Feed Spacer Research Articles

Optical coherence tomography for the in situ three-dimensional visualization and quantification of feed spacer channel fouling in reverse osmosis membrane modules

To understand the biofouling in membrane modules in more detail, three-dimensional in situ imaging of the feed spacer channel by means of optical coherence tomography (OCT) was applied. The biofilm formation was investigated in membrane fouling simulators (MFS) mimicking the feed spacer channel of spiral wound reverse osmosis membrane modules. To test the capability of OCT for the visualization of feed spacer fouling, river water and effluent of a waste water treatment plant were used as feed solutions. Additionally, the impact of the feed spacer geometry on the biofouling deposit accumulation was tested using a wide-meshed and small-meshed feed spacer, respectively. 3D OCT datasets were quantified and the determined amount of biofilm was correlated to the feed channel pressure drop (FCP). The study validates the application of OCT as imaging tool to time-resolved visualize and quantify the biofilm in feed spacer filled channels. The amount of biofilm correlated with the FCP. However, the feed spacer geometry and feed composition yielded differences in the structure and distribution of grown biofilm. Hence, the FCP as macroscopic measure does not necessarily allow to assume a certain biofilm structure or distribution. A more detailed understanding of the FCP development could be achieved by the high-resolution 3D imaging data provided by OCT. Hence, this study emphasizes the capabilities of OCT to investigate the biofouling development in membrane modules.

Experimental and numerical characterization of the water flow in spacer-filled channels of spiral-wound membranes

Micro-scale flow distribution in spacer-filled flow channels of spiral-wound membrane modules was determined with a particle image velocimetry system (PIV), aiming to elucidate the flow behaviour in spacer-filled flow channels. Two-dimensional water velocity fields were measured in a flow cell (representing the feed spacer-filled flow channel of a spiral wound reverse osmosis membrane module without permeate production) at several planes throughout the channel height. At linear flow velocities (volumetric flow rate per cross-section of the flow channel considering the channel porosity, also described as crossflow velocities) used in practice (0.074 and 0.163 m·s−1) the recorded flow was laminar with only slight unsteadiness in the upper velocity limit. At higher linear flow velocity (0.3 m·s−1) the flow was observed to be unsteady and with recirculation zones. Measurements made at different locations in the flow cell exhibited very similar flow patterns within all feed spacer mesh elements, thus revealing the same hydrodynamic conditions along the length of the flow channel. Three-dimensional (3-D) computational fluid dynamics simulations were performed using the same geometries and flow parameters as the experiments, based on steady laminar flow assumption. The numerical results were in good agreement (0.85–0.95 Bray–Curtis similarity) with the measured flow fields at linear velocities of 0.074 and 0.163 m·s−1, thus supporting the use of model-based studies in the optimization of feed spacer geometries and operational conditions of spiral wound membrane systems.

Dominant factors controlling the efficiency of two-phase flow cleaning in spiral-wound membrane elements

Two-phase flow cleaning has been successfully applied to control fouling in spiral wound membrane elements. This study focuses on its experimental optimization using a Taguchi Design of Experiment method (L-25 orthogonal arrays) to elucidate the influence of different factors and to reveal the important one(s) affecting the cleaning efficiency of two-phase flow cleaning. All possible combinations of the factors, i.e. feed type, spacer geometry, gas/liquid ratio, and liquid velocity, each at five levels were evaluated. The main effect of each factor on the efficiency of two-phase flow cleaning was measured by determining the performance response (mean of cleaning efficiency) and by calculating the mean signal-to-noise ratio. An analysis of variance was applied to calculate the relative contribution of each factor on the efficiency of two-phase flow cleaning. The results showed that the feed type is by far the most essential factor contributing to the cleaning efficiency. The spacer geometry is ranked second, followed by the gas/liquid ratio and the liquid velocity, which both have an only very minor effect on the cleaning performance. In terms of practical application, the operator should consider first the type of foulant prior to taking a decision on whether or not two-phase flow cleaning will be effective. Once the foulant type is defined, the use of the highest gas/liquid ratio, the highest liquid velocity, and the thickest feed spacer (diamond type) are recommended to achieve maximum two-phase flow cleaning efficiency.

Gypsum scaling and membrane integrity of osmotically driven membranes: The effect of membrane materials and operating conditions

The emerging thin film composite (TFC) forward osmosis (FO) and pressure retarded osmosis (PRO) membranes generally have better separation properties compared with their cellulose triacetate (CTA) counterparts. Nevertheless, their scaling performance has been rarely reported. In the current study, the phenomenon of membrane integrity loss as a result of scaling is reported for the first time for osmotically driven membrane processes (ODMPs). The results show that the TFC membrane suffered marked flux reduction during the scaling in the active-layer-facing-feed-solution (AL-FS) orientation, accompanied with the severe damage of the membrane active layer. The membrane integrity loss is attributed to the scale formation and growth in the confined space between the membrane and the feed spacer. Compared with the CTA membrane, the TFC was more prone to scaling and membrane damage due to its unfavorable physiochemical properties (presence of Ca2+ binding sites and ridge-and-valley roughness). Although antiscalant addition was shown to be effective for scaling control in AL-FS, it was ineffective in the active-layer-facing-draw-solution orientation. The current study reveals the critical need for scaling control in ODMP processes with respect to the membrane integrity and flux stability. The results also have far-reaching implications for FO and PRO membrane design and process operation.

Biofouling suppression of modified feed spacers: Localized and long-distance antibacterial activity

Biofouling hindering activity of two modified feed spacers was assessed. A ‘short-distance’ modification was attained by covalently binding polymeric quaternary ammonium groups (pQAs) to the spacer by atom transfer radical polymerization (ATRP). A ‘long-distance’ modification was based on embedding silver nanoparticles (nAg) in the polymeric spacer by sonochemical deposition. Cross-flow experiments in planar-membrane filtration cells using ultrafiltration membranes exposed to a mixed microbial enrichment were performed at conditions simulating spiral wound modules (0.15m·s−1 channel flow velocity).Physical properties of the modified spacers were evaluated using scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy. Microscopic analysis of the membranes adjacent to the modified spacers indicated the presence of a sporadic monolayer structure compared to a developed biofilm in the untreated spacers. SEM imaging and total organic carbon analysis of the attached layer revealed a low number of bacteria near the modified spacers and adjacent membranes.Both modified spacers displayed improved permeate fluxes compared to the control. nAg modified spacers displayed a somewhat steadier permeate flux profile within the length of the experiments probably due to the release of antibacterial active species. pQA modified spacers are assumed to have a more localized influence by direct contact on their surface.

Production of hydrogen by catalytic steam reforming of oxygenated model compounds on Ni-modified supported catalysts. Simulation and experimental study

Steam reforming of two representative components in the aqueous fraction of bio-oil, acetone and ethanol, was investigated over nickel based supported catalysts (Ni/Al2O3). The effect of Rh incorporation (Ni–Rh/Al2O3) and the properties of different γ-Al2O3 used as support was tested. Hydrogen-rich gas is produced at temperatures higher than 673 K in both acetone and ethanol reforming with maximum mole fraction of 0.6–0.7. The reactions involved and the possible routes were studied. Rh incorporation and nanostructured alumina affects the Ni metal size and the amount of carbon deposited onto catalysts’ surface after reforming tests; besides, acetone is also found as an important intermediate in ethanol reforming, and Rh improve hydrogen production with negligible intermediates presence. The effect of S/Acetone or S/Ethanol feed molar ratios, space velocity and temperature over acetone/ethanol conversion and over H2/CO ratio were modeled using the Matlab software in order to find the regions and the optimal operating conditions in both processes. Simulation results are consistent with the experimental data.

Early non-destructive biofouling detection and spatial distribution: Application of oxygen sensing optodes

Biofouling is a serious problem in reverse osmosis/nanofiltration (RO/NF) applications, reducing membrane performance. Early detection of biofouling plays an essential role in an adequate anti-biofouling strategy. Presently, fouling of membrane filtration systems is mainly determined by measuring changes in pressure drop, which is not exclusively linked to biofouling. Non-destructive imaging of oxygen concentrations (i) is specific for biological activity of biofilms and (ii) may enable earlier detection of biofilm accumulation than pressure drop.The objective of this study was to test whether transparent luminescent planar O2 optodes, in combination with a simple imaging system, can be used for early non-destructive biofouling detection.This biofouling detection is done by mapping the two-dimensional distribution of O2 concentrations and O2 decrease rates inside a membrane fouling simulator (MFS).Results show that at an early stage, biofouling development was detected by the oxygen sensing optodes while no significant increase in pressure drop was yet observed. Additionally, optodes could detect spatial heterogeneities in biofouling distribution at a micro scale. Biofilm development started mainly at the feed spacer crossings.The spatial and quantitative information on biological activity will lead to better understanding of the biofouling processes, contributing to the development of more effective biofouling control strategies.

Direct contact membrane distillation for the concentration of saline dairy effluent

The ability of direct contact membrane distillation to concentrate the waste effluent from salty whey, a by-product from the cheese making industry has been investigated. The effect of trace protein in the feed, cross-flow velocity and feed acidity were the factors examined. Flat Sheet PTFE membranes of nominal pore sizes 0.05, 0.22 and 0.45 μm were utilised. A decline in feed flux in the presence of trace protein in the feed was observed, but liquid penetration through the membrane could still be prevented by utilization of a membrane of smaller pore size, to achieve a final total solids concentration of ±30% w/w with water recovery from 37 to 83 %. The pressure-drop across the channel length was also predicted accounting for the feed spacer. To increase the channel length up to 1 m will require operation using the smallest pore size of 0.05 μm, unless very low cross-flow velocities are used. The fouling of the membrane is primarily governed by precipitation of a calcium phosphate salt. However, operation at low pH does not improve the flux or the final salt concentration significantly.

Effect of broiler breeder feeding programme and feeder space change at photostimulation using maize- or wheat-based diets on broiler progeny growth performance and leg health

1. The aim of this study was to evaluate the effects of diet type, maternal feeding programme at 29 weeks of age and breeder feeder space change at photostimulation on broiler progeny performance and leg health at 6 weeks of age.2. Fast-feathering Cobb 500 broiler breeders were fed on either maize- or wheat-based diets that had been formulated to have similar nutrient composition during growing and layer phases. Two feeding programmes, fast or flow, were used from 14 to 29 weeks of age. At 22 weeks, 69 females from each pen were placed in a layer house where feeder space was either similar to that in rearing (6.3 to 6.5 cm/female) or was increased from 6.3 to 8.4 cm/female. Eggs produced at 32 and 44 weeks of age were collected and incubated for two broiler experiments. A total of 16 male and 16 female one-d-old chicks were placed in floor pens in two experiments, respectively, with 6 and 4 replicate pens. Broiler gait scores and leg problem prevalence were evaluated at 6 weeks of age.3. Data were analysed as a 2 × 2 × 2 factorial design with diet type, feeding programme and feeder space change as main factors.4. The wheat diet increased the probability of observing crooked toes in broiler progeny compared to the use of maize, but only when breeders were fed according to the fast feeding programme and given similar feeder space as during rearing.5. Breeders given more feeder space in the laying period produced progeny with more locomotion problems compared with those provided similar feeder space, but only when maize was used and the slow feeding programme was applied to the breeders.6. The maternal feeding programme interacted with other factors to influence progeny leg health, but it did not solely influence walking ability or leg problems of progeny.7. In conclusion, an increased probability of observing walking impairment of broiler progeny was detected when breeders were given greater feeder space at photostimulation rather than no change and fed according to the slow feeding programme using maize diets in breeders and progeny.

Feeding site preference of Sitophilus zeamais (Coleoptera: Curculionidae) on maize grain

The possibility thatSitophilus zeamaisMotschulsky prefers a particular grain side and a grain side area for feeding, and the implications and value in maize programmes' breeding for resistance were investigated in the laboratory at ambient temperature (32.7 °C) and relative humidity (70.4%) using standard techniques. A total of 20 elite maize varieties were used: TZBRCOMP.2C1F1, TZBRELD3C5, PVASYN-3F2, PVASYN-6F2, DTSYN-11-W, BR9943DMRSR, IWDC3SNY-W, WHITEDTSTRSYN, 2008DTMA-YSTR and 2000SYNEE-WSTR, obtained from the Maize Breeding Unit of International Institute for Tropical Agriculture (IITA), Ibadan, Nigeria, and ILE-1-OB, IFEMAIZEHYBRID-1, IFEMAIZEHYBRID-2, IFEMAIZEHYBRID-5, IFEMAIZEHYBRID-6, ARTCOMPOSITE-A-Y, ARTCOMPOSITE-B-Y, ART/98/SW1-OB, ART/98/SW4-OB and ART/98/SW5-OB, obtained from the Institute of Agricultural Research and Training (IAR&T), Moor Plantation, Ibadan, Nigeria. The flat side of a maize grain at the point of penetration was significantly preferred (P≤ 0.05), possibly because it provided much needed space for anchorage and feeding. The South locus of the flattened side of a maize grain was also significantly preferred (P≤ 0.05), possibly because of its soft nature and proximity to the nutrient-rich embryo. The findings of the study have implications for seed viability and can be considered in breeding for resistance to infestation byS. zeamaisin stored maize.

Wean-to-finish feeder space availability effects on nursery and finishing pig performance and total tract digestibility in a commercial setting when feeding dried distillers grains with solubles1

The study objectives were to determine nursery phase feeder space allowance effects on pig performance when double stocked and, second, to determine feeder space allowance and dried distillers grains with solubles (DDGS) inclusion level effects on pig performance and nutrient digestibility during the growing-finishing phase. This study was performed on the same group of pigs within a commercial wean-finish system. For the nursery phase, a completely randomized design was used to compare 3 feeder space allowance treatments (2.1, 2.5, and 2.9 cm/pig). A total of 3,720 pigs were randomly allotted to same-sex pens (10 feeders/treatment) housing 62 pigs/pen. Thirty 7-hole, double-sided feeders were utilized in the study. Differing linear feeder space treatments were established by blocking off sections for the nursery and grow-finish portions of this experiment. All pigs were provided equal floor space (0.26 m2/pig). In the grow-finish phase, a total of 1,860 pigs (n = 60 pens) were utilized in a 2 × 3 factorial design with 3 feeder space allowances (4.1, 4.9, or 5.7 cm/pig) and 2 dietary DDGS treatments (30% [D30] or 60% [D60]). Fecal and diet samples were collected and analyzed to estimate apparent total tract digestibility percentage (ATTD %). In the nursery portion of the trial, there was no feeder space treatment effect on ADG, ADFI, or feed efficiency (P > 0.10) from weaning to d 56 postweaning or during any weigh period. In the grow-finish portion of the trial, feeder space allowance and DDGS inclusion level did not affect ADG, ADFI, or feed efficiency (P > 0.05) from d 57 postweaning to market. Pigs fed the D30 diet had greater HCW, percent yield, and loin depth than those on the D60 diet (P < 0.05). Pigs fed the D30 treatment had greater (P < 0.05) ATTD for DM and GE for both collection periods compared with those on the D60 treatment. In summary, feeder space allowance did not impact pig performance during the nursery or grow-finish production phases. Inclusion of DDGS at higher levels will decrease ADFI but not ADG or efficiency when isocaloric diets are fed. The inclusion level of DDGS does impact HCW and percent yield because of increasing intestinal weights when pigs are fed diets containing increasing DDGS inclusion rates. Dry matter and energy digestibility were greater in pigs fed the lower DDGS treatment.

Linking governance and ecosystem services: The case of Isla Mayor (Andalusia, Spain)

The widespread use of the Ecosystem Services (ESs) Framework in research studies has highlighted the fundamental role ecosystems play in the provision of human well-being. However, the long-term maintenance of healthy and resilient ecosystems requires more attention to the role played by human systems and their management practices on ecosystem functions. In this paper, we aim at advancing the understanding of how governance configurations relate to ESs. We focus on identifying institutions and rules that have triggered transformation in the supply and demand of ESs. For our case study, we have chosen Isla Mayor's social–ecological system, a small southern Spanish municipality with an intensive rice cultivation tradition, territorially linked (adjacent) to the bigger marshland National Park in the EU: “Doñana”. Our empirical analysis points to an enhancement of, and increased interaction among different ESs: (1) paddies as suitable living and feeding space for birds, (2) provision of quality rice, (3) local-based tourism principally aimed at bird watching in paddies and gastronomy. These improvements come with a co-managed multi-level governance system facilitating within and cross-scale interactions and the development of bridging institutions. Particularly, the way in which local actors adapt to new governance structures and rules, developing their own paths, becomes an essential explanation for the development of – apparently – more sustainable practices.

A study on characterization and methane dry reforming performance of Co–Ce/ZrO2 catalyst

The overall purpose of this study is to propose an effective Co-based non-PGM bimetallic carbon dioxide reforming of methane (CDRM) catalyst, and to find the optimal reaction conditions to be used in production of synthesis gas. In catalyst formulation, ZrO2, which has ability to produce surface oxygen, is used as a support. Ce is used as a promoter in order to increase oxygen storage capacity and to regulate surface oxygen transfer. Freshly reduced and spent catalysts were analyzed by SEM–EDX, HRTEM–EDX, TPO, Raman Spectroscopy and XPS. The reaction conditions were optimised through the use of an experimentally designed procedure using reaction temperature, CH4/CO2 feed ratio and space velocity as the parameters. XPS analysis of Co 2p indicated that additional oxidation of Co was limited during CDRM which may only lead to a small decrease in activity. EM measurements showed well dispersion of Co and Ce on the surface. No significant metal sintering was observed. Local changes in Co/Ce ratio were found to have pronounced effect on carbon formation. It is well known that methane dehydrogenation produces surface carbon as the side product covering the active site. The comparative analysis of extent of carbon coverage on Co and Ce sites of the freshly reduced and spent samples through metal mapping by SEM–EDX indirectly confirmed methane dehydrogenation is the primary function of Co sites, and there is no methane activation on Ce sites. A comparative evaluation of XPS Ce 3d obtained from freshly reduced and spent samples on the other hand showed the surface oxygen storage/transfer function of Ce through redox cycle. In the performance tests, high activity with stable performance was observed for 1/1CH4/CO2 feed ratio for CDRM over Co–Ce/ZrO2 catalyst.

Application of Cumene Technology Mathematical Model

The work describes the existing problems of the technological systems for cumene synthesis catalyst AlCl3. The paper describes the stages of development of the mathematical model ofbenzene alkylation with propylene. The model allows the calculation of composition of the product stream when changing process parameters of the plant: temperature, molar ratio of benzene/propylene, feed space velocity. The error of the model does not exceed 7.5%. The computer modeling system «Alkylation» is developed in Borland Delphi 7, and the module optimization of the process parameters is called «Optimization». The calculations allowed reducing the catalyst consumption in the alkylation reactor (to 10-15%) and increasing the cumene concentration in the product mixture (to 25% wt.). The concentration of n-propylbenzene in the output stream is 0.05 wt%. The recommendations for optimization of the industrial alkylation unit are presented for implementation at OJSC «Omsky Kautchuk».

Hydrogel-coated feed spacers in two-phase flow cleaning in spiral wound membrane elements: A novel platform for eco-friendly biofouling mitigation

Biofouling is still a major challenge in the application of nanofiltration and reverse osmosis membranes. Here we present a platform approach for environmentally friendly biofouling control using a combination of a hydrogel-coated feed spacer and two-phase flow cleaning. Neutral (polyHEMA-co-PEG10MA), cationic (polyDMAEMA) and anionic (polySPMA) hydrogels have been successfully grafted onto polypropylene (PP) feed spacers via plasma-mediated UV-polymerization. These coatings maintained their chemical stability after 7 days incubation in neutral (pH 7), acidic (pH 5) and basic (pH 9) environments. Anti-biofouling properties of these coatings were evaluated by Escherichia coli attachment assay and nanofiltration experiments at a TMP of 600 kPag using tap water with additional nutrients as feed and by using optical coherence tomography. Especially the anionic polySPMA-coated PP feed spacer shows reduced attachment of E. coli and biofouling in the spacer-filled narrow channels resulting in delayed biofilm growth. Employing this highly hydrophilic coating during removal of biofouling by two-phase flow cleaning also showed enhanced cleaning efficiency, feed channel pressure drop and flux recoveries. The strong hydrophilic nature and the presence of negative charge on polySPMA are most probably responsible for the improved antifouling behavior. A combination of polySPMA-coated PP feed spacers and two-phase flow cleaning therefore is promising and an environmentally friendly approach to control biofouling in NF/RO systems employing spiral-wound membrane modules.

Impact of Feed Spacer Filament Spacing on Mass Transport and Fouling Propensities of RO Membrane Surfaces

Material build-up on membrane surfaces is one of the vital challenges faced by Reverse Osmosis operations, leading to many operational and maintenance issues. To date, several modeling studies are dealt with flow behavior and concentration patterns for cross-flow membrane operations. However, the relative fouling propensities of top and bottom membrane surfaces are never addressed in any study for narrow channels filled with ladder-type spacers. In the present work, fluid flow patterns through different spacer configurations are visualized using ANSYS FLUENT by varying the dimensionless filament spacing, L (ratio of top or bottom filament spacing and channel height). Results clearly indicate that the average shear stress values at the top membrane surface are always higher (3–8 times) than that at the bottom membrane surface, but the mass transfer coefficient for the two walls yielded approximately similar average values having low to moderate filament spacings of L ≤ 3 (SP22 and SP33), indicating similar fouling propensities of membrane surfaces. Further increase in filament spacing with L ≥ 4 (SP44 and SP66), the average mass transfer coefficient for the top membrane indicated a sharp decline, suggesting high fouling propensity compared to bottom membrane which is not a desirable feature. Among the four spacer arrangements studied, SP44 (with L = 4) was found to be the optimal arrangement, yielding moderate pressure drop with nearly equal/higher area weighted values of mass transfer coefficient for the two walls and would lead to lower and equal fouling tendencies for top and bottom membrane surfaces, respectively.

Novel crossflow membrane cell with asymmetric channels: Design and pressure-retarded osmosis performance test

Pressure-retarded osmosis (PRO) has the potential to convert salinity gradient energy to hydraulic energy via a semi-permeable membrane. The performance of an osmotic membrane is usually evaluated using a crossflow membrane test cell with symmetric channels. However, it is not possible to test a PRO membrane under conditions of large differences in hydraulic pressure because the membrane cannot be supported in the channel inlet and outlet with a cavity. In this study, we present a newly designed PRO crossflow test cell that features asymmetric channels on both sides of the membrane. This asymmetric design prevents membrane bulge and rupture in the channel inlet and outlet even at an applied hydraulic pressure difference greater than 54.5bar. Excellent performance of this novel PRO cell was demonstrated using two different membranes at various operating pressures. To prevent membrane deformation at open spaces between spacer strands, a feed channel spacer should be selected carefully. Accordingly, the effects of tricot feed spacers with different voidages and thicknesses on PRO performance were also investigated.

Response of Shaver Brown Laying Hens to Different Feeding Space Allowances

Three hundred 18 week-old Shaver Brown pullets (1,477.40±7.41 g) were allotted to 5 treatment groups containing 3 replicates of 20 pullets each. The treatments consisted of five feeding space allowances (FSA): 5.60, 8.40, 11.20, 14.00 and 16.80 cm/bird. Data were collected on feed consumption, weight change, egg production and feed conversion ratio (feed: dozen eggs) for a period of 20 weeks (18 to 38 weeks of age). All data collected were subjected to analysis of Variance (ANOVA) and significant differences reported at 5% probability. Mean feed consumption, body weight change and egg weight were not significantly affected by FSA. Pullets given access to FSA of 11.20, 14.00 and 16.80 cm/bird laid the first egg at a significantly younger age compared to those given 5.60 and 8.40 cm/bird. Hen-day egg production (HDP) and egg mass increased with increasing FSA up to 14.00 cm/bird. Hen-day production did not differ between the groups given 8.40 and 16.8 cm/bird. Groups given 5.60 cm/bird consumed more feed per dozen egg produced. The lowest feed consumption per dozen eggs was recorded on 11.20 and 14.00 cm FSA per bird. These results suggest that keeping the feeding space between 11.00 and 14.00 cm/hen will improve flock uniformity and reduce cost of egg production of Shaver Brown hens in the study area.

Experimental characterization and numerical simulation of the anti-biofuling activity of nanosilver-modified feed spacers in membrane filtration

The antibiofouling activity of nanosilver-coated feed spacers was investigated in a crossflow membrane filtration-cell system simulating the conditions in spiral wound modules. Silver ions distribution in the feed channel and near the membrane was estimated by numerical simulation. Nanosilver was embedded into the polypropylene network using sonochemical deposition. The ability of the modified spacer to hinder biofilm development on the adjacent membranes was studied at 0.15ms−1 crossflow velocity, using polysulfone ultrafiltration membrane exposed to a mixed microbial enrichment. The physical properties of the modified spacer and biofilm development were evaluated using scanning electron microscopy and confocal laser scanning microscopy imaging (HRSEM and CLSM). HRSEM images depicted significantly less bacteria attached to the membranes near the modified spacer, mainly scattered with a sporadic monolayer structure. Dead/live staining assay by CLSM indicated a significantly higher percentage of dead bacteria cells attached to the membranes adjacent to the modified spacer in comparison to untreated spacers (control). The silver modified spacers displayed improved and steadier permeate flux along the 10 days of the run compared to the control. Flow and mass transfer simulations showed increased silver ions concentration near the membrane as a function of the spacer configuration. The life span of the modified spacer was evaluated as well.

Biofouling removal in spiral-wound nanofiltration elements using two-phase flow cleaning

Biofouling has detrimental effects on the feed channel pressure drop and the permeate flux in high-pressure membrane processes such as NF and RO. Two-phase flow cleaning is a chemical-free technique that is able to remove such biofilms. This paper presents a study into the effects of the gas/liquid ratio, feed spacer geometry, applied pressure and liquid velocity on the efficiency of two-phase flow cleaning in spiral-wound nanofiltration elements. A high-speed camera, optical coherence tomography and scanning electron microscopy were used to study biofouling and its removal. Our results show that two conditions must be met to ensure that a sufficiently high shear force is applied to biofilms on membrane and spacer surfaces. A good bubble distribution in the channel is the first requirement. While it is mainly the structure of the feed spacer that controls bubble flow and bubble size, a minimum gas/liquid ratio of 0.5 is necessary to achieve a good bubble distribution. The second condition is the use of a sufficiently high liquid velocity during cleaning. The bubble velocity was found to be 3.5–5.5 times as high as the used liquid velocity, and responsible for a marked improvement in the flux recovery.