Methods of Biodegradability Measurements

A variety of international standards and test methods have been created for biodegradability, product safety, and compost-derived products. The primary difference among these standards is the percentage of biodegradation necessary for adherence.

The main international organizations that have established standards or testing methods are:

  • American Society for Testing and Materials ( ASTM ),
  • European Committee for Standardization ( CEN ),
  • International Standards Organization ( ISO ),
  • Institute for Standards Research ( ISR ),
  • German Institute for Standardization ( DIN ),
  • Organic Reclamation and Composting Association ( ORCA ) (Belgium).

A family of ASTM standards addresses physical property deterioration under a variety of specific environmental conditions, including:

  • Simulated composting (D5509, D5512)
  • Simulated landfill (D5525)
  • Aerobic microbial activity (D5247)
  • Marine floating conditions (D5437).

A second group of ASTM standards addresses CO2 generation in aerobic environments, including:

  • Sewage sludge (D5209)
  • Activated sewage sludge (D5271)
  • Controlled composting (D5338).

A third group of ASTM standards addresses CH4/CO2 evolution in anaerobic environments, such as:

  • Anaerobic sewage sludge (D5210)
  • Anaerobic biodegradation (D5511)
  • Accelerated landfill (D5526).

D6400 differentiates between biodegradable and degradable plastics.

ASTM D5338-98 (Composting)

For biodegradable plastic materials to be accepted in composting plants, both biodegradability and disintegration are important.

Disintegration is the physical falling apart of the biodegradable plastic material, or more precisely of the product that has been made from it, into fine visually indistinguishable fragments at the end of a typical composting cycle.

Compostable material is understood to be a material in which:

  • the polymer chains break down under the action of microorganisms (bacteria, fungi, algae);
  • total mineralization is obtained (conversion into CO2 , H2O, inorganic compounds, and biomass under aerobic conditions); and
  • the mineralization rate is high and is compatible with the composting process.

Those materials showing a degree of biodegradation equivalent to that of cellulose (maximum permissible tolerance of 5%) are considered to meet the compostability criteria under these standards.

To measure compostability, the ASTM in the United States has developed the key standard ASTM D5338 – 98. This is a standard test method for determining aerobic biodegradation of plastic materials under controlled composting conditions.

In this method, the plastic is mixed with stabilized and mature compost derived from the organic fraction of municipal solid waste. The net production of CO2 is recorded relative to a control containing only mature compost.

After determination of the carbon content of the test substance, the percentage biodegradation can be calculated as the percentage of solid carbon of the test substance that has been converted into gaseous carbon in the form of CO2. In addition to carbon conversion, disintegration and weight loss can be evaluated.

To meet the ASTM D5338 – 98 standard, 60% of the original mass of a single-polymer material must mineralize in six months, whereas 90% must do so in a blend.

Materials should give way to intense microbial activity and be converted from carbon into carbon dioxide, biomass, and water. Materials should also begin to fragment, at which point disintegration begins. In this phase, the material must completely physically and visually disintegrate.

Ninety percent of the disintegrated material must not adversely affect the quality of the compost. Finally, even after land application, remaining materials should be safely converted into carbon dioxide by microorganisms. The resultant compost should not be toxic and should not deter plant growth.

ISO CD 14855 and the CEN test procedures are similar to ASTM D5338 – 98. The only difference is the temperature profile, which in both the ISO and the CEN procedures is continuously at 58°C, whereas in the ASTM test it follows a temperature profile of 35 – 58 – 50 – 35°C.

ASTM D5209-92 (Aerobic, Sewer Sludge)

The Sturm test (ASTM D5209 – 92) was developed to measure the biodegradability of both water-soluble and insoluble compounds in an aquatic environment. As with ASTM D5338 – 98, these tests are based on the measurement of CO2 produced during biodegradation of the plastic test material. The percentage biodegradation is calculated from the CO2 produced by the plastic sample in relation to the total theoretical amount.

Continuous aeration ensures that there is sufficient oxygen in the bioreactor at all times. The measurement of carbon dioxide evolved during degradation gives direct information on the bioconversion of the carbon backbone of the polymer to metabolic end products. A 10 mg plastic sample is used in the test. For this reason, it gives an artificially high breakdown rate. Although the measurement of evolved CO2 is very accurate, the rate of biodegradation can have an error of up to 80%.

The Technical Research Center of Finland (VTT) has developed a “headspace test” that, like the Sturm test, is also based on the measurement of carbon dioxide under aerobic conditions. The VTT test is performed in “headspace” bottles (volume 125 mL) containing 50 mL of a mineral nutrient medium and sewage sludge.

The benefits of the VTT test are its simplicity and the capacity for an extensive number of samples to be processed simultaneously, thereby allowing statistical evaluation. The carbon dioxide evolved during biodegradation is determined from the gas and liquid phases at weekly intervals.

ASTM D5210-92 (Anaerobic, Sewage Sludge)

Anaerobic degradation is biodegradation in the absence of oxygen. A mixed population of microorganisms is needed for complete degradation of the polymer. In the first stage, acidogenic bacteria convert organic substances into components of lower molecular weight, such as alcohols and short-chain fatty acids. Subsequently, acetogenic bacteria further degrade these substances into acetone, carbon dioxide, and hydrogen. In the final phase, methane and carbon dioxide are the end products of anaerobic degradation.

The standard test method for determining the anaerobic degradation of plastic materials is ASTM D5210 – 92. This test measures the amount of biogas released during polymer biodigestion by microorganisms. The biodegradation percentage is the ratio of biogas produced by the test sample in relation to the theoretical amount produced in the case of complete mineralization.

ASTM D5511-94 (High-solids Anaerobic Digestion)

The standard test method for determining the anaerobic biodegradation of plastic materials under high-solids anaerobic digestion conditions is ASTM D5511 – 94. This method evaluates the innate biodegradability of plastic in an anaerobic solid waste digester or a sanitary landfill under optimal conditions. The test measures the total biogas volume produced per unit weight of the sample. The percentage of biodegradation can be determined by calculating the amount of solid carbon in the sample that has been transformed into gaseous carbon in the form of CH 4 and CO 2, based on the carbon content of the test material.

Tests for Specific Disposal Environments

Tests are being developed to evaluate the generation and quality of humic substances in landfills, composting, and aquatic environments. Aerobic composting with activated vermiculite allows the recovery and quantification of polymeric residues, enabling more accurate determination of carbon balances and assessments of toxic compound generation and humus quality.

The disintegration properties of biodegradable plastics can be measured in pilot-scale or full-scale composting tests. The test substance undergoes a spontaneous composting process for 12 weeks. At the end of the process, sieving over 2 mm followed by precise sorting analysis is conducted. A bioplastic may pass the test at a specific thickness but fail at a higher thickness.

It is also important to demonstrate that bioplastic materials have no negative effect on compost quality. This involves chemical analyses, such as heavy metal evaluations, as required in the assessment of conventional compost quality. Additionally, ecotoxicity tests, including both plant germination tests and animal toxicity tests, are recommended. These tests ensure that small quantities of additives from plastics do not adversely affect compost quality.

International Standards Research

The Institute for Standard Research (ISR) has conducted studies on the performance of biodegradable plastics in both laboratory conditions and composting facilities. ISR has identified three criteria that plastics must meet in order to be considered compostable:

  • They must biodegrade at the same rate and to the same extent as known compostable materials such as garden waste and paper, and leave no persistent or toxic residues.
  • They must disintegrate during active composting so there are no visible or distinguishable fragments found on the screens.
  • They must display no ecotoxicity or phytotoxicity that might impact on the ability of the compost to support plant growth.

Three International Standards Organization (ISO) standards have set the criteria by which European biodegradable plastics are currently assessed. These are:

  • ISO 14855 (aerobic biodegradation under controlled conditions);
  • ISO 14852 (aerobic biodegradation in aqueous environments); and
  • ISO 15985 (anaerobic biodegradation in a high solids sewerage environment).

ISO 14855 is a controlled aerobic composting test, and ISO 14851 and ISO 14852 are biodegradability tests specifically designed for polymeric materials. An important part of assessing biodegradable plastics is testing for disintegration in the form in which they will ultimately be used.

Either a controlled pilot-scale test or a test in a full-scale aerobic composting treatment facility can be used. Because of the nature and conditions of such disintegration tests, the tests cannot differentiate between biodegradation and abiotic disintegration. However, they demonstrate that sufficient disintegration of the test materials has been achieved within the specified testing time.

The European Committee for Standardization (CEN) established the norm standard (CEN EN 13432) in 1999. The norm provides the European Commission’s European Directive on Packaging and Packaging Waste with appropriate technical regulations and standards. This norm is a reference point for all European producers, authorities, facility managers, and consumers.

The standard specifies requirements and procedures to determine the compostability of plastic packaging materials based on four main areas:

  • biodegradability,
  • disintegration during biological treatment,
  • effect on the biological treatment process, and
  • effect on the quality of the resulting compost.

Importantly, the packaging material that is intended for entering the bio-waste stream must be “ recognizable ” as biodegradable or compostable by the end-user.

Standard EN 13432 – Proof of Compostability of Plastic Products

Plastic products can demonstrate their compostability by meeting the harmonized European standard EN 13432. Compliance with recovery directives in reference to the European Packaging Directive 94/62 EC is based on this standard.

Scope of testing under EN 13432:

  • Chemical test: Disclosure of all constituents, and threshold values for heavy metals are to be adhered to.
  • Biodegradability in watery medium (oxygen consumption and production of CO2 ): Proof must be given that at least 90% of the organic material is converted into CO2 within six months.
  • Disintegration in compost: After three months ’ composting and subsequent sifting through a 2mm sieve, no more than 10% residue, relative to the original mass, may remain.
  • Practical test of compostability in a semi-industrial (or industrial) composting facility: No negative influence on the composting process is permitted.
  • Compost application: Examination of the effect of resultant compost on plant growth (agronomic test), ecotoxicity test.

The maximum thickness of a plastic material suitable for composting is determined by passing all of the tests required in standard practice composting operations. Passing individual tests is not sufficient. The European standard test methods are based on ISO standards 14851, 14852 (aerobic degradability in water), 14853 (anaerobic degradability in water), and 14855 (aerobic composting). The tests must be conducted by accredited laboratories. If a plastic product is advertised as “compostable” or “biodegradable,” European Bioplastics requires it to be approved according to EN 13432. The association has published information on so-called “degradable” or “oxo-degradable” plastic products since these terms are not always used correctly. Producers have voluntarily committed to product certification, which has been recognized by the European DG Enterprise.

The CEN 13432 is the most rigorous European standard for biodegradability. It can be applicable to packaging materials other than polymers, and includes the following tests and standards:

  • ISO 14855,
  • ISO 14855 (respirometric),
  • ISO 14852,
  • ASTM D5338 – 98,
  • ASTM D5511 – 94,
  • ASTM E1440 – 91,
  • modified OECD 207, and
  • CEN TC 261/SC4/WG2.

In order for a material to meet the standard, it cannot persist for more than six months under any of the aforementioned test conditions and must achieve a pass rate of at least 90%. Additionally, the material must not contain contaminants exceeding 50% of that found in “normal” compost, including Zn at 150 ppm, Cr at 50 ppm, Cu at 50 ppm, Mo at 1 ppm, Ni at 25 ppm, Se at 0.75 ppm, Cd at 0.5 ppm, As at 5 ppm, Pb at 50 ppm, F at 100 ppm, and Hg at 0.5 ppm, as outlined in EN 13432.

Other Standards

Several countries have their own certification programs for compostable plastics.

In Canada, the BNQ certification program for compostable plastic bags was established in September 2007. The program aims to make it easy for consumers and composting facilities to identify bags that are compostable. The certified products bear a mark shared between the BNQ and the Composting Council of Canada.

In Australia, standards based on EN prescribe procedures for determining biodegradability and disintegration of plastics using different methods, such as AS 4736-2006 for biodegradable plastics suitable for composting and other microbial treatment, AS 4351.1-1996 for determining and reporting biodegradability of organic compounds in an aqueous medium, and AS 4351.6-1996 for the determination of biodegradability of poorly soluble organic compounds. Certified compostable products are also subject to limited labeling requirements.

“OK Compost” Certification and Logo

The “OK Compost” logo indicates that a material is 100% compostable and biodegradable and can be used on the labeling of biodegradable plastics and other materials. AVI owns and manages the logo based on the CEN 13432 standard.

For certification, all laboratory tests must be performed on materials, intermediates, and additives to verify their chemical properties, ultimate biodegradability, and disintegration properties. Chemical testing is performed to ensure that harmful substances such as polychlorinated biphenyls (PCBs) and dioxins, as well as heavy metals such as lead, mercury, and cadmium, do not pass into the soil through the compost.

The testing methods specified for biodegradability and disintegration verify the complete degradation of materials within the processing period of normal composting plants. Additionally, an ecological nontoxicity test is prescribed to ensure that the plastics used do not have any adverse effect on the quality of the compost. The maximum compostable layer thickness is also determined.

If the results of the tests conform to the standard(s) and/or the certification scheme, the material, intermediate, or additive is registered and included in a positive list. Products made from registered materials, intermediates, and additives can be certified if they meet the maximum compostable layer thickness of the used materials or intermediates. Verification tests are performed to confirm that the same base materials as those declared on the certification application are being used. For this purpose, infrared spectra are recorded and compared.

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