Little Pro on 2016-03-23

Degradation is the process by which a chemical substance is broken down to smaller molecules by biotic means (biodegradability) or abiotic means (hydrolysis, photolysis or oxidisation). Half-lives (DT50) are used as measures of the stability and persistence of a chemical substance in the environment. Half-life (DT50) is defined as the time it takes for an amount of a compound to be reduced by half through degradation. It is a very important value for PBT assessment.


Biodegradation is the process by which organic substances are broken down by living organisms such as bacteria and fungi. Biodegradation can happen in surface water, sediment and soil.

The pass levels for ready biodegradability are 70% removal of DOC and 60% of ThOD or ThCO2 production for respirometric methods (OECD 301). The pass values have to be reached in a 10-d window within the 28-d period of the test.

If a substance is not readily biodegradable, an inherent biodegradability test may be conducted to assess whether the chemical substance has any potential for biodegradation under aerobic conditions.

Biodegradation Simulation Tests

Simulation tests aim at assessing the rate and extent of biodegradation in a laboratory system designed to represent either the aerobic treatment stage of STP or environmental compartments, such as fresh or marine surface water.

Abiotic Degradation

It mainly includes hydrolysis and photolysis. Hydrolysis in water is often dependent on pH. It half-life value will help to estimate how long a chemical substance will persist in an aqueous environment.

Degradation and Persistence Criterion

Under EU REACH regulation (Annex XIII), a substance fulfils the persistence criterion (P-) when:

  • the half-life in marine water is higher than 60 days, or
  • the half-life in fresh- or estuarine water is higher than 40 days, or
  • the half-life in marine sediment is higher than 180 days, or
  • the half-life in fresh- or estuarine water sediment is higher than 120 days, or
  • the half-life in soil is higher than 120 days.

US EPA uses the following persistence criterion (link):

  • Persistent: Half-life in water, soil, and sediment>60 days;
  • Very persistent: Half-life in water, soil, and sediment>180 days;

Testing Guidelines

  • Ready Biodegradability: OECD Test Guidelines No. 301 A-F: DOC Die-Away Test(TG 301 A), CO2 Evolution Test (TG 301 B), Modified MITI Test (I) (TG 301 C), Closed Bottle Test (TG 301 D), Modified OECD Screening Test (TG 301 E) and Manometric Respirometry Test (TG 301 F);
  • Inherent biodegradability: OECD Test Guidelines No. 302 A-C: Modified SCAS Test (TG 302 A), Zahn-Wellens/EMPA Test (TG 302 B) and Modified MITI Test (II) (TG 302 C);
  • Simulation Test: Activated Sludge Units (TG 303 A) and Biofilms (TG 303 B), Aerobic and Anaerobic Transformation in Soil (TG 307), Aerobic and Anaerobic Transformation in Aquatic Sediment Systems (TG 308); and Aerobic Mineralisation in Surface Water – Simulation Biodegradation Test (TG 309);
  • Hydrolysis: Hydrolysis as a Function of pH (revised OECD TG 111);
  • Photolysis: Phototransformation of Chemicals in Water – Direct Photolysis ( OECD TG 316).

All testing guidelines above can be accessed here.


You have learned the definition of degradation (biodegradation and abiotic), simulated tests, testing guidelines, dose descriptors (half-lives), and persistence criteria. .

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