Cake Compressibility Research Articles

Temporal resistance fluctuations during the initial filtration period of colloidal matter filtration

Membrane filtration processes with flux reversal for backwashing are employed to manage fouling during colloidal fluid treatment. This flux reversal accompanies filtration cycles where the total resistance increases over time. However, little is known about the initial phase of filtration. We focus on early filtration phase and present a microfluidic filtration system that allows precise observation of filtration resistances under constant flux or constant pressure operation for soft matter dead-end filtration with subsequent percolation of pure solvent. We identified temporal hydraulic resistance fluctuations during the start-up phase, which significantly differed from the steady-state condition. We focus on the experimental filtration of soft core–shell poly(N-isopropylacrylamide)-co-acrylic-acid microgels and observed a distinct peak in filter cake resistance with a subsequent reduction down to 50% of the initial peak resistance. To comprehend the phenomenon, we complement the experiments with temporal and spatial confocal microscopy studies linking cake compression and cake density gradients to its resistance. By comparing filtration, dialysis, and centrifugation as different microgel purification methods, we discover microgel shell degradation leading to detached polymer chains permeating in the downstream which causes the observed resistance reduction. This result demonstrates the permeation-induced degradation of macromolecular colloids during membrane filtration.

Theoretical model and parameter sensitivity analysis of the filter cake formation during slurry shield tunnelling

The filter cake formation during slurry shield tunneling in high permeability soil layers is complex and difficult to monitor. There is no reliable method to evaluate the mechanical characteristics during the filter cake formation. A theoretical model is proposed to describe the filter cake formation and development in this study. The finite difference method is used to simulate the whole filter cake formation process. In addition, the influence of the key bentonite slurry parameters and of the deep infiltration on the filter cake formation is investigated based on the proposed model. The results show that the excess pore water pressure within the filter cake is not uniformly distributed. The slurry support effect is manifested by a pressure drop generated on both sides of the filter cake. The development process of the filter cake can be summarized into three stages: prior infiltration filling of the filter cake, accumulation and thickening of the new filter cake, and compression and consolidation of the prior filter cake. The slurry deep infiltration results reflected by the membrane specific resistance have the most significant effect on the filter cake development. Compared with the slurry concentration, the viscosity has a greater influence on the formation and final thickness of the filter cake.

Destruction Mechanism of Filter Cake under Air Pressure during Chamber Opening

Abstract During chamber openings, the filter cake that has formed on the excavation face is thin and cannot be monitored directly, resulting in an unclear understanding of its destruction mechanism. To investigate the destruction mechanism of filter cake under air pressure, a series of air breakthrough tests (ABTs) were conducted in this study, during which the water discharge and pore pressure were monitored. The experimental results indicate that the filter cake undergoes significant compression during ABTs, but it tends to be stabilized after a testing time of 40,000 s. Cracks and pores are observed on the surface when the filter cake is destructed, and the cracks are mainly distributed between 80 and 100 % of the radius. The effective working time (EWT) of Filter Cake B with a compression time beyond 40,000 s is much longer than that of Filter Cake A with a compression time below 40,000 s. Because the EWT of Filter Cake B is much longer than that of Filter Cake A, the slurry composition that promotes the formation of Filter Cake B should be used as much as possible during the slurry shield. Additionally, it is proven that the EWT increases with higher slurry density and filter cake thickness, whereas it decreases with increasing slurry viscosity. The analysis of the filter cake destruction mechanism shows that the compression of filter cake, capillary effect, mechanical effect, and interfacial effect all play important roles in filter cake destruction. Except for the compression of the filter cake that can extend the EWT, the other effects will accelerate the filter cake destruction.

Modelling sludge dewatering in treatment reed bed considering sludge deposit formation

AbstractThe accumulation of sludge deposits is a crucial factor in the dewatering efficiency of sludge treatment reed bed (STRB). This paper presents an improved one-dimensional process-based mathematical model to simulate the dewatering mechanism in STRBs, in which the compressible cake filtration (CCF) theory was implemented to simulate the sludge deposits accumulation on the surface of the reed bed, while the varying sludge deposit thickness was accounted for using the moving mesh method. The proposed model also included the dual porosity variably saturated flow model and the Penman–Monteith equation to describe the dewatering through gravity drainage and evapotranspiration, respectively. The results from the model were validated with experimental data from laboratory-scale STRBs treating septage. The simulation results showed that considering the sludge deposit layer as a specific flow resistance effectively avoids the overprediction of water infiltration rate in the reed bed. The predicted results showed excellent agreement with the actual data, where only five cases of the root mean square error were above 10% compared to the average effluent flux. Further, the effect of evapotranspiration was found to be insignificant within a short-term simulation. The consideration of the influence of sludge deposit formation on drainage dewatering using the CCF model and moving mesh model has delivered a more robust simulation for sludge dewatering in STRBs, and the proposed model is capable of facilitating the understanding of the interactions between the sludge dewatering in STRB with respect to the bed characteristics, hydraulic load, and solid load.

Analytical and numerical modelling of the effect of filter medium resistance on the mechanical dewatering of wastes and industrial filter cakes

A nonlinear model is proposed to predict the effect of filter medium resistance on the filter cake compression and dewatering at constant pressure. This model is based on the Darcy-Terzaghi-Shirato filtration-compression equations presented in material coordinates and modified to consider the power-law pressure dependence of the specific cake resistance α. Above mentioned model is extended to consider the filter medium resistance Rmintroducing the number of Biot, Bi=αavWRm.Local profiles of compressive pressure and filter cake characteristics, as well as consolidation ratio and cake dryness, are simulated and compared for different moderately and highly compressible filter cakes (silica, activated sludge, Baker’s yeast, and CaCO3) based on the analytical and numerical solutions of the model. It is shown that the behavior of cake compression and dewatering significantly depends on the filter medium resistance Rm, especially at the small numbers of Bi≤1.

Protein fouling during constant-flux virus filtration: Mechanisms and modeling.

As biomanufacturers consider the transition from batch to continuous processing, it will be necessary to re-examine the design and operating conditions for many downstream processes. For example, the integration of virus removal filtration in continuous biomanufacturing will likely require operation at low and constant filtrate flux instead of the high (constant) transmembrane pressures (TMPs) currently employed in traditional batch processing. The objective of this study was to examine the effect of low operating filtrate flux (5-100 L/m2 /h) on protein fouling during normal flow filtration of human serum Immunoglobulin G (hIgG) through the Viresolve® Pro membrane, including a direct comparison of the fouling behavior during constant-flux and constant-pressure operation. The filter capacity, defined as the volumetric throughput of hIgG solution at which the TMP increased to 30 psi, showed a distinct minimum at intermediate filtrate flux (around 20-30 L/m2 /h). The fouling data were well-described using a previously-developed mechanistic model based on sequential pore blockage and cake filtration, suitably modified for operation at constant flux. Simple analytical expressions for the pressure profiles were developed in the limits of very low and high filtrate flux, enabling rapid estimation of the filter performance and capacity. The model calculations highlight the importance of both the pressure-dependent rate of pore blockage and the compressibility of the protein cake to the fouling behavior. These results provide important insights into the overall impact of constant-flux operation on the protein fouling behavior and filter capacity during virus removal filtration using the Viresolve® Pro membrane.

Effects of ceramsite derived from sewage sludge combined with the O3-FeCl3/PAM process on the dewatering of waste-activated sludge and investigation of dewatering mechanisms.

The high water content of waste-activated sludge restricts the subsequent disposal of sludge. The dehydration properties of O3, ferric chloride (FeCl3)/polyacrylamide, and sludge ceramsite sand (SCS) were studied. Simultaneously, the effect of combining the three was investigated to support the deep dehydration of waste-activated sludge. Experimental results showed that with13.42 mg/(g dry solids (DS)) of O3, 109.89 mg/(g DS) of FeCl3, and 100 mesh dosage of 70% DS of sludge ceramsite on weight, the highest sludge net yield was 7.13 kg/(m2·h) and the minimum specific resistance to filtration of sludge cake was 1.02 × 1012 (m/kg). Compared with the compressibility of the raw sludge, the compressibility of the sludge cake decreased by 37.48%. Moreover, the YN (net yield) increased by 73.55%. The results demonstrate that the structure of cracking, flocculation, and hydrophobic framework is the mechanism of sludge dewatering in this combined process. This combined treatment process provides a new perspective for the realization of deep dewatering of sludge and is anticipated to be a successful sludge dehydration method.

Rejection and fouling of track-etched microfiltration membranes by Acholeplasma laidlawii: Clues to mycoplasma behavior during “sterile” dead-end filtration

Mycoplasma contamination is commonplace during processing of biopharmaceuticals necessitating its sterile filtration. Consequently, we measured the passage of Acholeplasma laidlawii (a small bacterium lacking cell wall like mycoplasma) across track-etched membranes rated at 100 nm and 220 nm that are designated as “sterile filters” and analyzed the entire progression of fouling beginning with pore occlusion before transitioning to a cake. Initial biofouling was quantified to be intermediate blocking before bacteria formed highly compressible cakes. Under identical experimental conditions, phase inversion membranes of the same nominal pore rating rejected A. laidlawii to a significantly greater extent suggesting depth filtration and capture within the polymer matrix in contrast to surface capture by their track-etched counterparts. A facile fluorescence microscopy method to rapidly detect slow-growing A. laidlawii cells was demonstrated under conditions of low retention. Evidence of cake relaxation by depressurizing the filter and subsequent mobilization of individual cells upon re-pressurization is given. Multiple lines of evidence demonstrated the pressure-induced deformation of individual cells resulting in their incomplete rejection by membranes with pore ratings smaller than the bacterium's unstressed diameter: (i) complete retention of rigid silica colloids of similar size, shape, and zeta potential regardless of pressure in contrast to incomplete A. laidlawii removals, (ii) reduced bacterial removal by both membranes with increasing filtration pressure, (iii) 150–600 fold less permeable bacterial cakes at any given pressure compared with silica cakes, and (iv) presence of flattened and irregularly shaped A. laidlawii cells in electron micrographs whereas they were near perfect cocci at atmospheric pressure.

Optimization of the operating parameters in a membrane bioreactor operated in direct-flow mode by assessing threshold flux for compressibility

Direct-flow filtration (i.e., without membrane aeration during the filtration phase) is a challenging operation strategy in membrane bioreactors (MBRs). This study aims to provide a comprehensive approach for assessing optimal values for the main operating parameters (filtration time between backwashes and permeate flux) through the determination of the threshold flux for compressibility (Jth,c). A pilot-scale tertiary MBR was used for conducting filtration tests. Flux-step trials revealed the crucial role of cake compressibility in fouling deposition and reversibility. Accordingly, the compressible cake model was used to investigate Jth,c under different operating conditions and suspension characteristics. Based on this approach, lower filtration times (5–7 min) are recommended so that sustainable process productivity can be maximized. Specifically, decreasing filtration times from 12 to 7 min results in net permeate flux increase of more than 50%. A significant effect of suspension properties (biopolymer cluster content, BPC) on Jth,c was also found which decreased from 41.9 to 13.0 L/h m2 when increasing BPC from 0.3 to 4.6 mg/L. Long-term tests, under temporized and variable filtration time modes, demonstrate the feasibility of using the Jth,c as a guideline for optimizing filtration performance.

Reduced order modeling for compressible cake filtration processes using proper orthogonal decomposition

This work aims to increase the computational efficiency of a complex mathematical cake-filtration model with strong nonlinearities representing cake compression. To this end, we employ a hybrid data driven approach using the technique of proper orthogonal decomposition. Hereby, a few sample simulations from the initial system of partial differential equations are used as the foundation to find optimal, globally defined basis functions, which in return offer the possibility to build a reduced-order model. In summary, the dimension of the reduced order model is diminished by ≈98% compared to the full order model, which translates to a net decrease of ≈90 % computational time needed to solve a benchmark optimization problem. This significant numerical speed-up offers the possibility to use the reduced order model in further advanced process control and optimization methods.

Enhancing the Dewaterability of Oily Cold Rolling Mill Sludge Using Quicklime as a Conditioning Agent.

The high moisture and viscosity characteristics of oily cold rolling mill (CRM) sludge led to limitations in its recycling. In this paper, the results of using quicklime as a conditioning agent to improve the dewaterability of the oily CRM sludge cake were reported. Quicklime was selected as the best conditioning agent through conditioning-dewatering experiments because it could effectively reduce the viscosity of oily CRM sludge and improve the dewaterability of the oil sludge filter cake. The optimal conditioning effect was obtained when the quicklime dosage was 10%, the temperature was 60 °C, the liquid/solid ratio was 1.5:1, and the time was 30 min. The reduction of specific resistance to filtration was 95.9%, and the coefficient of compressibility of the filter cake decreased from 1.11 to 0.89. The dewatering rate increased from 9.0 to 52.6%. The oily CRM sludge cake formed an incompressible rigid porous structure because of conditioning with quicklime. In addition, after conditioning with quicklime, the oil content, chemical oxygen demand, and turbidity of the filtrate decreased, and the composition of the dried filter cake met the requirements of ironmaking raw materials. Using quicklime to condition the oily CRM sludge provided a green approach to waste recovery and sustainable management.

Dewatering of Arthrospira platensis microalgae suspension by electrofiltration

Harvesting microalgae has presented many challenges in the past, especially when taking into account the final dryness of the product and the energy requirements for the drying process. This study investigates the use of dead-end electrofiltration to dewater an Arthrospira platensis suspension under different pressures (1 to 4 bar) and electric current densities (30 to 80 A/m2). Filtration without an electric field helped reach a final cake dryness in the range of 13.3-15%. Electrofiltration was found to be a more effective dewatering method, reaching a final cake dryness of up to 20.8% with an energy consumption of 0.105 kWh/kg water removed, which is significantly lower than the energy consumption reported in literature. Increasing the electric current density helped increase the cake dryness, whereas increasing the operating pressure was not effective enough due to the high cake compressibility. Electrofiltration also modifies the filtrate and cake properties due to the electrolysis reactions.

Preparation of amphiphilic cationic polyacrylamide (CPAM) with cationic microblock structure to enhance printing and dyeing sludge dewatering and condition performance.

Flocculation is an important pretreatment technology for sludge dewatering, and the flocculant's performance is the key factor to determine the flocculation effect. Cationic polyacrylamide (CPAM) is commonly used in dewatering and conditioning of printing and dyeing sludge (PD sludge), and the research of high-efficiency flocculant is a hot spot in the field of PD sludge dewatering. Hydrophobic butylacrylate (BA) and (2-(Methacryloyloxy)ethyl) trimethylammonium chloride (DMC) were introduced into the copolymer, and amphiphilic (hydrophilic/lipophilic) CPAM, namely TP-ADB, with microblock structure was synthesized by ultrasonic initiated template copolymerization in this study. The functional group composition of TP-ADB was determined by FTIR and 1H NMR. Thermogravimetric analysis (TGA) showed that TP-ADB had good thermal stability. The amphiphilic rheological properties of the copolymer were measured according to the apparent viscosity. In addition, 1H NMR and TGA results confirmed the existence of microblock structure in the copolymer chain. The polymerization mechanism was discussed by association coefficient (KM) measurement. The results showed that the template copolymerization initiated by ultrasonic followed the law of free radical copolymerization. The pre-adsorption of DMC with sodium polyacrylate template (NaPAA) before the reaction confirmed that the template polymerization accorded with ZIP I mechanism. The cationic microblock structure and hydrophobic association of TP-ADB promoted the dewatering performance of PD sludge (FCMC = 72.9%, turbidity removal rate = 98.9%, SRF = 4.2 × 1012m·kg-1). Hydrophobic association enhanced the bridging, sweeping, and net catching effect, and promoted the growth of floc size and fractal dimension. Cationic microblock structure can produce compact floc with higher mechanical strength by enhancing electrical neutralization and electrical patching. As a skeleton, the compressibility of filter cake was reduced and the permeability was enhanced, and the PD sludge dewatering effect was significantly improved.

Optimal dosage strategies for filter aid filtration processes with compressible cakes

We establish optimal time-dependent dosage strategies for compressible filter aid materials, focusing on dead-end filter aid filtration processes operated in constant flow mode. To this end, we apply a direct optimal control approach by repeatedly evaluating a mathematical filtration model with filter aid concentration as the unknown time-dependent control function. Our algorithm iteratively constructs the sought-for trajectory to minimize a cost functional representing previously defined performance goals. We evaluate our optimal control approach for two different model formulations. In the first one, all impurity particles are separated by pure surface filtration on top of the existing filter cake, whereas in the second one, the model is extended toward depth filtration in the filter cake. Particularly for the latter, optimally controlled dosage strategies yield a significant performance improvement of up to 30% reduced filter aid consumption compared with the already optimized but constant filter aid dosage.

Dewatering of non-uniformly structured wet compacts under additional compressive pressure: Predictive model

Predictive model is developed to simulate the dewatering of non-uniformly structured wet compacts formed at different filtration pressures PF and then compressed under additional compressive pressure PE >PF. This model is based on the filtration-consolidation equation in which compressive pressure PS is replaced by the special power-law theta-function θ1/1−n=1+PSPa=P∗considering pressure dependence of the specific cake resistance, and transforming Darcy law to the linear form. Proposed model permits to cover industrially important dewatering processes, for which filtration pressure PF differs from the compression pressure PE, and 0 ≤ PF ≤ PE. In the particular cases of PF = 0 (uniformly structured semi-solid material) and PE = PF (filter cake formed and compressed at the same pressure) obtained analytical solutions are transformed to the previously obtained results. Several examples of the application of proposed model are presented for the highly and moderately compressible cakes (silica, activated sludge and CaCO3,). The profiles of θ- function and dimensionless compressive pressure P∗ are simulated and compared for different wet compacts based on the analytical and numerical solutions of the model. It is shown that the behavior of mechanical dewatering depends on the cake structure obtained after filtration. Consolidation ratio U of wet compacts with initially non-uniform structure formed by filtration differs from that of materials with initially uniform structure. Dryness of studied wet compacts d was also predicted based on the proposed model.

In-Situ Visualization and Characterization of Filter-Cake Deposition Using Time-Lapse Micro-CT Imaging

This work establishes and verifies a laboratory method for accurate, noninvasive, and nondestructive in-situ measurement of filter-cake, or mudcake, properties using high-resolution X-ray microcomputed tomography (micro-CT). Accurate in-situ characterization of mudcake properties is of vital importance for understanding mudcake deposition during mud-filtrate invasion and, more broadly, the deposition of filter cake during industrial filtration processes. The developed laboratory method involves the injection of pressurized drilling mud into a borehole located at the center of a cylindrical rock core sample. Radial mud-filtrate invasion is induced by maintaining the outside of the core sample at a lower pressure. Mudcake is deposited on the borehole wall while mud filtrate flows into the surrounding pore space of the core sample. Simultaneously, the core sample is continuously scanned using high-resolution X-ray micro-CT, allowing for three-dimensional (3D) in-situ visualization and characterization of the deposited mudcake. Analysis of experimental results is aided by a new set of filtration equations that are generalized for use with either water-based mud or invert emulsion drilling fluids, such as oil- or synthetic oil-based mud. Each experiment produces a time-series data set consisting of injected mud volume and the corresponding in-situ average mudcake thickness, porosity, and permeability. Results are presented for experiments involving injection of water- and synthetic oil-based drilling mud. We observe that experimental measurements agree well with modeled results when mudcake is thin compared to the size of the borehole—a condition that would be satisfied under most normal field conditions. Initial results also suggest that the developed method may be capable of providing accurate in-situ 3D measurements of local filter-cake properties during compressible cake filtration, which has significant implications for a wide range of industrial applications.

Material Properties in Cake Filtration – Discrete Element Simulation of Compression‐Permeability Cells

AbstractThis simulation study investigates the impact of material properties on the behavior of compressible filter cakes. A compression‐permeability (CP) cell is used as a standard experimental technique for characterizing compressible cakes. It contains a bidisperse particle packing simulated by the discrete element method. Using this setup, a set of crucial material properties is varied and their influence on the macroscopic compression behavior as described by established fitting equations is studied. Sobol's sensitivity analysis is performed, and the results indicate that Young's modulus, the coefficient of sliding friction, and the surface energy density have the strongest impact on cake solidity and compressibility.

Geotechnical Properties Determination of Thickened Fluid Fine Tailings

AbstractFluid fine tailings (FFT) comprising clayey-silt solids pose environmental and financial challenges. Currently, mining operators are depositing thickened FFT in deep-pits counting on self-weight consolidation to form stable ground. The motivation of this study is to model the long-term prospects of such deposits utilizing consolidation and direct shear strength measurements. The tests were conducted using scroll decanter centrifuge separated FFT sediment referred to as cake. Drained direct shear tests of the cake gave a linear Mohr–Coulomb failure envelope of 1.2 kPa cohesion intercept and 9° internal friction angle for normal stresses up to 1 MPa. Hydraulic conductivity of the cake was non-linear with normal stress decreasing to 1.7 × 10–11 m/s at 300 kPa. Consolidation results confirmed that the cake exhibits properties similar to those of active clay minerals. The cake compression index is governed by the same relationship as for active clays. The coefficient of consolidation for the cake was nearly constant and had a mean value of 2.25 × 10–3 m2/y, also similar to that of active clays. The void ratio—effective stress—hydraulic conductivity power law empirical relations were used to simulate settlement with a finite-strain model. Numerical results show that the top portion of an FFT deep-pit deposit remains in the liquid state while the lower portion whose maximum solids content converges to 74% is in plastic state. These mean that options that improve hydraulic conductivity and increase the shear strength of the thickened FFT should be integrated prior to final placement in order to create stable ground.

Potential Pitfalls in Membrane Fouling Evaluation: Merits of Data Representation as Resistance Instead of Flux Decline in Membrane Filtration.

The manner in which membrane-fouling experiments are conducted and how fouling performance data are represented have a strong impact on both how the data are interpreted and on the conclusions that may be drawn. We provide a couple of examples to prove that it is possible to obtain misleading conclusions from commonly used representations of fouling data. Although the illustrative example revolves around dead-end ultrafiltration, the underlying principles are applicable to a wider range of membrane processes. When choosing the experimental conditions and how to represent fouling data, there are three main factors that should be considered: (I) the foulant mass is principally related to the filtered volume; (II) the filtration flux can exacerbate fouling effects (e.g., concentration polarization and cake compression); and (III) the practice of normalization, as in dividing by an initial value, disregards the difference in driving force and divides the fouling effect by different numbers. Thus, a bias may occur that favors the experimental condition with the lower filtration flux and the less-permeable membrane. It is recommended to: (I) avoid relative fouling performance indicators, such as relative flux decline (J/J0); (II) use resistance vs. specific volume; and (III) use flux-controlled experiments for fouling performance evaluation.

Particle movements provoke avalanche-like compaction in soft colloid filter cakes

During soft matter filtration, colloids accumulate in a compressible porous cake layer on top of the membrane surface. The void size between the colloids predominantly defines the cake-specific permeation resistance and the corresponding filtration efficiency. While higher fluxes are beneficial for the process efficiency, they compress the cake and increase permeation resistance. However, it is not fully understood how soft particles behave during cake formation and how their compression influences the overall cake properties. This study visualizes the formation and compression process of soft filter cakes in microfluidic model systems. During cake formation, we analyze single-particle movements inside the filter cake voids and how they interact with the whole filter cake morphology. During cake compression, we visualize reversible and irreversible compression and distinguish the two phenomena. Finally, we confirm the compression phenomena by modeling the soft particle filter cake using a CFD-DEM approach. The results underline the importance of considering the compression history when describing the filter cake morphology and its related properties. Thus, this study links single colloid movements and filter cake compression to the overall cake behavior and narrows the gap between single colloid events and the filtration process.