ADAPTATION OF THE MICROBIAL ADHESION TO HYDROCARBON TEST (MATH) FOR MEASURING ACTIVATED-SLUDGE HYDROPHOBICITY

The ''microbial adhesion test to hydrocarbon'' (MATH) developed by Ros enberg et al. gives a cell hydrophobicity index (A% = percentage of ad hesion) and is easy to perform. However it is not applicable with any degree of accuracy to activated sludge even when dispersed by sonicati on, because of the presence of aggregated and free cells and of amphip hilic organic polymers from sludge which lead to the formation of a st able emulsion. The effect of emulsion formation is a decrease in bacte rial number from the aqueous phase by adhesion on to, and trapping bet ween, the droplets of octane. This induces an overestimation of the hy drophobicity of the bacterial suspension. To solve this problem, we pr opose modifications of the MATH which may be used for any bacterial su spension and which expresses the bacterial adsorption equilibrium betw een the surface of the octane droplets and the water phase. The test s tarts just as for the regular MATH by mixing the bacterial suspension and octane. The resulting emulsion is sampled, and then serially washe d 10 times with 5 ml of MilliQ water. By plotting the decreasing numbe rs of bacteria recovered in the washing solutions against the number o f bacteria adhering to octane droplets, a linear relationship is obtai ned the slope of which equals an equilibrium constant Ke. For the 5 sa mples of sludge tested, Ke (values from 1.3 to 3.2) appears to be a mo re accurate and sensitive parameter than A% (values from 62 to 86%). ( C) 1998 IAWQ. Published by Elsevier Science Ltd.