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 the anaerobic digestion of sewage sludge over periods of up to 3 days. The growth rate of anaerobic bacteria is much lower compared with that of aerobic microorganisms. For this reason, the test periods are longer for anaerobic methods than those used for aerobic bacteria. This method is applicable to substances, 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]. This method can provide information that is useful in predicting the likely effect of a test material on biogas production in anaerobic digesters. For example, only longer tests, which simulate working digesters more closely can indicate whether adaptation of the microorganisms to the test material can occur or whether compounds likely to be adsorbed onto sludge can build up to a toxic concentration over a longer period than allowed in this test. Information obtained by this part of ISO 13641 can also be helpful in choosing suitable initial mass concentrations for anaerobic biodegradability tests (e.g. ISO 11734). However, this part of ISO 13641 using undiluted sludge is less suitable for testing dilute digesting sludge than the method described in Part 2 of ISO 13641 where the mass concentrations are a hundredfold more dilute.