ISO 17556:2019 is a method for determining the ultimate aerobic biodegradability of plastic materials in soil by either measuring the oxygen demand in a closed respirometer or the amount of carbon dioxide evolved. The plastic material, which is the sole source of carbon and energy, is mixed with soil, and the mixture stands in a flask over a period of time during the which the amount of oxygen consumed (BOD) or the amount of carbon dioxide evolved is measured.

Provided that the evolved CO₂ absorbed in some way, such as using an alkaline solution, the BOD can be determined by measuring the amount of oxygen required to maintain a constant gas volume in a respirometer flask, or by measuring the change in volume or pressure (or a combination of the two). Alternatively, the amount of CO₂ evolved can be measured at intervals dependent on the biodegradation kinetics of the test substance by passing CO₂-free air over the soil and then determining the CO₂ content of the air.

The level of biodegradation, expressed as a percentage, is determined by comparing the BOD with the theoretical oxygen demand or by comparing the amount of CO₂ evolved with the theoretical amount. A normal test period is six months. The test may be shortened or extended until the plateau phase is reached, but the total test period should not exceed two years.

BPC Blue is a fully automated biodegradability testing system suitable for ISO 17556. Utilising an in-situ carbon dioxide absorption kit, BPC Blue functions as a closed-respirometer for continuous and on-line measurement of oxygen consumption or depletion caused by aerobic respiration of microorganisms. The instrument is suitable for all test protocols which the measurement principle is based on oxygen demand measurement.