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Ecological Engineering 57 (2013) 10– 16

Contents lists available at SciVerse ScienceDirect

Ecological Engineering

j ourna l ho me pa g e: www.elsev ier .com/ locate /eco leng

hort communication

ffect of aeration modes and influent COD/N ratios on the nitrogenemoval performance of vertical flow constructed wetland

ei Liu ∗, Xinhua Zhao, Nan Zhao, Zheng Shen, Mei Wang, Yuzhang Guo, Yinbo Xuchool of Environmental Science and Engineering, Tianjin University, No. 92, Weijin Road, Nankai District, Tianjin 300072, China

a r t i c l e i n f o

rticle history:eceived 8 October 2012eceived in revised form 9 March 2013ccepted 6 April 2013

eywords:

a b s t r a c t

Nitrification and denitrification have been proved to be the main pathways for nitrogen removal in con-structed wetlands (CWs), but they usually could not occur in a single wetland unit simultaneously due toconflicting oxygen demand. In this study, we employed two artificial aeration modes including continuousaeration (CA) and intermittent aeration (IA) in lab-scale vertical flow constructed wetlands (VFCWs) toinvestigate the nitrogen removal performance with different influent COD/N ratios. The artificial aeration

+

ertical flow constructed wetlanditrogen removaleration modeOD/N ratiolant assimilationissolved oxygen

significantly enhanced NH4 -N removal, especially for the wetland units with higher COD/N ratios. Thevariations of aeration modes and COD/N ratios also had a great effect on nitrogen removal. The IA unitshad a better performance on nitrogen removal compared to CA units when the COD/N ratio ranged from5 to 10. Plant biomass and nitrogen accumulation by plant were also studied. It was observed that plantsin IA and CA had less biomass and nitrogen uptake at lower COD/N ratios, but plants in IA could achievea better performance with higher COD/N ratios.

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. Introduction

Constructed wetlands (CWs) have received increasing atten-ion in the last decade due to their high potential for wastewaterreatment (Vymazal, 2005). The nitrogen removal performance islways the main focus when evaluating the treatment ability ofWs (Brix et al., 2001; Vymazal, 2007). Nitrification and denitri-cation are widely accepted as the main pathways for nitrogenemoval (Lin et al., 2002; Tanner et al., 2002). However, nitrificationequires aerobic condition while denitrification occurs with anaer-bic environment, which could not be fulfilled simultaneously inWs (Stottmeister et al., 2003). Denitrification is also sensitiveo variations of carbon sources (Gebremariam and Beutel, 2008;hang et al., 2010). In order to enhance nitrogen removal efficiency,ptimal manipulation of DO and organic matter is usually requiredn CWs (Dong et al., 2012; Zhao et al., 2010). Artificial aerationas been regarded as an effective way to control DO. Several stud-

es have reported that aerated CWs outperformed the non-aeratednes in nitrogen removal (Maltais-Landry et al., 2007; Ong et al.,010). Compared with continuous aeration, intermittent aeration

ould not only save the operational cost but also create differenticro-environments for effective nitrogen removal (Dong et al.,

012). Influent COD/N ratio could also significantly affect nitrogen

∗ Corresponding author. Tel.: +86 22 27408298; fax: +86 22 87402072.E-mail addresses: ebll@tju.edu.cn, llsd1987@163.com (L. Liu).

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925-8574/$ – see front matter © 2013 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.ecoleng.2013.04.019

© 2013 Elsevier B.V. All rights reserved.

emoval in CWs (Ding et al., 2012; Xia et al., 2008; Zhao et al., 2010).owever, there is little information on the interaction between

ntermittent aeration and COD/N ratio. Therefore, in this study,e comprehensively analyzed the effect of organic carbon addi-

ion and artificial aeration modes in the lab-scale vertical flowonstructed wetlands (VFCWs) to obtain optimal conditions underhich efficient nitrogen removal occurs.

The main purpose of this paper was to investigate how differenteration modes affect the DO profiles along the wetland units, ando evaluate the nitrogen removal performance of different aeration

odes with different COD/N ratios, and thus to identify optimalperation schemes and parameters for nitrogen removal.

. Materials and methods

.1. Treatment wetland and experimental design

Lab-scale cylindrical vertical flow constructed wetlandsVFCWs) were operated in a greenhouse environment with naturalunlight. They were composed of non-aerated units (NA), contin-ous aerated units (CA), and intermittent aerated units (IA). Ashown in Fig. 1, each wetland unit had a height of 60 cm and annner diameter of 18 cm. Slag with 8-15 mm in diameter was used

n the middle substrate layer (45 cm), and gravels were filled in theottom layer (10 cm) with 12-20 mm in diameter. Five samplingoints situated at 8, 15, 25, 40, 55 cm from the bottom were tappedlong the reactors. The water level was 3 cm below the surface of

L. Liu et al. / Ecological Engineering 57 (2013) 10– 16 11

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he wetland media (the position of the first sampling tap). Theetland units were planted with Iris pseudoacorus on April 2011

efore the experiment began. Iris pseudoacorus was transplantedrom natural wetland nearby. The initial plant biomass ranged from77 g/m2 to 636 g/m2 in wetland units.

IA and CA were installed with aeration systems which consistedf air compressors, air tubes and micro-bubble diffusers at a heightf 30 cm. The aerated wetlands received aeration at a flow rate of.18 m3/m2/h. A time switch controlled the IA systems. The IA sys-ems had 4 aerated/non-aerated cycles (A/N) every day and eachycle lasted for 6 hours. In each A/N cycle, the IA systems wererstly subject to aeration for three hours, and then have three hours

nterval without aeration. The aeration would begin at 0 AM, 6 AM,2 PM, and 6 PM, respectively.

The experiment was conducted from May to November 2011about 25 weeks). All the wetland units were flooded for twoeeks with tap water before the experiment. The synthetic waste-ater was prepared daily by mixing 0, 226.8, 453.6, 907.2 mg/L

H3COONa·3H2O (0, 100, 200, 400 mg/L COD), 152.8 mg/L NH4Cl40 mg/L TN), 17.5 mg/L KH2PO4, 7.3 mg/L CaCl2·2H2O, 4.5 mg/L

gSO4·7H2O, and 0.05 mg/L FeCl3·6H2O into tap water. COD/Natios of 0, 2.5, 5 and 10 were adopted in this study. The synthetic

tma1

ical flow constructed wetlands (VFCWs).

astewater was fed into IA systems during the first three hoursf each A/N cycle at a flow rate of 0.15 L/h (0.45 L/cycle) and theeeding would be ended in non-aerated phase. NA and CA werelso fed with identical flow rate at the same time for comparisonurpose. In order to ensure the same inflow rate and concentra-ion, the artificial wastewater was prepared in feed tanks and thenfluent loading rates of each experimental operation were shownn Table 1.

.2. Sample collection and analysis

The water samples were collected from the five sampling pointslong the wetland units twice a week. The water samples werenalyzed immediately for total nitrogen (TN), ammonia nitrogenNH4

+-N), nitrite nitrogen (NO2−-N), nitrate nitrogen (NO3

−-N)ccording to the Standard Methods (APHA, 2005). Chemical oxygenemand (COD) was analyzed with a HACH DR/2800 colorimeter.hysicochemical water parameters, such as dissolved oxygen (DO),

emperature, and pH, were determined in situ by DO meter and pH

eter, respectively. DO detection in IA was conducted in the non-erated period. Statistical analyses were determined by using SPSS9.0. Statistical significance was defined as p < 0.05 (SPSS, 2003).

12 L. Liu et al. / Ecological Engineering 57 (2013) 10– 16

Table 1Influent nutrient loadings (mean values ± SD), average removal rates, plant biomass and nitrogen uptake by plant.

Parameters COD/N=0:1 COD/N=2.5:1 COD/N=5:1 COD/N=10:1

NA CA IA NA CA IA NA CA IA NA CA IA

Influent COD loading rate (g/m2/d) 0.54 ± 0.08 6.18 ± 0.21 12.11 ± 0.29 23.23 ± 0.79Influent NH4

+-N loading rate (g/m2/d) 2.36 ± 0.13 2.45 ± 0.22 2.34 ± 0.17 2.27 ± 0.21Influent TN loading rate (g/m2/d) 2.48 ± 0.18 2.56 ± 0.17 2.41 ± 0.18 2.39 ± 0.18Average COD removal rate (g/m2/d) 0.19 0.40 0.35 4.18 5.29 5.18 8.39 10.51 10.06 15.34 20.19 19.13Average NH4

+-N removal rate (g/m2/d) 1.07 1.50 1.37 1.45 1.71 1.63 1.27 1.84 1.76 1.04 1.78 1.75Average TN removal rate (g/m2/d) 0.69 0.76 0.73 1.17 1.42 1.41 1.15 1.56 1.67 0.97 1.66 1.68Average effluent NO3

−-N (g/m2/d) 0.27 0.78 0.66 0.16 0.31 0.23 0.07 0.16 0.08 0.02 0.15 0.08Plant biomass (g/m2/d) 23.94 15.58 18.18 28.46 20.86 19.32 35.97 25.03 29.49 38.61 36.41 43.57Nitrogen uptake by plant (mg/m2/d) 126.37 89.68 73.91 199.21 167.82 185.48 253.24 226.57 257.87 264.78 292.94 287.07

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Plant contribution for N removal (%) 18.30% 11.85% 10.12% 16.97%

A: non-aerated wetland units; CA: continuous aerated wetland units; IA: intermit

.3. Plant harvesting and nitrogen storage

Plants samples in each wetland units were taken to measurehe initial biomass and nitrogen content. All parts of plants werearvested at the end of November. The plant biomass was cal-ulated based on harvested and initial biomass. The plants wereried at 80 ◦C to a constant weight. Subsamples of the dried plantsissues were powdered, and then they were homogenized. Theitrogen contents were determined according to the standard Kjel-ahl method.

.4. Mass removal rate and removal efficiency

The removal efficiency (E, %) and mass removal rate (R, g/m2/d)f nitrogen and organic matter were calculated according to Eqs.1) and (2), Nitrogen accumulation rate by plants (S, g/m2/d) wasalculated by Eq. (3):

= CiVi − CeVe

CiVi× 100% (1)

= CiVi − CeVe

A × HRT(2)

= N

A × T(3)

where Ci (mg/L) and Ce (mg/L) represents the concentrations ofnfluent and effluent, respectively. Vi (L) and Ve (L) are the influentnd effluent volume, respectively. A (m2) is the surface area of wet-and unit. HRT (day) stands for hydraulic retention time. N (g) is theitrogen content in plant biomass, and T (day) is the experimentalime.

. Results and discussion

.1. DO profile along wetland reactors

Combined nitrification and denitrification have been consideredo be the most important pathway for nitrogen removal, whichould be directly affected by DO (Jia et al., 2010; Sun and Austin,

007). Nitrification takes place in aerobic region while denitrifica-ion occurs with anaerobic/anoxic condition (Kadlec and Wallace,009). It was reported that DO concentration above 1.50 mg/L isssential for nitrification while denitrification usually occurs below.50 mg/L (Vymazal, 2007; Ye and Li, 2009). The distribution of DOlong NA, CA, and IA units at different COD/N ratios was shown

n Fig. 2. Although the VFCWs have higher oxygen transfer effi-iency than the horizontal flow ones, the DO concentrations alongA with different COD/N ratios were below 1 mg/L except the top

ayers. For different COD/N ratios, the DO detected at the top layer

ubi

1.79% 13.19% 21.93% 14.56% 15.46% 27.37% 17.63% 17.07%

erated wetland units.

f NA were in the range of 0.66-1.42 mg/L and less than 0.50 mg/Ln the 0-15 cm region above the bottom of VFCWs. Moreover, theO decreased with increasing COD/N ratios, which would ulti-ately inhibit the nitrification process. This phenomenon could

e explained by the fact that more oxygen would be consumed byhemical oxidation of excessive organic matter. In contrast, artifi-ial aeration could enhance the oxygen availability of the VFCWs.he DO levels increased from 0.27 mg/L and 0.23 mg/L at the lowerayer to 4.06 mg/L and 2.65 mg/L at the upper layer for CA andA, respectively. Thus, aerobic and anaerobic/anoxic regions couldxist in CA and IA simultaneously, especially for higher COD/Natios, which would facilitate nitrogen removal in planted wet-and units. It was also reported that the intermittent aeration couldesult in the stratification of biofilms and the formation of micro-erobic/anaerobic zones (Dong et al., 2012).

.2. Nitrification performance with different aeration modes andOD/N ratios

Nitrification, as an aerobic chemo-autotrophic microbial pro-ess, plays an important role in nitrogen removal (Albuquerquet al., 2009). As discussed above, DO deficit often occurs insidehe VFCWs, especially with excessive influent organic matterVymazal and Kröpfelová, 2008). The average NH4

+-N removalates were shown in Table 1. The nitrification performance of NAecreased with increasing COD/N ratios. Higher oxygen demandould present with increasing COD/N ratios due to the require-ent of mineralization of excess influent carbon source. Moreover,

he excessive organic matter could further restrain the activity ofutotrophic ammonia oxidation bacteria through DO competition.hus, the NH4

+-N removal is more sensitive to variations of COD/Natio in NA.

As illustrated in Fig. 3, higher seasonal fluctuations of NH4+-

removal were observed in NA with different COD/N ratios0.53–2.02 g/m2/d). In summer, NA could have relatively higherH4

+-N removal due to stronger microbial activity and plant pres-nce. For CA and IA, higher seasonal fluctuation occurred at theOD/N ratio of 0 (CA, 0.23-1.34 g/m2/d; IA, 0.36-1.17 g/m2/d). Theemoval rates of CA and IA were more stable with increasing carbondditions during the whole experimental period. In non-aeratedetlands, the microbial activity is closely related with temperature.

his was especially true in winter when the activity of microorgan-sms is inhibited (Maltais-Landry et al., 2009). On the other hand,erated wetland systems could be tolerant of seasonal and influentariation and contribute to consistent nitrification performance.

NH4+-N removal was lower at a COD/N ratio of 0 for all wetland

nits. This result may be attributed to the fact that limited car-on source could affect nitrogen transformation, and consequently

nhibit nitrification. The average removal efficiency of NH4+-N in

L. Liu et al. / Ecological Engineering 57 (2013) 10– 16 13

Fig. 2. Profiles of dissolved oxygen along wetland unit at COD/N ratios (A) 0:1, (B) 2.5:1, (C) 5:1, (D) 10:1, in NA, CA, and IA. The data used were mean values with the standardd ts; CA

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eviation throughout the whole experiment (n = 50). (NA: non-aerated wetland uni

A and IA were 73.19% and 70.02%, respectively. Compared withA, IA had relatively lower nitrification removal rates. The aer-tion systems could significantly improve NH4

+-N removal withncreasing carbon addition. As the COD/N ratio increased from.5 to 10, the average NH4

+-N removal efficiency of NA droppedrom 59.19% to 45.75%, while CA and IA had a better nitrificationerformance at higher COD/N ratios. Meanwhile, average effluentO3

−-N in NA were always lower than that in IA and CA, sug-esting a higher nitrification bioactivity for CA and IA (Table 1).hese results are consistent with the study of Maltais-Landry et al.2009) who reported that non-aerated wetlands exported N mostlys NH4

+-N, indicating that nitrogen load could not be fully nitrified.hey also demonstrated that aeration can significantly improveitrification performance.

.3. Nitrogen removal with different aeration modes and COD/Natios

Nitrogen removal rates in most CWs were low due to theirnability to provide suitable conditions for nitrification and deni-rification simultaneously (Ong et al., 2010; Zhao et al., 2011). Theverage TN removal rates were shown in Table 1. The averageN removal rates ranged from 0.69-1.17 g/m2/d, 0.76-1.66 g/m2/d,.73-1.68 g/m2/d for NA, CA, and IA, respectively. These results areonsistent with the removal rates (52-841 g/m2/per year) reportedy Tanner et al. (2002) and mean value of 0.8 g/m2/d for aeratedetlands reported by Maltais-Landry et al. (2009). Zhang et al.

2010) also utilized limited artificial aeration on CWs and reportedn average value of 4.99 g/m2/d, which is higher than the resultsf this study. The possible reason for the difference may be that

itrogen removal is usually affected by many factors, such as sup-orting media, nitrogen species, plant species, influent loadingates, hydraulic retention time, and seasons (Tanner et al., 2002;ymazal, 2007). Moreover, the average TN removal rates in IA and

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: continuous aerated wetland units; IA: intermittent aerated wetland units.).

A were higher than that in NA, which is in agreement with thetudy of Maltais-Landry et al. (2009) who reported that artificialeration could increase TN removal by 11–46%.

The weekly removal rates of TN were displayed in Fig. 3. Highereasonal fluctuations were observed for NA when artificial waste-ater with different COD/N ratios was fed (0.15-1.75 g/m2/d). They

ould achieve higher TN removal rates in summer. For CA andA, higher fluctuation occurred at the COD/N ratio of 0 and 2.5.he TN removal rates were lower in the first several weeks, andhen increased or fluctuated. At higher COD/N ratios, CA and IAould maintain a stable performance. VFCWs with aeration still hadigher TN removal rates in November.

At a COD/N ratio of 0, the average removal efficiency for TN was9.19%, 31.99%, 30.88% for NA, CA, and IA, respectively. As shown inable 1, the average effluent NO3

−-N for NA, CA, and IA maintainedt a high level due to the deficit of organic matter. Thus, artificialeration could also result in higher nitrate accumulation, whichould further inhibit denitrification. At a COD/N ratio of 2.5, deni-

rification performance was improved due to addition of carbonource. Compared with the case at the COD/N ratio of 0, a sharpecrease of average effluent NO3

−-N occurred in all the VFCWs,specially for CA. The average TN removal efficiency of NA, CA, andA increased from 29.19% to 48.07%, 31.99% to 58.29%, 30.88% to7.60%, respectively, which was also significantly higher than thatt the COD/N ratio of 0. The results indicated that artificial aera-ion could strengthen the nitrogen removal with the presence ofarbon source, but no statistically significant effect of intermittenteration on nitrogen removal was observed with the COD/N ratiof 2.5.

With an influent COD/N ratio of 5, NA achieved a relatively

igher TN removal efficiency than that obtained at COD/N ratiof 2.5 and 10. However, as shown in Fig. 2 and Fig. 3, NA couldot achieve a high level of NH4

+-N removal with low DO con-entration, which would adversely inhibit denitrification due to

14 L. Liu et al. / Ecological Engineering 57 (2013) 10– 16

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ig. 3. Weekly average removal rates of COD, TN, NH4+-N at different COD/N ratios

ontinuous aerated wetland units; IA: intermittent aerated wetland units.).

nsufficient supply of NO3−-N as electron acceptors. The average

N removal efficiencies for CA and IA were 67.69% and 72.52% at COD/N ratio of 5, and 72.81% and 73.69% at a COD/N ratio of 10,espectively. The aerated wetland systems achieved a significantlyigher TN removal efficiency compared to that with a COD/N ratiof 2.5. This circumstance could be explained by the fact that simul-aneous nitrification and denitrification could occur in a singleerated VFCW with particular designs. Higher COD loading rates for

A and IA could also benefit denitrification. Although IA had loweritrification rate compared to CA, it is particularly noted that IAad a better TN removal performance at higher COD/N ratios. Withufficient carbon source, intermittent artificial aeration could not

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, CA, and IA during the experimental period. (NA: non-aerated wetland units; CA:

nly provide optimal DO concentrations in the upper and bottomegions but also accelerate the diffusion of wastewater. IA couldlso have different micro-environments in the attached biofilms,nd thus facilitated nitrification and denitrification simultaneously.

It was observed that IA could achieve a higher NH4+-N removal

fficiency compared with NA. Meanwhile, IA could also enhancehe denitrification process through the stratification of aerobic andnaerobic/anoxic zones compared to CA. IA could achieve a rela-

ively higher TN removal efficiency than CA when the COD/N ratioanged from 5 to 10, suggesting that IA could be a suitable choiceor treatment of wastewater with higher COD/N ratios. Intermit-ent aeration could also limit the operation costs by decreasing

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he energy requirement while maintaining stable nitrogen removalerformance.

.4. Plant growth and uptake

The plants in CA and IA grew slower than those in NA at theeginning of operation. The plant growth was disturbed by aeration

n the acclimation period as the leaves turned to yellowish. But thelants reproduced well when they adapt to aeration and influentrtificial wastewater. Plant biomass and nitrogen uptake by plantere shown in Table 1. Plant biomass in NA, CA, and IA wereithin the range of 23.94-38.61 g/m2/d, 15.58-36.41 g/m2/d, 18.18-

3.57 g/m2/d, respectively. The plant biomass of NA was higherhan that in CA and IA at COD/N ratio of 0, 2.5, and 5, while plant in IAould achieve the highest plant biomass at COD/N ratio of 10. Theseesults indicated that continuous aeration could result in decreasef plant biomass, but intermittent aeration could increase the plantiomass at higher COD/N ratios. Moreover, plant biomass in allFCWs increased as COD/N ratio ranged from 0 to 10, suggesting

hat COD/N ratio could significantly affect plant growth. In terms ofitrogen uptake by plant, plant uptake in CA and IA was lower thanA at the COD ratio of 0 and 2.5. The nitrogen uptake by plant in IAas higher than that of CA at COD/N ratio of 5 and 10, while plants

n CA achieved the highest nitrogen uptake at COD/N ratio of 10.t was reported that nitrogen uptake via plant can be enhanced byeration since nitrogen in the form of nitrate and nitrite is suitableor plant assimilation (Vymazal and Kröpfelová, 2008). The plants

ay also have indirect effect on hydrology and evapotransporationates, which would affect nitrogen mass balance (Maltais-Landryt al., 2009).

In our study, at different COD/N ratios, plant uptake accountedor 16.97–27.37% of nitrogen removal in NA, while 11.79–17.63%nd 10.12–17.07% of nitrogen removal were achieved by plant in CAnd IA, respectively. Plant uptake accounted for a larger proportionf TN removal in NA, indicating that plant could be more impor-ant for nitrogen removal in VFCWs without aeration. No significantifference was found between CA and IA. Microbial processes andediment storage may also be the main pathways for nitrogenemoval due to suitable environment provided by aerated VFCWs.altais-Landry et al. (2009) also assessed the seasonal variation of

itrogen removal performance through employing artificial aer-tion and different plant species. They reported that 10–19% ofitrogen was removed by plant. Moreover, nitrification and den-

trificaition accounted for 47–62% of total nitrogen removal, andediment storage was 27–63%. Wu et al. (2013) also studied theitrogen mass balance and transformation for polluted river waternd reported that plant uptake removed 8–34% of the total nitro-en, while sediment storage accounted for 21–34% of nitrogenemoval. Nitrification and denitrification could remove up to 24% ofitrogen. Therefore, it is reasonable to assume that microbial nitri-cation/denitrification, plant uptake, and sediment storage are theain pathways for nitrogen removal for aerated VFCWs.

. Conclusions

Nitrification and denitrification could not occur in a single wet-and unit due to conflicting requirements for dissolved oxygen.herefore, a single non-aerated VFCW could not achieve a satisfieditrogen removal. Artificial aeration could effectively control the

ractions of aerobic and anaerobic regions with particular designs,

nd therefore significantly improve nitrogen removal in VFCWs.owever, the relatively higher operational cost was the majorbstacle for its widespread utilization. Compared to continuouseration, intermittent aeration could not only increase dissolved

Y

eering 57 (2013) 10– 16 15

xygen concentration but also reduce the operation cost. More-ver, the nitrogen removal efficiencies of intermittent aerationere higher than that of continuous aeration when the COD/N

atio ranged from 5 to 10, indicating that intermittent aerationould improve microbial activities and plant uptake with higherrganic loadings. Thus, intermittent aeration is a suitable choiceor nitrogen removal enhancement in constructed wetlands.

cknowledgements

This research has been supported by Special Fund Project forcience and Technology Innovation of Tianjin City, China (Projectumber: 08FDZDSF03200).

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