Bacterial population development and chemical characteristics of refuse decomposition in a simulated sanitary landfill

Abstract

Population development of key groups of bacteria involved in municipal refuse conversion to methane was measured from the time of initial incubation through the onset of methane production. Hemicellulolytic bacteria, cellulolytic bacteria, hydrogen-producing acetogens, and acetate- and H(2)-plus-CO(2)-utilizing methanogens were enumerated by the most-probable-number technique with media containing oat spelt xylan, ball-milled cellulose, butyrate, acetate, and H(2) plus CO(2), respectively. Refuse decomposition was monitored in multiple replicate laboratory-scale sanitary landfills. A laboratory-scale landfill was dismantled weekly for microbial and chemical analysis. Leachate was neutralized and recycled to ensure methanogenesis. The methane concentration of the sampled containers increased to 64% by day 69, at which time the maximum methane production rate, 929 liters of CH(4) per dry kg-year, was measured. Population increases of 2, 4, 5, 5, and 6 orders of magnitude were measured between fresh refuse and the methane production phase for the hemicellulolytic bacteria, cellulolytic bacteria, butyrate-catabolizing acetogens, and acetate- and H(2)-CO(2)-utilizing methanogens, respectively. The cellulolytic bacteria and acetogens increased more slowly than the methanogens and only after the onset of methane production. The initial decrease in the pH of the refuse ecosystem from 7.5 to 5.7 was attributed to the accumulation of acidic end products of sugar fermentation, to the low acid-consuming activity of the acetogenic and methanogenic bacteria, and to levels of oxygen and nitrate in the fresh refuse sufficient for oxidation of only 8% of the sugars to carbon dioxide and water. Cellulose and hemicellulose decomposition was most rapid after establishment of the methanogenic and acetogenic populations and a reduction in the initial accumulation of carboxylic acids. A total of 72% of these carbohydrates were degraded in the container sampled after 111 days. Initially acetate utilization, but ultimately polymer hydrolysis, limited the rate of refuse conversion to methane. Microbial and chemical composition data were combined to formulate an updated description of refuse decomposition in four phases: an aerobic phase, an anaerobic acid phase, an accelerated methane production phase, and a decelerated methane production phase.