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Remediation and Control Technologies
A serial biofiltration system for effective removal of lowconcentration nitrous oxide in oxic gas streams: mathematical modeling of reactor performance and experimental validation Hyun Yoon, Min Joon Song, Daehyun D. Kim, Fabrizio Sabba, and Sukhwan Yoon Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b05924 • Publication Date (Web): 23 Jan 2019 Downloaded from http://pubs.acs.org on January 24, 2019
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Abstract art 72x45mm (600 x 600 DPI)
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A serial biofiltration system for effective removal of low-concentration nitrous oxide in oxic
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gas streams: mathematical modeling of reactor performance and experimental validation
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Hyun Yoon1, Min Joon Song1, Daehyun D. Kim1, Fabrizio Sabba2 and Sukhwan Yoon1*
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Corresponding Author
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* Email:
[email protected] 7
1Department
of Civil and Environmental Engineering, KAIST, Daejeon, 34141, Korea
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2Department
of Civil and Environmental Engineering, Northwestern University, Evanston,
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IL, 60208, USA
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ABSTRACT
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Wastewater treatment plants (WWTPs) are among the major anthropogenic sources of N2O, a
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major greenhouse gas and ozone-depleting agent. We recently devised a zero-energy zero-
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carbon biofiltration system easily applicable to activated sludge-type WWTPs and performed
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lab-scale proof-of-concept experiments. The major drawback of the system was the
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diminished performance observed when fully oxic gas streams were treated. Here, a serial
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biofiltration system was tested as a potential improvement. A laboratory system with three
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serially-positioned biofilters, each receiving a separate feed of artificial wastewater, was fed
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N2O-containing gas streams of varied flowrates (200-2000 mL·min-1) and O2 concentrations
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(0-21%). Use of the serial setup substantially improved the reactor performance. Fed fully
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oxic gas at a flowrate of 1000 mL·min-1, the system removed N2O at an elimination capacity
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of 0.402±0.009 g N2O·m-3·h-1 (52.5% removal), which was approximately 2.4-fold higher
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than that achieved with a single biofilter, 0.171±0.024 g N2O·m-3·h-1. These data were used
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to validate the mathematical model developed to estimate the performance of the N2O
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biofiltration system. The Nash-Sutcliffe efficiency indices ranged from 0.78 to 0.93,
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confirming high predictability, and the model provided mechanistic insights into aerobic N2O
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removal and the performance enhancement achieved with the serial configuration.
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Keywords: Nitrous oxide; biofiltration; wastewater treatment plants; nitrous oxide reductase;
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biofilm modeling
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INTRODUCTION
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Biological nitrogen removal (BNR) is an integral component of modern wastewater treatment
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plants (WWTP). In the most typical form of modern WWTPs, i.e., activated sludge
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bioreactors in A2O (anaerobic/anoxic/oxic) configuration, NH4+ in the wastewater influent is
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biologically oxidized to NO3- in the oxic tank via nitrification and NO3- recycled to the
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anoxic tank is reduced to N2 via denitrification.1 Both nitrification and denitrification of the
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BNR process contribute to emissions of N2O, a potent greenhouse gas with approximately
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300-fold higher global warming potential per molecule than CO2 and the most influential
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ozone-depletion agent.2-4 The relative contributions of the two biological processes to the net
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N2O emission from the activated sludge process are largely unknown; however, in situ
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measurements of N2O fluxes from the anoxic, anaerobic, and oxic tanks have repeatedly
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confirmed that release of produced N2O occurs predominantly in the oxic tank due to the
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stripping effect.2,5,6 Thus, N2O is emitted in relatively low concentrations (usually 99.999%
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N2 gas and >99.999% air, each carrying 100 ppmv N2O (Deokyang Gas Co., Ulsan, Korea), at
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different ratios in a 5-L glass bottle that served as an antechamber to the biofilter A. N2O-
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containing N2 gas (no O2) was passed through the biofilters at the flowrate of 600 mL‧min-1
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during inoculation and the initial phase of biofiltration operation (for 330 hours; designated as
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phase 0) to ensure that N2O-reducing biofilms were sufficiently established. The gas flow rate
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was gradually increased to 2000 mL‧min-1 over the next 150 hours (phase 1 to 6). After the
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anoxic operation, biofiltration system was operated with varying flowrates (200-2000 mL‧min-
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1)
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was sustained for at least 5X EBRT (empty bed retention time), to ensure that O2 and N2O
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concentration measurements were taken after pseudo-steady state was established. Upon
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completion of the experiment (at t = 580 h), biofilters were dismantled and three sets of foams
and O2 concentrations (5-21%) for the next 430 hours (phases 7-29) (Table 1). Each phase
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were collected from the top, middle, and bottom thirds from each biofilter. The foams were
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stored separately at -20 ºC until use.
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Table 1. Description of the operating conditions of the lab-scale serial biofiltration system O2 mixing ratio (%)
Gas flowrate EBRT (mL∙min-1) (min) 200 400 600 1000 2000 158
a
80.5 40.3 26.9 16.1 8.1
0
5
10
15
21
1a, 6 2 3 4 5
7, 12 8 9 10 11
13, 18 14 15 16 17
19, 24 20 21 22 23
25 26 27 28 29
These numbers (1-29) correspond to the phase numbers in the header of Figure 2.
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Analytical procedures. The N2O concentrations were measured with HP6890 Series gas
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chromatograph equipped with an HP-PLOT/Q column (30 m x 0.320 mm inner diameter and
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20.00 μm film thickness) and an electron capture detector (Agilent, Palo Alto, CA) with the
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temperatures of injector, oven and detector set to 200, 85, and 250 ºC, respectively.12 Helium
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(>99.999% purity) and 95% Ar/5% CH4 mixed gas (Deokyang Gas Co., Ulsan, Korea) were
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used as the carrier gas and the make-up gas, respectively. A 1700-series gastight syringe
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(Hamilton Company, Reno, NV) was used to collect gas samples from the sampling ports at
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the inlets and outlets of the biofilters and manually inject the samples into the chromatograph.
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At each time point, triplicate measurements were taken with three-minute intervals. The
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removal efficiencies ( RE (%)
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the system inlet and the system outlet, respectively) and elimination capacities
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( EC (g N 2O m 3 h 1 )
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flowrate) were calculated with the measured N2O concentrations. The O2 concentrations in the
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biofilters were measured with a FireSting-O2 oxygen meter (Pyroscience, Aachen, Germany).
Cin Cout 100 , where Cin and Cout are N2O concentrations at Cin
Cin Cout Qgas , where V is the bed volume and Qgas is the gas V
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Analyses of the microbial community compositions. The biofilm samples for microbial
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community analyses were processed as previously described.8 The foams previously stored in
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50-mL plastic tubes at -20°C freezer were thawed on ice and 30 mL sterile double-distilled
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water (>18 MΩ·cm) was pipetted into the tubes. To detach the biofilms from the foams, tubes
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were vortexed for 5 minutes and sonicated for 10 minutes. Two-milliliter aliquots of the cell
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suspension were pipetted into autoclaved nuclease-free 2-mL Eppendorf tubes. After
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centrifuging at 10,000 x g, DNA was extracted from the cell pellets using DNeasy PowerSoil
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DNA Isolation kit (QIAGEN, Hilden, Germany). Amplicon sequencing targeting the
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hypervariable V6-8 region of the 16S rRNA gene (amplified with 926F: 5’-
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AAACTYAAAKGAATTGRCGG-3’ and 1392R: 5’-ACGGGCGGTGTGTRC-3’ primer set)
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was outsourced to Macrogen Inc. (Seoul, Korea), where an Illumina MiSeq platform was used
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for acquisition of sequence data. The sequences from three subsections of each biofilter were
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analyzed separately first and combined for further analyses. The raw sequence datasets have
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been deposited at the NCBI SRA database (http://www.ncbi.nlm.nih.gov/sra) and are
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accessible to the public (accession number: SRP150324). The raw sequence data from the
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previous study were also included in the analysis.
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The raw sequence datasets were processed using the QIIME pipeline v 1.9.1 with options set
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to default values.19 After quality trimming, the filtered reads were assigned into operational
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taxonomic units (OTUs) in the Greengenes v13.8 database using the USEARCH algorithm.20
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The reads with no matching sequences in the database were clustered de novo into new OTUs.21
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In either case, a cutoff value of 97% was used for OTU assignments. The new OTUs were
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assigned to taxa using RDP classifer against the Greengenes database and unclassified reads
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were discarded. Hierarchical clustering of the microbial community profiles was performed
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using the hclust command of the R package “stats”.
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Mathematical model development. A mathematical model was developed for estimation of
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biofiltration performance for N2O removal from gas streams with varying O2 concentrations
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and flowrate (thus, varying EBRT). Each biofilter was modeled as a fully-mixed reactor despite
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the longitudinal flow direction, as the longitudinal N2O concentration profiles indicated near-
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complete mixing of the gas phase due to the long EBRTs (8.05-80.50 minutes).8 The
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constricted connections between the biofilters and resulting high linear velocity ensured
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negligible mixing between the biofilters. The model was developed with well-founded
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assumptions based on reliable reference and experimental observations that 1) N2O reduction
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only occurs in the absence of dissolved O2 (
