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ISO 17556: Ultimate Aerobic Biodegradability of Polymers in Soil (ISO/IEC 17025:2017 Accredited)

ISO 17556 is an aerobic biodegradation test that determines the ultimate biodegradability of an organic material (mainly polymers/plastics) in soil relying on the microorganisms originally present in soil without the addition of a specific inoculum. When artificial (standard) soil is used, however, inoculum may come from mature compost. Two methods are specified, i.e., (I) oxygen demand in a respirometer, and (II) CO2 evolution. A maximum test duration of 6 months is allowed.

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CONCEPT

This method is designed to yield the optimum rate of biodegradation of a plastic material in a test soil by controlling the humidity of the soil, and to determine the ultimate biodegradability of the material.

The plastic material, which is the sole source of carbon and energy, is mixed with the soil. The mixture is allowed to stand in a flask over a period of time during which the amount of oxygen consumed or the amount of CO2 evolved is determined. 

The level of biodegradation, expressed as a percentage, is determined by comparing the oxygen consumption with the theoretical oxygen demand (ThOD) or by comparing the amount of CO2 evolved with the theoretical amount (ThCO2).

The test is terminated when a constant level of biodegradation has been attained or, at the latest, after 6 months.

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OECD 301 CO2 evolution and closed bottle test overview

ISO 17556 Key Summary

▶ Materials that can be tested:
     ➢ Natural and/or synthetic polymers, copolymers or mixtures of these;
     ➢ Plastic materials which contain additives such as plasticizers, colorants or other compounds;
     ➢ Water-soluble polymers.
▶ Test substance requirement:
     ➢ Chemical formula for calculating the ThOD, or total organic carbon (C%) for the ThCO2;
     ➢ Preferably in powder form with maximum size of 250 um diameter;
     ➢ Similar shapes of different substances should be used if they are to be compared;
     ➢ Preferably, plastic materials without additives such as plasticizers should be tested;
     ➢ Do not inhibit bacteria at the concentration tested.
▶ Measurement: oxygen consumption or CO2 evolution;
▶ Temperature: 20-28°C;
▶ Reference: a well-defined biodegradable polymer (e.g., microcrystalline cellulose powder, ashless cellulose filters or poly-β-hydroxybutyrate);
▶ Test substance dosage: for oxygen consumption method, use 100-300 mg of test substance to 100-300 g of soil (200 mg of substance to 200 g of soil is recommended). For CO2 evolution method, high test substance dosage can be used (e.g., 2500 mg for 200 g of soil);
▶ Soil: natural soil collected from the surface layer of fields and/or forests, and sieved to give particles of less than 5 mm, preferably less than 2 mm in size;
▶ Pre-exposed soil is generally not used, but could also be used depending on the purpose of the test;
▶ Soil characteristics: (a) total water-holding capacity, (b) pH, and (c) organic-matter content;
▶ Number of reactors: 3 for blank control, 3 for reference control, and 3 for test substance;
▶ Duration: normally up to 6 months; prolong the test up to 2 years if significant biodegradation is still observed after 6 months;
▶ Upon termination, determine pH, nitrate and nitrite to correct the calculated degree of biodegradation for nitrification (for the oxygen consumption method).

ISO 17556 Requirement and Applicability

Different testing methods are applicable for materials with different properties. Below is a summary of the applicability of ISO 17556. You can also find the applicability for many other methods on Aropha Resource Center
. 

TestAnalytical methodSample info required *Poorly solubleVolatileAdsorbing
Oxygen demand in a respirometerOxygenThOD*+++
CO2 evolutionCO2Organic carbon content*+-+

*"Sample info required" is the information needed to calculate the biodegradation percentages. This must be available for a selected method.
*"ThOD" can be easily calculated based on the chemical formula of the sample. Try our Online ThOD Calculator.
*"Organic carbon content" is the ratio of the organic carbon weight to the weight of the sample. It can be calculated by the chemical formula (e.g., acetic acid C2H4O2, carbon content is 12*2/(12*2+1*4+16*2)=40%). Try our Online C% Calculator. If the formula is unknown, we can send the sample out to a third party lab for you for analysis (normally $110 with a 10-day turnaround time).

Aropha Resource Center

To learn more about different types of biodegradation tests, their applicability, biodegradation mechanisms, and many other information such as case studies, publications, and blogs, please check our Aropha Resource Center
.

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