Effect Of Cross-flow Velocity Research Articles

Effect of cross-flow velocity on spray evolution and impingement characteristics of a multi-hole port fuel injector

The location and orientation of the injector play a crucial role in determining engine performance and emissions from spark ignition and dual-fuel compression ignition engines. This study focuses on the spray atomization and downstream mixing of gasoline injected from a multi-hole port fuel injector in a crossflow. This study employed the phase Doppler interferometry technique to extract the droplet size and velocity distributions for the flow confined in a circular duct with a diameter similar to the intake port of the dual-fuel compression ignition engine. The flow velocity was maintained at 10 m/s at 1 atm pressure and 299 K temperature. The spray characteristics were compared for the quiescent and crossflow cases. The spray evolution was analyzed using a high-speed imaging technique. Near wall impingement analysis has been carried out using the spray impingement models. The early stage spray evolution was similar for the quiescent and crossflow cases. The horizontal velocity of the spray was found to be ∼12 m/s at 20 mm downstream of the injector. The velocity remained similar for the flow and no-flow cases, as drag force was found to have an insignificant effect. The drag force was estimated to be one order of magnitude higher for the 15-μm droplet than the 50-μm droplet. The maximum Sauter mean diameter observed for the flow case inside the spray was 53 μm, which was 18% higher than the maximum Sauter mean diameter of the no-flow case. The droplet Sauter mean diameter increased along the spray due to the coalescence of slow-moving droplets. The droplet breakup was found to be insignificant downstream of the spray. The flow entrained the droplets smaller than 30 μm. The spray-wall impingement criterion estimated around 42% of droplets to bounce off the surface at 50 mm, compared to 22% without flow.

Experimental investigation of filter cake deposition under cross-flow and static filtration conditions

Effects of cross-flow velocity, filtration pressure, and solid concentration on filter cake properties are studied by using a new filtration setup that allows performing filtration tests under a wide range of operating conditions. The main feature of this setup is that pressure and cross-flow velocity are decoupled, so it was possible to independently investigate the effect of these variables on filter cake properties. Filtration tests have demonstrated that particle invasion into filter media pores occurs initially, leading to the formation of an external filter cake. The increased cross-flow velocity creates an erosive shear stress at the surface of the external cake, which limits cake growth, indirectly maintaining a high filtrate flow rate. This high filtrate rate imparts a considerable drag force onto suspended particles, driving a high count into the porous substrate where these particles could deposit. Increasing pressure amplifies this internal particle deposition, particularly where there is no or limited external filter cake to limit filtrate drag and screen particles at the cake surface. The effect of cross-flow velocity on pore blocking and filter cake structure is observed and demonstrated by CT scanning and scanning electronic microscopy.

Use of ultrafiltration ceramic membranes as a first step treatment for olive oil washing wastewater

Olive oil is a food product in great demand throughout the world, its production generating large volumes of wastewater with a high organic load, where phenolic compounds are present. These compounds have outstanding antioxidant characteristics, so their recovery is of great interest. In this way, the treatment of these wastewaters should be based on reusing water and, at the same time, on recovering valuable compounds. Ultrafiltration with ceramic membranes (5, 15 and 50 kDa) is proposed as a first treatment step for olive oil washing wastewater (OOWW) from the continuous two-phase centrifugation process. The effect of cross flow velocity and transmembrane pressure was evaluated against permeate flux values and the removal of turbidity, colour, chemical oxygen demand (COD), sugars and the phenolic compounds recovery. The CFV had a great influence on the removal of colour and turbidity. COD rejection increased with increasing TMP, being the highest rejection obtained under the most extreme conditions. The rejection of phenolic compounds and sugars did not show great variation between conditions. The 50 kDa membrane was the one that presented the largest permeate flux decline and the lowest permeability recovery after the cleaning, confirming the great fouling that this membrane suffered. The 15 kDa membrane at the operating conditions of 3 m.s-1 and 3 bar was observed as a good option to eliminate much of the colour (72%) and turbidity (99%) and to reduce considerably the organic load (54%) without greatly affecting the concentration of phenolic compounds (rejection of 21%) for future recovery.

Combining patterned membrane filtration and flocculation for economical microalgae harvesting

Membranes have a lot of potential for harvesting microalgae, but mainly membrane fouling and high harvesting costs linked to low fluxes are hampering their breakthrough. Patterned membranes can reduce fouling by enchancing local turbulences close to the membrane surface on one hand, and by increasing the active area per m2 of installed membrane on the other. Flocculation can further increase membrane permeance by increasing microalgal partical size and reducing the fraction of free organic matter in the feed. In current study, the effect of polyethylene glycol (PEG) in the casting solution of patterned polysulfone membranes was investigated to better tune the performance of the patterned membranes, together with the effects of cross-flow velocity and chitosan dosage on membrane fouling. The energy consumption and total harvesting cost, extrapolated to a full-scale microalgal harvesting, were then estimated. The patterned membrane prepared with a 28w% PEG concentration showed the highest clean water permeance (900±22 L/m2 h bar) and membrane permeance in a microalgal suspension (590±17 L/m2 h bar). Patterned membranes showed a lower filtration resistance (15% permeance decline at the end of filtration) than flat membranes (72%) at a cross-flow velocity of 0.0025 m/s. Increasing cross-flow velocity could increase membrane permeance in most cases. The highest stable membrane permeance (110±17 L/m2 h bar) and the lowest filtration resistance were achieved when combining patterned membrane filtration with flocculation at optimized chitosan dosage. A very low energy consumption (0.28 kWh/kg) and harvesting cost (0.16 €/kg) were achieved under these conditions.

Biofouling Mitigation Approaches during Water Recovery from Fermented Broth via Forward Osmosis.

Forward Osmosis (FO) is a promising technology that can offer sustainable solutions in the biorefinery wastewater and desalination fields, via low energy water recovery. However, microbial biomass and organic matter accumulation on membrane surfaces can hinder the water recovery and potentially lead to total membrane blockage. Biofouling development is a rather complex process and can be affected by several factors such as nutrient availability, chemical composition of the solutions, and hydrodynamic conditions. Therefore, operational parameters like cross-flow velocity and pH of the filtration solution have been proposed as effective biofouling mitigation strategies. Nevertheless, most of the studies have been conducted with the use of rather simple solutions. As a result, biofouling mitigation practices based on such studies might not be as effective when applying complex industrial mixtures. In the present study, the effect of cross-flow velocity, pH, and cell concentration of the feed solution was investigated, with the use of complex solutions during FO separation. Specifically, fermentation effluent and crude glycerol were used as a feed and draw solution, respectively, with the purpose of recirculating water by using FO alone. The effect of the abovementioned parameters on (i) ATP accumulation, (ii) organic foulant deposition, (iii) total water recovery, (iv) reverse glycerol flux, and (v) process butanol rejection has been studied. The main findings of the present study suggest that significant reduction of biofouling can be achieved as a combined effect of high-cross flow velocity and low feed solution pH. Furthermore, cell removal from the feed solution prior filtration may further assist the reduction of membrane blockage. These results may shed light on the challenging, but promising field of FO process dealing with complex industrial solutions.

Separation of glucose, other reducing sugars and phenolics from natural extract by nanofiltration: Effect of pressure and cross-flow velocity

This study considers the effect of cross-flow velocity and transmembrane pressure on flux and rejection behavior during nanofiltration of Eurasian water milfoil hydrolysate. Fractionation by nanofiltration was applied to obtain a glucose-rich permeate solution and a concentrated retentate solution of other sugars and phenolics. Nanofiltration was performed in a cross-flow rectangular flat-sheet cell (MaxiMem, Prozesstechnik GmbH). Membrane Microdyn Nadir™ NP030 with molecular weight cut-off 500 Da was used. The velocity and shear stress distribution on the membrane surface was modelled by CFD simulations. Mild hydrodynamic conditions (cross-flow velocity and pressure) are recommended based on the observed permeate flux and rejection difference between the targeted groups of compounds. The results are discussed in rapport with existing experimental observations in the literature.

A study of droplet deformation: The effect of crossflow velocity on jet fuel and biofuel droplets impinging onto a dry smooth surface

The aeronautical sector has been, in the last decade, one of those that most invested in more efficient and ecological solutions in order to reduce significantly greenhouse gas and pollutant emissions. The introduction of biofuels in fuel mixtures for aircraft engines is a promising alternative in this sector. The main objective of this experimental study is understanding the influence of crossflow variation on droplet deformation and consequent impact outcomes. An experimental facility was developed and validated to study the impact of single droplets onto a dry, smooth aluminium impact surface under the influence of several crossflow velocities. Different crossflow velocities of 4,5,6,7 and 8m/s were tested. A combination of conventional jet fuel and a biofuel was considered to understand the behavior of jet fuel and biofuel mixtures, and three fluids were used: 100% jet fuel, 75% jet fuel/25% biofuel and 50% jet fuel/50% biofuel. Several parameters, including velocity components, impact angle and eccentricity, were analysed for the different crossflow velocities, and the spread and splash regimes were also defined for the different fluids. The results display that, for each crossflow velocity, an increase in the droplet impact velocity causes a shift from the spread to the splash regime. The presence of a crossflow induces deformation on the droplet, altering its outcome. Ellipsoidal droplets promote the occurrence of spreading, whereas splashing tends to occur for spherical forms, corresponding to higher and lower eccentricity values, respectively. A substantial increase in the crossflow velocity leads to aerodynamic breakup of the droplet.

Harvesting of Isochrysis zhanjiangensis using ultrafiltration: Changes in the contribution ratios of cells and algogenic organic matter to membrane fouling under different cross-flow velocities

Increasing the cross-flow velocity can improve the permeate flux to a certain extent during cross-flow ultrafiltration harvesting of microalgae. In this study, the effect of cross-flow velocity on the fouling mechanisms of Isochrysis zhanjiangensis suspension, fragile algogenic organic matter-free cells and cell-free algogenic organic matter was investigated using different cross-flow velocities of 0.13, 0.26, 0.51 and 0.63 m·s−1. The fouling mechanism of algogenic organic matter and algae cells was analyzed by Hermia's model; in particular, the changes in their fouling ratios under different cross-flow velocities were further elucidated based on the criteria importance through intercriteria correlation. The results revealed that cake formation by algogenic organic matter and cells mainly induced membrane fouling at different cross-flow velocities. Notably, the contribution of algogenic organic matter and algae cells to membrane fouling varied with cross-flow velocity. The objective weight of fouling caused by algae cells decreased from 0.758 to 0.279, then increased to 0.560 after cell disruption and finally reached 0.946. This result suggested that the cell disruption significantly aggravated cellular fouling. In contrast, the objective weight of fouling caused by algogenic organic matter increased with cross-flow velocity, and cell disruption reversed the trend. For further insight, a model was developed based on the fouling contributions of algogenic organic matter and cells to cake formation; the model was capable of predicting fluxes under different cross-flow velocities. These findings not only clarify the fouling mechanism of I. zhanjiangensis cells and algogenic organic matter but also provide a valuable model that is instructive for the development of membrane filtration of fragile microorganisms.

Dewatering of sewage for nutrients and water recovery by Forward Osmosis (FO) using divalent draw solution

Forward osmosis (FO), based on the phenomenon of osmosis, is a breakthrough technology for concentrating different feeds including domestic sewage. Globally, sewage treatment facilities are increasingly focusing on recovery of resources such as water, nutrients (nitrogen and phosphorus) and energy from sludge. In this context, FO can be applied for sewage up-concentration followed by nutrients recovery from the concentrated stream. Using biomimetic aquaporin membranes (thin film composite FO membranes incorporating aquaporin proteins), this work investigated divalent magnesium chloride as draw solution for sewage concentration. The effect of crossflow velocity and draw concentration on water recovery, rejection of organics and nutrients and membrane fouling was studied. Higher draw solution concentration and cross flow velocity enhanced water recovery but concentration of organics (as chemical oxygen demand, COD) remained unaffected. Over 24 h, an average water flux of 5.3 L/m2h was obtained with recovery of water (57%), phosphate (75%), ammonia (66%) and organics (73%). Membrane fouling was confirmed by zeta potential and ATR-FTIR analysis. Membrane cleaning was required after every 24 h cycle to restore the performance. The findings indicate that sewage concentration by FO with divalent magnesium chloride draw solution is promising for water and nutrients recovery.

Transport and deposition of colloidal particles on a patterned membrane surface: Effect of cross-flow velocity and the size ratio of particle to surface pattern

In this study, the transport and deposition of colloidal particles onto a patterned membrane surface are analyzed by both numerical simulation and experiment. The channel Reynolds number (Re) and the size ratio of the particle to the pattern (a/h) are chosen as the major simulation parameters. With an increase in Re and a/h, the number of deposited particles decreases and the rate of permeate flux decline is reduced. Under certain specific combinations of the design parameters, the suspended particles are impeded to access the patterned membrane surface. Based on the particle trajectory analysis, it is claimed that “inaccessible zones”, where the suspended particles cannot be accessed by the flow, are formed near the patterned membrane surface, with the increase in Re and a/h. Cross-flow microfiltration experiments using a poly(methyl methacrylate) (PMMA) particle suspension are also conducted to verify the trend in particle deposition observed in simulation. As the radius of the PMMA particle increases, the number of deposited particles is reduced and the critical flux for the onset of the particle deposition is increased, which qualitatively matches well with simulation. It is found that a patterned membrane can reduce tremendous amounts of colloidal fouling by prohibiting the access of the suspended particles to the membrane surface with the appropriate choice of design parameters such as Re and a/h.

Application of forward osmosis technology in crude glycerol fermentation biorefinery-potential and challenges

Forward osmosis (FO) is a low energy-intensive process since the driving force for water transport is the osmotic pressure difference, Δπ, between the feed and draw solutions, separated by the FO membrane, where πdraw > πfeed. The potential of FO in wastewater treatment and desalination have been extensively studied; however, regeneration of the draw solution (thereby generating clean water) requires application of an energy-intensive process step like reverse osmosis (RO). In this study, the potential of applying FO for direct water recirculation from diluted fermentation effluent to concentrated feedstock, without the need for an energy-intensive regeneration step (e.g. RO), has been investigated. Butanol production during crude glycerol fermentation by Clostridium pasteurianum, has been selected as a model process and the effect of cross-flow velocity and the dilution of draw solution on the water flux during short-term experiments (200min), were investigated. Statistical analysis revealed that the dilution of the draw solution is the most influential factor for the water flux. Subsequent modelling of an integrated FO-fermentation process, showed that water recoveries could lead to substantial financial benefits, although the integrated FO-fermentation process demonstrated lower water flux than expected. FTIR analyses of the membrane surface implied that the decrease in water flux was due to the presence of proteins, polysaccharides and other extracellular polymeric substances on the membrane active layer, indicating the presence of a fouling layer. Based on these findings, possible fouling alleviation strategies and future research directions are discussed and proposed.

Numerical investigation of 3D bubble growth and detachment

Combined with new numerical techniques, the boundary element method is adopted to study the 3D bubble growth and detachment from a submerged nozzle under constant pressure conditions in this paper. During the bubble growth, an efficient mesh topology optimization technique is adopted to eliminate ill-shaped elements for large deformation problems. At the bubble detachment, a 3D bubble pinching algorithm is employed to deal with the topological changes automatically. In this study, the cases with or without liquid cross flow are simulated numerically and the mechanism of bubble dynamic behaviors is analyzed. 3D results without cross flow show good agreement with the experimental and axisymmetric results in the literature, which validates the 3D numerical model. Besides, with the existence of the cross flow, the effects of cross flow velocity and nozzle radius on the bubble detachment characteristics are studied. The simulation results show that the bubble frequency and shape at the detachment can be controlled by exerting different cross flow velocities.

Effect of cross-flow velocity, oil concentration and salinity on the critical flux of an oil-in-water emulsion in microfiltration

Microfiltration is an attractive means for treating oily wastewater, especially when the size of the oil droplets are micrometer-sized since the conventional techniques become deficient. A systematic study on the critical flux of oil-in-water emulsion, which behaves differently from other colloidal foulants with regards to deformation, coalescence and splitting, has not been carried out to date. This was the goal of the current study, which employed the Direct Observation Through the Membrane (DOTM) technique to characterize the critical flux of oil-in-water emulsions of various concentrations, and at various cross-flow velocities (CFV) and salt concentrations. Five observations can be highlighted here. Firstly, the oil droplets with a mean droplet diameter of approximately 5µm exhibited critical fluxes equal to or greater than latex particles of 10µm. This is likely due to the twin effects of membrane oleophobicity promoting back-transport of the oil foulant from the membrane and the presence of a droplet size distribution with larger drops that can enhance the shear-induced diffusion of the average drops. Secondly, the critical flux values did not agree with the model that is valid for the size range the mean droplet diameter falls in, but instead agreed with the model adapted for smaller particulate foulants. Thirdly, the increase in the critical flux with CFV was more significant for the lower oil concentration. Fourthly, a striping phenomenon was observed at higher oil concentrations and lower CFV values. Striping was not observed for latex particles. Fifthly, the critical flux decreased with salt concentration. These findings highlight the unique fouling behavior of oil-in-water emulsions in microfiltration.

Mini-review: novel non-destructive in situ biofilm characterization techniques in membrane systems

Membrane systems are commonly used in the water industry to produce potable water and for advanced wastewater treatment. One of the major drawbacks of membrane systems is biofilm formation (biofouling), which results in an unacceptable decline in membrane performance. Three novel in situ biofouling characterization techniques were assessed: (i) optical coherence tomography (OCT), (ii) planar optodes, and (iii) nuclear magnetic resonance (NMR). The first two techniques were assessed using a biofilm grown on the surface of nanofiltration (NF) membranes using a transparent membrane fouling simulator that accurately simulates spiral wound modules, modified for in situ biofilm imaging. For the NMR study, a spiral wound reverse osmosis membrane module was used. Results show that these techniques can provide information to reconstruct the biofilm accurately, either with 2-D (OCT, planar optodes and NMR), or 3-D (OCT and NMR) scans. These non-destructive tools can elucidate the interaction of hydrodynamics and mass transport on biofilm accumulation in membrane systems. Oxygen distribution in the biofilm can be mapped and linked to water flow and substrate characteristics; insights on the effect of crossflow velocity, flow stagnation, and feed spacer presence can be obtained, and in situ information on biofilm structure, thickness, and spatial distribution can be quantitatively assessed. The combination of these novel non-destructive in situ biofilm characterization techniques can provide real-time observation of biofilm formation at the mesoscale. The information obtained with these tools could potentially be used for further improvement in the design of membrane systems and operational parameters to reduce impact of biofouling on membrane performance.

Experimental and numerical assessment of impingement and mixing of urea–water sprays for nitric oxide reduction in diesel exhaust

This study presents a joint experimental and numerical investigation of a spray used in selective catalytic NOx reduction (SCR) after-treatment systems for Diesel engine exhaust gases. The focus lies on the impingement conditions and distribution and mixing mechanisms of the spray in cross-flowing exhaust gases. These aspects are vital to system performance but still not well described.A pressure-driven SCR injector is characterized in an optically accessible flow test rig at different temperatures and mass flow rates representative of Diesel engine exhaust gas conditions by means of Mie scattering and Phase Doppler Anemometry (PDA). The effects of cross-flow velocity and temperature on spray structure, droplet size and velocity distributions are assessed resulting in a comprehensive characterization of the spray. Results show that the dense spray core, with droplets up to 200μm, only moderately reduces in density as smaller droplets are entrained at high exhaust flows. Wall impingement conditions however vary substantially as impingement angles are shallower at higher cross-flow velocities.A detailed assessment of the numerical model is presented and validation is carried out at different measurement locations of interest. The predicted droplet size distributions and velocities follow the observed trends and impingement angles as well as spray film areas on the channel floor are also in agreement with the experimental data. The validated model is subsequently used to numerically study the mixing dynamics.The findings suggest that at low cross-flow conditions two counter-rotating kidney vortices are formed which entrain reflected droplets of the spray impinging on the channel floor, which leads to improved mixing. Vapor concentrations increase close to the side walls and reflected droplets lead to film-formation on the side walls. At higher cross-flow velocities, vortex formation is not evident and spray-wall interaction is less pronounced – impairing mixing and leading to a reduction in concentration uniformity at down-stream locations. Film formation and high vapor concentrations are restricted to the bottom channel centerline.

Effect of cross-flow velocity on membrane filtration performance in relation to membrane properties

Organic fouling of membrane is one of the barriers that limit the application of membrane in water separation. Study about the effect of cross-flow velocity (CFV) on fouling rate is important to ensure that the system is operating at optimal conditions. This paper investigates the influence of CFV on the permeate flux in cross-flow configuration of nanofiltration (NF) and reverse osmosis (RO) processes by treating an organic compound in aqueous suspension. A commercial Aldrich humic acid (AHA) of 15 mg/L was used as the feed solution. The experiments were conducted at different cross-flow velocities (i.e. 0.27, 0.39, and 0.60 m/s) under a constant driving pressure of 7 bar and at an operating temperature in the range of 22–25°C. The permeate flux for both NF and RO membranes tested with AHA foulant increased slightly with time at the cross-flow velocities of 0.39 and 0.60 m/s, respectively. Despite the increase in organic rejections with the increased CFV, the rejection performance of AHA suspension was only marginally affected by the variation of CFV. Low permeate rates (flux) and strong repulsive forces were the reasons of no fouling result observed in the NF and RO membranes throughout the studied duration (i.e. 8 h). Interpretations on the chemical (contact angle and elemental composition) and physical properties (visual inspection) of membranes have further confirmed this observation. To study the fouling behavior of these membranes, it is suggested that an AHA feed with higher concentration, an addition of calcium ions, a longer filtration duration, and a higher initial flux should be used.

Innovative ceramic hollow fiber membranes for recycling/reuse of oilfield produced water

The focus of this study was on the development and application of ceramic hollow fiber membrane (CHFM) technology for the treatment of oilfield produced water (PW) prior to disposal/discharge into the environment. PW refers to any fossil water that is brought to the surface along with crude oil or natural gas. It is a complex mixture of dispersed oil, dissolved organic compounds, suspended solids, production chemicals, heavy metals, and natural radioactive minerals. PW is difficult to handle/treat and represents the largest volume of waste associated with the oil and gas industry. It can have different potential impacts on offshore or onshore environments depending on where it is discharged. Therefore, the development of effective treatment technologies for PW is essential from both ecological and economic standpoints. The first stage of any treatment process for PW consists of a significant reduction in the level of dispersed hydrocarbons and suspended solids. In a second and often in a third stage, the oil and total carbon (TC) content is reduced by hydrocyclones and by micro- and ultrafiltration using membrane technologies. Due to legal requirements, final oil contents below 10 ppm are required in some regions before PW disposal to the environment. CHFMs represent a new generation in the development of inorganic membranes by offering the advantages of one membrane consisting of both inorganic material and hollow fiber geometry. In this study, the effect of cross-flow velocity on the CHFM performance at a low trans-membrane pressure of 0.5 bar, as well as the permeate quantity and quality, was investigated in terms of fouling behavior and efficiency of oil and TC removal. In the filtration system presented here, using ceramic hollow fiber ultrafiltration (UF) membranes with a d90 pore diameter of 40 nm, the removal of oil and TC from samples of tank dewatering produced water and oily model systems, was successfully demonstrated with a remarkable decontamination efficiency of >99.5% (oil) and 61–94% (TC), respectively.

Effect of cross-flow velocity on flow and heat transfer characteristics of impinging jet with low jet-to-plate distance

An effect of cross-flow velocity on flow and heat transfer characteristics of impinging jet in the case of low jet-to-plate distance at H = 2D was experimentally and numerically investigated. In the experiments, the air jet from orifice impingement on the wall of wind tunnel while a cross-flow was simultaneously induced normal to the jet flow. The jet velocity was fixed while the cross-flow velocity was varied corresponding to velocity ratios (jet velocity/cross-flow velocity) VR = 3, 5 and 7. The temperature distribution on an impinged surface was visualized by using thermochromic liquid crystal sheet (TLCs), and Nusselt number distribution was evaluated by using image processing method. The flow pattern on impingement surface was visualized by using oil film technique. The numerical simulation was carried out for a better understanding of the jet flow in the cross-flow. The results show that Nusselt number peak shifts downstream and the Nusselt number peak increases with increasing cross-flow velocity.

Colloidal metastability and membrane fouling – Effects of crossflow velocity, flux, salinity and colloid concentration

Microfiltration (MF) and ultrafiltration (UF) involving colloidal suspensions are often involved in separations, concentration and clarification processes in the food and beverage as well as other industries. The increase in concentration near the membrane surface owing to concentration polarization can cause some part of a colloidal fouling layer to become metastable whereby it can undergo a higher order phase transition to a more dense gel. This was confirmed via deadend filtration studies wherein the fouling layer thickness obtained from the transmembrane pressure (TMP) was compared with that determined directly via ultrasonic time-domain reflectometry (UTDR). Whereas entering this metastable state is thermodynamically driven, the transition from a colloidal suspension to a more dense gel is a rate or kinetically driven process. This phase transition is manifest by a marked rate of increase in the TMP that occurs after an filtration time that is dependent on the flux. A ‘threshold transition flux’ is identified below which the time required for the phase transition can be considerably delayed. Since removing this dense gel layer via conventional cleaning protocols is more difficult, determining an operating strategy whereby this transition to a more dense gel can be delayed is clearly of interest for the optimal operation of MF and UF processes. To this end the effects of crossflow velocity, flux, salinity and colloidal silica concentration on this metastability phenomenon are studied for a polyethersulfone UF membrane under crossflow and constant flux conditions. A lower crossflow velocity and higher flux, increased salinity and higher colloidal silica concentration decrease the time required for the transition to a dense gel.

Impact of biofilm accumulation on transmembrane and feed channel pressure drop: Effects of crossflow velocity, feed spacer and biodegradable nutrient

Biofilm formation causes performance loss in spiral-wound membrane systems. In this study a microfiltration membrane was used in experiments to simulate fouling in spiral-wound reverse osmosis (RO) and nanofiltration (NF) membrane modules without the influence of concentration polarization. The resistance of a microfiltration membrane is much lower than the intrinsic biofilm resistance, enabling the detection of biofilm accumulation in an early stage. The impact of biofilm accumulation on the transmembrane (biofilm) resistance and feed channel pressure drop as a function of the crossflow velocity (0.05 and 0.20 m s−1) and feed spacer presence was studied in transparent membrane biofouling monitors operated at a permeate flux of 20 L m−2 h−1. As biodegradable nutrient, acetate was dosed to the feed water (1.0 and 0.25 mg L−1 carbon) to enhance biofilm accumulation in the monitors. The studies showed that biofilm formation caused an increased transmembrane resistance and feed channel pressure drop. The effect was strongest at the highest crossflow velocity (0.2 m s−1) and in the presence of a feed spacer. Simulating conditions as currently applied in nanofiltration and reverse osmosis installations (crossflow velocity 0.2 m s−1 and standard feed spacer) showed that the impact of biofilm formation on performance, in terms of transmembrane and feed channel pressure drop, was strong. This emphasized the importance of hydrodynamics and feed spacer design. Biomass accumulation was related to the nutrient load (nutrient concentration and linear flow velocity). Reducing the nutrient concentration of the feed water enabled the application of higher crossflow velocities. Pretreatment to remove biodegradable nutrient and removal of biomass from the membrane elements played an important part to prevent or restrict biofouling.