The objective of this study was to examine a simple anaerobic gas production test for screening the toxicity of organic chemicals to anaerobic metabolism. Inhibition of anaerobic digestion can pose a problem in sludge treatment systems as it may limit the biodegradation of hazardous pollutants. The method used was based on a single measurement of the headspace pressures in closed bottles containing inoculated medium and serial dilutions of the test chemicals. Under the applied methanogenic conditions, the anaerobic gas production was particularly sensitive to surfactants and Cl and NO2 substituted phenols. E.g., the EC50 values for the surfactants alkyl dimethylbenzylammonium chloride and sodium alkyl ethersulfate were 6.7 and 11 mg/l. Two of the surfactants and two of the phenols were also examined in the Microtox bioassay and in the growth inhibition test with Pseudomonas putida as these tests are commonly used for screening the toxicity of chemicals. The anaerobic gas production test was considerably less sensitive to toxic chemicals than the Microtox bioassay. However, for the surfactants, the anaerobic tests gave lower effect concentrations than obtained in the tests with P. putida. Screening the toxicity of chemicals to anaerobic processes may predict the potential effects of pulse loadings to anaerobic digesters. Furthermore, knowledge about inhibitory concentrations may be very useful prior to anaerobic biodegradability testing.

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