Published online March 8, 2018
Journal of Environmental Quality
TECHNICAL REPORTS ORGANIC COMPOUNDS IN THE ENVIRONMENT
Removal of Pollutants in Different Landfill Leachate Treatment Processes on the Basis of Organic Matter Fractionation Mehdi Zolfaghari, Oumar Dia, Nouha Klai, Patrick Drogui,* Satinder Kaur Brar, Gerardo Buelna, and Rino Dubé
C
urrently, >600 landfills across Canada receive
Abstract
municipal and industrial waste, making it the first choice for waste management (Giroux, 2014). The leachates produced by these facilities are extremely polluted with carbon, nitrogen, and phosphorus (macropollutants), as well as micropollutants such as emerging contaminants and heavy metals (Renou et al., 2008; Liyan et al., 2009). In the early stages of landfilling, intense anaerobic activity produces leachate with a very high concentration of biodegradable carbon. After a decade, volatile fatty acids bind together to generate macromolecules in old landfill leachate. In the humification process, humic substances are also produced by the microbial degradation of biodegradable compounds (Chian, 1977; Kang et al., 2002; Kjeldsen et al., 2002). Because the brownish-colored humic substances are nonbiodegradable, with a biodegradation half velocity constant of 90 mg L−1 (Esparza-Soto and Westerhoff, 2003), the biochemical oxygen demand/chemical oxygen demand (BOD/COD) ratio in the leachate decreases from 0.6 to 50% in electro-coagulation and electrooxidation, respectively. Rejection of metals by nanofiltration was >80% and depended on the size and charge of cation. All in all, a combination of membrane bioreactor and nanofiltration seems to be the optimal process configuration for efficient treatment of old landfill leachate.
Core Ideas • Humic substances comprise half of the organic carbon in landfill leachate. • Given its size & interaction with sludge, fulvic acid is the most concerning type of carbon. • Organic carbon with size 0.5 to 10 kDa is rarely removed by biological treatment. • We study the fate of metal & hydrophobic emerging contaminants bound with humic substances. • Combination of biological treatment & nanofiltration is the most efficient treatment option.
M. Zolfaghari, O. Dia, N. Klai, P. Drogui, and S.K. Brar, Institut National de la Recherche Scientifique-Eau, Terre et Environnement (INRS-ETE), Univ. du Québec, 490 rue de la Couronne, Québec, QC, Canada, G1K 9A9; G. Buelna and R. Dubé, Centre de Recherche Industrielle du Québec (CRIQ), 333 rue Franquet, Québec, QC, Canada, G1P 4C7. Assigned to Associate Editor Peter Kopittke.
Copyright © American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America. 5585 Guilford Rd., Madison, WI 53711 USA. All rights reserved. J. Environ. Qual. 47:297–305 (2018) doi:10.2134/jeq2017.09.0360 Received 11 Sept. 2017. Accepted 11 Dec. 2017. *Corresponding author (
[email protected]).
Abbreviations: BF, biofiltration; BOD, biochemical oxygen demand; COD, chemical oxygen demand; DEHP, di(2-ethyl hexyl) phthalate; ECo, electro-coagulation; EO, electro-oxidation; MBR, membrane bioreactor; NF, nanofiltration; TOC, total organic carbon.
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plastic materials, its concentration could reach up to 460 mg L−1 in municipal landfill leachate (Zolfaghari et al., 2016a). Previous studies correlated the concentration of DEHP, dissolved organic matter, and suspended solids in landfill leachate (Bauer and Herrmann, 1998). Although water treatment technologies involving aerobic biological processes, such as membrane bioreactor (MBR; Ahmed and Lan, 2012), biofiltration (BF; Dia et al., 2016), and aerated lagoons (Mehmood et al., 2009), can handle a high concentration of ammonia (up to 1000 mg N-NH3 L−1), they cannot meet the regulations on color and COD (Ziyang et al., 2009). Numerous studies have investigated different physical and/or chemical posttreatment methods such as coagulation (Dia et al., 2016), electro-oxidation (EO; Zolfaghari et al., 2016b), nanofiltration (NF; Bohdziewicz et al., 2001), and adsorption (Liyan et al., 2009) for handling residual carbon (Renou et al., 2008), yet none of them proposed the best option for treatment. In this study, the performance of physical (NF and electrocoagulation [ECo]), biological (MBR and BF), and chemical (EO) processes for carbon and metal removal was investigated. Total carbon in the raw and treated leachate was quantified according to its type and size to better understand the fate of contaminants in different processes and select the best combination of treatment processes.
Materials and Methods Landfill Leachate
The municipal waste of Frampton district is sent to an open municipal landfill located ~60 km southeast of Quebec City, Québec, Canada. This landfill has operated since 1991 and accepts >1000 t of municipal waste daily. Although the quality and quantity of landfill leachate varies greatly by season, an average of 100 m3 d−1 was annually produced that had the characteristics of the old landfill leachate with low BOD/COD ratio. Due to the frost during the winter (November–March, with average temperature −3 to −8°C), the leachate treatment was stopped, and the small amount the leachate produced was gradually accumulated in two 20,000-m3 retention basins. The sample was directly taken from the inlet of the retention basin in December 2015. Since precipitation at this time of the year is received as snow, the leachate was much more concentrated compared with the summer season (May–September, with average temperature 19–24°C) (Table 1). Consequently, the average concentrations of COD, TOC, ammonia, and alkalinity in winter were 1.4, 1.5, 2.3, and 2 times higher than in summer, respectively.
Experimental Units The five main processes selected for this study were: (i) MBR as the representative of the suspended growth reactor; (ii) BF for the attached growth biological process; (iii) EO as the advanced oxidation process; (iv) ECo; and (v) NF as the advanced physical separation. The treated landfill leachate in this study was taken from the running pilots operated under optimum conditions.
Membrane Bioreactor The 5-L submerged MBR was equipped with ultrafiltration, with pore size of 0.04 mm and filtration surface area of 0.047 m2. The pilot was fully automated, and the filtration was controlled 298
by transmembrane pressure. All the accessories and reactor startup were fully explained in a previous study (Zolfaghari et al., 2016a).
Biofiltration The BF pilot consisted of a polyvinyl chloride (PVC) column with a height of 2 m and diameter of 0.2 m, filled by horizontal layers of wood and peat (Dia et al., 2016). Aeration was applied for the development of microorganisms on the surface of the media. Raw leachate added at the top of the bioreactor was filtered and biodegraded by attached microorganisms.
Electro-Oxidation The laboratory-scale 1-L Plexiglas reactor was used for EO of the leachate. The pilot was equipped with niobium coated with boron-doped diamond as the anode and titanium coated with platinum as the cathode, with a 2-cm gap between them. The pilot and affiliated equipment were depicted in a previous study (Zolfaghari et al., 2016b).
Electro-Coagulation A 1.5-L cylindrical reactor equipped with two central electrodes was used for ECo of biotreated leachate. The anode was made of alloy-based magnesium, with a length of 80 cm and diameter of 1.9 cm; the cylindrical stainless steel cathode was 72 cm in length and 5 cm in inner diameter. The process run-up and other equipment were discussed in a previous study (Dia et al., 2016). As the conductivity of leachate was always >6 ms cm−1 for both chemical oxidation and ECo processes, addition of salt was not required.
Nanofiltration The stainless steel NF pilot was 20 ´ 20 ´ 4 cm, equipped with reverse-osmosis booster pumps (Model E-36, Eigen Water Company), which provided 10 ´ 105 Pa of pressure and had a flow rate up to 75 mL min−1. Nanofiltration membranes were purchased from the Synder Filtration Company. The molecular weight cutoff of 1-m2 membranes was 100, 10, 1, and 0.5 kDa for LY, ST, XT, and NFW model membranes, respectively. The membranes were later cut to fit the experimental pilot. The reject/filtration ratio was fixed by a high-pressure proportioning valve in the range of 5 to 20 to minimize the fouling rate.
Sampling and Analysis Approximately 5 L of sample was taken from each pilot in its optimum condition (Table 2) for further analysis of type and size characterization of TOC, DEHP, and metals. All samples were frozen at −8°C before the analysis. The methods for analysis of COD, TOC, total nitrogen, DEHP, NH4+, NO3−, NO2−, PO43−, total and volatile solid, metal cations, alkalinity, and color were fully described in previous studies (Dia et al., 2016; Zolfaghari et al., 2016a, 2016b).
Characteristics of Landfill Leachate The organic carbon was divided into five main categories: (i) suspended solids, (ii) colloids, (iii) humic acid, (iv) fulvic acid, and (v) the hydrophilic fraction. As shown in Fig. 1, microfiltration with pore size 0.45 mm was required for separation of suspended solids. Colloids mainly consisted of fatty acid globules and humus retained by the hydrophilic ultrafiltration. Since dissolved humic acids formed precipitates at pH £ 2, the samples were acidified Journal of Environmental Quality
with a 6 M hydrochloric acid solution. Centrifugation at 1700g for 15 min was required to concentrate the precipitated humic acid. The pellet was dissolved using 0.1 M sodium hydroxide for the analysis of carbon content. Extraction of fulvic acid was performed by adding the nonionic macrospores XAD 8 amberlite resin in the ratio of 2:1 g of resin/sample. The sample was diluted with water and shaken for 2 h. The process was repeated three times for each sample to completely adsorb fulvic acid. Filtration was performed for complete removal of the residual solution from the resin. The carbon content of the filtrate was representative of the hydrophilic fraction. Desorption of fulvic acid from amberlite resin followed the same procedure, with addition of 0.1 M sodium hydroxide. As mentioned in the protocol developed by van Zomeren and Comans (2007), the concentration of each different part was considered in the analysis of total carbon content. The recovery of fulvic acid was calculated as the difference between the concentrations of the dissolved organic carbon and humic substances. For the characterization of organic carbon according to its size, 3 L of sample was circulated between the NF pilot and storage tank. Around 400 mL of the filtrate was taken from each membrane. To minimize the contamination, the membrane was replaced for each wastewater. Chemical oxygen demand and TOC were measured as representatives of organic matter concentration. Fractionation of organic matter and extraction of humic substances was performed twice, and the average value was reported. Furthermore, each analysis was performed in triplicate to calculate the analytical error.
Results and Discussion
Problematic Pollutants in the Effluent of Landfill-Leachate Treatment Processes Due to the complex composition of landfill leachate, the processes’ performance varied significantly with the time. The average concentration of pollutants in the landfill leachate during winter is provided in Table 1. Carbon and nitrogen were the main contaminants in the landfill leachate, at concentrations up to 600 mg L−1. As the nitrogen content of humic and fulvic acid is 1 to 2% (Kang et al., 2002), most of the nitrogen in the landfill leachate was in the form of dissolved ammonia (~96%) (Campagna et al., 2013). Similar to old landfill leachate with 40 to 60% of humic substance, humic acid comprised half the organic carbon in this study (Ziyang et al., 2009). On the other hand, suspended solids and colloids comprised only 7.7% of TOC, as filtration by the landfill itself blocked the passage of the undissolved fraction. Low phosphorus concentration (~5 mg L−1) in the leachate was due to biological assimilation in the landfill, adsorption by the waste, and precipitation by aluminum, magnesium, and calcium cations (Ziyang et al., 2009; Ahmed and Lan, 2012). The concentration of toxic metals, such as arsenic and lead, remained below 100 mg L−1. However, the concentration of iron, magnesium, and aluminum was >1 mg L−1. As shown in Table 2, the optimum operating condition for each process was determined by running the biological processes for >6 mo. Optimization of physical and chemical processes was
Table 1. Concentration of pollutants in each portion of the raw landfill leachate. Pollutant Carbon Nitrogen Phosphorous Magnesium Iron Aluminum Nickel Zinc Copper Lead Arsenic Cobalt
Humic acid
Fulvic acid
Free and hydrophilic
Colloids
Suspended solids
——————————————————————————— mg L−1 ——————————————————————————— 105,000 ± 12,000 156,000 ± 14,000 222,500 ± 21,000 28,100 ± 1,000 12,400 ± 1,000 8,750 ± 1,000 11,143 ± 2,000 632,000 ± 10,000 578 ± 300 1,210 ± 400 369 ± 100 557 ± 100 3,150 ± 1,000 225 ± 100 1,376 ± 100 N.D.† N.D. 106 ± 40 895 ± 40 5,831 ± 40 N.D. 374 ± 50 2,340 ± 50 1,433 ± 50 727 ± 50 64 ± 10 29 ± 10 544 ± 10 19 ± 10 59 ± 10 N.D. 58 ± 10 14 ± 10 296 ± 10 5 ± 10 N.D. 11 ± 10 250 ± 10 28 ± 10 28 ± 10 23 ± 5 14 ± 5 54 ± 5 17 ± 5 21 ± 5 N.D. 12 ± 5 31 ± 5 32 ± 5 19 ± 5 N.D. 12 ± 5 10 ± 5 35 ± 5 9±5 N.D. N.D. N.D. 3±5 60 ± 5
† N.D., not determined Table 2. Performance of landfill leachate treatment processes in the optimum operating conditions. Process Membrane bioreactor Biofiltration Electro-oxidation Electro-coagulation Nanofiltration
Operating conditions Hydraulic retention time = 32 h, solid retention time = 80 d, sludge concentration = 8.94 g L−1, average organic load rate = 1.3 g COD d−1 L−1
Performance (removal efficiencies)† NH4+ = 98%, COD = 63%, BOD = 96%, turbidity = 99.9%, Ptot = 67%, Ntot = 27%, color = 29%
NH4+ = 94%, COD = 35%, BOD = 94%, turbidity = 95%, Hydraulic loading rate = 0.17 m3 m−2 d−1, temperature = 22°C, Ntot = 19%, Ptot = 42% pH = 8.5 Current density = 26.5 mA cm−2, treatment time = 123 min, NH4+ = 12%, COD = 57%, Ptot = 25%, Ntot = 3.4%, color = 97%, electrode type = titanium/boron doped diamond temperature TOC = 36%, Ptot = 18% = 20 ± 1°C, voltage = 9.8 V Current density = 10 mA cm−2, treatment time = 30 min, NH4+ = 0%, COD = 53%, Color = 85%, Ptot = 98% electrode type = Mg/Fe, temperature = 20 ± 1°C, voltage = 13.2 V + Molecular weight cut-off = 500 Da, applied pressure = 8 ´ 105 Pa, NH4 = 0%, COD = 55%, TOC = 57%, Color = 92%, Ptot = 86% reject ratio = 5:1, filtration surface = 289 cm2
† COD, chemical oxygen demand; BOD, biochemical oxygen demand; Ptot, total phosphorus; Ntot, total nitrogen; TOC, total organic carbon. Journal of Environmental Quality 299
Fig. 1. Characterization of landfill leachate according to the size and type of organic carbon.
achieved by the central composite design. Proliferation of heterotrophic and autotrophic bacteria transformed degradable organic carbon and ammonia into CO2 and NO3− (Ahmed and Lan, 2012; Dia et al., 2016; Zolfaghari et al., 2016b). Therefore, using biological processes satisfied the regulation of the effluent for ammonia and BOD. Electro-coagulation and EO, on the other hand, effectively removed the color and phosphorus in the biotreated leachate (Table 2). Looking at the overall treatment performances, TOC and COD removal efficiency were not satisfactory; therefore, carbon fate remains the main concern for landfill-leachate process plants.
Fate of Dissolved Organic Carbon Analyzing the effluent showed that suspended solids were well removed in all processes. Membrane blockage in MBR and NF, filtration in BF, precipitation on the surface of electrode in EO, and capture by the sludge in ECo processes were 300
mechanisms for the removal of undissolved organic carbon. Similar to suspended solids, colloids were completely removed in MBR and NF, as hydrophilic ultrafiltration and NF blocked the passage of the hydrophobic basic fraction (Ziyang et al., 2009). Adsorption into the peat and biodegradation removed them in BF. Precipitation and electro-flotation in EO and ECo processes efficiently removed colloids (data not shown). According to Fig. 2a, the molecular weight of humic acid was >1 kDa. Therefore, humic acid was completely retained by NF. Around 80% of humic acid was removed through MBR, as the sludge could adsorb 70 mg g−1 of humic acid (Zolfaghari et al., 2016a), especially molecules larger than 10 kDa. More than 90% of humic acid was oxidized completely by hydroxyl radicals in the EO process. Partial oxidation of larger humic acid molecules increased the portion of organic carbon with a size of 0.5 to 1 kDa, leading to an increase in their biodegradability (Campagna et al., 2013). Humic acid has 3 pKa with negative site concentration of Journal of Environmental Quality
Fig. 2. Size characteristics of (a) humic acid; (b) fulvic acid; and (c) hydrophilic fraction for the raw landfill leachate (LFL) and the effluent of membrane bioreactor (MBR), biofiltration (BF), electro-oxidation (EO), electro-coagulation (ECo), and nanofiltration (NF) processes and their combination.
7.43 ´ 10−3 mol g−1 at neutral pH (Wightman and Fein, 2001); divalent and trivalent cations, such as Fe3+ and Al3+, act like a bridge between humic acid molecules. Therefore, humic acid could form larger floc and be completely removed by precipitation during the coagulation process (Matilainen et al., 2010). Since the peat already contained humic acid, it rarely adsorbed extra humic substances, leading to only 30% removal in BF. We deduced that ~82% of humic acid molecules larger than 10 kDa were fractured
by microorganisms or oxidation, resulting in three-quarters of humic acid molecules being smaller than 10 kDa. According to Fig. 2b, ~90% of fulvic acid molecules were smaller than 1 kDa. Therefore, its removal efficiency was ~64% by NF. Smaller fulvic acid molecules had five times lower adsorption capacity than humic acid (Moura et al., 2007; Zhang et al., 2009); hence, its removal efficiency in MBR was only 36%. There are three views about the higher biosorption of humic acid than
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fulvic acid. First, larger sized humic acid molecules showed better interaction with the sludge surface through weak Van der Waals forces (Kang et al., 2002). Second, the abundance of aromatic groups in humic acid is conducive to hydrophobic interaction with the surface of microorganisms (Moura et al., 2007). Third, other researchers propose ternary interaction among cations, humic acid, and sludge as the main mechanism of adsorption in the presence of minerals (Esparza-Soto and Westerhoff, 2003). Dissociation constants (pKa1,2,3) for humic acid are 2.5, 6.1, and 8.8 (Wightman and Fein, 2001), whereas for fulvic acid, they are 4.5, 7.3, and 9.5 (Esparza-Soto and Westerhoff, 2003); consequently, humic acids are stronger acids, with a higher concentration of negative charge on their surface that facilitates ternary interaction. The effect of cationic bridging is more pronounced in alkaline pH, whereas hydrophobic interaction is the most important mechanism of biosorption in acidic pH (Feng et al., 2008). Like humic acid, wood and peat adsorbed only 47% of fulvic acid in the BF process. Partial reaction of fulvic acid with hydroxyl radicals oxidized all fulvic acid molecules to a size smaller than 100 kDa, with total removal efficiency of 30%. Likewise, the ECo process completely precipitated fulvic acid molecules larger than 1 kDa. As for fulvic acid, ~90% of the hydrophilic portion had a molecular weight 460 mg L−1 (Kukkonen et al., 1990; Kang et al., 2002; Zolfaghari et al., 2014). The bioavailability of pollutants and sludge adsorption capacity depended on the strength of their bond with humic substances (Kukkonen et al., 1990; Liyan et al., 2009). There are different explanations behind the interaction of DEHP and organic matter. Highly hydrophobic DEHP adsorbed onto the organic matter, especially colloids and suspended solids, through hydrophobic interaction (Moura et al., 2007). Around 0.44 mg DEHP g−1 TOC was partitioned by the particulate 302
phase, which was 12 times higher than for the dissolved fraction. Di(2-ethyl hexyl) phthalate also had four hydrogen-bond acceptors, which formed hydrogen bonds with peptide groups present in humic acid and the hydrophilic fraction (Zheng et al., 2007). As humic acid, with its larger molecular size, has more hydrogen bond donors, it facilitated DEHP partitioning (Fig. 3). Finally, carboxylic groups bound in fulvic acid attached to the same group in DEHP (Zheng et al., 2007), leading to 31% of DEHP being partitioned in fulvic acid. In MBR, complete removal of the particulate fraction and considerable removal of humic acid removed almost 70% of DEHP (Fig. 3a). Incomplete removal of humic acid and suspended solids in BF, on the other hand, decreased DEHP concentration slightly, to ~25.6 mg L−1. As shown in Fig. 2 and 3, a high concentration of fulvic acid and hydrophilic fraction contained 50% of dissolved DEHP. These fractions, with a molecular weight of 0.5 to 100 kDa, held most of the DEHP in the effluent of biological processes. As the remaining humic acid was removed by the ECo process, DEHP removal efficiency rose to 88%. In the EO process, on the other hand, a high concentration of dissolved organic matter, especially humic substances, acted as a shield for DEHP (He et al., 2009; Wenk et al., 2011), dramatically decreasing its removal (70% in MBR and 50% in the BF process. In both EO and ECo processes, electro-deposition on the surface of the cathode and precipitation by hydroxide anions were the main removal pathways (Drogui et al., 2007; Heidmann and Calmano, 2008). Production of hydroxide ions by the cathodic Journal of Environmental Quality
Fig. 3. Di(2-ethyl hexyl) phthalate (DEHP) fractioning in the raw landfill leachate (LFL) and the effluent of membrane bioreactor (MBR), biofiltration (BF), electro-oxidation (EO), electro-coagulation (ECo), and nanofiltration (NF) processes and their combination.
reaction precipitated metal cations in the form of hydroxide. In the first case, the oxidation–reduction potential had an effect on metal reduction, resulting in a removal efficiency of 50% for most divalent and trivalent metal cations. As reported in literature (Fu and Wang 2011), the ECo process had >90% removal efficiency, no matter the type of metal (Fig. 4). Formation of aluminum and iron hydroxide complexes in the ECo could remove metals by coprecipitation (Heidmann and Calmano, 2008). Rejection of metals by NF depended on the charge and size of cations. As the diffusion coefficient of metal cations in water decreased, the rejection dramatically increased (Murthy and Chaudhari, 2009). In size, Pb2+ is 60% larger than Ni2+, which resulted in 10 times smaller diffusion coefficients in water (Sato et al., 1996); lead was almost rejected by NF, whereas 29% of nickel escaped (Fig. 4). As trivalent cations, such as iron and aluminum, are surrounded by more water molecules, they have
smaller diffusion coefficients, which resulted in removal efficiency >80%.
Process Selection on the Basis of Practical and Economical Aspects A combination of biological and physical-chemical processes for treatment of old landfill leachate showed high removal efficiency for the main pollutants, such as, TOC, COD, NH4+, and total metal concentration. Due to the presence of ammonia and organic carbon at a size
