Formation Of Fouling Layer Research Articles

Thermal ageing of proteinaceous fouling layers during the growth phase

Fouling in heat exchangers can complicate the characterisation and interpretation of thermal effects because of ageing phenomena that occur within the deposited fouling layer. The prevailing process temperatures between the liquid bulk and heat-transferring surfaces create a large thermal conductivity distribution according to the position of the layer within the deposit. During the growth phase, an interaction occurs between the fouling layer formation and ageing. Therefore, deposition and ageing should always be considered in combination to obtain a better understanding of fouling. This paper discusses an experimental method for determining temperature-dependent ageing, expressed as a change in thermal conductivity with time and along the cross section of the fouling layer. An experimental setup is presented that includes a newly developed flow channel and an experimental implementation of an ageing model. In the first experiments, proteinaceous fouling layers were generated from whey protein concentrate (WPC) with and without simulated milk ultrafiltrate (SMUF), applied for different durations to create different fouling layer thicknesses. The thermal conductivity increased more rapidly near the heat-transferring surface than for the entire fouling layer. These findings can be related to the temperatures within the sublayers.

A Novel Non-Intrusive Continuous Sensor (NICS) to estimate thermal conductivity of food products in manufacturing systems

Fouling is a critical issue in commercial food manufacturing. Fouling can cause biofilm formation and pose a threat to safety of the food products. Early detection of fouling can lead to informed decision making about the product’s safety and quality, and effective system cleaning to avoid biofilm formation. In this study, a Non-Intrusive Continuous Sensor (NICS) was designed to estimate the thermal conductivity of the product as they flow through the system at high temperatures as an indicator of fouling. Thermal properties of food products are important for product and process design and to ensure food safety. Online monitoring of thermal properties during production and development stages at higher processing temperatures, ∼140 °C like current aseptic processes, is not possible due to limitations in sensing technology and safety concerns due to high temperature and pressure conditions. Such an in-line and noninvasive sensor can provide information about fouling layer formation, food safety issues, and quality degradation of the products. A computational fluid dynamics model was developed to simulate the flow within the sensor and provide predicted data output. Glycerol, water, reconstituted non-fat dry milk (NFDM), and 4% potato starch solution were selected as products. The product thermal conductivities were estimated at high temperatures (∼140 °C). Scaled sensitivity coefficients and optimal experimental design were taken into consideration to improve the accuracy of parameter estimates. Glycerol, water, NFDM, and 4% potato starch were estimated to have thermal conductivities of 0.292 ± 0.006, 0.638 ± 0.013, 0.598 ± 0.010, 0.487 ± 0.009 W/(m K, respectively. The sensor’s novelty lies in the short duration of the experiments, the non-intrusive aspect of its measurements, and its implementation of sequential estimation.

Spatial variation of fouling behavior in high recovery nanofiltration for industrial reverse osmosis brine treatment towards zero liquid discharge

Nanofiltration (NF), as a cost-efficient pre-concentrating process, has been incorporated into Zero Liquid Discharge (ZLD) treatment system to improve economic feasibility. However, NF is required to operate with an extremely high recovery rate to achieve ZLD, which could cause severe and complex membrane fouling.Understanding the variation of fouling behavior with increase in recovery rate is crucial for the development of effective fouling control strategy. Spatial variation of fouling behavior in a 3-stage NF used for industrial RO brine treatment towards ZLD was investigated in present study. Distinctive fouling characteristics were observed in each operation stage. Membrane fouling was dominated by organics at lead stage, which could be completely removed via simple base cleaning. Scaling by deposition of bulk crystallization occurred on fouling layer at middle stage, which required a combination of acid and base cleaning for its removal. At the tail stage, more refractory scaling by surface crystallization accompanied by irreversible silica and ferric fouling led to the formation of fouling layer that was resistant to conventional chemical cleaning. Significant humics in preformed fouling layer could have facilitated the occurrence of surface crystallization. Minor elements present in the feed such as silica, humics and iron could cause significant fouling and alter preformed fouling layer that induced more complex and refractory fouling/scaling as recovery rate increased.

Polymer Film Heat Transfer Surfaces in Seawater Desalination: Fouling Layer Formation and Technology

AbstractThe emerging polyether ether ketone (PEEK) films were evaluated as future heat exchanger materials in seawater thermal desalination processes. Fouling resistance curves were measured in a pilot plant for pure PEEK films, talcum‐filled PEEK films, and stainless‐steel films with artificial seawater, showing that pure PEEK films have the advantage of weaker forces of adhesion or cohesion between the scaling materials and the film surface. Increasing the flow rate from 18 to 24 L min−1 results in decreases of both the fouling rate and the asymptotic values. This was attributed to the lower polarity of 10.4 %, the lower arithmetic average roughness of 0.06 μm, and the homogeneous topography of the PEEK film. This polymer film is adaptable to in situ cleaning in thermal seawater desalination processes.

Influence of laccase treatment on fouling layer formation in ultrafiltration of birch hot-water extract

Fouling problems caused by lignin have been limiting the use of membranes in the recovery of value-added materials from biorefinery streams. Fortunately, laccase-based catalytic pretreatments enable modification of lignin to the less fouling form. Because the mechanisms behind the fouling caused by ligneous compounds in wood-based streams are still not deeply understood, the aim of this study was to find out how laccase oxidation affects membrane fouling caused by pressurized hot-water extract. The effect of laccase on the fouling tendency of birch extract was explored both with adsorptive and pressure-driven experiments. The results suggested that laccase treatment increased the molar mass of lignin and improved significantly filtration capacity of commercial polyethersulphone membrane, possibly due to formation of less dense foulant layer during the filtration. Based on FTIR and BET results laccase oxidation decreased both adsorptive and pressure-driven fouling caused by lignin. However, decreases in pure water permeabilities were higher for the samples that were fouled with laccase treated extract. This may have originated from the transformation in the fouling mechanism from the pore narrowing to the pore blocking and could be prevented by the selection of a membrane with a different pore size.

Performance index improvement of a double-pipe cooler with MgO/water-ethylene glycol (50:50) nano-suspension

A series of tests was conducted to unlock the potential application of MgO/water-EG (ethylene glycol) nanofluids (NF) in a double-pipe heat exchanger (HEX). The overall heat transfer coefficient (HTC), the inlet temperature of the working fluid, the fluid pressure drop (FPD), friction factor (FF) and the hydraulic performance index of the NF within the HEX were experimentally measured. Fouling of nanoparticles (NPs) within the Hex was also studied and modelled using asymptotic particulate fouling model. Results showed that MgO NPs can enhance the HTC by 39% at Re=10,500 and wt.% = 0.3 in the turbulent regime. Also, the presence of MgO NPs augmented the FF and the FPD values. The former was enhanced 33.8%, while the latter was augmented by 37% both at wt.% = 0.3 and at Reynolds number = 10,500. Results also revealed that the formation of porous particulate fouling layer on the internal wall of the inner tube creates a fouling thermal resistance which changes asymptotically with time. Overall, MgO/water-ethylene glycol shows a great potential to be used as a coolant within a HEX.

Versatile zero valent iron applied in anaerobic membrane reactor for treating municipal wastewater: Performances and mechanisms

Comprehensive insight into the long-term impact of applying zero valent iron (ZVI) in anaerobic membrane reactor (AnMBR) was conducted here. ZVI showed its versatility for enhancing the performances of AnMBR including COD and TP removal, H2S removal and membrane fouling suppression in this study, and the iron ions releasing rate of ZVI had great influence on the enhancing performances. Two iron ions releasing states were used including “with electric field” and “without electric field”. Applying electric field could boost iron ions releasing rate by 12 times here. COD removal rate increased by 3% via coagulation effect with and without electric field. H2S in biogas was decreased from >500 ppm to <60 ppm via the formation of FeS precipitation with and without electric field. Applying ZVI with electric field increased the removal rate of TP by 50% via the formation of Fe3(PO4)2, but applying ZVI without electric field did not change the TP removal due to the insufficient iron ions. Utilizing ZVI without electric field suppressed the membrane fouling rate by 20% via the improvement of mixed liquor filterability, however utilizing ZVI with electric field accelerated the membrane fouling rate by 17 times due to the formation of dense Fe-enriched fouling layer with server inorganic and organic fouling. This study provided a novel approach to improve the performances of AnMBR by utilizing versatile ZVI. Meanwhile, this study enclosed that the iron ions releasing rate of ZVI is vital technical parameter for its application.

Characterization of fouling and concentration polarization in ion exchange membrane by in-situ electrochemical impedance spectroscopy

The electrochemical impedance spectroscopy (EIS) was used to monitor the ion transport of the ion exchange membrane (IEM) interface with ectopic monitoring. But the general monitoring methods were lack of the information in concentration polarization (CP) and fouling formation. In this paper, an in-situ method for monitoring the ion mass transfer by EIS was established which can distinguish the contribution of membrane fouling and CP. The results showed that the in-situ EIS could more comprehensively monitor the desalination process. Membrane fouling, especially the formation of fouling layer on membrane surface, promoted the CP at membrane/solution interface. This was mainly because the ion mass transfer was hindered by serious membrane fouling, so that the ion concentration near the membrane surface was higher than that of the solution, and this phenomenon was gradually aggravated with the increase of membrane fouling. The recovery of membrane performances were assessed by different cleaning methods, it was found that the chemical cleaning effect was superior to inverted electrode according to online EIS. When irreversible fouling was formed in the membrane and then the fouled membrane was chemically cleaned, the formation of the fouling layer was accelerated and the CP was aggravated during membrane reuse.

Improvement of particle deposition model using random function method

Particles in the environment not only threaten people's health, but the adhesion of particles on the equipment also threatens the safe operation of the equipment and the adhesion of particles in the heat exchange field also causes a large amount of energy waste. In this paper, the transportation and deposition processes of particles with different particle sizes were studied by numerical simulation. A random function method is introduced to improve the particle deposition model in order to consider the different characteristics between clean and fouled surfaces. As an example, the particle deposition characteristics of a two-dimensional channel were simulated using the clean surface model, fouling layer model, and random function method. The results show that as the particle size and inlet velocity increase, the gap between the three models increase. In particular, in the simulation of mixed particle size, the random function method better simulates the change in deposition rate during formation of fouling layer and the composition of fouling layer. Furthermore, the growth of the fouling was accelerated by the first layer of sediments, and the first layer of the sediments usually consisted of small particles.

Recovery of succinic acid from fermentation broth by forward osmosis-assisted crystallization process

In this study, osmotically driven forward osmosis (FO) was employed prior to crystallization process in the downstream recovery of bio-based succinic acid. The fermentation broth containing succinic acid was initially pretreated using activated carbon. Powdered activated carbon (PAC) showed its effectiveness for glucose, formic acid, and color removal while succinic acid concentration remained unaffected. The untreated and treated fermentation broths were then concentrated using the FO process. FO exhibited a remarkable enhancement of concentration factor (CF) by 3.9-fold for the treated broth, thus resulting in a final succinic acid concentration of 111.26 g/L. By contrast, higher flux loss and lower CF were observed for untreated broth, mainly due to the adverse effect of severe membrane fouling and cake layer formation. Succinic acid crystals were then successfully recovered from the FO-concentrated broth in the final crystallization step. The purity and yield of succinic acid crystals were 90.52% and 67.09%, respectively for treated broth. This work demonstrated the development of a feasible FO-crystallization process for the downstream recovery of bio-based succinic acid. The findings have important implications for practical applications of FO technology in the bioprocess industries.

Effect of crossflow regime on the deposit and cohesive strength of membrane surface fouling layers

Acquiring knowledge of the properties of membrane fouling layers is crucial to mitigating fouling and developing cleaning strategies. The cohesive strength of these fouling layers, which determines the cleaning requirement of the membrane, is nevertheless rarely investigated. Here we introduced fluid dynamic gauging (FDG) to the crossflow microfiltration of a wood material, namely microcrystalline cellulose (MCC, nominal particle size 20 μm, 95% (in volume) of the particles are bigger than 5.4 μm and smaller than 56.4 μm), to study in situ the cohesive strength of the membrane surface fouling formed under different crossflow regimes. Using regenerated cellulose membrane with a nominal pore size of 0.2 μm, filtration experiments with FDG measurement show that the crossflow regime can lead to the formation of surface fouling layers with distinct cohesive strength. Fouling formed in turbulent/transitional crossflow (Reynolds number, Reduct = 4170) was stronger and its removal required more liquid shear stress compared to the layers formed in laminar crossflow (Reduct = 1560). The fouling layers that can withstand the minimum shear of 35 Pa from the FDG sensor with turbulent/transitional crossflow were, on average 294 ± 10 μm thick, in contrast to those formed in laminar crossflow, which were significantly thinner (144 ± 73 μm at 35 Pa shear stress, p < 0.05). On the other hand, turbulent/transitional crossflow reduced material deposition significantly (p < 0.05). After 1000 s filtration, 0.117 ± 0.003 kg m−2 MCC were found on the turbulent/transitional crossflow membranes, compare to 0.134 ± 0.005 kg m−2 in the laminar crossflow situation. Moreover, a similar permeate flux was observed in all experiments. Therefore, this work also highlights the necessity of developing membrane cleaning protocols based on the fouling layer properties, rather than on the permeate flux decline.

Development of a Computational Fluid Dynamics Model for Predicting Fouling Process Using Dynamic Mesh Model

This article presents a comprehensive computational model capable of simulating fouling layer thickness evolution using dynamic mesh model. This computational methodology has been developed to reproduce the deposit generation during fouling process with an innovated work method. Dynamic mesh model, from Ansys Fluent software, and external routines have been used to implement this advanced numerical model which allows to move the boundaries of a region relative to other boundaries of the zone. The displacement of the nodes of the mesh is the mechanism that this model uses to adjust the geometry according to the fouling layer evolution. During the simulation process, the geometry under investigation is modified to reproduce the emergence and gradual change of the fouling layer. Different rules of deposition and removal of the fouling process can be implemented in the proposed algorithm. The direct interaction between fouling expressions and governing equations of the main flow is used to predict deposits formation and growth. In this article, numerical simulations of soot fouling layer formation have been presented. Deposit evolution has been calculated inside different heat exchanger technologies used in exhaust gas recirculation systems to analyze fouling process and to verify the advantages of this new computational strategy.

Laser induced fluorescence (LIF) technique visualizes and characterizes concentration polarization and fouling layer in the cross-flow nanofiltration

In-situ non-invasive observation of concentration polarization (CP) and fouling layer is of vital importance to understanding membrane filtration process and membrane permeability loss. This study developed a novel real-time technique, laser induced fluorescence (LIF) technique, to visualize CP development and fouling layer formation, and to measure solute concentration field on the membrane surface directly during cross-flow nanofiltration, which helped to quantitatively evaluate the effect of various operation conditions on membrane permeability. The solute concentration distribution on the membrane surface was successfully determined from the LIF images, and the fouling layer thickness was measured by series of processed images after binary subtraction. LIF results demonstrated that the fouling development experienced approximate two stages, i.e. rapid growth phase and consolidation phase, and visualized the transition from initial CP to the later fouling layer formation. Afterwards, the formed fouling layer tend to be steady with nearly unchanging concentration and thickness. Quantitative analysis proved that the operational hydraulic condition has a significant influence on the formation and stabilization of fouling layer. The developed method and obtained results were very useful to in-depth recognize the nanofiltration process and to understand the membrane fouling mechanism.

On the rejection and reversibility of fouling in ultrafiltration as assessed by hydraulic impedance spectroscopy

The manifestation of critical fluxes in membrane filtration has typically served to set an upper value above which detrimental fouling events and a substantial increase of membrane resistance occurs. Despite the usefulness of critical flux concepts for conducting sustainable membrane processes, the specific phenomena causing them cannot be easily identified via conventional membrane filtration modes. Usually arbitrary fouling rates are selected to delimit the boundary between under- and over-critical flux operation. Timescales respective for colloidal matter accumulation are overlooked.Frequency response analysis of transmembrane pressures has been recently presented as a highly-sensitive method to track fouling in ultrafiltration. This hydraulic impedance spectroscopy method allows not only measuring membrane resistances but also corresponding time constants related to colloidal matter accumulation. In this work, hydraulic impedance spectra are gained for the ultrafiltration of different model foulants to assess the origin of critical fluxes for two different membranes. A correlation among characteristic impedance features and the type of fouling could be established: the evolution of phase shift between flux and pressure at increasing fluxes allows identifying the formation of external fouling layers, while registering of hysteresis loops at reversed frequency sequences signals the development of irreversible internal fouling. The hydraulic impedance method emerges as a precise and more sensitive monitoring tool to diagnose the beginning and nature of critical fouling.

In situ coagulation versus pre-coagulation for gravity-driven membrane bioreactor during decentralized sewage treatment: Permeability stabilization, fouling layer formation and biological activity

Gravity-driven membrane filtration systems are promising for decentralized sewage treatment due to their low energy consumption and low maintenance. However, the low stable permeability/flux is currently limiting their wider application. With the ultimate goal of increasing permeability, the aim of this study was to evaluate the effect of coagulation (in situ coagulation and pre-coagulation) on the performance of a gravity-driven membrane bioreactor (GDMBR) during treatment of synthetic sewage. Results show that in situ coagulation significantly increased permeability (more than two-fold); however, no stabilization of permeability occurred over the whole operation, when non-coagulated and pre-coagulated reactors were compared. The high permeability observed was attributed to the accumulated aluminium floc in the reactor, which prevented formation of fluorescent microbial metabolites (aromatic and tryptophan proteins, as well as fulvic acids), and further avoided membrane pore blocking. In addition, the surface porosity of the fouling layer was improved (from 11.2% to 32.4% for non-coagulated and in situ coagulated reactors). The unstable permeability was possibly associated with lower biological processes within the fouling layer. These might include lower adenosine triphosphate (ATP) content and lower fluorescent metabolites from the extracellular polymeric substances (EPS) caused by the accumulated Al (compared with the control). On the other hand, pre-coagulation improved the level of stable permeability compared with the control (80 versus 40 L/m2 h bar), mainly because pre-coagulation decreased the EPS content and also maintained high ATP content of the fouling layer. In addition, both coagulation processes reduced the total filtration resistance, mainly the hydraulically reversible resistance and cake layer resistance, which could lower the cleaning frequency. Overall, coagulation could greatly increase the removal efficiency and improve the GDMBR permeability, which would make the process suitable for decentralized wastewater treatment.

Membrane filtration device for studying compression of fouling layers in membrane bioreactors.

A filtration devise was developed to assess compressibility of fouling layers in membrane bioreactors. The system consists of a flat sheet membrane with air scouring operated at constant transmembrane pressure to assess the influence of pressure on resistance of fouling layers. By fitting a mathematical model, three model parameters were obtained; a back transport parameter describing the kinetics of fouling layer formation, a specific fouling layer resistance, and a compressibility parameter. This stands out from other on-site filterability tests as model parameters to simulate filtration performance are obtained together with a characterization of compressibility. Tests on membrane bioreactor sludge showed high reproducibility. The methodology’s ability to assess compressibility was tested by filtrations of sludges from membrane bioreactors and conventional activated sludge wastewater treatment plants from three different sites. These proved that membrane bioreactor sludge showed higher compressibility than conventional activated sludge. In addition, detailed information on the underlying mechanisms of the difference in fouling propensity were obtained, as conventional activated sludge showed slower fouling formation, lower specific resistance and lower compressibility of fouling layers, which is explained by a higher degree of flocculation.

Real-time fouling monitoring with Raman spectroscopy

The possibility to detect and identify early stage membrane fouling in real-time is a key to understand fouling phenomena. In this study normal Raman spectrocopy was explored as a novel online tool for monitoring membrane fouling. Raman spectra were measured during cross-flow experiments in which vanillin was used with different concentrations as a model compound for organic foulants. The measured Raman spectra were analyzed and interpreted by using principal component analysis and external calibration in order to determine the adsorbed amount of vanillin during the fouling process. Fouling induced changes in pure water permeabilities and off-line measured FTIR spectra were also analyzed in order to confirm the results of the real-time monitoring system. It was found that an increase in the solution concentration increased the degree of vanillin adsorption and led to better water permeabilities. The procedure used is able to provide results that describe well the accumulation of foulants in a straightforward and semi-quantitative way. The results demonstrate that the introduced monitoring tool has potential for gaining new knowledge about fouling layer formation.

Fouling behavior of dissolved organic matter in nanofiltration membranes from a pilot-scale drinking water treatment plant: An autopsy study

This study investigated the fouling behavior and mechanisms of nanofiltration (NF) membranes fed with surface water pre-treated using a combined coagulation and microfiltration (CC–MF) process in a pilot-scale drinking water treatment plant through membrane autopsies and characterization of the desorbed foulants. The CC–MF process removed hydrophobic DOM components preferentially and mitigated biofouling of the NF membranes efficiently. This clearly shows that the fouling characteristics of the NF membranes can be influenced by the CC–MF process. The fouling formation of the NF membranes was dominated by chemically removable DOM components primarily consisting of hydrophilic fractions and inorganic scaling. However, the surfaces of the fouled NF membrane (contact angle=65°) were more hydrophobic than those of the virgin NF membrane (contact angle=57°). This is attributed to the differences in the fouling behavior of hydrophobic and hydrophilic DOM components in relation to the surface features of the NF membranes (i.e., surface zeta potential and contact angle). The initial formation of organic fouling layers on the NF membranes surfaces was caused by deposition of hydrophilic DOM components, which can decrease electrostatic repulsive forces between humic substances and the negatively charged NF membrane surfaces by reducing the negative surface zeta potential. Therefore, hydrophobic DOM components accumulated more readily onto the NF membrane surfaces. The covering hydrophilic organic fouling layers with hydrophobic fractions may intensify the subsequent adsorption of hydrophobic DOM components on the NF membrane surfaces by increasing the contact angle.

Rotating Disk-Assisted Cross-Flow Ultrafiltration of Sugar Beet Juice

Rotating disk module (RDM)-assisted cross-flow ultrafiltration of sugar beet juice, obtained by cold pulsed electric field-assisted pressing, was studied in full recycling and concentration modes. Results of trans-membrane pressure (TMP) stepping tests with 10- and 50-kDa membranes showed that filtration flux increased with rotation speed and TMP until 1000 rpm and 4 bar. A slight variation of filtration purity was observed for a rotation speed range of 0–2500 rpm. A TMP increase resulted in purity reduction due to the formation of thicker fouling layer. A full recycling test study with a 10-kDa membrane confirmed the influence of TMP and rotation speed on filtration flux. A rotation speed of 1000 rpm and a TMP of 4 bar were selected for sugar beet juice purification, by concentrating juice from 13 to 1 L. The results of concentration mode tests revealed that, with a 10-kDa membrane, the filtrate purity could reach 95.6 ± 0.5 %, which was comparable to the purity of sugar beet juice purified by conventional liming-carbonation method. The flux decline in cross-flow filtration was still important, justifying more effective anti-fouling technologies investigation.

Analysis of the fouling in heat exchangers by irreversible thermodynamics methods

The source of reduced efficiency of the heat exchanger during operation are the fouling layers. In the analysis of existing theoretical research methods of the fouling dynamics in heat exchangers, it was proposed to use a method of entropy production minimization for the growth analysis of the thermal resistance of the heat transfer wall with the fouling layers. At the design stage, the method has shown the possibility to determine the energy performance of the heat exchanger in the time function, to evaluate the irreversibility in the transition of the heat exchanger to the operation with different temperature conditions and qualitative compositions of flows, to assess the behavior of a single flow, without calculating the entire heat exchanger. The developed mathematical model of the formation dynamics of solid fouling layers on the heat exchange surface has allowed to introduce the approximate to the real values additional thermal resistances into the calculation at the design stage. The model more accurately describes the heat transfer and fluid dynamics, taking into account the uncertainty of the solid fouling layer formation. This approach to the fouling layer formation analysis allows to forecast the behavior of the individual flow in heat exchangers and improve repair schedule under the continuous operation of the heat exchanger.