Chemical Anaerobic Biodegradability Testing: Standards, Methods, and Applications

Anaerobic biodegradability testing evaluates the ability of chemical substances to decompose under oxygen-free conditions, such as those found in sludge digesters, sediments, and landfills, by measuring the biogas (methane and carbon dioxide) produced by microorganisms. This article provides a comprehensive overview of chemical anaerobic biodegradability testing, covering the technical principle, applicable sample types, test parameters, internationally recognized standards such as ISO 11734 and OECD 311, and typical regulatory and industrial applications.

Technical Overview

Anaerobic biodegradability testing determines the extent to which an organic chemical substance can be mineralized by microorganisms in the absence of molecular oxygen. Unlike aerobic biodegradation, which consumes oxygen and produces carbon dioxide and water, anaerobic biodegradation proceeds through hydrolysis, acidogenesis, acetogenesis, and methanogenesis, ultimately yielding a biogas composed primarily of methane (CH4) and carbon dioxide (CO2).

The standard test principle involves incubating the test substance as the sole additional carbon and energy source in a sealed vessel containing an anaerobic inoculum, typically digested sludge collected from a municipal wastewater treatment plant. The vessels are incubated in the dark at a controlled mesophilic temperature of 35 ± 2 °C for a test period of up to 60 days or longer. The extent of biodegradation is quantified by continuously or periodically measuring the volume or pressure increase of the biogas produced, and expressing the net gas production as a percentage of the theoretical gas production (ThGP) calculated from the carbon content of the test substance.

A complete test design includes several parallel vessels: a blank control containing only inoculum to correct for background gas production, a reference control using a well-degradable compound such as cellulose or sodium benzoate to confirm inoculum activity, an abiotic control to rule out non-biological gas generation, and a toxicity control to verify that the test substance does not inhibit the anaerobic microbial community at the tested concentration. Accredited third-party laboratories typically follow this design to ensure data quality and regulatory acceptability.

Testing Samples

Pure and Technical-Grade Chemical Substances

This category includes surfactants, organic solvents, monomers, intermediates, dyes, pesticides, and pharmaceutical ingredients. The purity, water solubility, and carbon content of the substance should be documented in advance, because these parameters directly affect the preparation of the test solution and the calculation of theoretical gas production.

Formulated Products and Mixtures

Detergents, cleaning agents, personal care formulations, lubricants, and other multi-component products can be evaluated as whole formulations. For mixtures, the total organic carbon content is used to calculate the theoretical gas production, and the test report should clearly state the composition basis of the calculation.

Polymeric and Plastic Materials

Biodegradable plastics, films, coatings, and polymer additives are commonly tested under high-solids anaerobic digestion conditions or in aqueous anaerobic systems. Samples are usually prepared as powders or films with defined particle size or surface area to ensure reproducible microbial access.

Poorly Soluble and Volatile Substances

Chemicals with low water solubility or high volatility require adapted dosing techniques, such as direct weighing onto inert carriers or dosing through the septum of a sealed vessel. These adaptations must be described in the test report because they influence the validity of the degradation curve.

Testing Items

Core and supplementary parameters typically reported for anaerobic biodegradability testing include:

• Ultimate anaerobic biodegradability, expressed as the percentage of theoretical gas production (% ThGP) achieved within the test period.
• Net biogas production volume, corrected for the blank control, together with the cumulative biodegradation curve over time.
• Methane and carbon dioxide composition of the produced biogas, usually determined by gas chromatography.
• Dissolved organic carbon (DOC) removal or total organic carbon (TOC) balance where aqueous phase analysis is applicable.
• Inhibition assessment based on the gas production of the toxicity control compared with the combined blank and reference vessels.
• Supporting process parameters such as pH, volatile fatty acids (VFA), alkalinity, and total solids and volatile solids of the inoculum.

Testing Methods

Several internationally recognized methods are available, and the selection depends on the nature of the sample and the intended regulatory or commercial purpose:

• ISO 11734: Water quality - Evaluation of the "ultimate" anaerobic biodegradability of organic compounds in digested sludge by measurement of biogas production.
• OECD Test Guideline 311: Anaerobic biodegradability of organic compounds in digested sludge by measurement of gas production.
• ASTM D5210: Determining the anaerobic biodegradation of plastic materials in the presence of municipal sewage sludge.
• ASTM D5511: Determining anaerobic biodegradation of plastic materials under high-solids anaerobic digestion conditions.
• ISO 14853: Determination of the ultimate anaerobic biodegradation of plastic materials in an aqueous system by measurement of biogas production.
• ISO 15985: Determination of the ultimate anaerobic biodegradation of plastic materials under high-solids anaerobic digestion conditions by measurement of biogas production.

Regardless of the specific standard, the general test procedure follows a consistent workflow. The inoculum is first washed and pre-conditioned to minimize background gas production. Test substance is then added at a defined concentration, commonly in the range of 20 to 100 mg of organic carbon per liter for aqueous tests, into replicate serum bottles. The bottles are flushed with an oxygen-free gas mixture, sealed with butyl rubber stoppers, and incubated at 35 ± 2 °C in the dark. Gas pressure or volume is monitored throughout the incubation, and headspace composition is analyzed by gas chromatography at defined intervals.

Interpretation criteria are generally consistent across methods. Biodegradation exceeding 60% of theoretical gas production within the test period is regarded as evidence of ultimate anaerobic biodegradability, values between approximately 20% and 60% indicate partial degradation, and values below 20% suggest that the substance is not biodegradable under the tested anaerobic conditions. The reference compound should normally reach a high degradation level by the end of the test, for example at least 60% of theoretical gas production, to confirm inoculum viability.

Applications

Anaerobic biodegradability data serve a wide range of regulatory, environmental, and commercial purposes:

• Chemical registration and notification under regulatory frameworks, where anaerobic degradation data support environmental fate assessment of substances that may reach digested sludge or anoxic sediment compartments.
• Environmental risk assessment, including persistence evaluation in sediment and saturated soil scenarios and screening within persistent, bioaccumulative, and toxic (PBT) assessment strategies.
• Wastewater treatment compatibility, helping operators confirm that process chemicals and formulation ingredients will not accumulate in anaerobic digesters or impair biogas recovery.
• Substantiation of biodegradable claims for detergents, surfactants, plastics, and consumer products, including support for product declarations and eco-label documentation.
• Research and development of green chemistry alternatives, providing comparative degradation data for new molecules, formulations, and polymeric materials before market introduction.
• Discharge permit support and industrial wastewater characterization for facilities discharging effluent to municipal treatment infrastructure.

FAQ

What is the difference between aerobic and anaerobic biodegradability testing?

Aerobic tests measure oxygen consumption or CO2 evolution in the presence of oxygen, while anaerobic tests measure methane and CO2 production in sealed, oxygen-free systems using digested sludge inoculum. Many substances degrade well aerobically but persist anaerobically, so the two tests describe complementary environmental fate pathways.

How long does an anaerobic biodegradability test take?

The standard incubation period is up to 60 days, although testing of slowly degrading substances or polymeric materials may be extended to 90 days or more when justified by the degradation curve and confirmed inoculum activity.

What inoculum is used for the test?

The most common inoculum is digested sludge from a well-operating municipal anaerobic digester, used either as washed sludge or diluted digestate. The inoculum must demonstrate satisfactory activity by degrading the reference compound to the required level within the test period.

How much sample is required?

For soluble organic chemicals, typically 20 to 100 grams of sample is sufficient for a full test with replicates and analytical confirmation. Polymeric materials may require larger quantities depending on the selected standard and the required number of replicates.

What result is considered a pass?

Biodegradation greater than 60% of the theoretical gas production within the test period is generally accepted as demonstrating ultimate anaerobic biodegradability. Results should always be interpreted together with the reference control performance and the validity criteria of the applied standard.

Can insoluble or volatile chemicals be tested?

Yes. Direct weighing of the substance into the test vessel, use of inert carriers or silica dispersion, and gas-tight dosing through septa are established adaptations that allow testing of poorly water-soluble and moderately volatile substances without compromising the anaerobic integrity of the system.