A New Approach to Characterize Biodegradation of Organics By Molecular Mass Distribution in Landfill Leachate
By Ha, Dong Yun Cho, Soon Haing; Kim, Young Kwon; Leung, Solomon W
ABSTRACT: This study provides biodegradability of organics in leachate according to their molecular mass distributions (<0.5, 0.5 to 1, 1 to 3, 10, and >10 KDa). Organics with molecular mass values lower than 0.5 KDa were the predominant species in the raw leachate nitrate, and the aerated lagoon process was very effective in treating these highly biodegradable organics; the Fenton’s oxidation process was very effective in treating not-so-biodegradable organics with molecular mass values higher than 0.5 KDa, but a portion of these organics were converted into organics <0.5 KDa after Fenton's oxidation. An oxygen uptake measurement using a respirometer was more sensitive than a conventional biochemical oxygen demand measurement to evaluate bioactivities, especially when bioactivities were low.
Water Environ. Res., 80, 748 (2008).
KEYWORDS: molecular mass, leachate, aerated lagoon, Fenton’s oxidation process, biodegradable organics.
Landfilling, which remains the most popular method of solid waste disposal in Korea and in other countries, causes serious environmental problems by discharging leachate. Leachate from landfill site includes organic matters (biodegradable and nonbiodegradable organic matter), ammonia-nitrogen, halogenated organics, inorganic salts, and toxic metals (Kirn et al., 2004). A recent survey indicated that 48 499 metric tons of domestic wastes are generated daily in Korea, of which, approximately 43% is disposed in landfills. In addition, 19% of 95 908 metric tons of industrial wastes and 11% of 2858 000 metric tons of hazardous wastes are treated and disposed in landfills (Ministry of Environment, 2002). It is difficult to set up treatment systems for leachate discharged from solid waste landfill, as a result of changing organic constituents as the landfill ages (Ragle et al., 1995). Because the biodegradability of organics is related to the molecular mass (MM) distribution of organic compounds, molecular mass distribution data of organics are often used as a guideline to design the treatment process for leachates.
To provide feasibility information for setting up an appropriate leachate treatment process, the biodegradability of organics, with respect to molecular mass distribution in leachate, was investigated. Biodegradability of raw leachate, molecular-mass- fractionated samples from effluent of an aerated lagoon, and Fenton’s oxidation were investigated, by comparing the conventional 20-day biochemical oxygen demand (BOD^sub 20^)/chemical oxygen demand (COD^sub Cr^) ratio with accumulated oxygen uptake (by respirometer). Results from the investigation were used as a basis for characterization of leachates, so a proper treatment system could be recommended.
Site Description and Sample Collections. Samples were collected at the Sudokwon Landfill Site (SLS) (Kimpo, South Korea). The SLS was established in 1992, and the landfill area is approximately 15 million m^sup 2^. The amount of waste disposed in the SLS is approximately 7 million metric tons/y, which includes household, food, and industrial waste. Approximately 10 200 m^sup 3^/d of leachate is collected, and facilities for leachate treatment consist of an aerated lagoon, followed by Fenton’s oxidation and biological nitrogen treatment (anoxic/aerobic process). Among the leachate- treatment facilities, samples at the effluents of the lagoon and Fenton’s oxidation were used for this study, and their operation conditions are given in Table 1 (Kim and Cho, 1999); raw leachate samples were also used in this study, as a reference concentration. General characteristics of the raw leachate are given in Table 2.
Samples (raw leachate included) were pretreated to remove suspending solids, by passing through a 0.45-um-pore-diameter membrane filter (Whatman, Florham Park, New Jersey). There was no significant difference in the dissolved organic carbon concentration for the pretreated samples when they were compared with samples before filtering.
Molecular mass separations were carried out using batch ultrafiltration stirred cells (model 8400, Arnicon [Belford, Massachusetts] 350-mL capacity, Figure 1). Samples were separated into 5 molecular mass fractions, as follows: <500 Da (<0.5 KDa), 500 to 1000 Da (0.5 to 1 KDa), 1000 to 3000 Da (1 to 3 KDa), 3000 to 10 000 Da (3 to 10 KDa), and > 10 000 Da (>10 KDa). The separation was assisted using 345 KPa (50 psi) nitrogen gas on top of the sample solution, with constant stirring of solution, to prevent concentration polarization. The concentration of filtrated and retained organics was measured by biochemical oxygen demand (BOD) and COD. Experimental conditions of the molecular mass separation are shown in Table 3.
Estimation of Biodegradability. To evaluate the biodegradability of the molecular-mass-fractionated samples (<0.5, 0.5 to 1, 1 to 3, 10, and >10 KDa), values of the BOD^sub 20^/COD^sub cr^ ratio and amount of oxygen uptake by electrolytic respirometer were analyzed in parallel. The BOD test has several limitations; one of limitations is that the sample is diluted in the BOD bottle, and, therefore, the degradation kinetics in the bottle cannot be directly compared with the treatment process without dilution. Another limitation is that the breakdown of organic matter in the BOD test is incomplete (Logan and Wagenseiler, 2000). In this study, oxygen uptake by electrolytic respirometer was used to compare with oxygen uptake by BOD measurement. Figure 2 shows the experimental procedures for evaluating the biodegradability of organics. Biodegradability by the BOD^sub 20^/COD^sub Cr^ ratio was calculating by dividing the BOD^sub 20^ by the COD^sub Cr^ value (ISO, 1994; Jeong and Jeong, 2004). In general, the CODCr of samples were first determined, then the samples were diluted to one-half the strength, and the BOD values of samples (5-day BOD [BOD^sub 5^] and BOD^sub 20^) were determined. Supernatant from the aerated lagoon process was used for seeding material after settling at room temperature (20 to 25[degrees]C) for 24 hours for the BOD tests. To prevent nitrogenous oxygen demand, 2-chloro-6-(trichloromethyl) pyridine was added to test samples as a nitrification inhibitor. Experiments for BOD^sub 20^, COD^sub Cr^, mixed liquor suspended solids (MLSS), and mixed liquor volatile suspended solids (MLVSS) measurements were carried out according to Standard Methods (APHA et al., 1998), with duplicate samples.
A respirometer (model BI-1000, Bioscience Inc. [Bethlehem, Pennsylvania], total volume 1.2 L) test was used for direct measurement of the oxygen consumption by microorganisms (ISO, 1992); 200 mg/L (COD^sub Cr^ basis, filtered) of organics from each testing category (raw leachate, lagoon effluent, and organics of various molecular mass) was added to the sample initially, as a background concentration. After the initial MLVSS of the sample was determined, 100 mg MLVSS/L of returned sludge from the aerated lagoon process was used as a seeding material. It should be noted that the initial MLVSS reported for raw leachate and lagoon effluent was measured before the addition of seeding material; thus, the second-day measurements of suspended solids were lower than the initial values, as a result of the dilution effect by the sludge volume. All samples were adjusted to approximately pH 7, and the experimental temperature of the respirometer was maintained at 25 +- 2[degrees]C; however, the biological activity constant (McGhee, 1991) of samples was adjusted to 20[degrees]C when oxygen uptake comparisons were made with BOD measurements. Approximately 2 mL of MgSO^sub 4^ . 7H^sub 2^O (0.41 M), CaCl^sub 2^ (0.25 M), and FeCl^sub 3^ . 6H^sub 2^O (0.018 M) trace element solutions were added to 1 L of prepared sample for the respirometer test (APHA et al., 1998). The oxygen uptake of each sample (blank and controlled) was monitored. An aliquot of 50 mL of sample was taken and analyzed for MLSS and MLVSS. All chemicals used were of analytical quality; double deionized water was used for dilution and sample preparations.
Results and Discussion
Characterization of Organic Carbons by Molecular Mass Distribution for Different Treatments. Organics with molecular mass values lower than 500 (<0.5 KDa) were the predominant species in the raw leachate, and the fraction of total organic concentration was 68.8% (COD^sub Cr^). Fractions of other molecular mass values (0.5 to 1, 1 to 3, 3 to 10, and >10 KDa) of samples were 10.9, 7.3, 6.0, and 7.0%, respectively (Figure 3). Therefore, treatment of organic matters <0.5 KDa in the raw leachate should take precedence over other organic matters.
Efficiency of organic destruction according to molecular mass distributions was very different in various treatment processes. In the aerated lagoon process, 86.3% of organics <0.5 KDa was removed, but only 58.5% of organics >0.5 KDa was removed. In the Fenton’s oxidation process, the removal efficiency of organics <0.5 KDa was 38.0%, but, for organics >0.5 KDa, was 84.9%. In other words, the removal of organics <0.5 KDa was superior in the biological process, but organics >0.5 KDa were predominantly removed by the chemical process. Thus, experimental results suggested that organic matters <0.5 KDa could be defined as biodegradable organic compounds and organic matters >0.5 KDa as nonbiodegradable (NBD) or poor aerobically biodegradable organics (Choi et al., 1997). It should be noted that the Fenton’s oxidation process could also convert a portion of organics >0.5 KDa into organics <0.5 KDa. Thus, removal of NBD organics could be enhanced by chemical oxidation, such as Fenton's oxidation. With the same samples, organic removals were measured by BOD^sub 5^, as shown in Figure 4; not surprisingly, the biodegradability of the samples measured by BOD^sub 5^ was quite different from the COD^sub cr^ measurements. In effluent from the aerated lagoon process, the BOD^sub 5^ value of organics >0.5 KDa was exceptionally low (<1%), which was unrealistic and could be misleading. It is a known fact that low-molecular-mass organics, such as plasticizers and polycyclic aromatic hydrocarbons, would not be effectively degraded by aerated treatment (Logan and Wagenseiler, 2000; Welander and Henrysson, 1998). When the concentration of biodegradable organics is low, accurate BOD^sub 5^ measurement is difficult. This is a disadvantage of using BOD to characterize leachate.
Alternative Calculation of Biodegradable and Nonbiodegradable Organics for Different Treatment Process. The concentration of NBD organics can be calculated by the difference between COD^sub Cr^ and BOD^sub 20^, as follows (Christensene et al., 1992):
NBD = COD^sub Cr^ (total organics) – BOD^sub 20^ (biodegradable organics) (1)
According to eq 1, we deduced that NBD organics in raw leachate consisted of 41.5% <0.5 KDa and 58.5% >0.5 KDa. Also, biodegradable organics were 78.2% <0.5 KDa and 21.8% >0.5 KDa. Over 94% of the biodegradable organic matter in the raw leachate was removed by the lagoon process, but only 30% of the NBD organic matter was removed (Figure 5). The removal of 30% NBD organics was probably the result of surface sorption sedimentation by biofloc instead of biological activities (Tsezos and Bell, 1989; Wang et al., 2000).
The removal efficiencies of biodegradable and NBD organics from the Fenton’s oxidation process were 27.7 and 73.2%, respectively. It was shown that the Fenton’s oxidation process can be effective in NBD organics removal; our results are in agreement with those obtained by Kirn et al. (2001). Overall, after the Fenton’s process, biodegradable and NBD organics remaining constituted 4.5 and 18.6% of the raw leachate, respectively. Additional treatment, such as the membrane process, would be required for the remaining NBD organics to be removed.
Biodegradability by BOD^sub 20^/COD^sub Cr^ Ratio. The BOD^sub 20^/ COD^sub Cr^ ratio values of molecular-mass-fractionated raw leachate are shown in Figure 6. The BOD^sub 20^/COD^sub Cr^ ratio of organics <0.5 KDa was approximately 0.8, which means that organics <0.5 KDa were highly biodegradable.
The BOD^sub 20^/COD^sub Cr^ ratio values of other samples (0.5 to 1, 1 to 3, 3 to 10, and >10 KDa) were 0.53, 0.43, 0.46, and 0.49, respectively. If the criterion of biodegradability was set at 0.5 for the BOD^sub 20^/COD^sub Cr^ value (ISO, 1994; Jeong and Jeong, 2004), then the organics >0.5 KDa in the raw leachate were biodegradable, to a certain degree; however, biodegradation took place slowly. The BOD^sub 20^/COD^sub Cr^ ratio values in effluent from the lagoon process were less than 0.2 in all of the molecular- mass-fractionated samples (Figure 7).
This means that organics in effluent from the lagoon process were mainly NBD organics, and the majority of the biodegradable organics were already consumed by the process. In effluent from the Fenton’s oxidation process, the BOD/COD^sub Cr^. ratio values of organics <0.5 and 0.5 to 1 KDa increased sharply after 10 to 13 days (Figure 8); this was because part of the NBD organic matter in the effluent from the lagoon process was converted to biodegradable organic matter by the chemical oxidation process.
Therefore, the raw leachate can be characterized as moderately biodegradable; effluent from the lagoon process can be described as nonbiodegradable; and effluent from the Fenton’s oxidation can be described as slowly biodegradable.
Biodegradability by Respirometer Oxygen Uptake Test. The amount of apparent oxygen uptake by the molecularmass-fractionated samples in the raw leachate (blank-corrected) increased in order of molecular mass; <0.5,0.5 to 1, >10, and 1 to 3 KDa uptake apparently followed a similar pattern as 3 to 10 KDa after the first 2 days (Figure 9). Microorganisms could easily degrade organics that were <3.5 and 0.5 to 1 KDa, because these matters were highly biodegradable. These results corresponded well with those of the ultimate BOD test (Figure 6), especially with the oxygen uptake line of organics >10 KDa that was higher than the lines of 1 to 3 and 3 to 10 KDa. It was presumed that microorganisms consumed oxygen and converted highmolecular-mass organics into medium-range-molecular- mass organics.
In effluent from the lagoon process, the oxygen uptake lines of molecular-mass-fractionated samples showed a similar pattern (Figure 10). In the lagoon, organic matters <0.5 KDa were almost all removed; thus, the small amount of remaining organics <0.5 KDa in the effluent had similar degrading trend with organics >0.5 KDa. Generally, microorganisms preferred to degrade organics within a certain range of molecular mass than a mixture of all organic masses. Also, the amounts of oxygen consumed from samples of the lagoon effluent were relatively lower than those of the raw leachate- evidence that that lagoon process was effective in removing organics with molecular mass <0.5 KDa.
Biodegradability by Mixed Liquor Volatile Suspended Solids and Mixed Liquor Suspended Solids. The concentrations of MLVSS and MLSS in the respirometer were compared with oxygen uptake measurements. For the raw leachate, changes of concentrations both in MLVSS and MLSS were very similar (Figure 11), especially for measurements of organics >1 KDa; after 1 day, they showed distinctive concentration increases after the initial lag periods, which corresponded well with the oxygen uptake measurements (Figure 9). However, the MLVSS and MLSS measurements also showed rapid concentration increases for organics with molecular mass values < 1 KDa in the raw leachate after 3 days. Combined with BOD and respirometer measurements (Figures 6 and 9), in which the results indicated that organics > 1 KDa were slowly degradable by microorganisms, we speculated that the mid-range-molecular-mass organics are more efficient for cell growth, and further research is needed to substantiate the observations.
Likewise, MLVSS and MLSS concentrations in effluent of the lagoon process only showed increases for organics <0.5 KDa (Figure 12). The results also agreed well with the rapid oxygen uptake in the lagoon effluent (Figure 10) after 2 days of lag period. The MLVSS and MLSS concentrations for organics >0.5 KDa showed no significant changes in the lagoon effluent, although the respirometer test showed increasing activity with time. We speculated that microbes continuously consumed oxygen and broke down the larger organic molecules into smaller fragments, but the converted organics were not used for cell growth, as most of the larger organics that could be used for cell growth were already consumed in the lagoon process.
Comparison of Biodegradability Between BOD^sub 20^/COD^sub Cr^. Ratio and Oxygen Uptake. As shown in Figure 13a, the measurement of differences of BOD^sub 20^/COD^sub Cr^ and oxygen uptake by respirometer from the raw leachate were almost negligible, but there appeared to be great differences between the values of BOD^sub 20^/ COD^sub Cr^ and oxygen uptake up by respirometer from lagoon effluent for organics >3 KDa (Figure 13b); the relative oxygen uptake values were all significantly higher. It should be noted that, although the relative differences of BOD^sub 20^/COD^sub Cr^ and oxygen uptake were significant, the absolute measurements of these values were comparatively small. Thus, the difference could not be easily observed between Figures 8 and 10, and this high sensitivity is the advantage of the respirometer over conventional BOD measurements. The respirometer measurements showed that, despite the slow degradation activities, there were measurable oxygen uptakes for organics >3 KDa in the effluent of the aerated lagoon.
To establish a proper leachate treatment system, biodegradability tests of molecular-mass-fractionated organics can provide valuable information for the feasibility of such a system. Conclusions of the biodegradability tests in our current system were as follows:
1. The aerated lagoon process was effective in removing organics with molecular mass values <0.5 KDa, and the Fenton's oxidation process was suitable for removing organics >0.5 KDa.
2. Regardless of molecular weight, most biodegradable organics were removed by the aerated lagoon process. The Fenton’s oxidation process was very effective in reducing NBD. The NBD organics were predominant in the final effluent from the sequential aerated lagoon- Fenton’s oxidation leachate treatment system, and these organics were <0.5 KDa. Further study is need to provide necessary information to remove the final NBD organics.
3. Organics with molecular mass values <0.5 KDa can be classified as highly biodegradable, and organics with molecular mass values >0.5 KDa can be classified as having low or medium biodegradability.
4. Oxygen uptake measurements by respirometer were very comparable with BOD measurements, but the respirometer measurements were more sensitive and especially useful when bioactivity was low.
Submitted for publication January 10, 2006; revised manuscript submitted September 28, 2007; accepted for publication February 6, 2008.
The deadline to submit Discussions of this paper is November 15, 2008. References
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Dong Yun Ha1, Soon Haing Cho1, Young Kwon Kim2, Solomon W. Leung3*
1 Division of Environmental, Civil, and Transportation Engineering, Ajou University, Suwon, Korea.
2 Department of Environmental Engineering, Hankyong National University, Ansung, Korea.
3 Civil and Environmental Engineering Department, College of Engineering, Idaho State University, Pocatello, Idaho.
* 921 S. 8th Ave, Stop 8060, College of Engineering, Idaho State University, Idaho, 83209; e-mail: firstname.lastname@example.org.
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