Hiroshi Nagaoka

Mass transfer mechanism in a porous riverbed

Abstract The mass transfer mechanism in a porous riverbed was investigated in two experimental open channels which have porous beds composed of 1.9 and 4.08 cm diameter ceramic balls, respectively. Vertical diffusion coefficients in the porous bed were determined by analyzing the diffusional pattern of tracer (NaCl) from the overlying water into the deeper region of the porous bed. Velocity profiles both over and under the water-bed interface and turbulence at the interface was measured. The diffusion coefficients in the porous bed were expressed by the product of velocity component and mixing length along every depth of the porous bed. Near the interface, the diffusion coefficient was expressed by the product of the turbulent intensity and the void scale of the porous layer. In the deeper region of the porous bed, the diffusion coefficient was expressed by the product of the time-averaged velocity and the void scale.

Effect of turbulence on nitrifying biofilms at non-limiting substrate conditions

Abstract The effect of turbulence on nitrifying biofilms was studied in five cylindrical PVC (polyvinyl chloride) reactors, each having ten biofilm sampling taps, over a period of 196 days. Bulk water in the reactors was stirred by paddles at 32, 92, 140, 278 and 500 rpm and the turbulent intensities measured at 10 mm from the wall were 0.6, 1.5, 2.6, 4.4 and 8.9 cm/s. Biofilms appeared as isolated colonies and continued to grow as filament-type biofilms. Higher turbulence resulted in higher NH4-N flux and higher areal biomass density. Turbulent diffusion of substrates and by-products in the vicinity of filament-type biofilms must have resulted in the above phenomena. Photographic observation of the biofilm surfaces on sampling taps showed uniform biofilm filaments at higher turbulent intensities and large variation in the height of filaments at low turbulent intensities. Substrate flux and biofilm structure (areal density, filament height and cross-sectional area of filament) are inter-related parameters and are strongly affected by turbulence near the biofilm. Substrate flux is expressed as a power function of turbulent intensity, volumetric density and substrate concentration for filament-type biofilm when substrates are non-limiting.

Biological self-purification in porous riverbed.

本研究では室内水路を用いて,河床が礫で構成された浸透層を有する場合に生物学的自浄作用に与える影響について考察した。河川における生物反応の一例として硝化作用をとり上げ,自浄能力の推定に用いた。水路の水理特性は塩水トレーサーを用いて測定した。浸透層内での自浄機構を表わす数学モデルを作り,浸透層では表面から粒子2個分までの深さの領域で物質混合が大きく,その結果表流水からの基質供給が十分で生物活性が大きいことを明らかにした。さらにシミュレーションによって自浄能力に与える生物反応速度定数,浸透層の深さの影響について考察した。礫で構成された河床は,生物膜に対する付着面積を増加させ,また表流水における適度の乱れによって生物活性を増加させることができると考えられるので,実際の河道設計においては,礫や岩を3～4層積み重ねることによって自浄能力の大幅な増加を期待できるはずである。