Treatment of Landfill Leachate By Electrochemical Oxidation and Anaerobic Process

By Li, Tinggang Li, Xiufen; Chen, Jian; Zhang, Guoping; Wang, Hongchun

ABSTRACT: The removal performance of typical refractory organic compounds in landfill leachate was investigated during the electrochemical (EC) oxidation and anaerobic process combined treatment system in this paper. The results indicated that the treatment of landfill leachate by the combined system was highly effective. The toxicity of leachate was notably decreased after the electrochemical oxidation process and the biodegradability was improved. The concentration of the organic acid with low molecular weight in the leachate increased from 28% to 90% based on the biodegradability assays after the EC oxidation process. The anaerobic digestion could further remove the residual organic compounds. At a hydraulic retention time (HRT) of 16 hours and an organic loading rate (OLR) of 8 kg COD/m^sup 3^ d, the concentration of COD, SS, ALK, VA, N-TKN, N-NH^sub 4^^sup +^ and P-PO^sub 3^^sup – ^ in UASB effluent were 532, 12, 6744, 400, 540, 455 and 11.6 mg/L, respectively, with approximately 90% removal efficiency of COD. The organic compounds in the landfill leachate revealed different degradation characteristics in the combined system, p- chloroaniline, bisphenol A, 6-methyl-2-phenyl-quinoline, dimethylnaphthaline and N’-(2-methyl-4-chlorophenyl)-N- cyclohexyformamidine, classified into the first group in this paper, were completely removed by the EC oxidation and did not reappear in the effluent of the UASB reactor. Phenylacetic acid, 3-methyl- indole and N-cyclohexyl-acetamide, called the second group, were completely removed, but reappeared in the UASB reactor. 4-methyl- phenol, 3,4-dihydroisoquinoline, 2(3H)-benzothiazolone, exo-2- hydroxycineole and benzothiazole, the third group, were degraded little in the EC oxidation process, but extensively removed by the anaerobic process. Benzoic acid, benzenepropanoic acid and 2-cyano- 3,5-dimethyl-1-hydroxypyrrole, the fourth group, concentration obviously increased in the EC process, but was completely removed in the UASB reactor. The content of volatile fatty acids (VFAs) markedly increased from 0.68% in the leachate to 16.18% in the effluent from the electrochemical oxidation process (EC^sub effl^). In addition, the degradation rate of organic compounds from the landfill leachate was different in the EC oxidation and anaerobic process. Water Environ. Res., 79, 514 (2007). KEYWORDS: landfill leachate, biodegradability, electrochemical oxidation, anaerobic digestion, UASB

doi:10.2175/106143006X115435

Introduction

Generally the landfill leachate consists of high concentrations of COD, ammonia nitrogen and heavy metals as well as showing brown color (Bae et al., 1999). In addition, as a landfill ages, it contains significant amounts of toxic and refractory organic matter such as phenolic, acidamide, heterocyclic compounds and polycyclic aromatic hydrocarbon compounds (Berrueta et al., 1996; Cameron and Koch, 1980). It permeates into ground and surface waters, contributing to their pollution, and hence it poses considerable hazards to the natural environment. Removal of COD and ammonia nitrogen from landfill leachate is at all times one of the most difficult problems and the most pressing issues worldwide (Lema et al., 1988). Treatment of leachate is complex and expensive; it generally requires various process applications. Physical, chemical and biological treatments, such as adsorption, precipitation, filtration, electrochemical oxidation, anaerobic and aerobic treatment, have been studied in landfill leachate.

It should be noted that electrochemical (EC) oxidation has become an attractive alternative for leachate treatment as a result of recent progress (Peters, 1999; Marttinen et al., 2002). Since it can partly convert toxic and refractory compounds into simple matter or biodegradable substances without secondary pollution to the environment (Chiang et al, 1997; Sheng and Chi, 1996), it is considered an environmental friendly technology (Genders and Weinberg, 1992; Keith, 1995; Sotiropoulos, 1997). The up-flow anaerobic sludge blanket (UASB) system is characterized by active granular sludge, which has superb settling characteristics and excellent methanogenic activity ensuring high-rate anaerobic reaction. Therefore, such a system allows the accumulation of high biomass concentration for a slow growth rate and the use of high hydraulic retention times. The UASB system is undoubtedly the most extensively applied anaerobic high-rate system in the world (Berrueta et al., 1996; Kennedy and Lentz, 2000).

There are numerous reports on either the electrochemical oxidation or the UASB treatment of leachate, however only a handful of reports have investigated the combination of electrochemical oxidation and use of high rate UASB reactor. Landfill leachate treatment normally was focused on the removal of COD and ammonia nitrogen, but little attention has been paid to the degradation characteristics of toxic and refractory organic matter during leachate treatment. This paper highlights the degradation characteristics of organic pollutants in leachates by the electrochemical oxidation and anaerobic process combined treatment system. In order to determine the degradation characteristics of organic compounds in leachate, it was necessary to investigate the change of the typical organic compounds in landfill leachate before and after the electrochemical oxidation and anaerobic treatment. It may provide technical assistance to further investigate the treatment technology of landfill leachate and the large-scale development of electrochemical oxidation and anaerobic process combined treatment systems.

Materials and Methods

Experiment set-up. The schematic representation of the experimental set-up is given in Figure 1. The electrochemical system, purchased from Yixing Environmental Equipment Co. Ltd. (Yixing City, Jiangsu Province, P.R. China, 214036), included an electrochemical reactor, a direct current source and a magnetic stirrer. The electrochemical reactor had a working volume of 1 L, with the anode made of an alloy of titanium, iridium and ruthenium, and the cathode made of graphite. The high-rate Plexiglas UASB, which was homemade in our lab, had a valid volume of 900 mL and was 17 cm in length. The experimental temperature was kept constant at 35 +- 1[degrees]C by a heat exchanger and a thermo-regulation box.

Landfill Leachate. Landfill leachate used in the study was taken from Taohua mountain municipal landfill, Wuxi city, China. This landfill is in operation for 13 years old and accepts municipal waste produced by approximately 4,320,000 citizens. During this time of the landfill, the most easily microbial degradable organic compounds in leachate have already been degraded, so the leachate contained a relatively high ammonia nitrogen concentration and a low ratio of BOD to COD. Characteristics of the leachate are shown in Table 1.

Seed sludge. The seed sludge used in the UASB reactor was taken from Taihushui beer wastewater treatment plant in Wuxi city, China. The reactor was inoculated with 650 mL of seed sludge.

Biodegradability of raw leachate and ECera. In order to investigate the toxicity of organic compounds in leachate and determinate their fate during treatment, it is necessary to carry out preliminary assessment of biodegradability. Biodegradability assays were carried out in closed Erlenmeyer flasks of 500 mL, which were operated in batch type with raw landfill leachate assay at pH of 7.78 and ECeffl assay at pH of 7.19 from EC oxidation. The experimental temperature was kept constant at 35 [degrees]C by a heat exchanger and a thermo-regulation box. A given mass of sludge was put into the flasks from the lower section of the UASB reactor and was diluted to 2.1 g MLVSS/L. Direct methane measurements were carried out by using Mariette flasks filled with an alkaline solution (25 g/L NaOH). COD and methane were measured periodically until it became constant (Soto et al; 1992; Berrueta et al, 1996). Anaerobic biodegradability, in terms of percentage organic matter used by microorganisms during anaerobic digestion, can be calculated on a COD basis, as the addition of the fraction of COD removed from the influent and the fraction of COD that remains as volatile acidity in the effluent (Soto et al, 1991).

Experimental procedure. For EC pretreatment of leachate, the key operating conditions included electrode distance, pH, the density of current and reaction time. The electrochemical oxidation experiments with continuous stirring were performed under the optimized conditions with a 10 mm electrode distance, 6.5V electrical potential, 15A current, 100 cm^sup 2^ electrode, 0.15 A/cm^sup 2^ current density and pH at 8. After the electrochemical oxidation for 1 hour, the effluent (Table 1) entered into the UASB reactor with a successful start-up. After the EC oxidation and anaerobic biological process combined system was stably operated, samples of the raw leachate, the effluent of electrochemical oxidation (EC^sub eff^) and the effluent of the UASB reactor (UASB^sub eff^) were taken for analysis of components.

Analytical methods. The analyzed parameters included phosphate (PO^sub 4^^sup 3-^), total KjeTKN), ammonia nitrogen (NH^sub 4^^sup +^), COD, BOD^sub 5^, suspended solids (SS), volatile suspended solids (VSS), total alkalinity (ALK), volatile acidity (VA), color and pH (APHA, 1995). All parameters were measured by the national standard methods. The samples was analyzed by gas chromatograph and mass spectrograph (GC-MS, Finnigan trace, USA). GC conditions were: The column was OV1701 with 30 m x 0.25 mm x 0.25 [mu]m. The temperature at the inlet was 250[degrees]C and increased gradually from the beginning of 40[degrees]C. The carried air was He. MS conditions were: the ionic source was EI with the radiated current of 150 [mu]A and the temperature of 200[degrees]C. The voltage of detector was 350 V. The internal standard method was used in calculating the mass percent of different compounds. In order to obtain and concentrate all compounds from the samples, the samples were extracted twice by dichloromethane (CH^sub 2^Cl^sub 2^) under the condition of alkaline pH 12 and acid pH 2, respectively. Then the two extracts were mixed and concentrated to 1 mL before being measured. Results and Discussion

Biodegradability assays of raw leachate and EC^sub effl^. Typical results for a lab-scale biodegradability assay of raw leachate and EC^sub effl^ are shown in Fig. 2. It indicated that only 28% of the organic acid with low molecular weight measured by COD in the raw leachate was biodegradable. This can be attributed to the fact that the treated leachate came from a 13 years landfill. Clearly, the degradation, which takes place in the waste dump itself together with the flushing out effect, has reduced the amount of biodegradable materials. However, the biodegradability was improved remarkably by using EC oxidation and 90% of COD was removed during 12 days. Also, as seen from Figure 2, the results of assays raw landfill leachate and ECeffl were carried out under different pH conditions without unified adjustment. More biogas during UASB treatment was produced than without EC oxidation indicating strong inhibition to anaerobic microbes of raw leachate. For example, the volume of CH^sub 4^ was 250 mL for effluent of EC at initial pH of 7.19, but was only 50 mL for raw landfill leachate at initial pH of 7.78.

The main typical organic components in leachate before and after treatment. The measured results of raw landfill leachate by GC-MS are listed in Table 2. It can be seen from Table 1 was observed that the refractory biodegradable compounds were over 70% of organic components in the raw landfill leachate analyzed by GC-M S. The main typical organic compounds of about 50% in raw landfill leachate were given in Table 2. Phenolic, nitrogen heterocyclic, polycyclic aromatic hydrocarbon and hydroxyl aromatic compounds were the predominant organic components in the aged leachate and were difficult to biodegrade.

The characteristics of landfill leachates before and after EC oxidation are given in Table 1. The value of COD in the effluent of the electrochemical reactor was 5320 mg/L with about 54% of COD removed, indicating that many organic compounds present in the landfill leachate were degraded by radicals formed by EC oxidation (Chiang, et al., 1995). The remaining COD mainly was the organic acid with low molecular weight and easy to be biodegraded.

A bench scale UASB reactor was operated continuously for 162 days untill steady state was achieved, including 64 days of the startup. The leachates, after being treated 1 hour in an optimal EC oxidation condition, acted as influent of the UASB reactor. The removal efficiencies of the UASB including the stage of start-up are presented in Table 3. During 162 days of operation, particularly from day 102 to day 143, the COD removal of UASB amounted to around 90%, indicating the system had reached steady-state. The effluent concentrations of COD, SS, ALK, VA, N-TKN, N-NH^sub 4^^sup +^ and P- PO^sub 3^^sup -^ were 532, 12, 6744, 400, 540, 455 and 11.6 mg/L, respectively, with a HRT of 16 hours and an OLR (Organic Loading Rate) of 8 kg COD/m^sup 3^ d. In this study OLR was higher than that of Berrueta et al. (1996) who only applied anaerobic digestion to treat leachate without pretreatment. It is apparent that pretreatment may be necessary to remove the organic matters with high molecular weight from leachate. The total removal of COD from the leachate in the combined treatment system reached 95%.

In addition, the highest value of biogas production with 46.2 mL CH^sub 4^/h was obtained at the OLR of 8 kg COD/m^sup 3^ d. As shown in Table 3, the methane conversion rate ranged from about 63 to 236 mL CH^sub 4^/g (COD removed) when the HRT was elevated from 10 to 48 hours. It was found that, under steady-state conditions in the UASB reactor, the quantity of methane formed per unit of biomass and unit of COD removal was 4.3 mL CH^sub 4^(/g COD removed) (g VSS), taking into account the fact that the maximum methane yield can be as much as 350 mL CH^sub 4^/g COD removed (Lema et al., 1991). The specific activity of methanogenic bacteria is an indicator. It can be said that the higher the quantity of biogas formed, the more the active biomass present in the granular sludge of the UASB reactor, consequently the more the quantity of organic pollutants removed from the landfill leachate. Under the same conditions, biogas production could potentially evaluate the activity of the methanogenic organism and inhibition of organics in the influent of the UASB reactor (ECeffl). It was also apparent by comparing Table 3 with Table 1 that pH of the effluent from the reactor was always higher than the influent and kept a constant differential between influent pH and effluent pH. The ratio of acidity to alkalinity in effluent always remained below 0.3, which is the maximum value observed for UASB reactor, within 162 days. It was very helpful to avoid the system breakdown when the inhibition of toxic matters and the acidification of the reactor took place. This result was consistent with that of Berrueta et al (1996). These observations indicated the system operation was stable based on taking all this into account.

The measured results of the different organic acids in the effluent of the electrochemical oxidation reactor and anaerobic reactor by GC-MS are listed in Table 4 and 5. Comparing results in Table 4 and 5 with Table 2, it can be seen that, after EC oxidation, the concentration of toxic and refractory organic components in the landfill leachate was considerably decreased and new substances, organic acids with low molecular weight, were formed as shown in Table 4. During the electrochemical oxidation process, free radicals with high oxidizing activity were formed, which attacked refractory organic compounds, for example, heterocyclic compounds and polycyclic aromatic hydrocarbons (Table 2). Then, the ring-opening reaction (Flesazr and Ploszynska, 1985; Chiang, 1995 and 1997) took place and the simple matters produced. It was evident that the electrochemical oxidation effectively removed or converted toxicant or refractory organic compounds into simple or biodegradable substances, for example organic acids, which were easily taken up by anaerobic microorganism. This is an indication that there was noticeable improvement in biodegradation characteristics. Subsequently, the biochemical method can be used. As shown in Table 5, most of the easily biodegradable matter was removed in the anaerobic biological process. Though the effluent of the UASB reactor contained some complicated organic compounds, the content of those residual substances was greatly reduced.

Degradation characteristics of the typical organic compounds in electrochemical oxidation and anaerobic biological process combined treatment system. In order to further observe the degradation characteristics of the typical organic compounds in the electrochemical oxidation and anaerobic biological process combined treatment system, the organic compounds were classified into four types as shown in Fig. 3. The organic compounds, which were completely removed by the electrochemical oxidation process and did not reappear in the effluent of the UASB reactor, are shown in Fig. 3(a). These compounds included p-chloroaniline, bisphenol A (p,p- isomer), dimethylnaphthaline, 6-methyl-2-phenyl-quinoline and N’-(2- methyl-4-chlorophenyl)-N-cyclohexyformamidine. The removal of these toxic compounds from the leachate was favorable for the following anaerobic biological treatment. It meant that advanced electrochemical oxidation technology was an effective pretreatment technique to landfill leachate.

It was also observed from Figure 3(b) that some organic pollutants from the leachate, such as phenylacetic acid, 3-methyl- indole and N-cyclohexyl-acetamide were completely oxidized in the electrochemical oxidation process, but occurred once more in the effluent of the UASB reactor. For example, the concentration of phenylacetic acid was 30.4 mg/L in the leachate. It was removed completely by the electrochemical oxidation; however, it was detected at a concentration of 4.1 mg/L in the effluent of the UASB reactor. This may indicates that other residual complicated organic compounds in the effluent of the electrochemical oxidation reactor were metabolized by the microorganisms and were further transformed into a small quantity of phenylacetic acid during the anaerobic digestion process.

The results in Figure 3(c) revealed that some organic compounds remained in the effluent of the electrochemical reactor, such as 4- methyl-phenol, 2(3H)- benzothiazolone, naphthoquinone, benzothiazole, exo-2-hydroxycineole and 3,4-dihydroisoquinoline. They were mostly removed by combining the electrochemical oxidation with the anaerobic digestion process. However, there was an evident difference in removal of contaminants between the electrochemical reactor and the UASB reactor. As summarized in Figure 3(c), the removal of benzothiazole mical oxidation process, but it was not further degraded in the anaerobic digestion. This result indicated benzothiazole was extremely biorefractory and very difficult to remove by a biological process. The observation was consistent with that of Charles (1988) and Wang (2002). The electrochemical oxidation process was more efficient in removing the refractory compounds than the anaerobic treatment. However, the concentration of exo-2-hydroxycineole was reduced from 9.7 mg/L to 9 mg/L by the electrochemical oxidation process with the removal of 7.22%, but rapidly decreased to 2.2 mg/L with the higher removal of 75.56% by the anaerobic digestion. The most unfused ring compounds, such as two-ring and three-ring aromatic hydrocarbons, could be degraded in anaerobic process, and those rates of degradation was also higher than that of fused ring compounds and heterocyclic compounds (Maldonado, 1999; Rothermich, 2002). The combined system was more effective for the treatment of landfill leachate from this point of view. Another degradation characteristic, as shown in Figure 3(d), was that the concentration of some organic compounds obviously increased after the electrochemical oxidation process, but sharply reduced in the effluent of UASB reactor with approximately 74 to 96% removal rate. These kinds of compounds included benzoic acid, benzenepropanoic acid and 2-cyano-l-hydroxy-5-methylpyrrole. It was noteworthy that free radicals were formed during the electrochemical oxidation. Organic compounds in the leachate were attacked by these free radicals. Thus, these complex organic compounds were converted into simple matter, mainly was organic acid with low molecular weight, which was easily removed during the anaerobic digestion.

The degradation characteristic and removal efficiency of organic compounds were different in the combined system. In addition, the electrochemical oxidation and anaerobic digestion played different roles in removing contaminants from the landfill leachate.

Volatile fatty acid (VFA) produced in the electrochemical oxidation process. The changes in VFAs in the leachate before and after the electrochemical oxidation process are shown in Table 6. The most substantial change observed was that most of the refractory organic compounds were decreased or removed completely. The content of VFAs was increased from 0.68% to 16.18% after the electrochemical oxidation process. For example, there was no acetic acid detected in the landfill leachate, but its content increased to 2.3% with a concentration of 32.2 mg/L in the effluent of the electrochemical process. The fact that VFAs occurred in the effluent of the electrochemical oxidation process indicated that some organic compounds were converted into simple compounds, easily degraded with improving biotreatment efficiency of the landfill leachate, and that the electrochemical oxidation, in particular electrocatalysis, reduced the toxicity and content of refractory organic compounds in the raw leachate.

Conclusions

The landfill leachate studied had a refractory matter fraction of over 70% in organic components. The biodegradable matter fraction of the leachate sharply increased from 28% to 90% after EC oxidation based on the biodegradability assays. For a long duration of 162 days operation, it can be seen that the UASB reactor could successfully treat the effluent of EC oxidation. The effluent concentration of COD, SS, ALK, VA, N-TKN, N-NH^sub 4^^sup +^ and P- PO^sub 3^^sup -^ from UASB were 532, 12, 6744, 400, 540, 455 and 11.6 mg/L, respectively, with the COD removal efficiency of about 90% at a HRT of 16 hours and an OLR of 8 kg COD/m^sup 3^ d. The degradation characteristics and removal efficiency of organic matters in landfill leachate were different in the electrochemical oxidation and anaerobic biological process combined treatment system. The electrochemical oxidation convened the complex compounds into easily biodegradable matter, or were rempved completely and the content of VFAs markedly increased from 0.68% to 16.18%. The biodegradability was greatly improved. The residual organic matter could be degraded by the following anaerobic digestion. The electrochemical oxidation and anaerobic digestion played different roles in removing contaminants from the landfill leachate.

Acknowledgements

This study was financially supported by International Cooperation Key Project (No. 2002DF000006) from Ministry of Science and Technology, The People’s Republic of China, for which great acknowledgment is given. The authors thank Mrs. Yuan Shushen and Mr. Wang Liping of the Research Center for Analyses and Measurements at Southern Yangtze University, China, for their invaluable advice with the GC-MS technical assistance. Also acknowledgment to Prof. Larry Hoffman for revising the syntactical errors.

Submitted for publication July 10, 2004; revised manuscript submitted April 17, 2006; accepted for publication May 9, 2006.

The deadline to submit Discussions of this paper is August 15, 2007.

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Tinggang Li1,2, Xiufen Li1,2, Jian Chen1,2*, Guoping Zhang3, Hongchun Wang3

1 Lab of Environmental Biotechnology, School of Biotechnology, Southern Yangtze University, Wuxi, Jiangsu, 214036, China.

2 Key Lab of Industrial Biotechnology, Ministry of Education, Southern Yangtze University, Wuxi 2)4036, Jiangsu, China.

3 Jiangsu Environmental Engineering & Technical Research Center, Yixing 214205, Jiangsu, China.

* Corresponding Author: Tel./Fax: +86-510-5888301, E-mail: jchen@ sytu.edu.cn.

Copyright Water Environment Federation May 2007

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