Long-Term Treatment and Disposal of Landfill Leachate

This proposal is in response to item 13 of the FCSHWM request for proposals asking for research that examines the long-term treatment and disposal options for landfill leachate. Mature landfill leachates will require a complex sequence of biological, physical, and chemical treatment processes to reach discharge limits. It is likely that ammonia-nitrogen, humics, and xenobiotic compounds in these leachates will determine when post-closure monitoring may end. This research project proposes to examine in laboratory experiments the capacity of the landfill to provide complete in-situ treatment of landfill leachates so that they may be released to the environment without adverse impact. Objectives and Methodology A new and promising trend in solid waste management is to treat the landfill as a bioreactor. Bioreactor landfills are controlled systems in which moisture control and/or air injection are used as enhancements to create a solid waste environment capable of actively degrading the readily biodegradable organic fraction of the waste. There are many advantages associated with treating the landfill as a bioreactor, including the rapid reduction of biodegradable organic compounds and heavy metals in leachate. However, challenges remain, including the persistence of ammonia-nitrogen in the leachate. Recirculating leachate increases the rate of ammonification and results in accumulation of higher levels of ammonia-nitrogen concentrations, even after the biodegradable organic fractions of the waste are removed. Concomitantly, although the biodegradability of leachate organic compounds declines with time, complex organic compounds such as humic substance and xenobiotic compounds, remain in solution. Thus, leachate, with time, will require a complex sequence of biological, physical, and chemical treatment processes to reach discharge limits. It is likely that ammonia-nitrogen, humics, and xenobiotic compounds will determine when the landfill is biologically stable and when post-closure monitoring may end. This research project proposes to examine in laboratory experiments the capacity of the landfill to provide complete in-situ treatment of landfill leachates so that they may be released to the environment without adverse impact. Currently, ammonia treatment in leachate removed from landfills is primarily practiced ex-situ via biological co-treatment at publicly owned treatment works or on site treatment via biological nitrification/denitrification processes. Nitrification/denitrification is advantageous because it completely removes nitrogen. The additional costs associated with ex-situ treatment of ammonia have made in-situ removal techniques attractive alternatives. A few in-situ, or partially in-situ studies have been conducted, however, the data required to enable adequate implementation of such processes at field-scale bioreactor landfills are lacking. Thus, this research will investigate in-situ nitrification and denitrification processes in solid waste environments, allowing for a more informed approach to designing and operating bioreactor landfills by addressing the following: • Determination of optimal environmental conditions and expected efficiencies for in-situ nitrification, • Determination of optimal environmental conditions and expected efficiencies for in-situ denitrification, • Development of an implementation strategy for in-situ nitrification and denitrification at field-scale, including an economic comparison with ex-situ treatment approaches, These objectives will be met by implementing laboratory microcosm studies in 1-L containers aimed at determining nitrification and denitrification rates as a function of ammonia and nitrate mass loadings, respectively, under different environmental conditions (i.e. temperature, waste age, and oxygen levels). Particular attention will be paid to potent greenhouse gas emissions, such as N2O and NO, as production will contribute to the global climate change problem. Laboratory studies will provide data necessary to determine the waste degradation stage necessary to establish nitrification and denitrification, dictating where each zone should be operated in a bioreactor landfill, the amount of oxygen required, dictating air flowrates needed in the nitrification zone, the acceptable loading of ammonia and nitrate as a function of environmental conditions, dictating leachate recirculation flowrates, the size of reaction zones needed for nitrification and denitrification, based on reaction rates, and the need for a buffer to control pH, based on pH levels in the microcosms. The results from the laboratory studies will be used to develop a strategy for a subsequent pilot-scale study at the Florida Bioreactor Landfill Demonstration at the New River Regional Landfill, as well as for future field-scale implementation. Further, this project will explore the treatment of refractory organics in mature landfill leachate (mainly humic materials and xenobiotic organic compounds) by means of combined external partial chemical oxidation followed by biological treatment within the landfill. Use of the proposed method is expected to promote safe discharge of leachate into natural bodies by removing humic and fulvic acids and xenobiotic organics. Humic and fulvic acids increase the mobility of groundwater contaminants such as heavy metals cadmium, nickel, and zinc (Christensen, et al., 1996). Hydrophobic organic contaminants are considered to be the principal organic precursors for trihalomethanes (potential carcinogens in drinking water, Reckhow, (1990)) and many xenobiotic compounds are suspected carcinogens (Chiou et al., 1986). Further as an in situ treatment, this method is expected to reduce the cost and environmental risk of transportation and ex situ treatment of leachate. Because recalcitrant organics are removed, postclosure costs associated with monitoring of groundwater, surface water, and air will decline, as will long-term environmental risks from leachate contamination. Finally, there is a potential for a methane gas production increase in landfills due to biological degradation of previously recalcitrant organics. This technique would be applied at a time when methane generation is low because waste has been bio-stabilized. Assuming the gas is collected and utilized, economic advantages will be realized. To achieve the project objective the following research phases will be conducted: 1) explore mechanisms and process requirements for the selected oxidants (Fenton’s reagent and ferrate) in batch experiments, 2) test the overall treatment scheme (in situ biodegradation of leachate, leachate oxidation, and reintroduction of leachate for further in situ biodegradation) through simulation of a bioreactor landfill using laboratory-scale reactors, and 3) study the economical aspects of applying this technique in bioreactor landfills. In the first phase leachate will be oxidized in batch reactors with the goal of optimizing the oxidation time, dose, and pH necessary to maximize the biodegradability of previously refractory compounds. Also during Phase I, the role of oxidant scavengers will be studied with the goal of increasing the efficiency of the oxidation reactions. Phase II is a laboratory proof-of-concept of the overall treatment scheme. Phase III will look at the cost/benefits of this treatment technique and a comparison with costs of other ex-situ treatment options. During Phases I and II the organic matter biodegradability will be measured using biochemical oxygen demand, chemical oxygen demand, total organic carbon, ultraviolet absorbance at 254 nm, biochemical methane potential, and molecular size analysis. Table 1 provides a schedule for the proposed research. Note that this is a two-year project; however, funds are requested for Year 1 only. Table 1. Project Gantt Chart