External detachment forces acting on evenly spaced biofilm colonies grown in a roto-torque reactor are calculated numerically using a finite element based 2-D CFD tool. The 2-D laminar shear flow around a biofilm colony is investigated for different shear rates corresponding to different rotating velocities of a roto-torque reactor. The effect of neighbouring biofilm colonies is accounted for by the choice of boundary conditions for the flow model. The forces acting on the biofilm colonies show an initial gradual increase as the spacing decreases from a large value up to a particular spacing beyond which they decrease rapidly with decreasing spacing. This trend is highly correlated with the detailed structure of the flow around the biofilm colonies.

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