Simultaneous removal of siloxanes and H2S from biogas using an aerobic biotrickling filter

Journal Pre-proof Simultaneous removal of siloxanes and H2 S from biogas using an aerobic biotrickling filter Yuyao Zhang (Investigation) (Data curation) (Writing - original draft), Kazuyuki Oshita (Conceptualization) (Supervision) (Writing - review and editing), Taketoshi Kusakabe (Methodology), Masaki Takaoka (Supervision), Yu Kawasaki (Conceptualization) (Methodology), Daisuke Minami (Conceptualization) (Methodology), Toshihiro Tanaka (Conceptualization)

PII:

S0304-3894(20)30175-8

DOI:

https://doi.org/10.1016/j.jhazmat.2020.122187

Reference:

HAZMAT 122187

To appear in:

Journal of Hazardous Materials

Received Date:

12 October 2019

Revised Date:

1 January 2020

Accepted Date:

24 January 2020

Please cite this article as: Zhang Y, Oshita K, Kusakabe T, Takaoka M, Kawasaki Y, Minami D, Tanaka T, Simultaneous removal of siloxanes and H2 S from biogas using an aerobic biotrickling filter, Journal of Hazardous Materials (2020), doi: https://doi.org/10.1016/j.jhazmat.2020.122187

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Simultaneous removal of siloxanes and H2S from biogas using an aerobic biotrickling filter Yuyao Zhang a, Kazuyuki Oshita a,*, Taketoshi Kusakabe a, Masaki Takaoka a, Yu Kawasaki b, Daisuke Minami b, Toshihiro Tanaka b a

Department of Environmental Engineering, Graduate School of Engineering, Kyoto University, C-cluster, Kyotodaigaku-katsura, Nishikyo-ku, Kyoto, 615-8540, Japan b

Ebara Jitsugyo Co., Ltd., Ginza, Chuo-ku, Tokyo, Japan

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* Corresponding author: Kazuyuki Oshita, E-mail address: [email protected]

Graphical abstract 1

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Highlights

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・Feasibility of simultaneous removal of siloxanes and H2S from biogas by an aerobic BTF was investigated.

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・Biodegradation of H2S in BTF followed first-order kinetics. ・Up to 52% removal of D5 was reached mainly by the chemical-absorption in acid recycling liquid.

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・D5 was finally abated to dimethylsilanediol and L2.

Abstract The feasibility of simultaneous removal of siloxane and H2S from biogas was investigated using an aerobic biotrickling filter (BTF). The biodegradation of H2S in the BTF followed a first-order kinetic model and more 2

than

95%

H2S

was

eliminated

within

a

residence

time

of

0.3

min.

The

removal

of

decamethylcyclopentasiloxane (D5) increased with longer empty bed residence time (EBRT). The partition test and microbial community analysis further reveals that up to 52% removal of D5 was reached mainly by the chemical-absorption in acid recycling liquid. Finally, D5 was converted into mixtures of dimethylsilanediol (DMSD) and hexamethyldisiloxane (L2) via ring-opening hydrolysis in acid liquid and ring-shrinking polyreaction using CH4 derived from biogas. These operational characteristics demonstrate that

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the abiotic removal of D5, in addition to biological removal of H2S in an aerobic BTF can significantly

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decrease the siloxane loading to the downstream siloxane removing units.

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Keywords: biogas, siloxane, decamethylcyclopentasiloxane, hydrogen sulfide, biotrickling filter

1. Introduction

Biogas, produced during the anaerobic conversion of organic matter, is a renewable biofuel with applications such as heating, power cogeneration, and vehicle fuel (Chen et al., 2015). However, the presence of trace compounds, such as H2S, and siloxanes affects the potential of energy recovery from biogas because they damage the biogas-processing equipment (Appels et al., 2008; Dewil et al., 2006; Dewil et al., 2007). To 3

increase the feasibility of obtaining energy from biogas, current methods especially for removing siloxane from biogas focus mainly on physicochemical processes, such as activated carbon adsorption, gas-liquid absorption, and refrigeration (Ajhar et al., 2010; Cabrera-Codony et al., 2014,2018; Jiang et al., 2016; Schweigkofler and Niessner, 2001). Most physicochemical processes are expensive in terms of chemical/energy costs and the regeneration of saturated adsorbent materials. The improper disposal of spent adsorbent materials also pose health and environmental risks (Cabrera-Codony et al., 2015). Thus, the

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biological removal of siloxane from biogas has received increasing attention due to its potentially high treatment efficacy and reduced operating costs (Wang et al., 2014b; Boada et al., 2020). In particular,

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biotrickling filters (BTFs) are a promising biological technique for controlling odors and volatile organic compounds (Oyarzún et al., 2003; Vikrant et al., 2017; Santos-Clotas et al., 2019).

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However, the biological removal of siloxane using a BTF still faces some challenges, such as the limited mass

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transfer from the gas to the liquid phase, as well as the biodegradation-resistant siloxanes (Popat and

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Deshusses, 2008). Although biosurfactants synthesized by microorganisms increase the solubility of siloxanes and facilitate biodegradation processes, long empty bed residence times (EBRT) are still required to achieve

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meaningful elimination capacity (EC) (Li et al., 2014). The EBRT is the length of time that gas must spend in

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contact with the inside chamber of BTF. It does not depend on the volume occupied by the physical media, so it is used by engineers as a convenient design criterion when designing odor control treatment processes. In an aerobic BTF inoculated with Thiobacillus thioparus for the removal of H2S, the main metabolites are sulfate and H+, which decreases the pH (Oyarzún et al., 2003). The acid recycling liquid generated by biological oxidation of H2S could be an effective method for accelerating siloxane transfer from the gas to the liquid phase (Schweigkofler and Niessner, 2001). Therefore, the simultaneous removal of siloxane and H2S from 4

biogas using a BTF could be an attractive cost-effective alternative to conventional physicochemical processes. To investigate its operational characteristics, biogas from a sludge anaerobic digester was used as the inlet gas of an aerobic BTF, which was operated continuously under five EBRTs. A gas-liquid partition test of D5 was also carried out to estimate the mass transfer coefficients of D5 from the gas to the acid recycling liquid of the BTF. Furthermore, microbial community in the recycling liquid of the BTF was analyzed to define the biodegradation mechanism of H2S and chemical degradation mechanism of D5. Finally, the degradation

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products of D5 in the recycling liquid were identified by gas chromatography-mass spectrometry (GC-MS)

2. Materials and methods

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2.1. Wastewater treatment plant (WWTP) configuration

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analysis, and used to deduce the degradation pathways.

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Biogas and liquid samples were collected from the BTF experimental apparatus installed in WWTP T in Japan.

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The capacity of WWTP T is 847,000 m3 wastewater/day, serving a population equivalent of 837,410. The influent wastewater collects rainwater flow into a primary sedimentation tank, which is treated in the

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biological treatment tank, settled in the final sedimentation tank, and then released into a river after

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disinfection with sodium hypochlorite. The primary sludge and waste activated sludge (WAS) generated from the primary and final sedimentation are thickened by gravity and a belt-type thickener, respectively. Only the thickened primary sludge is digested anaerobically in a digestion tank. The digestion temperature is maintained at 55℃ and the solids retention time (SRT) is set to 10 days. The digested sludge is mixed with thickened WAS and then dewatered in a belt-press dewatering device. Dewatered sludge is incinerated in a fluidized bed incinerator or carbonized by pyrolysis. Biogas from anaerobic digestion with no further 5

treatment, as one stream of gas supply, is provided to the BTF experimental apparatus. 2.2. BTF setups and operating conditions 2.2.1 BTF setups Fig.1 shows a schematic diagram of the BTF used for simultaneous removal of siloxanes and H2S from biogas. The BTF consisted of a clear polyvinyl chloride column with an inner diameter of 15 cm and a height of 1.2 m. The column had a packing height of 1.0 m and was filled with polypropylene carrier (KG-088-O15;

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KansaiKako Co., Osaka, Japan). The length, particle diameter and specific surface area of polypropylene carrier were 15 mm, 15 mm and 960 m2·m-3, respectively (Fig. S1). The influence of temperature on the

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removal of H2S from biogas using BTF was investigated under temperature range of 15ºC – 45ºC. High removal efficiency of H2S (above 95%) was obtained between 28ºC and 32ºC. The outer layer of the column

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was connected to a water heating system to maintain the inner temperature of the column at 30℃.

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The gas will be mixed with recycling liquid after flowing through the BTF, which leads to the loss of recycling

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liquid. Therefore, the condensation tower was set up immediately after the biological desulfurizer to concentrate and collect the recycling liquid which was mixed in treated gas. The inner temperature of the

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condensation tower was maintained at 15℃ by a water cooling system. The sulfur was generated and clogged

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the biological desulfurizer when the H2S concentrations of inlet gas was too high. We examined previously the range of H2S concentration that does not generate sulfur in our lab-experiments. The result shows that, (i) when the H2S concentration of inflow gas was in the range of 100-600 ppmv, H2S was oxidized to sulfuric acid without generating sulfur; (ii) sulfur was generated and clogged the biological desulfurizer once H2S concentrations was above 700 ppmv. Since the H2S concentrations of biogas from anaerobic digestion was in range of 400–1200 ppmv, recycling gas from outlet of condensation tower was used to dilute the biogas with 6

same flow rate. 2.2.2 Inoculum and synthetic mineral medium Initially, 5 L of anaerobic digested sludge from a digestion tank of WWTP T was inoculated in the BTF. Meanwhile, 35 L of recycling liquid from the nutrient solution tank was recirculated continuously through the packing of the BTF at a flowrate of 0.9 L·min-1 using a pump. A liquid distributor was installed at the top of the BTF to ensure homogenous liquid distribution. The pH of the recycling liquid was not controlled during

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the BTF operation. Except for water to supply in the recycling liquid irregularly, everyday 20 mL of mineral medium was added into the nutrient solution tank (40 L). The medium contained (mg·L-1): N, 6,618; P, 1,032;

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K, 1,120; Fe, 11.8; Mn, 0.4; Al, 1.0; Na, 25.4; Ca, 6.0; Mg, 9.8; S, 17.8; and B, 0.4. 2.2.3 Gas sampling

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Six sampling ports were set at heights of 0, 0.2, 0.4, 0.6, 0.8, and 1.0 m, respectively. The concentration of

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H2S in gas collected from the sampling ports of the BTF was monitored on-site using a gas detection tube

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(Gastec Co., Kanagawa, Japan). Type 4LL detection tubes were used for 0 to 100 ppmv of H2S, whereas 4H tubes were used for 100 to 2000 ppmv of H2S. Moreover, gas from different sampling ports was sampled in a

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Tedlar bag made of vinyl fluoride resin. The CH4, CO2, and H2S concentrations in the gas collected in the

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Tedlar bag were measured on site using a gas chromatograph (GA3000 PLUS; Geotechnical Instruments (UK) Ltd, Warwickshire, England) (Fig. S2). For the evaluation of siloxane concentrations of the gas samples, we used styrene-divinylbenzene copolymer beads (Sepabeads; Mitsubishi Chemical Corp., Tokyo, Japan) as adsorbents, which were packed (5 g) into the adsorption column, the two ends of which then was packed with glass wool. The gas from different sampling ports was sampled through the adsorbent at a flowrate 0.5 L·min1

for 30 min. 7

2.2.4 Experimental conditions The BTF was operated for 180 days under several conditions of inlet concentration, EBRT and inlet loading (Table 1). The inlet concentrations of siloxanes and H2S mainly depended on the characteristics of the biogas from the sludge anaerobic digester. The concentration of D5 in the biogas ranged from 22.1 to 61.4 mg·m-3, which was 10 times higher than the octamethylcyclotetrasiloxane (D4) concentration (2.4–7.8 mg·m-3). D5 accounted for 80–90% of the total siloxane in biogas and was thus selected as the target siloxane in this work.

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The loading (LH2S, D5, D4), removal efficiency (REH2S, D5, D4), and elimination capacity (ECH2S, D5, D4) of H2S, D5, and D4 were evaluated using Eq. (1), Eq. (2), and Eq. (3), respectively.

AH

(1)

RE 

(Cin  Cout ) Cout

EC 

(Cin  Cout )Q AH

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QCin

(2) (3)

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L

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where A is the cross-sectional area (m2) of the packing layer, H is the height (m) of the packing layer, Q is the inlet gas flow rate (L·min-1), Cin is the inlet concentration (mg·m-3) and Cout is the outlet concentration of the

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BTF (mg·m-3).

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The EBRTs were controlled by setting the inlet flow rate from 8 to 29 L·min-1. The air flow rate was set to

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maintain the O2 concentration of the inlet gas mixture as 1.0%. Phase 1 corresponded to the start-up period, which was carried out at an EBRT of 0.6 min. Due to the high inlet concentration of D5 (Cin, D5), the start-up period lasted 37 days at a high inlet loading of D5 (LD5) 3,000–4,000 mg·m-3·h-1. During the experimental period (Phase 2–6), the effect of the EBRT on the performance of the BTF was observed by increasing the EBRTs from 0.6 min to 1.5 min and to 2.2 min, then decreasing to 1.8 min and to 1.0 min in a stepwise manner. 2.3. Partition tests 8

To estimate the mass transfer coefficients of D5 from the gas to the recycling liquid of the BTF, partition tests for D5 between gas and liquid phases (deionized water, H2SO4 solution and recycling liquid) were carried out. The recycling liquid of the BTF under long-term stable operation contains some biomass, which adsorbs siloxanes and contributes to the removal of siloxanes from biogas. Therefore, synthetic H2SO4 solution with the same pH (1.5) as the recycling liquid was used to analyze the influence of the acid recycling liquid itself on the removal of D5 in the BTF under acidic conditions.

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As shown in Fig. 2, the partition test was mainly performed similar to the experimental process described previously (Popat and Deshusses, 2008). First, 250 mL flasks were filled with 100 mL of the test liquid, and

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gas phase D5 was injected into the headspace to obtain concentrations of 1,000 mg·m-3. The flasks were placed in a water bath shaker (BT300, Yamato Scientific Co., Ltd., Japan) to maintain the temperature at 30℃. The

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shaking frequency was set to 150 times/min and the stroke was 5 cm. To observe the effect of time on the

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partitioning of D5, gas and liquid samples were taken in a flask at selected time intervals (0.25, 0.5, 1.0, 2.0,

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4.0, and 11.0 h). To assess the D5 concentrations in the headspace of flasks, each flask was connected to a closed gas-circulating system including an adsorption column, air pump (APN085LV-1, Iwaki Co., Ltd.,

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Tokyo, Japan), and rotor flowmeter (Kofloc, Kojima Instruments Inc., Kyoto, Japan). The gas in the headspace

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was circulated at a flowrate of 300 mL/min for 10 min and the D5 in the headspace was trapped in the adsorbent.

Finally, based on the D5 concentrations in the headspace (CG*) and the liquid phase (CL*) of flasks, the gasliquid partition coefficients of D5 were calculated using Eq. (4).

P

*

CG * CL

(4)

2.4. Analysis of siloxanes and degradation productions 9

To evaluate the siloxane concentrations in the liquid samples, siloxanes were extracted previously before gas chromatography-mass spectrometry (GC-MS) analysis using the following method (Fig. S3a). First, 100 mL of recycling liquid sample and 50 mL of n-hexane (minimum purity 96%; Kanto Chemical Co. Inc., Tokyo, Japan) were added to a separating funnel then shaken for 5 min. The mixture was treated with 8 g of anhydrous Na2SO4 to remove water, transferred into a conical flask and stirred at an agitation rate of 1,000 revolutions per minute (rpm) for 20 min with a magnetic stirrer (SR100; Toyo Roshi Kaisha, Ltd, Tokyo, Japan). After

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the mixture was centrifuged (3,000 rpm, 3 min), the hexane layer was separated using a disposable pipette. In the case of gas samples, the siloxanes in adsorbents were Soxhlet-extracted with n-hexane for 10 h at 65 ℃,

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then analyzed by GC-MS under the same analytical conditions as Oshita et al. (2015).

Tetrahydrofuran (THF) has been demonstrated to be an excellent choice for extracting both siloxane and its

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metabolites (Li et al., 2014; Xu, 1999). The process for the extraction and derivatization of siloxanes and their

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degradation products from liquid samples is shown in Fig. S3b. First, the recycling liquid was centrifuged at

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3000 rpm for 3 min. Then, 15 g of NaCl and 6 mL of THF was added to 60 mL of supernatant, then vortexed for 10 min. After being centrifuged at 3000 rpm for 3 min, the upper THF was transferred into a new glass

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tube. Then, 6 mL of fresh THF was added to the supernatant and the extraction procedure was repeated. Next,

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three THF extracts were combined and 5 g of Na2SO4 was added to the glass tube, and shaken for 5 min. The mixture was centrifuged at 3000 rpm for 3 min, and the dry THF extracts were concentrated to approximately 0.5 mL by blowing nitrogen. The Si–OH groups in siloxane degradation products can strongly interact with GC columns. To protect Si–OH groups, the siloxane degradation products were derived into trimethylsilylated derivatives of silanols with bis(trimethylsilyl)trifluoroacetamide (BSTFA). The concentrated THF extracts were treated with equivalent amounts (v/v) of BSTFA, vortex-mixed for 5 min, and then shaken using a 10

horizontal shaker for 2 h at 25°C. To measure the siloxanes and trimethylsilylated derivatives of silanols by GC-MS, we used a GC2010/GCMS-QP2010 instrument (Shimadzu Co., Kyoto, Japan) with helium as the carrier gas (ZERO-A; Sumitomo Seika Chemicals Co., Osaka, Japan), supplied at a constant flow rate of 1 mL/min. An HP-5MS (Hewlett Packard) capillary column (length: 60 m, inner diameter: 0.25 mm, film thickness: 0.25 µm) was used. GC-MS was carried out in selected ion-monitoring (SIM) mode, and the molecular ion of each

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compound was scanned at a rate of 1.5–2.0 scans/s. The inlet and interface temperatures were maintained at 230 ℃ and 210 ℃, respectively. The column temperature was initially held at 40 ℃ for 4 min, increased to

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100 ℃ at a rate of 6 ℃/min, and then to 200 ℃ at 4 ℃/min. Finally, the column was heated to 280 ℃ at a rate of 30 ℃/min, and its temperature was maintained at 280 ℃ for 3 min.

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2.5.1 Sample collection and DNA extraction

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2.5. MiSeq sequencing of 16S rRNA and high-throughput sequence analysis

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The recycling liquid of the BTF on day 180 was collected for 16S rRNA gene analysis. DNA was extracted and purified using the Extrap Soil DNA Kit Plus ver.2 (Nippon Steel, Japan), and quantified using PicoGreen

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dsDNA Assay Kit (Invitrogen).

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2.5.2 16S rRNA library preparation and MiSeq sequencing The V4-V5 hypervariable regions of the 16S rRNA genes were PCR amplified using universal primers U515F (5′-GTGYCAGCMGCCGCGGTA-3′) and 926R (5′-CCGYCAATTCMTTTRAGTT-3′). The purified PCR amplicon was quantified with PicoGreen dsDNA Assay Kit (Invitrogen) and then pair-end sequenced by MiSeq (Illumina, USA). 2.5.3 Data processing and analyses 11

The sequence data were processed using QIIME (Quantitative Insights Into Microbial Ecology) Pipeline (Caporaso et al., 2010). The sample was rarefied to exhibit the lowest number of reads (10,000 sequences) for alpha-diversity (observed species, Chao1 richness estimator) analyses, for which the rarefaction curves were generated from the observed species. Sequences were then phylogenetically assigned using the Greengene (DeSantis et al., 2006) and the Silva's Living Tree Database Project (http://www.arb-silva.de/projects/livingtree/) classifiers and allocated to phylum, class, family, and species levels. Sequences were clustered into

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operational taxonomic units (OTUs) using a 97% identity threshold. OTUs with the abundances exceeding

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0.1% were selected to compare the microbial communities of the BTFs.

3. Results and discussion

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3.1. Overall performance of the BTF

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As shown in Fig. 3, the BTF was operated continuously under five EBRTs for 180 days. The BTF was initialized at an EBRT of 0.6 min by controlling the flow rate of biogas, recycling gas, and air at 14, 14 and 1

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L·min-1, respectively. During the start-up phase (0–37 d), the inlet concentration of H2S (Cin, H2S) was 350–400

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ppmv, and the inlet LH2S was 20–25 g·m-3·h-1. The REH2S increased from 70% to 95%. Furthermore, the pH of

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recycling liquid in the BTF also decreased from 6.9 to 1.5. These results indicated that 30 days was sufficient for the BTF to successfully start up and reach steady-state conditions. The RED5 increased gradually during the operational period, from 8% up to 20%, After starting up successfully, the BTF was operated under the pre-set experimental conditions and each phase lasted 25–30 days. During Phase 2 (38–68 d), the BTF was operated at an EBRT of 0.6 min with Cin, H2S of 200–400 ppmv and Cin,

D5

of 19–25 mg·m-3. The REH2S increased from 95% to 100% following stable operation. The 12

corresponding ECH2S varied from 19 to 26 g·m-3·h-1. Although the RED5 under the steady-state was relatively low (15–18%), a higher ECD5 of 330–410 mg·m-3·h-1 was achieved with high LD5 of 1,800–2,000 mg·m-3·h-1. Simultaneously, ECD4 was in the range of 19–32 mg·m-3·h-1, which was much lower than the range reported by other researchers (Li et al., 2014; Popat and Deshusses, 2008; Wang et al., 2014b). According to the report by Popat and Deshusses (2008), when the inlet concentration of D4 (Cin, D4) was maintained at 45 mg·m-3, the LD4 and ECD4 were 151 mg·m-3·h-1 and 63 mg·m-3·h-1, respectively. In the study by Popat and Deshusses

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(2008), synthetic gas containing only D4 was used as inlet gas to control the Cin, D4, whereas actual biogas from a sludge anaerobic digester was used in this study. Thus, a low ECD4 was achieved under a low Cin, D4

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value of 2.4–7.8 mg·m-3 using actual biogas. Moreover, Li et al. (2014) investigated the performance of BTF for D4 removal under Cin, D4 of 20 mg·m-3 and EBRT of 3.3 min, and observed an ECD4 of 200 mg·m-3·h-1 and

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RED4 of 60%. Compared to the above observation of RED4, the RED4 under the steady-state was relatively low

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(9–13%), which we mainly attributed to the short EBRT of 0.6 min.

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On day 69, the flow rates of the inlet gas decreased from 29 to 12 L·min-1, yielding an EBRT of 1.5 min during the Phase 3 (69–96 d). The RED5 increased gradually to a steady value of 29–38%, whereas the REH2S remained

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almost constant at 100%. To achieve an EBRT of 2.2 min, the inlet flow rate of the BTF was decreased from

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12 to 8 L·min-1 on day 97 (Phase 4). After a sudden drop to 15% of RED5, the RED5 increased gradually and reached a steady range of 50–59%. Then, we increased the inlet flow rates of the BTF from 8 to 10 L·min-1 during the Phase 5 (125–150 d), and to 18 L·min-1 during the Phase 6 (151–180 d). During Phases 5 and 6, RED5 followed a similar trend, decreasing suddenly to 15–18% just after the EBRT was changed, then increasing gradually to corresponding steady values of 30–51% and 29–40%, respectively. The time taken to reach a steady RED5 during the Phase 3–5 required only about 5–10 days, much shorter than the period of 20– 13

35 days during Phase 1 and 6. The Cin, D5 during the periods of 0–37 d and 151–166 days were 42–36 and 33– 46 mg·m-3, respectively. These values were higher than the Cin, D5 of 20–25 mg·m-3 during Phase 3–5. This indicated that the performance of the BTF tended toward a steady-state more easily and quickly under lower Cin, D5. Overall, when the EBRT was set at 0.6, 1.0, 1.5, 1.8 and 2.2 min, the corresponding average RED5 of the BTF were 19%, 30%, 33%, 37% and 52%, respectively. Moreover, all of the ECs of D5 under the EBRTs tested,

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except for 1.5 min, were approximately 400 mg·m-3·h-1. The Cin, H2S during Phase 3, of 200–250 ppmv, was much lower than that of other phases, indicating that simultaneous removal of siloxane and H2S by BTF can

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support a higher siloxane treatment capacity. The EBRT is a significant parameter governing the mass transfer of a pollutant from the gas phase into the aqueous or solid phase. By increasing the EBRT, the D5 was allowed

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a longer period to complete the mass transfer from the gas to the recycling liquid and the packing of the BTF.

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3.2. Removal profiles of H2S and D5

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Therefore, a higher RED5 was achieved at a longer EBRT.

Chung et al. (1996) and Yang and Allen (2012) evaluated the kinetics of H2S biodegradation in a biofilter

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using a mathematical model. The biodegradation rates were fitted by a first-order at low concentrations (< 200

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ppmv) and zero-order at high concentrations (> 400 ppmv) (Yang and Allen, 2012). Some researchers also considered the fact that first-order kinetics occur in the whole biofilter. To integrate the H2S removal profiles in the BTF under different EBRTs, the first-order kinetics was simplified as follows: C C in

 exp  -k 1t 

(5)

where C is the concentration (ppmv) of H2S in gas from different outlets; Cin is the concentration (ppmv) of H2S in inlet gas; k1 is the first-order reaction rate coefficient (min-1); and t is the average residence time of H2S 14

in BTF, defined as Eq. (6):

t  EBRTs

H H total

(6)

where Htotal is the total height (m) of the BTF and H is the height (m) of different gas outlets of the BTF. As shown in Fig. 4, we plotted the experimental values for the relative proportion of H2S concentration in gas from different sampling ports with respect to the inlet gas over the residence time of the H2S in the BTF. In

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the case of all EBRTs, only the experimental datas under stable phase were used for fitting to the mathematical expression corresponding to the first-order kinetic model of the BTF. A good agreement between the experimental and fitted data (R2= 0.94) was obtained. The first-order rate coefficient k1 was determined to be

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8 min-1. Combining Eq. (5) and the k1 of 9 min-1, more than 95% of H2S degraded within a residence time of

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0.3 min, which is in the commonly required EBRT range of 0.3–3.0 min (Chung et al., 1996; Jin et al., 2005;

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Oyarzún et al., 2003; Ramirez et al., 2009). The corresponding packing height was approximately 0.1–0.5 m for total EBRT of 0.6–2.2 min (Eq. (6)). A similar conclusion was also reported by López et al. (2016) that the

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majority of H2S removal took place in the first reactor bed in the co-current configuration. Moreover, the protein concentration of the biofilm along the BTF further confirmed that the majority of the biomass appeared

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to be located near the inlet of the BTF (Jiang and Tay, 2010).

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The reaction rate coefficient directly reflected the activity of microorganisms in the BTF, which depended on the Cin, H2S and the pH of recycling liquid (Chaiprapat et al., 2011). The value of k1 of 9 min-1 obtained in this work is equal to 0.15 s-1 for Cin, H2S range of 200–600 ppmv and pH 1.5, which was of the same order but slightly lower than the value of 0.54 s-1 obtained by Yang and Allen (2012) for Cin, H2S < 200 ppmv and pH 2.0–8.0. Compared to previous studies where the pH of the recycling liquid was controlled at 6.0–8.0 to avoid 15

packing acidification (Jiang and Tay, 2010; Ramirez et al., 2009; Vikromvarasiri et al., 2017), the pH of the recycling liquid in this study was not controlled, but was stabilized in the range of 1.0–2.0 by the BTF itself. Actually, Ferroplasma as a genus of the Archaea, is one of the major ferrous-iron oxidizing microbes in highly acidic environments with pH values of 0–2.0 (Golyshina and Timmis, 2005). Especially, F. acidiphilum strain YT is a chemoautotroph that grows optimally at pH 1.7 (Ferrer et al., 2007). Therefore, 85.5% of the sequence reads was affiliated with the F. acidiphilum in the acidic recycling liquid with pH 1.5 in this study.

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Moreover, it has been reported that acidic BTFs lead to a specialization of the bacterial community and the prevalence of Acidithiobacillus species (Arespacochaga et al., 2014; Charnnok et al., 2013; Li et al., 2012;

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Montebello et al., 2013; Tu et al., 2016). Similar observation was found in the case of the Bacteria that 9.5% of the sequence reads belong to the genus Acidithiobacillus, among which, the relative abundance of the

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species At. thiooxidans and At. caldus was 1.4% and 8.1%, respectively (Table 2). The integrated sulfur

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oxidation model with various sulfur oxidation pathways of At. caldus proposed by Chen et al. (2012), provides

na

a powerful illustration for the almost complete removal of H2S was achieved during all operating periods with Cin, H2S range of 200–600 ppmv (Fig. 3). Furthermore, Jin et al. (2005) reported that the BTF could be operated

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at pH values ranging from 2 to 4 without significant performance deterioration. Therefore, it was feasible not

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to control the pH of the recycling liquid. The acid BTF was more cost-effective in terms of reducing the chemical cost of maintaining the pH against acidification. Fig. 5 shows the D5 removal profile along the height of the packing in the BTF under EBRTs of 0.6, 1.5 and 2.2 min. The D5 concentrations of the gas from different outlets of the BTF were measured on days 67, 86 and 114, respectively. The D5 removal trend was linear with respect to the height of the BTF for an EBRT of 0.6 min, whereas nonlinear trends were observed in the cases of 1.5 and 2.2 min. Approximately 78–86% of 16

total removal occurred within a height of 0–0.6 m for EBRTs of 1.5–2.2 min. A similar gradient of the curve was previously obtained by Li et al. (2014) that the D4 was degraded primarily in the first 0.5 m of the packing. Generally, the removal of D5 in the BTF was mainly attributed to absorption by the recycling liquid, adsorption by the biomass, and biodegradation. In previous report (Li et al., 2014), the pH of the recycling liquid in the BTF was between 6.7 and 7.2. In our study, the pH of the recycling liquid was approximately 1.0–2.0 due to the biological oxidation of H2S. It had been already confirmed that more than 95% siloxane

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removal is possible using H2SO4 solutions (48 wt.%) at 60°C (Schweigkofler and Niessner, 2001). Similar observation was also reported by Wang et al. (2014b) that the nitrogen was transformed to the nitrate when

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the gas flowed through the BTF. Under acid conditions, the D4 was oxidized to the silanol with the nitrate as the oxidant. Moreover, the pressure drop in the BTF under EBRTs of 2.2 and 0.6 min was, respectively,

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approximately 50 and 140 Pa; meanwhile, the amount of biomass existed in the recycling liquid increased

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(Fig. S3). This indicated that biomass accumulation is more likely to occur in BTFs with shorter EBRTs, which

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promotes the adsorption of D5 from biogas.

In some literature references, Arthrobacter sp. (Sabourin et al., 1996), Proteus sp. and Klebsiella sp.

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(Rosciszewski et al., 1998), Pseudomonas sp. (Accettola et al., 2008; Li et al., 2014; Santos-Clotas et al., 2019)

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Phyllobacterium myrsinacearum (Wang et al., 2014b), and Methylibium sp. (Boada et al., 2020) have been identified for the biological removal of siloxane under neutral pH. However, the most abundant Bacteria consortium of the acid BTF in our study is Acidithiobacillus which was capable of degrading the H2S. Furthermore, previous evidence has shown that the biological removal of D4 was nearly zero under pH 2.0 and 12.0 if excluding the acid or base hydrolysis (Wang et al., 2014b). Thus, the dominant degradation mechanisms of siloxane in this acidic BTF were abiotic absorption by recycling liquid, as well as adsorption 17

by biomass.

3.3. Gas–liquid partition of D5 The effect of equilibration time on the partition of D4 and D5, between gas and liquid phases is shown in Table 3. The partition coefficients (P) of D4 between air and deionized water in this study were similar to previously reported values (Popat and Deshusses, 2008). These results confirm that the experimental setup,

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gas, and liquid sampling methods can be used to provide reliable P values of D5 between air and the tested liquids (deionized water, H2SO4 solution and recycling liquid). The P of D5 between air and deionized water

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was approximately two times lower than that of D4, indicating that D5 vapor tend to be partitioned into liquid phase (Table 3). This was consistent with their Henry’s law coefficients based on solubility and vapor pressure

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(Kochetkov et al., 2001).

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According to a previous report, as shown in Table 3, the P of D4 between air and mineral medium was

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approximately 5.2, which is higher than the value of P between air and deionized water (Popat and Deshusses, 2008). Thus, researchers concluded that mineral media with higher ionic strengths than deionized water

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increase the P of D4 (Hamelink et al., 1996). However, the P of D5 between air and deionized water stabilized

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at approximately 1.4, which was 1.3–2.8 times higher than that of D5 between air and acid liquids (H2SO4 solution and recycling liquid). This could be due to differences between the pH values of the deionized water, H2SO4 solution and recycling liquid used in this partition test. The pH of the H2SO4 solution and recycling liquid was approximately 1.5, whereas the value was 6.7 in the case of deionized water. As discussed in Section 3.2., the acid liquids (H2SO4 solution and recycling liquid) promoted the absorption of D5, which decreased the P of D5. 18

Although the pH of these acid liquids were the same, the P value of D5 between air and recycling liquid over the entire partition test was lower than that of D5 between air and H2SO4 solutions. As shown in Fig. S4, the recycling liquid of the BTF after long-term stable operation for 180 days contained some biomass. At the microcolony level, extracellular polymers contained higher molecular weight proteins and more hydrophobic substances (Wang et al., 2014a), which have strong capabilities to adsorb organic materials. This observation indicates that, except for being absorbed in the acid liquid, some D5 residues were partly adsorbed in the

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biomass flocs in the recycling liquid (Fig. S5). Moreover, approximately 1.0–2.0 h was required for D5 to reach an equilibrium state between the air and acid

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liquids (H2SO4 solutions and recycling liquid), whereas 11.0 h was required for the test between air and deionized water. The mass transfer was found to be the main factor contributing to the improvement of D4

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removal in neutral BTF (Li et al., 2014; Popat and Deshusses, 2008). Thus, the concentrations of D5 in the

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headspaces of the flasks, along with equilibration time of the partition test, were used to estimate the mass

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transfer coefficient of D5 in the BTF under extremely acidic conditions. According to the previous mass transfer model from gas to liquid (Popat and Deshusses, 2008), the mass transfer rate of D5 in the gas–liquid

d CG C   k L  G  C L  dt  P 

(7)

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VG

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interfacial region of the liquid film is expressed by Eq. (7).

where kL is the mass transfer coefficient of D5 (m·h-1); V G is the volume of headspace of the flask (150 mL); CG is the concentration (mg·m-3) of D5 in the headspace of flask; CL is the equilibrium concentration (mg·m3

) of D5 in the liquid phase of flask; and A is the interfacial area for mass transfer in the flasks during the

partition tests. Based on the geometry of the flask and shaken condition, the possible wetted area was estimated at a value of 0.005 m2. Integrating Eq. (7) from time 0 to t gives Eq. (8), in which c is a constant: 19

C G  c  exp(

k L  t )  C LP P VG

(8)

During the partition test, the adsorption of D5 on the wall of the flask also contributed to the loss of some D5. Because the adsorption equilibrium was reached relatively quickly, we did not quantify the effect of this partial loss of D5 in the measurement of kL. As shown in Fig. 6, the concentration profiles of D4 and D5 in the headspace of flask can be simulated using Eq. (8) with high degrees of fitting (R2 > 0.95), and the calculated

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parameters were shown in Table 4. Based on the total amount of D5 in the flask, the corresponding D5 recovery rate during the equilibration time was approximately 89–94% for deionized water and 62–69% for acid liquids (H2SO4 solution and recycling liquid), respectively (Fig. S6). The low D5 recovery rate was attributed to the

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hydrolysis of the D5, especially in acid liquids.

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The mass transfer coefficient of D5 between air and deionized water was 0.029 m·h-1, which was much lower

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than the value of D4, 0.085 m·h-1. This suggests that siloxanes with higher molecular weight faced greater resistance to mass transport through the gas-liquid interface into the liquid phase. As shown in Table 4,

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although the mass transfer coefficients of D5 between air and acid liquids (H2SO4 solution and recycling liquid) were much lower than that of D4 between air and deionized water, these values were much higher than that

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of D5 between air and deionized water. This indicates that the resistance to gas–liquid mass transfer decreased

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significantly in the case of acid liquids. Moreover, the mass transfer coefficient of D5 between air and recycling liquid was 0.059 m·h-1, which was higher than the value of 0.049 m·h-1 for the H2SO4 solution. This indicates that the biomass in the recycling liquid further accelerated the removal of D5 from gas. Although equilibrium of D5 between gas and liquid in the BTF was not achieved during the tested EBRTs of 0.6–2.2 min, the use of acid recycling liquid shortened the time required for D5 to reach equilibrium. Thus, it was 20

feasible to achieve simultaneous removal of siloxane and H2S directly from biogas using an aerobic BTF. 3.4. Siloxane degradation products and pathway To further elucidate the siloxane degradation mechanism in the aerobic BTF, we sampled the degradation products in the recycling liquid of the BTF on day 180, and extracted using THF and then trimethylsilylated using BSTFA. We noticed that, after derivatization by BSTFA, five more peaks could be distinguished, but only three of them were identified by GC-MS (Fig. 7). These were octamethyltrisiloxane (L3),

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decamethyltetrasiloxane (L4), and dodecamethylpentasiloxane (L5), which were derived from dimethylsilanediol (DMSD), tetramethtldisiloxane-1,3-diol, and hexamethyltrisiloxane-1,5-diol, respectively.

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The concentrations of L3, L4, and L5 were 4.43, 0.11, and 0.02 mg·L-1, which indicates that DMSD was the main degradation product of siloxane in the aerobic BTF. Similar results have been reported previously

re

(Spivack and Dorn, 1994; Xu, 1998; 1999). Spivack and Dorn (1994) investigated the hydrolysis of

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tetramethyldisiloxane-1,3-diol and hexamethyltrisiloxane-l,5-diol in aqueous solutions. They found that

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DMSD dominated the equilibrium in dilute aqueous solution. Although the rate-limiting step depended on the target siloxane, the ultimate polydimethylsiloxane degradation product in soil was water-soluble DMSD (Xu,

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1998; 1999). Moreover, the silicic acid and methanol were known as the byproducts from biological

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degradation of siloxane (Li et al., 2014). However, no detectable peak of trimethylsilylated derivative of silicic acid was observed in the chromatogram of the BSTFA derivatives. The result was exactly used as a stronger argument for the lack of biological degradation. As shown in Fig. 7, the profile of the THF extracts indicated that hexamethyldisiloxane (L2) and hexamethylcyclotrisiloxane (D3) were the main siloxanes in the cycling liquid. Their concentrations were 1.31 and 0.26 mg·L-1, respectively. Moreover, the concentration of L2 in BSTFA derivatives was 1.70 mg·L-1, 21

which was higher than the value of L2 in THF extracts. Correspondingly, the concentration of trimethylsilanol (TMSOH) was 0.39 mg·L-1, because L2 was derived from TMSOH. However, in this study, D5 and D4 were the main siloxanes, and L2 and D3 were not detected in the inlet gas of the BTF. It was previously reported that the free energy change of the transformation of D5 and D4 to TMSOH by CH4 utilization is thermodynamically favorable (Tansel and Surita, 2014). Therefore, the L2 and D3 in the recycling liquid were likely to be produced during the transformation of D5 to TMSOH.

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Based on the above discussion, we propose the following multistep process as the degradation pathway of cyclic D5 in the recycling liquid of BTF (Fig. 8). First, under acid recycling liquid conditions, cyclic D5

-p

undertook sulfate catalyzed ring-opening hydrolysis to form linear oligomeric tetramethtldisiloxane-1,3-diol and hexamethyltrisiloxane-1,5-diol. These linear silanols were further hydrolyzed into DMSD, which could

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also be polymerized reversely to produce these linear silanols. The concentrations of DMSD,

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tetramethtldisiloxane-1,3-diol and hexamethyltrisiloxane-1,5-diol in recycling liquid further indicated that the

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cleavage rate of Si-O bond through hydrolysis reactions was faster than that of reverse polymerization reactions in the acid recycling liquid. Simultaneously, with present of CH4 58%, D5 underwent a ring-

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shrinking reaction to produce D4 and D3, which further transformed into TMSOH. Self-condensation of

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degradation products containing hydroxy functions was common occurrence under acids conditions (Rücker and Kümmerer, 2015). Therefore, as the main ring-shrinking product, TMSOH was converted into the linear L2 by self-condensation. Although the hydrolysis of L2 to TMSOH occurred simultaneously, the concentrations of TMSOH and L2 indicated that self-condensation was particularly pronounced. Overall, the degradation products of cyclic D5 in the BTF were hydroxy end-blocked linear silanols, and cyclic and linear oligomers, the main compositions of which were DMSD and L2. 22

4. Conclusion The feasibility of simultaneous removal of siloxane and H2S directly from biogas was confirmed by an aerobic BTF that supports a high EC for D5 (400 mg·m-3·h-1) and H2S (26 g·m-3·h-1). Although the equilibrium between the D5 in the gas and the recycling liquid of the BTF was not achieved during the EBRTs tested (0.6– 2.2 min), the acid recycling liquid generated by the biodegradation of H2S enhanced the mass transfer of D5

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from biogas. Finally, the D5 was chemically degraded into mixtures of byproducts, such as DMSD and L2.

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Acknowledgments

We thank the staff members of sewage treatment facility T who supported us in the sampling process. A part

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of this study was supported financially by a grant for strategic international research network program for

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vitalizing brain circulation from JSPS.

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Credit Author Statement

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Yuyao Zhang: Investigation, Data curation, Writing- Original draft preparation. Kazuyuki Oshita: Conceptualization, Supervision, Writing- Reviewing and Editing Taketoshi Kusakabe: Methodology Masaki Takaoka: Supervision Yu Kawasaki: Conceptualization, Methodology Daisuke Minami: Conceptualization, Methodology Toshihiro Tanaka: Conceptualization Declaration of interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

23

References

biodegradation studies. Clean Technol. Envir. 10 (2), 211-218.

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Accettola, F., Guebitz, G.M., Schoeftner, R., 2008. Siloxane removal from biogas by biofiltration:

-p

Ajhar, M., Travesset, M., Yuce, S., Melin, T., 2010. Siloxane removal from landfill and digester gas - A technology overview. Bioresour. Technol. 101 (9), 2913-2923.

re

Appels, L., Baeyens, J., Dewil, R., 2008. Siloxane removal from biosolids by peroxidation. Energy Convers.

lP

Manage. 49 (10), 2859-2864.

na

Arespacochaga, N.D., Valderrama, C., Mesa, C., Bouchy, L., Cortina, J.L., 2014. Biogas biological desulphurisation under extremely acidic conditions for energetic valorisation in Solid Oxide Fuel Cells.

ur

Chem. Eng. J. 255, 677-685.

Jo

Boada, E., Santos-Clotas, E., Bertran S., Cabrera-Codony, A., Martín, M.J., Baneras, L., Gich, F., 2020. Potential use of Methylibium sp. as a biodegradation tool in organosilicon and volatile compounds removal for biogas upgrading. Chemosphere 240, 1-10. Cabrera-Codony, A., Gonzalez-Olmos, R., Martin, M.J., 2015. Regeneration of siloxane-exhausted activated carbon by advanced oxidation processes. J. Hazard. Mater. 285, 501-508. Cabrera-Codony, A., Montes-Moran, M.A., Sanchez-Polo, M., Martín, M.J., Gonzalez-Olmos, R., 2014. 24

Biogas Upgrading: Optimal Activated Carbon Properties for Siloxane Removal. Environ. Sci. Technol. 48 (12), 7187-7195. Cabrera-Codony, A., Santos-Clotas, E., Ania, C.O., Martín, M.J., 2018. Competitive siloxane adsorption in multicomponent gas streams for biogas upgrading. Chem. Eng. J. 344, 565-573. Caporaso, J.G., Kuczynski, J., Stombaugh, J., Bittinger, K., Bushman, F.D., Costello, E.K., Fierer, N., 2010. QIIME allows analysis of high-throughput community sequencing data. Nat. Methods.. 7 (5), 335-336.

ro of

Chaiprapat, S., Mardthing, R., Kantachote, D., Karnchanawong, S., 2011. Removal of hydrogen sulfide by complete aerobic oxidation in acidic biofiltration. Process Biochem. 46 (1), 344-352.

-p

Charnnok, B., Suksaroj, T., Boonswang, P., Chaiprapat, S., 2013. Oxidation of hydrogen sulfide in biogas using dissolved oxygen in the extreme acidic biofiltration operation. Bioresour. Technol. 131, 492-499.

re

Chen, L.X., Ren, Y.L., Lin, J.Q., Liu, X.M., Pang, X., Lin, J.Q., 2012. Acidithiobacillus caldus sulfur oxidation

na

mutant.. PLoS One 7 (9), e39470.

lP

model based on transcriptome analysis between the wild type and sulfur oxygenase reductase defective

Chen, X.Y., Vinh-Thang, H., Ramirez, A.A., Rodrigue, D., Kaliaguine, S., 2015. Membrane gas separation

ur

technologies for biogas upgrading. RSC Adv. 5 (31), 24399-24448.

Jo

Chung, Y.C., Huang, C., Tseng, C.P., 1996. Kinetics of Hydrogen Sulfide Oxidation by Immobilized Autotrophic and Heterotrophic Bacteria in Bioreactors. Biotechnol. Tech. 10 (10), 734-748. DeSantis, T.Z., Hugenholtz, P., Larsen, N., Rojas, M., Brodie, E.L., Keller, K., Huber, T., Dalevi, D., Hu, P., Andersen, G.L., 2006. Greengenes, a chimera-checked 16S rRNA gene database and workbench compatible with ARB. Appl. Environ. Microbiol. 72 (7), 5069-5072. Dewil, R., Appels, L., Baeyens, J., 2006. Energy use of biogas hampered by the presence of siloxanes. Energy 25

Convers. Manage. 47 (13-14), 1711-1722. Dewil, R., Appels, L., Baeyens, J., Buczynska, A., Van Vaeck, L., 2007. The analysis of volatile siloxanes in waste activated sludge. Talanta 74 (1), 14-19. Ferrer, M., Golyshina, O.V., Beloqui, A., Golyshin, P.N., Timmis, K.N., 2007. The cellular machinery of Ferroplasma acidiphilum is iron-protein-dominated. Nature 445 (7123), 91-94. Golyshina, O.V., Timmis, K.N., 2005. Ferroplasma and relatives, recently discovered cell wall-lacking

ro of

archaea making a living in extremely acid, heavy metal-rich environments. Environ. Microbiol. 7 (9), 1277-1288.

-p

Hamelink, J.L., Simon, P.B., Silberhorn, E.M., 1996. Henry’s Law Constant, volatilization rate, and aquatic half-life of octamethylcyclotetrasiloxane. Environ. Sci. Technol. 30, 1946-1952.

re

Jiang, T., Zhong, W., Jafari, T., Du, S., He, J., Fu, Y.J., Singh, P., Suib, S.L., 2016. Siloxane D4 adsorption by

lP

mesoporous aluminosilicates. Chem. Eng. J. 289, 356-364.

na

Jiang, X., Tay, J.H., 2010. Operational characteristics of efficient co-removal of H2S and NH3 in a horizontal biotrickling filter using exhausted carbon. J. Hazard. Mater. 176 (1-3), 638-643.

ur

Jin, Y., Veiga, M.C., Kennes, C., 2005. Autotrophic deodorization of hydrogen sulfide in a biotrickling filter.

Jo

J. Chem. Technol. Biot. 80 (9), 998-1004. Kochetkov, A., Smith, J.S., Raghunathan Ravikrishna, Valsaraj, K.T., Thibodeaux, L.J., 2001. Air-water partition constants for volatile methyl siloxanes. Environ. Toxicol. Chem. 20 (10), 2184-2188. Li, J., Ye, G., Sun, D., Sun, G., Zeng, X., Xu, J., Liang, S., 2012. Performances of two biotrickling filters in treating H2S-containing waste gases and analysis of corresponding bacterial communities by pyrosequencing. Appl. Microbiol. Biotechnol. 95 (6), 1633-1641. 26

Li, Y., Zhang, W., Xu, J., 2014. Siloxanes removal from biogas by a lab-scale biotrickling filter inoculated with Pseudomonas aeruginosa S240. J. Hazard. Mater. 275, 175-184. López, L.R., Bezerra, T., Mora, M., Lafuente, J., Gabriel, D., 2016. Influence of trickling liquid velocity and flow pattern in the improvement of oxygen transport in aerobic biotrickling filters for biogas desufurization. J. Chem. Technol. Biot. 91, 1031-1039. Montebello, A.M., Bezerra, T., Rovira, R., Rago, L., Lafuente, J., Gamisans, X., Campoy, S., Baeza, M.,

ro of

Gabriel, D., 2013. Operational aspects, pH transition and microbial shifts of a H2S desulfurizing biotrickling filter with random packing material. Chemosphere 93 (11), 2675-2682.

-p

Oshita, K., Fujime, M., Takaoka, M., Fujimori, T., Appels, L., Dewil, R., 2015. Siloxane removal and sludge

Energy Convers. Manage. 96, 384-391.

re

disintegration using thermo-alkaline treatments with air stripping prior to anaerobic sludge digestion.

lP

Oyarzún, P., Arancibia, F., Canales, C., Aroca, G.n.E., 2003. Biofiltration of high concentration of hydrogen

na

sulphide using Thiobacillus thioparus. Process Biochem. 39, 165-170. Popat, S.C., Deshusses, M.A., 2008. Biological Removal of Siloxanes from Landfill and Digester Gases:

ur

Opportunities and Challenges. Environ. Sci. Technol. 42, 8510-8515.

Jo

Ramirez, M., Gomez, J.M., Aroca, G., Cantero, D., 2009. Removal of hydrogen sulfide by immobilized Thiobacillus thioparus in a biotrickling filter packed with polyurethane foam. Bioresour. Technol. 100 (21), 4989-4995.

Rosciszewski, P., Lukasiak, J., Dorosz, A., Galinski, J., Szponar, M., 1998. Biodegradation of polyorganosiloxanes. Macromol. Symp. 130 (1), 337-346. Rücker, C., Kümmerer, K., 2015. Environmental chemistry of organosiloxanes. Chem. Rev. 115, 466-524. 27

Sabourin, C.L., Carpenter, J.C., Leib, T.K., Spivack, J.L., 1996. Biodegradation of dimethylsilanediol in soils. Appl. Environ. Microbiol. 62 (12), 4352-4360. Santos-Clotas, E., Cabrera-Codony, A., Boada, E., Gich, F., Munoz, R., Martín, M.J., 2019. Efficient removal of siloxanes and volatile organic compounds from sewage biogas by an anoxic biotrickling filter supplemented with activated carbon. Bioresour. Technol. 294 (122136). Schweigkofler, M., Niessner, R., 2001. Removal of siloxanes in biogases. J. Hazard. Mater. 83 (3), 183-196.

ro of

Spivack, J., Dorn, S.B., 1994. Hydrolysis of oligodimethylsiloxane-a,w-diols and the position of hydrolytic equilibrium. Environ. Sci. Technol. 28, 2345-2352.

-p

Tansel, B., Surita, S.C., 2014. Differences in volatile methyl siloxane (VMS) profiles in biogas from landfills and anaerobic digesters and energetics of VMS transformations. Waste Manage. 34 (11), 2271-2277.

re

Vikrant, K., Kim, K.-H., Szulejko, J.E., Pandey, S.K., Singh, R.S., Giri, B.S., Brown, R.J.C., Lee, S.H., 2017.

lP

Bio-filters for the treatment of VOCs and odors- A review. Asian J. Atmos. Environ. 11 (3), 139-152.

na

Vikromvarasiri, N., Juntranapaporn, J., Pisutpaisal, N., 2017. Performance of Paracoccus pantotrophus for H2S removal in biotrickling filter. International J. Hydrogen Energy 42 (45), 27820-27825.

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Wang, B.B., Chang, Q., Peng, D.C., Hou, Y.P., Li, H.J., Pei, L.Y., 2014a. A new classification paradigm of

Jo

extracellular polymeric substances (EPS) in activated sludge: separation and characterization of exopolymers between floc level and microcolony level. Water Res. 64, 53-60. Wang, J., Zhang, W., Xu, J., Li, Y., Xu, X., 2014b. Octamethylcyclotetrasiloxane removal using an isolated bacterial strain in the biotrickling filter. Biochem. Eng. J. 91, 46-52. Wasserbauer, R., Zadak, Z., 1990. Growth of Pseudomonas putida and P.fluorescens on silicone oils. Folia Microbiol. 35 (5), 384-393. 28

Tu,X., Li, J., Feng, R., Sun, G., Guo, J., 2016. Comparison of removal behavior of two biotrickling filters under transient condition and effect of pH on the bacterial communities. PLoS One 11 (5), e0155593. Xu, S., 1998. Hydrolysis of poly(dimethylsiloxanes) on clay minerals as influenced by exchangeable cations and moisture. Environ. Sci. Technol. 32, 3162-3168. Xu, S., 1999. Fate of Cyclic Methylsiloxanes in Solid. 1. The Degradation Pathway. Environ. Sci. Technol. 33, 603-608.

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Yang, Y., Allen, E.R., 2012. Biofiltration Control of Hydrogen Sulfide. 2. Kinetics, Biofilter Performance, and

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Maintenance. Air Waste 44 (11), 1315-1321.

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Figure Captions

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Fig. 1. Schematic diagram of the aerobic biotrickling filter (BTF).

30

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Fig. 2. Experimental process of the partition test.

31

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Fig. 3. Performance parameters of the BTF during the operating period.

32

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Fig. 4. H2S elimination profile with respect to the residence time in the BTF.

Fig. 5. Decamethylcyclopentasiloxane (D5) elimination profile from the top to the bottom of the packing in the BTF. 33

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Fig. 6. D4 and D5 profiles in the headspace of flask during the equilibration time of partition test (a and b,

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deionized water; c, H2SO4 solution; d, recycling liquid).

34

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Fig. 7. Total GC-MS chromatogram of the THF extracts and BSTFA derivatives from recycling liquid of the

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BTF on day 180.

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Fig. 8. Possible pathway for D5 and D4 degradation in the BTF.

36

Table 1 Experimental conditions tested along the biotrickling filter (BTF) operation. Period (d)

Inlet flow rate (L·min-1)

EBRT (min)

D5 (mg·m-3)

H2S (ppm)

1

0-37

29

0.6

31-42

350-400

3000-4000

20-25

2

38-68

29

0.6

20-25

200-400

1800-2000

19-26

3

69-96

12

1.5

17-25

200-250

700-1000

5-7

4

97-124

8

2.2

19-28

400-550

600-700

7-10

5

125-150

10

1.8

22-29

400-600

700-800

9-14

6

151-180

18

1.0

17-33

450-600

1000-1500

19-25

Inlet loading D5(mg·m-3·h-1) H2S(g·m-3·h-1)

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Phase

Inlet concentration

Table 3 Effect of the equilibration time on the partition coefficient of D4 and D5 between gas and liquid phases. Uncertainties reported are the standard errors. D5 Partition Coefficient

18.0±1.2

11.8±1.7

0.5

6.5±1.0

3.7±1.1

1.0

3.0±0.5

2.0±0.3

2.0

1.9±0.1

1.4±0.4

4.0

1.6±0.4

11.0 16.0

9.8±1.5

Air-Mineral medium*

11.6±0.2

-

19.3±6.4

1.4±0.0

9.8±0.4

-

16.4±3.6

0.5±0.1

8.9±1.4

10.2±0.8

9.3±1.5

0.6±0.0

4.3±1.5

3.1±0.3

5.7±1.0

1.4±0.1

0.5±0.1

4.0±0.6

2.8±0.4

5.5±0.8

1.4±0.0

1.1±0.1

-

3.1±0.9

-

-

-

-

-

-

1.8±0.1

5.2±1.0

na

lP

0.25

AirAir-Recycling Air-Deionized Deionized liquid water water*

-p

Air-H2SO4 solution

re

Equilibration Air-Deionized Time (h) water

D4 Partition Coefficient

ur

* Popat, S.C., Deshusses, M.A., 2008. Biological Removal of Siloxanes from Landfill and Digester Gases: Opportunities and Challenges. Environ. Sci. Technol. 42, 8510-8515.

Jo

Table 4 Mass transfer coefficients (kL) of D4 and D5 between gas and liquid phases. D5

D4

Siloxanes

Air-Deionized water Air-H2SO4 solution Air-Recycling liquid Air-Deionized water

pH

6.7±0.2

1.4±0.1

1.5±0.2

6.5±0.1

Equation

y=267 e-0.94t + 458

y=787 e-1.41t + 375

y=794 e-3.53t + 264

y=532 e-1.04t + 1767

kL (m·h-1)

0.029

0.049

0.059

0.085

R2

0.98

0.95

0.99

0.97

37

Table 2 Relative abundance of species with more than 0.1% of sequences of reads.

Taxonomical identity

Phylum

Class

Family

Genus

Species

re

-p

Domain

ro of

Relative abundance (%)

4.6

Ferroplasma

F. acidiphilum

85.5

Mycobacterium

M. longobardum

0.1

lP

Thermogymnomonas T. acidicola

Ferroplasmaceae

Jo

ur

na

Archaea Euryarchaeota Thermoplasmata

Bacteria Actinobacteria Actinobacteria

Mycobacteriaceae

38

At. thiooxidans

1.4

At. caldus

8.1

ro of

Proteobacteria γ-proteobacteria Acidithiobacillaceae Acidithiobacillus

Jo

ur

na

lP

re

-p

Total

39

99.7