Publications archive - Waste and recycling
Key departmental publications, e.g. annual reports, budget papers and program guidelines are available in our online archive.
Much of the material listed on these archived web pages has been superseded, or served a particular purpose at a particular time. It may contain references to activities or policies that have no current application. Many archived documents may link to web pages that have moved or no longer exist, or may refer to other documents that are no longer available.
Nolan-ITU Pty Ltd
Prepared in association with ExcelPlas Australia
The rate of biodegradation of biodegradable plastics is dependent on the disposal environment and its conditions.
The major disposal environments for biodegradable plastics are:
To a large extent, the nature of the biodegradable plastic application should dictate the disposal environment.
Most biodegradable plastics lend themselves well to composting systems. Composting and soil burial is the preferred disposal environment for most biodegradable plastics. Many cities around the world now compost garden organics, food wastes, cardboard, paper products, mixed municipal solid waste and sewage sludge.
The degradation mechanism of biodegradable plastics in a composting environment is primarily hydrolysis combined with aerobic and anaerobic microbial activity. Composting technologies range from windrowing to in-vessel and trench composting.
In composting, material is turned or an air blower distributes air under the pile to maintain aerobic conditions for faster degradation, redistribute material and moisture and to maintain porosity of the compost. The mechanical stress imposed by turning the compost piles often facilitates the initial physical disintegration of plastic items to a primary dimension of about one inch. Typically for full degradation, composting occurs over a 10 to 12 week period. The degradation products of aerobic composting are compost and CO2.
The following pre-treatment steps are required before beginning the composting process:
Compost increases soil organic content. Humus results from the degradation of lignin, carbohydrate and proteins, and is also formed when many biodegradable plastics degrade. Humus is beneficial to the soil as:
The four main factors which are generally considered as fundamental in order to define the compostability are outlined below.
The physical persistence of a plastic material in compost derived product is unacceptable. The initial plastic material must not be physically recognisable in the final compost. The total disappearance of a plastic material can be achieved by disintegration, dissolving or melting.
In nature, organic materials are recycled through mineralisation, a process by which the carbon atoms of organic materials are incorporated into the microbial biomass and then, under aerobic conditions, oxidised to CO2. Compostable synthetic materials are expected to behave in the same manner. Their biodegradability is necessary to avoid accumulation of man-made materials in the soil. If this were not the case, composting would simply be the process of transformation of 'visible' solid waste into 'invisible' waste by its dissemination into the soil.
Synthetic polymeric materials should not release toxic compounds into the final compost in order to prevent negative effects on the environment, organisms and humans via the food-chain. This issue is strictly linked to chemical persistence.
The addition of biodegradable plastics to composting processes must not affect the quality of the final compost product. The assessment for the quality of compost should not be confused with toxicity (i.e. a compost can be a good plant fertiliser and yet be contaminated with toxic molecules).
Anaerobic digestion, using thermophilic microbes to produce methane and compost, is also gaining support as an alternative to landfills. Methane production may be faster, more efficient and more predictable in this system and a useful end-product, compost, is also produced.
Sewage is an excellent environment for biodegradation of biodegradable plastics as there is a preponderance of microbes and high levels of nitrogen and phosphorous. Activated sewage sludge will convert approximately 60% of a biodegradable polymer to carbon dioxide while the remaining 40% will enter the sludge stream where, under anaerobic digestion, it will be converted to methane.
Any biodegradable polymer that meets the compostability criteria will degrade even faster in a sewage environment. A rapidly degradable (or soluble) polymer that will not restrict sewer systems is needed if it is to be disposed through wastewater treatment plants.
Biodegradable plastics have the potential to be used in various flushable sanitary product applications such as sanitary towels, colostomy bags and other absorbent products. In addition, effluent from recycling activities must be treated in wastewater treatment plants and may contain biodegradable plastics.
Flushable biodegradable plastics can have an adverse impact on wastewater treatment plants due to the very fast throughputs, typically 4 to 8 hours, from the drain to the plant. This length of time is insufficient for many biodegradable plastics to lose their structure. Such plastics can foul the wastewater treatment plant primary screens. Ill conceived biodegradable plastics would only contribute to this problem. Flushable biodegradable plastics can be evaluated in a laboratory using activated sludge as the test medium.
It is to be expected that if biodegradable plastics began to occupy a significant market share of the plastics market in Australia that some material would end up in plastics reprocessing facilities. This could have significant effects on the sorting procedures required and the quality of recycled end products. These issues are covered in more detail in Section 9.
A recent study by Japanese researchers found that when conventional low-density polyethylene film was under bioactive soil for almost 40 years, the surface of the film showed signs of biodegradation with the molecular weight dropping by half the original. The inner part of the sample was almost unchanged with the molecular weight being retained (Ohtake, 1998).
Although the majority of biodegradable plastics are more easily degraded in composting and soil disposal environments than conventional plastics, environmentally degradable polymers could increase the capacity of landfill sites by breaking down in a relatively short time and freeing other materials for degradation, such as food scraps in plastic bags. Biodegradable plastics will not degrade appreciably in a dry landfill, however, unless they contain sensitisers and pro-oxidants (Garcia et al, 1998), which are transition metal catalysts such as manganese stearate or cobalt stearate.
Biodegradable plastics also make a contribution to landfill gas production, and in landfills where gas is collected for use as an alternative energy source this can be a positive outcome. Conversely however, where gas utilisation systems are not in place, the presence of biodegradable plastics will increase greenhouse gas emissions. Anaerobic microbes in the presence of water in the landfill will consume natural products and produce methane, CO2, and humus. Typical landfill gas contains 50% methane and 45% CO2, with the balance composed of water and trace compounds. To compliment landfill gas production, degradable polymers need to be consumed by anaerobic microbes to produce methane at rates comparable to those generated by degradation of natural products (i.e. lignin and cellulose in paper and garden waste).
Biodegradable plastic films can also be used for degradable landfill covers (see Section 6.6).
The rate of biodegradation in marine environments is affected by the water temperature. In cold waters, the plastic material may still be in a form that could endanger marine life for an extended period of time. In some initial trials carried out by Plastral Fidene, a starch-PCL blend was found to degrade in 20 weeks in Queensland waters and 30 weeks in South Australian waters;. the same sample was found to fully degrade in 20-30 days in a compost environment (pers. comm. W. Hall, Plastral Fidene, 2002). Thus seasonal and climatic effects on biodegradation rates need to be considered in relevant applications.
As biodegradable plastics have degradation behaviours based on particular environmental conditions, many biodegradable plastics may not degrade rapidly in the intestines of marine species, and injury is likely to remain an outcome (see Section 10.1). No studies have been done to date on the rate of degradation of biodegradable plastics in the gut of marine species and indeed ethics approvals for such work would be very difficult to obtain.
Plastic litter causes aesthetic problems as well as trauma to wildlife resulting from entanglement and ingestion of plastic packaging materials and lightweight bags. Wildlife losses are an issue for the conservation of biodiversity, and losses due to litter have caused public concern.
Biodegradable plastics should not be regarded as a panacea to the visible plastic litter problem. The breakdown of biodegradable plastics is not instantaneous, and at the least requires the presence of microbes in order to biodegrade. For instance, shopping bags hanging from branches above the waterline of streams and rivers is a much publicised form of plastic litter. The visual impact of this will not be lessened by biodegradable shopping bags since plastic in such environments is not exposed to microbes and may take more than a year to begin to decompose. Where photodegradable plastics are utilised, similar problems can occur where the conditions required for full degradation are not met.