Toxicity assessment of common xenobiotic compounds on municipal activated sludge: comparison between respirometry and Microtox®

The toxicity of four xenobiotic compounds 3,5-dichlorophenol, formaldehyde, 4-nitrophenol and dichloromethane, representative of industrial wastewater contaminants was evaluated by a simple respirometric procedure set up on the basis of OECD Method 209 and by the Microtox® bioassay. Very good reproducibility was observed for both methods, the variation coefficients being in the range of 2–10% for the respirometric procedure and 6–15% for Microtox®, values that can be considered very good for a biological method. Comparison of EC50 data obtained with the two methods shows that in both cases 3,5-dichlorophenol is more toxic than other compounds investigated and dichloromethane has a very low toxicity value. Intermediate EC50 values were found for the two other chemicals, formaldehyde and 4-nitrophenol. Moreover, the Microtox® EC50 values are generally lower (except for dichloromethane) than the respirometric ones: these differences could be explained by the fact that the Microtox® method uses a pure culture of marine species and, therefore, should not necessarily be expected to behave like a community of activated sludge bacteria. In conclusion, both methods can be usefully applied for toxicity detection in wastewater treatment plants but it is advisable to take into account that Microtox® is more sensitive than respirometry in estimating the acute toxicity effect on the biomass operating in the plant.