CHARACTERISATION OF CAKE COMPRESSIBILITY IN DEAD-END MICROFILTRATION OF MICROBIAL SUSPENSIONS

Determination of Compressibility and Relaxation Behavior of Yeast Cell Sediments by Analytical Centrifugation and Comparison with Deposit Formation on Membrane Surfaces

Separation of cells from produced biomolecules is a challenging task in many biotechnological downstream operations due to deposit formation of the retained cells, affecting permeation of the target product. Compression and relaxation behavior of cell deposits formed during filtration are important factors affecting operational performance. The determination of these factors by flux or pressure stepping experiments is time- and labor-intensive. In this work, we propose a screening method by analytical centrifugation, which is capable of detecting small differences in compression and relaxation behavior induced by milieu changes, using a model system comprised of washed and unwashed yeast cells in the presence or absence of bovine serum albumin as a model target protein. The main effects observed were firstly the impact of pH value, affecting interaction of bovine serum albumin and yeast cells especially close to the isoelectric point, and secondly the effect of washing the yeast cells prior to analysis, where the presence of extracellular polymeric substances led to higher compressibility of the deposited cells. By comparing and validating the obtained results with dead-end filtration trials, the stabilizing role of bovine serum albumin in deposits formed at low pH values due to interactions with the yeast cells was underlined.

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.

Physical structure determines compression of membrane biofilms during Gravity Driven Membrane (GDM) ultrafiltration

Increasing transmembrane pressure (TMP) can compress and increase the hydraulic resistance of membrane biofilms. The purpose of the present study is to evaluate how compression of membrane biofilms occurs and how structural rearrangement can affect hydraulic resistance. Biofilms with heterogeneous and homogeneous physical structures were grown in membrane fouling simulators (MFS) in dead-end mode for 20 days with either (i) a nutrient enriched condition with a nutrient ratio of 100:30:10 (C: N: P), (ii) a phosphorus limitation (C: N: P ratio: 100:30:0), or (iii) river water (C: N: P ratio: ca. 100:10:1). The structural and hydraulic response of membrane biofilms to (a) changes in transmembrane pressures (0.06-0.1-0.5-0.1-0.06 bar) and (b) changes in permeate flux (10-15-20-15-10 L/m²/h) were investigated. Optical coherence tomography (OCT) was used to monitor biofilm structural response, and OCT images were processed to quantify changes in the mean biofilm thickness and relative roughness. Nutrient enriched and river water biofilms had heterogeneous physical structures with greater surface roughness (Ra' > 0.2) than homogeneous P limiting biofilms (Ra' < 0.2). Compression of biofilms with rough heterogeneous structures (Ra' > 0.2) was irreversible, indicated by irreversible decrease in surface roughness, partial relaxation in mean biofilm thickness and irreversible increase in hydraulic resistance. Compression of homogeneous biofilm (Ra' < 0.2) was on the other hand reversible, indicated by full relaxation of the biofilms structure and restoration of initial hydraulic resistance. Hydraulic response (i.e., change in the specific biofilm resistance) did not correspond with the change in physical structure of heterogeneous biofilms. The presented study provides a fundamental understanding of how biofilm physical structure can affect the biofilm's response to a change in TMP, with practical relevance for the operation of GDM filtration systems.

Analysis of effect of particles on cake layer compressibility during ultrafiltration of upflow biological activated carbon effluent

Three different hollow-fibre ultrafiltration (UF) membranes were applied to treat upflow biological activated carbon (UBAC) effluent to determine the characteristics of membrane biofouling by microorganisms and particles. At the beginning of filtration, the cake layer formed on the membrane was loose and highly compressible, and the trans-membrane pressure (TMP) rapidly increased. When compressed to a certain extent, cake layer with low compressibility was formed by the accumulated particles and resulted in slower TMP increment. Thus, the decreased compressibility of the cake layer formed on the UF membrane during filtration of UBAC effluent led to the rapid increase in TMP at the beginning and slow increment in subsequently. The results were confirmed by filtering Escherichia coli, Staphylococcus aureus and kaolinite mixed suspensions with flat-sheet UF membrane. Our findings provide a new insight into membrane biofouling control and may facilitate better membrane application in drinking water treatment.

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.

Blocking laws analysis of dead-end constant flux microfiltration of compressible cakes

New blocking law models for dead-end constant flux microfiltration of colloids forming cakes that compressed in a linear and power law manner were derived. Constant pressure and constant flux experiments were performed using bacteria, colloidal silica, and treated natural waters to validate these new models and quantitatively verify blocking law predictions on the role of cake compressibility in microfilter fouling. Statistically invariant values of cake specific resistance and compressibility were obtained for constant flux and constant pressure operation for each feed suspension. This suggests that colloids formed cakes whose hydraulic resistance is dominated by a morphology that did not depend on their mode of deposition, confirming that the cake permeability was determined by the instantaneous pressure. Additionally, an inverse relationship between extracellular polymeric substances (EPS) secreted by bacteria and hydrodynamic flux restoration procedures was obtained demonstrating the importance of linking EPS to backwashing frequency when bacteria are present in the feed water.