Crossflow filtration of yeast broth cultivated in molasses

A Brief Review on the Resistance-in-Series Model in Membrane Bioreactors (MBRs)

The cake layer deposited on the membrane modules of membrane bioreactors (MBRs), especially under a submerged configuration, represents a relevant and fundamental mechanism deeply influencing the development of membrane fouling. It negatively affects the total resistance to filtration, while exerting a positive effect as a "pre-filter" promoting the "dynamic membrane" that protects the physical membrane from internal fouling. These two opposite phenomena should be properly managed, where the submerged membranes are usually subjected to a periodical cake layer removal through ordinary (permeate backwashing and air scouring) and/or irregular cleaning actions (manual physical cleaning). In this context, the physical removal of the cake layer is needed to maintain the design filtration characteristics. Nevertheless, the proper evaluation of the effect of physical cleaning operations is still contradictory and under discussion, referring in particular to the correct evaluation of fouling mechanisms. The aim of the present work was to summarize the different aspects that influence the fouling investigations, based on simple models for the evaluation of the resistance to filtration due to the cake layer, through physical cleaning operations.

Experimental Study of Membrane Fouling during Crossflow Microfiltration of Yeast and Bacteria Suspensions: Towards an Analysis at the Microscopic Level

Microfiltration of model cell suspensions combining macroscopic and microscopic approaches was studied in order to better understand microbial membrane fouling mechanisms. The respective impact of Saccharomyces cerevisiae yeast and Escherichia coli bacteria on crossflow microfiltration performances was investigated using a multichannel ceramic 0.2 µm membrane. Pure yeast suspensions (5 µm ovoid cells) and mixtures of yeast and bacteria (1 to 2.5 µm rod shape cells) were considered in order to analyse the effect of interaction between these two microorganisms on fouling reversibility. The resistances varied significantly with the concentration and characteristics of the microorganisms. Membrane fouling with pure yeast suspension was mainly reversible. For yeast and bacteria mixed suspensions (6 g L⁻¹ yeast concentration) the increase in bacteria from 0.15 to 0.30 g L⁻¹ increased the percentage of normalized reversible resistance. At 10 g L⁻¹ yeast concentration, the addition of bacteria tends to increase the percentage of normalized irreversible resistance. For the objective of performing local analysis of fouling, an original filtration chamber allowing direct in situ observation of the cake by confocal laser scanning microscopy (CLSM) was designed, developed and validated. This device will be used in future studies to characterize cake structure at the microscopic scale.

Modeling the influence of slurry concentration on Saccharomyces cerevisiae cake porosity and resistance during microfiltration

Filtration of an isotonic suspension of baker's yeast through a 0.45-μm membrane was studied at two different pressures, 40 and 80 kPa, for yeast concentrations ranging from 0.14 to 51 kg/m(3) (dry weight). For a yeast volume fraction above 0.06 (~21.8 kg/m(3) ), the porosity of the yeast cake is less dependent on the suspension concentration. For highly diluted suspensions, the specific cake resistance approaches a minimum that depends on the filtration pressure. Correlation functions of cake porosity and specific cake resistance were obtained for the concentration range investigated showing that the Kozeny-Carman coefficient increases when the applied pressure increases. Both filtration pressure and slurry concentration can be process controlled. In the range of moderate yeast concentration, the filtrate flux may be increased by manipulating the filtration pressure and the slurry concentration, thereby improving the overall process efficiency. The complex behavior of yeast cakes at high slurry concentration can be described by a conventional model as long as part of yeast cells are assumed to form aggregates, which behave as single bigger particles. The aggregation effect may be accounted for using a binary mixture model.

Ni(2+) and H(2)PO(4)(-) uptake properties of compounds in the CaTiO(3)-CaFeO(2.5) system

A batch method was used to investigate the uptake of heavy metal cations and anions by the compounds in the CaTiO(3)-CaFeO(2.5) system, in which a series of oxygen vacancies was systematically introduced into a perovskite structure as the x-value of Ca(Fe(x)Ti(1-x))O(3-x/2) was increased. Samples of CaTiO(3), CaFe(0.1)Ti(0.9)O(2.95), CaFe(0.5)Ti(0.5)O(2.75), CaFe(0.67)Ti(0.33)O(2.67) and CaFeO(2.5) were prepared by solid mixing (SM), co-precipitation (CP) and gel evaporation (GE) methods. The resulting samples were calcined at temperatures between 400 and 1000 °C. The target crystalline phases differed according to the preparation method, but in most cases were formed at 700-800 °C. The Ni(2+) sorption isotherms of all the samples were fitted better by the Langmuir model than by the Freundlich model, while in the case of H(2)PO(4)(-) sorption isotherms, these were better fitted by the latter model. The uptake ability increased with increasing x value of the samples. The maximum values for the saturated sorption of Ni(2+) (Q(0)(Ni(2+)) = 2.83 mmol/g) and H(2)PO(4)(-) (K(F)(H(2)PO(4)(-)) = 2.95 mmol/g) were achieved for x = 1 (i.e. CaFeO(2.5)) sample.

Membrane fouling by cell-protein mixtures: In situ characterisation using multi-photon microscopy

Fouling of the membrane by cell and protein mixtures can result in severe flux declines, leading to the eventual need to clean or replace the membrane. In this study multi-photon microscopy, a fluorescence-based technique is used to 3-D image in situ the fouling of microfiltration membranes by suspensions containing combinations of washed yeast, bovine serum albumin (BSA) and ovalbumin. Appropriate fluorescent labelling allows the three foulant species to be clearly identified. Images correlate well with filtration data and clearly show the cake of yeast cells capturing protein aggregates. The proteins exhibited very different filtration behaviour. When filtering washed yeast together with ovalbumin and/or a 50:50 mixture by mass of BSA and ovalbumin, the ovalbumin fouling dominates the system. Capture of aggregates by the cake did not reduce fouling of the membrane by the protein and increased the resistance of the cake. For mixtures of BSA and washed yeast, the presence of a cake of yeast cells did reduce fouling of the membrane by the protein, however, the extra resistance due to the cake resulted in a flux lower than that when filtering BSA alone.

Broth conditions determining specific cake resistance during microfiltration of Bacillus subtilis

The effects of broth pH, pressure, temperature, and fermentation medium on specific cake resistance were studied for dead-end microfiltration of Bacillus subtilis. Decreases in pH and transmembrane pressure decreased the specific cake resistance for cells grown in both complex and defined media. With the complex medium, the reduction in resistance with temperature decrease did not offset the flux decrease caused by the increase in viscosity. The greatest decrease in specific cake resistance occurred with adjustment of pH to 7.5 for cells grown in defined medium. For those cells the change in pH resulted in aggregation leading to a large increase in flux.

Ionic strength effect on copper biosorption by Sphaerotilus natans: equilibrium study and dynamic modelling in membrane reactor

Biosorption of copper by Sphaerotilus natans in different conditions of ionic strength and pH was studied by performing sorption tests in batch and membrane reactors. Equilibrium batch tests evidenced the negative effect of ionic strength and the positive effect of pH on biosorption performances: the highest determined value for copper specific uptake, q, was about 60 mg/g at pH 6 and about 15 mg/g at pH 4. A competitive equilibrium model was successfully fitted to experimental data at different ionic strength levels to account for copper-sodium competition. In membrane reactor tests, experimental profiles of copper concentration in the permeate vs. time did not evidence a significant effect of ionic strength at low pH values (4 and 5). On the other hand a more remarkable effect of ionic strength on copper concentration in the permeate was observed at pH 6. Experimental profiles of continuous biosorption in the membrane reactors were successfully simulated by developing a dynamic model accounting for Cu-Na competition and for binding ability of cells fragments.

Murine leukemia virus clearance by flocculation and microfiltration

Clearance of murine leukemia virus from CHO cell suspensions by flocculation and microfiltration was investigated. Murine leukemia virus is a retrovirus that is recommended by the U.S. Food and Drug Administration for validating clearance of retrovirus-like particles. Due to biosafety considerations, an amphotropic murine leukemia virus vector (A-MLV) that is incapable of self-replication was used. Further, A-MLV is incapable of infecting CHO cells, thus ensuring that infection of the CHO cells in the feed did not result in a reduced virus titer in the permeate. The virus vector contains the gene for the enhanced green fluorescent protein (EGFP) to facilitate assaying for infectious virus particles. The virus particles are 80-130 nm in size. The feed streams were flocculated using a cationic polyelectrolyte. Microfiltration was conducted using 0.1 and 0.65 microm pore size hollow fiber membranes. The level of virus clearance in the permeate was determined. For the 0.1 microm pore size membranes a 1,000-fold reduction in the virus titer in the permeate was observed for feed streams consisting of A-MLV, A-MLV plus flocculant, A-MLV plus CHO cells, and A-MLV plus flocculant and CHO cells. While the flocculant had little effect on the level of virus clearance in the permeate for 0.1 microm pore size membranes, it did lead to higher permeate fluxes for the CHO cell feed streams. Virus clearance experiments conducted with 0.65 microm pore size membranes indicate little clearance of A-MLV from the permeate in the absence of flocculant. However, in the presence of flocculant the level of virus clearance in the permeate was similar to that observed for 0.1 microm pore size membranes. The results obtained here indicate that significant clearance of A-MLV is possible during tangential flow microfiltration. Addition of a flocculant is essential if the membrane pore size is greater than the diameter of the virus particles. Flocculation of the feed stream leads to an increase in the permeate flux.

Clearance of minute virus of mice by flocculation and microfiltration

Clearance of minute virus of mice (MVM) from CHO cell suspensions by flocculation and microfiltration has been investigated. MVM is a parvovirus that is recommended by the U.S. Food and Drug Administration for validating clearance of parvoviruses. The feed streams were flocculated using a cationic polyelectrolyte. Virus clearance in excess of 10,000-fold was obtained in the bulk permeate for flocculated feeds streams. However, the level of clearance was only about 10- to 100-fold for unflocculated feed streams. The results suggest that virus clearance involves interactions between the MVM particles, the cationic polyelectrolyte, and the CHO cells present. Validating virus clearance is a major concern in the biotechnology industry. New unit operations are frequently added to the purification train simply to validate virus clearance. However, many of these unit operations are less effective at validating clearance of nonenveloped viruses. Validating clearance of parvoviruses is often particularly problematic as they are nonenveloped and the virus particles are small (18 to 24 nm), making physical removal difficult. The results obtained herein indicate that addition of the cationic polyelectrolyte not only results in significant clearance of MVM but also leads to an increase in permeate flux.

Filtration of a bacterial fermentation broth: harvest conditions effects on cake hydraulic resistance

The hydraulic resistance of cakes formed during the ultrafiltration of Streptomyces pristinaespiralis broths has been investigated for different harvesting conditions. S. pristinaespiralis broth was harvested after the point of microorganism activity declines (0-h aged broth) and afterwards held for different durations of up to 16 h (16 aged broths). Aging behavior occurring between the end of microorganism activity and harvest was compared for different acidification procedures (pH) and the mechanisms for which the hydraulic resistance of the cake is affected by aging have been investigated. For broths harvested under conditions where the acidification is fixed at pH 2 or 3, hydraulic resistance associated with cake build-up is directly determined by the interactions between the cells. Holding broths beyond 5 h contributes to a release of a soluble component from the cell surface. Enhanced cell surface interactions then turn the cake structure into a more open one and reduce the specific hydraulic resistance. For broths harvested under conditions where the acidification is fixed at pH 4, hydraulic resistance associated with cake build-up is both determined by cell interactions and cell morphology. The cause of the increase in specific hydraulic resistance with aging is due to the binding of a soluble component released by the microorganisms, which decreases the cell surface interactions.

Removal of divalent heavy metal mixtures from water by Saccharomyces cerevisiae using crossflow microfiltration

The removal of heavy metal ions, Ni2+, Cu2+ and Pb2+ using yeast (Saccharomyces cerevisiae) as carriers in a crossflow microfiltration is investigated. The effects of yeast cell and electrolyte concentrations on the transient and steady-state permeate flux and metal ion rejections are established. It is found that the metal ion rejection reaches a plateau if yeast cell concentration is greater than approximately 2 g/l as a result of cell aggregation. The binding affinity of the metals to yeast cell is Pb2+ > Cu2+ > Ni2+, which is also reflected in the metal ion rejection under identical process conditions. Because of the formation of yeast cell flocks in the presence of Pb2+, permeate flux is also higher for this metal. The presence of NaCl decreases both rejection and permeate flux for Ni2+ and Cu2+ but not for Pb2+. When binary or ternary metal mixtures are used, the rejection of the individual metals is reduced except that of Pb2+. It is found that the pseudo-gel concentration is unaffected by the presence of metal ions.