Biodegradable Plastics from Waste Hamper Plant Growth in Compost

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A recent study by Polish scientists which found that plant growth may be hampered by composts that contain biodegradable plastics could have implications for waste management practices within the EU, according to the European Commission.

The Commission explained that while standards on the use and quality of compost exist in most EU Member States, differences in soil policy mean that these standards vary and only a few Member States allow compost production from mixed waste.

EU Commission Biodegradable Plastics from Waste Hamper Plant Growth in CompostThe need for quality criteria to ensure environmentally-safe compost is highlighted in the Waste Framework Directive (2008)1, and a Green Paper on the management of bio-waste in the EU2, produced in response to this Directive, states that separate collection of bio-waste is a successful waste management practice for producing high quality compost.

According to the Commission a total of 13.2 million tonnes of compost was produced in the EU in 2005 by around 3500 composting facilities - 1.4 million tonnes of this was from mixed waste.

Research

As part of the study, which was partly-funded through the EU’s Regional Development Fund, researchers from a number of Polish universities investigated the effects of composts containing biopolymers on seed germination and plant root growth, as well as their effects on other soil organisms in a series of experiments over a six-month period.

The scientists tested five samples of composts containing different combinations of three biopolymers: polyethylene C, thermoplastic maize starch and a compatibiliser, which varied in their starch content and density. A total of 92% of the compost’s content was plant material (wheat straw, rapeseed straw and pea waste), and 8% from the biopolymers.

According to the Commission the results revealed that seed germination of garden cress, white mustard and sorghum species was inhibited more than twice as much when the applied composts contained biopolymers.

The greatest negative effect on germination was seen with a compost mix in which polyethylene made up 65% of the biopolymer content. Root growth was also found to be twice as likely to be hindered.

Toxicity

The researchers also found that biopolymer-containing composts strongly inhibited growth of Heterocypris incongruens, an organism often used in scientific tests to indicate the toxicity of composts and sewage sludge. These composts also reduced the growth of Vibrio fischeri bacteria.

From the results, the Commission said that all of the composts containing biodegradable polymer materials could be classified using a risk assessment system3 at a higher toxicity level (class III – presenting an acute hazard) than those without (class II – presenting a low acute hazard).

Further testing was said to have demonstrated that composts containing biopolymers did not prevent compost maturing. Mature compost is more stable and useful to plants, because more nutrients are made available by soil microorganisms. However, the presence of biopolymers diluted the dry mass of solid organic matter in the compost which may limit its usefulness.

Current legislation in Poland (The Decree of the Minister of Agriculture and Rural Development (2008)) stipulates that composts to be used as organic fertilisers contain a minimum of 30% dry mass of solid organic matter.

With regard to the study’s findings the Commission said that it is important to assess the quality and potential environmental risks associated with composts prepared from different wastes.


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