Compost Natural Degradation Rate Testing: Principles, Standards, and Procedures

Compost natural degradation rate testing is a laboratory-based evaluation that quantifies how quickly and completely a material biodegrades under controlled composting conditions, providing scientific evidence for compostability claims, product certification, and regulatory compliance. Guided by internationally recognized standards such as ISO 14855, ASTM D5338, and EN 13432, the test measures carbon dioxide evolution from microbially degraded samples to calculate the ultimate biodegradation percentage. This article presents a comprehensive technical overview of the sample types, testing items, methods, and applications involved in compost biodegradation rate testing.

Technical Overview

Compost natural degradation rate testing, also referred to as controlled composting biodegradation testing, determines the ultimate aerobic biodegradability of materials under optimized, industrial-scale composting conditions. The method is applicable to plastics, polymer compounds, packaging products, natural fibers, and other organic materials that are claimed to be biodegradable or compostable.

The test principle is based on respirometry. The test material is mixed with a mature compost inoculum and incubated in bioreactors at a thermophilic temperature of approximately 58 °C, with a moisture content of around 50 percent and continuous aeration using humidified, carbon-dioxide-free air. Under these conditions, microorganisms metabolize the organic carbon in the sample and convert it into carbon dioxide, water, microbial biomass, and mineral salts. The carbon dioxide released from each reactor is measured at regular intervals by infrared analysis, gas chromatography, or titration, and the net biodegradation percentage is calculated by comparing the cumulative net CO2 production with the theoretical CO2 (ThCO2) that would be released if all organic carbon in the sample were completely mineralized.

The biodegradation percentage at time t is expressed as: Dt = [(CO2)T − (CO2)B] / ThCO2 × 100, where (CO2)T is the cumulative carbon dioxide from the test vessel and (CO2)B is the background carbon dioxide from the blank inoculum vessel. A cellulose reference material is tested in parallel to verify inoculum viability. Under EN 13432, EN 14995, and ASTM D6400, a material is generally regarded as biodegradable when at least 90 percent of its organic carbon is converted within 180 days, or when it reaches a level comparable to the cellulose reference.

• ISO 14855-1 and GB/T 19277.1 — ultimate aerobic biodegradability under controlled composting conditions by analysis of evolved carbon dioxide
• ISO 14855-2 — laboratory-scale gravimetric measurement of carbon dioxide
• ASTM D5338 — aerobic biodegradation of plastic materials under controlled composting conditions
• EN 14046 and EN 13432 — biodegradation and compostability requirements for packaging
• ISO 16929, EN 14045, and GB/T 19811 — disintegration testing under pilot-scale composting conditions

Testing Samples

Biodegradable Plastics and Polymer Resins

Thermoplastic materials engineered for biodegradation, including PLA, PBAT, PBS, PHA/PHB, PCL, and starch-based blends. Resin samples are typically cryogenically ground or cut into small fragments to promote uniform microbial attack, and their total organic carbon content is characterized before exposure.

Compostable Packaging and Consumer Products

Finished articles such as shopping bags, food waste bags, food containers, cups, cutlery, straws, coffee capsules, agricultural mulch films, and cosmetic packaging. When a certification scheme requires whole-product evaluation, finished goods are tested in the same form in which they are placed on the market.

Natural Fibers and Bio-Based Materials

Paper, paperboard, molded pulp, cotton, linen, jute, sisal, bamboo products, wood fibers, and bio-based nonwovens or textiles intended for compostable end-of-life pathways.

Additives, Coatings, and Other Material Forms

Liquids, pastes, adhesives, inks, coatings, and processing additives may be evaluated after adsorption onto inert carriers or direct incorporation into the compost matrix, provided the organic carbon content is accurately determined.

Testing Items

• Ultimate aerobic biodegradation rate — the cumulative percentage of organic carbon converted to carbon dioxide over the exposure period
• Biodegradation kinetics curve — biodegradation percentage versus time, used to identify lag phases, biodegradation velocity, and plateau behavior
• Disintegration degree — the mass fraction of sample fragments retained on or passing a 2 mm sieve after composting, with a target of at least 90 percent disintegration
• Sample characterization — total organic carbon (TOC), total inorganic carbon (TIC), dry solids, volatile solids, and ash content
• Compost quality parameters — pH, salinity, conductivity, volatile fatty acids, and ammonia of the resulting compost
• Ecotoxicity supporting tests — plant germination rate and biomass yield, plus heavy metal screening when required by full compostability schemes
• Process monitoring — reactor temperature, moisture, airflow rate, and exhaust carbon dioxide concentration throughout the test

Testing Methods

Sample Preparation and Characterization

Before testing, samples are dried to determine dry solids and volatile solids, and their carbon content is measured by elemental analysis or the difference between total and inorganic carbon. The test mass is calculated so that the ratio of total dry solids of the test material to the volatile solids of the inoculum falls within the range specified by ISO 14855-1, typically between 1:6 and 1:20.

Controlled Composting Biodegradation Test

A mature, sieved compost inoculum aged two to four months is charged into parallel reactors together with the test material. Reactors are held at 58 ± 2 °C in a temperature-controlled enclosure, aerated continuously with humidified air, and periodically fluffed and re-moistened to maintain microbial activity. Carbon dioxide in the exhaust gas is quantified throughout the test, and the run continues until the biodegradation curve reaches a plateau, with a maximum duration of six months. Validity criteria include blank inoculum respiration of 50 to 150 mg CO2 per gram of volatile solids during the first ten days and biodegradation of the cellulose reference exceeding 70 percent after 45 days.

Disintegration Test Under Pilot-Scale Composting Conditions

Following ISO 16929 or EN 14045, test specimens are mixed with fresh biowaste in a pilot-scale or full-scale composting pile for approximately twelve weeks, including a sustained thermophilic phase. The output is then screened through a 2 mm sieve, and the degree of disintegration is calculated from the recovered fragment mass. This physical fragmentation test is commonly combined with the biodegradation rate test to support overall compostability claims under EN 13432, ASTM D6400, or EN 14995.

Applications

• Certification and labeling support for compostable packaging and plastic products under EN 13432, ASTM D6400, EN 14995, and ISO 18606
• Research and development screening of biodegradable polymer formulations, plasticizers, fillers, and additives
• Regulatory compliance demonstrations for plastic reduction policies, extended producer responsibility schemes, and market access requirements
• Substantiation of environmental marketing claims for retailers, brand owners, and exporters
• Quality control verification of incoming resins and finished compostable goods during mass production
• Scientific research on structure-biodegradability relationships and novel bio-based materials

FAQ

What result is considered a pass for biodegradation rate?

Under EN 13432, ASTM D6400, and related schemes, at least 90 percent of the organic carbon in the material must be biodegraded to carbon dioxide within 180 days under controlled composting conditions, or the material must achieve at least 90 percent of the biodegradation level reached by the cellulose reference.

How long does the test take?

Most tests run between 90 and 180 days. The test is terminated once the biodegradation curve plateaus, and ISO 14855-1 sets an absolute maximum exposure period of six months.

How much sample is required?

Depending on reactor size and carbon content, approximately 100 to 500 grams of dry sample are typically sufficient. Samples should be provided as powders, granules, films, or cut fragments to ensure representative and reproducible results.

What is the difference between biodegradation rate and disintegration?

Biodegradation rate is a molecular-level measurement based on carbon dioxide evolution, indicating how much organic carbon is actually mineralized by microorganisms. Disintegration is a physical measurement of fragmentation, determined by sieving composted output through a 2 mm screen. Both are required for a complete compostability assessment.

Where should this testing be performed?

Testing should be conducted by accredited third-party laboratories operating in accordance with ISO/IEC 17025 and equipped with controlled composting bioreactors, gas analysis systems, and experienced technical personnel to ensure data reliability and regulatory acceptance.