What Is ASTM D5338 and Why Does It Matter?

The ASTM D5338 standard, formally titled “Standard Test Method for Determining Aerobic Biodegradation of Plastic Materials Under Controlled Composting Conditions”, is one of the most cited protocols in the biodegradable plastics industry. Developed by ASTM International, this method provides a laboratory-scale simulation of industrial composting conditions to measure how quickly and completely a plastic material breaks down through microbial action. For manufacturers, regulators, and environmental researchers, ASTM D5338 offers a reproducible, data-driven way to verify biodegradability claims and to compare the environmental fate of different polymer formulations.

Unlike simple disintegration tests that only assess physical fragmentation, ASTM D5338 measures the actual mineralization of carbon—conversion of the plastic’s carbon into carbon dioxide (CO₂)—over a defined period. This makes it a rigorous indicator of true biodegradation, distinguishing materials that merely break into microplastics from those that are fully consumed by microorganisms. The standard is widely referenced in compostability certification schemes and is often required for products seeking the “Biodegradable in Compost” label in North America and other regions.

Core Principles of the ASTM D5338 Test

At its heart, ASTM D5338 quantifies aerobic biodegradation by capturing and measuring the CO₂ released when microorganisms metabolize the carbon in a plastic sample. The test is run under conditions that mimic a well-managed industrial composting facility: a temperature of 58 ± 2°C (136 ± 4°F), a moisture content near 50–60%, and a nutrient-supplemented compost inoculum derived from mature, aerated compost. The plastic sample is ground or cut into small pieces to maximize surface area, then mixed with the inoculum in a sealed vessel. A continuous or periodic stream of CO₂-free air is passed through the reactor, and the exhaust is analyzed for CO₂ concentration. Over a typical 180-day test duration, the cumulative CO₂ evolution is compared to that of a positive control (e.g., cellulose powder) and a negative control (e.g., a non-biodegradable polyethylene film). The result is expressed as a percentage of the theoretical maximum CO₂ possible if all carbon in the sample were converted to CO₂.

Detailed Test Setup and Conditions

Inoculum Preparation

The compost inoculum must be fresh, active, and representative of industrial composting biology. ASTM D5338 specifies that the inoculum be sieved to remove large particles and then conditioned to ensure stable microbial activity. The compost should have a C:N ratio between 10:1 and 20:1, a pH near 7–8, and a moisture content of 50–60% of its water-holding capacity. This ensures that nutrients are not limiting and that the microorganisms are in a growth phase.

Sample Preparation

Plastic samples are typically ground to a particle size less than 2 mm to increase surface area and accelerate biodegradation. The exact amount of sample added depends on the carbon content: the standard recommends a total carbon input of approximately 25–50 mg per gram of compost. The sample and inoculum are mixed thoroughly and placed in a reactor vessel (usually a glass or stainless steel bottle of 1–3 liters). The vessel is sealed and connected to an air supply and CO₂ trap.

Temperature and Aeration Control

The reactors are incubated in a dark, temperature-controlled chamber set to 58 ± 2°C. This temperature is typical of the thermophilic phase of industrial composting, where microbial activity is highest. Air is supplied at a rate that maintains aerobic conditions without stripping moisture—typically 50–100 mL/min per reactor. The air is first passed through a CO₂ scrubber (soda lime or ascarite) to ensure zero background CO₂. The exhaust gas is then passed through a CO₂ trap (e.g., a solution of barium hydroxide or sodium hydroxide) that can be periodically titrated or analyzed via infrared gas analysis.

Duration and Monitoring

The standard test duration is 180 days, although shorter periods (e.g., 45 days for highly degradable materials) or longer periods (up to 1 year) can be used with justification. CO₂ evolution is measured at regular intervals—daily or weekly—and cumulative curves are plotted. At the end of the test, the residual material can be analyzed for remaining organic carbon, and the mass balance can be closed.

Data Interpretation and Acceptance Criteria

The primary metric is the percentage of biodegradation, calculated as:

% Biodegradation = (CO₂ sample − CO₂ blank) / Theoretical CO₂ × 100

where CO₂ sample is the cumulative CO₂ from the test reactor, CO₂ blank is the cumulative CO₂ from a reactor containing only compost inoculum (to account for background respiration), and theoretical CO₂ is the amount of CO₂ that would be produced if all carbon in the sample were oxidized.

A common benchmark for “ready biodegradability” under composting conditions is 90% absolute biodegradation (relative to a cellulose control) within 180 days, or 90% of the maximum biodegradation achieved by the control. However, ASTM D5338 itself does not set a pass/fail threshold—that is left to certification bodies like the Biodegradable Products Institute (BPI) or the European standard EN 13432. In practice, a material that reaches 60–70% mineralization might still be considered partially biodegradable, but it would not qualify for a compostability claim.

Comparisons with Other Biodegradability Standards

ASTM D5338 is often compared to the ISO 14855 series, which uses a similar approach. The two methods are technically equivalent for most purposes, although ISO 14855-1 uses a static reactor system while ASTM D5338 allows both static and dynamic (continuous flow) setups. The ISO 16929 and ASTM D6400 standards focus on disintegration (fragmentation) and ecotoxicity, not mineralization, and are complementary to D5338. For home composting conditions, ASTM D5929 (now withdrawn) or ISO 14855-2 at lower temperatures (25–30°C) may be more appropriate.

There is also the emerging ASTM D5511 standard for anaerobic biodegradation in landfills. The key difference is that D5338 requires aerobic, thermophilic conditions, which create much faster degradation rates than anaerobic or mesophilic environments. Material developers must be careful to match the test method to the intended end-of-life scenario (industrial compost vs. landfill vs. home compost).

Practical Applications of ASTM D5338

Product Development and R&D

Polymer scientists use D5338 to screen new formulations—such as PLA/PHA blends, starch composites, or proprietary biodegradable polyesters—to see how modifications (e.g., chain length, additives, branching) affect degradation rate. The test can also be used to evaluate the effect of pro-oxidant additives or enzyme coatings.

Certification and Labeling

Companies seeking to market their products as “compostable” typically submit D5338 data to third-party certifiers such as BPI or TÜV Austria. These certifiers often require additional tests (disintegration, ecotoxicity, heavy metal analysis) but rely on D5338 as the primary biodegradation evidence. A passing result allows the use of logos like the “Compostable” seedling logo in the EU or the BPI logo in North America.

Regulatory Compliance and Green Claims

In jurisdictions like the European Union under the Single-Use Plastics Directive (SUPD) and the Packaging and Packaging Waste Directive (PPWD), biodegradability claims must be supported by standardized testing. ASTM D5338 is accepted as equivalent to ISO 14855 for demonstrating that a plastic will biodegrade in industrial composting facilities. Similarly, the Federal Trade Commission (FTC) Green Guides in the United States advise that biodegradability claims be substantiated by scientific evidence—D5338 is the preferred method for compostable claims.

Environmental Impact Assessments

Life cycle assessment (LCA) practitioners often use D5338 data to model the end-of-life carbon emissions of biodegradable plastics. The CO₂ evolved during the test can be used to compute the global warming potential contribution of composting, and the residual carbon fraction can be used to estimate soil organic carbon sequestration potential.

Strengths and Limitations

Strengths

  • Rigorous and quantitative: Gives a precise, time-resolved measure of microbial mineralization.
  • Standardized and globally recognized: Accepted by certifiers, regulators, and industry bodies worldwide.
  • Reproducible: When conducted correctly, inter-laboratory variability is low, making it suitable for comparative studies.
  • Simulates real conditions: Temperature, moisture, and aeration mimic industrial compost facilities.

Limitations

  • Does not simulate home composting: The high temperature (58°C) is not representative of low-temperature back-yard compost piles.
  • Does not measure disintegration or ecotoxicity: A material may fully mineralize but leave toxic residues; D5338 alone is insufficient for a compostability claim.
  • Can be slow for recalcitrant materials: Some materials may only reach 20–30% degradation within 180 days, which does not mean they are non-biodegradable—only that they need more time or different conditions.
  • Sensitive to experimental variables: Inoculum quality, particle size, and C/N ratio can significantly affect results; careful controls are required.

ASTM International periodically reviews D5338, and recent discussions have focused on extending the test to include mesophilic (25–40°C) conditions for home compostability, and on harmonizing the test with ISO 14855 to reduce duplication. There is also growing interest in using real-time respirometry and stable isotope labeling to track the fate of specific carbon atoms within the polymer backbone. For marine biodegradability, ASTM D6691 (aerobic, seawater) and D7473 (anaerobic, sediment) are related but distinct protocols.

Manufacturers should also be aware that D5338 does not cover the rate of disintegration in a compost facility; for that, ASTM D6400 (Standard Specification for Compostable Plastics) integrates D5338 data with disintegration and ecotoxicity requirements. Thus, a product claiming full compostability must meet all criteria of D6400 or the equivalent EN 13432.

Best Practices for Testing Laboratories

Accurate D5338 testing requires rigorous attention to:

  • Inoculum selection: Use a consistent, quantified source of mature compost; avoid immature or anaerobic composts that skew background CO₂.
  • Control materials: Include a positive control (cellulose or polylactic acid) to verify microbial activity and a negative control (HDPE or LDPE) to ensure zero biodegradation.
  • Mass balance: At the end of the test, measure total organic carbon in the residual solids to account for any carbon not converted to CO₂ or incorporated into biomass.
  • Data reporting: Report cumulative CO₂ curves, percentage biodegradation at each time point, and a plot comparing the sample to the controls. Statistical analysis (e.g., using a first-order kinetics model) is often added for research publications.

Conclusion: The Role of ASTM D5338 in a Circular Economy

As the world moves toward reducing plastic pollution and enabling true circular material flows, standardized biodegradation testing becomes indispensable. ASTM D5338 provides the scientific backbone for verifying that a plastic can be metabolized by microorganisms in an industrial compost environment, converting waste into CO₂, water, and biomass—without toxic residues. While it is not a standalone solution (disintegration, ecotoxicity, and home-compost tests are also needed), it is the most widely accepted and technically robust method for measuring aerobic biodegradation. Manufacturers who invest in D5338 testing gain credible data to support environmental claims, satisfy regulators, and build consumer trust. Researchers continue to refine the method, pushing toward better simulation of real-world conditions and faster turnaround times. Ultimately, the standard is a key tool in the transition away from persistent plastics and toward materials that can safely return to the biological cycle.

For further reading, refer to the official ASTM D5338-15 standard, the ISO 14855-1 standard for international comparison, and certification guidelines from the Biodegradable Products Institute.