Disposal options for biodegradable plastics

Biodegradable plastics that break down in the environment can be used instead of traditional plastics to reduce the problem of plastic waste disposal. A new study highlights large differences in the rates of decomposition of these polymers, which can affect decisions on disposal methods.

Disposing of plastic waste is a serious problem because plastics barely decompose in the environment. Biodegradable polymers have been developed as substitutes for non-biodegradable plastic materials. Microorganisms (microbes) that occur naturally in the environment can break down these polymers into simpler compounds. Under aerobic (with oxygen) conditions, microbes consume oxygen to decompose biodegradable polymers primarily into carbon dioxide and water. Under anaerobic conditions (i.e. without oxygen) anaerobic microbes decompose biodegradable polymers primarily into methane and carbon dioxide.
In order to decide the most appropriate means of disposal for two types of biodegradable polymer, this study investigated how easily a poly(caprolactone) (PCL)-starch blend polymer (used in food packaging, for example) and a poly(butylene succinate) (PBS) polymer (used to produce disposable cups and cutlery, for example) decomposed. In addition, the biodegradation rates of both polymers under aerobic and anaerobic conditions were determined and the effects of burying both these biodegradable polymers in a landfill were observed.

Decomposition characteristics of samples of a PCL-starch blend and PBS-based polymer were tested in the laboratory under aerobic conditions, using microbes contained in activated sludge taken from a local municipal sewage treatment plant in Seoul, South Korea. Similar tests under anaerobic conditions were carried out on samples using microbes contained in anaerobic sludge taken from a local municipal sewage treatment plant in Seoul.

The chemical and physical structure of the polymers and the conditions of decomposition led to significant differences in the degradation characteristics of both biodegradable polymers.

The aerobic experiments found that 88 per cent of PCL-starch blend was degraded in 44 days. In contrast, only 31 per cent of the PBS polymer was degraded after 80 days. In addition, the PCL-starch blend polymer degraded at a rate seven times faster than the biodegradation rate of the PBS sample under aerobic conditions.

Under anaerobic experimental conditions, 83 per cent of the PCL-starch blend was degraded after 139 days whilst only 2 per cent of the PBS sample was degraded in 100 days.

Samples of each polymer were buried in a landfill site for 90 days and the appearance of the surfaces was compared before and after burial. The surface of the PCL-starch blend was smooth before burial, but after the 90 days in landfill, the surface was covered with numerous pinhole-sized holes, suggesting that there had been physical and chemical degradation caused by microbial activity. Although the surface of the PBS sample was no longer smooth after removal from landfill, relatively few pinholes were observed.

This study suggests that PCL-starch could be disposed of by composting, in landfills or in anaerobic digesters. The polymer could be. pre-treated using technologies, such as thermal, enzymatic, ultrasonic treatment and the application of adapted microorganisms, to reduce the volume of waste. Further work is needed to find suitable disposal routes for the PBS biodegradable polymer.

Source: Cho, H.S., Moon, H.S., Kim, M. et al. (2011) Biodegradability and biodegradation rate of poly(caprolactone)-starch blend and poly(butylene succinate) biodegradable polymer under aerobic and anaerobic environment. Waste Management. 31: 475-480.

Contact: jaeykim@snu.ac.kr