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Biodegradable bag standards

Posted: 17.04.09  |  Created by: Do Something


When is a plastic bag biodegradable or not? Do Something’s biodegradable plastics advisor, Dr Ranjith Jayasekara, takes us through the testing that bags need to pass before they can truly claim to be biodegradable.
There is a growing demand for environmentally responsible bags to replace the traditional polyethylene based plastic check-out bags. Reusable bags are the best alternative but when it comes to single-use bags, biodegradable bags are considered the next best alternative. Once disposed, they tend to disintegrate and eventually biodegrade in the environment.

Traditionally, the best known biodegradable bags have been paper bags. However, many plastic bags available in the market today claim to be biodegradable and/or compostable.  Unfortunately, many of those claims are often unsubstantiated. This leads to misleading claims, misled retailers and consumers and potential action by the ACCC.  

Therefore, it is necessary to have standards to assess plastic bags against their claims – standards that help to identify those plastic bags which are truly biodegradable and compostable.

The Australian standard

If a plastic bag claims to be biodegradable, it needs to have passed Australian standard AS 4736-2006. This standard provides an assessment criteria for plastic bags that are to be biodegraded in municipal and industrial aerobic composting facilities. This Australian standard is similar to the better known European EN 13432 standard, but has an additional requirement of a worm test.
In order to comply with the AS 4736-2006 Australian biodegradability standard, plastic bags need to achieve or meet the following requirements:

  • minimum of 90% biodegradation of plastic bags within 180 days in compost
  • minimum of 90% of plastic bags should disintegrate into less than 2mm pieces in compost within 12 weeks  
  • no toxic substances should be formed during the composting of plastic bags
  • hazardous substances such as heavy metals should not be present in plastic bags above the accepted levels
  • bag should contain more than 50% organic materials.

Biodegradability is an essential pre-requisite for compostability. A compostable bag is designed to biodegrade in a composting process. The ultimate fate of most plastic bags is disposal into municipal solid waste streams and therefore, their biodegradability is best determined by an aerobic composting test.

To claim compostability, plastic bags must demonstrate their biodegradation in mature compost and disintegration in one composting cycle. Also the resulting compost must meet the compost quality criteria.  

What questions should retailers ask of companies who claim that their bags are biodegradable or compostable?

Retailers should look for written evidence for compliance to the AS4736 standard such as a test report or a letter from a testing body. An example would be certification from a third party such as AIB Vincotte (OK compost), BPI (compostable) or DIN Certco (logo). Currently there is no certification body in Australia but one should be set up by June 2009.

Other additional things to ask for:

  • if the plastic bag complies to the EN 13432 standard, retailers should look for additional evidence that the bag bears no toxicity threat to earthworms once it is composted
  • the thickness of the plastic bag material being bought by the retailer should be equal or lower than the thickness used for the disintegration test. Retailers need to ensure that they’re buying the same material that has been tested
  • carefully check whether ink and pigments etc. have been included in the original report. If not ask for additional evidence for toxicity testing.

The standard in detail

All plastic bags that comply with AS 4736-2006 - and which are labelled as compostable - must meet the requirements specified under the following categories listed in the AS 4736-2006 standard.   

  • i) Characterisation
  • ii) Biodegradability
  • iii) Disintegration
  • iv) Compost quality
  • v) Recognisability

i) Characterisation

(a) Thickness
– the maximum thickness of the plastic bags should be equal or lower than the thickness of the plastic bag material that was used for disintegration test.

Disintegration rate decreases as thickness of the bags increases. That means bags made out of thick films will disintegrate slowly and won’t necessarily pass the disintegration test, even though bags made out of thinner films of the same material comply with the standard.

Therefore thickness of the bag being bought should be checked against the thickness of the certified bags.  

(b) Volatile solid content – this indicates the amount of organic matter in the material. It requires a minimum of 50% volatile solids. Put simply, more than 50% of the bags should be burnt out at elevated temperatures in the presence of oxygen.

(c) Heavy metals - The maximum allowable heavy metal contents of the plastic bags as measured in mg/kg on a dry basis are as follows.

mg/kg dry basis
Zinc (Zn)
Copper (Cu)
Nickel (Ni)
Cadmium (Cd)
Lead (Pb)   
Mercury (Hg)
Chromium (Cr)   
Molybdenum (Mo)
Selenium (Se)
Arsenic (As)
Fluorine (F)

ii) Biodegradability

A minimum of 90% of the plastic bag must biodegrade within 180 days when tested in compost. Testing has to be carried out according to the standard test method specified in AS 4736. Out of the few methods recommended, the most appropriate and logical one is AS ISO 14855 as this test relates to composting. Alternatively ISO 14851 or ISO 14852 can be used, if required. The test should be performed in such a way that all validity criteria specified in the test method must be met, otherwise the test should be disregarded.   

iii) Disintegration

This test measures the susceptibility of plastic bags breaking down into less than 2mm plastic fractions within a 12 weeks composting cycle. The recommended method to test disintegration is ISO 16929. More than 90% of the plastic bags should disintegrate into less than 2mm pieces within 12 weeks in one composting cycle for the bags to pass this test.

iv) Compost quality

(a) Assessment of negative effect

The presence of plastic bags in solid waste can influence the composting process as well as the resulting compost. Therefore, any potential negative effect of plastic bags, either on the composting process or the resulting compost, should be assessed based on the best available evaluation methods.  

Physical and chemical parameters of the composting process and the resulting compost - with and without addition of plastic bags - are to be measured and compared for any significant deviation of corresponding values.

Parameters such as total dry solids, volatile solids, salt content, pH, bulk density and presence of total nitrogen, ammonium nitrogen, phosphorus, magnesium and potassium are required to be determined according to AS 4454 standard guidelines. These must then be compared for any significant difference of two data sets. To pass the test, the difference should be insignificant.   

(b) Eco-toxicity

Effect on plant seedlings

During the degradation process, plastic bags can form different chemicals (metabolites) which are retained in the resulting compost. Those components may cause adverse effects on seedling emergence and early growth of higher plants.

Therefore, any toxicity effect in the resulting compost on seedling emergence and early growth of higher plant needs to be assessed based on the procedure specified in EN 13432 Appendix E (this is the European plastic bag biodegradability standard).

Percentage seedling emergence and biomass production in composts - prepared with and without addition of plastic bags - are calculated and compared to check whether there is any significant difference between two data sets. The difference should be statistically insignificant to pass the test.

Effect on earthworms

Fractions of plastic bags and their metabolites in the resulting compost can also have an adverse effect on the reproduction and survival of earthworms.

In order to assess this effect, Australian standard AS 4736 requires an additional test on earthworms. This test is not required by plastic bags that are tested to the European EN 13432 standard. This is why many environmentalists believe that the Australian AS 4736 standard is better than the European EN 13432 standard.

This is the only difference between those two standards but it is an important one. Toxicity on earthworms is assed by ASTM E 1676 and less than 10% difference between two data sets is required to pass this test.  

v) Recognisability

There should be a clear labelling on the bags indicating biodegradability or compostability. Such claims should be designed to help end users identify and differentiate the bags from other products of similar appearance.

The Australian AS 4736-2006 standard helps retailers and the public to identify those bags which are truly compostable. Plastic bags which don’t meet this standard are not recommended by the ‘Ban The Bag’ campaign for use in Australian stores.

Dr Ranjith Jayasekara is the Do Something advisor on biodegradable plastics.

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