This part of ISO 13641 specifies a screening method for assessing the potential toxicity of substances, mixtures, surface waters, groundwaters and wastewaters, 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) were adopted to a defined test period over several days. The inoculum can be collected from anaerobic sediments or from large, or laboratory scale, anaerobic digesters. This method is applicable to materials, soluble or insoluble in water, including volatile chemicals (see Reference [1] in the Bibliography). NOTE Special care is necessary with compounds of low water-solubility, and in these cases, see for example, ISO 10634. For general information on biotesting see ISO 5667-16[2]. Information obtained by this method can be helpful prior to anaerobic biodegradability testing with low inoculum mass concentrations and for estimations of the potential effects of chemicals and wastewater to anaerobic processes in habitats characterized by a relatively low anaerobic biomass, for example natural sediments and soils.