3905 Journal of Applied Sciences Research, 9(6): 3905-3913, 2013 ISSN 1819-544X This is a refereed journal and all articles are professionally screened and reviewed
ORIGINAL ARTICLES Pretreatment of Landfill Leachate by Ammonia Stripping 1
El-Gohary, F.A., 2Khater, M. and 3Gamal M. Kamel
1,3 2
Water Pollution Research Department, National Research Center, Cairo, Egypt Department of chemistry, Faculty of Science, Cairo University, Cairo, Egypt
ABSTRACT Ammonia stripping from landfill leachate has been investigated. The main objectives were to enhance biodegradability and to overcome ammonia toxicity to further biological treatment steps. The stripping process was carried out at various pH values ranging from 8.5 to 11. To investigate the impact of initial ammonia concentration on the stripping process, raw as well as diluted leachate (1 leachate: 1 water) were used for the present study. The efficiency of the treatment process was monitored by chemical analyses and Vibrio fischeri toxicity test. The chemical analyses covered ammonia nitrogen (NH4–N) and organic pollutants (COD and BOD5).The improvement in biodegradability was measured by the BOD/COD ratio. The highest removal efficiency of ammonia nitrogen (NH4-N) was obtained at pH value of 11 after air stripping for 24 hr. Average removal values were 94.5% for raw leachate and 96.9% for diluted leachate. The biodegradability, as expressed by the BOD/COD ratio, increased from 0.34 for raw leachate to 0.38 after ammonia stripping for 24 hr at pH values of 10 and 11. Kinetics of the rate of reaction for raw and diluted leachate indicated the preference of the use of raw leachate. Key words: Landfill, Leachate, ammonia stripping, biodegradability, toxicity, Vibrio fischeri. Introduction Landfill is currently the most widely used method for municipal solid waste disposal. Up to 95% of such waste collected worldwide is disposed of in landfills (El-Fadel et al., 1997). At a sanitary landfill, the waste is placed on the ground and extended in thin layers (cells). These layers are then compacted to reduce their volume and covered periodically with a suitable earth material. The degradation of the organic fraction of the wastes in the landfill, in combination with the percolating rain water produces a highly contaminated liquid called “leachate” (Kjeldsen et al., 2002 & Castrillón et al., 2010) Leachate emissions from landfill sites are of concern, primarily due to their toxic impact when released untreated into the environment (Farre et al., 2008). The impact of landfills is also long-term, due to the potential of landfills to generate leachates and emit biogas for many years after closure. At the same time, its quantity and quality varies with time, because deposited wastes are comprised of a wide range of inorganic, organic and/or xenobiotic compounds, which affects the composition and environmental impact of formed leachate. Its composition is therefore site and time specific, based on the characteristics of deposited wastes, rainfall regime that regulates the moisture level and landfill age (Kjeldsen et al., 2002 & Renou et al., 2008). In particular, the composition of landfill leachate varies generally depending on the age of the landfill. Even within a single landfill site variability is frequently evident. Significant components of leachate are heavy metals and degradable organics at the beginning of landfill operation, while persistent organic pollutants usually appear later as a result of biotic and abiotic processes in the system (Pivato et al., 2006). Among these substances there are several compounds classified as potentially hazardous: bioaccumulative, toxic, genotoxic, and they could have an endocrine disruptive effect (Marttinen et al., 2002). The common features of stabilized leachate are high strengths of ammonia nitrogen (NH4–N) and chemical oxygen demand (COD), as well as a low ratio of BOD5/COD ( Irene et al., 1996 & Jin-Song Guoa et al., 2010).The leachate composition from different sanitary landfills, as reported in literature (Renou et al., 2008 & Magda Cotmana et al., 2010), shows wide variations. COD could vary from 1000 to 70,900mgL−1, resulting in severe toxicity in many cases. The BOD5/COD ratio ranges from 0.04 to 0.70 and could decrease rapidly with the ageing of the landfill (Chian et al., 1976 Magda Cotmana et al., 2010) showing low biotreatability. With few exceptions, the pH of leachates lies in the range of 5.8–8.5, which is due to the biological activity inside the body of the landfill. It is also important to notice that the majority of total Kjeldahl nitrogen (TKN) is ammonia, which can be found at concentrations up to 13,000 mg L−1. Ammonium ions have environmental negative effects such as eutrophication of the rivers (Jacques Barbier et al., 2002) and are toxic for the aquatic life even
Corresponding Author: El-Gohary, Water Pollution Research Department, National Research Center, Cairo, Egypt E-mail:
[email protected]
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in very low concentrations (Furrer et al., 1996 & Jin-Song Guoa et al., 2010). Therefore, Leachate should be treated before reaching surface water bodies or ground water, because it can accelerate algae growth due to its high nutrient content, deplete dissolved oxygen in the streams, and cause toxic effects in the surrounding water environment. Careful site management can reduce the quality and pollution potential of the formed leachate, but it cannot completely eliminate it (Bull et al., 1983). Pollutants should be identified and treated properly, to avoid contamination of receiving environment (Wintgens et al., 2003). Treatment methods must be matched to the actual characteristics of the particular leachate (Zhang et al., 2006). To remove the majority of pollutants, biological methods are usually implemented (Henze et al., 1995). However, the application of biological treatment alone is not an option due to the leachate characteristics (Keenan et al., 1984). Furthermore, neither biological nor chemical treatment separately achieves high treatment efficiencies. There are two reasons for the low removal efficiency of each treatment system: (i) significant presence of high-molecular weight organics that are difficult to remove and (ii) inhibitory effects of organic compounds, inorganic salts and metals to activated sludge microorganisms (Keenan et al., 1984). Sometimes leachate is mixed with municipal wastewater and treated in conventional wastewater treatment plant, but this may cause problems because of the presence of harmful constituents, including ammonium nitrogen, which is usually present in high concentrations in mid to old-age landfills (Wang et al., 2000). Its high concentrations could cause difficulties to conventional aerobic activated sludge processes, due to the ammonia toxicity as well as low C/N ratio (Henze et al., 1995; Onay., 1998). These are the reasons, why combinations of several treatment methods including physical, chemical and biological processes are usually recommended. Air stripping, adsorption, and membrane filtration are major physical leachate treatment methods (Amokrane et al., 1997; Trebouet et al., 2001 & Jin-Song Guoa et al., 2010;); coagulation flocculation, chemical precipitation, and chemical and electrochemical oxidation methods are the common chemical methods used for the landfill leachate treatment (Amokrane et al., 1997; Lin et al., 2000; Trebouet et al., 2001; Marttinen et al., 2002 & Jin-Song Guoa et al., 2010). The most popular biological treatments of landfill leachate are the anaerobic digestion or aerobic activated sludge methods (Lema et al., 1988 & Jin-Song Guoa et al., 2010). Biological processes are quite effective to treat leachate, when applied to relatively younger leachates, but they are less efficient for the treatment of older ones (Amokrane et al., 1997 & Jin-Song Guoa et al., 2010). Biorefractory contaminants, contained mainly in older leachates, are not amenable to conventional biological processes, whereas the high ammonia content might also be inhibitory to activated sludge microorganisms (Li et al., 1999; Jin-Song Guoa et al., 2010). Furthermore, a supplementary addition of phosphorus is often necessary, as landfill leachates are generally phosphorus deficient (Amokrane et al., 1997 & Jin-Song Guoa et al., 2010). Therefore, physicochemical processes are often required to remove ammonia and other toxic compounds prior to biological treatment (Rautenbach et al., 1994; Kurniawan et al., 2006; Gotvajn et al., 2009; Renou et al., 2008 & Jin-Song Guoa et al., 2010). Ammonia can be removed from waste by biological nitrification/denitrification process (Hao et al., 1994; Chakchouk et al., 1995 & Jacques Barbier et al., 2002). Ammonium ions are oxidized to nitrate ions which are then reduced to molecular nitrogen. This treatment, however, produces high amounts of sludge (Rostron et al., 2001; Jacques Barbier et al., 2002). Nowadays, ammonium or air stripping is the most widely employed treatment for the removal of NH4-N from landfill leachate (Diamadopoulos., 1994; Collivignarelli et al., 1998; Marttinen et al., 2002; Jin-Song Guoa et al., 2010). NH4–N is transferred from the waste stream into the air and is then absorbed from the air into a strong acid such as sulphuric acid or directly fluxed into the ambient air. Ammonium stripping gives an NH4-N treatment performance in the range of 85–95% with concentrations ranging from 220 to 3260mgl−1 (Bonmat et al., 2003 & Jin-Song Guoa et al., 2010) Due to its toxicity, the aim of the present study was to reduce ammonia concentration in order to improve the efficiency of the next biological treatment step. The ammonia air stripping process was used for the present study. Materials and Methods The leachate used for this study was supplied from Borg El-Arab landfill which has been receiving solid waste since 2001. Site description: Borg El-Arab sanitary landfill: is located some 60 km away from the city of Alexandria to the west on Alexandria–Mattrouh road (Figure1). The total area of the site is approximately 375,000 square meters (2,500m x 150m). The site was a quarry used by Cement Company with an average depth of 15 m. The landfill life time was estimated at 12 years. Currently, according to environmental register the landfill receives a maximum amount of 3,500 ton/day of wastes produced from Alexandria Governorate and its districts. The average amount
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is 2,000 ton daily. For collection of leachate within Borg El-Arab landfill, 2% sloped bottoms are constructed. Borg El-Arab site layout is showed in Figure (2). In 2003, it was decided to install onsite leachate evaporation ponds. In 2009, a study has been carried out to establish two new evaporation ponds to accommodate the leachate generated from the new landfill cells that have been filled and closed as the leachate currently generated exceeds the capacity of the existing ponds.
Fig. 1: Map showing the location of landfill sites.
Fig. 2: Location of the Evaporation Ponds in Relation to the Landfill and Neighboring Activities. Leachate characterization: Leachate samples were provided from the drainage system at the landfill site. The collected samples were stored at 4°C and transferred to the laboratory for analyses and use for the experiments. All chemicals used were of analytical grade. Chemical Oxygen Demand (COD, biological oxygen demand (BOD5), NH3–N and pH were measured according to the Standard Methods for the Examination of Water and Wastewater (APHA 2008). The results of the analyses are presented in Table 1.
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Toxicity testing: The toxicity test was performed for raw and treated leachate using the inhibitory effect of wastewater samples on the Light Emission of Vibrio fischeri (Luminescent Bacteria Test). Toxicity was evaluated as 30 min EC50 values(Zgajnar Gotvajna et al., 2009).The inhibition in terms of EC (effective concentration) values was estimated by comparison of the rate in the test mixture with that in the control mixture containing no test material. Ammonia stripping The ammonia stripping experiments were performed in five Plexiglass reactors with an internal diameter of 5.5cm and height of 58.5cm. 2 L leachate samples were put in each reactor. Before conducting the experiments, the pH values of the leachate samples were adjusted to be 9.0, 9.5, 10.0 and 11.0 by using 1M NaOH .The last reactor was left without pH change as control. Samples were aerated by compressed outdoor air at a flow rate of 2 l/min. The aeration took place for 24 h and samples for different analyses were withdrawn periodically. All experiments were carried out at room temperature 25±2 ◦C. Rate of the reaction of ammonia stripping: The rate of reaction for ammonia stripping from raw as well as diluted leachate was determined using the following equation: R= -KC (R is the rate of the reaction) dc/dt=-KC LN C/C◦ = -KT (C is the concentration of ammonia, T is the time of the reaction) LN C◦/ C = KT (C◦ is the initial concentration of ammonia) To determine the rate of the reaction (K), LN C◦/ C was plotted versus the time of the reaction for raw as well as diluted (1:1) leachate . Results and Discussion Leachate Characteristics: The leachate samples collected from the landfill site were analyzed. The results are presented in Table 1. The landfill leachate was considered to have a low BOD5/COD ratio ranging from 0.342 to 0.352, with an average value of 0.347. Ammonia concentration (NH4–N) varied over a range from 5300 to 5736. Thus, it can be classified as stabilized or “old” and non-biodegradable leachate. Table 1: Leachate Characteristics site. Parameter pH CODtot BODtot BOD/COD NH4-N
Unit _ mgO2/l mgO2/l mgN/l
Range 8.1- 8.5 15600 -16800 5350 - 5930 0.342-0.352 5300 - 5736
Average 8.3 16200 5640 0.347 5518
Ammonia stripping: Ammonium or air stripping is the most widely employed treatment for the removal of NH4–N from landfill leachate ( Diamadopoulos et al., 1994; Collivignarelli et al., 1998; Marttinen et al., 2002 & Jin-Song Guoa et al., 2010). The process involves diffusion of large quantities of air in the reactors containing raw and diluted leachate (1:1) thus driving the ammonia from the liquid to the gas phase. The process was carried out under pH control as a critical parameter for the optimization of ammonia stripping. Changes of ammonium nitrogen versus time of air stripping at different pH values, for raw and diluted leachate are presented in Tables 2&3 and Figures 3. Available data indicated that the most effective removal of ammonia nitrogen was obtained at pH value of 11 and after 24 hr for both raw (94.5%) and diluted leachate (96.9%). At pH values of 10 and 11, NH4–N removal had significant linear increase up to 6hr. Thereafter, the increase in NH4–N removal was not significant. Therefore, a pH value of 10 and contact time of 6 hr have been selected as the optimum operating conditions for ammonia nitrogen removal from the leachate under investigation.
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These results are mainly due to the fact that the reaction of NH3 with water can be represented by Eq. (1). From this equation, raising the pH (as represented by the OH−) will drive the reaction to the left, increasing the concentration of NH3. This makes ammonia more easily removed by stripping. NH3 +H2O ↔ NH4+ +OH−
Eq. 1
Cheung K.C., et al., (1997) investigated pH as critical parameter for the optimization of ammonia stripping in a stirred tank. After one day, they achieved a significant ammonia removal between 86 and 93 % at pH greater than 11. Comparing the amount of ammonia removed using raw leachate with that when diluted leachate was used at constant pH value and contact time, indicates that the results are in favor of using raw leachate for ammonia stripping which can then be followed by dilution. Table 2: Changes in ammonium nitrogen concentration versus contact time of air stripping at different pH values. (a) Raw leachate pH value 8.5 9.0 9.5 10.0 NH4–N NH4–N %R NH4–N %R NH4–N %R NH4–N %R Conc. Conc. Conc. Conc. Conc. (mgN/l) (mgN/l) (mgN/l) (mgN/l) Contact Time (hr) Raw leachate 5572 _ 5572 _ 5572 _ 5572 _ 2 5420 2.7 4573 17.9 3920 29.6 3665 34.2 4 4970 10.8 4480 19.5 3500 37.1 3375 39.4 6 4685 15.9 4280 23.1 3360 39.6 1030 81.5 8 4490 19.4 4200 24.6 3080 44.7 950 83.0 24 3585 35.7 3500 37.1 1120 79.8 796 85.7 Table 3: Changes in ammonium nitrogen concentration versus contact time of air stripping at different PH values. (b)Diluted leachate (1:1) pH value 8.5 9.0 9.5 10.0 NH4–N NH4–N %R NH4–N %R NH4–N %R NH4–N %R Conc. Conc. Conc. Conc. Conc. (mgN/l) (mgN/l) (mgN/l) (mgN/l) Contact Time (hr) Raw leachate 2800 _ 2800 _ 2800 _ 2800 _ 2 2490 11.1 2242 19.9 1960 30 1879 32.9 4 2263 19.2 2212 21 1680 40 1683 39.9 6 2178 22.2 2100 25 1540 45 955 65.9 8 2117 24.4 2050 26.7 1520 45.7 899 67.9 24 1930 31.1 1800 37.7 660 76.4 531 81.8
(a) Raw leachate
(b) Diluted leachate
Fig. 3: The effect of pH value and contact time on NH4–N removal by air stripping.
11.0 NH4–N Conc. (mgN/l)
5572 2935 405 400 355 305
11.0 NH4–N Conc. (mgN/l)
2800 1458 373 368 143 88
%R
_ 47.3 92.7 92.8 93.6 94.5
%R
_ 47.9 86.7 86.9 94.9 96.9
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Rate of Ammonia removal: From the available results it can be seen that ammonia nitrogen removal under the same operating conditions is higher when raw leachate was used compared to the diluted leachate. After 6hr contact time and at pH value of 10, the ammonia was reduced by 4622 mgN/l. Corresponding value for diluted leachate was 1901 mgN/l. To confirm these results, kinetics of the reaction was determined. The results obtained are presented in Figure 4. From the available data it can be seen that the calculated reaction rate for raw leachate was 0.210 h-1 (R2=0.833) and for diluted leachate (1:1) it was 0.109 h-1 (R2=0.853). Obtained values are in accordance with the data reported in literature Silva et al., (2004) & Magda Cotmana et al., (2010). This indicates that ammonia nitrogen removal is a function of initial ammonia concentration. Studies carried out by Srinath et al., (1974) & Magda Cotmana et al., (2010) indicated that up to 150 mg/l ammonia, the stripping process followed a firstorder kinetics model. When the concentration dropped below 150 mg/l the kinetics changed. Based on the results obtained it has been decided to use raw leachate and then dilute it for further treatment steps.
(a) Raw leachate
(b) Diluted leachate
Fig. 4: Rate of reaction for the raw and diluted leachate (1:1). COD Reduction: In addition to ammonia removal, the process of air stripping involves the mass transfer of volatile organic contaminants from leachate to air. As expected COD removal increased with increasing pH value and aeration time. Air stripping of raw leachate for 24 hr at pH value of 11, reduced COD value by 27.3 % .Corresponding value for diluted leachate was 33.4%. From the results presented in Tables 4&5 and Figure 5, it can be seen that changing the pH value within the range from 8.5 to 9.5 did not affect the COD removal. Raising the pH value up to 10 and 11 caused significant increase in COD percentage removal values. Comparison of the COD reduction when raw leachate was used with that of diluted leachate showed the same trend as in case of ammonia removal. Table 4: Changes in COD values versus contact time of air stripping at different pH values. (a) Raw leachate PH 8.5 9.0 9.5 COD T.COD %R T.COD %R T.COD %R Conc. Conc. Conc. Conc. (mgO2/l) (mgO2/l) (mgO2/l) Contact Time (hr) Zero time 15222 0 15222 0 15222 0 2 hours 15080 0.9 15088 0.9 15077 0.9 4 hours 15020 1.3 15023 1.3 15022 1.3 6 hours 14740 3.2 14701 3.4 14690 3.5 8 hours 14490 4.8 14477 4.9 14410 5.3 24 hours 14050 7.7 14003 8 14000 8
10.0 T.COD Conc. (mgO2/l)
15222 14680 14575 14250 13370 13190
%R
0 3.6 4.3 6.4 12.2 13.3
11.0 T.COD %R Conc. (mgO2/l)
15222 13320 12633 12125 11620 11070
0 12.5 17 20.3 23.7 27.3
3911 J. Appl. Sci. Res., 9(6): 3905-3913, 2013 Table 5: Changes in COD values versus contact time of air stripping at different pH values. (b)Diluted leachate (1:1) PH 8.5 9.0 9.5 10 COD T.COD %R T.COD %R T.COD %R T.COD Conc. Conc. Conc. Conc. Conc. (mgO2/l) (mgO2/l) (mgO2/l) (mgO2/l) Contact time (hr) Zero time 7690 0 7690 0 7690 0 7690 2 hours 7605 1.1 7450 3.1 7435 3.3 7390 4 hours 7475 2.8 7220 6.1 7150 7 7120 6 hours 7370 4.2 7170 6.8 7060 8.2 6690 8 hours 7230 6 6750 12.2 6740 12.4 6605 24 hours 6910 10.1 6645 13.6 6620 13.9 5750
(a) Raw leachate
11 %R
T.COD Conc. (mgO2/l)
%R
0 3.9 7.4 13 14.1 25.2
7690 7375 7075 6660 6540 5120
0 4.1 8 13.4 15 33.4
(b) Diluted leachate
Fig. 5: The effect of pH and contact time on COD removal. Impact of ammonia stripping on biodegradability of raw leachate: In the present study the BODtot /CODtot ratio was taken as an indication for the biodegradability of the raw leachate before and after the stripping process. The results presented in Table 6 and Figure 6 show an increase in the BOD/COD ratio from 0.342 for raw leachate to 0.358 after air stripping for 6hr at pH value of 10. Increasing the contact time to 24hr at the same pH value of 10 increased the BOD/COD ratio to 0.377 and further to 0.3777 after 24 hr. Values for the experimental run carried out at pH 11 gave similar results. BOD/COD ratio increased from 0.342 to 0.362 after 6hr and to 0.3778 after 24 hr aeration. Based on these results it can be concluded that air stripping for 6 hr at a pH value of 10 is optimum for leachate air stripping to reduce ammonia concentration. Table 6: Changes in BOD/COD ratio at different operating condition. PH Raw Contact Time (hr) BOD 5350 COD 15600 BOD/COD 0.342
10 6 5210 14550 0.358
11 24 4980 13200 0.377
6 4528 12525 0.362
24 4333 11470 0.378
Toxicity reduction: The relative effectiveness of ammonia stripping for toxicity removal according to Vibrio fischeri is presented in Table 7.
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Fig. 6: Variations in BOD/COD ratio versus pH-value and stripping time. Table 7: Relative effect of ammonia stripping on toxicity removal according to Vibrio fischeri. Sample pH EC* 50% Toxicity Unit Raw 8.5 1.36 73.5 6hr 10 1.01 99 24 hr 10 1.01 99 4 hr 11 1.01 99 24 hr 11 1.01 99 * (Inhibition %)
Toxicity degree Very toxic Non toxic Non Toxic Non toxic Non Toxic
Conclusions: According to the laboratory scale data obtained, the following can be concluded: - Air stripping of ammonia is a promising pretreatment option for the removal of NH4-N and reduction of toxicity to microorganisms. -The optimum operating conditions for air stripping of ammonia from raw leachate is pH 10 and contact time of 6 hrs. - Ammonia stripping improved the biodegradability of landfill leachate and reduced its toxicity effect References Amokrane, A., C. Comel, J. Veron, 1997. Landfill leachate pretreatment by coagulation-flocculation. Water Res., 31: 2775-2782. Bonmat, A., X. Flotats, 2003. Air stripping of ammonia from pig slurry: characterization and feasibility as a preor post-treatment to mesophilic anerobic digestion. Waste Managent., 23: 261-272. Bull, P.S., J.V. Evans, R.M. Wechsler, 1983. Biological technology of the treatment of leachate from sanitary landfills, Water Research, 17(11): 1473-1481. Castrillón, L., Y. Fernández-Nava, M. Ulmanu, I. Anger, E. Marañón, 2010. Physico-chemical and biological treatment of MSW landfill leachate Waste Management, 30: 228-235. Chakchouk, M., G. Deiber, J.N. Foussard, H. Debellefontaine, 1995. Environ. Technol., 16: 645. Chian, E.S.K., F.B. DeWalle, 1976. Sanitary landfill leachates and their treatment, J. Environ. Eng. Div., 45: 411-431. Cheung, K.C., L.M. Chu, M.H. Wong, 1997. Ammonia stripping as a pretreatment for landfill leachate. Water Air Soil Pollut, 94: 209-221. Collivignarelli, C., G. Bertanza, M. Baldi, F. Avezzu, 1998. Ammonia stripping from MSW landfill leachate in bubble reactors: process modeling and optimization. Waste Management. Res., 16: 455-466. Diamadopoulos, E., 1994. Characterization and treatment of recirculation stabilized leachate, Water Res., 28: 2439-2445. El-Fadel, M., A.N. Findikakis, J.O. Leckie, 1997. Environmental impacts of solid waste landfilling, J. Environ. Manage, 50: 1-25. Hao, O.J., K.K. Phull, J.M. Chen, 2009. Wet Oxidation of TNT Red Water and Bacterial Toxicity of Treated Waste“, Water Res., 28: 283-290. Farre, M., S. Perez, L. Katiani, D. Barcelo, 2008. Fate and toxicity of emerging pollutants, their metabolites and transformation products in the aquatic environment TrAC, 27: 991-1007.
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