ISO 9439 Assessment of the Ultimate Aerobic Biodegradability of Organic Compounds in Aqueous Media - Carbon Dioxide Evolution Test

The conditions described in this standard do not always correspond to the optimum conditions that ensure maximum biodegradation. With this test system, microbially derived carbon dioxide is measured in traps through which gas from test vessels is passed. Some of the carbon dioxide remains in the vessels as dissolved inorganic carbon, and its concentration may increase as biodegradation progresses.

As organic carbon nears complete removal, the dissolved inorganic carbon concentration gradually decreases, tending to reach zero by the end of incubation. Therefore, to fully measure biogenically generated carbon dioxide, it is necessary to acidify the medium at the end of the test.

The standard "ISO 9439 Water quality - Assessment of ultimate aerobic biodegradability of organic compounds in an aqueous medium - Carbon dioxide evolution test" developed by the International Organization for Standardization (ISO) describes a method for assessing the ultimate biodegradability of organic compounds by aerobic microorganisms in an aqueous medium at a given concentration by means of carbon dioxide determination.

This method is valid for organic compounds that have the following properties:

• Soluble in water under test conditions. In this case, dissolved organic carbon removal may be determined as additional information.
• Slightly soluble in water under test conditions. In this case, special precautions may be required to ensure good dispersion of the compound (e.g. ISO 10634 standard).
• Non-volatile or having a negligible vapour pressure under the test conditions (for volatile substances, for example, the ISO 9408 or ISO 14593 standards apply).
• No inhibitory effect on test microorganisms at the concentration selected for the test. The presence of inhibitory effects can be determined as specified in this standard or using another method for determining the inhibitory effect of a compound on bacteria (e.g. ISO 8192 standard).

In principle, the biodegradability of organic compounds by aerobic microorganisms is determined using a static aqueous test system. The test mixture contains an inorganic medium, an organic compound at a concentration of 10 mg/L to 40 mg/L of organic carbon as the nominal sole carbon and energy source, and a mixed inoculum obtained from a wastewater treatment plant or other environmental source. The mixture is shaken in test vessels and aerated with carbon dioxide-free air, typically for up to 28 days.

Carbon dioxide formed during microbial degradation is retained in external containers, determined by a suitable analytical method, compared with the theoretical amount and expressed as a percentage.

For sufficiently water-soluble compounds, dissolved organic carbon removal can optionally be measured to obtain additional information on ultimate biodegradability. This can be done using the method provided, but a useful procedure is described in the appendix to this standard (Annex D) that allows the use of higher concentrations of the test compound and inoculum, thus increasing the biodegradability potential of the test. If a substance-specific analytical method is available, information on primary degradability can also be obtained.

Incubation should be carried out in the dark or in diffused light at a temperature between 20°C and 25°C that does not vary by more than plus/minus 2°C during the test period.

In short, the ISO 9439 standard provides a procedure for assessing the ultimate aerobic biodegradability of organic compounds in an aqueous environment by measuring the carbon dioxide produced during biodegradation. This standard is of critical importance to industries and researchers assessing the environmental impacts of chemical substances, particularly their ability to naturally degrade in water.

This standard is designed to assess the extent to which organic compounds are degraded by aerobic microorganisms in an aqueous environment. The method focuses on carbon dioxide evolution as a primary indicator of biodegradation, reflecting the conversion of organic carbon to carbon dioxide and water.

This standard applies to:

• Water-soluble organic compounds.
• Compounds with low water solubility should be dispersed with special dispersion measures (for example as specified in the ISO 10634 standard).
• Non-volatile compounds or compounds with negligible vapor pressure under test conditions.
• Compounds that do not inhibit the activity of test microorganisms at the selected concentration.

The primary objective of the ISO 9439 standard is to determine ultimate biodegradability—that is, the complete breakdown of a compound into carbon dioxide, water, and mineral salts through microbial activity. This is particularly important for assessing the environmental fate of chemicals such as lubricants, surfactants, fuels, and personal care products and ensuring they do not persist in aquatic ecosystems.

The ISO 9439 test method is performed in continuously ventilated bottles for at least 28 days using respirometry to measure carbon dioxide production. The key steps are:

• Test setup: The test sample is introduced into an aqueous medium containing aerobic microorganisms, typically obtained from activated sludge or similar inoculum. The system is ventilated with carbon dioxide-free air to ensure accurate measurement of carbon dioxide produced by biodegradation.
• Carbon dioxide measurement: The amount of carbon dioxide released is measured periodically, usually by keeping it in a solution (e.g., sodium hydroxide) and analyzing it by titration or other analytical methods.
• Calculating biodegradability: Percent biodegradability is calculated by comparing the measured carbon dioxide to the theoretical maximum carbon dioxide that would be produced if the compound were completely degraded. This provides a clear measure of biodegradability.

This test is designed to cover a wide range of compounds, including those with low solubility that are non-volatile and have no inhibitory effect on microorganisms. For poorly soluble substances, techniques similar to those described in ISO 10634 can be used to ensure appropriate dispersion.

The ISO 9439 standard is widely used in environmental science and regulatory contexts. It is particularly important for:

• Chemical safety assessments: This standard is typically used to meet regulatory requirements for chemical notification, such as the REACH framework of European Union countries or OECD guidelines.
• Product development: Industries that develop biodegradable products such as plastics, surfactants, or personal care products use this standard to verify environmental compatibility.
• Environmental risk assessment: This test assists in risk assessments for ecosystems by helping determine whether a compound will persist in aquatic environments.

Advanced laboratories offer ISO 9439 standard testing services serving industries that want to verify the biodegradability of their products.

While ISO 9439 is a powerful method, it has some limitations:

• Non-volatile requirement: The test is not suitable for volatile compounds, as aeration can cause loss of the test substance and falsify the results. For volatile substances, alternative standards such as ISO 9408 or ISO 14593 are recommended.
• Inhibition test: The test assumes that the compound does not inhibit microbial activity. If inhibition is suspected, additional testing (e.g., ISO 8192 standard) is required to confirm the suitability of the compound for testing.

Consequently, the ISO 9439 standard is a vital tool for assessing the aerobic biodegradability of organic compounds in aqueous environments. By measuring carbon dioxide evolution, it provides a reliable and standardized method for assessing how easily a substance can be degraded by microorganisms, providing critical data for environmental protection and regulatory compliance.

Our organization, which has been operating for many years and meticulously follows global developments in science and technology, employs a team of experienced professionals and a broad infrastructure to conduct testing, measurement, and analysis in a wide range of areas for businesses across all sectors. In this context, we also provide testing services in accordance with the ISO 9439 Water quality - Assessment of the ultimate aerobic biodegradability of organic compounds in aquatic environments - Carbon dioxide evolution test.