Improved bacterial nanocellulose production from glucose without the loss of quality by evaluating thirteen agitator configurations at low speed

Thirteen agitator configurations were investigated at low speed in stirred-tank reactors (STRs) to determine if improved crude bacterial nanocellulose (BNC) productivity can be achieved from glucose-based media while maintaining high BNC quality using Komagataeibacter xylinus ATCC 23770 as a model organism. A comparison of five single impellers showed the pitched blade (large) was the optimal impeller at 300 rpm. The BNC production was further increased by maintaining the pH at 5.0. Among the single helical ribbon and frame impellers and the combined impellers, the twin pitched blade provided the best results. The combined impellers at 150 rpm performed better than the single impellers, and after optimizing the agitation conditions, the twin pitched blade (large) and helical ribbon impellers performed the best at 100 rpm. The performances of different agitators at low speed during BNC production were related to how efficiently the agitators improved the oxygen mass transfer coefficient. The twin pitched blade (large) was verified as providing the optimum performance by an observed crude BNC production of 1.97 g (L×d)⁻¹ and a BNC crude yield of consumed glucose of 0.41 g g⁻¹, which were 2.25 and 2.37 times higher than the initial values observed using the single impeller respectively. Further characterization indicated that the BNC obtained at 100 rpm from the STR equipped with the optimal agitator maintained high degree of polymerization and crystallinity.     

Agitation,aeration and shear stress as key factors in inulinase production by Kluyveromyces marxianus

Factorial design and response surface analyses were used to optimize the production of inulinase (2,1-β-d-fructan fructanohydrolase, EC 3.2.1.7) by Kluyveromyces marxianus ATCC 16045, using sucrose as carbon source. Effects of aeration, agitation and type of impeller (disk turbine, marine, pitched blade) were studied in a batch stirred reactor. Two factorial designs 2² were carried out. Agitation speed varied from 50 to 550 rpm (revolution per minute), aeration rate from 0.5 to 2.0 vvm (air volume/broth volume·minute). It has been shown that the enzyme production was strongly influenced by mixing conditions, while aeration rate was shown to be less significant. Additionally, the increase in the agitation speed is limited by the death rate, which increases drastically at high speeds, lowering the enzyme production. Also, the impeller type has significant influence in the production, the disk impeller at 450 rpm and aeration at 1.0 vvm led to an activity of 121 UI/mL, while the pitched blade was shown to be the best impeller for this process, leading to the best production, 176 UI/mL, at 450 rpm and 1.0 vvm. The maximum shear stress for inulinase production was about 0.22 Pa, since higher values cause higher cell death rates, affecting the enzyme production. The same results were confirmed with another microorganism, which was also sensible to shear stress. Therefore, it has been concluded that in some cases, mainly when the microorganism is sensible to shear stress, the interaction between mass transfer and mechanical stress should be considered in scale up processes.     

Homogeneous batch cultures of<Emphasis Type="Italic"> Aspergillus oryzae</Emphasis> by elimination of wall growth in the Variomixing bioreactor

A novel principle for mixing and aeration in stirred bioreactors, named Variomixing, was developed. Four baffles are rotated intermittently at a rotational speed slower or similar to the speed of a centrally placed axial flow impeller. Rotational speeds of the baffles and impeller of 5–10 and 500–600 rpm, respectively, results in the highly turbulent flow regime characteristic of conventional bioreactors with high mixing and mass transfer capacities. Stagnant zones around crevices and crannies in which wall growth may commence are avoided since the baffles are never completely at rest. Increasing the rotational speed of the baffles (5 s every 5 min), so that it follows the speed of the impeller (500–600 rpm), cancels the effect of the baffles and a deep vortex and high peripheral liquid flow rates at the reactor wall develop. The vortex ensures that also the head-space of the reactor wall is flushed and any deposits removed. The filamentous fungus Aspergillus oryzae has been grown in batch cultures in the Variomixing bioreactor. Compared to conventional laboratory-scale bioreactors, in which more than 30% of all biomass was found attached to walls, less than 2% of the total A. oryzae biomass was found on the walls in the Variomixing bioreactor.     

Effect of the aeration rate and agitation speed on β-carotene production and morphology of Blakeslea trispora in a stirred tank reactor: mathematical modeling

The effect of aeration rate and agitation speed on β-carotene production and morphology of Blakeslea trispora in a stirred tank reactor was investigated. B. trispora formed hyphae, zygophores and zygospores during the fermentation. The zygospores were the morphological form responsible for β-carotene production. Both aeration and agitation significantly affected β-carotene concentration, productivity, biomass and the volumetric mass transfer coefficient (K_La). The highest β-carotene concentration (1.5 kg m⁻³) and the highest productivity (0.08 kg m⁻³ per day) were obtained at low impeller speed (150 rpm) and high aeration rate (1.5 vvm). Also, maximum productivity (0.08 kg m⁻³ per day) and biomass dry weight (26.4 kg m⁻³) were achieved at high agitation speed (500 rpm) and moderate aeration rate (1.0 vvm). Conversely, the highest value of K_La (0.33 s⁻¹) was observed at high agitation speed (500 rpm) and high aeration rate (1.5 vvm). The experiments were arranged according to a central composite statistical design. Response surface methodology was used to describe the effect of impeller speed and aeration rate on the most important fermentation parameters. In all cases, the fit of the model was found to be good. All fermentation parameters (except biomass concentration) were strongly affected by the interactions among the operation variables. β-Carotene concentration and productivity were significantly influenced by the aeration, agitation, and by the positive or negative quadratic effect of the aeration rate. Biomass concentration was principally related to the aeration rate, agitation speed, and the positive or negative quadratic effect of the impeller speed and aeration rate, respectively. Finally, the volumetric mass transfer coefficient was characterized by the significant effect of the agitation speed, while the aeration rate had a small effect on K_La.     

Optimization of the aeration and agitation speed of Aeribacillus palidus 418 exopolysaccharide production and the emulsifying properties of the product

The specific properties of exopolysaccharides (EPS) from thermophilic microorganisms have attracted interest in their optimized production. In this study, the ability of Aeribacillus pallidus 418 to grow and produce polysaccharide in a 5-l stirred tank bioreactor was investigated. Agitation rates of 100, 200, 600, 900, and 1100 revolutions per minute (rpm), at an air flow rate of 0.5 gas volumes per unit medium volume per minute (vvm), and aeration rates of 0.25, 0.5, 1.0, and 1.5 vvm, at an agitation rate of 900 rpm, were examined. A maximum EPS yield of 170 μg/ml has been registered in a single impeller bioreactor equipped with an original Narcissus impeller at agitation speed of 900 rpm, with an aeration rate of 0.5 vvm. The bioprocess oxygen uptake rate (OUR) and oxygen mass transfer coefficient (K_La) were evaluated. The emulsifying properties of the specific EPS produced by A. pallidus 418 were determined. Stable oil-in-water emulsions, a low level of separated water phase and high dispersion stability were found, which together demonstrate the prospects for the industrial exploration of EPS production. Enhanced synergism between the A. pallidus 418 synthesized EPS and various commercially used hydrocolloids was observed; superior synergy was achieved in combination with xanthan gum.     

Effects of pellet morphology on broth rheology in fermentations of Aspergillus terreus

Effects of pellet morphology on broth rheology are reported for pelleted submerged cultures of the lovastatin producing filamentous fungus Aspergillus terreus, growing in fluidized bed and stirred tank bioreactors. The pellet diameter and compactness were affected by the agitation intensity of the broth; however, the total biomass productivity was not affected. In fluidized beds and stirred tanks with agitation intensity of up to 300 rpm (impeller tip speed of 1.02 m s⁻¹), the fungal pellets were stable at diameters of up to about 2300 μm. In more intensely agitated stirred tanks (≥600 rpm; impeller tip speed of ≥2.03 m s⁻¹), the stable pellet size was only about ≤900 μm. The biomass concentration and the pellet diameter were the main factors that influenced the flow index and the consistency index of the power-law broths. Because the biomass productivity was the same in all experiments in a given type of reactor and the oxygen concentration was kept at ∼400% of air saturation, the pellet size and morphology were not influenced by oxygen mass transfer effects. Pellets were always dense in the core region and no necrosis of the biomass occurred.     

Expansion of human mesenchymal stem cells on microcarriers

The effects on human mesenchymal stem cell growth of choosing either of two spinner flask impeller geometries, two microcarrier concentrations and two cell concentrations (seeding densities) were investigated. Cytodex 3 microcarriers were not damaged when held at the minimum speed, N_JS, for their suspension, using either impeller, nor was there any observable damage to the cells. The maximum cell density was achieved after 8–10 days of culture with up to a 20-fold expansion in terms of cells per microcarrier. An increase in microcarrier concentration or seeding density generally had a deleterious or neutral effect, as previously observed for human fibroblast cultures. The choice of impeller was significant, as was incorporation of a 1 day delay before agitation to allow initial attachment of cells. The best conditions for cell expansion on the microcarriers in the flasks were 3,000 microcarriers ml⁻¹ (ca. 1 g dry weight l⁻¹), a seeding density of 5 cells per microcarrier with a 1 day delay before agitation began at N_JS (30 rpm), using a horizontally suspended flea impeller with an added vertical paddle. These findings were interpreted using Kolmogorov’s theory of isotropic turbulence.     

Influence of dissolved oxygen and shear conditions on clavulanic acid production by Streptomyces clavuligerus

Clavulanic acid (CA), a potent β-lactamase inhibitor, is produced by a filamentous bacterium. Here, the effect of DO and shear, expressed as impeller tip velocity, on CA production was examined. Cultivations were performed in a 4 L fermentor with speeds of 600, 800 and 1,000 rpm and a fixed air flow rate (0.5 vvm). Also, cultivation with automatic control of dissolved oxygen, at 50% air saturation, by varying stirrer speed and using a mixture of air and O₂ (10% v/v) in the inlet gas, and a cultivation with fixed stirrer speed of 800 rpm and air flow rate of 0.5 vvm, enriched with 10% v/v O₂, were performed. Significant variations in CA titer, CA production rate and O₂ uptake-rate were observed. It was also found that the DO level has no remarkable effect on CA production once a critical level is surpassed. The most significant improvement in CA production was related to high stirrer speeds.     

In situ filtration of Anchusa officinalis culture in a cell-retention stirred tank bioreactor

Summary The effect of agitation and aeration on filtration of Anchusa officinalis culture in a stirred tank bioreactor integrated with an internal filter unit was investigated. Increases in suction head of the pump that drove the filtration process were measured at impeller speeds of 100 and 200 rpm. Surprisingly, suction head attained at 200 rpm was about 40% higher than at 100 rpm. Direct observation of the cake deposition process in the reactor using a dilute cell suspension revealed that the filter cake formed at 100 rpm was thicker, but less compact. Aeration at 0.4 vvm was shown to have little effect on the filtration rate, since the bulk fluid flow was dominated by the impeller hydrodynamics. The initial flux can be recovered by filter backwashing with compressed air at a flow rate of 0.6 vvm for a duration of 5 minutes.     

Optimisation of the operational conditions of trichloroethylene degradation using Trametes versicolor under quinone redox cycling conditions using central composite design methodology

Extracellular radicals produced by Trametes versicolor under quinone redox cycling conditions can degrade a large variety of pollutant compounds, including trichloroethylene (TCE). This study investigated the effect of the agitation speed and the gas–liquid phase volume ratio on TCE degradation using central composite design (CCD) methodology for a future scale-up to a reactor system. The agitation speed ranged from 90 to 200 rpm, and the volume ratio ranged from 0.5 to 4.4. The results demonstrated the important and positive effect of the agitation speed and an interaction between the two factors on TCE degradation. Although the volume ratio did not have a significant effect if the agitation speed value was between 160 and 200 rpm, at lower speed values, the specific pollutant degradation was clearly more extensive at low volume ratios than at high volume ratios. The fitted response surface was validated by performing an experiment using the parameter combination in the model that maximised TCE degradation. The results of the experiments carried out using different biomass concentrations demonstrated that the biomass concentration had a positive effect on pollutant degradation if the amount of biomass present was lower than 1.6 g dry weight l⁻¹. The results show that the maximum TCE degradation was obtained at the highest speed (200 rpm), gas–liquid phase volume ratio (4.4), and a biomass concentration of 1.6 g dry weight l⁻¹.     

Effect of ammonium sulphate concentration and agitation speed on the kinetics of alginate production by Azotobacter vinelandii DSM576 in batch fermentation

Growth and alginate production by Azotobacter vinelandii DSM576 as a function of initial ammonium sulphate concentration (0.45–1.05 g l⁻¹) and agitation speed (300–700 rpm) were studied in batch fermentations at controlled pH. The time course of growth, alginate production and substrate consumption and the effect of nitrogen concentration and agitation speed on kinetic parameters and on maximum alginate molecular weight (MW) was modelled using empirical equations. The kinetics of growth, alginate production and polymerization were deeply affected by agitation speed and, to a lesser extent, by inorganic nitrogen concentration. Average and maximum specific growth rate and maximum alginate MW all increased with agitation speed, and were higher at intermediate ammonium sulphate concentration. Maximum alginate MW (>250,000) was obtained at high agitation speed (600–700 rpm) and alginate depolymerization was limited or did not occur at all when the agitation speed was higher than 500 rpm, while at 400 rpm depolymerization significantly reduced the alginate. However, alginate yield was negatively affected by increasing agitation speed. A good compromise between alginate yield (>2 g l⁻¹) and quality (MW>250,000) was obtained with agitation speed of 500–600 rpm and 0.75–0.90 g l⁻¹ of ammonium sulphate. Journal of Industrial Microbiology & Biotechnology (2000) 25, 242–248. Received 23 February 2000/ Accepted in revised form 04 August 2000     

Enhancement of pyruvate production by Torulopsis glabrata using a two-stage oxygen supply control strategy

The effect of agitation speeds on the performance of producing pyruvate by a multi-vitamin auxotrophic yeast, Torulopsis glabrata, was investigated in batch fermentation. High pyruvate yield on glucose (0.797 g g(-1)) was achieved under high agitation speed (700 rpm), but the glucose consumption rate was rather low (1.14 g l(-1) h(-1)). Glucose consumption was enhanced under low agitation speed (500 rpm), but the pyruvate yield on glucose decreased to 0.483 g g(-1). Glycerol production was observed under low agitation speed and decreased with increasing agitation speed. Based on process analysis and carbon flux distribution calculation, a two-stage oxygen supply control strategy was proposed, in which the agitation speed was controlled at 700 rpm in the first 16 h and then switched to 500 rpm. This was experimentally proven to be successful. Relatively high concentration of pyruvate (69.4 g l(-1)), high pyruvate yield on glucose (0.636 g g(-1)), and high glucose consumption rate (1.95 g l(-1)h(-1)) were achieved by applying this strategy. The productivity (1.24 g l(-1) h(-1)) was improved by 36%, 23% and 31%, respectively, compared with fermentations in which agitation speeds were kept constant at 700 rpm, 600 rpm, and 500 rpm. Experimental results indicate that the difference between the performances for producing pyruvate under a favorable state of oxygen supply (dissolved oxygen concentration >50%) was caused by the different regeneration pathways of NADH generated from glycolysis.     

Cultivation of Methylosinus trichosporium OB3b: III. production of particulate methane monooxygenase in continuous culture

Continous culture experiments with the obligatory methanotroph, Methylosinus trichosporium OB3b, were conducted to study the whole-cell methane monooxygenase (MMO) and nitrogenase activities in a nitrate minimal salts medium under oxygen-limited conditions with methane as the carbone source. The important variables investigated were the feed medium concentrations of copper and nitrate, CO(2) addition, the agitation speed, and the dilution rate. M. trichosporium OB3b required quantitative amounts of copper (2.6 x 10(-4) g Cu/g dry cell Wt) for the exclusive production of particulate MMo during continous culture growth. When the feed medium nitrate concentration was varied in the range of 5-50 mM, the whole-cell specific pMMO activity exhibited a maximum at 40 mM. The elimination of external CO(2) gassing decreased pMMO activity by more than 30%. The steady-state cell density increased continuously over a 300-700 rpm range of agitation speed, whereas, the pMMO activity became maximal at 400 rpm. Also, the pMMO activity increased with the dilution rate up to 0.06 h(-1) and remained constant thereafter. Maximal continuous pMMO productivity was, thus, achieved in Higgin's medium containing 10 muM Cu, 80 muM Fe, and 40 mM nitrate with an agitation speed of 500 rpm and a dilution rate of 0.06 h(-1). Nitrogenase activity, on the other hand, increased over a feed medium copper concentration of 2-15 muM, falling sharply at 20 muM, and it exhibited a minimum at 20 mM when the feed medium nitrate concentration was varied. (c) 1992 John Wiley & Sons, Inc.     

Comparison of O transfer characteristics between a new centrifugal impeller and a flat-bladed turbine impeller

The efficiency of O transfer by a novel centrifugal impeller was higher than that of a conventional flat-bladed turbine impeller at an agitation speed lower than 300 rpm. In addition, at the same agitation speed (200 and 300 rpm), the centrifugal impeller possessed smaller shear stress than the flat-bladed turbine impeller as evaluated by the changes in size distribution of granulated agar particles which were sheared with those two types of impeller.     

A novel centrifugal impeller bioreactor. II. Oxygen transfer and power consumption

The effects of the impeller configuration, aeration rate, and agitation speed on oxygen transfer coefficient K(L)a were studied in a newly designed centrifugal impeller bioreactor (5-L). The oxygen transfer rates in the novel bioreactor were also compared with those in a cell-lift bioreactor with comparable dimensions. The cell-lift impeller produced much higher surface oxygen transfer rates than the centrifugal one at an agitation speed over 200 rpm. This result was in good agreement with our observation that the cell-lift impeller produced much higher unfavorable turbulence. In addition, the experiments using granulated agar particles as pseudo plant cells indicated that the K(L)a value decreased steadily with an increase in agar particle concentration, and the centrifugal impeller still demonstrated a larger K(L)a than the cell lift up to a high pseudo cell concentration of 19.5 g dry weight (DW)/L (under 150 rpm and 0.20 vvm) or 22.3 g DW/L (under 200 rpm and 0.20 vvm). Furthermore, the correlation between power number and impeller Reynolds number for both the centrifugal and the cell-lift impellers was successfully obtained, which could be used for predicting the power input required by each impeller. From the results obtained, the centrifugal impeller bioreactor is expected to have great potential in its application to shear-sensitive biological systems.     

Production of chitosan oligosaccharides by chitosanase directly immobilized on an agar gel-coated multidisk impeller

An immobilized enzyme bioreactor consisting of an agar gel-coated multidisk impeller was developed for the hydrolysis of highly viscous chitosan solutions, and the operating conditions for the production of physiologically active chitosan oligosaccharides (pentamers and hexamers) were investigated. Chitosanase was directly immobilized on the agar gel-coated multidisk impeller by a multipoint attachment method. The high stability of the immobilized enzyme was confirmed by means of five repetitions of a batch hydrolysis reaction. When the enzyme activity at the support surface was relatively high, the yield of the target products was higher at an impeller speed of 2 s⁻¹ than at a speed of 1 s⁻¹. However, no significant increase in yield was observed at impeller speeds higher than 2 s⁻¹ in reactions at either of the two substrate concentrations tested (5 and 20 kg/m³). When the surface enzyme activity was low, the impeller speed did not affect the yield of the target products. The maximum yield of pentamers and hexamers increased as the surface enzyme activity decreased, and high yields (>30%) were obtained at activities below 160 U/m². From the viewpoint of productivity, the optimal surface-enzyme activity was about 340 U/m², and at that activity, the yield of target products was 22%. This yield was higher than that reported for conventional acid hydrolysis. To maximize both the productivity and the yield of the target products, the surface area for the immobilized enzyme should be increased. Our results suggest that it may be possible to obtain high yields of pentamers and hexamers of chitosan oligosaccharides from highly viscous chitosan solutions with this reactor.     

Production of <Emphasis Type="SmallCaps">l</Emphasis>-phenylalanine from glycerol by a recombinant <Emphasis Type="Italic">Escherichia coli</Emphasis>

The production of l-phenylalanine is conventionally carried out by fermentations that use glucose or sucrose as the carbon source. This work reports on the use of glycerol as an inexpensive and abundant sole carbon source for producing l-phenylalanine using the genetically modified bacterium Escherichia coli BL21(DE3). Fermentations were carried out at 37°C, pH 7.4, using a defined medium in a stirred tank bioreactor at various intensities of impeller agitation speeds (300–500 rpm corresponding to 0.97–1.62 m s⁻¹ impeller tip speed) and aeration rates (2–8 L min⁻¹, or 1–4 vvm). This highly aerobic fermentation required a good supply of oxygen, but intense agitation (impeller tip speed ~1.62 m s⁻¹) reduced the biomass and l-phenylalanine productivity, possibly because of shear sensitivity of the recombinant bacterium. Production of l-phenylalanine was apparently strongly associated with growth. Under the best operating conditions (1.30 m s⁻¹ impeller tip speed, 4 vvm aeration rate), the yield of l-phenylalanine on glycerol was 0.58 g g⁻¹, or more than twice the best yield attainable on sucrose (0.25 g g⁻¹). In the best case, the peak concentration of l-phenylalanine was 5.6 g L⁻¹, or comparable to values attained in batch fermentations that use glucose or sucrose. The use of glycerol for the commercial production of l-phenylalanine with E. coli BL21(DE3) has the potential to substantially reduce the cost of production compared to sucrose- and glucose-based fermentations.     

A Kinetic Study of the Polymorphic Transformation of Nimodipine and Indomethacin during High Shear Granulation

The objective of the present study was to investigate the mechanism, kinetics, and factors affecting the polymorphic transformation of nimodipine (NMD) and indomethacin (IMC) during high shear granulation. Granules containing active pharmaceutical ingredient, microcrystalline cellulose, and low-substituted hydroxypropylcellulose were prepared with ethanolic hydroxypropylcellulose solution, and the effects of independent process variables including impeller speed and granulating temperature were taken into consideration. Two polymorphs of the model drugs and granules were characterized by X-ray powder diffraction analysis and quantitatively determined by differential scanning calorimetry. A theoretical kinetic method of ten kinetic models was applied to analyze the polymorphic transformation of model drugs. The results obtained revealed that both the transformation of modification I to modification II of NMD and the transformation of the α form to the γ form of IMC followed a two-dimensional nuclei growth mechanism. The activation energy of transformation was calculated to be 7.933 and 56.09 kJ·mol⁻¹ from Arrhenius plot, respectively. Both the granulating temperature and the impeller speed affected the transformation rate of the drugs and, in particular, the high shear stress significantly accelerated the transformation process. By analyzing the growth mechanisms of granules in high-shear mixer, it was concluded that the polymorphic transformation of NMD and IMC took place in accordance with granule growth in a high-shear mixer.     

Cell morphology of extrusion foamed poly(lactic acid) using endothermic chemical foaming agent

Poly(lactic acid) (PLA) was foamed with an endothermic chemical foaming agent (CFA) through an extrusion process. The effects of polymer melt flow index, CFA content, and processing speed on the cellular structures, void fraction, and cell-population density of foamed PLA were investigated. The apparent melt viscosity of PLA was measured to understand the effect of melt index on the cell morphology of foamed PLA samples. The void fraction was strongly dependent on the PLA melt index. It increased with increasing melt index, reaching a maximum value, after which it decreased. Melt index showed no significant effect on the cell-population density of foamed samples within the narrow range studied. A gas containment limit was observed in PLA foamed with CFA. Both the void fraction and cell-population density increased with an initial increase in CFA content, reached a maximum value, and then decreased as CFA content continued to increase. The processing speed also affected the morphology of PLA foams. The void fraction reached a maximum value as the extruder’s screw speed increased to 40 rpm and a further increase in the processing speed tended to reduce the void fraction of foamed samples. By contrast, cell-population density increased one order of magnitude by increasing the screw speed from 20 to 120 rpm. The experimental results indicate that a homogeneous and finer cellular morphology could be successfully achieved in PLA foamed in an extrusion process with a proper combination of polymer melt flow index, CFA content, and processing speed.     

Influence of surface topography on biofilm development: Experiment and modeling

The effect of surface topography on the long-term development (≈10 weeks) of biofilms has been investigated using a monitoring technique based on images produced by a flat-bed scanner and initially developed for flat surfaces. The biofilm response to rotation speed changes in lab-scale rotating biological contactors (RBCs) has been studied. Two RBCs, each containing five discs (two with flat surfaces and three with rough surfaces) were run initially at two different rotation speeds: 4 rpm for reactor I and 40 rpm for reactor II. After 47 days, the rotation speed was increased in reactor I to 40 rpm and decreased in reactor II to 4 rpm. Prior to the rotation speed change, the biofilm on the flat discs underwent large detachments in both reactors, but the biofilm on rough discs was less extensively damaged. The increase in rotation speed induced large detachments of the biofilm in reactor I on all discs, but the biofilm on the rough discs recovered more effectively with faster regrowth. In reactor II, the decrease in rotation speed favored the development of the biofilm. Wall stress distributions obtained from CFD simulations on flat and rough discs at different rotation speeds were well correlated with experimental observations.