PLA anaerobic biodegradation testing quantifies how polylactic acid materials are converted into biogas (methane and carbon dioxide) by anaerobic microorganisms under controlled laboratory conditions, in accordance with international standards such as ISO 15985, ASTM D5511, and ASTM D5526. Reliable test data enables manufacturers to substantiate environmental claims, support certification, and optimize material formulations for anaerobic digestion and landfill end-of-life scenarios. This article explains the technical principles, sample requirements, test items, methods, and typical applications of anaerobic degradation testing for PLA materials.
Technical Overview
Polylactic acid (PLA) is a bio-based thermoplastic polyester produced from renewable feedstocks such as corn starch, sugarcane, or cellulose through fermentation of sugars into lactic acid followed by polymerization. Thanks to its processability, transparency, and favorable carbon footprint, PLA is widely used in packaging, disposable tableware, fibers, 3D printing filaments, and biomedical devices.
Although PLA is well known for its compostability under industrial aerobic composting conditions, real-world disposal routes frequently involve oxygen-limited environments such as anaerobic digesters, sanitary landfills, and deep sediments. Anaerobic biodegradation testing therefore measures the extent to which microorganisms in these environments can convert the carbon contained in PLA into methane (CH4) and carbon dioxide (CO2), collectively known as biogas, in the complete absence of oxygen.
The degradation of PLA follows a two-stage mechanism: hydrolysis of the ester linkages first cleaves the polymer chains into lower-molecular-weight oligomers and lactic acid, which are then mineralized by microorganisms. Because hydrolysis is strongly temperature-dependent, biodegradation of PLA under mesophilic anaerobic conditions (approximately 35 °C) is typically slower and may remain incomplete compared with thermophilic composting, which makes quantitative laboratory testing essential for evaluating real performance.
Testing Samples
Anaerobic biodegradation tests can be performed on PLA materials at virtually any stage of the product life cycle. The most common sample categories are described below.
PLA Resins and Pellets
Virgin or compounded PLA granules used by converters are usually cryogenically ground or cut to a defined particle size to ensure homogeneous distribution in the test vessel and consistent contact with the inoculum.
PLA Films, Sheets, and Bags
Cast or blown films, thermoforming sheets, and carrier bags are cut into small specimens of specified dimensions so that surface area and thickness, which strongly influence the hydrolysis rate, are standardized across all replicates.
Finished Products
Cups, cutlery, straws, coffee capsules, trays, fibers and nonwovens, 3D printed parts, and medical or pharmaceutical components can be tested in their representative market form, either intact or reduced in size in accordance with the applicable standard.
Modified and Composite PLA Materials
Blends such as PLA/PBAT, PLA/PHB, PLA/PBS, and PLA/starch, together with filled or reinforced grades, plasticized formulations, nanocomposites, and coated or multilayer structures, can be evaluated to compare formulation strategies and quantify the influence of additives on anaerobic biodegradability.
Testing Items
A typical anaerobic biodegradation study for PLA reports the following parameters:
- Ultimate anaerobic biodegradation rate, expressed as the percentage of theoretical biogas production (ThBP) released as CH4 and CO2
- Cumulative biogas volume and biogas yield per gram of test carbon or volatile solids
- Methane and carbon dioxide composition of the biogas, determined by gas chromatography
- Kinetic biodegradation curves versus incubation time, including lag phase, active biodegradation phase, and plateau phase
- Total organic carbon or elemental carbon content of the sample, required for calculating the theoretical biogas production
- Volatile solids, total solids, and pH of the inoculum and final digestate
- Residual sample mass, degree of disintegration, and visual observations after testing
- Molecular weight change of the residual polymer, determined by gel permeation chromatography, to correlate hydrolysis with mineralization
Testing Methods
ISO 15985 — High-Solids Anaerobic Digestion
ISO 15985 determines the ultimate anaerobic biodegradation of plastics under high-solids anaerobic-digestion conditions by measuring the volume of biogas produced. The test material is mixed with digested sludge inoculum at a high total solids content and incubated at a controlled mesophilic temperature of approximately 35 °C until the biodegradation curve reaches a plateau. The method is technically equivalent to ASTM D5511 and is widely used for European certification and research purposes.
ASTM D5511 — High-Solids Anaerobic Digestion (North America)
ASTM D5511 is the North American counterpart to ISO 15985. The test is commonly performed at 35 °C or under thermophilic conditions of approximately 50 to 55 °C, depending on the digester environment being simulated, and the generated biogas is measured manometrically or volumetrically. Because the method accelerates natural landfill processes, its results are frequently cited for landfill-biodegradability claims in the United States.
ASTM D5526 — Accelerated Landfill Conditions
ASTM D5526 evaluates the anaerobic biodegradation of plastic materials under accelerated landfill conditions over an extended duration. It is selected when a longer-term, landfill-relevant assessment is required beyond the scope of the standard high-solids digestion test.
ISO 11734 and ISO 14853 — Aqueous Anaerobic Systems
ISO 11734 assesses ultimate anaerobic biodegradability in diluted digested sludge, while ISO 14853 evaluates plastics in an aqueous anaerobic medium; both measure biogas evolution by pressure increase or volume displacement. These methods are relevant for products that may enter anaerobic aquatic environments such as sediments and lagoons.
General Test Procedure
The overall workflow is consistent across the above standards and consists of the following steps:
- Sample characterization: determine carbon content, dry mass, and volatile solids, and reduce the material to the particle size prescribed by the standard.
- Inoculum preparation: collect actively digested anaerobic sludge, pre-incubate it to deplete residual biodegradable carbon, and characterize its volatile solids content and pH.
- Vessel setup: prepare test vessels containing inoculum plus sample, blank vessels containing inoculum only, reference vessels containing microcrystalline cellulose as a positive control, and, where required, inhibition-control vessels containing both sample and cellulose.
- Incubation: maintain the sealed vessels at the specified temperature under strictly anaerobic conditions until the biodegradation curve reaches a plateau or the maximum test duration is reached.
- Biogas measurement: record cumulative biogas volume or headspace pressure at regular intervals and determine the CH4/CO2 ratio by gas chromatography.
- Calculation: express the result as the percentage of theoretical biogas production, where ThBP is derived from the measured carbon content of the sample.
- Validity checks: the run is considered valid when the cellulose reference achieves the biodegradation level required by the standard and the blank vessels produce only a low background gas volume.
Applications
Anaerobic biodegradation data for PLA materials support a wide range of industrial, regulatory, and scientific objectives:
- Substantiation of environmental and marketing claims in line with advertising standards for biodegradability statements
- Support for ecolabel and certification programs addressing anaerobic digestibility and landfill behavior
- Formulation development, including comparison of plasticizers, blend ratios, fillers, and processing aids
- Assessment of the compatibility of PLA packaging and foodservice items with anaerobic digestion infrastructure
- Input data for landfill gas generation models and municipal solid waste management planning
- Quality control for resin producers and converters verifying lot-to-lot consistency
- Academic and industrial research on structure-biodegradability relationships in aliphatic polyesters
FAQ
Does PLA biodegrade under anaerobic conditions?
Yes, PLA can be metabolized by anaerobic microorganisms, but the rate is governed primarily by hydrolysis, which is slow at mesophilic temperatures. Under standard 35 °C test conditions PLA often shows an extended lag phase and moderate biodegradation levels, whereas thermophilic conditions or appropriate pretreatment substantially accelerate degradation.
What is the difference between ISO 15985 and ASTM D5511?
The two methods are technically equivalent: both evaluate high-solids anaerobic digestion by measuring biogas volume. ISO 15985 is preferred in Europe and international markets, while ASTM D5511 dominates in North America, particularly for landfill-related claims.
How long does the test take?
Incubation continues until the biodegradation curve reaches a plateau. Runs commonly last from several weeks up to 60 days or longer, and slow-degrading materials such as unmodified PLA may require extended durations or modified conditions agreed in advance with the laboratory.
How much sample is required?
Typically 100 to 500 grams of material is sufficient for a full test series with replicate vessels, blanks, and reference controls, depending on the carbon content of the sample and the selected standard. Exact requirements are confirmed before testing begins.
What result is considered a pass?
Pass criteria are defined by the specific certification scheme rather than by the test method itself. Many specifications reference a biodegradation threshold on the order of 50 to 60 percent of theoretical biogas production, combined with valid reference and blank performance, so the requirements of the applicable program should always be verified.
Can finished products be tested, or only resins?
Finished products such as cups, films, cutlery, and fibers can be tested directly, either as market-ready specimens or reduced to the particle size specified in the standard, allowing the results to reflect the actual commercial article.