Visualization of Microbubble Distribution Inside Turbulent Boundary Layer Along Flat and Curved Solid Surfaces

How microbubbles behave inside turbulent boundary layers are investigated experimentally. Water electrolysis is applied for generation of microbubbles in water, of which electrodes are flash mounted on the solid wall in the upstream section of the measurement area. Four kinds of solid surfaces are examined to compare the microbubble distribution. For a circular cylinder of the radius R = 22 mm at Re = 5,000, we found that microbubbles depart from the surface earlier than the liquid boundary layer. For an elliptic cylinder of the curvature radius of R = 60 mm and a hydrofoil of NACA0040, microbubble injection made the separation point move downstream in the range of 9,000 < Re < 90,000. To compare the effect with the cases of flat solid surfaces (R = infinity), we visualized three-dimensional distribution of microbubbles with color-coded volumetric illumination technique. The result has shown formation of microbubble clusters intermittently, which has Coulomb potential due to negative electric charge on bubble interfaces.

Biodegradation of lipids from olive oil mill wastewaters in a stationary basket bioreactor with immobilized Bacillus spp. cells – Influence of internal diffusion

The study is focused on the external and internal mass transfers of lipids during their biodegradation process in a bioreactor with stationary basket bed of immobilized Bacillus spp. cells. By means of the lipid mass balance for a single particle of biocatalyst, considering the kinetic model adapted for the immobilized bacterial cells, specific mathematical models have been developed to estimate their mass flows in the liquid boundary layer surrounding the particle and inside the particle. The values of mass flows are significantly influenced by the internal diffusion velocity of lipids and the rate of their consumption, but also by the position inside the basket bed. These influences accumulated led to the appearance of a biological inactive region near the particle centre, its magnitude varying from 1.3 to 49.4% of the overall volume of particles.

Nitrification Kinetics and Simultaneous Removal of Biomass and Phosphorus in Rotating Biological Contactors

This paper presents some important aspects of the Rotating Biological Contactor. (1) Steady-state biofilm kinetics and its application to the design of an RBC aiming at nitrification : Using the proposed kinetics in which the flux of rate-limiting substrate is expressed as a function of the bulk substrate concentration, liquid boundary layer thickness, liquid film thickness, and molecular diffusion coefficient and intrinsic reaction rate of the substrate, the relationship between the bulk ammonia concentration and ammonia flux was predicted at various sizes and rotating speeds of disk. Experimental verification of the predicted results was also made. A new disk media, i.e., reticulated media with surface protrusions,was proposed to promote the external diffusion of soluble substrates to the biofilm, and to reduce the disk weight. (2) Simultaneous removal of the detached biomass and precipitated phosphorous in a two-storey RBC:A two-storey RBC, whose upper and lower parts function as the RBC trough and storage space of the detached biomass, was operated in a four-staged unit. Experimental investigation showed that the phosphorus precipitated by aluminium was adsorbed to the biofilm, and settled into the lower part as the detached biomass. The removal efficiency of the detached biomass was very high resulting in an effluent suspended solids concentration of about 10 ppm.