A NOVEL CONCEPT FOR EVALUATION OF BIOFILM ADHESION STRENGTH BY APPLYING TENSILE FORCE AND SHEAR FORCE

A novel methodology is proposed in this study to evaluate biofilm adhe sion strength in two different ways: by measuring detached biomass cau sed by tensile force and by shear force. Tensile force was provided by centrifuging biofilm-attached plates installed on rotary tables. Shea r force was provided by colliding biofilm-attached plates by gravity. Test biofilms consisting of denitrifiers were formed on the flat surfa ces of square (25 cm(2)) plates that had been submerged in a rectangul ar open-channel reactor. The detachment tests revealed that, although biofilm adhesion strength was relatively high at the earlier growth st age, it drastically decreased at the later stage. The most weakened lo cation toward biofilm depth was observed at the substratum surface, at which the adhesion strength by tensile force dropped from a several P a to below 1 Pa as biofilms became aged. The adhesion strength by shea r force was all the time more than 100 times as large as that by tensi le force, even though having a similar behavior. The proportion of cav ity, i.e., biofilm-absent area at the biofilm/substratum interface, in creased as biofilms became mature. Cavity formation was strongly respo nsible for lessening the adhesion strength. It is suggested that biofi lm slough-off is caused by the decline of adhesion strength by tensile force rather than by shear force. Copyright (C) 1996 IAWQ.