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.