Estimating Toxicity of Organic Chemicals to Activated‐Sludge Microorganisms

Inhibition of respiration rates $\left({\mathrm{IC}}_{50}\right)$ of activated‐sludge (AS) microorganisms and a commercial surrogate culture, Polytox, were measured using the respirometric technique for 50 organic chemicals. The correlation between the ${\mathrm{IC}}_{50}$ values for the two cultures was found to be highly significant with $r^2=0.928$, the Polytox cultures being more sensitive than AS cultures. Using AS experimental inhibition data for a training set of 40 chemicals, a single‐variable quantitative structure activity relationship (QSAR) model was developed to estimate ${\mathrm{IC}}_{50}$ for AS. When this QSAR model was tested on a testing set of 10 “new” chemicals, the predicted ${\mathrm{IC}}_{50}$ values agreed well with the experimentally measured values, with $r^2=0.844$ at $p=0.0002$. Inhibition data reported in the literature for 32 additional chemicals were also compared with predictions made by the QSAR model, and the agreement was satisfactory, with an overall $r^2$ of 0.798, at $p=0.0001$.