Compost biodegradation carbon dioxide (CO2) determination is the core laboratory method for quantifying the ultimate aerobic biodegradability of plastic and organic materials under controlled composting conditions, as specified in standards such as ISO 14855, ASTM D5338, and GB/T 19277. By measuring the evolved CO2 and comparing it with the theoretical CO2 production of the test material, this method provides the definitive biodegradation percentage required for compostability certification of biodegradable plastics. This article explains the testing principle, applicable sample types, test items, analytical methods, industrial applications, and frequently asked questions.
Technical Overview
Compost biodegradation carbon dioxide (CO2) determination is a respirometric test that quantifies how completely a material is mineralized by microorganisms in an aerobic composting environment. The test material is mixed with mature, active compost derived from organic waste and incubated under controlled thermophilic conditions, typically at 58 ± 2 °C, with continuous aeration using CO2-free air and maintained humidity. As microorganisms metabolize the carbon in the test material, they release CO2 as the primary metabolic end product.
The amount of CO2 evolved from the test vessel is measured continuously or periodically and compared with the theoretical amount of CO2 (ThCO2) that the test material could generate based on its total organic carbon content. The ratio of net CO2 evolution (test vessel minus blank compost vessel) to the theoretical CO2 yield, expressed as a percentage, is defined as the degree of biodegradation. A material is generally recognized as ultimately biodegradable when it achieves at least 90% biodegradation within 180 days, either in absolute terms or relative to the cellulose reference material.
Test validity is ensured through parallel control vessels. The blank control (compost without test material) quantifies the background respiration of the inoculum, the reference material (microcrystalline cellulose) demonstrates the activity of the compost, and the inert control (such as non-biodegradable polyethylene) confirms the specificity of the system. This test is a mandatory chemical characterization step in compostability certification schemes aligned with ISO 17088, EN 13432, EN 14995, and ASTM D6400.
Testing Samples
Biodegradable Plastic Resins and Compounds
Neat polymers and compounded formulations including PLA (polylactic acid), PBAT (polybutylene adipate terephthalate), PBS (polybutylene succinate), PHA/PHB (polyhydroxyalkanoates), PPC (polypropylene carbonate), PCL (polycaprolactone), starch-based alloys, and their modified or blended grades supplied as pellets, powders, or granules.
Finished Products and Packaging
Compostable packaging articles such as films, bags, rigid containers, bottles, cups, cutlery, straws, coffee capsules, labels, and adhesive systems intended for industrial composting streams. Products are typically tested after size reduction to particles of a few millimeters to ensure homogeneous exposure, unless the certification scheme requires testing of the article as marketed.
Agricultural and Horticultural Materials
Mulch films, planting pots, seed tapes, fertilizer coating materials, crop twine, and soil-biodegradable articles where mineralization behavior in biological treatment or soil-adjacent environments must be demonstrated.
Natural, Bio-based, and Fiber-based Materials
Paper and paperboard products, molded pulp, natural fibers, lignocellulosic materials, bio-waxes, biopolymers extracted from biomass, and additives such as plasticizers, processing aids, and colorant masterbatches used at representative concentrations in the final formulation.
Chemicals and Intermediate Materials
Organic chemicals, surfactants, lubricants, coatings, and intermediate raw materials whose ultimate aerobic biodegradability in a composting matrix needs to be characterized for regulatory dossiers or product development programs.
Testing Items
The determination package typically covers the following measurable outputs:
- Cumulative and time-resolved CO2 evolution from the test, blank, and reference vessels (mg CO2 per gram of volatile solids).
- Theoretical CO2 (ThCO2) calculation based on the measured total organic carbon or elemental carbon content of the test material.
- Degree of biodegradation (percentage of ThCO2), reported as both absolute biodegradation and biodegradation relative to cellulose.
- Biodegradation kinetics, including lag phase, biodegradation rate, plateau detection, and time to reach 60% and 90% thresholds.
- Compost inoculum characterization: pH, volatile solids, total dry solids, ash content, carbon-to-nitrogen ratio, and respiration activity.
- Test material characterization: total organic carbon, elemental composition (C, H, N), dry solids, and ash content as required for ThCO2 determination.
- Supplementary observations such as residual mass, visual disintegration, and pH drift of the compost matrix at the end of the test.
Testing Methods
ISO 14855-1 – Controlled Composting with Continuous CO2 Analysis
The test material is mixed with mature compost in static or dynamically agitated vessels held at 58 °C, aerated with humidified, CO2-free air. Carbon dioxide in the exhaust gas is quantified continuously using an infrared gas analyzer (IRGA) or an equivalent on-line detector, providing a high-resolution biodegradation curve suitable for certification purposes.
ISO 14855-2 – Gravimetric Determination of Evolved CO2
Exhaust air from each composting vessel is passed through columns filled with alkaline absorbents such as sodium hydroxide, potassium hydroxide, or soda lime. The mass gain of the absorption columns, attributable to captured CO2, is determined by gravimetry. This laboratory-scale approach requires no on-line analyzer and delivers results equivalent to the IRGA method when operated correctly.
ISO 14855-3 – Supplementary Aerobic Composting Method
This part extends the controlled composting framework with an alternative incubation configuration, allowing laboratories flexibility in vessel design and aeration management while retaining the same biodegradation calculation basis.
ASTM D5338 – Aerobic Biodegradation under Controlled Composting
The North American equivalent to ISO 14855-1, widely specified for compostability claims in markets governed by ASTM D6400. The method uses similar thermophilic incubation, cellulose reference validation, and CO2-based biodegradation calculation.
GB/T 19277 Series – Chinese National Standard
GB/T 19277.1 and GB/T 19277.2 define the controlled composting biodegradation test with CO2 analysis and gravimetric absorption, respectively, and constitute the reference method for biodegradability evaluation in the Chinese market, harmonized with the ISO 14855 framework.
Supporting Analytical Techniques
- Titrimetric CO2 determination, in which CO2 is absorbed in barium hydroxide or sodium hydroxide solution and the residual alkali is back-titrated with standardized acid.
- Elemental analysis (CHN) or total organic carbon analysis for establishing the theoretical CO2 potential of the material.
- Continuous monitoring of temperature, airflow, humidity, and oxygen content to document compliance with the aerobic incubation conditions throughout the test.
Calculation and Validation Criteria
The biodegradation percentage is calculated as the net cumulative CO2 of the test vessel (after blank subtraction) divided by ThCO2 and multiplied by 100. The test is considered valid when the cellulose reference reaches more than 70% biodegradation after 45 days, the blank vessels produce a background CO2 level within the range of 50 to 150 mg CO2 per gram of volatile solids over the first ten days, the inoculum pH remains in a physiologically acceptable range, and replicate vessels agree within approximately 20 percentage points at plateau. The standard test duration extends up to six months and may be concluded earlier once the biodegradation curve reaches a plateau with all validity criteria satisfied.
Applications
Compost biodegradation CO2 determination serves as the scientific backbone for a wide range of commercial and regulatory objectives:
- Industrial compostability certification of packaging and consumer products under frameworks referencing ISO 17088, EN 13432, EN 14995, and ASTM D6400, where a minimum 90% biodegradation result within 180 days is a mandatory requirement.
- Research and development of biodegradable polymer formulations, additives, and processing technologies, using biodegradation kinetics to compare candidate grades and optimize blend ratios.
- Regulatory compliance and market access documentation for manufacturers exporting compostable products to regions with mandatory compostability legislation.
- Quality control and incoming material verification for converters purchasing biodegradable resins, ensuring lot-to-lot consistency of mineralization performance.
- Environmental product declarations, carbon footprint studies, and sustainability reporting that require experimentally measured biodegradation endpoints rather than literature assumptions.
- Academic and industrial research into structure-biodegradability relationships of novel bio-based polymers and naturally derived materials.
Accredited third-party laboratories equipped with multi-channel respirometric composting systems can run the full control set required by the standards and issue test reports that are accepted by certification bodies, brand owners, and regulatory authorities worldwide.
FAQ
What is the pass criterion for biodegradation in this test?
A material is regarded as ultimately biodegradable under controlled composting conditions when it achieves at least 90% biodegradation in absolute terms, or at least 90% of the biodegradation level reached by the cellulose reference, within a maximum test period of 180 days.
How long does the test take?
The incubation period is up to 180 days. The test is terminated when the biodegradation curve reaches a plateau; many materials complete their biodegradation phase within 90 to 150 days. Additional time is required for sample preparation, inoculum conditioning, elemental analysis, and reporting.
Why is the CO2 measurement corrected with a blank control?
Mature compost itself contains residual biodegradable organic matter that continues to respire and release CO2. The blank vessel quantifies this background emission so that only the CO2 attributable to the test material is counted in the biodegradation calculation.
What is the role of the cellulose reference material?
Cellulose is a well-characterized, readily biodegradable positive control. Its performance verifies that the compost inoculum is sufficiently active. If cellulose does not exceed 70% biodegradation within 45 days, the test is invalidated and must be repeated with a more active inoculum.
How is biodegradation different from disintegration?
Biodegradation measures molecular conversion of the material into CO2, water, and biomass, whereas disintegration measures physical fragmentation into particles small enough to pass through a 2 mm sieve after a composting cycle. Both, together with ecotoxicity and heavy metal limits, are required for full compostability certification.
What sample form and quantity are required?
Test materials are usually reduced to particles below approximately 5 mm to ensure uniform mixing with the compost. The required quantity is calculated to deliver a suitable organic carbon loading in the vessel, commonly in the range of several grams to tens of grams depending on vessel size and the carbon content of the material.
Can home composting claims be supported by this test?
Industrial composting tests run at 58 °C do not by themselves demonstrate home compostability. For home composting claims, similar respirometric testing is performed at lower ambient temperatures over extended durations in accordance with the relevant national and international specifications.