The controlled natural degradation composting test is a laboratory-scale respirometric method used to evaluate the ultimate aerobic biodegradability of plastics, polymers, and organic materials under simulated industrial composting conditions. Based on international standards such as ISO 14855, ASTM D5338, EN 14046, and GB/T 19277.1, the test quantifies the carbon dioxide evolved by microorganisms to calculate the biodegradation percentage of a material within a defined test period. This article provides a detailed technical overview of the test principle, sample requirements, testing items, experimental procedures, validity criteria, and typical industrial applications.
Technical Overview
The controlled natural degradation composting test, also known as the controlled aerobic composting biodegradation test, is a standardized laboratory procedure that determines how completely a material is mineralized by the microbial community of an active compost ecosystem. The method reproduces the thermophilic phase of industrial composting inside a closed, instrumented system, allowing the entire biodegradation process to be monitored with high accuracy and reproducibility.
The core principle of the test is respirometry. The test material is blended with a mature compost inoculum and incubated at 58 °C ± 2 °C under controlled aeration, humidity, and nutrient conditions. As microorganisms metabolize the carbon contained in the sample, they release carbon dioxide. The evolved CO2 is captured and quantified continuously or at regular intervals, and the cumulative CO2 production is compared with the theoretical CO2 (ThCO2) that would be released if all carbon in the sample were completely converted to CO2. This ratio expresses the degree of biodegradation as a percentage.
The method is embedded in the major compostability frameworks used worldwide, including:
- ISO 14855-1 and ISO 14855-2 — determination of the ultimate aerobic biodegradability of plastic materials under controlled composting conditions by analysis of evolved carbon dioxide
- ASTM D5338 — standard test method for determining aerobic biodegradation of plastic materials under controlled composting conditions
- EN 14046 — ultimate aerobic biodegradability of packaging materials under controlled composting conditions
- GB/T 19277.1 — the corresponding Chinese national standard for controlled composting biodegradability testing
- EN 13432, ASTM D6400, ISO 18606, and GB/T 41010 — compostability certification and product frameworks that cite the controlled composting biodegradation test as a core requirement
Because temperature, aeration, moisture, and inoculum activity are all controlled and validated, the test yields reproducible biodegradation kinetics that cannot be obtained from uncontrolled field exposure. A material is generally regarded as having demonstrated satisfactory biodegradability when it reaches at least 90 percent biodegradation within 180 days, or at least 90 percent of the biodegradation level achieved by the cellulose reference material tested in parallel.
Testing Samples
Biodegradable Polymers and Compounds
Typical test materials include poly(lactic acid) (PLA), poly(butylene adipate-co-terephthalate) (PBAT), polybutylene succinate (PBS), polyhydroxyalkanoates (PHA/PHB), polycaprolactone (PCL), poly(propylene carbonate) (PPC), polyglycolic acid (PGA), starch-based blends, cellulose derivatives, and bio-based composites. Raw resins, masterbatches, and polymer additives can also be evaluated to characterize their contribution to the overall biodegradation profile.
Natural and Fiber-Based Materials
Natural polymers such as cellulose, lignin, and chitosan, together with paper, paperboard, bagasse, bamboo fiber, wood pulp, and molded fiber products, are frequently submitted for testing to substantiate their inherent biodegradability and support compostable product claims.
Finished Products and Product Components
Compostable carrier bags, food containers, cups, cutlery, straws, wrapping films, agricultural mulch films, coffee capsules, labels, adhesives, coatings, and printing inks can all be assessed, either as complete articles or as isolated components whose individual degradation behavior must be demonstrated.
Sample Preparation and Characterization
Materials are preferably reduced to a powder or granular form with a particle size of approximately 250 μm or less, typically by cryogenic milling, to maximize surface area and ensure uniform microbial access. Films may alternatively be cut into small pieces where the degradation of the actual article form is of interest. Before testing, the sample must be characterized for total organic carbon content (by elemental analysis), total dry solids (dried at approximately 105 °C), and volatile solids (ash content after ignition at approximately 550 °C), because these values are required to calculate the theoretical CO2 production.
Testing Items
The controlled composting test delivers a complete biodegradation profile of the submitted material. The principal testing items include:
- Ultimate aerobic biodegradation degree, expressed as the percentage of theoretical CO2 released
- Cumulative CO2 evolution and biodegradation kinetics, presented as a time-dependent biodegradation curve
- Theoretical CO2 (ThCO2) calculated from the measured carbon content of the sample
- Total dry solids and volatile solids of both the sample and the compost inoculum
- Process monitoring parameters, including temperature, pH, moisture content, and airflow rate throughout the incubation
- Optional supplementary assessments commonly required for full compostability evaluation, such as disintegration behavior, compost quality, plant ecotoxicity (germination and growth), and heavy metal content
Testing Methods
Test System and Apparatus
The standard setup consists of airtight composting vessels, typically 2 to 5 liters in capacity, maintained in an incubator at 58 °C ± 2 °C. CO2-free, humidified air is passed through each vessel at a controlled flow rate, generally in the range of 30 to 150 mL per minute depending on vessel size. Carbon dioxide in the exhaust gas is quantified either by an infrared gas analyzer, which enables continuous monitoring, or by absorption in an alkaline solution such as barium hydroxide or sodium hydroxide followed by titration with standardized hydrochloric acid.
Inoculum Preparation
The inoculum is mature compost, typically aged two to six months, derived from the organic fraction of municipal or garden waste. It is screened to a particle size of approximately 10 mm or less and adjusted to a water content of roughly 50 to 55 percent. The inoculum must contain an active microbial population with volatile solids content within the range specified by the applicable standard, and its respiration activity is verified through the blank vessels to confirm it is neither too active nor too depleted.
Experimental Procedure
- Characterize the test material: determine carbon content, dry solids, and volatile solids, and calculate ThCO2.
- Homogeneously mix the test material with the inoculum, typically at a dry-mass ratio of approximately 1:6, corresponding, for example, to 100 g of sample combined with about 600 g of wet compost in an ASTM D5338 configuration.
- Prepare the vessel set in at least triplicate: test vessels containing sample plus inoculum, blank vessels containing inoculum only, and reference vessels containing microcrystalline cellulose plus inoculum.
- Incubate all vessels at 58 °C ± 2 °C with continuous aeration and moisture maintained at approximately 50 to 60 percent throughout the test.
- Measure CO2 evolution at regular intervals, typically daily during the initial high-activity phase and weekly thereafter, while recording temperature, pH, and moisture.
- Continue the test until the biodegradation curve reaches a plateau phase, with a maximum duration of 180 days.
Calculation of Biodegradation
The net CO2 produced by the sample is obtained by subtracting the cumulative CO2 of the blank vessels from that of the test vessels. The biodegradation degree is then calculated as the net cumulative CO2 divided by ThCO2 multiplied by 100 percent, where ThCO2 equals the sample mass multiplied by its carbon fraction and by the molar mass ratio 44/12. The same calculation is applied to the cellulose reference to normalize the results.
Validity Criteria
- The cellulose reference material must achieve more than 70 percent biodegradation within approximately 45 days of incubation.
- The blank vessels should evolve within the activity window specified by the standard, commonly 50 to 150 mg of CO2 per gram of inoculum volatile solids during the first ten days.
- The relative deviation among replicate vessels at the plateau phase should not exceed 20 percent.
- The test is valid only if it is terminated at the plateau of biodegradation, with a maximum incubation period of 180 days.
- A passing result for certification purposes generally requires at least 90 percent biodegradation in absolute terms or relative to the cellulose reference.
Applications
The controlled natural degradation composting test is a cornerstone of biodegradable material development and compostability conformity assessment across multiple industries:
- Compostability certification under EN 13432, ASTM D6400, and ISO 18606, where the biodegradation result is the first and most critical technical gate for packaging and plastic products seeking industrial compostable labeling.
- Research and development of new biodegradable polymers, enabling formulation comparison, additive screening, and degradation kinetics modeling under standardized, reproducible conditions.
- Regulatory compliance and market access, supporting declarations required by plastic restriction policies, packaging recovery regulations, and national biodegradable product specifications such as GB/T 41010.
- Substantiation of environmental marketing claims, providing accredited third-party laboratory evidence for statements regarding biodegradability and compostability.
- Acceptance decisions by industrial composting operators, who rely on standardized biodegradation and disintegration data to approve incoming materials for organic recycling streams.
- Academic research and life-cycle assessment, supplying quantitative mineralization data used in environmental modeling, carbon footprint studies, and scientific publications.
FAQ
How does the controlled composting test differ from aqueous biodegradation tests?
The controlled composting test is performed in a solid, thermophilic compost matrix at 58 °C and is specifically designed to represent industrial composting, whereas aqueous methods such as ISO 14851 and ISO 14852 assess biodegradation in a liquid, activated-sludge medium at moderate temperatures. The composting test is the reference method for industrial compostability claims.
How long does the test take?
The incubation continues until the biodegradation curve reaches a plateau, which typically occurs between 90 and 180 days depending on the material. The maximum permitted duration under the certification standards is 180 days.
What sample form and quantity are required?
A powder or granule of 250 μm or smaller is preferred to maximize microbial access, although films and product fragments can be tested when the degradation of the actual article form must be demonstrated. Typical charges range from tens of grams in small-scale ISO 14855-2 systems to approximately 100 g of material in standard ASTM D5338 vessels. Elemental carbon analysis must be performed on the sample before testing.
Does a high biodegradation percentage mean the product is fully compostable?
Not by itself. Under EN 13432, ASTM D6400, and related frameworks, full compostability additionally requires demonstration of disintegration in a composting environment, absence of negative effects on the composting process and the resulting compost, compliance with heavy metal limits, and passing ecotoxicity assessments using plant germination and growth tests.
What is the difference between biodegradation and disintegration?
Biodegradation refers to the molecular breakdown of the material by microorganisms into CO2, water, and biomass, measured respirometrically in this test. Disintegration refers to the physical fragmentation of the material so that no residue larger than 2 mm remains after composting, assessed using pilot-scale or laboratory-scale methods such as ISO 16929 and ISO 20200. Both properties are required for compostability certification.
Can finished products be tested, or only raw materials?
Both can be tested. Raw resins and additives are commonly evaluated during development, while finished articles such as bags, containers, and mulch films are tested for certification. Finished products may also require component-level testing when they contain multiple materials such as inks, adhesives, or coatings.