Adam Sochacki
email: adam.sochacki@polsl.pl

Joanna Surmacz-Górska

Olivier Faure
email: ofaure@emse.fr

Bernard Guy
email: bguy@emse.fr

Polishing of synthetic electroplating wastewater in microcosm upflow constructed wetlands: Effect of operating conditions

Keywords: electroplating wastewater, metals, cyanides, constructed wetlands, upflow, sulfates Abbreviations

This paper addresses the issue of polishing electroplating wastewater in subsurface vertical flow constructed wetland. Electroplating wastewater treatment or polishing in constructed wetlands (CWs) was studied to a very limited degree. Four types of microcosm upflow constructed wetlands were selected for the experiment based on type of bed media and the presence or absence of vegetation (Phragmites australis). The main objective of the system design was to promote metals removal by precipitation as sulfides mediated by sulfate reducing bacteria. The experimental system consisted of eight PVC columns (diameter 20 cm) filled up to. 63.5 cm with either gravel or gravel-peat mixture and was operated for one year, polishing synthetic electroplating wastewater. The wastewater used in this study contained metals (Cu, Ni, Pb and Zn), cyanides and sulfates, which typically occur in electroplating wastewater. During the experiment the effect of the following factors was studied: presence of vegetation, hydraulic loading, wastewater strength, type of bed media, and addition of lactates as external carbon source for the columns with gravel. The results showed that the uplow columns can efficiently remove metals (with the exception of Pb) and cyanides. It was observed that the removal efficiency of Cu, Ni and Zn increases with increasing influent concentration of metals and increasing hydraulic loading rate. The role of plants and bed media was minor. The addition of lactates to the feed of the columns with gravel was found to improve the reduction of metals concentration in the passing wastewater.

Introduction

Electroplating is a process of applying metal coating to an object to enhance its protective properties and alter its appearance, or both [START_REF] Bartkiewicz | Oczyszczanie ścieków przemysłowych (Industrial wastewater treatment)[END_REF]. High concentrations of toxic pollutants in the electroplating effluents (metals and cyanides often in hundreds of mg/L [START_REF] Bartkiewicz | Oczyszczanie ścieków przemysłowych (Industrial wastewater treatment)[END_REF]) preclude direct application of biological methods for their treatment. Thus, constructed wetlands (CWs) are not an alternative to conventional physical-chemical methods of electroplating wastewater treatment. The term 'polishing' is understood as a treatment of wastewater, pretreated by conventional methods, to the level that allows discharge into sewer or the environment. Thus, the polishing process would be employed to reduce residual amounts of metals and cyanides, which are invariably present in the electroplating wastewater. It is noteworthy that conventional primary and secondary unit processes at municipal wastewater treatment plants are not fully adequate for efficient removal of heavy metals and cyanides of electroplating origin, and the biological processes may be inhibited by these contaminants if the biomass was not previously adapted [START_REF] Torrens | Activated sludge and cyanide: a deadly combination[END_REF][START_REF] Kiptoo | Speciation studies of nickel and chromium in wastewater from an electroplating plant[END_REF][START_REF] Henriques | Activated sludge inhibition by chemical stressors-a comprehensive study[END_REF]. Moreover, treatment of wastewater containing metals by activated sludge impairs the quality of the excess sludge precluding its agricultural application [START_REF] Chipasa | Accumulation and fate of selected heavy metals in a biological wastewater treatment system[END_REF]. Thus, even the polishing of wastewater discharged into municipal sewer should be important from the environmental perspective. The application of CWs for the treatment of electroplating wastewater was reported scarcely in the literature. Ranieri and Young [START_REF] Ranieri | Clogging influence on metals migration and removal in subsurface flow constructed wetlands[END_REF] studied the removal of Ni and Cr (and other contaminants) from municipal wastewater containing about 20% of electroplating and textile wastewater. This study consisted in monitoring of a full-scale CW with horizontal subsurface flow. The treatment of electroplating effluents was also investigated in the system with water hyacinth in Asia [START_REF] Yeoh | Use of water hyacinth in wastewater treatment[END_REF][START_REF] Abbasi | Aquatic plants based water treatment systems in Asia, in: Aquatic Plants for Water Treatment and Resource Recovery[END_REF].

Vymazal and Kröpfelová [START_REF] Vymazal | Wastewater Treatment in Constructed Wetlands with Horizontal Sub-surface Flow[END_REF] referred to the work of Shroff [START_REF] Shroff | Reuse of water and sludge for cultivation of variety of value added botanical species[END_REF] that was dealing to some extent with the treatment of electroplating wastewater. The important feature of metals is that they can only be removed from wastewater but cannot be destroyed. This allows assumption that metals are accumulated in wetland substrate or biota [START_REF] Ranieri | Clogging influence on metals migration and removal in subsurface flow constructed wetlands[END_REF]. The removal of metals in subsurface CWs is dominated by four mechanisms: adsorption, filtration and sedimentation, association with metal oxides and hydroxides, and precipitation as sulfides. The extent of these processes depends on the type of the CWs, pH, redox potential, influent composition, the dominant plant species and microbial activity [START_REF] Kosolapov | Microbial processes of heavy metal removal from carbon-deficient effluents in constructed wetlands[END_REF]. Since, in general, metal sulfides are less soluble than their carbonate or hydroxide counterparts, this process allows achieving more complete precipitation and stability over a broader pH range [START_REF] Blais | Metals Precipitation from Effluents: Review[END_REF]. Some of the metals which react with sulfides to form highly insoluble metal sulfides are: Cu, Fe, Ni, Pb, and Zn.

These metals are often present in electroplating wastewater [START_REF] Wallace | Small-Scale Constructed Wetland Treatment Systems: Feasibility, Design Criteria, and O&M Requirements[END_REF]. The key requirements for sulfate-reducing systems are: anaerobic conditions (oxidation-reduction potential below -100 mV), electron donors (simple organic compounds), microbial groups capable of utilizing inorganic sulfur compounds as electron acceptors; inorganic sulfur compounds (as electron acceptors) [START_REF] Younger | Mine Water: Hydrology, Pollution, Remediation[END_REF][START_REF]Engineering guidelines for the passive remediation of acidic and/or metalliferrous mine drainage and similar wastewaters[END_REF][START_REF] Reddy | Biogeochemistry of Wetlands: Science and Applications[END_REF]. The major factor limiting the application of microbial sulfate reduction to the removal of metals from carbon deficient industrial wastewaters (such as electroplating wastewater) in CWs is the availability of carbon and energy sources to drive the process. In order to stimulate sulfate-reducing bacteria (SRB) in the case of carbon-deficient effluents, a proper carbon source should be provided to enhance their growth and to cause other bacteria to remove the oxygen from the environment [START_REF] Kosolapov | Microbial processes of heavy metal removal from carbon-deficient effluents in constructed wetlands[END_REF]. Unlike for natural wetlands, sulfates are not limiting substance in CWs treating industrial wastewater, as sulfuric acid is often used to adjust pH of wastewater [START_REF] Kosolapov | Microbial processes of heavy metal removal from carbon-deficient effluents in constructed wetlands[END_REF]. The conditions favorable for SRB may occur in various types of wetland systems, but the upflow (UF) CWs appear viable solution. They are characterized by the fact that their bed is constantly saturated and permanently flooded over the surface. UF CWs are, in fact, mostly applied when anaerobic treatment processes should be promoted, such as bacterial sulfate reduction [START_REF] Fonder | Systematic Classification, Nomenclature and Reporting for Constructed Treatment Wetlands[END_REF]. Importantly, also cyanides can be degraded under anaerobic conditions in various biotic and abiotic processes [START_REF] Dash | Cyanide in industrial wastewaters and its removal: A review on biotreatment[END_REF].

The goals of the experiment were to study the feasibility of polishing electroplating wastewater and to study the effect of hydraulic loading, vegetation, type of bed media, wastewater strength, and addition of external carbon source.

2

Materials and methods

Microcosm constructed wetlands

The experimental system consisted of eight PVC columns (height, H = 80 cm, diameter, d = 20 cm) filled up to 63.5 cm with either mineral or mixed organic-mineral media. The active bed area (A) corresponding to these dimensions was 0.0314 m 2 , the bed volume (V) was 20 L.

The columns were operated in a UF saturated mode with a 6.5-cm layer of water above the bed media preventing air from penetrating into the substrate. Thus, water depth (h) in the system was invariably 70 cm and the corresponding nominal wetland volume (V nom ) was 22 L.

These columns were fed from the bottom by peristaltic pump Masterflex ® L/S ® 7523-80 using PharMed ® BPT tubing (2.79 mm) and the outlet was situated 70 cm above the bottom. The schematic of column used in the experiment is shown in Fig. 1.

(a) (b) Figure 1 (a) Cross section of a column used in the experiment (all dimensions in cm; in scale), and (b) photo of the experimental system

The main objective of the system design was to promote metals (Cu, Ni, Pb, Zn) removal by precipitation as sulfides mediated by SRB, which, in general, are obligate anaerobes.

Anaerobic conditions in the columns were promoted by their construction and operation mode. The columns were filled with quartz gravel (3-8 mm) or equivolume gravel-peat mixture (hereafter referred to as 'peat'). The inlet zone at the bottom of the columns with peat contained 10-cm layer of the gravel (as shown in Fig. 1). The pH value of the peat was 3.5-4.5; organic matter content 89.9% of DW, and water-holding capacity 746 mL/L. The bed media were inoculated with SRB using anaerobic sludge (mesophilic anaerobic digestion) with initial 14-day batch of wastewater to all the columns prior to the start-up of the experiment. Selected columns were planted with rhizomes of Phragmites australis (Cav.)

Trin. ex Steud and Phalaris arundinacea L. Four types of UF-CWs were selected for the experiment based on type of bed media and the presence or absence of vegetation. Eight columns were used in this study, since each type of column was duplicated. The following types of columns were used:

-columns filled with peat and planted (UF-PP),

-columns filled with peat and unplanted (UF-PU),

-columns filled with gravel and planted (UF-GP),

-and columns filled with gravel and unplanted (UF-GU).

The was equal in the both influents at 500 mg/L (resulting from dissolution Na 2 SO 4 ), and the Pb concentration was equal at 1 mg/L throughout the experiment. The final concentration of sulfates was higher (up to 860 mg/L) in the feed because of sulfates in the tap water used. The concentration of Cu, Ni and Zn was raised from 1 mg/L in the low-strength feed to 5 mg/L in the high-strength feed. In week 33 the inflow rate (Q in ) to the columns was decreased from 3.25 to 2.00 mL/min. In week 44 of the experiment lactates were started to be added to the influent fed to the columns with gravel (UF-GP, UF-GU). Lactates were added to the feed as 80% lactic acid and the dosage was calculated based on the assumption that a target ratio COD/  2 4 SO is 0.3. This value was based on the study of Song et al. [19] where it was found to be the optimum ratio. The concentration of sulfates used for calculation was 800 mg/L (real concentration was close to this value) and the corresponding COD of lactates was 240 mg O 2 /L. In week 48 cyanide was started to be added to all the types of columns. Cyanide was added as KCN in a dosage equal to 2.0 mg/L of total cyanide. All the operation periods are given in Tab. 1.

Table 1

Operation chart for the experimental system (grey shading indicates relevant periods)

Wastewater analysis

Effluent was sampled from the columns by either syringe from the outlet zones of the columns or collected to plastic bottles connected by plastic hose to the outflow stubs of the columns. Influent samples were taken by syringe or by immersion of sample vials, depending on water level. Metals (Cu, Fe, Mn, Ni, Pb, Zn) were analyzed with flame atomic adsorption spectrometry (FAAS; Varian Spectra AA 800) after acidifying filtered samples with 65% HNO 3 to pH<2. The detection limits for metal analysis by FAAS were 0.0015, 0.0040, 0.0020, 0.0010 mg/L for Cu, Ni, Pb, Zn, respectively. Standard deviation was calculated for each measurement series and was less than 5% for each metal considered. The analysis was performed using duplicate samples. Readily liberated cyanides were determined using Merck Cuvette test 1.09701.0001. In this method (for samples decomposed by digestion) cyanide ions react with a chlorinating agent to form cyanogen chloride, which in turn reacts with 1,3dimethylbarbituric acid to form a violet dye (pyridine-free König reaction). This dye was determined photometrically using Merck Spectroquant NOVA 400 spectrophotometer. Standard deviation of the method was ±0.0039 mg/L, coefficient of the variation of the method was ±1.5%, and confidence interval was ±0.010 mg/L. Chemical oxygen demand (COD) of wastewater was determined using standard method of reflux digestion and titration ISO 6060:1989 [START_REF]Water quality --Determination of the chemical oxygen demand[END_REF]. Sulfate was determined using Merck Cuvette test 1.14791.0001. In this method (for samples decomposed by digestion) sulfate ions react with barium iodate, liberating iodate ions in the process. These oxidize tannin to a brown-red compound that was determined photometrically using Merck Spectroquant NOVA 400 spectrophotometer.

Standard deviation of the method was ±4.8 mg/L, coefficient of the variation of the method was ±2.6%, and confidence interval was ±9 mg/L.

Statistical analysis

Most of the subsets of the performance data had non-normal distribution, therefore nonparametric statistical test were used to compare these subsets. Median and the median absolute deviation (MAD) were used as descriptors of the central tendency and dispersion of data distributions for non-normally distributed data. The Shapiro-Wilk W test was employed to test for normality of the data. Dependent groups were analyzed using the Wilcoxon matched pair test and independent groups were analyzed using the Mann-Whitney U test.

Differences were considered statistically significant if p < 0.05. Statistical testing was performed using the STATISTICA 10 software [START_REF] Statsoft | STATISTICA (data analysis software system), version 10[END_REF].

3

Results and discussion

Overall treatment efficiency for metals

The experimental system was used to treat synthetic electroplating wastewater in four types of columns, of which some were left unplanted. The unplanted columns were used as a reference for the planted systems and were not considered CWs in this paper. The median values of the removal efficiency for metals in all the columns during the whole experiment (weeks 1-53) are presented in Fig.

Metals removal in the course of the experiment

The median data as shown in Fig. 2 facilitate comparison of several systems but does not allow visualizing the time course of the process. Figure 3

Treatment efficiency of CWs for cyanides

Cyanides are an inherent compound of the electroplating wastewater. In the experiment system cyanides were added to the high-strength feed starting from week 48. The influent and effluent cyanide concentration and the removal efficiencies are presented in Tab. The results presented in Tab. 2 indicate that the removal of cyanides was higher than 91% in all the UF columns. All the effluent concentrations were statistically different from the corresponding influent concentrations. The differences in removal between the columns are not pronounced and the effect of bed media and vegetation cannot be clearly demonstrated.

Effect of operational conditions on the treatment efficiency

The design of the experimental system was intended to allow for comparison of the effect of various operational conditions and configuration of the columns. The overall averaged data for the whole experiment or the data presented on the time scale are not suitable for analyzing the results in this regard. In this section the effect of various factors is assessed based on the data paired according to the effect of interest. This is, each pair comprises two data sets, one set where the factor with a potential effect on the treatment efficiency occurred and the other set, in which this factor was absent. The analyzed data were divided into relevant periods to enable meaningful conclusions drawn from the comparison. The significance of the difference between two relevant data sets was assessed statistically.

Effect of wastewater strength

The experimental system was fed with either low-or high-strength wastewater, which was intended to simulate the treated electroplating effluent and pretreated electroplating effluent, respectively. The low-strength influent was fed in the beginning of the experiment (weeks 1-17) and the high-strength was fed in the remaining period (weeks 18-53). The operational conditions of the experimental system were changed after week 33 (see Tab. 1), therefore the effect of wastewater strength can be assed properly only for the periods of weeks 1-17 for low-strength wastewater, and week 18-32 for high strength wastewater. The results from these periods were compared to assess whether the strength of the influent affected the treatment efficiency. It should be noted that these two influents were not fed into the columns in parallel, so the apparent difference between the treatment efficiencies in these periods cannot be solely attributed to the effect of the wastewater strength but could also be caused by the system's maturation. The comparison of the removal efficiency of the UF columns between the periods when the system was fed with low-strength wastewater (weeks 1-17) and highstrength wastewater (weeks 18-32) is presented in Tab. 3.

Table 3

Influent quality and treatment efficiencies (median ± MAD) in the UF-PP and UF-GP columns fed with low-(Low) and high-strength influent (High), (weeks: 1-17 for low-strength influent, n = 8; 18-32 for high-strength influent, n = 8) Based on the results presented in Tab. 3 it can be seen that all the differences between the removal efficiencies in the given periods for the UF-PP columns were statistically significant, in contrast to the to the UF-GP columns where none of the differences were of statistical significance. In the high-strength-influent period the operation of the UF-GP was unstable for two weeks because of tubing clogging and puncture, which could cause that outlying data points appeared in the data set for this period. It was, observed however, that the type of influent affected the operation of the UF columns in a similar manner. The removal of Cu was cca. 2% higher, in both types of columns, when the high-strength influent was used. Also, the removal of Ni was more efficient, by cca. 16% , when the high-strength influent was fed to the UF-PP columns. It was the only exception to the similarity between the UF columns, when the Ni removal was lower by 5.5% during the period when the high-strength influent was fed to the UF-GP columns. It was probably owing to saturation of the gravel, which has lower sorption capacity than peat. The same observation as for Cu and Ni (only in the UF-PP columns) was observed for Zn, which was removed to a higher extent in both types of the planted UF columns when these columns were fed with the high-strength influent. The efficiency of the Zn removal increased by cca. 10% and 4% for the UF-PP and UF-GP CWs, respectively. In contrary to the above findings (with the exception of the Ni removal in the UF-GP CWs), the Pb removal efficiency was markedly lower, by cca. 30% and 24%, in the UF-PP and UF-GP CWs, respectively, when the high-strength wastewater was fed into these CWs. However, in this period influent concentrations of only Cu, Ni and Zn were increased from 1 to 5 mg/L, but Pb concentration remained unadjusted at 1 mg/L. This may suggest that the Pb removal was hindered by the increased concentration of the other metals.

Metal

Effect of vegetation

Plants are an inherent feature of CWs [START_REF] Brix | Do macrophytes play a role in constructed treatment wetlands?[END_REF]. The significance of their effect on the removal of metals depends on many factors. Only a minor fraction of metals is removed by emergent hydrophytes in the subsurface-flow CWs [START_REF] Marchand | Metal and metalloid removal in constructed wetlands, with emphasis on the importance of plants and standardized measurements: A review[END_REF]. The effect of plants on the removal of metals in the experimental system is presented in Tab. 4 where the data for the planted and unplanted UF columns are compared.

Table 4

The comparison between the treatment efficiencies (median ± MAD, %) in the planted and unplanted UF columns, (weeks 3-42; n = 18-21) The differences between the data sets for the planted and unplanted counterpart columns were statistically significant for all the metals, except Pb. The differences between the removal of Cu in the planted and unplanted UF columns filled with either gravel or peat are less or equal 1%. The results for Ni are somewhat contradictory, as the presence of plants enhanced the treatment efficiency in the UF-PP columns by cca. 7.2%, but it attenuated the removal of Ni in the UF-GP CWs by about 8%. The same observation is valid for the removal of Zn, which was higher (by only 1.6%) in the UF-PP CWs than in the unplanted counterpart (UF-PU CWs), but was lower, by about 6%, in the UF-GP than in the unplanted counterpart (UF-GU CWs). The removal of Pb was higher in the unplanted UF columns, both in these filled with gravel or peat, and differed by 4% and 2%, respectively. The number of stems per column was less than 10, thus probably, the below-ground biomass was insufficient as a substrate for biofilm and as a means of oxygen transfer into the bed media. Thus, neither biotic nor abiotic processes occurring in the planted columns were not affected significantly by the plants. The differences observed between the removal efficiencies in the planted and unplanted columns were probably due to the fact that the plants modified the hydraulic permeability of the upper part of the columns, thus affecting the removal.

Metal

Effect of hydraulic loading rate

The hydraulic loading rate (HLR) is an operational parameter that can be used to control the efficiency of the treatment in CWs. The complexity of the processes occurring in CWs for the removal of metals and cyanides causes that it is difficult to assume any value of the hydraulic retention time (HRT) in the beginning of the experiment as the optimum one. For this reason, two values of the hydraulic loading rate were studied: 0.15 and 0.09 m/d, which corresponded to the HRT values of 1.6 -2.1 d, and 2.7 -3.4 d, respectively. The comparison of the effect of HLR on the removal efficiency of metals is given in Tab. 5. The influent quality was not modified in the period, for which the data are given. The results given in Tab. 5 indicate that the treatment efficiency was lower for the lower values (0.09 m/d) of the HLR in both UF-PP and UF-GP columns. Only the removal of Ni in the UF-PP columns at both HLRs was virtually equal and the removal efficiency of Cu was higher for the lower HLR in the UF-GP columns. Most of the differences were not statistically significant with the exception of Pb in the UF-PP columns. The above findings suggest that the higher HLR is more advisable value rather than the lower one, however, two remarks should be made. Firstly, the decreased efficiency corresponding to the lower HLR can result from the chronology of the experiment. For example, the progressive saturation of the adsorption sites on the bed media could elucidate the decreased efficiency. In fact, it was observed that the effect of HLR was more pronounced in the columns with gravel (UF-GP)

especially for Ni and Zn, because gravel has much lower binding affinity for metals than peat.

Secondly, only two values of the HLR were investigated, therefore no meaningful general conclusions can be drawn for a wider range of the values for this parameter. Similar findings were, however, presented in [START_REF] Yadav | Removal of chromium and nickel from aqueous solution in constructed wetland: Mass balance, adsorption-desorption and FTIR study[END_REF]. It was shown in this article that the removal of Ni and Cr was highest at lower hydraulic retention time (24 h) decreased markedly at HRT of 48 h for Ni, and slightly for Cr. The authors did not explain the causative factor. The other authors found that hydraulic loading rate had no effect on metal removal efficiency in subsurface-flow CWs [START_REF] Song | Lead and zinc removal by laboratory-scale constructed wetlands[END_REF] or that increased HLR enhanced the removal of metals in sulfate-reducing lab-scale reactors packed with a mixture of organic and inorganic materials [START_REF] Neculita | Effectiveness of sulfate-reducing passive bioreactors for treating highly contaminated acid mine drainage: I. Effect of hydraulic retention time[END_REF], in pilot-scale horizontal flow CWs [START_REF] Gikas | Removal of Iron, Chromium and Lead from Waste Water by Horizontal Subsurface Flow Constructed Wetlands[END_REF], and in vertical flow microcosm CWs [START_REF] Yadav | The removal of heavy metals in wetland microcosms: Effects of bed depth, plant species, and metal mobility[END_REF].

Effect of substrate type (internal carbon source)

The CWs have the ability to reduce metal concentrations in the wastewater passing through.

The removal of metals can occur under either oxic or anoxic conditions. In the experiment reported in this paper the removal processes were investigated under anoxic conditions. It was expected that the predominant removal process would be the precipitation of metals with biogenic sulfides. The generation of sulfides is mediated by SRB, which need organic carbon as an electron donor. The experimental system was designed to provide organic carbon as an internal source in the bed media, which was peat. Not all of the columns contained organic substrate so that the comparison of the effect of the internal carbon source would be possible.

The data shown in Tab. 6 represent the treatment efficiency in weeks 33 -43, this is, before the addition of external carbon source to the wastewater in gravel-and peat-filled columns.

Table 6

The comparison between the treatment efficiencies (median ± MAD, %) in the planted columns filled with gravel (UF-GP) or peat (UF-PP), (weeks 33-43; n = 4-5) The effect of substrate type in the planted UF columns was statistically significant for the removal of Cu, Ni and Pb which was invariably in the columns with gravel (UF-GP). The removal efficiencies were not statistically different for Pb, but interestingly the removal was higher (by 7.0%) in the UF-GP columns. The removal of Cu was almost the same in both types of the discussed columns (only 0.7% higher in the columns with peat). Interestingly Scholz et al. [START_REF] Scholz | Comparison of filter media, plant communities and microbiology within constructed wetlands treating wastewater containing heavy metals[END_REF] also found that internal carbon source does not enhance the removal of Pb and Zn in vertical-flow microcosm CWs, however, under aerobic conditions. The removal efficiencies were much higher in the columns with peat, this is, by 32.8% for Ni, and 20.7%

Metal

for Zn. This suggests that peat affects the removal of Ni and Zn. The effect of peat can be probably related to adsorption processes and the stimulation of SRB activity. The second hypothesis can be corroborated by the comparison of sulfates removal in the UF-PP and UF-GP columns. The removal of sulfates was low, however, positive in the columns with peat, whereas sulfates were released from the columns with gravel. Neculita et al. [START_REF] Neculita | Passive treatment of acid mine drainage in bioreactors using sulfate-reducing bacteriacritical review and research needs[END_REF] emphasized that solid organic sources do not significantly promote the activity of SRB unless used as a mixture of various carbon sources.

Effect of the external carbon source addition

The treatment of wastewater containing metals in anaerobic CWs is often based on the assumption that precipitation of metals with biogenic sulfides would be the predominant removal process. As it was already mentioned in this paper (Section1) the activity of the SRB depends on the availability of simple organic compounds, amongst other things. The source of these compounds may origin from the inside or the outside of the system. The former solution was applied when the experimental system was constructed by adding organic media to the bed media as peat. The latter source of organic carbon in the form of lactate was started to be added to the influent of the columns with mineral media (gravel) in week 44.

The main goal of adding the external carbon source to the influent was to enhance the removal of metals. The comparison of the metal removal efficiencies in the columns treating the influent with and without the external carbon source is given in Tab. 7 for weeks 44-53 of the experiment. The results given in Tab. 7 suggest that the addition of the external carbons source may allow treatment with comparable metal removal efficiency. It can be seen in Tab. 7 that statistically significant difference between the removal efficiencies was only for Pb, which was removed about 20% more efficiently in the UF-GP columns, than in the peat columns (UF-PP). The removal of sulfates is higher in the columns with peat, but the removal is low in all the types of columns presented in Tab. 7.

The data presented in Tab. 7 are of particular interest when the design decisions are to be made, but they furnish no information if the treatment efficiency was improved by the addition of the external carbon source. This information can be obtained, for example, by comparing the data in Tab. 6 and Tab. 7. More direct comparison of the treatment efficiency before and after the addition of the external carbon source is presented in Tab. 8 for weeks 45-53, this is with one-week lag relative to the moment when lactates were started to be added to the influent, and for weeks 33-44 when lactates were not added. The addition of the external carbon source improved the removal efficiency of metals and sulfates as shown in Tab. 8. The statistically significant differences were observed for Cu, Ni, Zn and sulfates in the UF-GP columns. The most pronounced improvement of the treatment efficiency was obtained for Ni (by about 32%) in the UF-GP columns. Only the removal efficiency of Cu in the UF-GP columns increased by less than 4%. The other removal improvements were higher. The addition of external carbon source (as glucose) was also reported to enhance the metal removal efficiency in subsurface flow microcosms by Lesage et al. [START_REF] Lesage | Effects of sorption, sulphate reduction, and Phragmites australis on the removal of heavy metals in subsurface flow constructed wetland microcosms[END_REF].

One of the drawbacks of using external carbon sources is that it increases the COD of the wastewater. This issue, however, must not be considered without comparing the effect of external and internal carbon sourced as a cause for residual COD in the effluent. The advantage of using external carbon sources is that CWs amended with these materials are less prone to clogging then wetlands with internal carbon source such as peat [START_REF] Kennedy | Hydraulics of Peat Filters Treating Septic Tank Effluent[END_REF]. Additional aspect that should be addressed is the release of reduced sulfur species from the system. They are more toxic to the environment than sulfate and may cause odor nuisance in some cases.

The influent and effluent COD and concentrations and the corresponding removal are shown in Tab. 9. Also the ratio COD/  2 4 SO in the influent is given as it is an essential parameter for the bacterial sulfate reduction. The results in Tab. 9 show that, even though, COD of the wastewater with the external carbon source was about 9-fold higher than the influent COD of the wastewater without the external carbon source the effluent COD of the former wastewater is lower than for the latter. Also the effluent concentrations of sulfates in all the types of columns are comparable. Taking into account the influent and effluent concentration of sulfates it can be assumed that there is a possibility that reduced species may appear in the effluent in comparable amounts from the columns with the internal and external carbon source. It should also be mentioned that some of the reduced sulfur species were probably oxidized to elemental sulfur in the outflow zone of the columns or were volatilized. The COD/

 2 4
SO ratio in the influent with the external carbon source was 0.32, this is, near the intended value of 0.3, which was proved optimum for the bacterial sulfate reduction as reported by Song et al. [START_REF] Song | Influence of electron donor and toxic materials on the activity of sulfate reducing bacteria for the treatment of electroplating wastewater[END_REF]. This ratio was much lower for the influent without the external carbon source, namely 0.04, however, the reduction of sulfates occurred as the organic-carbon source was in the media, so the actual ratio is unknown.

Noteworthy, no steady state was reached for the removal of metals after the addition of the external carbon source as it was indicated in Fig. 3e-h. This suggests that the treatment efficiency was still increasing making the external carbon source a reasonable alternative to the internal carbon source used in the experiment.

Conclusions

This study furnished new information and expanded the existing knowledge on industrial wastewater treatment in constructed wetlands. In fact, this is, to the best knowledge, the first publication studying the removal of metals and cyanide from electroplating wastewater in subsurface flow constructed wetlands. The aim of this study was to study the polishing of the electroplating wastewater pretreated using physical-chemical methods. High removal efficiency was achieved for all the metals of concern: Cu, Ni, Zn, save Pb, depending on the influent quality and the operation mode of a CW. Also the removal of cyanides was invariably above 90% in all the types of columns. Higher influent concentrations of Cu, Ni and Zn resulted in higher removal efficiency. The removal of Pb was decreased by elevated concentrations of other metals in the effluent. Higher HLR allowed for higher removal efficiency of metals in the UF columns. The role of plants in the removal of metals and cyanide was minor. Moreover, the observed effect of the presence of vegetation on the treatment efficiency was different for the metallic contaminants, thus it is difficult to draw definite conclusion. Application of peat as internal carbon source proved to affect significantly only the removal of Ni and Zn, causing increased removal efficiency as compared to gravel substrate. The removal of metals and sulfate was increased when lactate as an external carbon source was added to the influent to stimulate the activity of SRB. The use of lactate allows achieving higher treatment efficiency than in the counterpart constructed wetlands with the internal carbon source and comparable effluent concentration of organic matter in the effluent.

5

Figure 2

 2 Figure 2Median removal efficiencies (and max. and min. values indicated by whiskers; in %) in the columns of the experimental system during the experiment (n = 24-26), weeks 1-53

3

 3 presents the influent and effluent concentrations of Cu, Ni, Pb and Zn, respectively, on time scale. These graphs allow for observation of time-dependent changes due to internal factors and external factors, which are changes introduced intentionally (or sometimes occurring stochastically) during the experiment. The internal factors that govern the treatment in the studied CWs can be sorption capacity for metals, stimulation or inhibition of microorganisms, clogging, etc. The external factors can be controlled and monitored (e.g. flow rate, feed composition), monitored but uncontrolled (e.g. temperature) and uncontrolled and non-monitored. The internal behavior of the system can be to some extent caused by the external factors controlled during the experiment. For example, the activity of SRB can be increased providing more advantageous conditions for their growth. The trendlines in Fig. 3 represent two-period moving averages. Data for UF-PP columns Data for UF-GP columns Cu, Ni, Pb and Zn concentrations in the influent and effluent of the UF-PP (ad), UF-GP (e-h) columns as a function of time; concentration, (mg/L) on ordinate, time (weeks) on abscissa; weeks 1-53; n = 33, influent; n = 23-26, effluent It can be observed in Fig. 3 that the relation between effluent concentration and operation mode is not clearly visible for Cu, Pb and Zn. The exception is the data for Ni concentration in the effluent of both types of planted columns. It can be observed in Fig. 3b that the effluent Ni concentration decreased below 0.1 mg/L when high-strength feed was applied in the UF-PP columns. In the UF-GP columns (Fig. 3f), however, the influent and effluent conetrations of Ni were proportional. The effect of various operation mode was presented and discussed in further sections. The concentration of Cu in the effluent of the UF-PP and UF-GP columns was most data points in the range of 0.01 to 0.1 mg/L. For Ni, Pb and Zn effluent concetration most of data points were in the range of 0.1 to 1 mg/L with the exception of Ni in the UF-PP columns. It was also observed that the effluent concentration of Cu, Ni and Pb were stable or increasing towards the end of the experiment. In contrast, it was observed that the effluent concetration of Zn was slightly decreasing. It should be noted that the decrease of effluent metal concetration in the UF-GP columns (after week 44) can be attributed to the addition of lactates serving as external carbon source. It was observed that the addition of lactates caused the most significant decrease of metal effluent concentration in the case of Ni. The effect of cyanide addition to the feed (week 48) is not evident.

  2.

		100%
		90%
		80%
		70%
	Removal	40% 50% 60%
		30%
		20%
		10%
		0%
		UF-PP	UF-PU		UF-GP	UF-GU
		Cu	Ni	Pb	Zn

Table 2

 2 2. The removal efficiencies given in Tab. 2 were calculated based on median influent and effluent concentrations. The influent concentration of cyanides were different for columns with mineral and organic media, as these two types of columns were fed with wastewater with lactate and without lactate, respectively, from separate tanks The influent and effluent concentrations (median ± MAD) of readily liberated cyanides an corresponding removal efficiencies in all the types of columns (weeks 48-53; n = 5-8)

			Column type
			UF-PP UF-PU UF-GP UF-GU
	Influent	Median 1.043 1 1.043 1 1.280 2 1.280 2
	mg/L	MAD	0.534	0.534	0.851	0.851
	Effluent	Median 0.056	0.085	0.085	0.080
	mg/L	MAD	0.043	0.065	0.044	0.028
	Removal
	%		94.7% 91.9% 93.4% 93.8%

1

for the feed without lactates,

2

for the feed with lactates

Table 5

 5 Comparison of metal removal efficiency (median ± MAD, %) depending on the hydraulic loading in the UF columns

	Metal UF-PP 1	UF-GP 2
		0.15 m/d 0.09 m/d 0.15 m/d 0.09 m/d
	Cu	99.6±0.2 95.4±3.6 99.1±0.7 94.9±6.1
	Ni Pb	99.0±0.3 99.2±0.1 75.5±2.0 66.1±4.2 69.1±38.3 39.7±16.1 55.5±24.7 52.0±21.2
	Zn	95.6±1.4 94.8±2.9 89.3±7.5 74.0±11.2

Table 7

 7

			Comparison between metal removal efficiencies (median ± MAD, %) in
			columns treating wastewater with external carbon source (ECS) and without external carbon source (No ECS) external carbon source (weeks 44-53, n = 5-8)
	Substance UF columns
			UF-PP	UF-GP
			No ECS	ECS
	Cu Ni Pb Zn		96.0±1.3 98.1±0.6 98.1±0.8 97.8±0.3 46.2±32.6 69.6±9.8 97.1±0.6 93.5±3.0
	2 4 SO		10.7±3.5 4.0±21.9
	Statistically significant differences are shown in bold

Table 8

 8 Statistically significant differences are shown in bold

		Comparison between metal removal efficiencies (Median ± MAD, %) before (before ECS) (weeks 33-43, n = 4-5) and after addition of external carbon source (after ECS) to the feed of the UF-GP columns (weeks 45-53, n = 4-7)
	Substance UF-GP
		Before ECS After ECS
	Cu	94.9±6.1	98.2±0.5

Table 9

 9 COD and sulfates removal in the experimental system in weeks 44-49

		COD Concentration 1	Removal 2	 Concentration 1 2 4 SO	Removal 2	Influent COD/ SO 2 4	
	Influent	[mg O 2 /L] 32±2	[%]	[mg/L] 860±47.5	[%]	ratio 0.04
	Effluent from:
	UF-PP Influent+ECS 3 273±21 94±6	-197.0	742.5±36.25 855±40	13.7	0.32
	Effluent from:
	UF-GP	69±11	74.6	775±70	9.4

weeks

21-32 (0.15 m/d; n = 7), weeks 33-47 (0.09 m/d, n = 7), 2 weeks 21-32 (0.15 m/d; n = 7), weeks 33-43 (0.09 m/d, n = 5)

median±MAD,

calculated based on the median values for the influent and effluent; n = 4-6 for COD; n = 5-9 for sulfates; data for weeks 44-49,

external carbon source

Acknowledgements

The project was supported by Grant N N523 561938 from the Polish Ministry of Science and Higher Education. The lead author was granted scholarship in the Project "SWIFT (Scholarships Supporting Innovative Forum of Technologies)" POKL.08.02.01-24-005/10 cofinanced by the European Union under the European Social Fund.


Parini Alberto
email: alberto.parini@unife.it

Gaetano Bellanca
email: gaetano.bellanca@unife.it

Andrea Annoni
email: andrea.annoni@gmail.com

Francesco Morichetti
email: morichetti@elet.polimi.it

Andrea Melloni
email: melloni@elet.polimi.it

Michael J Strain
email: michael.strain@strath.ac.uk

Marc Sorel
email: marc.sorel@glasgow.ac.uk

Mathilde Gay
email: mathilde.gay@enssat.fr

Christelle Pareige
email: christelle.pareige@enssat.fr

Laurent Bramerie
email: bramerie@enssat.fr

M Thual
email: thual@univ-rennes1.fr

BER Evaluation of a Passive SOI WDM Router

Keywords: Optical switching devices, photonic integrated circuits, optical interconnections, microring resonators, Silicon on Insulator, Bit-Error-Rate

ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

I. INTRODUCTION

HIP multiprocessor architectures represent the state of the art in the design of high performance digital integrated systems. Currently, silicon technology allows the manufacturing of hundreds of computational cores on the same circuit die, hence reaching a high grade of parallelism and scalability. Due to the large number of interconnected functional entities, the data exchange among them can only be implemented by means of a shared network infrastructure (Network on Chip -NoC). As the number of linked cores increases, the burden of the communication infrastructure in terms of bandwidth demand and power consumption becomes predominant, such as to constitute the actual upper bound to a further expansion of the system. To overcome this limit, an attractive solution is to realize optical-based interconnections among the cores (Optical Network-on-Chip (ONoC)) [START_REF] Poon | Cascaded Microresonator-Based Matrix Switch for Silicon On-Chip Optical Interconnection[END_REF]. Indeed, photonic-based systems can achieve extremely high bandwidths through wavelength division multiplexing (WDM), while maintaining a sustainable power dissipation level [START_REF] Shacham | On the Design of a Photonic Network-on-Chip[END_REF]. In ONoCs literature, router topologies are usually classified into two families: active and passive networks. In active networks all rings have the same radius, and their resonances are tuned via thermo-optic or electro-optic effects. This allows a dynamic "on the fly" allocation of the routing paths, but requires extra tuning circuitry. In passive networks, on the contrary, the routing paths are set once during design, by suitably placing sets of rings with different radii. Passive solutions reduce the technological complexity of the circuit, by avoiding the presence of tuning mechanisms, but impose stricter constraints at architectural and system levels. While active topologies have been extensively analyzed [START_REF] Sherwood-Droz | Optical 4×4 hitless silicon router for optical networks-on-chip (NoC)[END_REF][START_REF] Biberman | Thermally active 4x4 non-blocking switch for networkson-chip[END_REF][START_REF] Lee | Multi-wavelength message routing in a non-blocking fourport bidirectional switch fabric for silicon photonic networks-on-chip[END_REF], in this contribution we report on the Bit Error Rate (BER) characterization of a passive topology with four input and four output ports, realized in Silicon-on-Insulator (SOI) technology. The switching capabilities are rated through BER curves for each significant routing path in a 10 Gbit/s single channel configuration and in a 3-channel (3×10 Gbit/s) WDM operating mode. The single channel BER, measured by exciting only one channel, allows to evaluate loss in performance due to the propagation in the component. On the other hand, WDM BER assesses the robustness of the device with respect to crosstalk effects when several channels Alberto Parini, Gaetano Bellanca, Andrea Annoni, Francesco Morichetti, Andrea Melloni, Michael J. Strain, Marc Sorel, Mathilde Gay, Christelle Pareige, Laurent Bramerie, and Monique Thual BER Evaluation of a Passive SOI WDM Router C Fig. 1. Left: sketch of the characterized GWOR topology with four input and four output ports. The wavelength-dependent routing is achieved through the choice of different radii for rings R1 and R2. Right: top-down microscope image of the fabricated device. The highlighted region is the switching core of the router. In the real device, to ease measurements, the input ports are all positioned on the left side of the chip whereas the output ones are on the right. For this reason, some optical circuitry (bend, crossing) have been added in the final layout.

propagate together, thus giving a direct estimation of the discrimination capability of the router [START_REF] Parini | Bit Error Rate Performance Evaluation of a Silicon-On-Insulator Optical-Network-On-Chip Router in a WDM Configuration[END_REF].

The article is structured as follows: section II describes the main operating characteristics of the investigated device, with fabrication details highlighted in section III. In section IV, the setup used for BER measurements is illustrated. Finally, results are reported and discussed in section V.

II. DESCRIPTION AND OPERATION

Fig. 1 shows the topology of the characterized four input, four output Generic Wavelength-routed Optical Router (4×4 GWOR) structure [START_REF] Tan | On a Scalable, Non-Blocking Optical Router for Photonic Networks-on-Chip Designs[END_REF]. The device consists of four pairs of rings, positioned at the corners of a square network layout (see Fig. 1). The rings labeled R1 have a slightly different round trip length (i.e. different resonant wavelength) with respect to the ones labeled R2. This spectral interleaving in the 1550 nm wavelength range provides the routing assignment of Tab 1.

As the device is topologically invariant under step rotations of 180° (see Fig. 1), and orthogonal input ports differ only with regards to the first encountered ring resonator routing wavelength, we have decided to characterize the device using the West-In port.

By injecting in this input port three signals with different wavelengths, namely λ 0 , λ 1 and λ 2 , the routing paths inside the device are the following:

1. λ 0 is a through (out of resonance) wavelength for both R1 and R2 ring pairs; consequently, the signal at this wavelength is directly routed from West-In to East-Out (through path);

2. λ 1 is a drop wavelength for the R1 rings; the associated data stream is then redirected toward the North-Out port (drop path driven by R1 rings);

3. λ 2 is a drop wavelength for the R2 ring family, but not for the R1 one; the associated data stream is therefore routed to the South-Out port (drop path driven by R2 rings).

In this device, U-turn paths, i.e. West-In to West-Out, are topologically forbidden (a computational core, in fact, does not need to communicate with itself).

III. DEVICE FABRICATION AND CHARACTERIZATION

The proposed integrated GWOR network was fabricated on a commercial Silicon-on-Insulator (SOI) wafer with a 220 nm thick silicon layer on a 2 µm thick oxide buffer layer. The channel waveguide was patterned using electron-beam lithography on a hydrogen silsesquioxane (HSQ) resist, followed by inductively coupled plasma etching [START_REF] Gnan | [END_REF]. The silicon core has a width of 480 nm and is buried under an oxide bi-layer made of 550 nm-thick HSQ layer and 350 nm-thick SiO 2 films, respectively. Propagation loss is about 3 dB/cm for TE polarization at a wavelength of 1550 nm. Inverse tapered waveguide sections buried in a SU-8 polymer waveguide were realized at the chip's end facets. These mode adapters reduce the coupling loss with tapered lensed fibers to (5±1 dB)/facet as well as the spurious Fabry-Perot resonance effects due to optical back-reflections at waveguide terminations.

Circuit design and mask generation was produced by using a commercial simulator [START_REF]ASPIC[END_REF] exploiting a building block approach for photonic integrated circuits [START_REF] Melati | Validation of the building block based approach for the design of photonic integrated circuits[END_REF]. The bending radius of the resonators of the GWOR is 20 m for rings R1 and 18 m for rings R2, resulting in a free spectral range FSR of 552 GHz and 613 GHz, respectively. The finesse of all the rings is about 35, corresponding to a drop port transmission bandwidth of 15.5 GHz (R1) and 17.5 GHz (R2). The bus-to-ring power coupling K = 7.5% is obtained through zero-length directional couplers with a gap distance of 150 nm between the straight bus waveguide and the bent waveguide of the ring. The waveguide crossings of the GWOR are optimized by introducing linear-shaped tapers with an overall footprint of 15 × 15 m 2 , in order to reduce both insertion losses (<0.7 dB) and cross-talk between orthogonal waveguides (<-30 dB). The overall footprint of the GWOR device is about 0.8 mm 2 .

IV. BER EVALUATION SETUP Fig. 2 shows the measured transmission spectra of the three output ports in the spectral range 1551 nm-1553.5 nm. Within this range, three possible carrier wavelengths corresponding to the previously described routing paths of the device are λ 0 = 1551.32 nm (through path between the West-In and the East-Out ports), λ 1 = 1552.11 nm (resonance of the R1 pairs) and λ 2 = 1553.13 nm (resonance of the R2 pairs). On the same figure, the black solid thin line shows the spectra of the three laser sources tuned at λ 0 , λ 1 and λ 2 , respectively. As can be observed from Fig. 2, the total losses (path losses + facet losses) measured at each channel wavelength are 23.3 dB at λ 0 , 33.78 dB at λ 1 and 35.81 dB at λ 2 . As previously stated, the BER of each channel is measured in two different configurations: single channel and 3-channel WDM.

Tab. 1: Routing table of the 4 × 4-GWOR North Out East Out South Out West Out North In - λ 2 λ 0 λ 1 East In λ 2 - λ 1 λ 0 South In λ 0 λ 1 - λ 2 West In λ 1 λ 0 λ 2 -
In the single channel scenario, only the carrier wavelength which corresponds to the path under test is launched into the West-In port of the DUT. In the WDM configuration, on the contrary, the BER for each output port is evaluated by injected simultaneously into the West-In port of the DUT a comb of three contiguous wavelengths associated with the three routing paths of the device. Fig. 3 shows in details the experimental setup employed for the BER measurements in the WDM configuration [START_REF] Nguyen | 100-Gb/s Wavelength Division Demultiplexing Using a Photonic Crystal Four-Channel Drop Filter[END_REF]. It must be pointed out that the setup for the single channel measurement is the same, except that in this case only the source pertinent to the examined path (channel under test) is active.

When operating in WDM configuration, two of the three wavelengths coming from the laser sources are combined with a 3 dB coupler and sent to the Mach-Zehnder modulator MZ2; the other channel, on the contrary, is sent to MZ1. The modulators receive two 10 Gbit/s binary NRZ input streams of 2 31 -1 Pseudo Random Bit Sequences (PRBS). MZ1 modulates the channel under test (in turns, λ 0 , λ 1 and λ 2 ), whereas MZ2 modulates the adjacent two interfering channels (in turns, the pairs (λ 1 , λ 2 ), (λ 0 , λ 2 ), and (λ 0 , λ 1 )).

This two modulator-based configuration guarantees that the data stream of the channel under test is not correlated to the data stream of the other two interfering channels. A further 3 dB coupler merges these signals before passing them through an erbium doped fiber amplifier (EDFA1). After the amplification stage, the signals are adjusted on a TE polarized mode by a Polarization Controller (PC) to avoid excitation of TM resonances, and finally coupled into the DUT. Microlensed fibers, with a mode field diameter of 2 m, provide an efficient input and output coupling to the DUT.

Moreover at the injection side, when operating in WDM, the power magnitude of the three wavelengths is kept equalized, in order to make the router to operate with a balanced comb.

V. RESULTS AND DISCUSSION

We evaluated the power penalty in single channel and in WDM configurations for a Bit Error Rate ranging between 10 -4 and 10 -10 , with respect to the Back to Back (B2B) experiment.

In the B2B experiment, the DUT is replaced with an optical loss stage, whose total attenuation matches the one introduced by the DUT on the observed port. Consequently, the B2B BER is a measure of the power penalty caused by the degradation of the optical signal to noise ratio, due exclusively to intrinsic losses. Results obtained for the different routing paths are summarized in the following:

NORTH OUT: This is a drop path driven by the R1 ring resonances. The routing wavelength associated with this port is λ 1 , while λ 0 and λ 2 represent the interfering signals when operating in WDM configuration. The top left panel in Fig. 4 shows the BER curves obtained in single channel (red line with circles) and in WDM (red line with triangles) operation, respectively. As one can note, the single channel configuration performs better than the corresponding B2B experiment (black dashed line). This apparently counterintuitive effect should not be surprising; in fact, since the device under test is a cascade of resonant filters, it can slightly modify the shape of the propagating pulses due to the filter selectivity, and give a better adaptation of the signal to the receiver, with a consequent increase of the sensitivity. Actually, this effect takes place for the north-out port and the south-out port, whose routing paths are associated with a drop transit (filtering) in rings R1 and R2, respectively. At a BER of 10 -9 , the power penalty measured in the WDM configuration with respect to the single channel one is only 0.4 dB; therefore, it is possible to assert that the associated routing path is only affected by intrinsic penalties due to the propagation, and is substantially robust with respect to crosstalk from the other channels. Moreover, the eye diagrams in both operating modes are nearly coincident, except for a slightly higher noise level affecting the eye in the WDM case.

SOUTH OUT: This drop path is driven by the R2 ring resonances. The associated wavelength is now λ 2 (right peak in the spectrum of the signal presented in Fig. 2), whereas λ 0 and λ 1 are the interfering WDM signals. From the measured transmission spectrum of Fig. 2 (green dash-dotted line) one can see that the rejection of λ 0 with respect to λ 2 is 15 dB, whereas the rejection of λ 1 is more than 25 dB. As a consequence of the high selectivity of this channel, the crosstalk can be considered negligible. This is confirmed by the corresponding WDM BER curve, which does not show any significant penalty with respect to the one measured in single channel configuration (only 0.14 dB at a BER of 10 -9 ). The robustness with respect to crosstalk is pointed out also by the eye diagrams, which are fully open in both cases.

EAST OUT: Unlike the previous two, this output port is a through-path for both R1 and R2 ring families. The associated wavelength is now λ 0 , while λ 1 and λ 2 are the interfering WDM signals. The BER of the single channel shows no significant penalties with respect to the B2B experiment, as claimed by the corresponding eye diagram. On the contrary, when operating in WDM regime, the BER curve exhibits a power penalty of 7 dB, (for a BER of 10 -9 ), which is induced by the strong crosstalk of the interfering channels. It must be noted that the low performance on this path is determined by the poor rejection of the north-eastern R2 ring couple. In fact, by observing the spectrum in Fig. 2 (blue solid line) one can note that the relative rejection of λ 2 with respect to λ 0 is of only 3 dB (right notch in the same figure). Due to this poor rejection, a high fraction of the λ 2 signal is not filtered out and remains on the bus waveguide, giving rise to interference on this output. This effect is also confirmed by the presence of multiple tracks in the eye diagram of Fig. 4. To assure a better performance on this path, R2 nodes must be precisely tuned. To this purpose, photo-induced treatments [START_REF] Canciamilla | Photo-induced trimming of chalcogenide-assisted silicon waveguides[END_REF][START_REF] Bachman | Permanent fine tuning of silicon microring devices by femtosecond laser surface amorphization and ablation[END_REF] can be exploited for a post-fabrication compensation of fabrication tolerances, as well as to trim the circuit response.

VI. CONCLUSION

The switching capabilities and the robustness to crosstalk of a four input -four output GWOR device realized in SOI technology have been evaluated through BER measurements in single (10 Gbit/s) and 3 channel (3×10 Gbit/s) WDM configurations. Comparisons between BER performance in single and multichannel operative conditions enable a clear understanding of the main sources of transmission quality deterioration in the system, pointing out the most critical paths across the router. Indeed, results show that the poor rejection of a switching node due to fabrication issues introduces a crosstalk power penalty of 7 dB on the through path. Surprisingly, the drop path driven by the faulty node is not penalized (power penalties lower than 0.5 dB), thus highlighting the stronger robustness of drop-port routed channels to crosstalk effects.

Fig. 2 .

 2 Fig. 2. Spectra of the three output ports of the device (blue solid line for East-Out, red dashed line for North-Out and green dash-dotted line for South-Out), together with the spectra of the three laser sources used for the BER estimations (black thin solid line).

Fig. 3 .

 3 Fig. 3. Experimental setup for BER evaluation in single channel and WDM configuration. Inset (a) shows the eye diagram of the input signal when operating in single channel configuration, whereas inset (b) shows the eye diagram of the input signal in WDM configuration.

Fig. 4 .

 4 Fig. 4. BER curves in single channel and in 3-channel WDM configurations for the three output ports of the device. Beside each plot the eye-diagrams corresponding to both scenarios are reported.

This work is supported by the Italian Ministry of the University and Research through PHOTONICA FIRB 2008 and SAPPHIRE PRIN 2009 Projects. Alberto Parini thanks the Programma Operativo FESR 2007-2013 of the Emilia Romagna Region -Attività I.1.1 for the financial support to his research activity. We acknowledge the support from the technical staff of the JWNC at Glasgow University.

A.