Total Nitrogen Removal of Municipal Solid Waste Leachate Using Hybrid Constructed Wetlands

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1 Total Nitrogen Removal of Municipal Solid Waste Leachate Using Hybrid Constructed Wetlands Richardson.V.P.S 1, Meetiyagoda.T.A.O.K 2, Jinadasa. K.B.S.N 1, Gamage.P.D 2 1 Department of Civil Engineering Faculty of Engineering University of Peradeniya SRI LANKA 2 Department of Limnology Faculty of Fisheries and Marine Sciences Technology, University of Ruhuna SRI LANKA stevenprassanna@gmail.com Abstract: The treatment efficiency three individual hybrid constructed wetlands containing three macrophytes Cyperus alternifolius umbrella palm), Scirpus atrovirens (green bulrush) and Typha angustifolia (narrow leave cattail) were evaluated in this study. In batch experiments 74%, 60% and 55% total nitrogen removal was observed from the each unit respectively. Evidences for the transformation of ammonium-n into nitrate-n were observed from the obtained results of the vertical unit effluent. The concentration reduction of nitrate-n from the horizontal unit effluent could be attributed to the removal of nitrate-n by denitrification process. However, there is no any significant removal were attained among the evaluated macrophytes, as the overall average efficiency of all three macrophytes was approximately equal. Keywords: Hybrid constructed wetlands, Leachate treatment, Nitrogen removal 1. INTRODUCTION The landfill is the most common technology used to dispose municipal solid residues in developing countries like Sri Lanka. Open dumping is used as an inadequate waste management practice of disposing solid waste and which is widely known to yield a wastewater product called leachate. The leachate is formed as a result of multiple chemical and biological reactions of solid waste within the landfill (Daabes et al., 2013). Landfill leachates contain a high concentration of organic matters and inorganic ions, including heavy metals (Baun et al., 2000) and chemical oxygen demand, volatile suspended solids and electrical conductivity (Huan-jung et al., 2006). Landfills without proper management and leachate treatment facilities become a prominent source of pollution that contaminates soil, surface water bodies and ground water (Percival et al., 1997; Kjeldsen and Christophersen, 2001). Further, it changes the ecological balance, pollutes the quality of air and creates ideal places for breeding of disease vectors. Therefore, it is essential to implement the adequate treatment facilities to treat the leachates in order to comply with regulations and standards of discharge into the environment. In the recent years, constructed wetlands (CWs ) have received tremendous interests for water quality enhancement due to the less labor intensive, low operational cost, and easy maintenance nature (Yeh et al., 2006). Constructed wetlands are engineered systems that have been designed and constructed to create ecological condition same to natural wetlands for treating wastewater in different physical, chemical and biological conditions (Wallace and Knight, 2006). They are utilizing the advantage of many of the processes that occur in natural wetlands, but do so within a more controlled environment (Hammer and Bastian, 1989). All types of CWs are attached growth bioreactor (Kadlec, 1989) while media material and roots, stems, leaves, and litter of wetland vegetation provide the surface for microbial attachment (USEPA 1993; Sinclair, 2000). 134

2 Depending on type of macrophytes and flow regime constructed wetlands could categorize into several units (Brix and Schierup, 1989; Vymazal and Kröpfelová, 2008). Based on the type of macrophytes CWs are classified into 4 groups that called, free-floating, floating-leaved, submerged and emergent and depending on wastewater flow regime CWs could be named as Free Water Surface (FWS), Subsurface Flow (SSF). Also subsurface flow constructed wetland subdivided to two types according to direction known as horizontal and vertical subsurface flow (Imfeld et al., 2009). Wetlands have been found to be effective in treating many pollutants and pathogens simultaneously. The principal pollutant removal mechanisms include biological processes such as microbial metabolic activity and plant uptake as well as physico-chemical processes such as sedimentation, adsorption and precipitation at the water-sediment, root-sediment and plant-water interfaces (Reddy and DeBusk, 1987). However, rate of biological treatment mechanisms are determined by several factors such as ph, temperature, redox potential, oxygen availability and presence of electron acceptors/ donors. There are sufficient studies to indicate some roles being played by wetland plants on Nitrogen compounds. The major removal mechanism of organic nitrogen is the sequential processes of ammonification, nitrification and denitrification. Ammonification is a complex and energy releasing process where organic nitrogen is biologically converted into ammonia-n (Kadlec and Knight, 1996). Ammonification rates are fastest in aerobic zone and the rate of ammonification is dependent on temperature, ph, nutrient availability and prevailing soil conditions (Reddy and Patrick, 1984). Ammonia volatilization is a physico-chemical process where ammonium-n is known to be in equilibrium between gaseous and hydroxyl forms. Volatilization may be a significant route for nitrogen removal in constructed wetlands with open water surface where algal assemblages can create high ph values during the day through their photosynthetic activity (Brix and Schierup, 1989). Vymazal, J. (2007) stated that Removal of ammonia is limited due to lack of oxygen in the filtration bed as a consequence of permanent waterlogged conditions. Nitrification is usually defined as the biological oxidation of ammonium to nitrate with nitrite as an intermediate in the reaction sequence (Wallace and Nicholas, 1969). The oxidation of ammonium to nitrate is a two- step process (Hauck, 1984). The first step, the oxidation of ammonium to nitrite, is occur under aerobic conditions and the second step, the oxidation of nitrite to nitrate, is performed under facultative conditions. Nitrate-N reduction in wetland is through two processes including denitrification and nitrogen uptake by macrophytes. The latter process is important only if the plant is harvested (Seidel, 1976). Denitrification is anoxic process in which nitrate is converted into nitrogen gas where nitrogen oxides (in ionic and gaseous forms) serve as terminal electron acceptors (Hauck, 1984) Usually organic compounds are acting as an electron donating substrate and the energy which has been resulting through this reaction is used support the respiration of those denitrifying microorganisms (Hauck, 1984). Reddy and Patrick, (1984) reported, selection of substrate for the energy generation, either oxygen or nitrogen would be accomplished by the availability of oxygen in the wetland matrix. Nitrogen assimilation is another mechanism of nitrogen removal by a plant, which converts inorganic nitrogen forms into organic compounds that serve as building blocks for cells and tissues. The two forms of nitrogen generally used for assimilation are ammonia and nitrate nitrogen (Kadlec and Knight, 1996). However, if the wetland is not harvested, the nutrients which have been incorporated into the plant tissue will be returned to the system. Matrix adsorption is another removal mechanism of nitrogen where the ionized ammonia has been adsorbed solution through a cation exchange reaction with detritus, inorganic sediments or soils (Vymazl, 2007). Removal of nitrogen compounds in CWs is governed mainly by microbial nitrification and denitrification, while other mechanisms such as plant assimilation, matrix adsorption and ammonia volatilization are generally of less importance (Green et al., 1998). Various types of constructed wetlands differ in their main design characteristics as well as in the processes which are responsible for pollution removal. Vertical flow constructed wetlands remove successfully ammonia-n but very limited denitrification takes place in these systems (Seidel, 1966). On the other hand, horizontal-flow constructed wetlands provide good conditions for denitrification but the ability of the system to nitrify ammonia is very limited (Vymazl, 2007). 135

3 Single-stage constructed wetlands cannot achieve high removal of total nitrogen due to their inability to provide both aerobic and anaerobic conditions at the same time (Vymazal, 2007). In order to achieve effective removal of total nitrogen VF CWs could be combined with HF CWs which, in contrast, do not nitrify but provide suitable conditions for reduction of nitrate formed during nitrification in VF beds (Vymazal, et al., 2008; Brix, 2005). In combined systems, the advantages of the HF and VF systems can be combined to complement each other. It is possible to produce an effluent low in BOD, which is fully nitrified and partly denitrified and hence has a much lower total-n concentrations (Cooper, 1999). The main objective of this project work is to evaluate the nitrogen removal efficiency of hybrid constructed wetlands on the basis of three locally adaptative plant species in municipal solid waste leachate treatment and to desseminate the knowledge for the local authorities for adaptation. 2. MATERIALS AND METHODS 2.1 Implementation Wetland Unit Three individual hybrid constructed wetlands (Vertical flow followed by horizontal flow) were constructed and filled up with suitable substrate media. Three locally adaptive plant species were selected for this study namely, Umbrella Palm (Cyperus alternifolius), Green Bulrush (Scirpus atrovirens) and Narrow Leave Cattail (Typha angustifolia). All three plant species were separately planted in three units individually. Three overhead tanks were installed at the site to supply the leachate in a pre-determined flow rate (31.5 L/Day) to satisfy the required hydraulic retention time (7 days). Evaluation of the all three wetland unit is expected to analyze on weekly basis for subsequent 10 weeks duration. 2.2 Leachate Collection and Characterization Municipal solid waste leachate was collected from a nearby dumpsite called Gohagoda on weelky basis for subsequent10 weeks. Physico-chemical characterization of the leachate has been conducted according to the standard methodolgies for Total nitrogen (TN), ammonical-n, nitrate-n, Bilogical Oxygen Demand (BOD), Chemical Oxygen Demand (COD) and other physical parameters. 2.3 System Evaluation Prior to the flow of leachate, wetland system was allowed to saturate with deionized water for adequate time. Whole wetland system was ran by 5% diluted leachate for initial five weeks and 25% diluted leachate for rest of the 5 weeks. Effluent leachate samples from the vertical flow units and horizontal flow units were collected individually by weekly basis for all three wetland units during the estimated evaluation duration. Collected samples were brought to the laboratory and analyzed for Total nitrogen, ammonical-n, nitrate-n, BOD, COD and ph as per the standard methodologies. 3. RESULTS AND DISCUSSION 3.1 Ammonia-N Removal Ammonia- N concentration in the leachate was ranges between mg/l for 5% dilution and mg/L for 25 % dilution. It is observed that, vertical effluents of umbrella palm, green bulrush and narrow leaf catail has shown an average removal of 82%, 85% and 71% for the ammonium-n respectively. There is no any significant level of ammonia-n removal was observed from the analyzed samples of horizontal unit effluent. This can be explained by the deficiency of oxygen in the horizonatl unit(vymazal, 2007). This observation is agrees well with that of (Reddy and Patric, 1984). However, there was no significant difference in overall removal efficiencies observed between the three macrophyte systems (figure 1). 136

4 3.2 Nitrate-N Results Figure 1 Variation of Ammonium-N for the total unit effluent Nitrate-N concentration in the leachate was reported in a minor level, ranges between mg/l for 5% diluted leachate and % for 25% diluted sample. Vertical effluent of all plant species has shown elevated levels of nitrate-n than in the influent concentration (figure 2). This would be explained by the transformation of ammonium-n into nitrate-n by aerobic microorganisms and possible volatalization of ammonium-n into ammonia gas (Reddy and patrick, 1984). However, rate of transformation was not consisitent throughout the evaluation period. It is interesting to note that, attenuated levels of nitrate-n in horizontal effluent than the respective values of the vertical effluent, could be attributed to the transformation of nitrate-n into nitrogen gas by anaerobic bacterial activities (Lin et al., 2002). Direct plant uptake also contributed to the partly reduction of nitrate-n of the feed water. However, it is usually less importance compared to the denitrification process (Dendene et al., 1993).Overall average of nitrate-n reduction was reached on the total effluent at a value of 70%, 65% and 55% (Figure 3) for the each plant species respectively. Figure 2 Variation of Nitrate-N for the vertical unit effluent 137

5 3.3 Total Nitrogen Results Figure 3 Variation of Nitrate-N for the total unit effluent Total nitrogen concentration in the leachate was observed as mg/L for the 5% diluted leachate and mg/l for 25% diluted leachate. The concentration reduction from inlet to outlet for the umbrella palm, green bulrush and narrow leaf catail has shown 74%, 60% and 55% respectively. Figure 4: Variation of Total nitrogen concentration 4. CONCLUSION The experimental results have demonstrated that hybrid wetlands provided a suitable environment for microorganisms to decompose or transform pollutants to assist in nitrogen removal in leachates. Results of the vertical effluent revealed that the removal efficiencies of ammonium-n higher in vertical wetland unit than the horizontal wetland unit. Evidences for the assistance of horizontal unit in denitrification have been established by the attenuated level of nitrate-n concentrations in the horizontal unit effluent than in the vertical effluent. Umbrella palm contributed strongly to the removal of total nitrogen than other two macrophytes. 138

6 REFERENCES Baun, A., Jensen, S.D., Bjerg, P., Christensen, T.H., Nyholm, N., 2000, Toxicity of organic chemical pollution in groundwater down-gradient of a landfill (Grindsted, Denmark). Environ. Science Technology. 34, pp Brix H, and Schierup, H. 1989, The use of macrophytes in water pollution control. AMBIO, 18 pp Brix, H., 2005, The use of vertical flow constructed wetlands for on-site treatment of domestic waste water: New Danish guidelines. Ecol. Eng, 25, pp Daabes, M.A., Qdais, H.A. and Alsyouri, H. 2013, Assessment of heavy metals and organics in municipal solid waste leachates from landfills with different ages in Jordan. Journal of Environmental Protection, 4, pp Dendene M.A., Rolland T, Tremolieres M, Cariener R 1993, Effect of ammonium ions on the net photosynthesis of three species of Eldoea. Aquatic Botany 46(3 4), pp Green, M., Friedler, E. and Safrai, I. 1998, Enhancing nitrification in vertical flow constructed wetland utilizing a passive air pump. Water Research, 32 (12), pp Hammer D.A., Bastian, RK. 1989, Wetlands ecosystems: natural water purifiers In: Hammer DA, editor. Constructed wetlands for wastewater treatment. Chelsea, Michigan: Lewis Publishers; pp Hauck, R.D., 1984, Atmospheric nitrogen chemistry nitrification, denitrification and their relationships. In the handbook of environmental chemistry (ed. O. Hutzinger), vol. 1, part C (The natural environment and biogeochemical cycles), pp Berlin, Germany: Springer- Verlag. Huan-jung, F., Shu, H.Y., Yang, H.S. and Chen, W.C. 2006, Characteristics of landfill leachates in central Taiwan. Taiwan Sci. Total Environ. 361, pp Imfeld G, Braeckevelt M, Kuschk P, et al. 2009, Monitoring and assessing processes of organic chemicals removal in constructed wetlands,chemosphere, pp Kadlec R.H., 1989, Hydrological factors in wetland water treatment. In: Constructed Wetlands for Wastewater Treatment (Hammer DA, ed). pp , Lewis Publishers, USA Kadlec, R., and Knight, R., 1996, Treatment wetlands Lewis Publisher, Boca Raton, FL, USA. Kjeldsen. P. and Christophersen. M., 2001, Composition of Leachate from Old Landfills in Denmark,Waste Management and Research, 19(3), pp Lin, Y., Jing S, Wang T., Lee D., 2002, Effects of macrophytes and external carbon sources on nitrate removal from ground water in constructed wetlands, Environmental Pollution 119(3), pp Percival, L.J., Senior, E. and Southway, C. 1997, Treatment of high strength leachate from a closed co- disposal landfill site in South Africa. Water South Africa 23. Reddy, K.R. and De-Busk, T.A., 1987, State of the art utilization of aquatic plants in water pollution control. Water Science and Technology 19(10), pp Reddy, K.R. and Patrick, W.H. (eds), 1984, Nitrogen transformation and loss in flooded soils and sediments. CRC Critical Review Environmental Control 13, pp Seidel, K., 1966, Reinigung von Gew assern durch h ohere Pflanzen, Naturwissenschaften, vol. 53(12), pp Seidel, K., 1976, Macrophytes and Water Purification. In Biological Control of Water Pollution; Tourbier, J., Pierson, R.W., Eds.; Pennsylvania University Press: Philadelphia, PA, USA, pp

7 Sinclair K., 2000, Guidelines for Using Free Water Surface Constructed Wetlands to Treat Municipal Sewage. Queensland Department of Natural Resources, Australia USEPA, 1993, Subsurface Flow Constructed Wetlands for Wastewater Treatment: A Technology Assessment, EPA 832-R , Office of Water, Washington, USA. Vymazal, J., 2007, Removal of nutrients in various types of constructed wetlands, Sci. Tot. Environ. 380, pp Vymazal, J. and Kröpfelová, L. Wastewater Treatment in Constructed Wetlands with Horizontal Sub- Surface Flow; Springer: Dordrecht, The Netherlands, Yeh, T. Y., Chuang, C.C. and Ju, C.H., 2006, Pollutants transformation and removal within constructed wetlands hybrid systems, Proceedings of the 4th WSEAS Int. Conf. on HEAT TRANSFER, THERMAL ENGINEERING and ENVIRONMENT, Elounda, Greece, pp Wallace, S.D., Knight R.L., 2006, Small scale constructed wetland treatment systems, Water Environment Research Foundation, IWA Publishing, London, UK Wallace, W and Nicholas, D.J.D., 1969, the biochemistry of nitrifying microorganisms. Biol. Rev. 44, pp