OECD 209: Activated sludge respiration inhibition test

Assessment of the inhibitory effects of chemicals on activated sludge microorganisms by measuring oxygen uptake according to OECD Test Guideline 209 (equivalent to EU Method C.11 under Commission Regulation (EC) No. 440/2008). The test is applicable to water-soluble, poorly soluble, and volatile substances, with appropriate measures taken to maintain exposure concentrations for volatile compounds.

Activated sludge from a wastewater treatment plant receiving predominantly domestic sewage is exposed to a geometric series of test substance concentrations together with synthetic sewage feed and dilution water for 3 hours at 20 ± 2°C and pH 7.5 ± 0.2. Oxygen uptake is measured using a dissolved oxygen electrode in a closed measurement cell. Where required, total respiration can be partitioned into:

  • Heterotrophic respiration (carbon oxidation)

  • Nitrification (ammonium oxidation), determined by measuring oxygen uptake with and without N-allylthiourea, a specific nitrification inhibitor

An optional shorter contact time (e.g., 30 minutes) is available for rapidly degrading or volatile compounds. Quality controls include blanks, optional abiotic controls (no inoculum), and a reference substance (typically 3,5-dichlorophenol) to confirm sludge sensitivity.

Results are reported as oxygen uptake rates and percentage inhibition relative to controls. Primary endpoints are ECx values (e.g., EC50) and/or NOEC, expressed in mg/L. Results also identify non-inhibitory concentrations suitable for use in subsequent biodegradability testing.

The method is accepted in REACH registration dossiers submitted to ECHA, where it fulfills the information requirement for toxicity to microorganisms. The closely related U.S. EPA guideline OPPTS 850.6800 (Modified Activated Sludge, Respiration Inhibition Test for Sparingly Soluble Chemicals) covers a modified version of the same test for poorly soluble substances.

Suitable sample matrices
Chemicals, mixtures
Available quality systems
Accredited testing laboratory
Standard
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