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Influence of recirculation in a lab-scale vertical flow constructed wetland on the treatment efficiency of landfill leachate
Bioresource Technology 101 (2010) 1756–1761
Contents lists available at ScienceDirect
Bioresource Technology journal homepage: www.elsevier.com/locate/biortech
Influence of recirculation in a lab-scale vertical flow constructed wetland on the treatment efficiency of landfill leachate Silviya Lavrova, Bogdana Koumanova * Department of Chemical Engineering, University of Chemical Technology and Metallurgy, 8 Kliment Ohridsky Blvd., 1756 Sofia, Bulgaria
a r t i c l e
i n f o
Article history: Received 22 June 2009 Received in revised form 6 October 2009 Accepted 12 October 2009 Available online 24 November 2009 Keywords: Landfill leachate Constructed wetland Phragmites australis
a b s t r a c t Landfill leachate taken from a landfill situated in the north-western region of Bulgaria has been treated in a laboratory scale vertical flow constructed wetland (VF-CW) at different flow rates (40, 60 and 82 ml min 1) and recirculation ratios (time of water running through wetland to time of quiet water – 1:1; 1:2; 1:3). Young Phragmites australis was planted on the top layer of the reactor. The low flow rate (40 ml min 1) and recirculation ratio of 1:3 allowed removal efficiencies of 96% for COD (in 8 days), 92% for BOD5 (in 3 days), 100% for ammonia (in 5 days) and 100% for total phosphorus (in 2 days). At the highest flow rate studied (82 ml min 1) and shorter quiet period (recirculation ratio 1:1) the water needs longer period of treatment (2 days more according to COD). The results of this study indicate that both flow rate and recirculation ratio should be taken into account for proper design of VF-CW. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction Leachate quality varies throughout the operational life of a landfill (Kulikowska and Klimiuk, 2008) and long after its closure. There are three broad and overlapping phases of waste decomposition, in which chemical and biological processes give rise to both landfill gas and leachate during and beyond the active life of the site (Robinson, 1996). The landfill leachates are capable of producing severe environmental impacts especially in vulnerable recipients such as aquifers and surface waters. These effects may include eutrophication or toxic effects on aquatic organisms resulting from ammonia, heavy metals or organic compounds. Ammonia nitrogen concentrations often present more of a long-term problem, than the leaching of degradable organic substances such as volatile fatty acids. A sustainable low cost solution for leachate management is the constructed wetland system. They utilize anaerobic and aerobic reactions to break down, immobilize or incorporate organic substances and other contaminants from polluted effluent (Randerson and Slater, 2005; Sleytr et al., 2007; Batzias and Siontorou, 2008; Wojciechowska and Obarska-Pempkowiak, 2008a,b). Martin and Moshiri (1994) and Bulc et al. (1997) found that constructed wetlands are effective in reducing organic components and nutrient loading of landfill leachate. Mathewson and Mathewson (1998) concluded that a treated wetland offered the most cost effective and environmentally sound technique to treat the landfill leachate. * Corresponding author. Tel.: +359 2 83 61 302; fax: +359 2 868 54 68. E-mail addresses: [email protected] (S. Lavrova), [email protected] (B. Koumanova). 0960-8524/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.biortech.2009.10.028
The wastewater treatment efficiency depends on the design of the wetlands and their bed media characteristics (Aslam et al., 2007; Tuszynska and Obarska-Pempkowiak, 2008; Torrens et al., 2009; Albuqureque et al., 2009). There are surface and subsurface flow systems (IWA, 2000; Kadlec and Wallace, 2008; Vymazal and Kropfelova, 2008). Subsurface flow systems are two types – horizontal and vertical. Vertical flow constructed wetlands (vegetated submerged beds) possess greater oxygen transport ability than horizontal flow systems. They are more effective for the removal of organic matter and ammonia nitrogen from wastewaters through aerobic microbial activities (Sun et al., 2003; Obarska-Pompkowiak et al., 2008; Yalcuk and Ugurlu, 2009). The subsurface flow constructed wetlands act as fixed-film bioreactors (EPA, 2000). The wetland systems are filled with different media types typically planted with the same species of emergent vegetation present in the marshes. Kadlec and Knight (1996) listed 37 families of vascular plants that have been used in water quality treatment. The primary role of vegetation in a wetland treatment system is to recycle nutrients in the waste into a harvestable crop, but vegetation plays a distinct role in each treatment process. The vegetation is the support media for biological activity. Also it can maintain long-term infiltration rates. The presence of vegetation increases the wetland evapotranspiration efficiency (Bialowiec and Wojnowska-Baryla, 2007, 2008). The ammonia nitrogen is one of the significant dangerous components in landfill leachates. The transformation of the nitrogenous pollutants in VF-CW was studied by Sun and Austin (2007). Connolly et al. (2003) studied the fate of ammonia nitrogen (NH4-N) in a lab-scale downflow reed bed system treating an artificial landfill leachate. Decrease of the NH4-N level of the leachate was observed when the reed beds were saturated. For
S. Lavrova, B. Koumanova / Bioresource Technology 101 (2010) 1756–1761
artificial leachates with NH4-N levels of 150 ± 5 mg/l, an average removal rate of 44% in a 3 h treatment was achieved. This study also demonstrated that in general a greater rate of effluent recirculation around downflow reed beds gives higher NH4-N removal. The aim of this study is to evaluate the removal of organic matter (as COD and BOD5), nutrients (nitrogen and phosphorus), metals and inorganic compounds in a lab-scale VF-CW for treatment of landfill leachate using different flow rates and recirculation ratios.
2. Methods Landfill leachate was taken from a landfill situated in the northwestern region in Bulgaria. After collection, the landfill leachate was settled overnight and then the supernatant was diluted with tap water (volume ratio 1:1). The characteristics of the diluted leachate are summarized in Table 1. The experiments were carried out in a laboratory system, consisting of an influent sedimentation tank, vertical flow constructed wetland (VF-CW), peristaltic pump and effluent tank (Fig. 1). After sedimentation and dilution the supernatant was allowed to flow into a Plexiglass VF-CW, 123 mm in diameter and 900 mm in height. The VF-CW column was filled with a 300-mm
Table 1 Characteristics of the landfill leachate. Parameter
Influent
COD, mg/l BOD5, mg/l Ammonia nitrogen (NH4-N), mg/l Nitrate nitrogen (NO3-N), mg/l Total phosphorus (TP), mg/l pH Salinity, ‰ Conductivity, lS cm 1 Total dissolved solids (TDS), mg/l Suspended solids (SS), mg/l
2800 204.2 198.4 1.9 5.5 7.9 2.5 4710 2460 2720
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high bottom layer of 35–55 mm round gravel and a top layer with a height of 500 mm of 5–25 mm gravel. Young Phragmites australis was planted in the top layer of the VF-CW. The VF-CW was operated continuously in a recirculation regime. The experiments with three different flow rates (40, 60 and 82 ml min 1, respectively) were carried out. Experiments with three different recirculation ratios at every flow rate were conducted (the time of water running through the wetland to the time of quiet water in it – 1:1; 1:2; 1:3). Thus the water was running for 1 hour and then was left quiet for 1, 2 or 3 h, respectively. The effluent was analyzed for chemical oxygen demand (COD), biochemical oxygen demand (BOD5), ammonia nitrogen (NH4-N), nitrate nitrogen (NO3-N), dissolved oxygen (DO) and total suspended solids (TSS) according to standard methods (APHA– AWWA–WEF, 1999). pH, conductivity, salinity and total dissolved solids (TDS) has been analyzed by SENSION 156, Hach Lange. Metals content also has been determined using atomic absorption spectroscopy (Perkin Elmer-323). 3. Results and discussion 3.1. COD and BOD5 removal The comparison of the COD and BOD5 values of the treated wastewater during the experiments is illustrated in Figs. 2 and 3. The data demonstrate the influence of the recirculation at three different flow rates on the treatment ability of the system. The decreasing of COD values after 5 days from the beginning is fast (Fig. 2). Then the process slowed down. The efficiency at the 5th day (recirculation ratio 1:1) was 67% – at a flow rate of 82 ml min 1, 81% – at flow rate 60 ml min 1 and 90% – at flow rate 40 ml min 1. The efficiency at recirculation ratio 1:2 was 78%, 86% and 90%, respectively, and at recirculation ratio 1:3 it was 78%, 90% and 96%. COD decreased slower when the flow rate was higher. As shown in Fig. 3 the elimination of BOD5 occurs fast in most cases during the initial 5 days. The efficiency of BOD5 removal at a recirculation ratio of 1:1 was 72%, 85% and 92% for flow rate
Fig. 1. Lab-scale vertical flow constructed wetland system.
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Fig. 2. COD values (error bars show the standard deviation (n = 5)) during the experiments at different flow rates and recirculation ratios (1:1; 1:2; 1:3).
82, 60 and 40 ml min 1, respectively. At recirculation ratio 1:2 it was 83%, 92% and 93% for the corresponding flow rates. The efficiency was 91% at flow rate 82 ml min 1 and recirculation ratio 1:3. The same efficiency was obtained after 4 days at flow rate 60 ml min 1 and after 3 days at flow rate 40 ml min 1. It was observed that the longer the water remained quiet in VF-CW, the faster COD and BOD5 decreased. 3.2. Nitrogen removal The importance of nitrogen removal is comparable with that for organic carbon, toxic compounds and metals removal during the leachate treatment in VF-CW. Ammonium removal by nitrification in constructed wetlands differing in design and purpose has been reported (Faulwetter et al., 2009). It is known that autotrophic nitrification consists of two successive aerobic reactions, the conversion of ammonium to nitrite by ammonium oxidizing bacteria and the conversion of nitrite to nitrate by nitrite oxidizing bacteria. The concentration of ammonia nitrogen in the influent used in this
Fig. 3. BOD5 values (error bars show the standard deviation (n = 5)) during the experiments at different flow rates and recirculation ratios (1:1; 1:2; 1:3).
study (Table 1) was relatively high. So, it was interesting to record the changes of NH4-N and NO3-N values during the leachate treatment at different flow rates and recirculation ratios. The influence of these parameters on the ammonium depletion is illustrated in Fig. 4. It was exhausted completely during the experiments. The decrease of ammonia nitrogen could be the collective result of volatilization, nitrification, plant uptake in wetland system and immobilization. At the same time NO3-N increased depending on the flow rate and the recirculation ratios (Fig. 5). In all cases the values increased during the first 1–2 days. After this period the curves shape depended on the experimental conditions. The concentration of NO3-N has increased faster when the flow rate was lower. The influence of the recirculation ratios was opposite. During the experiments at different conditions DO was measured and the values were from 5.2 to 8 mg/l. It is known that the concentration of 1 mg/l is sufficient for oxidation of ammonium (Hammer and Hammer, 2001). The lack of the denitrification during the treatment can be the result of lower activity of denitrifying bacteria in the system. Vymazal (2007) reported that VF-CW removes successfully NH4-
S. Lavrova, B. Koumanova / Bioresource Technology 101 (2010) 1756–1761
Fig. 4. NH4-N values (error bars show the standard deviation (n = 5)) during the experiments at different flow rates and recirculation ratios (1:1; 1:2; 1:3).
N but the denitrification is very limited in these systems. It was also well established that carbon availability plays an important role in both synthesis and activity of denitrifying enzymes as well as general support of the denitrifying population. The lack of organic carbon sources is thought to prevent significant levels of denitrification (Schipper et al., 1993).
3.3. Phosphorus removal Phosphorus removal in wetland treatment systems occurs through adsorption, plant uptake, complexation, and precipitation (Watson et al., 1989). The value of TP in the treated leachate was relatively low (5.5 mg/l). It was established that TP removal follows the same tendency as NH4-N removal. During the first 2 days a significant TP elimination occured (Fig. 6). That a higher flow rate leads to a longer period needed for treatment of elimination of TP. At the same time the change of the recirculation ratio from 1:1 to 1:3 (water stays quiet in VF-CW longer)
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Fig. 5. NO3-N values (error bars show the standard deviation (n = 5)) during the experiments at different flow rates and recirculation ratios (1:1; 1:2; 1:3).
leads to shorten period of elimination (e.g., at flow rate 82 ml min 1 the TP removal was 41.8%, 60% and 67.3% for 1:1, 1:2 and 1:3 recirculation ratios during 8, 6 and 4 days, correspondingly). 3.4. pH, salinity and metals content It was established that during the experiments pH slightly decreased from 7.9 to 7.5 and the salinity also decreased from 2.5‰ to 1.9‰. TDS gradually decreased from 2460 to 1778 mg dm 3. The values of TSS varied from 1.91 to 3.96 g/l. Landfill leachate conductivity decreased from 4710 to 3408 lS cm 1 accompanied with decreasing in the metal concentrations. Table 2 shows the variations of elements concentration during the treatment at recirculation ratio 1:3 and 40 ml min 1 flow rate. The initial concentrations of some elements were low (for Cu, Zn, Bi, Cr, Mn and Al – less than 0.3 lg/ml). The initial concentra-
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Table 2 Metal ions changes during the treatment at 40 ml min
1
flow rate of the wastewater and recirculation ratio 1:3. 1
Time, days
Concentration of the elements, lg ml Cu
Zn
Bi
Mg
Ca
Cr
Mn
Fe
K
Na
Al
0 1 2 3 4 5 6 7 8 9 10
0.02 0.02 0.05 0.05 0.05 0.08 0.05 0.05 0.02 0.08 0.08
0.25 0.23 0.05 0.14 0.24 0.18 0 0.06 0.04 0.14 0
0.2 0 0.2 0 0 0 0 0 0 0 0
0.98 0.83 0.79 0.84 0.88 0.86 0.67 0.74 0.7 0.68 0.68
71 64 61 64 66 65 56 60 58 65 56
0.27 0.19 0.19 0.13 0.15 0.12 0.04 0.13 0.13 0.08 0.08
0.15 0 0.02 0.02 0.03 0.02 0 0.02 0 0 0
3.63 1.45 0.77 1.32 2.36 1.77 0.5 1.5 0.95 1.05 0.45
526 429.5 401.5 416 438 419 377.5 389.5 384.5 370 367.5
480 446 454 450 465 443 442 443 439 427 439
0.325 0.106 0.087 0.062 0.275 0.15 0.133 0.062 0.087 0.375 0.075
recirculation ratio 1:3 and 40 ml min 1 flow rate these concentrations decreased by 21% for Ca, 31% for K and only 8% for Na.
4. Conclusions The influence of recirculation in a lab-scale vertical flow constructed wetland on the treatment efficiency of landfill leachate at different flow rates was studied. It was established that the higher flow rate leads to longer period needed for treatment. The recirculation ratios also influence the purification process. Alternating between water movement through the VF-CW and stagnant periods resulted in a varying extent of purification, and the longer the stagnant period of the water in VF-CW the shorter the period for obtaining the desired characteristics of the effluent water. References
Fig. 6. TP values (error bars show the standard deviation (n = 5)) during the experiments at different flow rates and recirculation ratios (1:1; 1:2; 1:3).
tion of Mg and Fe was 0.98 and 3.63 lg/ml, respectively. Significantly higher initial concentrations were measured for Ca (71 lg/ ml), K (526 lg/ml) and Na (480 lg/ml). During the treatment at
Albuqureque, A., Olivera, J., Semitela, S., Amaral, L., 2009. Influence of bed media characteristics on ammonia and nitrate removal in shallow horizontal subsurface flow constructed wetlands. Bioresour. Technol. 100, 6269–6277. APHA–AWWA–WEF, 1999. Standard Methods for the Examination of Water and Wastewater, 20th ed. American Public Health Association, Washington, DC, USA. Aslam, M.M., Malik, M., Baig, M.A., Quazi, I.A., Iqbal, J., 2007. Treatment performance of compost-based and gravel-based vertical flow wetlands operated identically for refinery wastewater treatment in Pakistan. Ecol. Eng. 30, 34–42. Batzias, A.F., Siontorou, C.G., 2008. A new scheme for biomonitoring heavy metal concentrations in semi-natural wetlands. J. Hazard. Mater. 158, 340–358. Bialowiec, A., Wojnowska-Baryla, I., 2007. The efficiency of landfill leachate evatranspiration in the soil–plant system with reed Phragmites australis. Ecohydrology and Hydrobiology 7 (3–4), 331–337. Bialowiec, A., Wojnowska-Baryla, I., 2008. The landfill leachate evatranspiration in soil–plant system with reed – Phragmites australis. Int. J. Environ. Waste Manage. 2 (6), 526–539. Bulc, T., Vrhovsek, D., Kikanja, V., Haber, R., Perfler, R., Laber, J., Cooper, P., 1997. The use of constructed wetland for landfill leachate treatment. Water Sci. Technol. 35, 301–306. Connolly, R., Zhao, Y., Sun, G., Allen, A., 2003. Removal of ammoniacal-nitrogen from an artificial landfill leachate in downflow reed beds. Process Biochem. 39 (12), 1971–1976. Faulwetter, J.L., Gagnon, V., Sundberg, C., Chazarenc, F., Burr, M.D., Brisson, J., Camper, A.K., Stein, O.R., 2009. Microbial processes influencing performance of treatment wetlands: a review. Ecol. Eng. 35, 987–1004. Hammer, M.J., Hammer Jr., M.J., 2001. Water and Wastewater Technology, fourth ed. Prentice-Hall, Inc., Upper Saddle River, NJ, USA. IWA, 2000. Constructed Wetlands for Pollution Control: Processes, Performance, Design and Operation, Scientific and Technical Report No. 8. International Water Association, London, UK. Kadlec, R.H., Knight, R.L., 1996. Treatment Wetlands. Lewis Publications, Boca Raton, FL. Kadlec, R.H., Wallace, S.D., 2008. Treatment Wetlands, second ed. CRC Press, Boca Raton, USA. Kulikowska, D., Klimiuk, E., 2008. The effect of landfill age on municipal leachate composition. Bioresour. Technol. 99 (13), 5981–5985. Martin, C.D., Moshiri, G.A., 1994. Nutrient reduction in an in-series constructed wetland system treating landfill leachate. Water Sci. Technol. 29, 267–272. Mathewson, C.C., Mathewson, H.A., 1998. Designing a wetland for wastewater treatment – a truly interdisciplinary effort. In: International Association for
S. Lavrova, B. Koumanova / Bioresource Technology 101 (2010) 1756–1761 Engineering Geology and the Environment, International Congress, vol. 5, no. 8. Vancouver. Obarska-Pompkowiak, H., Gajewska, M., Wojciechowska, E., 2008. Application of vertical flow CW for highly contaminated wastewater treatment. In: Eleventh IWA International Conference on Wetland Systems Technology in Water Pollution Control, 2–7 November, Madhya Pradesh, India, vol. II. pp. 918–924. Randerson, P.F., Slater, F.M., 2005. The role of willow plants in the treatment of ironrich landfill leachate. In: Proceedings of Sixth International Conference on Environmental Engineering, 26–27 May, Vilnius, Lithuania, vol. 1. Vilnius Gediminas Technical University, pp. 420–424. Robinson, H.D., 1996. A review of the composition of leachates from domestic wastes in landfill sites. Department of the Environment (DoE), Wastes Technical Division, UK Technical Aspects of Controlled Waste Management Series, CWM072-95. Schipper, L.A., Cooper, A.B., Harfoot, C.G., Dyck, W.J., 1993. Regulators of denitrification in an organic riparian soil. Soil Biol. Biochem. 25 (7), 925–933. Sleytr, K., Tietz, A., Langergraber, G., Haberl, R., 2007. Investigation of bacterial removal during the filtration process in constructed wetlands. Sci. Total Environ. 380, 173–180. Sun, G., Austin, D., 2007. Completely autotrophic nitrogen-removal over nitrite in lab-scale constructed wetlands: evidence from a mass balance study. Chemosphere 68, 1120–1128. Sun, G., Gray, K.R., Biddlestone, A.J., Allen, S.J., Cooper, D.J., 2003. Effect of effluent recirculation on the performance of a reed bed system treating agricultural wastewater. Process Biochem. 39, 351–357. Torrens, A., Molle, P., Boutin, C., Salgot, M., 2009. Impact of design and operation variables on the performance of vertical-flow constructed wetlands and intermittent sand filters treating pond effluent. Water Res. 43, 1851–1858.
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Tuszynska, A., Obarska-Pempkowiak, H., 2008. Dependence between quality and removal effectiveness of organic matter in hybrid constructed wetlands. Bioresour. Technol. 99, 6010–6016. US Environmental Protection Agency (EPA/625/R-99/010), 2000. Constructed Wetlands Treatment of Municipal Wastewaters. Vymazal, J., 2007. Removal of nutrients in various types of constructed wetlands. Sci. Total Environ. 380, 48–65. Vymazal, J., Kropfelova, L., 2008. Wastewater Treatment in Constructed Wetlands with Horizontal Sub-surface Flow. Series of Environmental Pollution, vol. 14. Springer, Germany. Watson, J.T., Reed, S.C., Kadlec, R.H., Knight, R.L., Whitehouse, A.E., 1989. Performance expectations and loading rates for constructed wetlands. In: Hammer, D.A. (Ed.), Constructed Wetlands for Wastewater Treatment. Municipal, Industrial and Agricultural. Lewis Publishers, Chelsea, Michigan, pp. 319–358. Wojciechowska, E., Obarska-Pempkowiak, H., 2008a. Landfill leachate treatment at a pilot plant using hydrophyte systems. In: Pawlowska, M., Pawlowski, L. (Eds.), Management of Pollutants from Landfills and Sludge. Taylor and Francis, Balkema, London, UK, pp. 205–210. Wojciechowska, E., Obarska-Pempkowiak, H., 2008b. Performance of reed beds supplied with municipal landfill leachate. In: Vymazal, Jan (Ed.), Wastewater Treatment, Plant Dynamics and Management in Constructed and Natural Wetlands. Springer, The Netherlands, pp. 251–265. Yalcuk, A., Ugurlu, A., 2009. Comparison of horizontal and vertical constructed wetland systems for landfill leachate treatment. Bioresour. Technol. 100 (9), 2521–2526.
Contents lists available at ScienceDirect
Bioresource Technology journal homepage: www.elsevier.com/locate/biortech
Influence of recirculation in a lab-scale vertical flow constructed wetland on the treatment efficiency of landfill leachate Silviya Lavrova, Bogdana Koumanova * Department of Chemical Engineering, University of Chemical Technology and Metallurgy, 8 Kliment Ohridsky Blvd., 1756 Sofia, Bulgaria
a r t i c l e
i n f o
Article history: Received 22 June 2009 Received in revised form 6 October 2009 Accepted 12 October 2009 Available online 24 November 2009 Keywords: Landfill leachate Constructed wetland Phragmites australis
a b s t r a c t Landfill leachate taken from a landfill situated in the north-western region of Bulgaria has been treated in a laboratory scale vertical flow constructed wetland (VF-CW) at different flow rates (40, 60 and 82 ml min 1) and recirculation ratios (time of water running through wetland to time of quiet water – 1:1; 1:2; 1:3). Young Phragmites australis was planted on the top layer of the reactor. The low flow rate (40 ml min 1) and recirculation ratio of 1:3 allowed removal efficiencies of 96% for COD (in 8 days), 92% for BOD5 (in 3 days), 100% for ammonia (in 5 days) and 100% for total phosphorus (in 2 days). At the highest flow rate studied (82 ml min 1) and shorter quiet period (recirculation ratio 1:1) the water needs longer period of treatment (2 days more according to COD). The results of this study indicate that both flow rate and recirculation ratio should be taken into account for proper design of VF-CW. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction Leachate quality varies throughout the operational life of a landfill (Kulikowska and Klimiuk, 2008) and long after its closure. There are three broad and overlapping phases of waste decomposition, in which chemical and biological processes give rise to both landfill gas and leachate during and beyond the active life of the site (Robinson, 1996). The landfill leachates are capable of producing severe environmental impacts especially in vulnerable recipients such as aquifers and surface waters. These effects may include eutrophication or toxic effects on aquatic organisms resulting from ammonia, heavy metals or organic compounds. Ammonia nitrogen concentrations often present more of a long-term problem, than the leaching of degradable organic substances such as volatile fatty acids. A sustainable low cost solution for leachate management is the constructed wetland system. They utilize anaerobic and aerobic reactions to break down, immobilize or incorporate organic substances and other contaminants from polluted effluent (Randerson and Slater, 2005; Sleytr et al., 2007; Batzias and Siontorou, 2008; Wojciechowska and Obarska-Pempkowiak, 2008a,b). Martin and Moshiri (1994) and Bulc et al. (1997) found that constructed wetlands are effective in reducing organic components and nutrient loading of landfill leachate. Mathewson and Mathewson (1998) concluded that a treated wetland offered the most cost effective and environmentally sound technique to treat the landfill leachate. * Corresponding author. Tel.: +359 2 83 61 302; fax: +359 2 868 54 68. E-mail addresses: [email protected] (S. Lavrova), [email protected] (B. Koumanova). 0960-8524/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.biortech.2009.10.028
The wastewater treatment efficiency depends on the design of the wetlands and their bed media characteristics (Aslam et al., 2007; Tuszynska and Obarska-Pempkowiak, 2008; Torrens et al., 2009; Albuqureque et al., 2009). There are surface and subsurface flow systems (IWA, 2000; Kadlec and Wallace, 2008; Vymazal and Kropfelova, 2008). Subsurface flow systems are two types – horizontal and vertical. Vertical flow constructed wetlands (vegetated submerged beds) possess greater oxygen transport ability than horizontal flow systems. They are more effective for the removal of organic matter and ammonia nitrogen from wastewaters through aerobic microbial activities (Sun et al., 2003; Obarska-Pompkowiak et al., 2008; Yalcuk and Ugurlu, 2009). The subsurface flow constructed wetlands act as fixed-film bioreactors (EPA, 2000). The wetland systems are filled with different media types typically planted with the same species of emergent vegetation present in the marshes. Kadlec and Knight (1996) listed 37 families of vascular plants that have been used in water quality treatment. The primary role of vegetation in a wetland treatment system is to recycle nutrients in the waste into a harvestable crop, but vegetation plays a distinct role in each treatment process. The vegetation is the support media for biological activity. Also it can maintain long-term infiltration rates. The presence of vegetation increases the wetland evapotranspiration efficiency (Bialowiec and Wojnowska-Baryla, 2007, 2008). The ammonia nitrogen is one of the significant dangerous components in landfill leachates. The transformation of the nitrogenous pollutants in VF-CW was studied by Sun and Austin (2007). Connolly et al. (2003) studied the fate of ammonia nitrogen (NH4-N) in a lab-scale downflow reed bed system treating an artificial landfill leachate. Decrease of the NH4-N level of the leachate was observed when the reed beds were saturated. For
S. Lavrova, B. Koumanova / Bioresource Technology 101 (2010) 1756–1761
artificial leachates with NH4-N levels of 150 ± 5 mg/l, an average removal rate of 44% in a 3 h treatment was achieved. This study also demonstrated that in general a greater rate of effluent recirculation around downflow reed beds gives higher NH4-N removal. The aim of this study is to evaluate the removal of organic matter (as COD and BOD5), nutrients (nitrogen and phosphorus), metals and inorganic compounds in a lab-scale VF-CW for treatment of landfill leachate using different flow rates and recirculation ratios.
2. Methods Landfill leachate was taken from a landfill situated in the northwestern region in Bulgaria. After collection, the landfill leachate was settled overnight and then the supernatant was diluted with tap water (volume ratio 1:1). The characteristics of the diluted leachate are summarized in Table 1. The experiments were carried out in a laboratory system, consisting of an influent sedimentation tank, vertical flow constructed wetland (VF-CW), peristaltic pump and effluent tank (Fig. 1). After sedimentation and dilution the supernatant was allowed to flow into a Plexiglass VF-CW, 123 mm in diameter and 900 mm in height. The VF-CW column was filled with a 300-mm
Table 1 Characteristics of the landfill leachate. Parameter
Influent
COD, mg/l BOD5, mg/l Ammonia nitrogen (NH4-N), mg/l Nitrate nitrogen (NO3-N), mg/l Total phosphorus (TP), mg/l pH Salinity, ‰ Conductivity, lS cm 1 Total dissolved solids (TDS), mg/l Suspended solids (SS), mg/l
2800 204.2 198.4 1.9 5.5 7.9 2.5 4710 2460 2720
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high bottom layer of 35–55 mm round gravel and a top layer with a height of 500 mm of 5–25 mm gravel. Young Phragmites australis was planted in the top layer of the VF-CW. The VF-CW was operated continuously in a recirculation regime. The experiments with three different flow rates (40, 60 and 82 ml min 1, respectively) were carried out. Experiments with three different recirculation ratios at every flow rate were conducted (the time of water running through the wetland to the time of quiet water in it – 1:1; 1:2; 1:3). Thus the water was running for 1 hour and then was left quiet for 1, 2 or 3 h, respectively. The effluent was analyzed for chemical oxygen demand (COD), biochemical oxygen demand (BOD5), ammonia nitrogen (NH4-N), nitrate nitrogen (NO3-N), dissolved oxygen (DO) and total suspended solids (TSS) according to standard methods (APHA– AWWA–WEF, 1999). pH, conductivity, salinity and total dissolved solids (TDS) has been analyzed by SENSION 156, Hach Lange. Metals content also has been determined using atomic absorption spectroscopy (Perkin Elmer-323). 3. Results and discussion 3.1. COD and BOD5 removal The comparison of the COD and BOD5 values of the treated wastewater during the experiments is illustrated in Figs. 2 and 3. The data demonstrate the influence of the recirculation at three different flow rates on the treatment ability of the system. The decreasing of COD values after 5 days from the beginning is fast (Fig. 2). Then the process slowed down. The efficiency at the 5th day (recirculation ratio 1:1) was 67% – at a flow rate of 82 ml min 1, 81% – at flow rate 60 ml min 1 and 90% – at flow rate 40 ml min 1. The efficiency at recirculation ratio 1:2 was 78%, 86% and 90%, respectively, and at recirculation ratio 1:3 it was 78%, 90% and 96%. COD decreased slower when the flow rate was higher. As shown in Fig. 3 the elimination of BOD5 occurs fast in most cases during the initial 5 days. The efficiency of BOD5 removal at a recirculation ratio of 1:1 was 72%, 85% and 92% for flow rate
Fig. 1. Lab-scale vertical flow constructed wetland system.
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Fig. 2. COD values (error bars show the standard deviation (n = 5)) during the experiments at different flow rates and recirculation ratios (1:1; 1:2; 1:3).
82, 60 and 40 ml min 1, respectively. At recirculation ratio 1:2 it was 83%, 92% and 93% for the corresponding flow rates. The efficiency was 91% at flow rate 82 ml min 1 and recirculation ratio 1:3. The same efficiency was obtained after 4 days at flow rate 60 ml min 1 and after 3 days at flow rate 40 ml min 1. It was observed that the longer the water remained quiet in VF-CW, the faster COD and BOD5 decreased. 3.2. Nitrogen removal The importance of nitrogen removal is comparable with that for organic carbon, toxic compounds and metals removal during the leachate treatment in VF-CW. Ammonium removal by nitrification in constructed wetlands differing in design and purpose has been reported (Faulwetter et al., 2009). It is known that autotrophic nitrification consists of two successive aerobic reactions, the conversion of ammonium to nitrite by ammonium oxidizing bacteria and the conversion of nitrite to nitrate by nitrite oxidizing bacteria. The concentration of ammonia nitrogen in the influent used in this
Fig. 3. BOD5 values (error bars show the standard deviation (n = 5)) during the experiments at different flow rates and recirculation ratios (1:1; 1:2; 1:3).
study (Table 1) was relatively high. So, it was interesting to record the changes of NH4-N and NO3-N values during the leachate treatment at different flow rates and recirculation ratios. The influence of these parameters on the ammonium depletion is illustrated in Fig. 4. It was exhausted completely during the experiments. The decrease of ammonia nitrogen could be the collective result of volatilization, nitrification, plant uptake in wetland system and immobilization. At the same time NO3-N increased depending on the flow rate and the recirculation ratios (Fig. 5). In all cases the values increased during the first 1–2 days. After this period the curves shape depended on the experimental conditions. The concentration of NO3-N has increased faster when the flow rate was lower. The influence of the recirculation ratios was opposite. During the experiments at different conditions DO was measured and the values were from 5.2 to 8 mg/l. It is known that the concentration of 1 mg/l is sufficient for oxidation of ammonium (Hammer and Hammer, 2001). The lack of the denitrification during the treatment can be the result of lower activity of denitrifying bacteria in the system. Vymazal (2007) reported that VF-CW removes successfully NH4-
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Fig. 4. NH4-N values (error bars show the standard deviation (n = 5)) during the experiments at different flow rates and recirculation ratios (1:1; 1:2; 1:3).
N but the denitrification is very limited in these systems. It was also well established that carbon availability plays an important role in both synthesis and activity of denitrifying enzymes as well as general support of the denitrifying population. The lack of organic carbon sources is thought to prevent significant levels of denitrification (Schipper et al., 1993).
3.3. Phosphorus removal Phosphorus removal in wetland treatment systems occurs through adsorption, plant uptake, complexation, and precipitation (Watson et al., 1989). The value of TP in the treated leachate was relatively low (5.5 mg/l). It was established that TP removal follows the same tendency as NH4-N removal. During the first 2 days a significant TP elimination occured (Fig. 6). That a higher flow rate leads to a longer period needed for treatment of elimination of TP. At the same time the change of the recirculation ratio from 1:1 to 1:3 (water stays quiet in VF-CW longer)
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Fig. 5. NO3-N values (error bars show the standard deviation (n = 5)) during the experiments at different flow rates and recirculation ratios (1:1; 1:2; 1:3).
leads to shorten period of elimination (e.g., at flow rate 82 ml min 1 the TP removal was 41.8%, 60% and 67.3% for 1:1, 1:2 and 1:3 recirculation ratios during 8, 6 and 4 days, correspondingly). 3.4. pH, salinity and metals content It was established that during the experiments pH slightly decreased from 7.9 to 7.5 and the salinity also decreased from 2.5‰ to 1.9‰. TDS gradually decreased from 2460 to 1778 mg dm 3. The values of TSS varied from 1.91 to 3.96 g/l. Landfill leachate conductivity decreased from 4710 to 3408 lS cm 1 accompanied with decreasing in the metal concentrations. Table 2 shows the variations of elements concentration during the treatment at recirculation ratio 1:3 and 40 ml min 1 flow rate. The initial concentrations of some elements were low (for Cu, Zn, Bi, Cr, Mn and Al – less than 0.3 lg/ml). The initial concentra-
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Table 2 Metal ions changes during the treatment at 40 ml min
1
flow rate of the wastewater and recirculation ratio 1:3. 1
Time, days
Concentration of the elements, lg ml Cu
Zn
Bi
Mg
Ca
Cr
Mn
Fe
K
Na
Al
0 1 2 3 4 5 6 7 8 9 10
0.02 0.02 0.05 0.05 0.05 0.08 0.05 0.05 0.02 0.08 0.08
0.25 0.23 0.05 0.14 0.24 0.18 0 0.06 0.04 0.14 0
0.2 0 0.2 0 0 0 0 0 0 0 0
0.98 0.83 0.79 0.84 0.88 0.86 0.67 0.74 0.7 0.68 0.68
71 64 61 64 66 65 56 60 58 65 56
0.27 0.19 0.19 0.13 0.15 0.12 0.04 0.13 0.13 0.08 0.08
0.15 0 0.02 0.02 0.03 0.02 0 0.02 0 0 0
3.63 1.45 0.77 1.32 2.36 1.77 0.5 1.5 0.95 1.05 0.45
526 429.5 401.5 416 438 419 377.5 389.5 384.5 370 367.5
480 446 454 450 465 443 442 443 439 427 439
0.325 0.106 0.087 0.062 0.275 0.15 0.133 0.062 0.087 0.375 0.075
recirculation ratio 1:3 and 40 ml min 1 flow rate these concentrations decreased by 21% for Ca, 31% for K and only 8% for Na.
4. Conclusions The influence of recirculation in a lab-scale vertical flow constructed wetland on the treatment efficiency of landfill leachate at different flow rates was studied. It was established that the higher flow rate leads to longer period needed for treatment. The recirculation ratios also influence the purification process. Alternating between water movement through the VF-CW and stagnant periods resulted in a varying extent of purification, and the longer the stagnant period of the water in VF-CW the shorter the period for obtaining the desired characteristics of the effluent water. References
Fig. 6. TP values (error bars show the standard deviation (n = 5)) during the experiments at different flow rates and recirculation ratios (1:1; 1:2; 1:3).
tion of Mg and Fe was 0.98 and 3.63 lg/ml, respectively. Significantly higher initial concentrations were measured for Ca (71 lg/ ml), K (526 lg/ml) and Na (480 lg/ml). During the treatment at
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