Print version ISSN 0104-6632
Braz. J. Chem. Eng. vol.29 no.2 São Paulo Apr./June 2012
A. RoodbariI; R. Nabizadeh NodehiI; A. H. MahviI,II,III,*; S. NasseriI; M. H. DehghaniI; M. AlimohammadiI
ISchool of Public Health, Tehran University of Medical Sciences, Tehran, Iran. E-mail: email@example.com, firstname.lastname@example.org, email@example.com, firstname.lastname@example.org, email@example.com
IINational Institute of Health Research, Tehran University of Medical Sciences, Tehran, Iran
IIICenter for Solid Waste Research, Institute for Environmental Research, Tehran University of Medical Sciences, Phone: + 98 21 88954914, Fax: + 98 21 66462267, Enghelab Avenue, School of Public Health, Tehran, Iran
Landfill leachate is one of the most important sources of toxic organic compounds for ground and surface waters. Advanced oxidation processes can offer an effective and environmentally friendly method for pretreatment of landfill leachates. In this study, an ultrasonic process was used for the pre-treatment of landfill leachate with the objective of improving its overall biodegradability, evaluated in terms of the BOD5/COD ratio, up to a value compatible with biological treatment. Under optimized experimental conditions (pH of 10, power of 110 watts, frequency of 60 kHz, TiO2 concentration of 5 mg/L and exposure time of 120 min), this method showed suitability for partial removal of chemical oxygen demand (COD). The biodegradability was significantly improved (BOD5/COD increased from 0.210 to 0.786) which allowed an almost total removal of COD by a sequential activated sludge process.
Keywords: Landfill leachate; Biodegradability; ultrasound.
Leachate consists of many different organic and inorganic compounds that may be either dissolved or suspended (Bila et al., 2005). These compounds pose a potential pollution problem for local ground and surface waters (Esplugas et al., 2004). Leachate from mature landfills is typically characterized by a high ammonium (NH4+) content, low biodegradability (low BOD5/COD ratio) and a high fraction of refractory and large organic molecules such as humic and fulvic acids (Acgdacg and Sponza, 2005), but leachate from young landfills contains low organic compound concentrations (Li et al., 2010). Usually young landfill leachates are treated more easily as compared to the old one (Koh et al., 2004). Leachate is classified as stabilized (mature), intermediate, or fresh (young) given BOD5/COD values of <0.2, 0.20.5, and >0.5, respectively (Robinson et al., 1983; Sun et al., 2010; Pilli et al., 2011). Some methods have been reported for the removal organics from landfill, such as biological treatment (aerobic, anaerobic and anoxic processes), physical methods (sedimentation, air stripping, adsorption, and membrane filtration) (Çeçen et al., 2003; Dehghani et al., 2007) and chemical methods (chemical precipitation (Lim et al., 2009; Kettunen et al., 1996), chemical oxidation (Zhao et al., 2010; Kurniawan et al., 2006), reverse osmosis, granular activated carbon (Peixoto et al., 2010), ion exchange resins and electrochemical oxidation (Bashir et al., 2010; Schuk et al., 1986)). In general, biological processes are preferred for the treatment of leachates with a high ratio of biochemical oxygen demand to chemical oxygen demand (BODU/COD) (Peixoto et al., 2009) and of fresh leachates containing mainly volatile fatty acids (Abdul Aziz et al., 2011). Compact physical and chemical methods have been suggested for the treatment of old, stabilized and dilute leachates with low biodegradability (Kurniawan et al., 2004; Kargi et al., 2004). Because of the variation in leachate composition and the wide range of pollutants contained in leachate, it is difficult to predict a treatment technique that will be effective for leachate (Atmaca, 2009; Mahvi, 2009). Usually a combination of physical, chemical and biological methods is used for effective treatment of landfill leachate, since it is difficult to obtain satisfactory results by using any one of those methods alone (Kurniawan et al., 2004). Traditionally, the degradation of organic compounds can be achieved by advanced oxidation processes (AOPs) (Anglada et al., 2011; Kargi et al., 2003). AOPs have been used to enhance the biotreatability of liquids containing various organic compounds that are non-biodegradable and/or toxic to common microorganisms (Chiang et al., 1995; Cortez et al., 2011). The ultrasonic process is one of AOPs and involves pyrolysis phenomena (thermal degradation) and the generation of the hydroxyl radical (OH), which has a very high oxidation potential and is able to oxidize almost all organic pollutants and volatile matter such as NH3 (Mahvi et al., 2009). Although this process is very effective for complete mineralization of pollutants, if it is applied as the only treatment process, it will be expensive. However, if the ultrasonic process is used as a pre-treatment with other methods such as biological processes, its use can be optimized and be more useful for leachate treatment (Ben Yahmed et al., 2009; Lim et al., 2010). The objective of this study was to investigate the effect of the ultrasonic process as a pre-treatment for leachate biodegradability improvement.
MATERIALS AND METHODS
Samples of landfill leachate were obtained from a municipal landfill site (over 10 years old) located in Shahrood (Semnan, Iran). All leachate samples were collected from leachate lift stations or storage tanks, stored at 3ºC, and tested within 2 days of collecting the samples. The main chemical characteristics of the raw leachate are summarized in Table 1. The ammonium concentrations were analyzed with a C203 8 parameter test meter (Hanna Electronics Co., Ltd.). The pH was measured by a Benchtop pH Meter (Cole-Parmer Co., Ltd). The pH meter was calibrated before each use with pH 3, 7 and 10 buffer solutions. BOD and COD measurements were determined following Standard Methods 5210 and 5220, respectively. Reagents and standard chemicals were purchased from Hach Co., except for the BOD buffer solution, which was prepared according to Standard Method 5210. Ferrous sulphate (FeSO4·7H2O), sulphuric acid and hydrogen peroxide (Merck, 30 wt. %) were of analytical grade. BOD check standards were performed with each batch of BOD measurements. The results were considered good when the value of the BOD check standard fell within the range of 198 ± 30.5 mg /L. The average ± standard deviation of the BOD check standards for the entire duration of the project was 169 ± 29.0 mg /L, which demonstrates good results given the inherent variability in BOD measurements. COD check standards were also performed with each batch of COD measurements. A COD standard solution of 1000 mg /L was diluted to 200 and 500 mg /L to ensure the accuracy of COD measurements. The relative difference for calibration check standards (RDcal) is defined as the absolute difference of the check standard concentration and the known concentration all divided by the known concentration. The RDcal for COD was <10% for the entire duration of the project.
As shown in Figure 1, for the laboratory experiments a cylindrical shaped Plexiglas reactor with total volume of 1 L was prepared. The solution in the reactor was mixed with a magnetic stirrer, while sufficient aeration was provided by a compressor connected to a porous stone located in the bottom of the reactor. The compressor was used to ensure a completely mixed condition in the reactor. The ultrasonic source was a Model UGMA-5000 ultrasound generator with three (30, 45 and 60 kHz) transducers having a titanium probe with 20 mm diameter. The power input could be adjusted continuously from 60 to 120 W. A leachate sample of 1000 ml was sonicated in a covered cylindrical glass vessel. Aeration was supplied by a Model SALWAT air compressor. The sludge was continuously aerated using aeration pumps.
After the optimization by factorial design, the ultrasonic irradiation was applied in the treatment of raw leachates using a batch mode. At first, the raw leachate sample was filtered through filter paper (0.45) to remove any suspended solid impurity. Then the sample was adjusted to the required pH with H2SO4 or NaOH. Then different scenarios were tested with regard to power intensities of 70 and 110 W, frequencies of 30, 45 and 60 kHz, reaction times of 30, 60, 90 and 120 minutes and pH of 3, 7 and 10. For determining the effect of catalyst on sonication efficiency, 5 mg/L of TiO2 and ZnO have been used separately. To evaluate the effect of ultrasonic irradiation on the biodegradability of raw and sonochemically pre-treated leachates, BOD5 and COD of both samples were measured.
The activated sludge system was applied in cylindrical aeration glass-vessels (30 cm internal diameter and 60 cm in height). The system was aerated by using air pumps placed at the bottom of the reactors. The initial volume of the culture was 150 mL, which was completed to 300mL with substrates (leachate, pre-treated leachate or glucose) at the beginning of each cycle. The pH was controlled by a probe and adjusted to 7.0 by using H2SO4 or NaOH. The oxygen concentration was monitored by using an O2 probe, located at the top of the reactor. All the experiments were carried out in duplicate and at room temperature (2025ºC) for periods of 72 h. For COD determinations, samples (10 ml each) were taken every 12 h, after they had been centrifuged and filtered through a 0.45 mm Millipore filter (De Morais and Zamora, 2005).
RESULTS AND DISCUSSION
Characterization of the Raw Landfill Leachate
The main chemical characteristics of raw leachate are summarized in Table 1. When the biodegradability ratio (BOD5/COD) is lower than 0.25 and the pH near to 8, the samples can be considered to be moderately stabilized leachates (Wang et al., 2009). In most cases, intensive and sophisticated physicochemical processes are necessary for the treatment of mature leachates.
Effect of the Sonocatalytic Process on the Biodegradability of Leachate
The results indicated that the sonocatalytic process can improve leachate biodegradability (BOD5/COD ratio). The BOD5/COD ratio for raw leachate was 0.210, but it reached 0.786 (with TiO2) and 0.783 (with ZnO) after sonication. Independent Samples T-test showed that there is significant difference between the BOD5/COD ratios of raw leachate and pretreated leachate with the sonocatalytic process (pvalue =0.000 for both TiO2 and ZnO). The results indicated that the system operates with great efficiency (BOD5/COD ratio=0.786) at pH 10, power of 110 watts, frequency of 60 kHz and TiO2 concentration of 5 mg/L.
Effect of the Ultrasound Power Input on Biodegradability Improvement
Figure 2 shows the effect of power input on leachate biodegradability for TiO2 and ZnO. As shown in this figure, the power input clearly improves biodegradability. Independent Samples T-test (Table 2) showed that there is a significant difference between the BOD5/COD ratios of raw leachate and pretreated leachate at different powers. (pvalue =0.000 for both TiO2 and ZnO).
According to sonochemistry theory, when the ultrasound intensity reaches or exceeds the cavitation threshold, bubbles will be formed easily and the cavities collapse violently. Increasing the ultrasonic power will increase the energy of cavitation, lowering the threshold limit of cavitation, and enhancing the quantity of cavitation bubbles (Kurniawan and Lo, 2009). In other words, at higher intensities, the concentration of hydroxyl radicals and mass transfer are higher, which leads to more degradation of organic material (Chianese et al., 1999) and also more biodegradable intermediate compounds. The efficiency of ratio improvement then increases with the increase of ultrasonic intensity.
Effect of Exposure Time on Biodegradability Improvement
Results indicated that the exposure time somehow improved biodegradability. Figure 3 shows the effect of exposure time on leachate biodegradability for TiO2 and ZnO. A One-Way ANOVA test (Table 3) showed that there is no significant difference between the BOD5/COD ratios of raw leachate and pretreated leachate with sonocatalytic process at different exposure times (pvalue=0.467 for TiO2 and 0.398 for ZnO).
Effect of Frequency on Biodegradability Improvement
Results indicated that the frequency clearly improve biodegradability. Figure 4 shows the effect of frequency on leachate biodegradability for TiO2 and ZnO. A One-Way Anova test (Table 4) showed that there is significant difference between the BOD5/COD ratios of raw leachate and pretreated leachate with the sonocatalytic process at different frequencies (pvalue =0.000 for both TiO2 and ZnO). Results of the Tukey statistical test (Table 5) also showed that there are significant differences between frequencies of 30 and 60 kHz (pvalue =0.000) and between 45 and 60 kHz (pvalue =0.000).
Effect of pH on Biodegradability Improvement
Results indicated that the pH somehow improves biodegradability. Figure 5 shows the effect of pH on leachate biodegradability for TiO2 and ZnO. A One-Way ANOVA test (Table 6) showed that there is no significant difference between the BOD5/COD ratios of raw leachate and pretreated leachate with the sonocatalytic process at different pHs (pvalue =0.503 for TiO2 and 0.170 for ZnO).
Effect of Type of Catalyst on Biodegradability Improvement by Ultrasound
Figure 6 shows the effect of the type of catalyst on the leachate biodegradability (BOD5/COD ratio). Results showed that the effects of the two catalysts on leachate biodegradability were similar but an Independent Samples T-test (Table 7) indicated that there is no significant difference between the BOD5/COD ratios of raw leachate and pretreated leachate with TiO2 and ZnO (pvalue =0.287).
Concurrent Effect of Power Input and Frequency on Biodegradability Improvement by Ultrasound
As mentioned above, power and frequency were effective parameters (statistically significant) for biodegradability improvement by the sonocatalytic process. The Univariate statistic test (Table 8) showed that estimates of the size of the effect were 88.6 % for power and 74.9 % for frequency but the concurrent effect was smaller (44.4 %).
Biodegradability Changes During Ultrasonic Decomposition
Initially, the biodegradability of the leachates was evaluated through the evolution of the BOD5/COD ratio. For untreated samples, this parameter has values of about 0.21, while ultrasonic treatment for 120 min permits its enhancement up to values near 0.786, which represents substantial biodegradability according to the current literature (Bae et al., 1999; Pi et al., 2009). This result indicates that the ultrasonic process can break down or rearrange molecular structures of organic matter and convert the non-biodegradable organics to more biodegradable forms. This is a fact of remarkable importance in the case of the application of an integrated chemicalbiological system to leachate treatment (De Morais and Zamora, 2005). In general, it is admitted that an ultrasonic process can transform recalcitrant organic compounds into easily biodegradable products, improving the efficiency and reducing the cost of further biological steps. In a second phase, raw and pre-treated leachate was submitted to a biological degradation process using a sequential batch reactor. The evolution of the COD during the biological treatment (Figure 7) confirms the low biodegradability of raw leachates, which achieve a maximal COD removal of about 30% at 72 h of treatment. On the other hand, the COD of pre-treated leachates decreases progressively, attaining COD removal higher than 90% at the end of the 72-h cycle. Additionally, the use of ultrasonically pre-treated samples favored the preservation of the physical characteristics of the biological sludge, which could be corroborated by the measurement of traditional physical parameters and microscopic observation.
Landfill leachates contain some macromolecular organic substances that are resistant to biological degradation. With very low biodegradability ratios (BOD5/COD), usually lower than 0.21, these complex matrixes show a recognized resistance to conventional activated sludge systems. When applied as a relatively brief pre-treatment, the sonocatalytic process induces several modifications of the matrix, which results in a significant enhancement of its biodegradability. For this reason, the integrated chemicalbiological systems proposed here represent a suitable solution for the treatment of landfill leachate samples with an efficient remediation of the relevant parameter (COD).
Authors would like to thank the research office of Tehran University of Medical Sciences.
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(Submitted: July 24, 2011 ; Revised: October 22, 2011 ; Accepted: November 2, 2011)