Degradation of biodegradable plastics under controlled composting conditions

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Courtesy of ORBIT e.V.

The management of solid waste is a growing concern in many countries. Municipal solid waste is a major component of the total solid waste generated by society, and the fraction of municipal solid waste made up of plastic materials continues to increase. The composting of plastic materials along with organic wastes will help society to usefully recycle more of its solid waste and divert it from landfills and other less desirable waste-management options. Therefore, the use of composting has gained some attention, although the composting of municipal refuse is not yet widespread. At this time, no more than 30 composting plants are operating in Japan. One reason for this is that compost products contain significant quantities of impurities, including plastic, which cannot be accepted by farmers. Separating the materials unsuitable for the composting process from municipal refuse is very costly, and even when an excellent separator is used, the compost products still suffer from some contamination by plastics. Accordingly, many kinds of biodegradable plastic materials have been developed (e.g., Doi & Fukuda, 1994) which are expected to effectively promote the composting of organic waste.

Numerous studies have been done on degradation of biodegradable plastic in the composting process (Buchanan et al., 1995; Choi & Park, 1996; Gilmore et al., 1992; Gu et al., 1993; Johnson et al., 1993; Tosin et al., 1996; Yue et al., 1996). These studies have mainly been attempted to evaluate the degradability of biodegradable plastic during composting by measuring the weight loss of the film. Recently, however, various test methods have been designed to determine the percentage of aerobic degradation of plastic materials when exposed to a controlled composting environment under laboratory conditions (ASTM, 1993; CEN, 1994). Two methods, ASTM D5338-92 and CEN have been developed for determining such degradability with highly reproducible data and a short test period. In both these methods, matured compost is mixed with a biodegradable plastic and composted for 45 days, after which the percentage of decomposition is determined based on the difference in the amount of CO2 evolution from matured compost absent from any biodegradable plastic. We here used fresh waste as a substitute for matured compost, and mixed it with biodegradable plastic to simulate the vigorous degradation of organic materials in a practical composting process. In addition, we investigated the effects of temperature and inoculum on the degradability of the biodegradable plastic by using the bench-scale reactor under well-controlled laboratory conditions. Although many studies have been done on the degradability of biodegradable plastics in the composting process, few studies have examined the effects of composting conditions such as temperature and the inoculation of seeding material on the degradability of biodegradable plastic (Ohtaki et al., 1997). Moreover, we compared degradability of different plastics in the same, well controlled composting conditions that has not been seen in previous studies.

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