Anaerobic biodegradation testing of degradable plastics evaluates how polymeric materials decompose in oxygen-free environments by measuring biogas (methane and carbon dioxide) production under controlled laboratory conditions. This testing is essential for manufacturers seeking certification, regulatory compliance, and scientific validation of environmental claims for biodegradable products. This article provides a comprehensive technical overview of test principles, applicable standards, sample requirements, testing procedures, and typical applications.
Technical Overview
Anaerobic biodegradation testing is a laboratory-based evaluation method used to determine the ultimate biodegradability of plastic materials in oxygen-free environments. In these tests, plastic samples are introduced into sealed vessels containing an anaerobic inoculum, typically derived from actively digesting sludge from wastewater treatment plants or industrial anaerobic digesters. Microorganisms within the inoculum metabolize the organic carbon in the test material and convert it into biogas, which consists primarily of methane (CH4) and carbon dioxide (CO2).
The extent of biodegradation is quantified by comparing the cumulative amount of carbon converted to biogas against the theoretical carbon content of the test material. The result is expressed as a percentage of biodegradation, which indicates how completely the polymer is mineralized under anaerobic conditions. This testing approach simulates real-world disposal scenarios such as anaerobic digesters, deep landfill layers, and sludge treatment systems, where oxygen is absent or severely limited.
Unlike aerobic composting tests, anaerobic tests generate methane as a key metabolic end product, making gas volume measurement and gas composition analysis critical to accurate results. Tests typically run for 30 to 60 days or longer, depending on the standard applied and the biodegradation kinetics of the material.
Testing Samples
Biodegradable Polymer Resins
Raw polymer materials constitute the primary category of test samples. These include commonly studied biodegradable plastics such as polylactic acid (PLA), polyhydroxyalkanoates (PHA), polybutylene succinate (PBS), polybutylene adipate terephthalate (PBAT), polycaprolactone (PCL), and starch-based blends. Resin samples are usually ground or cut into small particles to increase surface area and ensure uniform microbial access.
Finished Products and Articles
Manufactured goods intended for anaerobic disposal or digestion pathways can also be evaluated directly. Typical product samples include:
- Compostable and biodegradable carrier bags and films
- Food packaging containers, trays, and wraps
- Disposable cutlery, plates, and straws
- Coffee capsules and single-serve beverage pods
- Agricultural mulch films and horticultural products
- Hygiene and personal care product components
Reference and Control Materials
Each test series requires control materials to validate the activity of the inoculum and the reliability of the test system. These include a positive reference such as cellulose or a known biodegradable polymer, a blank control containing only inoculum, and optionally a negative reference such as polyethylene to confirm that non-biodegradable materials do not produce significant biogas.
Testing Items
Anaerobic biodegradation testing typically encompasses the following measurement parameters:
- Ultimate degree of anaerobic biodegradation, expressed as a percentage of theoretical carbon conversion
- Cumulative biogas production, including total volume and pressure measurements
- Methane (CH4) and carbon dioxide (CO2) content of the evolved biogas
- Total organic carbon (TOC) and total carbon content of the test material
- Volatile solids and total dry solids of the inoculum and test mixture
- pH, volatile fatty acids, and ammonia levels for digestion system stability monitoring
- Disintegration and visual assessment of residual material after testing
Pass criteria vary depending on the applicable standard or certification scheme, but a common requirement is that the test material achieves a biodegradation degree comparable to the positive reference, with many certification programs requiring at least 50 to 90 percent biodegradation relative to cellulose within the specified test duration.
Testing Methods
Aqueous Low-Solids Anaerobic Test
ISO 14853 specifies a method for determining the ultimate anaerobic biodegradation of plastics in an aqueous system by measuring biogas production. The test is conducted at a mesophilic temperature of approximately 35 degrees Celsius using diluted digested sludge as the inoculum. This method is well suited to materials that disperse or hydrolyze in liquid media and provides a rigorous assessment of intrinsic biodegradability.
Slurry Anaerobic Digestion Test
ISO 13975 evaluates the ultimate anaerobic biodegradation of plastic materials in controlled slurry digestion systems. This method more closely replicates industrial anaerobic digestion conditions, with higher solids content and a mixed slurry environment. It is particularly relevant for products such as food waste collection bags that are expected to enter anaerobic digestion facilities together with organic waste streams.
High-Solids Anaerobic Digestion Test
ASTM D5511 and ISO 15985 describe methods for determining anaerobic biodegradation under high-solids anaerobic digestion conditions, simulating the environment of a biologically active landfill. These tests typically operate under thermophilic or accelerated mesophilic conditions with solids content above 20 percent, providing insight into long-term landfill behavior of degradable plastics.
Sewage Sludge Anaerobic Test
ASTM D5210 determines the anaerobic biodegradation of plastic materials in the presence of municipal sewage sludge. The test uses a seed culture prepared from actively digesting sludge and measures the conversion of plastic carbon into carbon dioxide and methane over an incubation period, generally conducted at approximately 35 degrees Celsius.
General Test Procedure
Regardless of the specific standard, the general workflow follows a consistent sequence:
- Sample preparation, including drying, weighing, and particle size reduction as required by the standard
- Inoculum preparation, activity verification, and pre-conditioning under anaerobic conditions
- Test vessel loading with inoculum, test material, mineral medium, and controls in triplicate
- Incubation at controlled temperature with continuous or periodic agitation in a sealed environment
- Periodic measurement of biogas volume, pressure, and gas composition using gas chromatography or infrared analysis
- Data correction using blank controls and calculation of biodegradation percentage against theoretical carbon content
- Final reporting with biodegradation curves, gas composition data, and validity confirmation from control results
Testing should be performed by accredited third-party laboratories or qualified in-house facilities with validated gas measurement systems, experienced microbiology staff, and strict anaerobic handling protocols to ensure data integrity and repeatability.
Applications
Anaerobic biodegradation test data supports a wide range of industrial, regulatory, and commercial objectives:
- Certification support for compostable and biodegradable plastic products under schemes such as EN 13432, which includes anaerobic digestion as an optional treatment pathway
- Scientific substantiation of environmental marketing claims regarding end-of-life performance
- Research and development screening of new biodegradable polymer formulations, additives, and blends
- Compatibility assessment for products entering anaerobic digestion facilities, such as food waste liners and organic packaging
- Landfill behavior evaluation to understand methane generation potential and long-term residue stability
- Regulatory submissions and compliance documentation for market access in regions with plastic restriction policies
- Quality control verification for incoming resins and finished biodegradable product batches
Industries that commonly rely on these tests include packaging manufacturing, consumer goods, agriculture, food service, waste management, and polymer production. The results help stakeholders make informed decisions about material selection, waste treatment infrastructure, and product environmental positioning.
FAQ
What is the difference between anaerobic and aerobic biodegradation testing?
Aerobic testing, such as ISO 14855 or ASTM D5338, measures conversion of plastic carbon into carbon dioxide in the presence of oxygen, simulating composting conditions. Anaerobic testing measures conversion into both methane and carbon dioxide in oxygen-free environments, simulating digesters and landfills. The microbial communities, gas measurement methods, and end products differ significantly between the two approaches.
How long does an anaerobic biodegradation test take?
Typical test durations range from 30 to 60 days, although some standards allow extension until the biodegradation curve reaches a plateau. Slowly degrading materials or high-solids tests may require longer incubation periods to capture the ultimate biodegradation potential accurately.
What biodegradation level is considered a pass?
Acceptance thresholds depend on the applicable standard and certification scheme. Many programs require the test material to reach at least 50 percent, and often 90 percent, of the biodegradation level achieved by the cellulose positive reference within the test period. Absolute thresholds of 60 percent or higher within a defined timeframe are also common in certification frameworks.
Why is methane measurement important?
Methane is a primary metabolic end product of anaerobic degradation and often represents more than half of the carbon converted in a well-functioning digestion system. Measuring both methane and carbon dioxide ensures accurate carbon balance calculations and provides valuable data on potential biogas recovery or greenhouse gas considerations.
Can any plastic be tested anaerobically?
Yes, both biodegradable and conventional plastics can be evaluated. However, conventional petroleum-based plastics such as polyethylene and polypropylene typically show negligible biodegradation, which is why they are frequently used as negative references. Materials containing inorganic carbonates or additives that release CO2 abiotically require careful interpretation to avoid overestimating biodegradation.
Which sample form should be submitted for testing?
Standards generally require samples in powder, granule, film, or small fragment form with defined particle size limits to promote uniform microbial attack. Finished products are usually cut or milled before testing. Laboratories can often assist with sample preparation to meet the specific requirements of the chosen standard.