The “microbial adhesion test to hydrocarbon” (MATH) developed by Rosenberg et al. gives a cell hydrophobicity index (A% = percentage of adhesion) and is easy to perform. However it is not applicable with any degree of accuracy to activated sludge even when dispersed by sonication, because of the presence of aggregated and free cells and of amphiphilic organic polymers from sludge which lead to the formation of a stable emulsion. The effect of emulsion formation is a decrease in bacterial number from the aqueous phase by adhesion on to, and trapping between, the droplets of octane. This induces an overestimation of the hydrophobicity of the bacterial suspension.

To solve this problem, we propose modifications of the MATH which may be used for any bacterial suspension and which expresses the bacterial adsorption equilibrium between the surface of the octane droplets and the water phase. The test starts just as for the regular MATH by mixing the bacterial suspension and octane. The resulting emulsion is sampled, and then serially washed 10 times with 5 ml of MilliQ water. By plotting the decreasing numbers of bacteria recovered in the washing solutions against the number of bacteria adhering to octane droplets, a linear relationship is obtained the slope of which equals an equilibrium constant Ke. For the 5 samples of sludge tested, Ke (values from 1.3 to 3.2) appears to be a more accurate and sensitive parameter than A% (values from 62 to 86%).

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