ISO 9439:1999

Overview - ISO 9439:1999 (Carbon dioxide evolution test)

ISO 9439:1999 specifies a laboratory method to evaluate the ultimate aerobic biodegradability of organic compounds in aqueous media by measuring microbially produced carbon dioxide (CO2). The standard is intended for compounds that are water‑soluble (or can be dispersed), non‑volatile (or negligible vapor pressure) and not inhibitory to test microorganisms. The test measures CO2 evolved over an incubation period (typically up to 28 days) and expresses biodegradation as a percentage of the theoretical CO2 (ThCO2) yield.

Key topics and technical requirements

• Test principle: static aqueous system containing inorganic medium, test compound as sole carbon source (10–40 mg/L organic carbon), and a mixed environmental inoculum; aeration with CO2‑free air and trapping/analysis of evolved CO2.
• Inoculum: activated sludge or environmental samples (wastewater, surface water); preconditioning or pre‑exposure options are described. Inoculum quantity and quality must ensure sufficient biodegradative activity and meet validity criteria.
• Environmental conditions: incubation in the dark or diffused light at 20–25 °C (±2 °C). Agitation and controlled aeration are required.
• Apparatus and analysis: test vessels with CO2‑impermeable tubing, CO2‑free air supply (example flowrates provided), external CO2 traps and analytical methods (CO2 or DIC analyzers or titrimetric determination after absorption). DOC measurement is optional (Annex D).
• Quality and validity: use of reference compounds (e.g., aniline, sodium benzoate), inhibition checks, calculation against ThCO2, and reporting requirements are defined. Safety warning: activated sludge may contain pathogens-handle with appropriate precautions.

Practical applications and users

ISO 9439 is used to:

• Determine ultimate biodegradability for regulatory dossiers, environmental risk assessments and product stewardship.
• Support chemical manufacturers, environmental testing laboratories and consultants in characterizing persistence of organic substances in aquatic environments.
• Compare biodegradation performance of novel compounds, intermediates or formulations under aerobic conditions. Primary users include environmental laboratories, wastewater treatment operators, regulatory bodies, ecotoxicologists and R&D teams in chemical and pharmaceutical industries.

Related standards

• ISO 14593, ISO 15462 - alternative CO2‑based or biodegradation methods
• ISO 10634 - guidance for poorly water‑soluble compounds
• ISO 9408 - methods for volatile substances
• ISO 8192, ISO 9887, ISO 9888, ISO 11923 - related biodegradation, inhibition and analytical procedures

Keywords: ISO 9439:1999, carbon dioxide evolution test, aerobic biodegradability, water quality, DOC, ThCO2, activated sludge, CO2 measurement.