This part of ISO 13641 specifies a screening method for assessing the potential toxicity of substances, mixtures, surface waters, groundwaters and waste waters, effluents, sludges or other environmental samples by determining the production of biogas (carbon dioxide and methane) from muds, sediments and other anaerobic environments with low biomass concentration. The growth rate of anaerobic bacteria is much lower, compared with that of aerobic microorganisms. For this reason, the test periods in anaerobic methods are longer than in those with aerobic bacteria. The conditions of this test (for example amount of inoculum and substrate in the test bottles) we