Biodegradability and Certifi cation Procedures
5.3 Norms
5.3.2 Compost Biodegradation Tests
5.3.2.1 Controlled Composting Test
In the fi rst years of the existence of biodegradable plastics, it became clear that the aerobic, aquatic biodegradation tests were not appropriate to evaluate the biodegradation of these polymers in composting. The environmental conditions are of course very different:
high temperatures up to 60-65 °C in composting as opposed to ambient temperature in water, different moisture content, etc. An important difference is also the activity
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155 of fungi and actinomycetes. Whereas in water these organisms can be detected but are not really active, in compost they are dominantly present and very active. It has been known for a long time that fungi can degrade some materials much better and faster than bacteria. The biodegradation of lignin by white rot fungi is a well-known example [37, 38]. This observation led to the development of a novel biodegradation method in which a composting process was simulated as good as possible while still measuring the biodegradation based on carbon conversion very precisely [39, 40]. This test procedure became fi rst standardised at ASTM level: ASTM D5338-92 [41]. Later, and after a few modifi cations it became adopted at ISO level in 1999 as ISO 14855 [42].
• ISO 14855 – Determination of the ultimate aerobic biodegradability and disintegration of plastic materials under controlled composting conditions - Method by analysis of evolved carbon dioxide
Principle: The test item is mixed with mature compost and incubated under batch conditions at 58 °C under optimum oxygen and moisture conditions. The mature compost acts at the same time as the carrier matrix, the source of microorganisms and the source of nutrients.
The mixture is continuously aerated with carbon dioxide-free air. The exhaust air is analysed for carbon dioxide. A schematic layout of the test is given in Figure 5.4.
Figure 5.4 Schematic layout of controlled composting test
1 2 3
4
6
5
9 7 8
With 1 = air 4 = headspace 7 = CO2 removal system 2 = CO2 free air 5 = test compost mixture 8 = composting vessel 3 = exhaust air 6 = NaOH solution 9 = CO2 determination system
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The maximum test duration is six months while a typical minimum duration is 45 days.
The carbon dioxide produced during the composting is measured continuously or at regular intervals. After subtracting the background carbon dioxide production from the blank compost inoculum (without any extra carbon source addition), the percentage of biodegradation is determined by the net amount of carbon of the test item that is converted to carbon dioxide. Also the rate of biodegradation can be established. A positive reference control, cellulose, is tested in parallel to check the activity of the inoculum. Strict requirements are imposed on the results for cellulose to validate the test. The test item is preferably added in the form of fi ne powder. However, when fi lm samples or formed products are added, the test procedure also allows an evaluation of the disintegration under composting conditions.
In Figure 5.5 an example is given of a carbon dioxide biodegradation curve of the blank compost inoculum and the cellulose positive reference, each in three replicates. From the difference in carbon dioxide the net biodegradation can be calculated and graphically represented as in Figure 5.6.
The ISO procedure was adopted in a new edition of the ASTM standard, ASTM D5338-98e1 [41] and can also be found back in the European norm EN 14046 [43] which is expanding the fi eld of application from plastics to all possible types of packaging materials.
In Japan, the ISO 14855 was adopted without changes as JIS K 6953 [44].
Figure 5.5 Carbon dioxide evolution curve
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157 5.3.2.2 Mineral Bed Composting Test
The two major types of tests for determining the biodegradation of plastics each show some specifi c advantages and specifi c disadvantages. The aquatic tests allow not only a measurement of biodegradation through carbon dioxide production or oxygen consumption but also the measurement of biomass and (dissolved) metabolites. However, the aquatic environment is a poor simulation of a composting environment and biodegradation is much less aggressive. In contrast, the controlled composting test is a much better simulation of the composting environment and shows a high fungal activity. Yet, because of the complex compost matrix a precise measurement of metabolites and biomass is not possible.
In an effort to combine the possibilities and advantages of both types of tests, a novel test method was developed in which the compost matrix is replaced by a mineral, inert medium [45, 46]. In this novel test procedure the carrier matrix consists of vermiculite which is a type of expanded clay mineral with an overall physical structure and water holding capacity behaviour very similar to compost. This medium is inoculated with a compost extract and brought to the right moisture content by the addition of a mineral medium, which at the same time is introducing the necessary nutrients. The physical structure and the compost eluate inoculum initiate a microbiological (aggressive) activity similar to the activity in a real composting pile. At the same time the inert matrix permits the extraction and determination of metabolites and biomass.
The absence of an extra carbon source also prevents the phenomenon of a ‘priming effect’
in a mineral bed test. The addition of a readily degradable and energy-rich test material in a conventional controlled composting test sometimes results in an extra stimulated activity
Figure 5.6 Biodegradation curve
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of the compost matrix and a higher background production of carbon dioxide compared to the blank compost reactors. As a result the net carbon dioxide production in the test reactors is overestimated and a biodegradation percentage above 100% can be obtained.
The absence of an extra carbon source under the form of compost in the mineral bed test prevents this extra activity or priming effect.
The novel method using the vermiculite, mineral inert matrix has been proposed at the ISO TC61/SC 5/WG 22 as an amendment to the ISO 14855 [42].
• ISO 14855 Amendment 1: Use of a mineral bed instead of mature compost
At the end of 2001 the proposal is at the stage of a draft amendment (‘DAM 1’). Some further research and development is still needed but it is expected that the new method will be an offi cial ISO standard within a short term.
5.3.2.3 Other Compost Biodegradation Tests
Another, more drastic approach to improve the precision of the measurement of CO2 production and biodegradation and ascertain that effectively the CO2 derived from a test material is determined, is by the use of 14C radiolabelled test material. An aerobic aquatic test procedure as well as an aerobic, composting test procedure using such material in which the production of 14CO2 is measured by absorption and liquid scintillation counting has been developed in the ASTM subcommittee D20.96 on degradable plastics and has been published as ASTM D6340-98 [47].
• ASTM D6340-98 - Standard Test Methods for Determining Aerobic Biodegradation of Radiolabelled Plastic Materials in an Aqueous or Compost Environment
Besides norms for biodegradation tests in which the biodegradation is measured directly, some exposure tests have also been standardised, e.g., ASTM D5509-96 [48] and ASTM D5512-96 [49]. In these tests plastic test items are exposed to composting conditions and these conditions are precisely defi ned. After a certain period of incubation a loss of property is measured, such as weight, molecular weight, tensile strength, tear resistance, etc. It may be clear that these parameters are only secondary results of biodegradation and are no proof of a complete biodegradation and mineralisation as shown in Equation 5.1 of Figure 5.3.