Pilot Scale Performance of the Electro-Oxidation of Landfill Leachate

Feb 16, 2009 - Angela Anglada, Ane Urtiaga and Inmaculada Ortiz* .... Orchideh Azizi , David Hubler , Glenn Schrader , James Farrell , and Brian P. Ch...
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Environ. Sci. Technol. 2009, 43, 2035–2040

Pilot Scale Performance of the Electro-Oxidation of Landfill Leachate at Boron-Doped Diamond Anodes ANGELA ANGLADA, ANE URTIAGA, AND INMACULADA ORTIZ* Department of Chemical Engineering, University of Cantabria, Avenida de los Castros s/n, 39005 Santander, Spain

Received September 29, 2008. Revised manuscript received January 15, 2009. Accepted January 15, 2009.

During the electrochemical oxidation of real wastewaters, the different species present in the effluent may interact creating complex scenarios making the prediction of the behavior of the whole system difficult. In this paper the different phenomena that occur during the electro-oxidation process of landfill leachate at a pilot plant scale with boron-doped diamond (BDD) anodes are elucidated. The total BDD anode area of the pilot plant was 1.05 m2. The evolution of the concentration of chloride ions, chlorate, and inorganic carbon and the value of pH and redox potential were found to be inter-related. In turn, the concentration of chloride affected the oxidation of ammonia, which took place through indirect oxidation by active chlorine. Moreover, chloride ions competed with organic matter to be oxidized at the anode. The effect of current density was also investigated. Organic matter and ammonia oxidation were highly influenced by the applied current density value. A change in the mechanism of organic matter oxidation was observed when high current densities were applied. Two mathematical models, previously applied to the oxidation of synthetic wastewaters in the literature, were able to predict the evolution of chemical oxygen demand and ammonia for low current density values.

Introduction The generation of municipal solid waste is rapidly growing in developed countries due to their increasingly affluent lifestyles and to their continuing industrial and commercial growth. The sanitary landfill method for the ultimate disposal of solid waste material continues to be widely accepted and used due to its economic advantages (1). One of the main problems generated by this common practice is the production of leachate, which is known for its complex composition. Landfill leachates contain a large number of compounds, some of which can create a threat to human health and the environment if released into the natural waters. Consequently, leachates need to be treated in order to meet the standards for its discharge into the sewer or its direct disposal into surface water (2). According to literature, the biological refractory nature of landfill leachates makes it necessary to develop alternative methods other than biodegradation to effectively reduce the contaminant loading of these effluents (3). * Corresponding author phone: 34 942201585; fax: 34 942201591; e-mail: [email protected]. 10.1021/es802748c CCC: $40.75

Published on Web 02/16/2009

 2009 American Chemical Society

In recent years, the application of electrochemical processes for the treatment of polluted wastewaters has gained increasing interest. This technology has been successfully applied to the degradation of both synthetic (4-7) and industrial wastewaters (8-10). In the case of landfill leachate, the effect of anode material and several operating factors such as pH, current density, chloride concentration, as well as added electrolytes have been investigated (11-13). On boron-doped diamond anodes, almost complete mineralization of organic matter with very high current efficiencies is obtained. The oxidation of organic compounds is known to occur, depending on the applied potential, through direct electron transfer in the potential region before oxygen evolution and indirect oxidation via electrogenerated hydroxyl radicals in the potential region of oxygen evolution. Moreover, several models that describe the electrochemical treatment of wastewater containing organic pollutants have been developed (14-17). These models were able to reproduce the experimental results with great accuracy. However, although they reproduced the experimental data obtained during the electrochemical oxidation of synthetic wastewaters contaminated with a wide variety of organic pollutants such as triazines (18), acetic acid, phenol (19), and textile dyes (20) with great precision, they still have to be validated for real wastewaters with complex and unknown detailed composition. During the treatment of real wastewaters, which are usually multicomponent mixtures of various pollutants, competitive reactions may occur at the anode surface, and the different species may interact creating complex scenarios that are difficult to describe. A proper understanding of the system behavior is necessary in order to be able to design the process. To achieve this, consistent mathematical models that describe the processes and reactions involved in the electro-oxidation of multicomponent mixtures of various pollutants have to be developed. This knowledge can be conveniently obtained from the study of the process at a pilot plant scale. In this work, the electrochemical treatment of landfill leachate, an especially troublesome byproduct of landfilling of municipal waste, was studied. The different phenomena that occur during the electro-oxidation process of landfill leachate at different applied current densities is presented. Two mathematical models (19, 21) that were used to describe the oxidation kinetics of synthetic wastewaters have been used to predict the evolution of chemical oxygen demand (COD) and ammonia. The experiments were carried out at a novel electrolysis plant with a total anodic area of 1.05 m2. A boron-doped diamond on silicon anode and a stainless steel cathode were employed.

Experimental Section Wastewater Characterization. The landfill leachate was collected from March 2008 to June 2008 from the municipal landfill site of Meruelo in Cantabria, Spain. The raw leachate was initially treated on site by a biological process of activated sludge to reduce biodegradable organic compounds and ammonia. The physicochemical characteristics of the biologically pretreated leachate are shown in Table 1. As can be seen, the effluent of the aerated sludge system is still a highly contaminated stream, with an average concentration of COD (860 mg/L) and ammonium (780 mg/L) above the disposal limits. Also, the landfill leachate presents a high value of the electrical conductivity, due to the high concentration of chloride anions, permitting the application of electrochemical oxidation without the addition of more electrolytes. VOL. 43, NO. 6, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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TABLE 1. Characterization of Pretreated Leachate Used as Feed in this Work parameter

range

average

pH conductivity (mS/cm) dissolved organic carbon, DOC (mg/L) COD (mgO2/L) [NH4+] (mg/L) anion concentrations chloride (mg/L) nitirite (mg/L) nitrate (mg/L) sulfate (mg/L) chlorate (mg/L)

8.05-8.23 8.7-10.7

8.16 9.4

300-390

337

770-970 630-900

860 780

1500-1600 10-250 5-1730 80-120 0-40

1537 95 1110 98 32

Pilot Plant Scale System. In Figure 1 a schematic diagram of the pilot plant is shown. The detailed design and construction of the unit was commissioned to Adamant Technologies. The elements of the pilot plant can be grouped in three sectors: the feeding system, the electrooxidation unit, and the power supply, instrumentation, and control unit. The feeding system includes a tank of 750 L (1) and three pumps disposed in parallel (2) that continuously feed the electrolyte to be treated into the electrooxidation unit (3). The tank has a low and high level switch (4), and cooling water is circulated through a refrigeration coil located at the bottom part of the feed tank. As the fluid leaves the tank it is distributed in three treatment lines disposed in parallel, with one pump per way and five DiaCell sets (5), containing each set ten DiaCells (anode-cathode pair). This gives a total of 150 cells [10 (cell set-1) × (5 sets line-1) × 3 (lines)]; the total anode surface of the pilot plant is 1.05 m2. Both the DiaCell sets and the individual cells within each set are arranged in parallel. The electrode materials are stainless steel for the cathode and boron-doped diamond (BDD) on silicon as the anode. Electric power is supplied by three power rectifiers (6) with a maximum output of 750 A and 16 V. The pilot plant includes also conductivity, temperature, pH, and ORP probes (7), an external hydrogen sensor (8), and a ventilation system (9). The unit is operated by means of a PLC (10). All experiments were conducted in discontinuous mode. A volume of 230 L of biologically pretreated leachate was oxidized in each experiment. In all cases, the leachate was previously ultrafiltrated in a ZeeWeed 10 hollow-fiber membrane module (nominal pore size ) 0.04 µm) to avoid the input of large particles in the electrochemical compartments. The flow rate was adjusted to 300 L/min per line. Analytical Determinations. Samples were withdrawn at regular time intervals and preserved in a refrigerator at 4 °C. DOC analysis were performed using a TOC-V CPH Shimadzu, and COD was determined by closed reflux and colorimetric method following the analytical procedure 5220D from Standard Methods (22). Ammonium nitrogen concentration was obtained by distillation and titration according to the Standard Method 4500 (22). Ion chromatography was used to determine the concentrations of inorganic anions.

Results and Discussion The effect of current density on the performance of electrochemical oxidation of landfill leachate was studied by varying the applied current density in the range 300-1200 A/m2. Next, an attempt at elucidating qualitatively the different phenomena that occur during the electrooxidation process of such a complex effluent as is landfill leachate, is made. To accomplish this a wide variety of data is presented; 2036

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DOC concentration, COD, chloride concentration, ammonium nitrogen concentration, inorganic carbon concentration (IC), chloride concentration, pH value, redox potential, and nitrate ion concentration. Figure 2 shows the effect of the applied current density on the changes in COD and DOC removal during the electrooxidation of landfill leachate. An increase in the current density from 300 to 450 A/m2 scarcely affects the removal rate of organic matter. However, at higher current densities the oxidation levels increase with the current density. In all cases, the mineralization rate (DOC removal) is smaller than that of COD. Although complete removal of COD was achieved for those experiments performed at 600, 900, and 1200 A/m2, a residual fraction of refractory carbon remains at the end of the treatment. In addition, the COD/DOC ratio decreased with time reaching values lower than 0.5. Similar findings have been reported previously in the literature, and this behavior has been explained in terms of the accumulation of refractory compounds, mainly short chain carboxylic acids, as final products (23). The low value of the theoretical COD/ TOC ratio for these products (i.e., 0.5 for maleic acid) lends support to this assumption and could explain the remaining carbon concentrations observed. In the literature, the electrochemical oxidation of organic matter has been described by three oxidation mechanisms. These mechanisms are: direct electrooxidation (24), hydroxyl radical mediated oxidation (14), and oxidation mediated by oxidants generated during the treatment from the salts contained in the waste (25). In Figure 2a, the experimental results are compared with those predicted by a model proposed in the literature (19) that assumes direct electrochemical reaction and mass transport limitations. This model has been validated with a wide range of organic compounds such as phenol (26), 4-nitrophenol (27), and 2-naphtol (24). The equation that describes the trend of COD during electrochemical oxidation is given below

[ ( )]

COD(t) ) COD0 exp -

Akm t V

(1)

The COD concentration during the electrochemical treatment is calculated by means of the initial value of COD (COD0), the area of the electrode (A), the volume of leachate in the tank (V), and the value of the mass transport coefficient (km). The latter was determined by means of the limitingcurrent technique using a ferricyanide/ferrocyanide/Na2CO3 electrolyte solution. Comparison of the experimental and simulated data, calculated using eq 1, reveals that this model satisfactorily predicts the evolution of COD when a current density of 300 and 450 A/m2 is applied. At higher current densities, the oxidation rate of COD is higher than that predicted by the model. This suggests a change in the oxidation mechanism of organic matter. At high current densities, mediated electrochemical oxidation processes such as indirect oxidation by hydroxyl radicals and by electrogenerated oxidants (from the oxidation of the electrolyte support) have a strong influence. The electrochemical oxidation of wastes containing chloride ions is said to lead to the formation of “active chlorine” (25). Although “active chlorine” has a high stability and oxidation capacity, an increase in chloride concentration in the range of 1420 mg/L < Cl- < 8570 mg/L was observed to have a slight adverse effect on the kinetics of COD removal during the electrochemical oxidation of landfill leachate on the laboratory scale (28, 29). This suggests that hydroxyl radical mediated oxidation, which has been observed to contribute to the oxidation process in the literature (30), coexists with direct oxidation at high current densities. An increase of the bOH radical concentration with the applied current density would account for the variation of the oxidation rate of organic compounds.

FIGURE 1. Schematic diagram of the pilot plant: 1, feed tank; 2, pumps; 3, Diacell unit; 4, low and high level switch; 5, DiaCell set; 6, power rectifiers; 7, probes; 8, hydrogen sensor; 9, ventilation system; 10, PLC.

FIGURE 3. Influence of the applied current density (b, j ) 300 Am2-; O, j ) 450 Am2-; 2, j ) 600 Am2-; 0, j ) 900 Am2-; 9, j ) 1200 Am2-) on the evolution of [NH4+]/[NH4+]0. Initial conditions were as follows: COD0 ≈ 1000 mg/L; DOC0 ≈ 360 mg/ L; [IC]0 ≈ 490 mg/L; [NH4+]0 ≈ 783 mg/L; pH0 ≈ 8.2; [Cl-]0 ≈ 1640 mg/L. All lines correspond to simulated data obtained using the mathematical model described by eqs 2-5.

FIGURE 2. Influence of the applied current density (b, j ) 300 Am2-; O, j ) 450 Am2-; 2, j ) 600 Am2-; 0, j ) 900 Am2-; 9, j ) 1200 Am2-) on the evolution of (a) COD/COD0 and (b) DOC/ DOC0. Initial conditions were as follows: COD0 ≈ 1000 mg/L; DOC0 ≈ 360 mg/L; [IC]0 ≈ 490 mg/L; [NH4+]0 ≈ 783 mg/L; pH0 ≈ 8.2; [Cl-]0 ≈ 1640 mg/L. The line represents the simulated data obtained using the mathematical model described by eq 1. Figure 3 shows the normalized ammonium concentration profiles during the electrochemical treatment of landfill leachate. This figure shows the influence of the applied current density. The comparison of Figure 2a with Figure 3 shows that the influence of the applied current density is much more significant in the case of ammonium oxidation than in the case of COD removal. Furthermore, whereas COD degradation profiles show an exponential rate for all applied current densities, the trend of ammonium removal is observed to vary with the applied current density. An initial delay in the curved evolutions of ammonium removal is observed at low current densities and gradually disappears as the current density increases. In addition, ammonium removal occurs at a slower rate than that of COD. These results can be attributed to the fact

that, at lower current densities, direct oxidation of COD is favored against chlorine evolution at the anode. Ammonia removal was found to take place, on the laboratory scale, through indirect oxidation by active chlorine formed during the electrochemical process (28, 29). During indirect oxidation, chlorine evolution occurs at the anode. At pH < 3.3, aqueous chlorine is the predominant species whereas at higher bulk pH values; its diffusion away from the anode is coupled to its dismutation reaction to form chloric (I) acid at pH < 7.5 and chloric (I) ions at pH > 7.5 (5). During indirect oxidation of NH4+, HOCl reacts with NH4+ through breakpoint chlorination reactions to regenerate Cl-. Consequently, theoretically, chloride concentration should remain constant. Nevertheless, chloride concentration can decrease for different reasons: AOX formation, stripping of chlorine, and chlorate formation at alkaline pH. Chloride concentration decreased during the whole treatment process and was highly affected by the current density (Figure 4a). Two zones can be distinguished in the chloride concentration profiles. In the first one chloride concentration decreases slowly with time and in the second one, a rapid decrease in chloride concentration is observed. This shift in the chloride concentration profile occurs when the pH value drops from an average value of 8.2 to 3 (Figure 4b), which in turn happens when the inorganic carbon (IC) concentration reaches a low value (Figure 4c). At alkaline pH, chloride concentration seems to decrease due to chlorate formation. Figure 5a shows the change in VOL. 43, NO. 6, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 5. (a) Formation of chlorate during the electrochemical treatment of landfill leachate. Influence of the applied current density: b, j ) 300 Am2-; O, j ) 450 Am2-; 2, j ) 600 Am2-; 0, j ) 900 Am2-; 9, j ) 1200 Am2-. (b) Percentage of chloride ions eliminated in the form of chlorate. Initial conditions were as follows: COD0 ≈ 1000 mg/L; DOC0 ≈ 360 mg/L; [IC]0 ≈ 490 mg/L; [NH4+]0 ≈ 783 mg/L; pH0 ≈ 8.2; [Cl-]0 ≈ 1640 mg/L.

FIGURE 4. Evolution with time of (a) chloride concentration, (b) pH, and (c) IC concentration during electrochemical treatment of landfill leachate. Influence of the applied current density: b, j ) 300 Am2-; O, j ) 450 Am2-; 2, j ) 600 Am2-; 0, j ) 900 Am2-; 9, j ) 1200 Am2-. Initial conditions were as follows: COD0 ≈ 1000 mg/L; DOC0 ≈ 360 mg/L; [IC]0 ≈ 490 mg/L; [NH4+]0 ≈ 783 mg/L; pH0 ≈ 8.2; [Cl-]0 ≈ 1640 mg/L. Solid lines in Figure 4c represent the linear fitting of the experimental data. chlorate concentration with treatment time as a function of the applied current density. For current density values of 300, 450, and 600 A/m2 chlorate concentration increases during the whole treatment process. However, when a current density of 900 and 1200 A/m2 is applied, chlorate concentration reaches a maximum value when the pH turns acidic. This is in accordance with the fact that chlorate formation takes place at alkaline pH (4, 31). Nevertheless, despite the higher ClO3- concentrations detected at high current densities, as the applied current density increases the percentage of chloride eliminated in the form of chlorate decreased with the applied current density from almost 38 to 26% for 300 and 1200 A/m2, respectively (Figure 5b). At acidic pH, the depletion of chloride concentration seems to be accelerated due to the formation of gaseous chlorine. It has already been mentioned that, at a pH value 2038

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FIGURE 6. Effect of the applied current density on the evolution with time of the redox potential: b, j ) 300 Am2-; O, j ) 450 Am2-; 2, j ) 600 Am2-; 0, j ) 900 Am2-; 9, j ) 1200 Am2-. Initial conditions were as follows: COD0 ≈ 1000 mg/L; DOC0 ≈ 360 mg/L; [IC]0 ≈ 490 mg/L; [NH4+]0 ≈ 783 mg/L; pH0 ≈ 8.2; [Cl-]0 ≈ 1640 mg/L. of 3, aqueous chlorine is the predominant species. Despite the use of a mechanical ventilation system, a strong smell of chlorine was detected at acidic pH. This sustains the hypotheses that at acidic pH, the loss of chloride ions is due to the formation of gaseous chlorine. In addition, the presence of a higher chlorine concentration could also be the source of the abrupt transition of ORP from 300 to 900 mV (Figure 6) as chlorine is a stronger oxidant than HOCl. Moreover, the rate of chloride depletion increased with the current density (Figure 4a). As a consequence, at 900 and 1200 A/m2 chloride concentration decreases rapidly with time

TABLE 2. Values of k′ at the Different Applied Current Densities current density (A/m2)

k′ (h-2)

R2

300 450 600 900 1200

0.019 0.036 0.051 0.114 0.160

0.989 0.98 0.993 0.814 0.732

and goes below 200 mg/L during the last 2 h of the process. As a result of the low concentration of Cl- at the end of the process, ammonium removal rate becomes negligible. This indicates that ammonium removal is due to indirect oxidation by electro-generated “active chlorine”. Likewise, Li and Liu (4) observed that the oxidation of ammonia was primarily due to the indirect oxidation by HOCl and that it was controlled by the rate of chlorine evolution at the anode. Besides, ammonia removal was highly affected by the applied current density and Cl- concentration. The evolution of nitrate concentration with time was also determined. For all the applied current densities, the percentage of N-NH4+ oxidized to N-NO3- increased during the first two hours of the process and then a plateau was reached. The average plateau levels varied in the range 63-80%. Nitrate formation was found to stop when either ammonium had been completely removed or when chloride concentration had disappeared. Szpyrkowicz and Radaelli (21) described the kinetics of decolorisation of a simulated textile wastewater by means of a second-order rate constant. In the present study this model (eq 2-4), which has been previously used to describe the decolorisation of textile wastewater, has been used to predict the evolution of ammonium based on the hypothesis that both degradations take place due to indirect oxidation promoted by active chlorine. d[NH+ 4] ) -k[NH+ 4 ][Cl2] dt

(2)

In eq 2, k is the second-order rate constant (mol-1 s-1) and [Cl2] is the concentration of dissolved active chlorine. The rate of chlorine loss reactions due to the cathodic reduction of active chlorine, to the anodic oxidation to ClO3-, and to the homogeneous reaction with NH4+ were supposed to be much lower than the production rate. Thus, the variation of chlorine concentration (till saturation value is reached) with time can be described by equation d[Cl2] φjA ) dt nFV

(3)

where φ is the current efficiency for chlorine evolution. The current efficiency for chlorine evolution depends on the current density, mass transport rate coefficient, and chloride concentration. This model assumes that chloride concentration and hence φ remain constant throughout the oxidation process and that Cl2 evolution is the main anodic reaction. Consequently, the substitution of the integrated form of eq 3 in eq 2 gives ln

[NH+ 4 ]t [NH+ 4 ]0

)-

kφjAt2 ) -k ′ t2 4nFV

(4)

Values of k′ (h-2) were calculated for the different applied current densities from the slopes of the logarithms of [NH4+]t/ [NH4+]0 vs the square of the electrolysis time. The values of the slope (k′) obtained from the exponential fitting together with the correlation coefficients are given in Table 2. It can

be observed, that although at low current densities (300-600 A/m2) the value of the correlation coefficient is higher than 0.98; at high current densities (900 and 1200 A/m2) its value is lower than 0.82. At high current densities a rapid decrease in chloride concentration occurs (Figure 4a), thus failing to comply with one of the model’s assumptions: that the faradaic efficiency for chlorine evolution remains constant throughout the electro-oxidation process. Therefore, the model reported in eqs 2-4 does not predict the behavior of ammonium oxidation at 900 and 1200 A/m2 satisfactorily. As a result, the values of k′ at 900 and 1200 A/m2 were left out of the linear fitting of the data in Table 2 (eq 5). k′ ) 1.07 × 10-4 × japl - 1.292 × 10-2 (R2 ) 0.996)

(5)

Although the value of k′ increased linearly with the applied current density, in accordance to the proposed kinetic model, the linear regression did not pass through the origin. The linear regression crossed the current density axis at around 120 A/m2. This value corresponds to the minimum current density that has to be applied to oxidize chlorine and hence ammonium. A minimum current density value has to be applied in order to oxidize ammonium as chlorine evolution is not the main reaction at the anode. Besides chlorine evolution, direct oxidation of organic matter, present in the wastewater at high concentrations, occurs at the anode and is favored against chlorine evolution at low current densities. Figure 3 shows a comparison of the experimental and simulated data obtained by calculating the value of k′ with eq 5. Simulated profiles are represented by the solid lines. It can be concluded that the kinetic model, developed on the basis of indirect oxidation by electrogenerated active chlorine, is able to predict the evolution of the concentration of the pollutant as a function of the current density, in the range 300-600 A/m2, and of the chloride concentration, during the electrochemical oxidation of such a complex matrix as is landfill leachate. Good agreement between the experimental and modeling results, with standard deviations ranging from 2.6 to 9.5%, was obtained.

Implications for Deployment of Electro-Oxidation Technology Electrochemical oxidation of landfill leachate, by means of BDD anodes, is an efficient technology that is able to oxidize COD completely and almost all ammonia under appropriate conditions. Although total mineralization of the organic matter is achieved, partial oxidation of ammonia to nitrate anions occurs. Therefore, a subsequent separation step is required in order to obtain an effluent with suitable characteristics for its reuse (i.e., irrigation) or for its discharge to natural water sources. It should be highlighted that the conventional biological method of nitrification/denitrification is usually preferred, as it is probably the cheapest method to eliminate organic matter and nitrogen from leachate. However, when the biological process is hampered by toxic substances and/or by the presence of biorefractory organics, which is usually the case of old landfill leachate (28), an alternative treatment or an additional treatment stage is needed. An attractive alternative is the membrane bioreactor technology (32, 33) which is able to reduce ammonia levels of 1200-1500 mg/L in the raw leachate down to 5-15 mg/L (34). Nonetheless, this technology is also unable to remove biorecalcitrant compounds which are in part responsible for the color of the leachate. Besides electrochemical oxidation, several advanced oxidation processes (i.e., Fenton, O3/H2O2, O3/UV, H2O2/ UV) have been proposed as efficient alternatives for mineralization of recalcitrant organics in landfill leachate (35). However, these techniques also present the disadvantages of their high costs, the frequent use of reagents, and, in the VOL. 43, NO. 6, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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case of Fenton oxidation, the generation of sludge. Thus, the development of a sustainable process based on the integration of efficient technologies is of paramount importance. If appropriately combined, electrochemical oxidation is envisaged to make a meaningful contribution to integrated treatment processes. At any rate, the selection of the best option relies on a thorough understanding of the behavior of each technology, described by means of suitable mathematical models and parameters. In this respect, this work contributes to the understanding of electrically driven oxidation of biorecalcitrant landfill leachates by identifying the different phenomena that occur during the electrooxidation process of landfill leachate at a pilot plant scale with BDD, anodes and by describing the oxidation kinetics of COD and ammonium by suitable mathematical models.

Acknowledgments Financial support from the Spanish Ministry of Environment (Project 546/2006/2-2.5), the Ministry of Education and Science (Projects PET2005-0169 and CTM2006-00317), and the collaboration of MARE, S.A. is gratefully acknowledged. A. Anglada also thanks the MEC for a FPU research grant.

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