An Integrated Process, Fenton Reaction−Ultrafiltration, for the

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An Integrated Process, Fenton Reaction-Ultrafiltration, for the Treatment of Landfill Leachate: Pilot Plant Operation and Analysis Oscar Primo, Ana Rueda, Marı´a J. Rivero, and Inmaculada Ortiz* Departamento de Ingenierı´a Quı´mica y Quı´mica Inorga´ nica, E.T.S.I.I. y T., UniVersidad de Cantabria, AVenida de los Castros s/n, 39005 Santander, Spain

The treatment of a mature landfill leachate, from a municipal landfill located in northern Spain (Cantabria), by an integrated system [Fenton reaction-neutralization-ultrafiltration (UF)] was carried out in a pilot plant that operated either in batch and continuous mode. The initial average chemical oxygen demand (COD) concentration of the leachate was 2100 mg/L, with a biological oxygen demand (BOD5)/COD ratio of 0.08. The sequence of stages implemented was (i) Fenton oxidation, (ii) neutralization of Fenton’s effluent to pH 7, and (iii) ultrafiltration with submerged membranes. The influence of different parameters, such as redox potential evolution, H2O2/COD mass ratio, and hydraulic retention time, was investigated in the Fenton oxidation process. The effect of neutralization and UF steps was also studied. COD reduction after the integrated treatment was up to 80%. The final effluent was free of solids, color, and iron. The results demonstrated the high potential of Fenton’s reagent in combination with membrane filtration in the remediation of recalcitrant wastewaters on a pilot plant continuously operated. 1. Introduction Landfill is one of the most widely employed methods for the disposal of municipal solid wastes (MSW). Up to 95% total MSW collected worldwide is disposed of in landfills.1,2 Some alternative methods, such as recycling, composting, and incineration, are nowadays very much encouraged, but even incineration creates a residue of approximately 10-20% that must be ultimately landfilled. Leachate is made up of rain that passes through a landfill site and liquids that are generated by the breakdown of the wastes within the landfill. It has high concentration of organic and inorganic contaminants, including humic acids, ammonia nitrogen, heavy metals, xenobiotics, and inorganic salts.1,3 Untreated leachates can mix with groundwater or surface waters and contribute to the pollution of the environment.3 Consequently, leachates need to be treated to meet the standards for its discharge into the sewer or its direct disposal into surface water. As a result, one of the major issues to deal with at present is the collection, storage, and suitable treatment of landfill leachates. The main alternatives to treat leachates are biological treatments and physicochemical treatments. The treatability of landfill leachate depends on its composition and characteristics. The biological treatment, has been shown to be very effective when leachate biological oxygen demand (BOD5)/chemical oxygen demand (COD) ratio is high, but this ratio generally decreases when the age of the landfill increases, because most of the organic compounds in the stabilized leachate have high molecular weight and are refractory compounds that are not easily biodegradable.3-7 Advanced oxidation processes (AOPs) have been applied to reduce the organic load or toxicity of several wastewaters from different origins.8-10 AOPs are based on the generation of hydroxyl free radicals, which have a high electrochemical oxidant potential. These processes have been proposed in recent years as an effective alternative for mineralization of recalcitrant organic compounds in landfill leachate. However, these tech* To whom correspondence should be addressed. Phone: 34 942201585. Fax: 34 942201591. E-mail: [email protected].

niques are not economically acceptable as an individual process. So, it is important to determine the best treatment option as well as the optimal operation conditions required to achieve the maximum removal of recalcitrant compounds. A significant decrease of overall leachate treatment cost could be obtained by the combination of AOPs with a biological process and/or with other physical-chemical technologies, but the compatibility of these combinations should be proved.11-15 The Fenton process is one of the most common AOPs, and it is capable of extensively degrading organic contaminants in a variety of wastewater streams. It has been successfully applied to the degradation of different industrial wastewaters, including pharmaceutical,16 textile,17,18 cork processing,19 or paper pulp20 industrial effluents. It is easy to operate and the technology required is simple. The reagents are readily available, easy to store, and relatively safe to handle. Energy input is not necessary to activate hydrogen peroxide, and the process can be performed at ambient temperature. Moreover, it commonly requires a relatively short reaction time compared with other AOPs.21,22 The use of the Fenton process to reduce BOD5 and COD of landfill leachates has been described in the literature. Deng and Englehardt23 have reviewed the current knowledge of the Fenton treatment of landfill leachate. In comparison with active benchscale research on the Fenton process, the reported practical application on pilot plant scale is scarce.24-28 The Fenton process is defined as the catalytic generation of hydroxyl radicals (•OH) resulting from the chain reaction between ferrous ion and hydrogen peroxide, and the oxidation of organic compounds (RH) by Fenton’s reagent can proceed by the following chain reactions:3,29

Fe2+ + H2O2 f Fe3+ + OH- + •OH

(1)

Fe2+ + •OH f Fe3+ + OH-

(2)

RH + •OH f H2O + R•

(3)

R• + Fe3+ f R++ Fe2+

(4)

The typical Fenton wastewater treatment process includes four stages: oxidation, neutralization, coagulation/flocculation, and solid-liquid separation.3

10.1021/ie071111a CCC: $40.75 © 2008 American Chemical Society Published on Web 01/01/2008

Ind. Eng. Chem. Res., Vol. 47, No. 3, 2008 947 Table 1. Landfill Leachate Average Composition parameter

unit

range

average

pH conductivity total suspended solids COD TOC BOD5 N-NH4 ClSO42Na+ K+ Ca2+ Mg2+ Fe (total) Mn2+ Sn2+

mS/cm mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L

7.61-7.96 7.23-10.3 280-354 900-2100 1100-1750 100-150 1100-1400 1737-2077 430-516 1177-1640 755-1005 149-177 67.5-90 1.75-3.4 1.15-1.5 1.5-1.9

7.7 9.1 295 1750 1300 120 1225 1876 500 1364 845 155 73 2.04 1.25 1.46

Coagulation is due to precipitation of ferric oxyhydroxides after the neutralization stage. Lau et al.11 and Wang et al.30 reported that the oxidation and coagulation were responsible for approximately 20% and 80% of overall COD removal, respectively, in Fenton treatment of a biologically stabilized leachate. Usually, solid-liquid separation is carried out in sedimentation tanks, but a big area is needed for the large-dimensioned settling tanks, and in addition, sometimes the solid particles resist settling. Therefore, membrane processes appear as suitable technologies to increase the efficiency of the separation process with low space requirements. The membranes can be fitted either outside or within the tank. Both configurations can be employed to obtain an effluent without suspended solids. In this work, this separation was achieved using submerged ultrafiltration membranes, because it is the configuration that uses less space and works with higher concentrations of suspended solids. Ultrafiltration membranes have pore sizes between 1 and 20 nm and are designed to provide high retention of colloid and other macromolecules and to produce a high-quality effluent. Other advantages of this technology in contrast to conventional processes are its modular design and high capacity. The aim of this work is to evaluate the efficiency of an integrated process that combines Fenton oxidation followed by neutralization and solid-liquid separation with submerged ultrafiltration membranes, for the treatment of a stabilized landfill leachate on pilot plant scale, with the main objective of reducing organic matter expressed as COD and color. 2. Materials and Methods 2.1. Landfill Leachate Characterization. Leachate was collected from a municipal landfill located in Cantabria, a region in the north of Spain. The total area of the landfill was 40 000 m2 and about 250 000-300 000 tons of municipal solid waste were disposed there per year. Leachate generation in the landfill was about 500-800 m3/day. It was drained out with perforated pipes and stored in pools with 4000 m3 capacity. The leachate was initially treated in situ by a biological process to reduce biodegradable organic compounds and ammonia. Fenton treatment was studied as an alternative treatment or a pretreatment to increase the biodegradability or reduce the toxicity before the conventional biological process. The average physicochemical characteristics of the raw leachate are summarized in Table 1. It was an alkaline mixture of dark brown color. The BOD5/ COD ratio indicated the low biodegradability. Other major components present in the leachate were ammonium and chloride.

2.2. Materials. The materials used in the experiments were FeSO4‚7H2O (Panreac) as source of catalyst, 96% H2SO4 (Panreac) and Ca(OH)2 solution (Calcinor, S.A.) or 50% NaOH (Panreac) to adjust the pH value, and 35% w/w H2O2 (Solvay Interox, S.A.). All chemicals were of analytical grade. Deionized water supplied by a Milli-Q water purification unit (Millipore Waters) was used to prepare diluted samples. 2.3. Pilot Plant. The landfill leachate treatment in the pilot plant consisted of three steps: Fenton oxidation, neutralization, and ultrafiltration with submerged membranes. In the Fenton oxidation, iron was added as catalyst, so it had to be removed after the reaction. Therefore, a neutralization step was required. To obtain a final effluent with low solids concentration and low turbidity, conventional clarification sometimes offers technical problems, so ultrafiltration with submerged membranes was tested on pilot plant scale. Fenton experiments were carried out using a 130 L polyethylene tank. Mixing was provided by a speed motor (VLA, Milton Roy) connected to an epoxy-coated steel shaft. The acidic condition in the reactor (pH ) 3) was controlled with an automatic pH controller (Dositec pH, ITC) that added sulfuric acid when necessary. Three pumps supplied reagents solutions (H2O2 and Fe2+) and landfill leachate to the reactor (Prominent Beta BT4a, Hanna BL 1.5, and Prominent Vario C, respectively) according to the treatment conditions. At the end of the reaction, the Fenton effluent was neutralized in a 280 L stirred polyethylene tank. The tank was equipped with an automatic pH controller (Dositec pH, ITC). Ca(OH)2 or NaOH was added to keep pH value at 7. Ultrafiltration was carried out using a submerged membranes unit (ZeeWeed 10, Zenon). Membrane fouling was prevented by air scouring and with periodical backwashing of the membranes. A ZeeWeed 10 hollow-fiber membrane module (nominal pore size ) 0.04 µm, nominal surface area ) 0.93 m2) was immersed in a 250 L polyethylene tank. Figure 1 shows a schematic flowsheet of the leachate treatment in the pilot plant. The Fenton leachate treatment was carried out in batch and continuous operation. The influence of the main experimental variables in the process efficiency was studied. Table 2 shows the experimental conditions used in the different experiments. 2.4. Analytical Determinations. Samples were withdrawn at regular time intervals from the three tanks and preserved in a refrigerator at 4 °C, in accordance with standard methods.31 Prior to the analytical determinations, large particles and suspended solids were removed by filtration with 0.45 µm polypropylene syringe filters. Total organic carbon (TOC) analyses were performed using an TOC-V CPH (Shimadzu), and COD was determined by a closed reflux and colorimetric method (Spectroquant NOVA 400, Merck) following the analytical procedure 5220D from ref 31. To analyze the COD concentration of the treated samples, interferences caused by residual H2O2 were avoided using sodium hydrogen sulfite solution (40% w/v). BOD5 was measured by incubation during 5 days at 20 °C using Oxitop bottles. Ammonium nitrogen concentration was obtained by distillation and titration according to Standard Method 4500.31 Ion chromatography (Dionex 120 IC with IonPac AS9-HC column) was used to determine the concentrations of inorganic anions and organic acids, whereas metals in leachate were measured by induced coupled plasma (Perkin-Elmer, Plasma 400). Total suspended solids (TSS) retained by a glass filter of 0.45 µm and dried at 103-105 °C were determined gravimetrically

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Figure 1. Pilot plant diagram. Table 2. Experimental Conditions Used in Fenton Leachate Treatment: (a) Batch Experiments and (b) Continuous Operation (a) Batch Experiments: Reaction Time ) 60 min, pH0 ) 3.0 expt

H2O2/COD0 (mass)

H2O2/Fe2+ (mass)

Fe2+ (g/L)

1 2 3 4 5 6

2.5 4 10 2.5 4 10

3.75 6 10 1.875 3 5

1 2

(b) Continuous operation: pH ) 3.0 expt

H2O2/COD0 (mass)

H2O2/Fe2+ (mass)

HRT (h)

1 2 3 4 5

5 3.3 3.3 1.6 3.3

11 7 7 3 7

2 2 3 2 1.25

(Standard Method 5420D).31 The hydrogen peroxide concentration was monitored by iodometric titration. The oxidation-reduction potential, pH, and temperature of the reaction medium was followed with a Hanna HI8424 instrument. Conductivity was measured with a Crison CM 35 conductivimeter. 3. Results and Discussion 3.1. Fenton Oxidation. Initially, the definition of the operational conditions for the pilot scale tests was based on several experiments carried out on laboratory scale.32 The pilot plant was operated for 9 months. 3.1.1. Batch Experiments. First, batch experiments were carried out to check the reproducibility of the process on the pilot plant scale. All experiments were carried out in triplicate with an experimental error lower than 6%. The reactor was filled with 100 L of leachate and the pH was adjusted to 3-3.5 with sulfuric acid. Then, reagents were added, and the total reaction time was 60 min.

Figure 2. COD removal efficiencies for Fenton’s reaction at different H2O2/ COD mass ratios in batch experiments (reaction time ) 60 min, pH0 ) 3.0).

The efficiency of Fenton oxidation to remove organic compounds, expressed as COD, was investigated by varying the initial H2O2/COD weight ratio from 2.5 to 10.0 with Fe2+ concentrations of 1 and 2 g/L. The results are shown in Figure 2. For the range of work, the initial H2O2/COD ratios scarcely affected to the COD removal, which ranged between 57% and 61% when the iron concentration was 2 g/L. This fact could be due to the presence of a refractory organic fraction that was not susceptible to Fenton oxidation (i.e., low molecular weight organic acids). Slightly better results were achieved when 1 g/L Fe2+ was used, with all the H2O2/COD ratio values reaching COD removals up to 74%. The increase of H2O2/Fe2+ ratio could lead to a predominant role of chemical oxidation versus coagulation33 or to the scavenging effect of the excess of Fe2+ on hydroxyl radicals (reaction 2). Some of the Fe3+ ions formed can be reduced to Fe2+ through reaction 4, although the rate of reduction is several orders of magnitude lower than that of Fe2+ to Fe3+ conversion.3 Furthermore, the use of a small concentration of iron meant a lower generation of iron sludge at the end

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Figure 3. Contribution of organic acids to the final COD value in batch experiments (reaction time ) 60 min, pH0 ) 3.0).

Figure 5. Start up of Fenton’s reaction in continuous operation (Fe2+ ) 1 g/L, HRT ) 120 min, pH0 ) 3.0).

Figure 4. Effect of reaction time on COD removal efficiency (Fe2+ ) 1 g/L, H2O2/COD ) 4.0, pH0 ) 3.0).

of the treatment. The maximum COD removal (74%) was achieved by working with 1 g/L Fe2+ and H2O2/COD ) 4. Chemical oxidation processes can break down or rearrange molecular structures of organic matter and convert the nonbiodegradable organic compounds to more biodegradable forms.13 As the Fenton degradation proceeds, low molecular weight acids accumulate, because they are resistant to oxidation by this treatment. The concentration of these organic acids was measured and its equivalent COD concentration calculated. Therefore, the greater the extent of Fenton’s oxidation, the greater the amount of acid byproducts formed, which are more amenable to biodegradation.25,34 Acetic, formic, and oxalic acids were the three main organic acids detected in the effluent of the Fenton process. Formic acid was the most recalcitrant of these compounds to Fenton oxidation. Figure 3 shows the percentage of the final COD attributed to the sum of concentrations of the organic acids in the applied conditions in the batch Fenton experiments. It could be observed that final acid concentrations increased, up to 55% of final COD, when the initial H2O2/COD ratio increased. The effect of the oxidation time on the Fenton process was tested working at the best experimental conditions. Figure 4 shows the increase of COD reduction as a function of reaction time. The results demonstrated that COD was rapidly removed in the first 30 min when the reagents concentrations were higher, and therefore, the oxidation occurred more intensely. Foam was formed on the top layer of the leachate and proved the formation of carbon dioxide as the oxidation reaction proceeded. The removal of COD was not significant after 60 min of reaction time. However, longer times (120 min) were used in order to avoid the presence of residual hydrogen peroxide in the effluent. 3.1.2. Continuous Experiments. According to the results obtained in the batch experiments, initial Fenton experiments

Figure 6. COD and TOC removal efficiencies for Fenton’s reaction at different HRT (Fe2+ ) 1 g/L, H2O2/COD ) 3.3, pH0 ) 3.0).

were carried out to analyze the behavior of the continuous process working at a residence time of 2 h. H2O2/COD0 mass ratio was 5 and H2O2/Fe2+ mass ratio was 11, at pH 3 and room temperature. Redox potential, which could be a helpful parameter in continuous experiments, as its value is related to the oxidant capacity of the reaction medium, was continuously recorded.25,35 Figure 5 shows the value of redox potential and COD removal percentage in the Fenton reactor. When the COD concentration decreased, the redox potential increased, and when the oxidation reaction was almost finished, these parameters remained stable. 60% of COD removal was reached. A final value of 590 mV for the redox potential at pH 3 was obtained. These results confirmed that the Fenton treatment can be monitored by measuring the oxidation reduction potential. This is very helpful from a technical point of view, as the possibility of monitoring a process on-line simplifies the process control. Effect of the Hydraulic Retention Time. The hydraulic retention time (HRT) is a very important parameter to design a process operating in continuous mode at full scale. It is related not only to the treatment efficiency but also to the size of the reactor. The design of the pilot plant employed in this work limited the treatment capacity up to a maximum value of 70 L/h; next, a set of experiments was performed in order to check the behavior of the system and the influence of the hydraulic retention time in the yield of Fenton oxidation process, working with initial average COD and TOC concentrations of 2100 and 1100 mg/L, respectively, and with H2O2/COD mass ratio of 3.3 and H2O2/Fe2+ mass ratio of 7. Figure 6 shows the COD and TOC removal efficiencies achieved in the reactor working at

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Figure 7. COD removal efficiencies for Fenton’s reaction at different H2O2/ COD mass ratios in continuous operation (Fe2+ ) 1 g/L, HRT ) 120 min, pH0 ) 3.0).

Figure 8. TSS concentration and vacuum pressure in the UF process during the continuous operation (Fe2+ ) 1 g/L, pH0 ) 3.0).

three different hydraulic retention times. The highest COD and TOC removal efficiencies in the reactor (78 and 71%, respectively) were achieved for a retention hydraulic time of 3 h, but after comparison with the results obtained after 2 h, it was concluded that the slight increase of efficiency did not justify working at longer times than 2 h. Effect of H2O2/COD Mass Ratio. The ratio of hydrogen peroxide to ferrous iron (H2O2/Fe2+) and the ratio of hydrogen peroxide to organic matter (H2O2/COD) are key issues in the Fenton process. The H2O2/Fe2+ mass ratio can greatly fluctuate according to the type of pollutants and to the matrix effect in complex wastewaters.5,18,29,36-39 For this reason, and considering the results obtained in batch experiments on the pilot plant, different experiments working with H2O2/Fe2+ mass ratios of 3, 7, and 11 (H2O2/COD mass ratios 1.6, 3.3, and 5, respectively) were performed. Figure 7 indicates that the optimal H2O2/COD mass ratio to achieve the maximum COD removal is around 3.3. As it can be seen, when H2O2 was overdosed (H2O2/COD ) 5), •OH radicals were scavenged through reaction 3, decreasing the COD removal efficiency. In batch experiments, 76% COD removal was achieved after 120 min of reaction using a ratio H2O2/COD ) 4 and 1 g/L Fe2+. In continuous operation, and taking into account the experimental error, similar results were obtained, i.e., 78% COD reduction after 2 h of oxidation, but employing less hydrogen peroxide (H2O2/COD ) 3.3). A higher yield of hydrogen peroxide was obtained in continuous mode. Two-Step Process. Working with complex wastewaters such as landfill leachates makes it very difficult to characterize the chemical compounds present during the oxidation reaction. Therefore, although the presence of short chain organic acids, which are recalcitrant to Fenton oxidation, could be expected from batch experiments, additional experiments were carried out to study the possibility of increasing the COD removal in a second Fenton oxidation step. The experimental conditions employed in this additional reaction were the optimum conditions found in the previous experiments (H2O2/COD ) 3.3 and H2O2/Fe2+ ) 7). The residual H2O2 concentration in the effluent of the first Fenton reaction was considered to calculate the H2O2 dosage in the second step. It was observed that COD did not decrease considerably after a second Fenton reaction. Thus, the results obtained showed that after the first oxidation process the remaining organic material was recalcitrant to treatment by the Fenton process. 3.2. Neutralization. Not only oxidation but also the coagulation step contributed to the removal of organic constituents, though the effect of coagulation has not been well-determined.3

In this study, the pH selected to carry out the precipitation of iron as well as the degradation of residual hydrogen peroxide was 7. In the neutralization step, iron precipitates were formed and, consequently, organic matter associated with iron was removed from the solution. In batch experiments, a solution of 50% NaOH was used to set the pH at the desired value. COD reduction ranged between 10% and 18% of the Fenton effluent concentration according to the operational conditions. This low efficiency could be due to the fact that low molecular weight oxidative byproducts were more difficult to precipitate than high molecular weight compounds.3 When the pilot plant was operated in continuous mode, a solution of 40 g/L Ca(OH)2 (pH 7) was used in the neutralization step. The achieved COD removal ranged between 16 and 30%, and the sulfate concentrations decreased to 5000 mg/L. The lower solubility of CaSO4 helped to remove more ions compared to the 11 000 mg/L of sulfates obtained after neutralization with NaOH. Lo´pez et al.25 worked at pH 8.5 and added Ca(OH)2 and a cationic polyelectrolyte to remove residual ferric ions, obtaining a COD removal of 18%. All these results demonstrate that the neutralization is a polishing step, and better results are obtained when Ca(OH)2 is used because an additional reduction of sulfate ions is achieved. 3.3. Ultrafiltration. The ultrafiltration step achieved the removal of the precipitated solid compounds in the neutralization step, obtaining a clean, colorless effluent with residual iron concentration lower than 1 mg/L. The COD removal was low because the remaining organic compounds were constituted by small molecular structures as result of the oxidation treatment and they could not be retained by the membrane. In batch experimental operation of the Fenton reaction, the ultrafiltration unit was running in continuous mode for approximately 50 h and the permeate flow was in the range of 30-50 L/h. Submerged membranes showed good behavior without fouling problems and kept a constant permeate flow. The amount of total solids in the tank was higher than 10 g/L without an increase in the transmembrane pressure, which was kept constant at 10 psi. The COD in the effluent ranged between 350 and 400 mg/L. In continuous operation, the ultrafiltration process reached a low additional removal of residual COD. The amount of total solids in the ultrafiltration tank varied between 10 and 24 g/L, increasing transmembrane pressure from 10 to 45 psi in 55 h of operation time when the permeate flow increased from 30 to 70 L/h (Figure 8). Figure 9 shows the COD concentration in each step in the different experimental conditions. The lowest COD concentra-

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Figure 9. COD concentration decrease after the integrated process (Fe2+ ) 1 g/L, pH0 ) 3.0). Table 3. Percentages of Removal in the Different Steps of the Treatment for Different Modes of Operation batcha

treatment step leachate tank Fenton’s reactor neutralization ultrafiltration tank final effluent

continuousb

COD COD TOC removal TSS removal removal (%) (mg/L) (%) (%) 0 74 79 80

350 10000 ndc

0 78 81 83

0 69 81 84

TSS (mg/L) 400 1000 3500 10000-24000 nd

a Batch operation: H O /COD ) 4; Fe2+ ) 1 g/L, HRT ) 1 h. 2 2 Continuous operation: H2O2/COD ) 3.3; Fe2+ ) 1 g/L, HRT ) 2 h. c nd ) not detectable. b

tion achieved in the final effluent (after Fenton reaction, neutralization, and UF) was approximately 400 mg/L. Considering that this value is higher than the threshold value (160 mg/ L) for direct disposal into surface waters in the Spanish legislation, that the final oxidation products are small chain organic acids, and that the BOD5/COD ratio increased from the initial value of 0.08 up to 0.36, a final biological treatment could be a good option in the definition of the whole process. 3.4. Comparison of Results in Batch and Continuous Operation. Table 3 summarizes the reduction percentages of several parameters during the treatment steps achieved working in batch and continuous modes and using the best operational conditions found experimentally: 1 g/L Fe2+ and H2O2/COD ) 4 for batch operation and 1 g/L Fe2+ and H2O2/COD ) 3.3 for continuous experiments. As expected, similar results with a total COD reduction around 80% in the final effluent were achieved under both operation modes. This effluent was free of solids, iron, and hydrogen peroxide. The pilot plant treatment combining Fenton oxidation and UF attested a high potential to remove organic matter and color, working in both batch and continuous mode. In the final effluent, the ammonium concentration (1100 mg/ L) and inorganic salts (5000 mg/L sulfates) still remained. Thus, a total polishing of the effluent could be achieved by incorporation of a process with high efficiency in ammonium removal (MBR, electrochemical oxidation) followed by a membrane unit, such as reverse osmosis, for the removal of inorganic salts. 4. Conclusions In this work, the optimal operation conditions of an integrated process, Fenton-UF, applied to the treatment of landfill leachate have been defined. Starting with the information obtained at

laboratory scale, the analysis was carried out in a pilot plant with an average capacity of 50 L/h. The progress of Fenton’s treatment could be instrumentally monitored by measuring the redox potential evolution during leachate oxidation. The best operational conditions were determined. Working in batch mode, the COD reduction achieved was approximately equal to 76% using 1 g/L Fe2+ and a H2O2/ COD ratio of 4. The residual COD was attributed to low molecular weight organic acids (mainly, acetic, formic, and oxalic acids). In continuous operation, the maximum amounts of COD and TOC that could be removed by this system were 78 and 69%, respectively. Such removal values were achieved working with a H2O2/COD mass ratio of 3.3 and with a hydraulic retention time in the reactor of 2 h. After Fenton’s reaction, the residual iron was precipitated, using a solution of 50% NaOH for the batch experiments and 40 g/L Ca(OH)2 solution in the continuous operation, at pH 7. Neutralization using Ca(OH)2 led to better COD removal and higher reduction of sulfate ions. Ultrafiltration using submerged membranes separated the sludge, yielding a colorless final effluent without suspended solids. Submerged membranes showed good behavior without fouling problems and kept a fixed permeate flow of 50 L/h with an amount of solids in the tank higher than 10 g/L without increasing transmembrane pressure. Neutralization and UF processes caused an additional low removal of residual COD. They could be considered as polishing steps in the removal of COD. Continuous operation and batch experiments achieved similar results. A slightly higher hydrogen peroxide efficiency was obtained in continuous mode. The pilot plant treatment combining Fenton oxidation and UF attested a high potential to remove recalcitrant organic matter and color from landfill leachates, allowing for an easy removal of the residual biodegradable COD and ammonia with further treatment. Acknowledgment Financial support from the Spanish Ministry of Environment (project 546/2006/2-2.5) and MARE, S.A. collaboration is gratefully acknowledged. O.P. and A.R. also thank the Leonardo Torres Quevedo Foundation and University of Cantabria, respectively, for a predoctoral research grant. Literature Cited (1) Kurniawan, T. A.; Lo, W.; Chan, G. Y. S. Radicals-catalyzed oxidation reactions for degradation of recalcitrant compounds from landfill leachate. Chem. Eng. J. 2006, 125, 35-57. (2) Diamadopoulos, E. Characterization and treatment of recirculationstabilized leachate. Wat. Res. 1994, 28, 2439-2445. (3) Deng, Y. Physical and oxidative removal of organics during Fenton treatment of mature municipal landfill leachate. J. Hazard. Mater. 2007, 27, 380-388. (4) Kang, Y. W.; Hwang, K. Y. Effects of reaction conditions on the oxidation efficiency in the Fenton process. Wat. Res. 2000, 34, 2786-2790. (5) Wang, F.; Smith, D. W.; Gamal El-Din, M. Application of advanced oxidation methods for landfill leachate treatmentsA review. J. EnViron. Eng. Sci. 2003, 2, 413-427. (6) Kurniawan, T. A.; Lo, W.; Chan, G. Y. S. Physico-chemical treatments for removal of recalcitrant contaminants from landfill leachate. J. Hazard. Mater. 2005, 129, 80-100. (7) Wiszniowski, J.; Robert, D.; Surmacz-Gorska, J.; Miksch, K.; Weber, J. V. Landfill leachate treatment methods: A review. EnViron. Chem. Lett. 2006, 4, 51-61. (8) Legrini, O.; Oliveros, E.; Braun, A. M. Photochemical processes for water treatment. Chem. ReV. 1993, 93, 671-698. (9) Andreozzi, R.; Caprio, V.; Insola, A.; Marotta, R. Advanced oxidation processes (AOP) for water purification and recovery. Catal. Today 1999, 53, 51-59.

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(10) Gogate, P.; Pandit, A. A review of imperative technologies for wastewater treatment II: Hybrid methods. AdV. EnViron. Res. 2004, 8, 553597. (11) Lau, I. W. C.; Wang, P.; Fang, H. H. P. Organic removal of anaerobically treated leachate by Fenton coagulation. J. EnViron. Eng. 2001, 27, 666-669. (12) Rivas, F. J.; Beltra´n, F.; Carvalho, F.; Gimeno, O.; Frades, J. Study of different integrated physical-chemical + adsorption processes for landfill leachate remediation. Ind. Eng. Chem. Res. 2005, 44, 2871-2878. (13) Lopez de Morais, J.; Peralta Zamora, P. Use of advanced oxidation processes to improve the biodegradability of mature landfill leachates. J. Hazard. Mater. 2005, 123, 181-186. (14) Battistoni, P.; Boccadoro, R.; Bolzonella, D.; Pezzoli, S. Optimization of chemical and physical pretreatments in a platform for the treatment of liquid industrial wastes. Ind. Eng. Chem. Res. 2001, 40, 4506-4512. (15) Santos, A.; Yustos, P.; Rodrı´guez, S.; Garcı´a-Ochoa, F.; Gracia, M. Decolorization of textile dyes by wet oxidation using activated carbon as catalyst. Ind. Eng. Chem. Res. 2007, 46, 2423-2427. (16) Tekin, H.; Bilkay, O.; Ataberk, S.; Balta, T.; Ceribasi, I. H.; Sanin, F. D.; Dilek, F. B.; Yetis, U. Use of Fenton oxidation to improve the biodegradability of pharmaceutical wastewater. J. Hazard. Mater. 2006, 136, 258-265. (17) Arslan, I.; Teksoy, S. Acid dyebath effluent pre-treatment using Fenton’s reagent: Process optimization, reaction kinetics and effects on acute toxicity. Dyes Pigm. 2005, 73, 31-39. (18) Gulkaya, I.; Surucu, G. A.; Dilek, F. B. Importance of H2O2/Fe2+ ratio in Fenton’s treatment of a carpet dyeing wastewater. J. Hazard. Mater. 2006, 126, 763-769. (19) Beltra´n, J.; Domı´nguez, J. R.; Lo´pez, R. Advanced oxidation of cork-processing wastewater using Fenton’s reagent: Kinetics and stoichiometry. J. Chem. Technol. Biotechnol. 2004, 79, 407-412. (20) Pe´rez, M.; Torrades, F.; Garcı´a-Hortal, J. A.; Dome´nech, X.; Peral, J. Removal of organic contaminants in paper pulp treatment effluents under Fenton and photo-Fenton conditions. Appl. Catal. B: EnViron. 2002, 36, 63-74. (21) Pignatello, J.; Oliveros, E.; MacKay, A. Advanced oxidation processes for organic contaminant destruction based on the Fenton reaction and related chemistry. Crit. ReV. EnViron. Sci. Tech. 2006, 36, 1-84. (22) Lopez, A.; Mascolo, G.; Detomaso, A.; Lovecchio, G.; Villani, G. Temperature activated degradation (mineralization) of 4-chloro-3-methyl phenol by Fenton’s reagent. Chemosphere 2005, 59, 397-403. (23) Deng, Y.; Englehardt, J. D. Treatment of landfill leachate by Fenton process. Water Res. 2007, 146, 334-340. (24) Yoon, J.; Kim, Y.; Huh, J.; Lee, Y.; Lee, D. Roles of oxidation and coagulation in Fenton process for the removal of organics in landfill leachate. J. Ind. Eng. Chem. 2002, 8, 410-418. (25) Lopez, A.; Pagano, M.; Volpe, A.; Di Pinto, C. Fenton’s pretreatment of mature landfill leachate. Chemosphere 2004, 54, 1005-1010.

(26) Zhang, H.; Choi, H. J.; Huang, C. Treatment of landfill leachate by Fenton’s reagent in a continuous stirred tank reactor. J. Hazard. Mater. 2006, 136, 618-623. (27) Bautista, P.; Mohedano, A. F.; Gilarranz, M. A.; Casas, J. A.; Rodrı´guez, J. J. Application of Fenton oxidation to cosmetic wastewaters treatment. J. Hazard. Mater. 2006, 143, 128-134. (28) Tambosi, J.; Di Dominico, M.; Schirmer, W.; Jose´, H. J.; Moreira, R. Treatment of paper and pulp wastewater and removal of odorous compounds by a Fenton-like process at the pilot scale. J. Chem. Technol. Biotechnol. 2006, 81, 1426-1432. (29) Zhang, H.; Choi, H. J.; Huang, C. Optimization of Fenton process for the treatment of landfill leachate. J. Hazard. Mater. 2005, 125, 166174. (30) Wang, P.; Lau, I. C. W.; Fang, H. H. P.; Zhou, D. Landfill leachate treatment with combined UASB and Fenton coagulation. J. EnViron. Sci. Health A 2000, 35, 1981-1988. (31) Standard Methods for Examination of Water and Wastewater, 20th ed.; American Public Health Association (APHA): Washington, DC, 1998. (32) Primo, O.; Rivero, M. J.; Ortiz, I. Photo Fenton process as an efficient alternative to the treatment of landfill leachates. J. Hazard. Mater. In press. (33) Neyens, E.; Baeyens, J. A review of classic Fenton’s peroxidation as an advanced oxidation technique. J. Hazard. Mater. 2003, 98, 33-50. (34) Santos, A.; Yustos, P.; Cordero, T.; Gomis, S.; Rodrı´guez, S.; Garcı´a-Ochoa, F. Catalytic wet oxidation of phenol on active carbon: Stability, phenol conversion and mineralization. Catal. Today 2005, 102103, 213-218. (35) Guedes, A. M. F. M.; Madeira, L. M. P.; Boaventura, R. A. R.; Costa, C. A. V. Fenton oxidation of cork cooking wastewatersOverall kinetic analysis. Water Res. 2003, 37, 3061-3069. (36) Tang, W. Z.; Huang, C. P. Stoichometry of Fenton’s reagent in the oxidation of chlorinated aliphatic organic pollutants. EnViron. Technol. 1997, 18, 13-23. (37) Esplugas, S.; Gime´nez, J.; Contreras, S.; Pascual, E.; Rodrı´guez, M. Comparison of different advanced oxidation processes for phenol degradation. Water Res. 2002, 36, 1034-1042. (38) Ksibi, M. Chemical oxidation with hydrogen peroxide for domestic water treatment. Chem. Eng. J. 2006, 119, 161-165. (39) Di Iaconi, C.; Ramadori, R.; Lopez, A. Combined biological and chemical degradation for treating a mature municipal landfill leachate. Biochem. Eng. J. 2006, 31, 118-124.

ReceiVed for reView August 14, 2007 ReVised manuscript receiVed October 30, 2007 Accepted October 31, 2007 IE071111A