A photosynthetic rotating annular bioreactor (Taylor-Couette type flow) for phototrophic biofilm cultures.

In their natural environment, the structure and functioning of microbial communities from river phototrophic biofilms are driven by biotic and abiotic factors. An understanding of the mechanisms that mediate the community structure, its dynamics and the biological succession processes during phototrophic biofilm development can be gained using laboratory-scale systems operating with controlled parameters. For this purpose, we present the design and description of a new prototype of a rotating annular bioreactor (RAB) (Taylor-Couette type flow, liquid working volume of 5.04 L) specifically adapted for the cultivation and investigation of phototrophic biofilms. The innovation lies in the presence of a modular source of light inside of the system, with the biofilm colonization and development taking place on the stationary outer cylinder (onto 32 removable polyethylene plates). The biofilm cultures were investigated under controlled turbulent flowing conditions and nutrients were provided using a synthetic medium (tap water supplemented with nitrate, phosphate and silica) to favour the biofilm growth. The hydrodynamic features of the water flow were characterized using a tracer method, showing behaviour corresponding to a completely mixed reactor. Shear stress forces on the surface of plates were also quantified by computer simulations and correlated with the rotational speed of the inner cylinder. Two phototrophic biofilm development experiments were performed for periods of 6.7 and 7 weeks with different inoculation procedures and illumination intensities. For both experiments, biofilm biomasses exhibited linear growth kinetics and produced 4.2 and 2.4 mg cm(-)² of ash-free dry matter. Algal and bacterial community structures were assessed by microscopy and T-RFLP, respectively, and the two experiments were different but revealed similar temporal dynamics. Our study confirmed the performance and multipurpose nature of such an innovative photosynthetic bioreactor for phototrophic biofilm investigations.

Orientation and detachment dynamics of Bacillus spores from stainless steel under controlled shear flow: modelling of the adhesion force.

Shear-flow induced spore detachment was performed under well-controlled laminar flow conditions, in a specially-designed shear stress flow chamber. By comparing detachment profiles of a panel of four strains, belonging to the B. cereus group (B. cereus and B. thuringiensis) and to less related Bacillus species (B. pumilus), it was shown that the spore ability of attaching to stainless steel, probed under dynamic conditions, was mainly affected by the presence (and number) of appendages. Adhesion force between the B. cereus 98/4 strain and stainless steel was quantified at nanoscale. To this aim, detachment results were combined with a theoretical modelling, based on the balance of hydrodynamic forces and torque exerted over a simplified spore model with a spherical form. The wall shear stress, required to remove 50% of the spores initially attached to stainless steel, was determined. On this basis, an adhesion force of 930 ± 390 pN was obtained. Real-time re-orientation of B. cereus 98/4 spores was experimentally established, by using a high-speed camera for tracking the motions of individual spores with high temporal and spatial resolution. Even though tethered to stainless steel without any detachment occurring, spores kept mobile on the substratum, probably due to the existence of discrete bonds or local clusters of anchoring sites, and tended to re-orientate in the flow direction, for minimizing hydrodynamic forces and torque exerted by fluid flow. A significant heterogeneity within the population was also observed, with the co-existence of both moving and immobile spores.

Influence of a transverse flowrate on the oxygen transfer performance in heterogeneous aeration: case of hydro-ejectors.

This paper deals with the scaling of aeration devices, and more specifically hydro-ejectors, in the case of heterogeneous aeration. Because the transfer of oxygen only occurs in a very small part of the volume of the treatment basin, the transfer performance of the aerator depends on the device itself and on the surrounding flow characteristics. First experiments were achieved with a 10 L mechanically agitated reactor in order to operate at a known kLa and liquid flowrate Q. The results show that the oxygen transfer capacity of the reactor is used to a greater or lesser extent depending on the flowrate which passes through the bubbling region. When a hydro-ejector is concerned, the oxygen transfer occurs inside an aerated zone of about 2 m3; experiments carried out with an industrially scaled HE in a 120 m3 test basin allowed to estimate that the kLa in this zone is about 700-800 h(-1). Applying a compartment model, it is then possible to determine the oxygen transfer capacity of the HE as a function of the transverse liquid flowrate. While this OC is 3 kg O2/h under the test basin conditions, it reaches up to 12 kg O2/h under industrial flow conditions. This value was obtained in the aerobic biological treatment of the washing waters of a sugar refinery where two 33,000 m3 basins aerated by 152 HE could degrade 35 t/d of COD.