Short Communication
Treatment of landfill leachate by ozone-based
advanced oxidation processes
Jerry J. Wu
a,*, Chih-Chao Wu
a, Hong-Wen Ma
b, Chia-Chi Chang
aaDepartment of Environmental Engineering and Science, Feng Chia University, Taichung 407, Taiwan bGraduate Institute of Environmental Engineering, National Taiwan University, Taipei, Taiwan
Received 17 July 2002; received in revised form 7 October 2003; accepted 7 October 2003
Abstract
In this study, laboratory experiments are conducted to compare the efficacy using several ozone-based advanced oxidation processes (AOPs), such as O3, O3/H2O2, and O3/UV, to treat landfill leachate. Raw leachate was initially
coagulated by ferric chloride (FeCl3) at the experimental-determined optimal dosage of 900 mg l1, and the ozone-based
AOPs were subsequently applied. Results indicate that all AOPs would result in a significant increase on the ratio of BOD5/COD from 0.06 to 0.5 at the applied ozone dosage of 1.2 g l1. The increase on biodegradability for ozonated
leachate indicates that these AOPs would be beneficial to the subsequent biological treatment process. To better explain the alteration of high organic molecules after oxidation, ultrafiltration was used to separate the leachate by several molecular weight cutoffs (MWCO). The COD distribution for coagulated leachate is 34% for MWCO > 10 kDa, 7% for MWCO between 5 and 10 kDa, 22% for MWCO between 1 and 5 kDa, and 37% for MWCO < 1 kDa. Following ozonation or AOPs, the predominant distribution of COD would be obviously shifted to the MWCO less than 1000 g mol1 (72–85%) over the other MWCO ranges. In addition, Gel Permeation Chromatograph (GPC) analysis has
showed a substantial agreement on the cleavage of larger organic compounds into smaller ones. O3/UV was found to be
the most effective approach among these ozone-based AOPs to enhancing the biodegradability and eliminating the color of leachate.
Ó 2003 Elsevier Ltd. All rights reserved.
Keywords: Ozone-based advanced oxidation processes; Landfill leachate; Biodegradability; Gel permeate chromatography; ADMI; Fourier transformed infrared
1. Introduction
The fact for the serious contamination into the receiv-ing water by the effluent of landfill leachate has increas-ingly attracted more concerns worldwide. Fortunately, the remarkable growth in economics and living standard has accelerated the development of water and wastewater purification technology. It is well acknowledged by the
fact that landfill leachate released by municipal wastes contains large quantities of non-biodegradable and toxic constituents including organic and inorganic substances and that several treatment steps must be adopted to achieve a satisfactory removal of hazardous pollutants. Landfill leachate is commonly generated as a result of water from precipitation, surface run-off, and infiltration or intrusion of groundwater percolating through the landfill (Tsai et al., 1997; Ince, 1998; €ooman and Rosqvist, 1999). Leachate characteristics are determined by the sit-ting, the design and the means of operation of the landfill, and also depend on the condition of waste biodegradation through time. It is rather complicated that contaminants in the landfill leachate are released from the waste due to
*
Corresponding author. Tel.: 24517250; fax: +886-4-24517686.
E-mail address:jjwu@fcu.edu.tw(J.J. Wu).
0045-6535/$ - see front matter Ó 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.chemosphere.2003.10.006
successive biological, chemical, and physical processes. Baig et al. (1999) classified landfill leachate into three types, such as young, intermediate, and stabilized, according to its different landfill ages.
Conventionally, Landfill leachate is treated by the biological processes to eliminate its organic loading in the effluent. Aerobic and anaerobic biological processes have been reported to remove a substantial fraction of biological oxygen demand (BOD), chemical oxygen de-mand (COD) and ammonia from old solid-waste landfill leachate (Dzombak et al., 1990). Although Nedwell and Reynolds (1996) indicated that the use of two-stage anaerobic processes involving methanogenic and sulfur-reducing digestions results in a steady state removal efficiency of COD up to 97%, the most difficult aspects for the old landfill site is the degradation of those re-calcitrant organic compounds within a reasonable period by bio-treatment. In the past decade, advanced oxida-tion technologies including ozonaoxida-tion have been pro-posed and suggested as an auxiliary approach or alternative for the management of landfill leachate (Bigot et al., 1995). Although many of the previous re-searches using ozone or advanced oxidation processes (AOPs) have demonstrated the effectiveness in elimi-nating COD or total organic carbon (TOC) for the landfill leachate (Wable et al., 1993; Bigot et al., 1994; Kim et al., 1997; Steensen, 1997; Wenzel et al., 1999), most of them only used AOPs on the final stabilization prior to discharging into the receiving water body. There-fore, in this study we have attempted using ozone-based AOPs as pretreatment to decompose the recalcitrant organic matters and increase their biodegradation for the subsequent bio-associated treatment. Therefore, the objectives in this paper could be summarized as (1) to evaluate the feasibility of preoxidation of ozone-based AOPs to increase the biodegradability and (2) to com-pare the efficacy of different ozone-based AOPs on oxi-dizing the organic compounds.
2. Materials and methods
2.1. Leachate collection and characteristics
Raw leachate was collected from the wastewater treatment plant of Chen–Shi–Li municipal landfill lo-cated close to Tainan city in southern Taiwan. Upon collection, the leachate was stored at 4°C to minimize any further change that might occur in its physico-chemical and biological properties until the experiments and analyses were carried out within a week. The physicochemical characteristics of the raw leachate used in our study were analyzed and listed in Table 1. According to the characteristics of pHand biodegrad-ability in the collected raw leachate, it is apparent that the raw leachate would fall into the category of
stabi-lized form as it has higher pHvalue (>7.5) and lower BOD5/COD (<0.1) due to the slow release of refractory
organic matter from the landfill site.
2.2. Ozone and AOPs experiments
Ozone was generated from dried pure oxygen by corona discharge using an ozone generator (Ozonia RXO-5), which can produce 6% ozone concentration (w/w) in the oxygen enriched gas stream. To better main-tain the performance of the ozone system, the oxygen was dried using a molecular sieve before entering into the ozone generator. A 4-l semi-batch reactor, which was made of pyrex glass with the dimensions of 12 cm diameter and 50 cm tall, was used to facilitate the oper-ation of all ozone-based oxidoper-ation processes (Fig. 1). Ozone was introduced through a porous fritted diffuser that can produce fairly fine bubbles with diameter less than 1 mm, which had been determined using cam-era with close-up lens and image analysis software Matrox Inspector 2.0 (Chang, 2001). The gas flowrate was regulated at 1 l min1 by a mass flow controller
(Brooks 5850E). The gaseous ozone concentrations in the inlet and outlet stream were determined spectro-photometrically by the absorbance of ozone measured in a 2-mm flow-through quartz cuvette at the wavelength 258 nm. An extinction coefficient of 3000 M1cm1was
used to convert absorbances into concentration units (Nowell and Hoignee, 1988). The inlet and outlet gaseous ozone concentrations in this study were calculated as 40 and 0 mg l1, respectively. Since ozone dosage is
typi-cally defined as the product of gas flowrate, ozone concentration, and ozonation period, divided by the reactor volume, the desired dosage of 1.2 g l1 would
take 30 min to achieve. To reduce foaming caused by the bubbling of ozone into the leachate, 5 ml of anti-foaming agent, Anti-foam 289 (Sigma Co.) was added to the leachate prior to ozonation. All experiments were operated at 25 °C using a water jacket around the reactor. During the experimental period, the leachate in
Table 1
Characteristics analysis of raw landfill leachate used in this study
Parameter Mean value or concentration
pH8.1a COD 6500 mg l1 TOC 4000 mg l1 BOD5 500 mg l1 NH3-N 5500 mg l1 Cl 6000 mg l1 Alkalinity 650 mg l1as CaCO 3
True color 12 000 ADMIb
a
The median value.
b
American Dye Manufacturers Institute. 998 J.J. Wu et al. / Chemosphere 54 (2004) 997–1003
the reactor was pumped continuously through a closed loop equipped with a chamber where all sensors of pH and dissolved ozone monitor were placed and with a sampling port where the leachate could be withdrawn for chemical analyses. The hydrogen peroxide (30%) injection system was initiated using a syringe pump to deliver a fixed flowrate, depending on the ratio to the applied ozone dosage, into the reactor. The input ratio of H2O2 to ozone by weight was controlled at 1.0 g g1,
which had been previously determined to be the optimal ratio (Chang, 2001). In addition, a high-pressure mer-cury UV lamp (Hanovia Co.) with an input energy of 200 W and output intensity of 5.1 mW cm2was placed
at the center of the reactor and shielded by a vertically immersed quartz jacket which was circulated with cold
water to avoid heat accumulation. A black box placed outside the reactor was employed to minimize any penetration of UV light when UV-enhanced facility was operated.
2.3. Analytical methods
The characteristic analyses of the raw and oxidized leachate including BOD5, COD, and ammonia nitrogen,
were performed according to Standard Methods (APHA, 1998). Chloride ions and TOC were measured using ion chromatography (Dionex, DX-100) and TOC analyzer (Tekmar Dohrmann, Apollo 9000), respectively. Color was determined according to the ADMI method pro-posed by American Dye Manufactures Institute using
three different wavelengths, including 438, 540, and 590 nm. To better understand the change of molecular weight distribution after oxidation, both gel permeation chromatograph (GPC) and ultrafiltration membrane separation were employed. GPC was performed using a HPLC system (Shimadzu LC-10) and a refractive-index
detector. The TSKgel G3000PWXL column (TOSOH
Co.), which is especially suitable for the analysis of molecular weight within the range of 1–8.5 kDa, was used to separate the organics in the leachate into several groups. The pump flowrate was set at 0.6 ml min1. The
average molecular weight determined by GPC chroma-tography could be expressed as below (Dickenson and Amy, 2000). Mn¼ X CiMi=Ci Mw¼ X CiMi2= X CiMi D¼ Mw=Mn
where Mn is the average molecular weight based on the
calculation by molecule number; Mw is the average
molecular weight based on the calculation by weight composition of molecules; Ci is the area or number of
specific molecular weight (Mi) from GPC
chromatogra-phy; Mi is the molecular weight; D is the distribution
coefficient of average molecular weight and D = 1. The UF system (Amicon Co.) was utilized to separate or-ganics by passing leachate sample through the ultracel PL membranes with several molecular weight cutoffs (MWCO) of 1, 5, and 10 kDa. Furthermore, the fourier transform infrared spectroscopy (FTIR-8201, Shimadzu Co.) was also used to characterize and observe the change of functional groups in the leachate.
3. Results and discussion
3.1. Coagulation by ferric chloride
Raw leachate collected from the landfill site was initially coagulated by ferric chloride before all ozone-based advanced oxidation processes were applied. According to the evaluation of jar tests in the labora-tory, the most optimal coagulant dosage for COD re-moval of the raw leachate was determined to be 900 mg l1. Under this circumstance the total COD
de-creased approximately 60% (from 6500 to 2500 mg l1).
Similarly, the significant change on the percentage dis-tributions of COD at different MWCOs was observed from 53% to 34% for the molecular weight range greater than 10 kDa (Fig. 2), indicating that coagulation mechanism would result in the substantial removal of larger organic molecules. The average molecular weight (Mw) based upon the calculation of its weight
compo-sition in GPC chromatography was also found to
de-crease significantly from 75 to 11 kDa. It appears that larger molecule portions of raw leachate can be effec-tively removed by ferric chloride coagulation. However, the ratio of BOD5/COD, a traditional indicator of
bio-degradability, of the leachate after coagulation is still below 0.1. It can be predicted that the difficulty would take place if the subsequent biological treatment process is directly applied to decompose these residual organic matters in the effluent of coagulation treatment. There-fore, the ozone-based oxidation processes is proposed in the following session on highlighting the enhancement of biodegradability of landfill leachate and removal of color.
3.2. Ozone-based advanced oxidation processes
In this study, three kinds of oxidation processes including ozone, ozone/UV, and ozone/hydrogen per-oxide were provided to treat the landfill leachate after coagulation. It is apparent that all of the ratios of BOD5
to COD in Fig. 3(a) were observed to increase from 0.1 to 0.5 during the period of applying 1.2 g l1 ozone
dosage regardless of the combination with UV or hydrogen peroxide. This result indicates that the bio-degradability of the leachate has been substantially im-proved after oxidation reaction. Nevertheless, ozone/UV could be figured out to have a better oxidation ability at the initial oxidation period by comparing with the other two processes in Fig. 3(a). The hydroxyl radicals, in-duced from the decomposition of ozone or either from the initiation of hydrogen peroxide or UV lamp, are unable to further accelerate the degradation of non-biodegradable organics primarily due to the presence of greater alkalinity, such as bicarbonate and carbonate, acting as the OHradical scavenger (Langlais et al., 1991; Masten et al., 1996) in the leachate. Moreover, the effect of pHvalues was found to increase approximately from 8.1 to 8.6 for all ozone-based AOPs used in this study. The increase in pHupon sparging with ozone is thought to be due to the stripping of carbon dioxide and volatile fatty acids (Wu et al., 1998a).
Considering the influence of oxidation on molecular weight distribution, Fig. 2 using UF separation shows that COD distribution of the coagulated leachate is 34% for MWCO > 10 kDa, 7% for MWCO between 5 and 10 kDa, 22% for MWCO between 1 and 5 kDa, and 37% for MWCO < 1 kDa. After applying ozonation process alone, the predominant COD distribution would be significantly shifted to the MWCO less than 1 kDa (72%), which appears that the organic compounds with larger molecular weight has been oxidized into smaller ones. As such, ozone/UV behaves much further destruc-tion for the large organic compounds into small pieces with the 85% portion in COD distribution less than 1 kDa. Similarly, the use of GPC separation also gives a clear evidence (Table 2) that average molecular weight
(Mw) of the organic compounds in the leachate decreases
from 10 800 g mol1to 7170 g mol1by ozonation alone,
to 7800 g mol1 by ozone/UV, and to 6080 g mol1 by
ozone/hydrogen peroxide. Another critical parameter, distribution coefficient defined by D¼ Mw=Mn, was used
to assess the distribution range of molecular weights. The larger D value means more broaden distribution of molecular weights would be perceived. Table 2 indicates the d value was observed to decrease from 3.4 to 2.2 by the ozone-based oxidation processes. Thus, the trend of becoming into smaller molecules from larger organic compound by chemical oxidation processes would be closely related to the enhancement of biodegradability. Vogel et al. (2000) presented a useful parameter for characterizing oxidation process by using the mean oxidation number of carbon (MOC):
MOCs¼ 4 1:5ðCOD=TOCÞ
The changes of MOC during the course of oxidation reaction were observed in Fig. 3(b). Ozone/UV leads to the most rapid increase in the oxidation state of carbo-naceous compounds than the other two processes. It is essentially consistent to the observation for the change of BOD5/COD. Nevertheless, the oxidation states finally
achieve a stable level after ozone dosage greater than 0.6 g l1 for all of three oxidation approaches.
For the removal of organic parameters such as TOC, it can be found in Fig. 3(c) that ozonation process alone has the least efficiency in reducing TOC (15% removal), which can be attributed to the destruction mechanisms partially governed by direct reaction of ozone molecules, which are known to react slowly and to end up in car-boxylic acids, aldehydes, and ketones, rather than CO2.
Although the ozone process alone has the ability to produce hydroxyl radicals via the decomposition path-way of radical-type chained reactions (Langlais et al., 1991), the net production efficiency of free radicals is still less than the other ozone-based AOPs used in this re-search. Since the ozone-based AOPs could not decom-pose appreciable amount of organic components in the leachate at the ozone dosage used in this experiment, a biological process would be recommended to operate after the oxidation treatment by degrading the residual organic intermediates into final stable products, such as carbon dioxide and water. Typically, color of leachate is another environmentally concerned issue. As ozone has been widely utilized as a reliable technique and alter-native for decolorizing effluent of the dyeing industry (Wu et al., 1998b), the removal of color of leachate is also included in this session. Color of leachate has been measured to decrease significantly with the increase of ozone dosage as seen in Fig. 3(d). Since most colors are
mainly originated from the unsaturated bonds and specific functional groups, ozone and hydroxyl radicals would be very attractive on specifically attacking the conjugated chains that impart color to the organic molecules.
Fourier transform infrared spectroscopy (FTIR) has been frequently utilized in characterizing the functional groups of the organics for many environmental analyses. In Fig. 4, it is apparent that the hydroxyl group intensity
increases after ozone oxidation, which results in the substitution of functional groups of the organic inter-mediates associated with the electrophilic reaction by adding into the oxygen atoms. In contrast, the intensity representing the specific stretch wavenumbers of double or triple bond between carbons has been observed to diminish after ozonation mainly due to the mechanism of cyclo addition (Langlais et al., 1991).
4. Conclusion
This research can lead to several conclusions sum-marized as below:
1. Coagulation pretreatment can effectively remove varieties of large molecules in the raw landfill leach-ate and aid on the subsequent treatment of ozone-based advanced oxidation processes.
2. Biodegradability indicated by BOD5/COD of landfill
leachate can be significantly enhanced when applied ozone dosage till 1.2 g l1. Ozone/UV is found to be
the best oxidation approach among the three types of selected processes. Leachate separated by UF membrane and GPC has demonstrated the composi-tion shifting from larger molecules into smaller ones. Thus, it is thought to have a direct relationship with the increase of leachate biodegradability.
3. The ozone-based advanced processes are demon-strated to decolorize the leachate for 90% removal. Nevertheless, the removal of TOC is potentially lim-ited even though rather high dosage of ozone is ap-plied. This result implies that ozone-based AOPs should be an excellent alternative prior to using bio-logical treatment process.
4. The FTIR analysis provides useful information in characterizing the change of functional groups of leachate after ozonation reaction.
Fig. 3. (a) BOD5/COD change, (b) MOC change, (c)
percent-age change of TOC, and (d) percentpercent-age change of color (ADMI) of leachate at different ozone-base AOPs and ozone dosage.
Table 2
Calculation of average molecular weight on GPC separation Mn Mw dðMw=MnÞ After coagulation 3190 10 800 3.4 O3 3070 7170 2.4 O3/UV 3010 7800 2.6 H2O2/O3 2810 6080 2.2 4000 3500 3000 2500 2000 1500 1000 0 20 40 60 80 100 C=C C-H C-H C=C O-H Before Ozonation After Ozonation Percent Transmittance cm-1
Fig. 4. FTIR spectra of leachate before and after ozonation process.
Acknowledgements
The authors wish to thank the National Science Council of the Republic of China for the financial support.
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