ultrasonication > Ozone >

341

Acid. This is because the physical pre-treatment such as ultrasonication not only disintegrate

342

flocs of microorganism and lysis zoogloea cell in sludge but also keep the chemical characters

343

of released compounds less changed for subsequent digestion process. Whereas pre-treatment

344

related to chemical reaction may change biological properties of the compounds/nutrients

345

through esterification, caramelization (for hydrocarbons) or maillard reaction (for amino

346

acids) thus affect the capability of fermentation and methanation.

347

Nevertheless, energy consumption/agent cost needs to be considered during full scale

348

application as it in certain rang is in linear correlation with the increment of BMP capacity.

349

Studies using ultrasonic pre-treatment to enhance the LAD of sewage sludge have shown that

350

in the laboratory stage, the system operates as an energy consuming process as evidenced by

351

the ratio of input energy to output energy range from 35.6 to 198.7[23, 40-43]. The input-

352

output energy ratio of this study is 37.07~43.54, which belongs to the range of low level

353

compared with others’ results. Nevertheless, in full-scale applications, the system achieves

354

net energy yield due to the increased capacity of sludge pre-treatment at a time and further

355

optimization of methanization process[44, 45]. In addition, the DAD technology has the

19

356

advantages of lower energy requirements for heating and less energy loss, as well as avoiding

357

extra energy for liquid residue treatment, which all contribute to more economical and

358

energy-saving in DAD over LAD.

359

5. Conclusion

360

All the tested pre-treatment methods showed significant improvement in DAD of sewage

361

sludge in term of hydrolysation and methanation. Ultrasonic pre-treatment showed the best

362

result among the single treatment. Combined pre-treatment (ultrasonication-ozone) showed

363

a better result than all the single pre-treatments, i.e. the BMP increased by 138.2% over

364

control. Likewise, it resulted in 53.7%, 63.7% and 57.3% more reduction in TS, VS, COD

365

respectively compared with control. Nevertheless, physical pre-treatment shows advantage

366

over chemical methods, which proves the effectiveness on cell disintegration and release of

367

nutrients is important, but the state of compounds for methanogenesis is more critical. The

368

results will provide some technical guidance and support for the further application of DAD

369

pre-treatment technology. Calculations show that the use of ultrasound combined with ozone

370

pre-treatment to improve DAD of sewage sludge is feasible from both technical and

371

economic/energy perspectives. In future work, the technology should be up-scaled to have

372

better evaluation on economic and energy profile. Multiple indicator parameters (such as

373

protein, polysaccharide content) also need to be establish for a comprehensive evaluation of

374

DAD of dewatered sewage sludge and other waste material.

20

375 376 377 378 379 380 381 382 383 384 385 386 387 388 389 390 391 392 393 394 395 396 397 398 399 400 401 402 403 404 405 406 407 408 409 410 411 412 413 414 415 416 417 418 419 420 421 422 423 424

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[23] T. Onyeche, O. Schläfer, H. Bormann, C. Schröder, M. Sievers, Ultrasonic cell disruption of stabilised sludge with subsequent anaerobic digestion, Ultrasonics, 40 (2002) 31-35. [24] X. Yin, X. Lu, P. Han, Y. Wang, Ultrasonic treatment on activated sewage sludge from petroplant for reduction, Ultrasonics, 44 (2006) e397-e399. [25] A.B. Aylin, O. Yenigün, A. Erdinçler, Ultrasound assisted biogas production from co-digestion of wastewater sludges and agricultural wastes: Comparison with microwave pre-treatment, Ultrasonics Sonochemistry, 40 (2017). [26] E. Elbeshbishy, S. Aldin, H. Hafez, G. Nakhla, M. Ray, Impact of ultrasonication of hog manure on anaerobic digestability, Ultrasonics Sonochemistry, 18 (2011) 164-171. [27] V. Fernández-Cegrí, D.L.R. Ma, F. Raposo, R. Borja, Impact of ultrasonic pretreatment under different operational conditions on the mesophilic anaerobic digestion of sunflower oil cake in batch mode, Ultrasonics Sonochemistry, 19 (2012) 1003-1010. [28] J. Yiying, L. Huan, R.B. Mahar, W. Zhiyu, N. Yongfeng, Combined alkaline and ultrasonic pretreatment of sludge before aerobic digestion, Journal of Environmental Sciences, 21 (2009) 279284. [29] X. Liu, H. Liu, J. Chen, G. Du, J. Chen, Enhancement of solubilization and acidification of waste activated sludge by pretreatment, Waste Management, 28 (2008) 2614-2622. [30] S. Sahinkaya, Disintegration of municipal waste activated sludge by simultaneous combination of acid and ultrasonic pretreatment, Process Safety and Environmental Protection, 93 (2015) 201205. [31] G. Xu, S. Chen, J. Shi, S. Wang, G. Zhu, Combination treatment of ultrasound and ozone for improving solubilization and anaerobic biodegradability of waste activated sludge, Journal of Hazardous Materials, 180 (2010) 340-346. [32] X.B. Tian, A.P. Trzcinski, L.L. Lin, W.J. Ng, Impact of ozone assisted ultrasonication pretreatment on anaerobic digestibility of sewage sludge, Journal of Environmental Sciences, 33 (2015) 29-38. [33] A. Salihu, M.Z. Alam, Pretreatment Methods of Organic Wastes for Biogas Production, Journal of Applied Sciences, 16 (2016) 124. [34] M.C. Rand, A.E. Greenberg, M.J. Taras, M.C. Rand, A.E. Greenberg, M.J. Taras, Standard methods for the examination of water and wastewater. 14th edition, APHA, 1976. [35] W.F. Owen, D.C. Stuckey, J.B. Healy, L.Y. Young, P.L. Mccarty, Bioassay for Monitoring Biochemical Methane Potential and Anaerobic Toxicity, Water Research, 13 (1979) 485-492. [36] N.N. Zhai, T. Zhang, D.X. Yin, G.H. Yang, X.J. Wang, G.X. Ren, Y.Z. Feng, Effect of initial pH on anaerobic co-digestion of kitchen waste and cow manure, Waste Management, 38 (2015) 126131. [37] C. Bougrier, C. Albasi, J.-P. Delgenès, H. Carrère, Effect of ultrasonic, thermal and ozone pretreatments on waste activated sludge solubilisation and anaerobic biodegradability, Chemical Engineering and Processing: Process Intensification, 45 (2006) 711-718. [38] M. Anjum, N.H. Al-Makishah, M.A. Barakat, Wastewater sludge stabilization using pretreatment methods, Process Safety & Environmental Protection, 102 (2016) 615-632. [39] z. Fang, L. Dong, C. Zhenqian, Fractal study on moisture permeability and efective difusivity under ultrasound, CHINA SCIENCEPAPER, 8 (2013) 816-819. [40] V. Riau, M.A. De la Rubia, M. Perez, Upgrading the temperature-phased anaerobic digestion of waste activated sludge by ultrasonic pretreatment, Chemical Engineering Journal, 259 (2015) 672681. [41] C. Bougrier, H. Carrere, J.P. Delgenes, Solubilisation of waste-activated sludge by ultrasonic treatment, Chemical Engineering Journal, 106 (2005) 163-169. [42] C.M. Braguglia, G. Mininni, A. Gianico, Is sonication effective to improve biogas production and solids reduction in excess sludge digestion?, Water Science & Technology, 57 (2008) 479-483. [43] M.R. Salsabil, A. Prorot, M. Casellas, C. Dagot, Pre-treatment of activated sludge: Effect of sonication on aerobic and anaerobic digestibility, Chemical Engineering Journal, 148 (2009) 327335. [44] U. Neis, K. Nickel, A. Lunden, Improving anaerobic and aerobic degradation by ultrasonic disintegration of biomass, J Environ Sci Heal A, 43 (2008) 1541-1545.

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[45] S. Perez-Elvira, M. Fdz-Polanco, F.I. Plaza, G. Garralon, F. Fdz-Polanco, Ultrasound pretreatment for anaerobic digestion improvement, Water Science and Technology, 60 (2009) 15251532.

23

482

483

Colored Figures

Graphical Abstracts

484 485 486

24

487

488

Fig.1.

489

490 491

Fig.1. Schematic diagram of the bio-methane production test: 1- Gas collection and measurement bottle, 2- Anaerobic reactor, 3- Sweeping gas inlet, 4-Water bath.

492 493

25

494

Fig.2.

495

496

Fig.2. Reduction percentage of TS, VS and COD (%) after dry anaerobic digestion under

497

different pre-treatment conditions (A) Acidification; (B) Ozone; (C) Ultrasonication; (D)

498

ULS-ozone combined.

499

26

500

Fig.3.

501

502

Fig.3. Initial and final value of pH and TVFA in dry anaerobic digestion system after

503

different pre-treatment. (A) Acidification; (B) Ozone; (C) Ultrasonication; (D) ULS-ozone

504

combined.

505 506 507 508 509 510

27

511

Fig.4.

512 513

Fig.4. Comparison of biological methane potential (BMP) in dry anaerobic digestion

514

system after different pre-treatment. (A) Acidification; (B) Ozone; (C) Ultrasonication; (D)

515

ULS-ozone combined.

516 517

28

518

Fig.5.

519

Fig.5. Cumulative methane production in dry anaerobic digestion system after different

520

pre-treatment. (A) Acidification; (B) Ozone; (C) Ultrasonication; (D) ULS-ozone combined.

521 522

29

523

524

Fig.6.

Fig.6. Correlation between COD degradation rate and energy input

525

30

526

527 528

Fig.7.

Fig.7. Energy input-output ratio (Einput/Eoutput) of combined pre-treatment, energy ratio(η) required to remove per unit TS.

529

31

530

Tables

531

532

Table.1. Characteristics of dewatered sewage sludge and inoculum

Parameters

Dewatered sewage sludge

Inoculum

pH

7

6.9

Moisture Content (%)

79.0

74.3

TS (%)

21.0

25.7

VS (%) dry base

47.9

57.9

TVFA (mg/L) dry base

1173.1

1212.2

TCOD (%)

14.3

17.7

533

534

Table.2. Acid pre-treatment for dewatered sludge Mass of HCl added

Acidified pH of

pH of

(mL/kg wet sludge)

dewatered sludge

neutralized sludge

Control

0.00

7.10±0.5

7.10±0.5

pH 5

0.68

4.78±0.5

7.00±0.5

pH 4

1.37

4.13±0.5

7.10±0.5

pH 3

2.74

3.21±0.5

7.10±0.5

pH 2

5.47

1.98±0.5

7.00±0.5

Experiment

535 536

32

537

Table.3. Characteristics of dewatered sludge after pre-treatment

Treatment

Pre-treatment time

TS (%)

VS (%)

COD (%)

Control

——

17.77

47.91

14.31

17.15

47.90

14.29

( -0.62) *

(-0.01)

(-0.02)

16.33

46.55

14.19

48 hours (prior to

(-1.44)

(-1.36)

(-0.12)

neutralize)

16.01

46.00

14.1

(-1.76)

(-1.91)

(-0.21)

15.44

45.55

13.89

(-2.33)

(-2.36)

(-0.42)

17.73

47.89

14.26

(-0.04)

(-0.02)

(-0.05)

17.64

47.85

14.19

(-0.13)

(-0.06)

(-0.12)

17.59

47.81

14.10

(-0.18)

(-0.10)

(-0.21)

17.35

47.79

13.89

(-0.42)

(-0.12)

(-0.42)

17.71

47.87

14.28

(-0.06)

(-0.04)

(-0.03)

17.71

47.86

14.21

(-0.06)

(-0.05)

(-0.10)

17.69

47.83

14.05

(-0.08)

(-0.08)

(-0.26)

Acid-pH5

Acid-pH4

Acid-pH3

Acid-pH2

Ozone-0.05gO3/gVS

Ozone-0.07gO3/gVS 30 minutes Ozone-0.09gO3/gVS

Ozone-0.11gO3/gVS

Ultrasonication-4.5 kJ/gVS

Ultrasonication-11.5 kJ/gVS

30 minutes

Ultrasonication-22.5 kJ/gVS

33

Ultrasonication-53.5 kJ/gVS

Combined-53.5 -0.05 (kJ/gVS- gO3/gVS)

Combined-53.5-0.07 (kJ/gVS- gO3/gVS) 30 minutes Combined-53.5 -0.09 (kJ/gVS- gO3/gVS)

Combined-53.5 -0.011 (kJ/gVS- gO3/gVS) 538

17.68

47.80

13.99

(-0.09)

(-0.11)

(-0.32)

17.48

47.81

14.38

(-0.29)

(-0.10)

(-0.07)

17.40

47.76

14.31

(-0.37)

(-0.15)

(0)

17.35

47.66

14.21

(-0.42)

(-0.25)

(-0.10)

17.28

47.31

14.08

(-0.49)

(-0.60)

(-0.23)

*number in parenthesis means reduction/increase over the control trail

539

34

540

Black and white Figures

541

Graphical Abstracts

542 543

35

544

Fig.1.

545

546 547

Fig.1. Schematic diagram of the bio-methane production test: 1- Gas collection and measurement bottle, 2- Anaerobic reactor, 3- Sweeping gas inlet, 4-Water bath.

548 549

36

550

Fig.2.

551

552

Fig.2. Reduction percentage of TS, VS and COD (%) after dry anaerobic digestion under

553

different pre-treatment conditions (A) Acidification; (B) Ozone; (C) Ultrasonication; (D)

554

ULS-ozone combined.

555 556 557

37

558

Fig.3.

559

560

Fig.3. Initial and final value of pH and TVFA in dry anaerobic digestion system after

561

different pre-treatment. (A) Acidification; (B) Ozone; (C) Ultrasonication; (D) ULS-ozone

562

combined.

563 564 565 566 567

38

568

Fig.4.

569 570

Fig.4. Comparison of biological methane potential (BMP) in dry anaerobic digestion

571

system after different pre-treatment. (A) Acidification; (B) Ozone; (C) Ultrasonication; (D)

572

ULS-ozone combined.

573 574 575 576 577

39

578

Fig.5.

579

Fig.5. Cumulative methane production in dry anaerobic digestion system after different

580

pre-treatment. (A) Acidification; (B) Ozone; (C) Ultrasonication; (D) ULS-ozone combined.

581

40

582

583

Fig.6.

Fig.6. Correlation between COD degradation rate and energy input

584

41

585

Fig.7.

586

Fig.7. Energy input-output ratio (Einput/Eoutput) of combined pre-treatment, energy

587

ratio(η) required to remove per unit TS.

588

Highlights

589 590 591 592 593 594 595 596 597 598 599 600



Different pre-treatment methods were investigated and evaluated systematically in terms of BMP and biodegradation.



Ultrasonic pre-treatment had the best result among the single technologies and physical technology reflects better results than chemical technologies.



Combined pre-treatment (ultrasonication-ozone) showed more significant enhancement than single methods as evidenced by 138.2% higher BMP over control.



Ultrasonication combined with ozone pre-treatment has potential for economic and energy conservation.

42