Assessment of biodegradability of plastics under simulated composting conditions in a laboratory test system

Abstract

An automated laboratory-scale test system was developed for measuring the aerobic biodegradability of degradable plastics under simulated composting conditions. Biodegradation was monitored by measuring microbial carbon dioxide formation and oxygen consumption. Completeness of biodegradation was assessed in an aquatic test by conducting a carbon mass balance. The percentage of plastic carbon degraded to carbon dioxide, biomass and water-soluble byproducts were determined. The rate of biodegradation under simulated composting conditions was measured in a fixed-bed system with mature compost. A time-dependent temperature profile was applied to simulate the natural self-heating of a composting process. The aquatic test was conducted at a constant temperature of 30°C as well as with a temperature profile. The rate of biodegradation was significantly higher in the aqueous environment. Equal degrees of mineralization were reached in the aquatic and the fixed-bed system only if the same temperature profile was applied. Conducting the aquatic test at a constant temperature of 30°C less microbial carbon dioxide formation was observed. However, a carbon mass balance revealed that taking into consideration the portion of the plastic's carbon incorporated into biomass, a similar degree of biodegradation was reached. Consequently, the measurement of microbial carbon dioxide production is not sufficient to assess the extent of biodegradation of plastics. In the heterogeneous matrix ‘compost’, it is not feasible to assess the completeness of biodegradation due to limited possibilities to analyze degradation intermediates and biomass growth. Therefore, a new fixed-bed system with an inert, carbon-free packing material was developed. The inert material was inoculated with an aqueous eluate from compost. First results showed biodegradation rates similar to a compost environment.