ISO 13641-2:2003

Overview

ISO 13641-2:2003 - Water quality - Determination of inhibition of gas production of anaerobic bacteria - Part 2: Test for low biomass concentrations - is an ISO screening method for assessing the potential toxicity of substances, mixtures, surface waters, groundwaters, wastewaters, effluents and sludges by measuring biogas (methane + carbon dioxide) production from anaerobic inocula that have low biomass. The method is designed for settings where anaerobic growth rates are slow (natural sediments, soils, low-biomass digesters) and provides a standardized way to detect inhibition of anaerobic activity.

Key topics and technical requirements

• Scope & purpose: Screening for inhibition of anaerobic gas production at low inoculum mass concentration; applicable to soluble or insoluble materials including volatile chemicals (with caveats for poorly water‑soluble compounds).
• Test principle: Incubate mixtures of a defined inoculum and degradable substrate with varying concentrations of test material; measure biogas by headspace pressure increase before and after acidification (to release CO2 from carbonates). Calculate percentage inhibition and derive effective concentration values (e.g., ECx).
• Inoculum: Sourced from anaerobic sediments or full‑scale/laboratory digesters. Target final mass concentration: 0.20 g/L ± 0.05 g/L total dry solids. pH adjusted to ~7 ± 0.5.
• Conditions: Typical incubation temperature 35 °C, extended incubation times compared with aerobic tests. Test bottles are sealed, gas‑tight and pressure‑resistant (glass not recommended due to explosion risk).
• Reagents & media: De-aerated dilution water, high‑purity nitrogen for oxygen exclusion, phosphoric acid for CO2 release; 10× concentrated test medium and trace element solutions are specified. Reducing agents (sodium sulfide or titanium(III) citrate) ensure anaerobic conditions.
• Apparatus & measurement: Gas‑tight bottles with suitable septa, precision pressure meter (needle attachment and venting), microsyringes, optional glove box for anaerobic manipulations. Calibration procedures are provided (Annex A).
• Safety & interferences: Sludge may contain pathogens and inflammable gases - handle with care; avoid glass where possible; follow national biosafety rules. Special attention required for low-solubility compounds and volatile chemicals.

Practical applications and users

• Environmental laboratories performing toxicity screening of chemicals, industrial effluents, and contaminated sediments.
• Wastewater treatment operators and researchers assessing potential inhibitory effects on anaerobic digestion processes at low biomass concentrations.
• Regulatory agencies and consultants evaluating environmental risks in anaerobic habitats (natural sediments, soils) where biomass is limited.
• Useful as a precursor to detailed anaerobic biodegradability testing or process impact studies.

Related standards

• ISO 13641-1 (Part 1: General test) - broader methodology for inhibition of gas production.
• ISO 10634 - guidance for preparing poorly water‑soluble compounds for biodegradability tests.
• ISO 5667-16 - general information on biotesting and sampling.
• ISO 11923 - methods for total solids determination (referenced for inoculum mass concentration).

Keywords: ISO 13641-2:2003, water quality, anaerobic bacteria, inhibition of gas production, low biomass concentrations, biogas, methane, toxicity screening, anaerobic sediments, inoculum.