OECD 311 Anaerobic Biodegradability of Organic Compounds in Digested Sludge: Measurement of Gas Production

The standard “OECD 311 Anaerobic biodegradability of organic compounds in digested sludge: By measurement of gas production”, published under the OECD guidelines on responsible business conduct for multinational enterprises (OECD guidelines), describes a screening method for assessing the potential anaerobic biodegradability of organic chemicals under certain conditions.

Washed digestate sludge containing low concentrations of inorganic carbon (up to 10 mg/L) is diluted approximately tenfold to a total solids concentration of 1 g/L to 3 g/L and incubated with 20 to 100 mg C/L of test substance in closed containers at 35°C plus/minus 2°C for up to 60 days. The activity of the sludge is measured by running parallel blank controls. A reference substance (phenol, sodium benzoate, polyethylene glycol 400) is tested in parallel to control the procedure.

At least three test vessels are prepared for the test substance, blank controls, reference substance, inhibition controls, and pressure control chambers. The increase in headspace pressure resulting from carbon dioxide and methane production in the vessels is measured. Inorganic carbon is measured at the end of the test. The amount of carbon (inorganic plus methane) produced by biodegradation of the test substance is calculated from the net gas production and net IC50 formation in the liquid phase, exceeding the blank control values. The amount of biodegradation is calculated from the total IC50 and methane-C50 produced as a percentage of the measured or calculated amount of carbon added as the test substance.

The OECD 311 standard assesses the potential anaerobic biodegradability of organic chemicals under conditions similar to those in anaerobic digesters, such as those found in wastewater treatment plants. This test is critical for assessing the environmental impact of chemicals, particularly surfactants used in detergents and cleaning products, by determining their ability to biodegrade in an oxygen-free environment.

This test, a standard method developed by the Organisation for Economic Co-operation and Development (OECD), is designed to assess the degradability of organic compounds by microorganisms under anaerobic conditions, such as in sewage sludge digesters, sediments, or groundwater. Unlike aerobic biodegradation tests (e.g., the OECD 301 series), which evaluate degradation in the presence of oxygen, the OECD 311 standard focuses on anaerobic environments, where methane and carbon dioxide are the primary end products of degradation. This test is particularly important for compliance with regulations such as the European Union eco-label, which requires surfactants to be biodegradable under both aerobic and anaerobic conditions.

This test method serves as a screening tool to determine ultimate anaerobic biodegradability, which refers to the complete breakdown of a compound into simpler molecules such as methane, carbon dioxide, and water. It is suitable for a wide range of organic compounds, including water-soluble and water-insoluble substances such as detergents, surfactants, industrial chemicals, and polymers.

The OECD 311 test is based on the standard “ISO 11734 Water quality - Assessment of the "ultimate" anaerobic biodegradability of organic compounds in digested sludge - Method for measuring biogas production” developed by the International Organization for Standardization (ISO) and includes the following basic steps:

• Test medium: Washed digested sludge from a wastewater treatment plant, primarily used for municipal wastewater treatment, is used as the inoculum. The sludge is diluted to a total solids concentration of 1 g/L to 3 g/L and contains low levels of inorganic carbon (up to 10 mg/L) to minimize interference.
• Test conditions: Sludge is incubated with test substance at concentrations of 20 to 100 mg C/L in closed containers at a controlled temperature of 35°C plus/minus 2°C for up to 60 days. This period simulates the conditions of an anaerobic digester.
• Controls and references: To ensure accuracy, testing includes:
  • Blank controls to measure base gas production from mud.
  • Reference substances (e.g. phenol, sodium benzoate or polyethylene glycol 400) to verify the activity of the sludge.
  • Inhibition controls to assess whether the test substance inhibits microbial activity.
  • Pressure control rooms to monitor gas production.
• Measurement: Biodegradation is measured by monitoring the increase in pore pressure caused by the production of carbon dioxide and methane. The amount of inorganic carbon in the liquid phase is also measured at the end of the test. The extent of biodegradation is calculated as the percentage of total carbon produced (inorganic carbon and methane) relative to the carbon content of the test substance.

The OECD 311 test is widely used in regulatory frameworks to assess the environmental fate of chemicals. It is particularly important for:

• Regulatory compliance: In European Union countries, this test is accepted to demonstrate anaerobic biodegradability for eco-label certifications, such as surfactants in detergents.
• Environmental risk assessment: Helps assess the persistence of chemicals in anaerobic environments such as sediments, groundwater or anaerobic digesters.
• Product development: Manufacturers of chemicals, detergents, and cleaning products use this testing to ensure their products meet environmental standards.

Although the OECD 311 test is a powerful screening tool, it has some limitations:

• Test duration: The standard 60-day period may not capture the full degradation potential of slowly biodegradable compounds.
• Inoculum variability: The microbial composition of the sludge can vary depending on its source, which can affect reproducibility.
• No pre-adaptation: The test prohibits pre-adaptation of the vaccine to the test substance, which may result in longer lag phases (up to 20 days or more) and may not reflect real-world scenarios where adapted microbes are present.
• Screening nature: As a screening test, OECD 311 may not fully simulate complex environmental conditions and further testing may be required for definitive results.

As a result, the OECD 311 standard is a critical tool for assessing the anaerobic biodegradability of organic compounds and provides essential data for environmental risk assessments and regulatory compliance. By simulating the conditions in anaerobic digesters, it helps ensure that chemicals, particularly surfactants, do not persist in anaerobic environments. Despite its limitations, the test remains a cornerstone of environmental safety testing, with ongoing work to develop and improve its methodologies to meet modern regulatory and scientific needs.

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 OECD 311 standard: "Anaerobic biodegradability of organic compounds in digested sludge: Measurement of gas production."