Removal of Nitrogen Oxide Based on Anammox through Fe (II) EDTA

Jun 1, 2017 - State Key Laboratory of Coal Mine Disaster Dynamics and Control, Chongqing University, Chongqing 400044, P. R. China. ‡. Department of...
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Removal of Nitrogen Oxide Based on Anammox through Fe(II)EDTA Absorption Daijun Zhang,*,†,‡ Lulu Ren,‡ Zongbao Yao,‡,§ Xinyu Wan,‡ Peili Lu,†,‡ and Xiaoting Zhang‡ †

State Key Laboratory of Coal Mine Disaster Dynamics and Control, Chongqing University, Chongqing 400044, P. R. China Department of Environmental Science, Chongqing University, Chongqing 400044, P. R. China § State Key Laboratory of Lake Science and Environment, Nanjing Institute of Geography and Limnology, Chinese Academy of Sciences, Nanjing 210008, China ‡

S Supporting Information *

ABSTRACT: Nitrogen oxides in flue gas cause considerable environmental and health problems. In this study, the NO removal through Fe(II)EDTA absorption based anaerobic ammonium oxidation (Anammox) reduction was first evaluated in batch tests. The removal efficiency and removal rate of NO reached 74.5%−85.4% and 1.62−15.48 μmol NO gVSS−1 h−1 with Fe(II)EDTA absorption, respectively. The optimal temperature and pH for Fe(II)EDTA absorption based Anammox reduction of NO was 30 °C and 7.0 in the range of temperature from 25 to 45 °C and pH from 6.5 to 8.0, respectively. The maximum specific substrate utilization rate and half-saturation constant of Fe(II)EDTA-NO were experimentally determined as 0.09 mmol N gVSS−1 h−1 and 1.338 mM, respectively. Then, a sequencing batch reactor (SBR1) performing Fe(II)EDTA absorption based Anammox reduction for NO removal was continuously operated for 35 days. The NO removal performance of SBR1 decreased gradually, and the NO removal efficiency and NO removal rate decreased from 86.65% and 17.26 μmol gVSS−1 h−1 to 43.64% and 12.75 μmol gVSS−1 h−1, respectively. Meanwhile, the biomass, the diversity, and abundance of the bacterial community continually decreased in the sludge of SBR1, indicating that the reactor could not stably run when the Fe(II)EDTA-NO concentration was 0.09 mM. However, with improving Fe(II)EDTA-NO concentration and daily addition of a little biomass from an expanded granular sludge blanket reactor (EGSB) Anammox reactor, another sequencing batch reactor (SBR2) had stably run for 55 days. The average NO removal efficiency and NO removal rate reached 85.04% and 3.97 mM NO d−1 (or 106.15 μmol gVSS−1 h−1), respectively. This is very interesting to ammonia based wet flue gas desulfurization, as the produced wastewater containing (NH4)2SO3 could be purified by Anammox to produce enough biomass. on temperature and toxic nitrite concentrations.10,11 Kartal et al.9 reported that there was hardly any detectable N2O in the effluent gas stream with the influent NO concentration increased to 3500 ppm. Furthermore, the appropriate ferrous ethylene diamine tetra acetic acid (Fe(II)EDTA) could enhance Anammox due to accelerating the growth of anaerobic ammonium oxidation bacteria (AnAOB).12 Therefore, a Fe(II)EDTA absorption-Anammox integrated process can be used, in theory, for NO removal. This study aims to investigate the NO removal properties of a Fe(II)EDTA-absorption based Anammox-reduction process, not considering the impacts of oxygen and sulfur dioxide in flue gas. The NO removal rates at different physicochemical conditions were investigated, and the relevant kinetics were studied. Two gastight SBRs with and without addition of exogenous Anammox sludge, respectively, were operated for long periods to study the stability of NO removal through Fe(II)EDTA absorption based Anammox reduction. The bacterial community structure in SBR1 without addition of exogenous Anammox sludge was characterized.

1. INTRODUCTION The emission of NO into the atmosphere is a considerable environmental and health problem. As well-established and commonly used technologies to remove NO from industrial flue gas, selective catalytic reduction (SCR) and noncatalytic reduction (SNCR) suffer from prohibitive cost and the risk of secondary pollutants production.1,2 Some biotechnologies have also been developed to remove NO from industrial flue gas,3 of which the chemical absorption based biological reduction is a promising approach. A chemical absorption−denitrification integrated NO removal process was suggested to absorb the NO with Fe(II)EDTA and subsequently reduce it through denitrification, of which the NO removal efficiency is 88%.4 However, in this process, an extra organic carbon source is needed as electron donor,4 and nitrous oxide (N2O), as an intermediate, would emit. Nitrogen shortcut technologies by anammox organism-like biofilms and autotrophic bacteria offer significant cost savings over traditional biological nitrogen removal and have apparent advantages to treat wastewater.5,6 As an intermediate of Anammox, NO can be reduced to dinitrogen (N2) together with ammonium (NH4+).7 It was demonstrated that Candidatus Brocadia anammoxidans could tolerate up to 600 ppm (approximately 1 mg of NO day−1 NO load)8 or even up to 3500 ppm of NO.9 Biological treatment for nitrites by anammox organism in water treatment systems was dependent © XXXX American Chemical Society

Received: January 11, 2017 Revised: June 1, 2017

A

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2. MATERIALS AND METHODS

Table 2. Batch Tests To Investigate the Effect of Temperature and pH

2.1. Chemicals. Na 2 EDTA(99%), FeSO 4 ·7H 2 O(99%), (NH4)2SO4, EDTA, H3BO4, MnCl2·4H2O, CuSO4·5H2O, ZnSO4· 7H2O, NiCl2·6H2O, Na2SeO4·10H2O, Na2MoO4·2H2O, and Na2WO4· 2H2O were obtained from Chongqing Chuandong Chemical Co., Ltd., China. NO-containing mixture gas (with Ar) was provided by Southwest Chemical Research and Design Institute, China. 2.2. Batch Tests. 2.2.1. Effect of Fe(II)EDTA Absorption, Temperature, and pH. A group of batch tests were conducted to investigate the effect of the Fe(II)EDTA on the absorption. The sludge with the specific Anammox activity (SAA) of 455 mg N gVSS−1 d−1 was taken from an Anammox SBR,13 washed three times with a nutrient solution, and placed in a gastight batch reactor of 500 mL working volume. Synthetic wastewater containing NH4+-N 75 mg/L (added in (NH4)2SO4) and 0.36 mM Fe(II)EDTA or no Fe(II)EDTA was used as influent (Table 1). The NO-containing mixture gas (with

NO

NO

NH4+-N

Fe(II)EDTA

Batch tests

ppm

mg L−1

mM

1

50 100 50 75 100 150 250 500 50 75 100 150 250 500

75 75 75 75 75 75 75 75 75 75 75 75

2

3

NH4+-N −1

Temperature

Batch tests

Number

ppm

mM

mg L

°C

pH

The effect of temperature

1 2 3 4 5 6 7 8 9

500 500 500 500 500 500 500 500 500

0.36 0.36 0.36 0.36 0.36 0.36 0.36 0.36 0.36

40 40 40 40 40 40 40 40 40

25 30 35 40 45 30 30 30 30

7 7 7 7 7 6.5 7.0 7.5 8.0

The effect of pH

Table 1. Batch Tests To Investigate the Effect of Fe(II)EDTA Absorption

Fe(II) EDTA

nutrient solution and placed in a gastight butyl lithium vial with an effective volume of 1000 mL. The initial concentrations of 1.0 mM Fe(II)EDTA-NO and 50 mg L−1 NH4+-N were set up. The mineral components were the same as mentioned above. The temperature and pH were kept as 30 ± 1 °C and 7.5, respectively. During the 10-h experiment, the samples were collected every 0.5 h for chemical analysis. The multisubstrate kinetic model for NO Anammox reduction (eq 1)14 was established and calibrated with AQUASIM software.

qNO = qNOmax ·

SNH4+

SNO · KNH4+ + SNH4+ KNO + SNO

(1)

where qNO is the specific substrate utilization rate (mmol N L−1 gVSS−1 h−1), qNOmax is the maximum substrate utilization rate (mmol N L−1 gVSS−1 h−1), S is the substrate concentration (mmol N L−1), KNH4+ is the half-saturation constant of ammonium (mmol N L−1), and qNO is the half-saturation constant of NO. The concentrations of Fe(II)EDTA-NO and ammonium from the batch tests were used as input data. The half-saturation constant of NH4+ (16.64 mg N L−1) was taken from our previous report.15 2.3. Long-Term Operation of SBRs Performing NO Removal through Fe(II)EDTA Absorption Based Anammox Reduction. Two gastight SBRs with working volume of 3 L were used to perform NO removal through Fe(II)EDTA absorption based Anammox reduction with and without addition of exogenous Anammox sludge, respectively. The NO concentration of the inflow and outflow of these two reactors was tested to calculate the removal rate and efficiency (eq 2 and eq 3). The removal rate was calculated with the equation below:

0.36 0.36 0.36 0.36 0.36 0.36

Ar) was then aerated into the gastight batch reactor at 40 mL min−1. Fe(II)EDTA was prepared with Na2EDTA and FeSO4·7H2O (1:1). The mineral matter was dosed additionally as follows (g L−1): NaH2PO4 0.05, CaCl2·2H2O 0.3, MgSO4·7H2O 0.3, KHCO3 1.25, FeSO4 0.00625, EDTA 0.00625, and 1.25 mL L−1 of microamount element solution. The microamount element solution consisted of (g L−1): EDTA 15, H3BO4 0.014, MnCl2·4H2O 0.99, CuSO4·5H2O 0.25, ZnSO4 ·7H 2 O 0.43, NiCl 2 ·6H 2 O 0.19, Na 2 SeO 4 ·10H2 O 0.21, Na2MoO4·2H2O 0.22, and Na2WO4·2H2O 0.050. HCl (1 mol/L) or NaOH (1 mol/L) was used to adjust the pH to 7.5. The wall of the gastight reactor contains a hollow interlayer. Warm water circulated between the interlayer and a temperature-controlled water bath (HH series, Shanghai Jiangxing Instrument Co., Ltd., China.) to keep the temperature of the reactor at 30 ± 1 °C. The stirring frequency of the reactor was controlled at 200 rpm by a magnetic stirrer. DO and pH were measured with a dissolved oxygen analyzer (JPBJ-608, Shanghai Jingke Industry Co., Ltd., China) and a pH electrode (pH2100e, METTLER TOLEDO) installed in the reactor. The anaerobic environment was obtained by flushing N2 (99.999%) into the headspace of the gastight reactor continually. Samples were collected every 0.5 h during the experiment for chemical analysis. Another two batch tests were conducted to study the effects of different temperatures of 25 to 45 °C and pH values of 6.5 to 8.0 (Table 2). The 500 ppm of the NO-containing mixture gas (with Ar) was aerated into the batch reactor at 40 mL min−1. Synthetic wastewater containing 0.36 mM Fe(II)EDTA, 40 NH4+-N mg/L, and the mineral matter was used as influent. The sludge source and composition of themineral matter were the same as aforementioned. 2.2.2. Kinetics of Fe(II)EDTA-NO Anammox Reduction. The sludge taken from the Anammox SBR13 was washed three times with a

RNO = or

(Q input − Q output )NtCNO , input VmV

RNO = (CNO , influent − CNO , effluent )N

(2)

−1

where RNO is the NO removal rate, mMd ; Qinput is the air flow rate of the input gas, m3/min; N is the number of cycles per day, d−1; t is the aeration time per cycle, min; CNO,input is the NO concentration of the input gas, ppm; Vm is the molar volume, L/mol; V is the volume of the reactor, L; CNO,ef fluent is the NO concentration of the influent, mM; CNO,ef fluent is the NO concentration of the effluent, mM. The removal efficiency was calculated with the equation below: ENO = or

CNO , input − CNO , output

ENO =

CNO , input

× 100%

CNO , influent − CNO , effluent CNO , influent

× 100% (3)

where ENO is the NO removal efficiency, %; CNO,input is the NO concentration of the input gas, ppm; CNO,output is the NO concentration of the output gas, ppm. B

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Figure 1. Profiles of NO in the control tests without NH4+ and Fe(II)EDTA addition. 2.3.1. Long-Term Operation without Addition of Exogenous Anammox Sludge. SBR1 without addition of exogenous Anammox sludge was operated for 35 days. The reactor was operated with a 240 min cycle: began with 10 min feeding with synthetic water; simultaneously, the NO-containing mixture gas (with Ar) aerated into the reactor for 210 min for NO removal; and then followed with 20 min of settling and 10 min of draining. The flow rate and the concentration of the aerated gas were 40 mL/min and 500 ppm, respectively. The sludge with the SAA of 455 mg N g of VSS−1 d−1 from another Anammox SBR13 was inoculated in the SBR1 reactor with the initial sludge concentration of 1.89 g of TSS L−1. The concentrations of Fe(II)EDTA and NH4+ in the influent were 0.09 mM and 50 mg L−1, respectively. The remaining components and the operation conditions were the same as those in the batch tests. Two sludge samples (at day 1 and day 35) were collected from the reactor to follow the variation of microbial community structure by high throughout sequencing (Sangon Biotech (Shanghai) Co., Ltd.). 2.3.2. Long-Term Operation with Addition of Exogenous Anammox Sludge. In order to improve the NO removal rate, SBR2 with addition of exogenous Anammox sludge was operated for 55 days. The reactor was operated with a 240 min cycle: began with 10 min feeding with the effluent from a separated 3 L scrubber;

simultaneously, the SBR2(3L) started to stir and react for 210 min for NO removal; and then followed with 20 min of settling and 10 min of draining. The separated 3 L scrubber was operated in batch mode at a gas flow rate of 150−200 mL min−1, in which 5000 ppm of NOcontaining mixture gas (with Ar) was chemically adsorbed in a solution of 2 mM Fe(II)EDTA. The inoculated sludge with the SAA of 3.53 g N g of VSS−1 d−1 was taken from a long-term operated Anammox EGSB reactor, which had been operated for more than 240 days with a total nitrogen removal rate of 6 kg N m−3 d −1. In addition, 100 mL of exogenous Anammox sludge (MLSS 2.24 g L−1, MLVSS 1.70 g L−1) from the Anammox EGSB reactor was added into SBR2 every day. 2.4. High Throughout Sequencing and Analysis of the Microbial Community. 2.4.1. DNA Extraction. The collected Anammox sludge sample (0.1 g) was placed in a sterile Eppendorf vial, and 1.5 mL of DNA lysis buffer was dripped into the vial. DNA extraction was performed by using a Soil DNA Extraction Kit (OMEGA BIO-TEK, Norcross, GA, USA) following the instructions described by Zhou et al. (1996).16 Subsequently, 1% agarose electrophoresis (EP) was used to test the obtained DNA sample. A microultraviolet spectrophotometer was used to determine the concentration and purity of the sample. The DNA was used for C

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Energy & Fuels functional genes amplification and quantification, and Miseq high throughout sequencing and analysis. 2.4.2. High Throughout Sequencing and Analysis of the Microbial Community. The purified DNA from each sample was used as template for amplification. The V4−V5 hypervariable regions of bacterial 16S rDNAs were amplified using the primer set of the primer-pairs 515F (GTGCCAGCMGCCGCGG) and 907R(CCGTCAATTCMTTTRAGTTT).17 The length of the amplified fragment was 411bp. The thermal profile consisted of 1 cycle of 3 min at 94 °C, followed by 5 cycles of 30 s at 94 °C, 20 s at 45 °C, 30 s at 65 °C, and 20 cycles of 20 s at 94 °C, 20 s at 55 °C, 30 s at 72 °C, and 1 cycle of 5 min at 72 °C. Then a unique barcode was used to sort multiple samples in each Illumina sequencing run. The 16S rDNA V4−V5 fragment of each sample was amplified with a barcoded primer, the second thermal profile consisted of 1 cycle of 3 min at 95 °C, followed by 5 cycles of 30 s at 95 °C, 20 s at 55 °C, 30 s at 72 °C, and 1 cycle of 20 s at 72 °C. The PCR amplification systems are presented in Tables S3 and S4 (shown in the Supporting Information). All PCR products of the samples were pooled as one sample for PE sequencing using the Illumina sequencing instrument according to the Barcoded Illumina Paired End Sequencing (BIPES) procedure. After sequencing, the PE reads were overlapped to construct full-length V4− V5 fragments, which were further separated into their original samples according to the barcode sequences. All raw reads have been deposited into the National Center for Biotechnology Information(NCBI) Sequence Read Archive (SRA) database (http://www.ncbi.nlm.nih. gov/sra/) with an accession number of SRR5383653. All sequences that contained one or more ambiguous reads (N’s), those that did not have a recognizable reverse primer sequence, and those that contained more than six errors in the primers were removed. Qualified sequences were clustered into operational taxonomic units (OTUs) using uparse (version 7.1) by setting 3% and 5% distance levels, respectively. Rarefaction curves and Alpha diversity were generated using mother (version v.1.30.1) for each sample. The sequences were taxonomically classified using the RDP Classifier (version 2.7) with a set confidence threshold of 70%. After phylogenetic allocation of the sequences down to the phylum and genus levels, the relative abundance of a given phylogenetic group was set as the number of sequences affiliated with that group divided by the total number of sequences per sample. 2.5. Chemical Analysis. Consistent with APHA (2005) protocols,18 NH4+ and NO2− were analyzed using colorimetric methods, and NO3− was measured spectrophotometrically. Mixed liquid suspended solids (MLSS) and mixed liquor volatile suspended solids (MLVSS) were measured using the standard methods.18 The concentration of ferrous ions and total iron in solution was determined by a modified 1,10-phenanthroline colorimetric method at 510 nm with a spectrophotometer (model T6-1650F, PERSEE (Beijing) Instruments Co., Ltd.). Fe(III) concentrations were calculated from the difference between total Fe and Fe(II).4,19 The concentration of Fe(II)EDTA-NO was measured at 420 nm.20 The concentration of NO was determined using a chemiluminescent NOx analyzer (Thermo, model 42i-HL). The N2H4 concentration was measured using the spectrophotometric method,21 and the interference of NO2− was eliminated by adding 0.5% sulfuric acid.22

Figure 2. Effect of Fe(II)EDTA absorption on NO removal.

The NO removal rate achieved at 500 ppm of NO in the batch tests without Fe(II)EDTA addition was not significantly higher (p-value > 0.05) than that at 75 ppm, 100 ppm, 150 ppm, or 250 ppm and was significantly higher (p-value < 0.05) than that at 50 ppm, as shown in Table S1 (shown in the Supporting Information). The NO removal rate achieved at 500 ppm of NO in the batch tests with Fe(II)EDTA addition was significantly higher (p-value < 0.005) than that at other NO concentrations, as shown in Table S1 (shown in the Supporting Information). Figure 3 shows the result of the NO removal rate at different temperatures. It can be seen that the maximum NO removal

Figure 3. NO removal rate at different temperatures.

3. RESULTS 3.1. Effect of Fe(II)EDTA Absorption, Temperature, and pH. The concentrations of NO were kept constant in the control tests without NH4+ and Fe(II)EDTA addition (Figure 1), indicating very low solubility of NO in water. The effect of Fe(II)EDTA absorption on NO removal by Anammox is shown in Figure 2. The NO removal rates were quite low with NH4+ (75 mg N L−1) and no Fe(II)EDTA addition. However, the NO removal rates sharply increased when 0.36 mM Fe(II)EDTA was added. These results indicate that Fe(II)EDTA absorption enhanced the solubility of NO, thereby increasing the NO removal rate by Anammox.

rate was obtained at 30 °C, suggesting that the optimal temperature for NO removal is 30 °C in the range of temperature from 25 to 45 °C. the NO removal rate achieved at the optimal temperature (30 °C) was not significantly higher (p-value > 0.05) than that at 25 °C, 35 °C, or 40 °C and significantly higher (p-value < 0.05) than that at 45 °C, as shown in Table S2 (shown in the Supporting Information). Figure 4 reveals that the maximum NO removal rate was achieved at pH 7.0, manifesting the optimal pH for NO removal is 7.0 with the pH range from 6.5 to 8.0. The NO removal rate achieved at the optimal pH (7.0) was not D

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In order to improve the NO removal rate, the SBR2 performing the Fe(II)EDTA absorption based Anammox reduction of NO with addition of exogenous high-activity Anammox sludge ran stably for 55 d (Figure 7). In the first 28 days, the input Fe(II)EDTA-NO concentration was about 0.5 mM, and the average NO removal rate was 2.56 mM NO d−1. In the last 27 days, the input Fe(II)EDTA-NO concentration increased to about 0.8 mM. The average NO removal efficiency, NO removal rate, and TN removal rate reached 85.04%, 3.97 mM NO d−1(or 106.15 μmol of N g of VSS−1 h−1), and 69.22 g N m−3 d−1, respectively. The maximum NO removal efficiency and NO removal rate reached 86.82% and 4.18 mM NO d−1, respectively. 3.4. Structure of Bacterial Community in the Sludge. The MiSeq high throughout sequencing results of the sludge from the SBR1 (Figure 8) showed that the coverages of B1 (sludge of day 1) and B2 (sludge of day 35) were 0.98 and 0.96, indicating that samples B1 and B2 can represent the microbial community structure of the sludge. The most abundant phyla in the two samples were Proteobacteria (accounting for 48.06− 49.13% of the total reads), Chlorof lexi (23.99−25.65%), and Planctomycetes (16.81−25.82%). These three phyla accounted for more than 90% of the total reads in the two samples. Other dominant phyla accounted for 6.30−8.38% of the total reads, including Firmicutes, Chlorobi, Gemmatimonadetes, Acidobacteria, Armatimonadetes, and Bacteroidetes. The sequence number of B2 was only 42.04% of B1. The lower ACE and Chao of B2 than B1 showed the lower bacteria abundance of B2. Also, the Shannon−Wiener and Simpson’s diversity showed that the diversity of B2 was lower than B1. The sequence number of Proteobacteria, Chlorof lexi, Planctomycetes, and Chlorobi decreased by 62.50%, 65.69%, 35.42%, and 28.07%, respectively. In the two samples, Candidatus Kuenenia genus dominated in the AnAOB of phylum Planctomycetes. In the sample B2, although the reads of Ca. Kuenenia genus decreased by 31.95%, the relative abundance of Ca. Kuenenia increased by 62.19%.

Figure 4. NO removal rate at different pH values.

significantly higher (p-value > 0.05) than at other pH values, as shown in Table S2 (shown in the Supporting Information). 3.2. Kinetics of Fe(II)EDTA-NO Anammox Reduction. The experimental data of the batch tests were fitted using eq 1,14 and the maximum specific substrate utilization rate and half-saturation constant of NO were 0.09 mmol N gVSS−1 h−1 and 1.338 mM, respectively. The results were presented in Figure 5. The high correlation coefficient (0.99) indicated that eq 1 accurately described the removal of NO in the Fe(II)EDTA absorption based Anammox reduction.

4. DISCUSSION In the NO removal process of this study, NO was first absorbed into aqueous solution in the form of Fe(II)EDTA-NO and reduced to the intermediate hydrazine (N2H4) by the hydrazine Synthase (HZS) with NH4+ as electron donor, and N2H4 was then subsequently oxidized by the hydrazine dehydrogenase (HDH) to the end product of N2.23 Maybe there is another possible coexisting reaction mechanism to Fe2+ oxidation and NO reduction to N2 mediated by AnAOB, which is related to nitrate-dependent Fe2+ oxidation by AnAOB.24 Because nitrate reduction to N2 must go through the intermediate NO, Fe2+ has the capability to reduce NO. This is consistent with the result that no Fe(II)EDTA-NO and N2H4 were detected after the batch tests while the NO was removed. The physiological state of the AnAOB can be affected by temperature and pH.25 Low temperatures (19 °C) and high pH (9.8−10.5) appear to favor the absorption of NO by Fe(II)EDTA.26 In contrast, AnAOB is mesophilic and pH neutral. As the absorption of NO by Fe(II)EDTA is an instantaneous reaction,27 temperature and pH change the activity of AnAOB so as to influence the NO removal rate in the Fe(II)EDTA absorption based Anammox reduction process. Therefore, the optimal temperature and pH of NO removal depend on the optimal temperature and pH for growth of AnAOB.

Figure 5. Kinetic of Fe(II)EDTA-NO Anammox reduction.

3.3. Long-Term Operation of the SBR. The SBR1 performing the Fe(II)EDTA absorption based Anammox reduction of NO without addition of exogenous Anammox sludge was operated for 3 5d (Figure 6). The NO concentration in the off-gas increased from 67.12 to 282.26 ppm. The NO removal efficiency and NO removal rate decreased from 86.65% and 17.26 μmol g of VSS−1 h−1 to 43.64% and 12.75 μmol g of VSS−1 h−1, respectively. In addition, the MLSS and MLVSS of the reactor decreased from 1.89 g L−1 and 1.55 g L−1 to 1.01 g L−1 and 0.82 g L−1, respectively. E

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Figure 6. Performance of SBR1 without addition of exogenous Anammox sludge.

In order to mitigate the negative effect on the AnAOB by high concentration of Fe(II)28 and Fe(II)-NO,29 during the long-term operation of SBR1 without addition of exogenous Anammox sludge, the Fe(II)EDTA was controlled at a very low concentration (0.09 mM). The maximum NO removal rate of the reactor was 0.37 mM d−1, much lower than that (1.3 mM) reported in a previous study.29 NO was first absorbed and chelated with Fe(II)EDTA and subsequently reduced by anammox bacteria. The absorption ratio of Fe(II)EDTA to NO is 1:1. As the Fe(II)EDTA concentration used in this study(0.09 mM) is lower than theirs (3.5 mM),29 the absorption of NO is much less, leading to the poor NO removal rate. The MLVSS in the reactor decreased by 47%, and the NO removal rate decreased by 26%. These results indicated that the reactor could not run stably. Because of the limitation of Fe(II)EDTA in the reactor, the concentration of soluble NO in the form of Fe(II)EDTA-NO and bioavailable to AnAOB was much lower than the half-saturation constant of NO (1.338 mM), which could not support the biomass growth. This was reasonable because nitrite was not dosed in the reactor to avoid the oxidation of Fe(II)EDTA by nitrite and the decrease of NO absorption efficiency. In the Anammox metabolism, the oxidation of nitrite to nitrate is needed to provide the electrons for cell carbon synthesis.30

As a low concentration Fe(II)EDTA-NO was used as electron acceptor, the growth of AnAOB was limited. Although AnAOB can hardly grow at low concentration of Fe(II)EDTANO, Fe(II)EDTA absorption based Anammox reduction still has potential to remove NO from flue gas. AnAOB has a relatively low endogenous decay coefficient.31 In this study, the determined decay rate of AnAOB biomass in SBR1 without addition of exogenous Anammox sludge was 0.009 d−1, very close to that reported by Zhang et al.31 and significantly lower than its maximum biomass growth rate (0.06 d−1).15 The lost capacity of Anammox could be supplemented by addition of exogenous Anammox sludge, as this method was used to strengthen the resistance of AnAOB to the toxicity of organic matter.32 In this study, the SBR2 with exogenous addition of Anammox biomass ran stably for 55 d at an average NO removal rate of 3.97 mM NO d−1. Whether the anammox reactor can operate stably is related to the toxicity of NO, Fe, and EDTA,33 and may also be related to the type of reactor. Biofilms are probably more resistant to toxic substances. Although the reactor can be operated stably with exogenous addition of Anammox biomass, this also increased the difficulty of operating the reactor, such as sludge concentration and sludge retention time control. Therefore, further investigation should be conducted to find more sustainable strategies. It is necessary to explore a more efficient anammox reactor for F

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Figure 7. Performance of SBR2 with addition of exogenous Anammox sludge.

reduction of NO is a potential approach of flue gas denitrification. The results of the microbial community structure demonstrated that the diversity and abundance of the bacterial community continually decreased in the sludge of SBR1 without addition of exogenous Anammox sludge. This was consistent with the decreasing performance of NO removal in the reactor. The decrease of the sequence number of many phylum bacteria was due to the toxicity of NO.23 And the differences among the tolerances of bacteria to toxic NO caused the different decreases of their sequence numbers, although Kartal et al.35 reported that 3500 ppm of NO could not inhibit the activity of AnAOB. Especially, the increasing relative abundance of Ca. Kuenenia was related to Anammox reduction of NO. This is in agreement with the report by Wang et al.29

removing NO based on AnAOB growth and to investigate its operating conditions. For instance, EGSB-like biofilm reactors could be used to domesticate anammox bacteria with NO for long-term to strengthen the tolerance of AnAOB to toxic substances. Moreover, the concentration of Fe(II)EDTA-NO could be appropriately decreased and hydraulic retention time can be properly extended to weaken inhibition of toxic substances on Anammox. In fact, large amounts of (NH4)2SO3 and small amounts of NH4HSO3 are produced in the process of ammonia based wet flue gas desulfurization.34 The produced wastewater containing high ammonium and low organic carbon can be purified by Anammox. The produced high-activity Anammox sludge can be used in the process of Fe(II)EDTA absorption based Anammox reduction for the flue gas denitrification. The maximum NO removal rate of 4.18 mM NO d−1 in SBR2 with exogenous addition of Anammox biomass was little lower than that in Fe(II)EDTA absorption−denitrification (5.60 mM NO d−1),4 2 times higher than that (2.02 mM d−1) by Wang et al.29 Moreover, the used concentration of Fe(II)EDTA was 2 mM in this study, much lower than the 20 mM usually used in a chemical absorption−denitrification integrated NO removal process.4,29 In addition, Anammox has many advantages over denitrification, such as no organic carbon needs, lower sludge production, and no greenhouse gas (N2O) emission.35 The (NH4)2SO3 and NH4HSO3 contained in the wastewater could be used as a reductant for scrubber liquid regeneration,34 so as to eliminate the impact of oxygen in flue gas on Anammox. Therefore, the Fe(II)EDTA absorption based Anammox

5. CONCLUSIONS The optimal temperature and pH for Fe(II)EDTA absorption based Anammox reduction of NO were 30 °C and 7.0 in the range of temperature from 25 to 45 °C and pH from 6.5 to 8.0, respectively. The maximum specific substrate utilization rate and half-saturation constant of NO were 0.09 mmol N gVSS−1 h−1 and 1.338 mM, respectively. SBR1 without exogenous addition of Anammox biomass could not run stably for long-term. The removal of NO by Fe(II)EDTA absorption based Anammox reduction was stable for long-term in SBR2 with exogenous addition of Anammox G

DOI: 10.1021/acs.energyfuels.7b00119 Energy Fuels XXXX, XXX, XXX−XXX

Energy & Fuels

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Article

AUTHOR INFORMATION

Corresponding Author

*Address: Department of Environmental Science, Chongqing University, Chongqing 400030, P. R. China. Tel.: +86-02365105875. Fax: +86-023-65105875. E-mail address: dzhang@ cqu.edu.cn. ORCID

Daijun Zhang: 0000-0002-3737-1388 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The partial financial support from the Scientific Research Foundation (2011DA105287-ZD201505) of State Key Laboratory of Coal Mine Disaster Dynamics and Control, the Natural Science Foundation, China (NSF 51078365) and the Fundamental Research Funds for the Central Universities (CDJZR 13245501) is gratefully acknowledged.

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Figure 8. Distribution of the dominate genus of Planctomycetes.

biomass. The average NO removal rate reached 3.97 mM NO d−1. The biomass, the diversity, and the abundance of the bacterial community in SBR1 continually decreased. The increasing relative abundance of Ca. Kuenenia was related to NO Anammox reduction.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.7b00119. NO removal rate of batch test performing Fe(II)EDTA absorption and significant differences (p-values) compared with the NO removal rate at highest NO concentration (Table S1); NO removal rate of batch tests performing effect of temperature and pH and significant differences (p-values) compared with the NO removal rate at optimal temperature and pH (Table S2); the first PCR reaction system (Table S3); the second PCR reaction system (Table S4); profiles of NO removal by Anammox without Fe(II)EDTA addition (Figure S1); profiles of NO removal by Anammox with Fe(II)EDTA addition (Figure S2); profiles of NO concentration with different temperatures (Figure S3); and profiles of NO concentration with different pH values (Figure S4). (PDF) H

DOI: 10.1021/acs.energyfuels.7b00119 Energy Fuels XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.energyfuels.7b00119 Energy Fuels XXXX, XXX, XXX−XXX