Design of a MultifunctionalPermeable Reactive Barrier for theTreatment of Landfill LeachateContamination: Laboratory ColumnEvaluationT H O M A S V A N N O O T E N , L U D O D I E L S ,A N D L E E N B A S T I A E N S *

Flemish Institute for Technological Research (VITO),Separation and Conversion Technologies,Boeretang 200, 2400 Mol, Belgium

Received June 20, 2008. Revised manuscript receivedSeptember 29, 2008. Accepted September 30, 2008.

This study describes a laboratory-scale multifunctionalpermeable reactive barrier (multibarrier) for the removal ofammonium (NH4

+: 313 ( 51 mg N L-1), adsorbable organichalogens (AOX: 0.71( 0.25 mg Cl L-1), chemical oxygen demand(COD: 389 ( 36 mg L-1), and toxicity from leachate originatingfrom a 40-year-old Belgian landfill. The complexity of thecontamination required a sequential setup combining differentreactive materials and removal processes. All target con-taminants could be removed to levels below the regulatorydischarge limits. Ammonium was efficiently removed in a firstmicrobial nitrification compartment, which was equippedwith diffusive oxygen emitters to ensure a sufficient oxygensupply. Ammonium was mainly oxidized to nitrite and to a lesserextent to nitrate, with an average mass recovery of 96%.Remaining ammonium concentrations could be further removedby ion exchange in a second compartment filled withclinoptilolite, exhibiting a total ammonium removal capacity of46.7 mg N per g of clinoptilolite. A third microbial denitrificationcompartment, fed with sodium butyrate as a carbon source, wasused to remove nitrate and nitrite formed in the firstcompartment. Maximum nitrification and denitrification ratesat 12 °C indicated that hydraulic retention times of ∼62 h and∼32 h were required in the columns to remove 400 mg NL-1 by nitrification and denitrification, respectively. Leachatetoxicity decreased to background levels together with the removalof ammonium and its oxidation products. AOX and CODwere efficiently removed by sorption in an additional compartmentfilled with granular activated carbon.

IntroductionLandfilling is still the most common method of municipalsolid waste disposal in most countries and the constructionof new landfills is ongoing due to the increasing trend ofwaste generation. Infiltration of rainwater results in extensiveamounts of landfill leachate, generally enriched in organicmatter, ammonium, and metals, which is threatening thesurrounding soil, groundwater, and surface water (1). Modernlandfills are well-engineered facilities designed to isolate thewaste from the environment by installation of impermeable

liners and leachate collection systems (2). Leachate is pumpedout of the landfill and treated externally to remove organicand inorganic contaminants. However, many older landfillswere built without leachate collection systems or with faultyliner systems, thereby causing serious groundwater pollution(1, 2). The extensive size of many of these contaminated sitesrenders conventional ex situ remediation techniques eco-nomically and technically unfeasible, due to the longtreatment duration and huge costs in soil excavation,groundwater pumping, and processing of the contaminatedsubstances. Passive in situ remediation techniques such aspermeable reactive barriers (PRBs) can offer a cost-effectiveway to control large-scale contaminated groundwater plumes(3). PRBs involve the placement of a reactive material orother amendments in the flow path of the contaminant plumeto create a zone where contaminants are immobilized ortransformed to nontoxic products by physical-chemical orbiological processes (4). The complexity of leachate con-tamination, however, requires a combination of differentreactive materials and removal processes, which can beachieved in a multifunctional PRB (multibarrier). Multi-functional PRBs have been evaluated for the removal ofmixtures of chlorinated aliphatic hydrocarbons (CAHs), BTEXcompounds, and heavy metals (5), and mixtures of polycyclicaromatic hydrocarbons (PAHs) and BTEX compounds (6).

This study describes a laboratory-scale multibarrier forthe removal of ammonium (NH4

+), adsorbable organichalogens (AOX), chemical oxygen demand (COD), and toxicityfrom leachate originating from a 40-year-old Belgian landfill.The concept is based on a sequence of microbial degradation(nitrification-denitrification) and abiotic sorption processesfor contaminant removal. A first nitrifying zone, equippedwith diffusive oxygen emitters, was used to convert am-monium into nitrate and nitrite (together called NOx

-) bymicrobial nitrification. This zone was followed by a secondzone containing granular clinoptilolite for the removal ofremaining NH4

+ concentrations by ion exchange (7-9). Athird denitrifying zone, fed with an external carbon source,was used to remove the NOx

- formed in the nitrificationzone. An additional zone containing granular activatedcarbon (GAC) was installed for the removal of AOX and CODby sorption. A schematic overview of the concept is presentedin Figure S1 of the Supporting Information. A columnexperiment was set up and operated at room temperatureas well as groundwater temperature (12 °C) to evaluate theperformance of the multibarrier concept. When properlyinstalled, the proposed multibarrier might be useful for thesemipassive treatment of leachate during the aftercare periodof old landfills, thereby replacing conventional energy-consuming wastewater treatment systems. On the other hand,the system can be installed downgradient of leaking landfillsfor the remediation of contaminated groundwater plumes.

Experimental SectionDescription of the Landfill Site. The landfill Hooge Maey islocated in Antwerp, Belgium, and is one of the oldest activeFlemish landfills, with dumping activities going on since 1967.This study focuses on a completed part of the landfill whichis in the methanogenic phase. The area has a size of 39 haand contains a heterogeneous mixture of industrial andmunicipal wastes. The area is confined by a cover layerconsisting of a low permeability layer covered by a highdensity polyethylene (HDPE) geomembrane, a drainage layer,and soil. The geomembrane is anchored in a vertical claydike surrounding the area. At a depth of ∼5 m, a thick,underlying natural clay layer isolates the waste from the

* Corresponding author phone: +3214335179; fax: +3214580523;e-mail: leen.bastiaens@vito.be.

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groundwater. Leachate has accumulated in the landfill dueto upwelling groundwater and infiltration of rainwater beforeconfinement. Leachate samples were collected from thelandfill drainage system between April 2007 and April 2008,and analyzed according to standard methods. An overviewof the physical and chemical characteristics is presented inTable S1. In general, the leachate is characterized by elevatedconcentrations of NH4

+ (313 ( 51 mg N L-1), AOX (0.71 (0.25 mg Cl L-1), and COD (389 ( 36 mg L-1). The leachatehas a temperature of 12.5 ( 0.9 °C, an electrical conductivityof 51 ( 4 mS cm-1, and a near-neutral pH (7.4 ( 0.2).

Description of the Column Setup. An overview of thecolumn setup is given in Figure 1. Two polyacrylate columns(height, 66 cm; inside diameter, 4 cm) were set up to simulatethe nitrification zone for ammonium removal. The columnswere partly filled with landfill leachate. Coarse sand (1-2mm) was slowly added to fill the first 6 cm of the columns,while the columns were gently tapped to achieve a densesolution-saturated packing of the material. One column wasinoculated by adding 220 mL of a 2-fold diluted sludge sampleoriginating from the aerated nitrification compartment ofthe landfill wastewater treatment system. The columnpacking was completed by adding coarse sand to the liquidphase, thereby ensuring a homogeneous spreading of theinoculum. The second column was not inoculated and servedas a control column. Oxygen was slowly released to thecolumns by using diffusive oxygen emitters, consisting of a2 m long silicon tubing (2.0 mm i.d., 4.0 mm o.d.) loopedaround a PVC cylinder. One end of the coils was connected

via a manifold to a pressurized gas cylinder, and the otherend was connected to a venting valve which opened 8 timesper day for 1 min. By pressurizing the silicon tubing (0.2-0.5bar), oxygen could diffuse through the tubing walls anddissolve into the landfill leachate due to the imposed chemicalgradient. Both columns were equipped with 3 oxygen emittersplaced in series. Landfill leachate was kept under nitrogenatmosphere in 5 L bottles at 2.5 °C. The leachate was pumpedin an upward flow through the columns with a flow rateranging from 3 to 19 mL h-1, corresponding to a pore watervelocity ranging from 0.16 to 1.05 m day-1. The control columnwas poisoned by the addition of sodium azide to the feedline with a syringe pump, resulting in a final concentrationof 235 mg L-1 in the column. Both columns were equippedwith pressure transducers (Sensortechnics) at the inlet portto monitor the permeability of the column packing. The outletport of each column was connected to a capped 12 mL vial.The column effluent was pumped from this vial into a secondglass column (height, 5 cm; i.d., 2.4 cm) which was filled withgranular clinoptilolite (Zeolite Products, 1.0-2.5 mm, porosity0.56) to simulate the ion exchange zone. The clinoptilolitecompartment of the biotic column system was followed bya third glass column (height, 25 cm; i.d., 2.4 cm) to simulatethe denitrification zone. The column was filled with coarsesand (porosity 0.39) and inoculated with 40 mL of the dilutedsludge that was used to inoculate the nitrification columns.Sodium butyrate was added as an external carbon source tothe feed line with a syringe pump, resulting in a finalconcentration of ∼350 mg L-1 in the feed line. Butyrate wasselected as it is also used to promote denitrification in thewastewater treatment plant of the landfill site. The leachatewas pumped from the vials into the subsequent columns ata flow rate ranging from 2.5 to 11 mL h-1, corresponding toa pore water velocity ranging from 0.23 to 1.01 m day-1 forthe clinoptilolite columns and 0.33 to 1.51 m day-1 for thedenitrification column. After 127 days of operation, thedenitrification column was replaced by a glass column(height, 5 cm; i.d., 2.4 cm) filled with GAC (Desotec, 1.0-2.5mm, porosity 0.83), for the removal of AOX and COD bysorption. The whole column system was operated at labora-tory temperature during the first 96 days of operation. In theperiod thereafter, the column system was operated in acooling room at 12 °C.

Sampling Procedures and Analysis. Periodically, liquidsamples (∼10 mL) were collected from the inlet and outletof the columns by connecting glass syringes to samplingports and allowing the liquid to flow into the syringes.Ammonium, nitrate, and nitrite were analyzed using Spec-troquant cell tests (Merck). Dissolved oxygen (DO) concen-trations were determined with a Clark-type electrode model781/781b oxygen meter (Strathkelvin Instruments). Butyratesamples were prepared for analysis by adding 0.5 mL ofsample to 2 mL of H2SO4 50%. Fatty acids were extractedwith diethylether and analyzed on a Trace GC-FID (Ther-moquest) with a 15 m AT-1000 column using helium as carriergas at a flow rate of 6 mL min-1.

Total COD was determined according to the DutchStandard Method NEN 6633:2006/A1 (10) and AOX wasmeasured following ISO 9562:2004. Toxicity was measuredby the Microtox test, based on the use of nonpathogenicbioluminescent marine bacteria (Vibrio fischeri). Toxicitylevels are related to the inhibition of bacterial light emissionafter exposure to the sample. Flow rates were calculated byweighing column effluents collected during a certain opera-tion period.

ResultsAmmonium Removal by Nitrification. The landfill leachatehad an average influent DO concentration of 1.3 ( 0.5 mgL-1 and an ammonium influent concentration ranging from

FIGURE 1. Schematic overview of the sequential labora-tory-scale multibarrier system, comprising a viable column trainand a column train poisoned with sodium azide.

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200 to 400 mg N L-1. The presence of diffusive oxygen emittersin the nitrification columns resulted in a nonlimiting supplyof oxygen. When the column system was operated atlaboratory temperature (first 96 days of operation), effluentDO concentrations of 24.0 ( 9.7 mg L-1 and 35.1 ( 5.0 mgL-1 were recorded for the viable nitrification column and thepoisoned control column, respectively, indicating microbialoxygen consumption in the former. After 96 days of operation,the column operation temperature was changed fromlaboratory temperature to groundwater temperature (12 °C).As a consequence, the solubility of oxygen in the leachateincreased, resulting in effluent DO concentrations of 51.6 (5.8 mg L-1 and 57.5 ( 5.0 mg L-1 for the viable nitrificationcolumn and the poisoned control column, respectively.Despite the slow release of oxygen, some gas accumulationcould be observed in the pores of the coarse sand packing,particularly in the abiotic control column where oxygen wasnot microbially consumed. However, gas accumulation didnot affect column permeability as indicated by the constantpressure recorded with the pressure transducers (data notshown).

An overview of ammonium removal in the nitrificationcolumns is presented in Figure 2. During the first 26 days ofoperation, ammonium was only partially (14( 3%) removedin the viable column. To improve ammonium removal, thehydraulic retention time (HRT) in the columns was increasedin the next period (day 26 to day 50) from ∼1.4 to ∼3.1 daysby lowering the flow rate. Ammonium removal graduallyincreased to >98% after 39 days of operation. In the periodthereafter (day 50 to day 95), the flow rate was graduallyincreased and ammonium removal rates reached a maximumof 3.1( 0.6 mg N h-1 (Figure 2D), corresponding to a removalcapacity of 400 mg N L-1 at a HRT of ∼38 h. Ammonium wasmainly oxidized to nitrite, and to a lesser extent to nitrate(Figure 3). Mass balance calculations agreed reasonably wellduring the complete operation period, with an average massrecovery of 96% and an average difference between am-monium removal and NOx

- formation of 19 mg N L-1. In thepoisoned control column, ammonium removal was <6%during the complete operation period, with no formation ofnitrate or nitrite. After 96 days of operation, the flow rate wasdecreased and the column operation temperature waschanged from laboratory temperature to groundwater tem-perature (12 °C). As a consequence of the temperaturedecrease, ammonium removal rates in the viable columndecreased by 39% and reached a maximum of 1.9 ( 0.1 mgN h-1 after 103 days of operation, corresponding to a removalcapacity of 400 mg N L-1 at a HRT of ∼62 h. During thecomplete experiment, no NOx

- could be detected in theinfluent bottles kept at 2.5 °C, indicating that no nitrificationtook place in the bottles. An overview of the maximumnitrification rates is presented in Table S2.

Ammonium Removal by Ion Exchange on Clinoptilolite.Ammonium removal in the clinoptilolite compartments ispresented in Figure 4. During the first 26 days of operation,the poisoned clinoptilolite column received 305 ( 4 mg NL-1 ammonium, while the viable clinoptilolite columnreceived only 267 ( 2 mg N L-1 ammonium, due to partialnitrification in the upgradient compartment. Within thisperiod, 96 ( 3% of the incoming ammonium was removedin the columns. The clinoptilolite packing of both columnswas saturated with ammonium after 28 days of operation,exhibiting a total ammonium removal capacity of 46.7 and49.6 mg N per g clinoptilolite for the viable and the poisonedcolumn, respectively. In the period thereafter, no significantammonium removal (<5%) could be observed in thepoisoned column. In the viable column, the incomingammonium concentrations were gradually decreasing to zeroat day 39, due to increasing nitrification rates in theupgradient compartment. As a consequence, approximately

15% of the captured ammonium (up to 93.5 mg L-1) wasreleased from the saturated clinoptilolite material within 28days of operation, while no significant release could beobserved in the poisoned column. Nitrate and nitriteconcentrations were unaffected by the clinoptilolite. DOconcentrations were partly decreased (58(32%) after passagethrough the column.

FIGURE 2. Overview of flow rate (A) and nitrogen removal (Band C: mg L-1; D: mg h-1) in the viable nitrification column (b),the poisoned control column (O), and the denitrification column(0). The dashed vertical line indicates day 96 when the columnoperation temperature was changed from room temperature to12 °C.

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Nitrate/Nitrite Removal by Denitrification. Oxygen,originating from the nitrification compartment, was com-pletely removed in the denitrification compartment. Incom-ing NOx

- concentrations gradually increased to 285 mg NL-1 during the first 50 days of operation, due to increasingnitrification rates in the upgradient nitrification compartment(Figure 3). Within this period, nearly all NOx

- (96( 3%) couldbe removed by denitrification, resulting in the formation ofgas bubbles (nitrogen) which could be observed in the poresof the sand packing. Similar to the nitrification columns, theflow rate of the denitrification column was increased in thenext period (day 50 to 95) to reach a maximum denitrificationrate of 1.40 ( 0.31 mg N h-1 (Figure 2D), corresponding toa NOx

- removal capacity of 400 mg L-1 at a HRT of ∼12 h.In addition, remaining ammonium concentrations were alsopartly removed in the denitrification column (13.6( 10.7 mgN L-1), probably due to the microbial use of ammonium asa nitrogen source for protein synthesis (11). When the columnoperation temperature was changed from laboratory tem-perature to groundwater temperature (12 °C), denitrificationrates decreased by 62% to 0.53(0.13 mg N h-1, correspondingto a NOx

- removal capacity of 400 mg N L-1 at a HRT of ∼32h. In general, most of the added sodium butyrate (∼95%)was consumed. An overview of the maxium denitrificationrates is presented in Table S2.

Toxicity Removal. After 74 days of operation, toxicity ofleachate samples from the viable column system wasmeasured by the Microtox test and related to concentrationsof ammonium, nitrate, and nitrite (Figure 5). Toxicity wasmeasured after exposure times of 5, 15, and 30 min, andexpressed as EC50 values, i.e., the effective concentration atwhich 50% inhibition occurs (thus, the lower the EC50 value,the more toxic the sample). Similar values were obtained forthe three exposure times. The influent leachate solutioncontained 263 mg N L-1 ammonium and exhibited an EC50

value of 28.7 ( 2.5%, corresponding to a slight toxicity. Afterpassage through the nitrification column, the ammonium

level decreased to 53 mg N L-1, together with a partial toxicityreduction (EC50: 55.5 ( 4.0%). Due to nitrification, nitrateand nitrite concentrations increased from almost zero to102 and 127 mg N L-1, respectively, thereby contributing tothe remaining toxicity of the leachate. Passage through theammonium-saturated clinoptilolite column did not signifi-cantly affect nitrogen levels and toxicity of the leachate. Inthe denitrification column, however, nitrate and nitrite wereremoved and the toxicity response was reduced to back-ground levels (EC50: >90%). Toxicity of samples from theviable column system was not compared with that of samplesfrom the control column system, as the latter was poisonedwith sodium azide.

AOX and COD Removal. AOX and COD concentrationsof the influent leachate were 0.70 ( 0.20 mg Cl L-1 and 395( 32 mg L-1, respectively. No significant removal of AOX andCOD could be observed in the nitrification, clinoptilolite,and denitrification compartment (data not shown). After 127

FIGURE 3. Overview of ammonium removal (•) and formation of nitrate (dark gray bars) and nitrite (light gray bars) in the viablenitrification column.

FIGURE 4. Overview of influent (b) and effluent (O) ammonium concentrations (mg N L1) in the clinoptilolite compartments of theviable (A) and the poisoned (B) column system.

FIGURE 5. Overview of toxicity levels (gray bars) andconcentrations of ammonium (b), nitrate (0), and nitrite (9)along the viable column system, measured after 74 days ofoperation. Toxicity data are shown as mean ( standarddeviation of duplicate measurements after 5, 15, and 30 minexposure time.

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days of operation, the denitrification compartment wasreplaced by a GAC compartment for the removal of AOX andCOD by sorption. During the complete operation period ofthe GAC compartment (54 days), leachate concentrations ofAOX and COD were respectively reduced to 0.24 ( 0.13 mgCl L-1 and 203 ( 59 mg L-1, indicating that the regulatorydischarge limits of the landfill wastewater treatment system(0.4 mg Cl L-1 and 300 mg L-1, respectively) could be reached.As no saturation of the GAC could be reached within theoperation period of the column, only minimal sorptioncapacities of 0.58 mg Cl g-1 GAC and 268 mg g-1 GAC couldbe determined for AOX and COD removal, respectively.

Discussion

Ammonium is one of the major components of landfillleachate in the long term, as there is no mechanism for itsdegradation under methanogenic conditions (2). Microbialnitrification-denitrification is the most studied process forammonium removal from wastewaters, although differenttreatment alternatives have been reported (12). Nitrificationrequires the presence of sufficiently high DO concentrations,due to high concentrations of dissolved organic carbon inlandfill leachate and the competitive interaction betweennitrifying bacteria and faster-growing heterotrophs for DO(13). In contrast to commonly used aerated reactors orlagoons, PRBs cannot be provided with oxygen by sparging,as accumulation of gas bubbles would drastically reduce thepermeability of the barrier material and create preferentialstreamlines. Therefore, a novel technology consisting ofsilicon-based diffusive oxygen emitters was used in this studyfor the delivery of oxygen. Unlike sparging techniques, theslow release of oxygen from the silicon emitters allows anefficient uptake by bacteria without wasting too much oxygen.Similar polymer-based diffusion systems were previouslyused for in situ bioremediation of groundwater contaminatedwith methyl tertiary butyl ether (MTBE) (14), ammonium(15), and pesticides (16). The use of diffusive oxygen emittersresulted into a sufficient and homogeneous oxygen deliverythroughout the columns, without negatively affecting thecolumn permeability. Sufficiently high nitrification rates werereached within 39 days of operation to reduce the ammoniumconcentration of the influent leachate (∼300 mg N L-1) to<6mg N L-1. Due to an efficient removal of nitrate and nitritein the denitrification compartment, a total nitrogen outputof <10 mg L-1 could be reached in the multibarrier system,which is much below the regulatory discharge limit of thelandfill wastewater treatment system (40 mg N L-1). Maxi-mum nitrification and denitrification rates corresponded toa removal capacity of 400 mg N L-1 at HRTs of ∼38 h and∼12 h, respectively. These findings are in accordance withrepresentative literature data. By using polymer mats foroxygen delivery in sand-filled columns operated at roomtemperature, Patterson et al. (15) achieved nitrification half-lifes of ∼0.25 days, which allow the removal of almost 400mg L-1 in ∼36 h. Denitrification rates reported by Pattersonet al. (15), however, were significantly lower compared tothe denitrification rates reported in this study. By supplyingthe columns with ethanol as a carbon source via polymermats, they achieved denitrification half-lifes of ∼0.34 days,indicating that it would take >49 h to remove 400 mg N L-1.

In this study, maximum nitrogen removal rates were alsodetermined at groundwater temperature (12 °C), as nitrifica-tion and denitrification are significantly affected by theambient temperature (17). The temperature decrease resultedin a 39% reduction of the nitrification rate, which is somewhatlower than the reduction that is conventially assumed (50%)with a temperature decrease of 10 °C (18). Denitrificationrates were more sensitive to the lower temperature anddecreased by 62%. Although denitrifiers are generally con-

sidered to be less sensitive to temperature decreases thannitrifiers, the opposite was also reported by Ilies and Mavinic(17) who studied biological treatment of landfill leachate.

The use of clinoptilolite is another well-known methodto remove ammonium from aqueous solutions by ionexchange (7-9). The material is of special interest due to itslow cost, the relative simplicity of application and operation(9), and the suitability for use in PRBs (8). In this study,ammonium concentrations could be efficiently removed to<8 mg N L-1 in the clinoptilolite compartments. However,the major drawback is the need for replacement and properdisposal of the material after saturation. Therefore, theclinoptilolite compartments should rather be considered asa downgradient buffer zone for ammonium removal whenmicrobial nitrification is insufficient. In particular during thestart-up phase of the multibarrier when nitrifying bacteriaare still in the lag phase, or as a consequence of ammoniumshock loads, variations in toxicity levels, and seasonaltemperature changes. The ammonium removal capacity ofclinoptilolite increases with an increase of the ammoniumconcentration in solution, due to a higher solute concentra-tion gradient which is the driving force for the ion exchangereaction (9). Therefore, clinoptilolite is partially releasingammonium when inflowing ammonium concentrations arelower than the degree of saturation of the material. As aconsequence, the clinoptilolite compartment should befollowed by a second nitrification compartment to removethe released ammonium. The ammonium release causes apartial regeneration of the clinoptilolite material, therebyextending its longevity.

Risk assessment of landfill leachate is traditionally basedon chemical analyses of specific compounds present in theleachate, but does not take into account interactions amongchemicals or toxic degradation products in a complex mixture(2). Microtox tests, however, are among the most frequentlyused methods to characterize the toxicity of landfill leachate(2) and were therefore used in this study to integrate thebiological effect of all the constituents of the leachate.Leachate toxicity decreased to background levels whilemigrating through the multibarrier system, in parallel withconcentrations of ammonium or its oxidation products.Although other parameters can also play a role, this findingmay indicate a relation between ammonium and the observedtoxicity, which is in accordance with the findings of otherresearchers (2).

Various physical, chemical, and biological methods havebeen described for the removal of AOX from wastewaters,but many of them are uneconomical when applied at field-scale operations (19). Chemical reduction of AOX in thepresence of granular zerovalent iron (Fe0) was evaluated inseparate batch and column experiments, but resulted in apoor removal efficiency (<40%) (data not shown). AlthoughFe0 is known to degrade a wide range of chlorinatedhydrocarbons (3), the landfill leachate is likely containing arange of other halogenic compounds that are unaffected byFe0. Similarly, the expected complexity of the AOX composi-tion will probably limit the efficiency of microbial degrada-tion. Therefore, it was decided to remove AOX together withCOD by sorption on GAC. Similar to the use of clinoptilolite,the major drawback is the need for replacement of thematerial after saturation.

Field-Scale Implications. The proposed sequential multi-barrier system was successfully removing all target con-taminants to levels below the regulatory discharge limits ofthe landfill wastewater treatment system. Maximum nitri-fication and denitrification rates at 12 °C indicated that HRTsof ∼62 h and ∼32 h are required to remove 400 mg N L-1 bynitrification and denitrification, respectively. The describedHRTs are feasible in a field-scale configuration. Supposingthat a reactive gate is installed to treat the described landfill

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leachate contamination at a flow velocity of 0.5 m day-1, thegate should contain a 1.3 m thick nitrification compartmentand a 0.7 m thick denitrification compartment, provided thatoxygen and butyrate are supplied as efficiently as in thecolumn system. Diffusive oxygen transmitters can be appliedin the field in the form of mats of silicon tubing woven intoa mesh frame (20). Technical-grade butyrate or otherrelatively cheap liquid carbon sources can be easily injectedin the barrier material. The possible release of ammoniumfrom the clinoptilolite material requires an additionalnitrification compartment after the clinoptilolite compart-ment to ensure complete ammonium removal. In addition,a field-scale configuration should allow that saturatedclinoptilolite and GAC can be replaced during operation,which can be achieved by using removable/replaceable“cassettes” (21).

The results indicate that the proposed concept haspotential to replace conventional wastewater treatmentsystems during the aftercare period of old confined landfillsfor the treatment of leachate with characteristics similar tothe leachate characteristics described here. Leachate orig-inating from newer and active landfills obviously requires adifferent approach, as it can contain much higher concen-trations of pollutants (e.g.,>2000 mg N L-1 ammonium) andtoxic substances (2), which are expected to inhibit microbialnitrification-denitrification processes. Additionally, the pro-posed concept is also promising for the remediation ofgroundwater contaminated by leaking landfills. Scale-up toa field pilot-scale installation at the landfill site is currentlyunderway.

AcknowledgmentsThis work was cofunded by the EU Life project MULTIBAR-DEM (LIFE06 ENV/B/000359). We thank J. Vos, S. Vangeel,and J. Maes for technical support to this study, the Inter-communale Hooge Maey for offering leachate and sludgesamples, R. Van Oers and R. Seynaeve for kindly providingthe sorption materials, and all the partners of the MULTI-BARDEM project for the fruitful discussions.

Supporting Information AvailableSchematic overview of the multibarrier concept, table withleachate characteristics, table with maximum nitrificationand denitrification rates. This information is available freeof charge via the Internet at http://pubs.acs.org.

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