Environ. Sci. Technol. 2008, 42, 8890–8895
This study describes a laboratory-scale multifunctional permeable reactive barrier (multibarrier) for the removal of ammonium (NH4+: 313 ( 51 mg N L-1), adsorbable organic halogens (AOX: 0.71 ( 0.25 mg Cl L-1), chemical oxygen demand (COD: 389 ( 36 mg L-1), and toxicity from leachate originating from a 40-year-old Belgian landfill. The complexity of the contamination required a sequential setup combining different reactive materials and removal processes. All target contaminants could be removed to levels below the regulatory discharge limits. Ammonium was efficiently removed in a first microbial nitrification compartment, which was equipped with diffusive oxygen emitters to ensure a sufficient oxygen supply. Ammonium was mainly oxidized to nitrite and to a lesser extent to nitrate, with an average mass recovery of 96%. Remaining ammonium concentrations could be further removed by ion exchange in a second compartment filled with clinoptilolite, exhibiting a total ammonium removal capacity of 46.7 mg N per g of clinoptilolite. A third microbial denitrification compartment, fed with sodium butyrate as a carbon source, was used to remove nitrate and nitrite formed in the first compartment. Maximum nitrification and denitrification rates at 12 °C indicated that hydraulic retention times of ∼62 h and ∼32 h were required in the columns to remove 400 mg N L-1 by nitrification and denitrification, respectively. Leachate toxicity decreased to background levels together with the removal of ammonium and its oxidation products. AOX and COD were efficiently removed by sorption in an additional compartment filled with granular activated carbon.
liners and leachate collection systems (2). Leachate is pumped out of the landfill and treated externally to remove organic and inorganic contaminants. However, many older landfills were built without leachate collection systems or with faulty liner systems, thereby causing serious groundwater pollution (1, 2). The extensive size of many of these contaminated sites renders conventional ex situ remediation techniques economically and technically unfeasible, due to the long treatment duration and huge costs in soil excavation, groundwater pumping, and processing of the contaminated substances. Passive in situ remediation techniques such as permeable reactive barriers (PRBs) can offer a cost-effective way to control large-scale contaminated groundwater plumes (3). PRBs involve the placement of a reactive material or other amendments in the flow path of the contaminant plume to create a zone where contaminants are immobilized or transformed to nontoxic products by physical-chemical or biological processes (4). The complexity of leachate contamination, however, requires a combination of different reactive materials and removal processes, which can be achieved in a multifunctional PRB (multibarrier). Multifunctional PRBs have been evaluated for the removal of mixtures of chlorinated aliphatic hydrocarbons (CAHs), BTEX compounds, and heavy metals (5), and mixtures of polycyclic aromatic hydrocarbons (PAHs) and BTEX compounds (6). This study describes a laboratory-scale multibarrier for the removal of ammonium (NH4+), adsorbable organic halogens (AOX), chemical oxygen demand (COD), and toxicity from leachate originating from a 40-year-old Belgian landfill. The concept is based on a sequence of microbial degradation (nitrification-denitrification) and abiotic sorption processes for contaminant removal. A first nitrifying zone, equipped with diffusive oxygen emitters, was used to convert ammonium into nitrate and nitrite (together called NOx-) by microbial nitrification. This zone was followed by a second zone containing granular clinoptilolite for the removal of remaining NH4+ concentrations by ion exchange (7-9). A third denitrifying zone, fed with an external carbon source, was used to remove the NOx- formed in the nitrification zone. An additional zone containing granular activated carbon (GAC) was installed for the removal of AOX and COD by sorption. A schematic overview of the concept is presented in Figure S1 of the Supporting Information. A column experiment was set up and operated at room temperature as well as groundwater temperature (12 °C) to evaluate the performance of the multibarrier concept. When properly installed, the proposed multibarrier might be useful for the semipassive treatment of leachate during the aftercare period of old landfills, thereby replacing conventional energyconsuming wastewater treatment systems. On the other hand, the system can be installed downgradient of leaking landfills for the remediation of contaminated groundwater plumes.
Introduction
Experimental Section
Landfilling is still the most common method of municipal solid waste disposal in most countries and the construction of new landfills is ongoing due to the increasing trend of waste generation. Infiltration of rainwater results in extensive amounts of landfill leachate, generally enriched in organic matter, ammonium, and metals, which is threatening the surrounding soil, groundwater, and surface water (1). Modern landfills are well-engineered facilities designed to isolate the waste from the environment by installation of impermeable
Description of the Landfill Site. The landfill Hooge Maey is located in Antwerp, Belgium, and is one of the oldest active Flemish landfills, with dumping activities going on since 1967. This study focuses on a completed part of the landfill which is in the methanogenic phase. The area has a size of 39 ha and contains a heterogeneous mixture of industrial and municipal wastes. The area is confined by a cover layer consisting of a low permeability layer covered by a high density polyethylene (HDPE) geomembrane, a drainage layer, and soil. The geomembrane is anchored in a vertical clay dike surrounding the area. At a depth of ∼5 m, a thick, underlying natural clay layer isolates the waste from the
Design of a Multifunctional Permeable Reactive Barrier for the Treatment of Landfill Leachate Contamination: Laboratory Column Evaluation THOMAS VAN NOOTEN, LUDO DIELS, AND LEEN BASTIAENS* Flemish Institute for Technological Research (VITO), Separation and Conversion Technologies, Boeretang 200, 2400 Mol, Belgium
Received June 20, 2008. Revised manuscript received September 29, 2008. Accepted September 30, 2008.
* Corresponding author phone: +3214335179; fax: +3214580523; e-mail:
[email protected]. 8890
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 42, NO. 23, 2008
10.1021/es801704t CCC: $40.75
2008 American Chemical Society
Published on Web 10/29/2008
FIGURE 1. Schematic overview of the sequential laboratory-scale multibarrier system, comprising a viable column train and a column train poisoned with sodium azide. groundwater. Leachate has accumulated in the landfill due to upwelling groundwater and infiltration of rainwater before confinement. Leachate samples were collected from the landfill drainage system between April 2007 and April 2008, and analyzed according to standard methods. An overview of the physical and chemical characteristics is presented in Table S1. In general, the leachate is characterized by elevated concentrations 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 leachate has a temperature of 12.5 ( 0.9 °C, an electrical conductivity of 51 ( 4 mS cm-1, and a near-neutral pH (7.4 ( 0.2). Description of the Column Setup. An overview of the column setup is given in Figure 1. Two polyacrylate columns (height, 66 cm; inside diameter, 4 cm) were set up to simulate the nitrification zone for ammonium removal. The columns were partly filled with landfill leachate. Coarse sand (1-2 mm) was slowly added to fill the first 6 cm of the columns, while the columns were gently tapped to achieve a dense solution-saturated packing of the material. One column was inoculated by adding 220 mL of a 2-fold diluted sludge sample originating from the aerated nitrification compartment of the landfill wastewater treatment system. The column packing was completed by adding coarse sand to the liquid phase, thereby ensuring a homogeneous spreading of the inoculum. The second column was not inoculated and served as a control column. Oxygen was slowly released to the columns by using diffusive oxygen emitters, consisting of a 2 m long silicon tubing (2.0 mm i.d., 4.0 mm o.d.) looped around a PVC cylinder. One end of the coils was connected
via a manifold to a pressurized gas cylinder, and the other end was connected to a venting valve which opened 8 times per day for 1 min. By pressurizing the silicon tubing (0.2-0.5 bar), oxygen could diffuse through the tubing walls and dissolve into the landfill leachate due to the imposed chemical gradient. Both columns were equipped with 3 oxygen emitters placed in series. Landfill leachate was kept under nitrogen atmosphere in 5 L bottles at 2.5 °C. The leachate was pumped in an upward flow through the columns with a flow rate ranging from 3 to 19 mL h-1, corresponding to a pore water velocity ranging from 0.16 to 1.05 m day-1. The control column was poisoned by the addition of sodium azide to the feed line with a syringe pump, resulting in a final concentration of 235 mg L-1 in the column. Both columns were equipped with pressure transducers (Sensortechnics) at the inlet port to monitor the permeability of the column packing. The outlet port of each column was connected to a capped 12 mL vial. The column effluent was pumped from this vial into a second glass column (height, 5 cm; i.d., 2.4 cm) which was filled with granular clinoptilolite (Zeolite Products, 1.0-2.5 mm, porosity 0.56) to simulate the ion exchange zone. The clinoptilolite compartment of the biotic column system was followed by a third glass column (height, 25 cm; i.d., 2.4 cm) to simulate the denitrification zone. The column was filled with coarse sand (porosity 0.39) and inoculated with 40 mL of the diluted sludge that was used to inoculate the nitrification columns. Sodium butyrate was added as an external carbon source to the feed line with a syringe pump, resulting in a final concentration of ∼350 mg L-1 in the feed line. Butyrate was selected as it is also used to promote denitrification in the wastewater treatment plant of the landfill site. The leachate was pumped from the vials into the subsequent columns at a flow rate ranging from 2.5 to 11 mL h-1, corresponding to a pore water velocity ranging from 0.23 to 1.01 m day-1 for the clinoptilolite columns and 0.33 to 1.51 m day-1 for the denitrification column. After 127 days of operation, the denitrification column was replaced by a glass column (height, 5 cm; i.d., 2.4 cm) filled with GAC (Desotec, 1.0-2.5 mm, porosity 0.83), for the removal of AOX and COD by sorption. The whole column system was operated at laboratory temperature during the first 96 days of operation. In the period thereafter, the column system was operated in a cooling room at 12 °C. Sampling Procedures and Analysis. Periodically, liquid samples (∼10 mL) were collected from the inlet and outlet of the columns by connecting glass syringes to sampling ports and allowing the liquid to flow into the syringes. Ammonium, nitrate, and nitrite were analyzed using Spectroquant cell tests (Merck). Dissolved oxygen (DO) concentrations were determined with a Clark-type electrode model 781/781b oxygen meter (Strathkelvin Instruments). Butyrate samples were prepared for analysis by adding 0.5 mL of sample to 2 mL of H2SO4 50%. Fatty acids were extracted with diethylether and analyzed on a Trace GC-FID (Thermoquest) with a 15 m AT-1000 column using helium as carrier gas at a flow rate of 6 mL min-1. Total COD was determined according to the Dutch Standard Method NEN 6633:2006/A1 (10) and AOX was measured following ISO 9562:2004. Toxicity was measured by the Microtox test, based on the use of nonpathogenic bioluminescent marine bacteria (Vibrio fischeri). Toxicity levels are related to the inhibition of bacterial light emission after exposure to the sample. Flow rates were calculated by weighing column effluents collected during a certain operation period.
Results Ammonium Removal by Nitrification. The landfill leachate had an average influent DO concentration of 1.3 ( 0.5 mg L-1 and an ammonium influent concentration ranging from VOL. 42, NO. 23, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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200 to 400 mg N L-1. The presence of diffusive oxygen emitters in the nitrification columns resulted in a nonlimiting supply of oxygen. When the column system was operated at laboratory temperature (first 96 days of operation), effluent DO concentrations of 24.0 ( 9.7 mg L-1 and 35.1 ( 5.0 mg L-1 were recorded for the viable nitrification column and the poisoned control column, respectively, indicating microbial oxygen consumption in the former. After 96 days of operation, the column operation temperature was changed from laboratory temperature to groundwater temperature (12 °C). As a consequence, the solubility of oxygen in the leachate increased, resulting in effluent DO concentrations of 51.6 ( 5.8 mg L-1 and 57.5 ( 5.0 mg L-1 for the viable nitrification column and the poisoned control column, respectively. Despite the slow release of oxygen, some gas accumulation could be observed in the pores of the coarse sand packing, particularly in the abiotic control column where oxygen was not microbially consumed. However, gas accumulation did not affect column permeability as indicated by the constant pressure recorded with the pressure transducers (data not shown). An overview of ammonium removal in the nitrification columns is presented in Figure 2. During the first 26 days of operation, ammonium was only partially (14 ( 3%) removed in the viable column. To improve ammonium removal, the hydraulic retention time (HRT) in the columns was increased in the next period (day 26 to day 50) from ∼1.4 to ∼3.1 days by lowering the flow rate. Ammonium removal gradually increased to >98% after 39 days of operation. In the period thereafter (day 50 to day 95), the flow rate was gradually increased and ammonium removal rates reached a maximum of 3.1 ( 0.6 mg N h-1 (Figure 2D), corresponding to a removal capacity of 400 mg N L-1 at a HRT of ∼38 h. Ammonium was mainly oxidized to nitrite, and to a lesser extent to nitrate (Figure 3). Mass balance calculations agreed reasonably well during the complete operation period, with an average mass recovery of 96% and an average difference between ammonium removal and NOx- formation of 19 mg N L-1. In the poisoned control column, ammonium removal was
