Evaluation of wastewater toxicity: comparative study between Microtox® and activated sludge oxygen uptake inhibition

Microtox® is a frequent toxicity tool for the screening of wastewaters discharged into wastewater treatment plants. There is currently an increasing controversy between this test and others using activated sludge. A Microtox® and electrolytic respirometry comparative study for toxicity determination has been performed. Seven organic and five inorganic toxic compounds have been assessed for comparing both methods. Microtox® proved to have a higher sensitivity to toxicants but was less representative of effects on activated sludge compared to respirometry. For instance, assays accomplished with LAS, a biodegradable reference surfactant, showed a toxic effect by Microtox® but good biodegradability and no toxicity in respirometry. This could be explained by the different nature of the biological material used, as Microtox® utilises the seawater Vibrio fischeri, whereas respirometry uses the bacterial consortium in activated sludge. For the evaluation of the potential toxicity of a compound on a WWTP, the preferred biological material be used should be activated sludge itself. Results obtained with any other biological material would be just an approach to reality.