Compost residue testing evaluates the residual materials remaining after the controlled composting of biodegradable plastics, packaging, and organic waste, verifying both the completeness of disintegration and the safety of the resulting compost. Conducted in accordance with recognized standards such as EN 13432, ASTM D6400, and ISO 16929, these tests quantify sieve residues, heavy metals, and ecotoxicological effects to confirm that a material degrades without compromising compost quality. This article outlines the technical principles, sample types, testing items, methods, and typical applications of compost residue testing.
Technical Overview
Compost residue testing refers to the systematic examination of solid residues and compost fractions that remain after a composting trial or after industrial composting of biodegradable materials. When plastics, packaging, or organic products are claimed to be compostable, the degradation process must be validated not only through biodegradation measurements, but also through an assessment of what physically and chemically remains at the end of the process. Residue testing therefore addresses two key questions: how much of the original material fails to disintegrate, and whether the residual fraction introduces harmful substances or toxic effects into the compost.
The most widely applied residue criterion originates from compostability specifications such as EN 13432, ASTM D6400, ISO 17088, and GB/T 20197. These specifications require that, after a defined composting period under controlled conditions, no more than 10 percent of the original dry mass of the test material may remain as fragments larger than 2 mm when the compost is passed through a standardized sieve. In parallel, the resulting compost must meet chemical limits for heavy metals and must not produce adverse effects in ecotoxicity tests, ensuring that residues do not impair compost quality or soil health.
Residue testing is typically performed as the final stage of a full compostability evaluation, following disintegration testing (for example per ISO 16929, ISO 20200, or EN 14045) and laboratory biodegradation testing (for example per ISO 14855, ASTM D5338, or EN 14046). Accredited third-party laboratories combine gravimetric sieve analysis, chemical characterization, and biological assays to deliver a complete residue assessment suitable for certification, product development, and regulatory compliance.
Testing Samples
Biodegradable Plastic Resins and Compounds
Granules, films, sheets, and molded specimens of biodegradable polymers such as PLA, PBAT, PBS, PHA/PHB, polycaprolactone, and starch-based or cellulose-based blends are tested after controlled composting to determine their disintegration residue and contribution to compost contamination.
Finished Compostable Products
Consumer and industrial products including compostable bags, food packaging, coffee capsules, cutlery, cups, plates, straws, agricultural mulch films, labels, adhesives, and nonwoven textiles are tested in their end-use form, since additives, inks, laminates, and coatings can significantly influence the residue profile.
Compost and Residual Fractions from Composting Facilities
Finished compost, oversize screening fractions, and sieved residues collected from industrial composting or anaerobic-aerobic treatment plants may be tested directly to verify product quality, quantify plastic fragments, and confirm compliance with compost quality and fertilizer regulations.
Testing Items
Depending on the applicable standard and the purpose of the evaluation, compost residue testing typically covers the following parameters:
- Disintegration degree and sieve residue rate (mass of fragments greater than 2 mm relative to original dry mass)
- Residual fragment size distribution and visual characterization of non-degraded particles
- Dry solids and volatile solids of the residual fraction and of the final compost
- Heavy metals and regulated elements, including arsenic, cadmium, chromium, copper, mercury, nickel, lead, selenium, zinc, cobalt, molybdenum, and fluorine
- pH value, electrical conductivity, and salinity of the compost extract
- Nutrient content, including total nitrogen, ammonium nitrogen, phosphorus, and potassium
- Seed germination rate and germination index using reference plant species
- Plant biomass production (shoot yield comparison against blank compost)
- Ecotoxicity endpoints such as earthworm survival, daphnia immobilization, or algae growth inhibition, where required
Testing Methods
Residue evaluation begins with a controlled composting trial. In a pilot-scale or laboratory disintegration test conducted per ISO 16929, EN 14045, ISO 20200, or GB/T 19811, the test material is mixed with mature compost and incubated at thermophilic temperatures for a defined period, commonly up to 12 weeks. In laboratory biodegradation tests per ISO 14855-1, ISO 14855-2, ASTM D5338, or GB/T 19277, the material is composted in respirometric vessels where carbon dioxide evolution is monitored, and the residual solids are recovered at the end of the run.
After composting, the residue is quantified by sieve analysis. The entire compost mass is dried to constant weight and passed through a 2 mm sieve, often with washing assistance depending on the method. The fraction retained on the sieve is separated, cleaned of compost adherents, dried, and weighed. Residue fragments are then examined visually or under magnification to distinguish test material fragments from inert compost components, and the residue rate is calculated as a percentage of the original dry mass of the test material. Compliance is achieved when this value does not exceed 10 percent.
Chemical characterization of the compost and residues is performed using validated analytical techniques. Heavy metals are determined after microwave acid digestion by inductively coupled plasma mass spectrometry, inductively coupled plasma optical emission spectrometry, or atomic absorption spectroscopy, and results are compared against the limit values listed in EN 13432, ASTM D6400, or applicable national compost regulations. Volatile solids are determined by ignition at 550 degrees Celsius, and pH and conductivity are measured in water extracts.
Ecotoxicity of the compost containing residues is evaluated by biological assays. The plant growth test per OECD Guideline 208, referenced by EN 13432, compares seedling emergence and biomass of species such as barley, radish, or cress grown in sample compost and in blank compost; germination and biomass in the sample compost must each reach at least 90 percent of the blank values. Where required, acute earthworm toxicity testing per OECD Guideline 207 and aquatic assays such as daphnia or algae tests per OECD Guidelines 202 and 201 may be added to detect harmful leachable substances originating from the degraded material or its additives.
Applications
Compost residue testing supports a wide range of industrial, regulatory, and scientific purposes:
- Compostability certification support: generating the disintegration, chemical, and ecotoxicity data required by certification programs based on EN 13432, ASTM D6400, ISO 18606, ISO 17088, or GB/T 20197
- Product development: comparing the disintegration behavior of polymer formulations, thicknesses, additives, pigments, and coating systems during material design
- Regulatory compliance: demonstrating conformity with packaging and compost regulations in the European Union, North America, and China for market access and labeling claims
- Compost plant quality control: monitoring plastic fragment contamination in finished compost and screening fractions to meet compost quality and fertilizer standards
- Claim substantiation: providing laboratory evidence for marketing statements regarding compostability, disintegration, and environmental safety
- Academic and industrial research: studying the degradation pathways of novel biopolymers, blends, and bio-based composites under realistic composting conditions
FAQ
What is the pass criterion for residue in compost residue testing?
Under EN 13432, ASTM D6400, ISO 17088, and equivalent specifications, the mass of test material fragments remaining on a 2 mm sieve after composting must not exceed 10 percent of the original dry mass of the specimen. In addition, the resulting compost must comply with heavy metal limits and show at least 90 percent of blank compost performance in germination and plant biomass tests.
How long does a compost residue test take?
The composting phase typically lasts up to 12 weeks for disintegration trials at pilot or laboratory scale, while respirometric biodegradation tests may run up to 6 months. Including conditioning, chemical analysis, and plant growth assays, which require several additional weeks, a complete residue evaluation usually spans 3 to 9 months depending on the selected standards.
What sample quantities are required?
Disintegration tests generally require 200 to 500 grams of test material cut to the dimensions specified by the standard, together with sufficient mature compost as inoculum. Biodegradation tests on powdered or granulated material typically require 100 to 500 grams. Exact quantities depend on the test method and laboratory setup, and should be confirmed with the testing laboratory in advance.
Is residue testing alone sufficient to prove compostability?
No. Residue and disintegration testing is one component of a full compostability assessment. A complete evaluation also includes characterization of the material, ultimate aerobic biodegradation testing of at least 90 percent within the specified period, and confirmatory chemical and ecotoxicological analyses. Only the combined data package supports a compostability claim.
Which sample forms can be tested?
Practically any solid biodegradable article can be evaluated, including films, injection-molded and thermoformed parts, laminates, coated papers, textiles, and foams. Because disintegration depends strongly on wall thickness, samples should be tested at the maximum thickness intended for the commercial product.