testing for biodegradation

TESTING PROTOCOLS FOR BIODEGRADATION

Mrs.G.D.SHAH (M.E. POLYMER TECH.)
I/C HEAD OF PLASTICS ENGG.DEPT.
GOVT. POLYTECHNIC, AHMEDABAD
GUJARAT, INDIA.
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ABSTRACT


Enormous work is being carried out in the field of biodegradable polymers. Hence it would be beneficial for the researchers if standard test methods are available for the measurement of the degree of degradation. A range of international standards, and test methods are developed specifically for biodegradability. The main international organizations that have established standards or testing methods are listed and the important test methods are described as in the standards and book references.

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Laboratory test protocols are usually evaluation of environmental degradation under simulated conditions to which a particular polymer will be exposed on disposal. Correlation with real world exposure is more difficult for biodegradation than photo degradation because the environment for biodegradation widely differ in microbial composition, pH, temperature, moisture, etc. hence can not be readily reproduced. In early years, the only tests to establish biodegradability were related to the microbial growth, weight loss, tensile and other physical properties losses. These all are indirect measurements of biodegradation often leads to results that are difficult to reproduce from laboratory to laboratory, giving rise to confusion on the susceptibility to biodegradation of a given polymer.
STANDARD ORGANISATIONS FOR TESTING: There are number of international standards, and test methods, developed specifically for biodegradability, product safety, and also for compost derived products. The main international organizations that have established standards or testing methods are:
i. American Society for Testing and Materials (ASTM) (www.astm.org);
ii. European Standardization Committee (CEN) (www.cenorm.be);
iii. International Standards Organization (ISO) (www.iso.org);
iv. Institute for Standards Research (ISR),
v. German Institute for Standardization (DIN); and
vi. Organic Reclamation and Composting Association (ORCA) (Belgium).
AMERICAN SOCIETY FOR TESTING AND MATERIALS (ASTM) 1. Standard Practice for Determining Degradation End Point in Degradable Polyethylene and Polypropylene Using a Tensile Test D3826-98(2002)
Degradation-end point (a brittle point) is determined for degradable polyethylene/polypropylene films and sheeting less than 1.0 mm (0.04 in.) thick. It does not determine the rate and degree of degradation of a polyethylene/polypropylene film or sheet. Tensile properties of plastics 1.0 mm (0.04 in.) or greater in thickness is determined by Test Method D 638.
2. Standard Test Method for Determining Anaerobic biodegradation of Plastic Materials Under High-Solids Anaerobic-Digestion Conditions D5511-94
This test method covers the determination of the degree and rate of anaerobic biodegradation of plastic materials in high-solids anaerobic conditions. The test materials are exposed to a methanogenic inoculum derived from anaerobic digesters operating only on pretreated household waste. The anaerobic decomposition takes place under high-solids (more than 30% total solids) and static non-mixed conditions.
3. Standard Test Method for Determining the Aerobic biodegradation of Plastic Materials in an Activated-Sludge-Wastewater-Treatment System D5271-93
This test method measures the degree and rate of aerobic biodegradation of plastic materials (including formulation additives which may be biodegradable) on exposure to activated-sludge biomass in the concentration range from 0.1 to 2.5 g/L mixed-liquor volatile suspended solids (MLVSS) under laboratory conditions.
4. Standard Test Method for Determining Aerobic biodegradation in Soil of Plastic Materials or Residual Plastic Materials after Composting D5988-96
This test method determines the degree and rate of aerobic biodegradation of synthetic plastic materials (including formulation additives that may be biodegradable) in contact with soil, or a mixture of soil and mature compost, under laboratory conditions.
5. Standard Test Method for Determining the Aerobic biodegradation of Plastic Materials in the Presence of Municipal Sewage Sludge D5209-92
This test method covers the determination of the degree and rate of aerobic biodegradation of synthetic plastic materials (including formulation additives that may be biodegradable) on exposure to activated-sewage sludge inoculums under laboratory conditions.
6. Standard Test Method for Determining the Aerobic Biodegradability of Degradable Plastics by Specific Microorganisms D5247-92
This test method describes the procedures required to carry out a pure-culture study for evaluating the biodegradation of degradable plastics in submerged culture under aerobic conditions. Degradation will be evaluated by weight loss, tensile strength loss, percent-elongation loss and changes in molecular-weight distribution.
7. Standard Test Method for Determining Weight Loss From Plastic Materials Exposed to Simulated Municipal Solid-Waste (MSW) Aerobic Compost Environment D6003-96
This test method determines the degree and rate of aerobic biodegradation of plastic materials exposed to a controlled composting environment. Aerobic composting takes place in an environment where temperature, aeration, and humidity are closely monitored and controlled.Biodegradability of the plastic is assessed by determining the amount of weight loss from samples exposed to a biologically active compost relative to the weight loss from samples exposed to a "poisoned" control.
8. Standard Practice for Exposing Plastics to a Simulated Active Landfill Environment D5525-94a
The plastic is exposed to a specific test environment. The test environment is a laboratory-scale reactor that simulates a landfill with enhanced biological activity. Biological activity is enhanced by adding moisture, recirculating leachate, and heating to 35°C. Plastic exposure occurs in the presence of a media undergoing anaerobic degradation. The standard media used in the practice simulates a municipal solid-waste stream.
9. Standard Test Method for Determining Anaerobic biodegradation of Plastic Materials under Accelerated Landfill Conditions WK9579 Revision of D5526-94(2002)
Landfills begin as aerobic environments and transform into anaerobic environments over time and as they are filled. Hence a need to have a biodegradation standard for both environments. Also, there is a need to measure the degree of degradation / disintegration in the aerobic phase over time which will correlate with the packing of landfill sites and volume availability.
10. New Standard Specification for Determining the Biodegradability of Plastics. Under Accelerated Anaerobic Conditions WK10726
The extent of biodegradation of plastics under anaerobic conditions is determined. The degradation of these materials should not introduce any hazardous chemicals into the landfills that may pose a risk in the future.
11. GROWTH RATINGS G-21-70 AND GZZ-76
These tests are developed for assessing the resistance of plastics to fungal and bacterial growth. Fungi like aspergillus niger,pennicillium at 28-30°C temperature at 85% RH for 21 days is used. Bacteria like pseudomonas aeruginosa incubated at 35-57°C for minimum 21 days are used. After suitable time the growth is assessed in terms of % surface covered. The test gives quick results. It is easy to do and give indication of biodegradation. But it is not conclusive for biodegradation of polymer.
INTERNATIONAL STANDARDS ORGANIZATION (ISO) (WWW.ISO.ORG): (4, 6, 7, 8, 10, 13, 14)
Three International Standards Organization (ISO) standards have set the criteria by which
European biodegradable plastics are currently assessed. These are:
i. ISO 14855 (aerobic biodegradation under controlled conditions);
ii. ISO 14852 (aerobic biodegradation in aqueous environments); and
iii. 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 it will be ultimately used. Either a controlled pilot-scale test or a test in a full-scale aerobic composting treatment facility can be used. Due to the nature and conditions of such disintegration tests, the tests cannot differentiate between biodegradation and abiotic disintegration, but instead demonstrates that sufficient disintegration of the test materials has been achieved within the specified testing time.
ISO 846 (1978):
It specifies the use of a mixture five strains of fungi over a period of at least 28 days at 30 ±2°C and 95-100% RH. The results of attack are measured by visual examination for growth.
Method A is used to determine the ability of plastics to act as the carbon and nitrogen source for the growth of microorganisms.
Method B is used to determine the fungi toxic properties of plastics.

COMPOST TOXICITY TESTS
For a comprehensive assessment of toxicity associated with compost applications, plastics can be tested on both plant and animal species. A number of polyester types were tested including a plasticized cellulose acetate, an aliphatic polyester, polyhydroxybutyrate-co-hydroxyvalerate and polycaprolactone. Cell culture medium with serum was used as the extraction medium.
PLANT PHYTOTOXICITY TESTING
While a product may not negatively impact plant growth in the short term, over time it could become phytotoxic due to the build-up of inorganic materials, which could potentially lead to a reduction in soil productivity. For this reason some manufacturers use plant phytotoxicity testing on the finished compost that contains degraded polymers. Phytotoxicity testing can be conducted on two classes of flowering plants. These are monocots (plants with one seed leaf) and dicots (plants with two seed leaf). Representatives from both of these classes are typically used in toxicity testing - summer barley to represent monocots and cress to represent dicots. Tests involve measuring the yield of both of these plants obtained from the test compost and from control compost.
ANIMAL TOXICITY TEST
Animal testing is generally carried out using earthworms (as representative soil dwelling organisms) and Daphnia (as representative aquatic organisms). Earthworms are very sensitive to toxicants. Since earthworm feeds on soil, they are suitable for testing the toxicity of compost. In the acute toxicity test, earthworms are exposed to high concentrations of the test material for short periods of time. Earthworms are exposed to soil and compost in varying amounts. Following 14 days of exposure, the number of surviving earthworms is counted and weighed and the percent survival rate is calculated. The earthworms are exposed to several mixture ratios of compost and soil mixtures.
Compost worms are used for testing the toxicity of biodegradable plastic residues. These worms are very sensitive to metals such as tin, zinc, and heavy metals and high acidity. For this test worms are cleaned and accurately weighed at intervals over 28 days. The compost worm toxicity test is considered to be an accurate method. The toxicity test can establish whether degradation products present in liquids pose any problem to surface water bodies. In the test, Daphnia are placed in test solutions for 24 hours. After exposure the number of surviving organisms is counted and the percent mortality is calculated.
SOIL BURIAL TEST BS 4618 SEC.4.5 1974
Soil burial is a traditional way to test samples for degradation because of its similarity to actual conditions of waste disposal. It lacks reproducibility because of the difficulties in controlling climatic factors and the population of various biological systems that are involved. Generally the samples are buried in soil for periods of up to two years. At the end of the resting period, changes in properties like loss in weight, mechanical strength, shape etc. are studied. It provides qualitative indication of biodegradation.

DIFFERENCE BETWEEN STANDARDS FOR BIODEGRADATION
The main point of differentiation between the various international standards is the percentage of biodegradation required for compliance. This is an important issue that is under discussion at ISO level. The compliance requirements for the key standards are shown in Table 6.1.