OSCILLATION CHARACTERISTICS OF BIOFILM STREAMERS IN TURBULENT FLOWINGWATER AS RELATED TO DRAG AND PRESSURE-DROP

Mixed population biofilms consisting of Pseudomonas aeruginosa, P. flu orescens, and Klebsiella pneumoniae were grown in a flow cell under tu rbulent conditions with a water flow velocity of 18 cm/s (Reynolds num ber, Re, = 1192). After 7 days the biofilms were patchy and consisted of cell clusters and streamers (filamentous structures attached to the downstream edge of the clusters) separated by interstitial channels. The cell clusters ranged in size from 25 to 750 mu m in diameter. The largest clusters were approximately 85 mu m thick. The streamers, whic h were up to 3 mm long, oscillated laterally in the flow. The motion o f the streamers was recorded at various flow velocities up to 50.5 cm/ s (Re 3351) using confocal scanning laser microscopy. The resulting ti me traces were evaluated by image analysis and fast Fourier transform analysis (FFT). The amplitude of the motion increased with flow veloci ty in a sigmoidal shaped curve, reaching a plateau at an average fluid flow velocity of approximately 25 cm/s (Re 1656). The motion of the s treamers was possibly limited by the flexibility of the biofilm materi al. FFT indicated that the frequency of oscillation was directly propo rtional to the average flow velocity (u((ave))) below 9.5 cm/s (Re 629 ). At u((ave)) greater than 9.5 cm/s, oscillation frequencies were abo ve our measurable frequency range (0.12-6.7 Hz). The oscillation frequ ency was related to the flow velocity by the Strouhal relationship, su ggesting that the oscillations were possibly caused by vortex shedding from the upstream biofilm clusters. A loss coefficient (k) was used t o assess the influence of biofilm accumulation on pressure drop. The k across the flow cell colonized with biofilm was 2.2 times greater tha n the k across a clean flow cell. (C) 1998 John Wiley & Sons.