The objective of this research is to investigate mass transfer mechanism in biofilms under oscillatory flow conditions, which provides valuable information for the understanding of self-purification by biofilms under oscillatory flow conditions like in coastal areas. A mathematical model was developed to describe substrate profiles in biofilms under oscillatory flow conditions, which suggested that substrate uptake rate by biofilms is proportional to the square root of the Reynolds number. A laboratory-scale channel with a wave generator was prepared, in which plastic plates were used as substratum for biofilms. Glucose was used as a substrate. Velocity and turbulence profiles near the biofilm were measured by a laser Doppler velocimeter. The cycle of the wave generator was changed and its short-term effect on the substrate uptake rate by the biofilm was measured. The substrate uptake rate decreased with the decrease of the Reynolds number of the wave motion according to a power law with a coefficient of 0.6, which suggests that substrate transport in biofilms under oscillatory flow conditions is carried out by turbulent diffusion caused by oscillatory flow motions over biofilms.

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