Inhibition concentrations of 50 organic chemicals acting singly and jointly in different combinations on activated sludge microorganisms and a surrogate test microbial culture, Polytox, were measured using the respirometric technique. The suitability of the surrogate test culture in toxicity studies was evaluated against activated sludge microorganisms. The toxicity correlation between the two organisms was highly significant with coefficient of determination of 0.922. The joint toxic effects of several 2-, 8-, and 10-component mixtures on the surrogate cultures were found to be simply additive. Using the single chemical toxicity data, Quantitative Structure Activity Relationship (QSAR) models were developed to relate toxicity to molecular structures of the chemicals. The application of these QSAR models in predicting joint effects in six 2-, six 8-, and ten 10-component mixtures was demonstrated. These predictions agreed with the experimentally measured values with coefficient of determination of 0.866 at p < 0.0001.

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