Effects of leachate recirculation on anaerobic treatment of municipal solid waste Chayanon Sawatdeenarunat ABSTRACT Anaerobic bioreactor landfills are the most popular method to treat municipal solid waste. Leachate recirculation is one of the methods to apply in bioreactor landfills to eliminate the leachate treatment and ground water contamination. The leachate recirculation could enhance landfill’s performance both in term of municipal solid waste (MSW) treatment and landfill gas production. The waste degradation is accelerated by the leachate recirculation as well as the landfill’s gas production rate is higher. Moreover, the proportion of methane in landfill gas is higher with leachate recirculation than those without recirculation. Nevertheless, chloride and ammonia nitrogen might accumulate in landfills and could inhibit methanogen bacteria. A key point of the recirculation rate of the bioreactor landfill is the balance of hydraulic pattern and biological process. KEYWORDS Municipal Solid Waste, bioreactor Landfills, Leachate recirculation, Anaerobic treatment INTRODUCTION Municipal Solid Waste (MSW) is a waste type which includes predominantly household waste with sometimes the addition of commercial wastes collected by a municipality within a given area. MSW is increasing in almost all part of the world. This is may be due to the population continues increasing in past ten decades. Bioreactor landfills are the most popular treatment system using for treating MSW at this time. Characteristics of MSW are different depending on its source and the people’s lifestyles in that area. The Municipal Solid Waste management is shown in figure 1 and the characteristics of MSW are shown in figure 2. Figure 1 MSW management (http://www.eia.doe.gov)Table 1 Characteristics of MSW in the United States, year 2000 and 2005. (www.epa.gov) Leachate is the refuse from landfills and the compositions vary regarding the age of each landfills and the characteristics of MSW. Leachate is caused by the percolation of precipitation through MSW and react with the products of decomposition, chemical and other materials. Normally, leachate is anoxic, acidic, high COD, high sulfate and metal ion. The samples of leachate characteristics are shown below. Table 2 Characteristics of leachate in conventional and recirculation landfills. (www.epa.gov)ANAEROBIC DIGESTION Anaerobic digestion is the reaction in which other compounds are used as the electron acceptor instead of oxygen. The reactions which could occur in anaerobic digestion of organic waste such as organic removal, denitrification, sulfide and sulfate removal are shown in figure 2. Figure 2 Reactions in anaerobic condition. The transformation of complex compounds into biogas in anaerobic processes could be identified to four important stages as shown in figure 3. Figure 3 Four stages of organic transformation in anaerobic condition (www. water.me.vccs.edu). 1. Hydrolysis stage: Complex compounds are hydrolysed into dissolved compounds with a low molecular weight. Protein is degraded to aminoacid, carbohydrate is transformed to monosaccharides and disaccharides and lipid are converted to long-chain fatty acid. 2. Acidogenesis stage: The dissolved compounds are extracted to simple organic compound such as volatile fatty acid, alcohol, lactic acid and mineral compounds such as carbon hydroxide, hydrogen, ammonia and hydrogen sulfide. 3. Acetogenesis stage or fermentation: The products from acidogenesis stage are converted to suitable substrate of methanogen microorganism such as acetate, hydrogen and carbon dioxide.4. Methanogenesis stage: This stage is often the limiting step of anaerobic digestion. In this step, carbon dioxide and hydrogen are converted to methane by hydrogenotrophic bacteria and acetate is converted to methane by methanogen bacteria (lettinga and Van Haandel,1990). An anaerobic digestion technology is applied widely in landfills to treat MSW. The advantages of this technology are low consuming energy, producing landfill gas which could be utilized to renewable energy and high ability to treat high organic waste such as MSW. Leachate treatment is become the serious problem in almost all landfills all over the world. Because leachate with high toxic compound could contaminate ground water and cause the serious environmental problem. Recirculation is one of the most popular method to reduce leachate problem. Leachate recirculation could provide moisture to landfills and helps to enhance the efficiency of anaerobic digestion in term of reducing long-term environmental outcome, liability of waste storage and improving the economic of landfills (Reinhart and Townsend, 1997). EFFECTS OF LEACHATE RECIRCULATION ON ORGANIC COMPOUNDS REMOVAL Since, the boundary between each stage of an anaerobic digestion could not be determined exactly, COD, BOD and methane production are used to depict. The results from many studies in laboratory indicated that the lechate recirculation showed exact advantage on COD removal. The reactors with leachate recirculation showed significantly rapid degradation and also reached stabilization more quickly than those without recirculation. This might be because the lechate recirculation could decrease acidogenesis and methanogenesis period (Francois et al., 2007), (Chan et al., 2002). Moreover, the effects of leachate recirculation are clearly more effective in anaerobic reaction than those in aerobic reaction (Bilgili et al., 2007). Figure 4 the effect of leachate recirculation on COD removal (Francois et al., 2007). From figure 4, it was indicated that the leachate characteristics of bioreactor (JR,VR) and conventional landfills (J,V) is similar in term of COD. But in acidogenesis stage, COD concentration in leachate of bioreactor landfills was higher. It was possible that the dryness of conventional landfills and the poor contact between leachate and MSW make fewer leaching opportunity. The concentrations of volatile acid were different in leachate from landfills with and with out recirculation. In conventional landfills, the majority in volatile acid is butyrate but for bioreactor landfills, acetate has the highest concentration. pH value normally increase with decreasing of organic compounds in leachate. Since recirculated leachate had higher organic compounds when compared with single-pass leachate, it could be treated inside landfills and repeatedly recirculated back into the landfills