DEODORIZATION AND WASTEWATER TREATMENT STUDY OF LANDFILL LEACHATE

Transcription

1 Journal of the Chinese Institute of Environmental Engineering, Vol. 16, No. 1, pp (006) DEODORIZATION AND WASTEWATER TREATMENT STUDY OF LANDFILL LEACHATE Jyh-Herng Chen, 1 Yung-Chien Hsu,, * Hsiang-Cheng Yang and Chau-Hung Chou 1 Department of Materials & Mineral Resources Engineering National Taipei University of Technology Taipei 106, Taiwan Department of Chemical Engineering National Taiwan University of Science and Technology Taipei 106, Taiwan Key Words : Ozonation, landfill leachate, deodorization, coagulation, BOD/COD ABSTRACT A sequential ozonation of landfill leachate was conducted to evaluate the preozonation effect on the efficiency of deodorization and coagulation treatment and the biodegradability of landfill leachate. For the deodorization of landfill leachate, hydrogen sulfide can be readily removed by ozonation, but the ozonation treatment of ammonia is not effective. For the coagulation treatment, the coagulation efficiency can be improved by the preozonation, provided that the ozone dosage is very low. For the biodegradability of landfill leachate, the BOD/COD of raw landfill leachate was increased from 0.18 to 0.8 after deodorization and coagulation process. The second ozonation at ph 8 and 10 further increased the BOD/COD to a maximum value equal to 0.34 and 0.45, respectively. The corresponding ozone dosages were equal to 61.9 and 450 mg L -1. Preozonation does not change the molecular weight distribution of organic compounds in landfill leachate, whereas the surface properties of organic compounds are modified. However, during the secondary ozonation, the organic compounds in landfill leachate are decomposed by ozone to become smaller molecules, which are more biodegradable. INTRODUCTION In Taiwan, 71% of garbage is treated in landfill. The garbage water content in Taiwan is about 55%, which is higher than that in Japan (48%) and western countries (5%) due to the difference in dietary habit [1]. In addition, the high average annual rainfall in Taiwan causes the landfill sites to generate a large amount of leachate. The release of landfill leachate can cause secondary environmental problems. Landfill leachate contains high levels of organic compounds, inorganic salts and heavy metals, resulting in wastewater with deep color and unpleasant odor. The composition of the leachate varies with the landfill age. Leachate from landfill sites has foul odor and high COD value, which is difficult to be removed. The aged landfill leachate exhibits very low BOD/COD ratio, usually below 0.1, indicating a low biodegradability of the leachate organics. The landfill gas contains methane, carbon dioxide, nitrogen, oxygen, hydrogen, hydrogen sulfide, ammonia and trace amounts of volatile fatty acid [,3]. Among the compounds of landfill gases, hydrogen sulfide, ammonia and volatile fatty acid are odorous compounds [3]. For the treatment of toxic hydrogen sulfide gas, caustic solution absorption and iron sponge adsorption are often adopted [4]. Other treatment methods of landfill gas include biological deodorization, combustion, neutralization and oxidation [5,6]. For the removal of COD from landfill leachate, the conventional method is a chemical coagulation process combined with biological treatment. However, the concentration of COD of thus treated landfill leachate is too high to meet the increasingly higher effluent standards (00 mg L -1, EPA, Taiwan). In order to meet the stricter effluent standards, several new landfill leachate treatment processes, such as reverse osmosis, electrolysis, activated carbon and ozonation, have been widely studied [7-10]. Ozone is a powerful oxidant. It undergoes selfdecomposition at high ph and releases hydroxyl free radicals, which act as a stronger oxidant than ozone molecules [11]. Ozonation has been applied to disin- * Corresponding author Hsu@ch.ntust.edu.tw.

2 3 Journal of the Chinese Institute of Environmental Engineering, Vol. 16, No. 1 (006) fection, deodorization and non-biodegradable wastewaters treatment [1-14]. With a suitable ozonation treatment, the coagulation efficiency of wastewater can be enhanced [15-17]. Ozonation can also reduce the toxicity of organic compounds and increase the biodegradability of wastewater [18,19]. Individual ozonation processes of landfill leachate have been widely studied, including deodorization, ozonation/coagulation and ozonation/biotreatment. A better understanding of the combined effects of these unit processes is important in order to establish an effective treatment process for landfill leachate. This study used the landfill leachate of an old landfill site (northern Taiwan) to evaluate the feasibility of ozonation of landfill leachate. A three-stage sequential process for landfill leachate treatment, including preozonation to deodorize leachate, coagulation to remove COD, and post ozonation to enhance biodegradability, was conducted. The performance efficiencies of deodorization, coagulation and biodegradability enhancement were investigated. MATERIALS AND EXPERIMENTAL METHODS The ozonation system includes three major parts: gas-inducing reactor [0], ozone generator and ozone detector. The gas-inducing reactor has two in-series 45 o pitched-blade turbines enclosed by a draft tube. Detailed configuration of this reactor is similar to that in the study of Hsu and Huang [0]. The ozone generator (Trailigaz, France) can provide gas flow rate ranging from 0 to 900 NL h -1. The maximum ozone production rate is 9 g O 3 h -1, when air is used to generate ozone. When pure oxygen is used, the maximum ozone production rate is 18 g O 3 h -1. The ozone concentrations of input and output gas streams were measured with an ozone UV photometry analyzer (Ld SOZ-30C Seki Electronics Co., Japan) using 54 nm wavelength. The detection range of ozone concentration was 0-00 g Nm -3 ± 0.%. The reactor was filled with landfill leachate (7 L) and the ph of leachate was adjusted to proper value using H SO 4 and NaOH solution. The agitation speed was adjusted to 1500 rpm, while the ozone gas was simultaneously introduced into the reactor. The flow rate of ozone gas was controlled at 10 NL h -1. The input ozone concentrations for preozonation and ozonation were controlled at 15 mg NL -1 and 0 mg NL -1, respectively. For the air stripping experiment, operation conditions were the same as ozonation. At proper time intervals, samples were taken to determine the BOD and COD of leachate, concentration of odorous compounds in the gas phase and the coagulation efficiency of preozonized leachate. The flow diagram of the experimental apparatuses is shown in Fig Air T. C Exit of gas 1. Ozone generator 10. ph meter. Air compressor 11. Vapor condensing device 3. Oxygen cylinder 1. Liquid sampling port 4. Moisture removal device 13. Mechanical seal 5. Ozone analyzer 14. NaOH solution vessel 6. Motor 15. Gas pipe for ozone 7. Gas-inducing reactor C.W. Cooling water 8. Dosing pump T.C. Thermal couple 9. Thermostatic jacket Fig. 1. Flow diagram of the experimental apparatus. BOD was measured with BOD Trak apparatus (Model 173B, HACH, U.S.A), which determines and converts the pressure drop in BOD bottle to BOD value. Prior to the BOD measurement, the samples were subjected to aeration for min to remove residual ozone, then the samples were kept in dark for 1 h. Hach COD digester and COD reagents were used to digest the oxidizable organics at 150 C. A UV/Vis spectrophotometer (Hach DR-4000) was used to determine the COD value of the digested samples at 60 nm. A Metrosonics ep-7400 gas sensor (Metrosonics, U.S.A) was used to determine the concentration of hydrogen sulfide and ammonia. The molecular weight distribution of leachate organics was measured by gel filtration chromatography with a C-16/17 glass column on sephadex G75 (Pharmacia Biotech, USA). The coagulation efficiency was determined using a jar test apparatus with six jars (JT-6, SK-1057, Taiwan). There were six standard coagulation vessels. Each vessel was equipped with a standard Ruston turbine impeller. The mixing speeds determined in the preexperiments were 150 rpm (rapid coagulation mixing) and 30 rpm (slow flocculation mixing). Background test showed that coagulation with 4000 mg L -1 of ferric chloride and at ph 8 could achieve the optimal coagulation. In this study, all the coagulation processes, therefore, were conducted under this optimal condition. The ADMI (American Dye Manufacturers Institute) value was determined using a built-in program in

3 Chen et al.: Deodorization and Treatment of Leachate 33 Table 1. Characteristics of the landfill leachate. COD 1100±35 BOD 190.5±37 BOD/COD 0.18 ADMI Value 700±15 NH 3 -N 607±10 SS 10±0 ph 7.9±0. H S ND t(s) Fig. 3. Ammonia removal by preozonation and air stripping of landfill leachate at ph = 8 (Gas flow rate = 10 NL h -1, inlet ozone = 0 mg NL -1, agitation speed = 1500±10 rpm). Fig.. Concentration of H S in gas phase during preozonation and air stripping of landfill leachate (Gas flow rate = 10 NL h -1, inlet ozone = 0 mg NL -1, agitation speed = 1500±10 rpm, ph = 8). UV/Vis spectrophotometer (Hach DR-4000) that scaned the visible light wavelength from 400 to 700 nm at 10 nm increment. The landfill leachate used in this study was obtained from an old landfill site near Taipei County, Taiwan. The compositions of the landfill leachate are shown in Table 1. The BOD/COD ratio of the leachate was about 0.18 and the ph was about 8. The sequential three-stage process was used in the present study. In the first stage, the landfill leachate was deodorized and preozonized. The coagulation treatment of preozonized leachate was conducted in the second stage. In the final stage, the landfill leachate was further treated by ozonation. 1. Deodorization t(s) RESULTS AND DISCUSSION Figure compares the variations of H S concentration in the gas phase for both air stripping and preozonation conditions of landfill leachate. For air stripping, the concentration of H S in the gas phase first increased from.4 ppm to a maximum value of 3.8 ppm. Then, the concentration of H S decreased to approximately 0 ppm after 350 s. For preozonation, the concentration of H S decreased continuously and rapidly from.4 ppm to approximately 0 ppm within 50 s. The continuous decrease of H S in the gas phase under ozonation indicates the decrease of H S concentration in the liquid phase resulting from the fast decomposition of H S by ozonation. Consequently, the emission of H S gas into the environment significantly decreased compared to that with air stripping. The ozonation of hydrogen sulfide in the liquid phase can be determined by measuring the sulfate concentration during the ozonation [1,]. However, our experimental results showed that the sulfate concentration was too low to be detectable, owing to the low initial concentration of hydrogen sulfide in this landfill leachate sample. Figure 3 shows the variation of ammonia concentration in gas phase for ozonation and air stripping of landfill leachate. Since ammonium is only minimally reactive with ozone [3,4], the variation of ammonia concentration in the gas phase was mainly by air stripping. For both ozonation and air stripping conditions, ammonia concentration curve shows a peak value, due to the fact that ammonia in the landfill leachate was stripped into the gas phase. Nevertheless, the two curves are different in the first 00 s, then, the two curves coincide with each other. In the first 00 s, the ammonia concentration in the gas phase for ozonation was slightly lower than that of air stripping. Since the landfill leachate initially contains various organic compounds, the ozoantion of those compounds will cause the consumption of ozone gas. Therefore, the

4 34 Journal of the Chinese Institute of Environmental Engineering, Vol. 16, No. 1 (006) Fig. 4. Effect of preozonation on the COD removal efficiency, R COD, by coagulation treatment of preozonized landfill leachate (ph = 8, FeCl 3 = 4000 mg L -1 ). output gas flow rate of ozonation will be lower than that of air stripping. For stripping after 00 s, the two curves overlap, indicating same stripping effect for both conditions. According to the previous study [5], the ozone outlet concentration started to rapidly increase after about 00 s of preozonation, indicating that ozone in the liquid phase is no longer consumed by organic compounds. Therefore, the stripping gases flow rates for both conditions become almost the same. Volatile fatty acid is one type of odorous compound often found in newer landfill. Nevertheless, for the old landfill leachate in this study, the amount of volatile fatty acids was very low. Therefore, the removal of volatile fatty acids was not studied here.. Effect of coagulation Figure 4 shows the effect of preozonation on the coagulation efficiency of preozonized landfill leachate. The coagulation treatment efficiency is evaluated by the COD removal efficiency. R COD is defined as 1- COD /COD 1, where COD 1 and COD represent the CODs of preozonized leachate and the coagulation treated leachate, respectively. For the raw water treated only by coagulation, the COD removal efficiency was equal to 0.45 (dotted line). It is clear from Fig. 4 that preozonation not only deodorizes the landfill leachate but also improves the coagulation efficiency, provided that the ozone dosage is low. However, excess ozone dosage will slightly decrease the COD removal efficiency of coagulation treatment. Farvardin and Collins [15] investigated the enhancement of preozonation on the coagulation efficiency of two simulation humic solutions and found that the optimal ozone dosages were 0.18 and 0.5 mg L -1, respec- (Da) Fig. 5. Molecular weight distribution of organic compounds in raw leachate, preozonized leachate and coagulated leachate (Preozonation: ozone dosage = 3.5 mg L -1, ph = 8, FeCl 3 = 4000 mg L -1 ). tively. Based on these results, it is comcluded that the primary mechanism for the improvement of coagulation efficiency of soluble organics is owing to that ozone reacts with organic compounds to increase the amount of carboxylic acid groups on the organic compounds. The carboxylic acid groups can easily combine with coagulant by chelating, resulting in higher COD removal [15,17]. Figure 5 shows molecular weight distributions of organic compounds in raw leachate, preozonized leachate and coagulated leachate. The molecular weight distributions were not significantly changed before and after preozonation. It is deduced that the major preozonation mechanism is to change the surface chemical properties, such as generation of carboxylic acid, of the organic compounds in the landfill leachate. However, after coagulation the compounds of molecular weight between 500 and 5000 Da were significantly reduced. Therefore, the coagulation is most effective for removing organics with molecular weight higher than 500 Da. 3. Effects of Ozonation on Biodegradability of Landfill Leachate One of the major concerns about landfill leachate is the biodegradability, i.e. BOD/COD ratio. The BOD/COD ratio of landfill leachate was increased from 0.18 to 0.8 after deodorization/coagulation process. Since biological treatment generally requires that the BOD/COD ratio should be higher than 0.40 [19], the second ozonation was conducted to further increase the biodegradability of landfill leachate. Figure 6 shows the effect of ozonation on the biodegradeability (BOD/COD) of landfill leachate. For ozonation at ph 8, the BOD/COD ratio first increased with

5 Chen et al.: Deodorization and Treatment of Leachate 35 BOD/COD (-) ozone dosage, until this ratio had reached a maximum value of 0.34 at an ozone dosage equal to 61.9 mg L -1. For a higher ozone dosage, the BOD/COD ratio decreased. This may be because the microorganisms used in this study were not suitable for the intermediate products generated during ozonation at ph 8. Figure 6 also shows that for ph 10, the BOD/COD ratio increased continuously from 0.8 to 0.45 at an ozone dosage equal to 450 mg L -1, indicating that less toxic intermediate products may be generated during ozonation at ph 10. Due to the complex nature of landfill leachate, it was not able to identify the intermediate products. However, at higher ph, the ozonation of organic compounds is more effective than that at lower ph. It is therefore deduced that the intermediate products at higher ph should be less toxic than those at ph 8. Figure 7 shows the comparison of molecular weight distribution of organic compounds in leachate before and after second ozonation. Ozonation was conducted at ozone dosage equal to 450 mg L -1, which is the optimal dosage for ozonation at ph 10. The results show that ozonation lowered the amounts of higher molecular weight compounds (molecular weight within 1-5 kda), consequently, the amounts of lower molecular weight compounds (< 1 kda) increased. Therefore, it is clear that the increase in BOD/COD from 0.8 to 0.45 (Fig. 6) was mainly due to the degradation of high molecular weight organic compounds (such as humic acids) into smaller molecules, which are more easily decomposed by microbes. Fig. 6. Variations in biodegradability (BOD/COD) at ph 8 and 10 during secondary ozonation. (Da) Fig. 7. Molecular weight distribution of organic compounds in raw leachate after coagulation, and raw leachate after coagulation and secondary ozonation (Secondary ozonation: ph = 10, ozone dose = 450 mg L -1, FeCl 3 = 4000 mg L -1 ). CONCLUSIONS A sequential treatment of landfill leachate was conducted to evaluate the deodorization of landfill leachate and the wastewater treatment. During the deodorization, hydrogen sulfide can be readily converted to sulfate by ozonation. However, the ozonation treatment of ammonia is not effective. For the coagulation treatment, the coagulation efficiency can be improved by the preozonation at low ozone dosage. Excess ozone dosage, on the other hand, will decrease the coagulation efficiency. The molecular weight distribution of the organic compounds in the landfill leachate remains almost the same, during the preozonation. It is deduced that the coagulation is most effective for removing organics with molecular weight higher than 500 Da. For the biodegradability of landfill leachate (BOD/COD), the BOD/COD ratio of raw landfill leachate was increased from 0.18 to 0.8 after deodorization and coagulation process. The second ozonation at ph 8 and 10 further increased the BOD/COD ratio to a maximum value equal to 0.34 and 0.45, respectively. The corresponding ozone dosages were equal to 61.9 and 450 mg L -1. Preozonation does not change the molecular weight distribution of organic compounds in landfill leachate, whereas the surface properties of organic compounds are modified. However, during the secondary ozonation, the organic compounds in landfill leachate are decomposed by ozone to become smaller molecules, which are more biodegradable. Based on previous conclusions, we suggest that the sequential process is applicable to the treatment of this landfill leachate. ACKNOWLEDGMENTS The authors thank the National Science Council

6 36 Journal of the Chinese Institute of Environmental Engineering, Vol. 16, No. 1 (006) of Taiwan, R.O.C. for the financial support under Project No: NSC E REFERENCES 1. Chen, H.L., H.L. Zheng and Z.Z. Wu, Research of nature and amount of leachate from hillside landfill. J. Agr. Forest. (in Chinese), 44(1), 17-6 (1995).. He, C.T., D.J. Herman, R.G. Minet and T.T. Tsotsis, A catalytic/sorption hybrid process for landfill gas cleanup. Ind. Eng. Chem. Res., 36(10), (1997). 3. Gourdon R., C. Comel, P. Vermande and J. Veron, Kinetics of acetate, propionate and butyrate removal in the treatment of a semi-synthetic landfill leachate on anaerobic filter. Biotechnol. Bioeng., 33(9), (1989). 4. Flynn, B.E., Sulfide treatment technologies for sour landfill gas. Waste Age, 6(9), (1996). 5. Kapahi, G. and M. Gross, Biofiltration for VOC and ammonia emissions control. Biocycle, 36(), (1995). 6. Kenson, R.E., Controlling Odors. Chem. Eng.- New York, 88(), (1981). 7. Di Palma, L., P. Ferrantelli, C. Merli and E. Petrucci, Treatment of industrial landfill leachate by means of evaporation and reverse osmosis. Waste Manage., (8), (00). 8. Chiang, L.C., J.E. Chang and T.C. Wen, Indirect oxidation effect in electrochemical oxidation treatment of landfill leachate. Water Res., 9(), (1995). 9. Rodríguez, J., L. Castrillón, E. Marañón, H. Sastre and E. Fernández, Removal of non-biodegradable organic matter from landfill leachates by adsorption. Water Res., 38(14-15), (004). 10. Bila, D.M., A.F. Montalvão, A.C. Silva and M. Dezotti, Ozonation of a landfill leachate: evaluation of toxicity removal and biodegradability improvement. J. Hazard. Mater., 117(-3), 35-4 (005). 11. Staehelin, J. and J. Hoigne, Decomposition of ozone in water in the presence of organic solutes acting as promoters and inhibitors of radical chain reactions. Environ. Sci. Technol., 19(1), (1985). 1. Liakou, S., S. Pavlou and G. Lyberatos, Ozonation of azo dyes. Water Sci. Technol., 35(4), (1997). 13. Beltran, F.J., J.M. Encinar, and J.F. Garcia-Araya, Oxidation by ozone and chlorine dioxide of two distillery wastewater contaminants: gallic acid and epicatechin. Water Res., 7(6), (1993). 14. Thorell, B., H. Broen, A. Grimvall, A. Nystroem, and R. Savenhed, Characterization and identification of odorous compounds in ozonated waters. Water Sci. Technol., 5(), (199). 15. Farvardin, M.R. and A.G. Collins, Preozonation as an aid in the coagulation of humic substancesoptimum preozonation dose. Water Res., 3(3), (1989). 16. Jekel, M.R., Flocculation effects of ozone. Ozone- Sci. Eng., 16(1), (1994). 17. Chandrakanth, M.S. and G.L. Amy, Effects of ozone on the colloidal stability and aggregation of particles coated with natural organic matter. Environ. Sci. Technol., 30(), (1996). 18. Karrer N.J., G. Ryhiner and E. Heinzle, Applicability test for combined biologicalchemical treatment of wastewaters containing biorefractory compounds. Water Res., 31(5), (1997). 19. Marco, A., S. Esplugas and G. Saum, How and why combine chemical and biological processes for wastewater treatment. Water Sci. Technol., 35(4), (1997). 0. Hsu, Y.C. and C.J. Huang, Characteristics of a new gas-induced reactor. AIChE J., 4(11), (1996). 1. Watkins, B.D., S.M. Hengemuehle, H.L. Person, M.T. Yokoyama and S.J. Masten, Ozonation of swine manure wastes to control odors and reduce the concentrations of pathogens and toxic fermentation metabolites. Ozone-Sci. Eng., 19(5), (1997).. Lloyd, F.M., Controlling reduced sulfur compounds. Environ. Technol., 7(5), (1997). 3. Hwang, Y., T. Matsuo, K. Hanaki and N. Suzuki, Removal of odorous compounds in wastewater by using activated carbon, ozonation and aerated biofilter. Water Res., 8(11), (1994). 4. Kuo, C.H., F. Yuan and D.O. Hill, Kinetic of oxidation of ammonia in solution containing ozone with or without hydrogen peroxide. Ind. Eng. Chem. Res., 36(10), (1997). 5. Chou, C.H., Landfill Leachate Treatment by Ozone. Master Thesis, Institute of Chemical Engineering, National Taiwan University of Science and Technology, Taiwan (000). Discussions of this paper may appear in the discussion section of a future issue. All discussions should be submitted to the Editor-in-Chief within six months of publication. Manuscript Received: April 1, 005 Revision Received: July 14, 005 and Accepted: September 9, 005

7 Chen et al.: Deodorization and Treatment of Leachate 37 垃圾掩埋場滲透水脫臭及其廢水處理探討 1 陳志恆,* 徐永錢 楊顯整 周肇宏 1 國立台北科技大學材料及資源工程系 國立台灣科技大學化學工程系 關鍵詞 : 臭氧化 垃圾掩埋場滲出水 脫臭 混凝 BOD/COD 摘 要 為評估垃圾掩埋場滲透水之脫臭及其廢水處理, 本研究採用序列處理系統進行探討可行之處理方案 關於脫臭方面, 結果顯示臭氧脫臭對於硫化氫有迅速去除之效果, 但對於氨則較不易去除 另外, 少量之預臭氧處理對於混凝有明顯效果, 如果增加臭氧量, 反而會降低其混凝作用 為了判斷微生物分解之難易度, 本研究採用 BOD/COD 比值作為指標 結果發現, 經過脫臭及混凝兩序列處理後, 其 BOD/COD 可以由原廢水之 0.18 上升至 0.8 為了進一步提高 BOD/COD 比值, 本研究於 ph 8 及 10 進行第二次臭氧化, 結果其最大 BOD/COD 比值分別可達 0.34 及 0.45, 而其所需注入之臭氧劑量分別為 61.9 及 450 mg L -1 預臭氧處理垃圾掩埋場滲透水後, 並不會改變滲透水中有機組成之分子量大小分佈, 而僅改變其表面化性 但第二次臭氧化可將該滲透水中的有機組成, 分解成較小之分子, 進而提高生物分解之效果