Waste Management & Research (1995) 13, 123-136 BIODEGRADABILITY OF THE ORGANIC FRACTION OF MUNICIPAL SOLID WASTE IN A HIGH-SOLIDS ANAEROBIC DIGESTER Masoud Kayhanian Department of Civil and Environmental Engineering, University of California at Davis, Davis, California 95616, U.S.A. (Received 25 October 1993, accepted in revised form 18 March 1994) Three methods were used to estimate the ultimate biodegradability of the organic fraction of municipal solid waste. These methods included: long-term batch digestion studies, measurement of lignin content, and chemostat studies. The ultimate bio- degradability values obtained from these methods were compared to a field operation using a pilot scale, high-solids, complete-mix, thermophilic, anaerobic digestion process. The biodegradability obtained from the pilot study, at a mass retention time of 30 days, was approximately 83 and 81% of the estimated values obtained from the lignin content and the batch study, respectively. In addition, it has been shown that the contents of the biodegradable volatile solids affects the prediction of biogas production rate, the computation of the organic loading rate, and feedstock C/N ratio. Key Words--Biodegradability, biodegradable volatile solids (BVS), biodegradable organic fraction of municipal solids waste (BOF/MSW), high-solids anaerobic digestion process. 1. Introduction Biological transformations can generally be classified as either aerobic or anaerobic processes. While each organic waste may contain a constant ultimate biodegradable fraction, practical biodegradability for an aerobic process may be different than for an anaerobic process under similar conditions. Additionally, in practice, factors such as particle size, time, and environmental conditions (i.e. temperature, nutrient requirements, etc.) will influence the final outcome of biodegradation. For example, because the right conditions do not exist in most landfills, the biodegradability estimated from analytical tests will usually over-estimate the real biodegradation that occurs in landfills. It is clear that the issue of biodegradability is broad, and many factors must be considered to completely describe all substrates and various biological transformation processes. The aim of this paper is not to completely describe all aspects of biodegradability. For practical purposes, this paper was prepared to deal with the biodegradability of the organic fraction of municipal solid waste in an in-vessel, high-solids, anaerobic digestion process. A typical organic fraction of municipal solid waste (MSW) in the U.S.A., consisting primarily of paper, yard waste, food waste, and other organic waste, including plastics, comprises approximately 70% of the waste stream. Paper, yard waste, and food waste make up the majority of the organic fraction, comprising approximately 53% of the waste stream which is collectively called the biodegradable organic fraction of MSW 0734-242X/95/020123+ 14 $08.00/0 © 1995 ISWA