The purpose of this work is two-fold: 1) to understand the phenomenon of the biodegradation process of municipal solid waste (MSW) in leachate recirculated bioreactors, and 2) to create a realistic predictive model based on this understanding which is capable of supporting a laboratory-to-field relationship for bioreactor landfills. Biodegradation is best described by loss of mass; however, primary researchers have assumed the phenomenon to be purely volume loss and modeled best by mechanical processes using a conservation of energy approach. It is suggested that the phenomenon requires a fundamental understanding of biodegradation process which results in a loss of mass, and therefore an understanding of the conservation of mass must be considered.
It is difficult to measure and predict volume change of a heterogeneous MSW material as well as the change in mass in the field. In the laboratory, under controlled conditions, changes in volume and mass can be determined destructively and a relationship between mass and volume changes can be obtained. Changes in volume in the laboratory are related to the corresponding volumes of gas produced as the MSW degrades. From these measurements, vertical strain (settlement) of MSW landfill and the state of biodegradation as a function of time can be estimated. This is the basis of the research conducted herein. Characteristics curves depicting percent biodegradation and vertical strain as functions of time for a given composition of MSW can be developed.
Four homogenized sample sets, each consisting of composite, readily, moderately, and slowly degradable MSW are prepared and tested in separate bioreactors. These are connected to an electronic gas flow meter, leachate recirculation tubing, and subjected to leachate over a period of approximately 260 days to simulate a landfill environment. Gas production, settlement, and other physical and engineering parameters are measured as these conditions vary. Approximately 72%, 93%, and 62% of the calculated theoretical total gas potentials of 6.23, 9.04, and 8.43 cubic feet per pound waste for composite, readily, and moderately degradable bioreactor sets are collected. From the laboratory program, it is determined that characteristic curves for any composite MSW sample could be developed from the results of the readily, moderately and slowly biodegradable MSW samples using weighted averaging techniques. In a landfill, lifts of MSW, placed at different times, will degrade at different rates and are at different states of biodegradation. A method to determine the average state of biodegradation for such a condition is developed to assist in field validation.
Field validation of the laboratory models based on newly developed characteristic curves is performed on two MSW bioreactor landfills. The first, Cape May County Municipal Utilities Authority bioreactor landfill, exhibits MSW composition similar to the composite waste sample tested in the laboratory. It is determined that the percent biodegradation predicted by the model developed here in is between 3 and 14 percent of the actual field results, and the settlement values predicted by the developed model are in very close agreement with those observed in the field. Similar agreement is obtained using the method for the second, Yolo County landfill located in California, with different MSW composition and environmental factors.
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