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It is imperative to design and develop a technology which would completely remove pollutants from contaminated waters due to agricultural, industrial, and domestic uses. For the first time, we discovered that graphene incorporated composite nanofibers (TZB-Gr) (Titanium dioxide-Zinc Oxide-Bismuth Oxide-Graphene) with a band gap of (2.5 eV) can effectively activate organic dyes under visible-light and UV-light irradiation to produce active ·O2− and ·OH radicals. The produced radicals are powerful oxidizing species to degrade most of the organic pollutant to become CO2 and H2O. TZB-Gr demonstrated a higher activity than TZB (Titanium dioxide-Zinc Oxide-Bismuth Oxide) and P25 (Commercial TiO2 nanoparticles). Kinetic study of composite nanofibers (NFs) was carried out. Furthermore, a reasonable catalytic mechanism of the TZB-Gr (NFs) was proposed, based on electron-hole pairs and recombination of photogenerated charges. It was shown that graphene-based nanofiber photocatalysis is superior to that on transition metal oxide (TZB) and P25 in degradation of a dye (methylene blue, MB) and (rhodamine B, RhB) in water, therefore providing a novel strategy for environmental remediation.

Nano-aerosols of size 100 nm and below are manifested everywhere in various urban micro-environments. In outdoor, the vehicular emission during traffic jam can release nano-aerosols with concentration above 200 million/m3 (200/cm3). Superimposed on vehicular emission are smog particles, around 10–50 nm, generated from the photochemical reaction of hydrocarbon and NOx in the presence of sunlight. These nano-aerosols, by virtue of their small sizes, can be inhaled readily into our bodies leading possibly to various chronic diseases. Air-borne viruses from influenza to epidemic viruses, which can lead to acute sickness and even death, are also in the same size range of 100 nm. Despite microfibers are being commonly used today in filters, there has been limited studies on filtration of real aerosols using microfiber filters, let alone nanofibrous filters, as most studies used simulated aerosols (e.g. sodium chloride) or test dust. Also, in standard sodium chloride test, only monodispersed aerosol size is allowed to challenge the filter. On the other hand, in reality aerosols of all sizes challenge simultaneously the filter. In this study, for the first time we have used nanofiber filter in a portable test filter set-up to filter polydispersed aerosols from the micro-environment near busy traffic area where aerosols comprise of both vehicular and atmospheric pollutants with size range between 10 and 400 nm. Real-time measurements are carried out with the portable test filter. The test results are compared to the theoretical correlation from Payet with diffusion correction at small Peclet number (Pe). We have found good comparison between test results with theoretical correlations using equivalent aerodynamic diameter for the aerosols with face velocity from 1 to 11 cm/s. A new dimensionless parameter, specific filter resistance (also established independently by Buckingham-π approach), is defined for the first time. It measures the flow drag on fibers to the amount of fibers present in the filter, the lower is the specific filter resistance the better is the filter. For our test condition, it is approximately 1.0–1.2. We have also improved the filtration efficiency of the filter by increasing the fiber basis weight by stacking up two layers of nanofibers. We have demonstrated that the efficiency increases while both the quality factor and specific filter resistance remain constant. We have also investigated the single fiber efficiency due to diffusion and found that at low velocity and low Peclet number (Pe 0.07 m/s) and large Peclet number (>10), there is small deviation from the theory, which is probably due to the smaller aerosols collecting by the larger aerosols upstream of the filter with an airstream consisting of polydispersed aerosol distribution challenging the filter. This has not been realized previously as tests conducted were mostly using monodispersed aerosol size distribution for which aerosol-aerosol interaction was absent.

Filtration of neutrally charged nano- and sub-micron aerosols (50–500 nm) using PVDF electret nanofiber filters based on dielectrophoretic effect was investigated experimentally for the first time. Dielectrophoretic effect is refers to inducing dipole charges on neutrally charged aerosols when they are in close proximity to the charged nanofibers; subsequently, the polarized aerosols are captured electrostatically by the charged fibers. Given the nanofiber diameter is less than 500 nm, the electrostatic attraction force in the proximity of the fiber was very strong, resulting in good aerosol capture without incurring higher pressure drop across the filter. We were able to charge a PVDF nanofiber filter using corona discharge and the performance of the filter remained relatively unchanged within 90 days of repeated testing with quality factor well reaching 0.14 Pa−1 or higher. Optimization of charging parameters was carried out and charge density on filters indicated by surface potential (SP) was found to increase with increasing charging voltage and decreasing charging distance. Correspondingly, filtration efficiency increased due to the enhanced electrostatic attraction between charged fibers and polarized aerosols. The influence of filter basis weight (W) on the filtration performance of electret filters was studied, which showed that the improvement in filtration performance was insignificant with limited increase in filtration efficiency and higher pressure drop. Although higher fiber amount led to higher surface potential, it also caused unsatisfactory charge density increment and higher electrical interference between adjacent fibers due to higher packing density. To enhance the performance of PVDF electret filters, a ‘multi-layering’ approach was proposed for the first time on electret nanofiber filter whereby the total charged fibers were distributed in thinner stack-up layers, with each layer being isolated by a porous permeable substrate, to reduce the electrostatic interference among fibers of adjacent layers. The latter has been the key culprit in reducing the performance when charged fibers were all stacked into a single layer, resulting in overlapping of intense electrostatic fields from different nanofibers. Moreover, through distributing fibers to multilayers, the filter porosity was increased, leading to lower air flow resistance and pressure drop. Therefore, compared with a filter with only one layer, a multi-layer filter with similar total basis weight of fibers had much better performance, which was further verified by halving the basis weight of each individual layer (WL) while maintaining the overall value. The single-fiber efficiency based on dielectrophoretic effect confirms larger aerosols got better charge induction resulting in stronger dipoles and better capture. The face velocity dictates the retention time and increasing retention time proves beneficial for dielectrophoretic capture. Multilayering could reduce electrostatic interference, thereby facilitating increasing basis weight of fibers to be used in a filter to achieve high capture efficiency. Moreover, filter durability/stability was investigated, which indicates the multi-layer PVDF electret filters could maintain high performance and are promising for long-term use. It was concluded that multilayer PVDF electret filters have a great performance in aerosol removal and a potential filtration enhancement mechanism was elaborated.

Nano-aerosols are airborne aerosols in the range of 100 nm. They are present abundantly in both pollutants and viruses, both of which affect human health. Filtering these nano-aerosols leads to high-pressure drop in cabin filters due to their small sizes forming impermeable dust cake, and prior to cake formation the capture efficiency is rather poor with conventional microfiber filters. We have investigated a composite filter with a microfiber layer positioned immediately upstream of a nanofiber layer. Such composite filter provides high efficiency and low-pressure drop during all phases of filtration of nano-aerosols: initial depth filtration, transition, and final cake filtration. The behavior of the composite filter together with various filter benchmarks (single and composite filters of microfiber/nanofiber filters) has been investigated by monitoring the pressure drop, especially during cake filtration. Using the measurements together with Ergun/Kozeny-Carman equation and material balance we come up with a new method to determine the solid volume fraction and permeability of the cake. The grade efficiency for the most penetrating particle is also measured concurrently to define the incipient point to cake formation and provide material balance. Most importantly, for the first time we have confirmed that the cake ultimately forms on the composite filter is due to the cake that originates from the upstream microfiber layer and not from the downstream nanofiber layer. Therefore, the pressure drop for the composite filter can be sustained relatively low throughout the filtration. On the other hand, the aerosol capture efficiency stays very high from start-to-end of filtration, with high initial filtration efficiency courtesy of the nanofiber filter installed downstream of the composite filter. The price to pay is a slightly higher pressure drop for the composite filter in the clean unloaded state, but this is insignificant during aerosol loading as most pressure drop is attributed to that of the cake. For the first time we have introduced, also independently using Buckingham-Pi analysis, a brand new dimensionless parameter, Beta, which measures the equivalent “thickness” of additional cake deposition (in molten form) to the viscous flow resistance “path length” due to the additional cake deposit. A high-capacity filter aimed for heavy aerosol loading should have large Beta that results in low solidosity, high porosity, and high permeability for the cake. In our study, the microfiber-nanofiber composite filter has the highest Beta of nearly 4 and also highest efficiency. Despite high efficiency the single nanofiber filter has much lower Beta of 2, while the single microfiber filter has Beta of 3, yet much lower efficiency, especially at initial filtration. Using the microfiber-nanofiber composite filter design, we can “engineer the cake” deposited on the filter to be more permeable and with least flow resistance during aerosol loading to attain high Beta values. This is an innovative approach to increase the badly needed aerosol storage capacity with low pressure drop for the nanofiber filter while maintaining high efficiency throughout the aerosol loading for extended filtration applications. The highly permeable aerosol cake can also be cleaned by backpulsed-and-backblow readily for filter reuse.

Photocatalytic removal of Dairy effluent (DE) was studied by using TiO2-GeO2 and TiO2-CdO nanofibers (NFs), produced by electrospinning method. These NFs were characterized by SEM, TEM and XRD studies. The TiO2-GeO2 and TiO2-CdO NFs were smooth and continuous, with an average diameter of about 273 nm and 256 nm respectively, and held their nanofibrous morphology even after more than 9 h of photocatalytic removal of DE under visible light irradiation. TiO2-GeO2 and TiO2-CdO NFs were effective materials for removal of DE, even after many runs and cycles. TiO2-GeO2 and TiO2-CdO NFs showed a maximum removal of 65% and 75%, respectively, after 3 h. The TiO2-GeO2 and TiO2-CdO NFs also showed excellent results in hydrogen release.

A thin, highly porous, nanofiber filter with low-pressure drop start out in “depth filtration” in a clean state with aerosols mostly captured by the fibers in the filter. For nanofibers with high capture efficiency, the fibers in the region upstream of the filter facing the challenging aerosol flow tends to capture more aerosols. The capture aerosols in turn can capture more incoming aerosols in this region leaving much less aerosols escaping downstream of the filter. Furthermore, these captured aerosols, forming dendritic structures, reduce the size of flowable pores in the region. This “skin region” upstream of the filter can be quite thin as compared to the entire filter thickness, yet it accounts for the large fraction of the aerosols captured in the filter as well as large fraction of the pressure drop across the filter. With more aerosols loading, the openings of the flowable pores in the skin region get blocked by aerosol dendrites that bridge across captured aerosols and fibers. These aerosol bridges subsequently build-up above the filter surface in the “surface filtration” regime. As more aerosol bridges stack-up on existing ones and interact with each other, they form ultimately a continuous cake layer on the filter surface. The pressure drop due to the skin effect, which is a combination of both pore filling in the skin region in the filter and the aerosol bridges, can be relatively low (i.e. low-skin effect) for low nanofiber packing density and a thin nanofiber filter. However, the establishment of the skin region during depth filtration and the aerosol bridging effect above the filter surface are still two essential processes prior to formation of the cake on the filter surface. Two experiments and related analysis on loading of thin nanofiber filters using nano-aerosols are used to investigate this scenario. It has been found that in the course of aerosol bridging, the pressure drop behaves predominantly concave upward with increasing specific aerosol deposit. This contrast with previous study with high-skin effect for a thick, less-porous nanofiber filter for which aerosol bridging predominantly yields a concave downward behavior with specific aerosol deposit during pressure drop excursion. Most importantly, the bridging model that predicts successfully the high-skin effect can also predict the low-skin effect where pressure drop has a concave upward behavior with increasing specific aerosol deposit. Finally, we have demonstrated the aerosol holding capacity for a thin, highly porous, nanofiber filter can still be significant, with a small fraction of the total aerosols captured by depth filtration in the filter, and majority of deposited aerosols above the filter in the surface filtration regime, partly in the aerosol bridges and mostly in the cake.

Highlights • Cyclic loading/cleaning nanofiber filter with nano-aerosols. • Thin skin layer upstream of filter controls loading behavior, permeability drops to 10–20% of initial value. • Capillary model for aerosol deposit, pore bridging, cake model match 6-cycle loading tests. • Transit from depth-to-surface-filtration – tests and modelling., Nano-aerosols from viruses to virgin pollutant particulates from combustion, 100 nm or smaller, are harmful to our health as they penetrate readily into our body causing various diseases. Nanofiber filter can capture effectively these nano-aerosols. However, over time the pressure drop increases dramatically and cleaning of the filter by backpulse/backblow is essential for filter reuse. The cyclic loading-and-cleaning of a nanofiber filter has been investigated for the first time experimentally and theoretically. The “skin” layer, a thin region upstream of the nanofiber filter, plays a pivoting role in controlling the pressure drop excursion of the filter. We model the skin layer to be made up of numerous fine capillaries and examine how continuous aerosols deposited in the capillaries affect rapid rise in pressure drop followed by bridging of aerosols across the capillary openings leading to more bridging and ultimately formation of cake on top of the bridges and filter surface. We have been able to describe the deposition of aerosols in the capillary pores for depth filtration, the deposition of aerosols in the cake (surface filtration), and the intermediate bridging regime between these two. We can depict the complete pressure drop excursion including the S-shaped curve behavior from depth filtration transiting to surface filtration for a filter with severe skin effect. Our prediction matches extremely well with the 6 cycles of loading/cleaning on a 280-nm nanofiber filter subject to challenging nano-aerosols, 50–400 nm. During cyclic loading and cleaning, the porosity and permeability in the skin layer for our experiment drop to 68% and 11–21% of their original values, respectively, and the effective pore diameter also drops from 1.2 to 0.6 μm.

Novel technologies have been on demand to develop improved photocatalyst for gas purification. Graphene has been used to improve the performance of photonic devices based on its high charge conductivity as well as other unique properties. Traditional approach uses discrete graphene sheets with sparse, sporadic deposition of semiconductor crystals (TiO 2 ) as photocatalyst, which results in poor light harvesting and electron-hole recombination at the sheet edges. In our novel configuration, the edge effect has been eliminated by having the graphene sheets being rolled up into “spiral rolls” inserted in the 80 nm TiO 2 /ZnO/Bi 2 O 3 (TZB) nanofiber, and free electrons can only travel unidirectional in the graphene rolls along the axis of the nanofiber. The nanofiber is fabricated with its surface packed with 10-nm size TZB nanocrystallites that increases the surface area, thereby improving light harvesting. Further, the addition of ZnO and Bi 2 O 3 reduce the band-gap energy of the composite facilitating harvesting of the visible light spectrum. Other than fast transport of electrons to sites where photocatalysis is needed, the graphene roll (exposed in between the pores of the TZB nanocrystallites) has proven to harvest more energy from the entire UV–vis spectrum and almost doubled the already large surface area of the nanofibers when maximum amount of graphene is added into the nanofibers. Note the larger surface area of the photocatalyst facilitates pollutant gas molecule adsorption, which is the first and an important step prior to photocatalysis. The TZB-Gr photocatalyst, after optimized with as much as 26.5% by mass of graphene in the nanofibers, has superior photoactivity in degradation of NO under solar irradiation. It is 35% higher than nanofibers with just TZB alone, at least 17% higher than traditional approach with discrete graphene sheets (with edge effect) with TZB particles deposited on the sheets, and 10 times better than 25-nm TiO 2 nanoparticles (P25). Other photonic devices, such as solar cells, and non-photonic devices, such as bio-chemical sensors and lithium batteries, can also benefit from the innovative configuration of semiconductor nanofibers with inserted graphene for the low band-gap energy, fast charge transit, long electron life time, reduced recombination rate, and large surface area.

Highlights • Filtration of COVID-19 + airborne carrier, simulated by ambient aerosols 100 nm. • Ambient aerosol - Irregular shape, aerosol interactions, different properties. • Added electrostatic efficiency 100% – 180%X mechanical efficiency for aerosol >80 nm. • 6-layer stack-up charged nanofibers 525 nm fiber diameter, max electrostatic capture, reduce △p. • 50, 100, 300 nm. Ambient aerosol:88%, 88% 96%; NaCl aerosol:92%, 94%, 98%(N98) efficiency. • △P 2.65 mm water, 1/10 ≪ N95 respirator 25–35 mm water≪ N98, Quality Factor = 0.1 – 0.13/Pa., The novel coronavirus (COVID-19), average size 100 nm, can be aerosolized by cough, sneeze, speech and breath of infected persons. The airborne carrier for the COVID-19 can be tiny droplets and particulates from infected person, fine suspended mists (humidity) in air, or ambient aerosols in air. To-date, unfortunately there are no test standards for nano-aerosols (≤100 nm). A goal in our study is to develop air filters (e.g. respirator, facemask, ventilator, medical breathing filter/system) with 90% capture on 100-nm airborne COVID-19 with pressure drop of less than 30 Pa (3.1 mm water). There are two challenges. First, this airborne bio-nanoaerosol (combined virus and carrier) is amorphous unlike cubic NaCl crystals. Second, unlike standard laboratory tests on NaCl and test oil (DOP) droplets, these polydispersed aerosols all challenge the filter simultaneously and they are of different sizes and can interact among themselves complicating the filtration process. For the first time, we have studied these two effects using ambient aerosols (simulating the bio-nanoaerosols of coronavirus plus carrier of different shapes and sizes) to challenge electrostatically charged multilayer/multimodule nanofiber filters. This problem is fundamentally complicated due to mechanical and electrostatic interactions among aerosols of different sizes with induced charges of different magnitudes. The test filters were arranged in 2, 4, and 6 multiple-modules stack-up with each module having 0.765 g/m2 of charged PVDF nanofibers (mean diameter 525 ± 191 nm). This configuration minimized electrical interference among neighboring charged nanofibers and reduced flow resistance in the filter. For ambient aerosol size>80 nm (applicable to the smallest COVID-19), the electrostatic effect contributes 100–180% more efficiency to the existing mechanical efficiency (due to diffusion and interception) depending on the number of modules in the filter. By stacking-up modules to increase fiber basis weight in the filter, a 6-layer charged nanofiber filter achieved 88%, 88% and 96% filtration efficiency for, respectively, 55-nm, 100-nm and 300-nm ambient aerosol. This is very close to attaining our set goal of 90%-efficiency on the 100-nm ambient aerosol. The pressure drop for the 6-layer nanofiber filter was only 26 Pa (2.65 mm water column) which was below our limit of 30 Pa (3.1 mm water). For the test multi-module filters, a high ‘quality factor’ (efficiency-to-pressure-drop ratio) of about 0.1 to 0.13 Pa−1 can be consistently maintained, which was far better than conventional filters. Using the same PVDF 6-layer charged nanofiber filter, laboratory tests results using monodispersed NaCl aerosols of 50, 100, and 300 nm yielded filtration efficiency, respectively, 92%, 94% and 98% (qualified for 'N98 standard') with same pressure drop of 26 Pa. The 2–6% discrepancy in efficiency for the NaCl aerosols was primarily attributed to the absence of interaction among aerosols of different sizes using monodispersed NaCl aerosols in the laboratory. This discrepancy can be further reduced with increasing number of modules in the filter and for larger 300-nm aerosol. The 6-layer charged nanofiber filter was qualified as a 'N98 respirator' (98% capture efficiency for 300-nm NaCl aerosols) but with pressure drop of only 2.65-mm water which was 1/10 below conventional N95 with 25-mm (exhaling) to 35-mm (inhaling) water column! The 6-layer charged PVDF nanofiber filter provides good personal protection against airborne COVID-19 virus and nano-aerosols from pollution based on the N98 standard, yet it is at least 10X more breathable than a conventional N95 respirator.

The World Health Organization declared the novel coronavirus (COVID-19) outbreak as a pandemic on March 12, 2020. Within four months since outbreak in December 2019, over 2.6 million people have been infected across 210 countries around the globe with over 180,000 deaths. COVID-19 has a size of 60–140 nm with mean size of 100 nm (i.e. nano-aerosol). The virus can be airborne by attaching to human secretion (fine particles, nasal/saliva droplets) of infected person or suspended fine particulates in air. While NIOSH has standardized N95, N99 and N100 respirators set at 300-nm aerosol, to-date there is no filter standards, nor special filter technologies, tailored for capturing airborne viruses and 100-nm nano-aerosols. The latter also are present in high number concentration in atmospheric pollutants. This study addresses developing novel charged PVDF nanofiber filter technology to effectively capture the fast-spreading, deadly airborne coronavirus, especially COVID-19, with our target aerosol size set at 100 nm (nano-aerosol), and not 300 nm. The virus and its attached aerosol were simulated by sodium chloride aerosols, 50–500 nm, generated from sub-micron aerosol generator. PVDF nanofibers, which were uniform in diameter, straight and bead-free, were produced with average fiber diameters 84, 191, 349 and 525 nm, respectively, with excellent morphology. The fibers were subsequently electrostatically charged by corona discharge. The amounts of charged fibers in a filter were increased to achieve high efficiency of 90% for the virus filter but the electrical interference between neighbouring fibers resulted in progressively marginal increase in efficiency yet much higher pressure drop across the filter. The quality factor which measured the efficiency-to-pressure-drop kept decreasing. By redistributing the fibers in the filter into several modules with lower fiber packing density, with each module separated by a permeable, electrical-insulator material, the electrical interference between neighboring charged fibers was reduced, if not fully mitigated. Also, the additional scrim materials introduced macropores into the filter together with lower fiber packing density in each module both further reduced the airflow resistance. With this approach, the quality factor can maintain relatively constant with increasing fiber amounts to achieve high filter efficiency. The optimal amounts of fiber in each module depended on the diameter of fibers in the module. Small fiber diameter that has already high performance required small amounts of fibers per module. In contrast, large diameter fiber required larger amounts of fibers per module to compensate for the poorer performance provided it did not incur significantly additional pressure drop. This approach was applied to develop four new nanofiber filters tailored for capturing 100-nm airborne COVID-19 to achieve over 90% efficiency with pressure drop not to exceed 30 Pa (3.1 mm water). One filter developed meeting the 90% efficiency has ultralow pressure drop of only 18 Pa (1.9 mm water) while another filter meeting the 30 Pa limit has high efficiency reaching 94%. These optimized filters based on rigorous engineering approach provide the badly needed technology for protecting the general public from the deadly airborne COVID-19 and other viruses, as well as nano-aerosols from air pollution which lead to undesirable chronic diseases.

Centrifugation accompanied by flocculation of fine biosolids is often utilized in wastewater treatment. Unfortunately, commonly used laboratory jar tests often over-estimate the size of the flocculated solids (flocs) that can be realized in-situ in the centrifuge as the fragile flocs can be easily broken by shear and turbulence during feed acceleration in the centrifuge. Currently, there is no satisfactory method to predict in-situ floc size in the centrifuge, despite the floc size is critical to separation and solids recovery. The difficulty in making predictions also leads to inaccuracy in predicting solids recovery by decanter centrifuge and scale-up between centrifuges of different sizes, designs, and operating conditions, which is an even more serious issue. This study attempts to address these hurdles. In this study, first the flow pattern in a decanter centrifuge in form of moving layer at the pool surface is demonstrated by two different experiments. Second, a model on separation of suspended flocs in the moving layer in a centrifuge is developed. Further, a two-parameter model is proposed to represent the floc size distribution wherein the first parameter represents the minimum floc size (primary particle without coagulation or flocculation) and the second parameter represents the median floc size. A closed form analytical solution for the model is obtained with results expressed by the ratio of minimum-to-median floc size and a dimensionless Leung number which measures feed rate to the clarification rate of the centrifuge. Third, a Buckingham-π analysis has been conducted on separation in a moving layer under centrifugal field confirming these two governing dimensionless parameters can also be derived independently from the more basic dimensionless π groups. Fourth, by matching the solids recovery obtained from field tests with the model prediction, the median floc size can be inferred. In this matching process, the minimum floc size is assumed to be equal to the size of the primary, unflocculated solids. Fifth, four sets of tests (over 20 runs) have been carried out using two decanter centrifuges of different sizes and designs operating, respectively, under different pool depths, feed rates, polymer dosages and rotation speeds in a wastewater treatment plant processing mixed sludge with 48% primary sludge and 52% waste activated sludge. The median floc size inferred from the present method is in the range of 4–9 mm from the three series of tests (14 tests). The improved feed acceleration design with speed matching closely that of the rotating pool results in less pool turbulence, larger flocs, and lower polymer dosage (7 kg polymer/ton sludge); while the poorer feed acceleration with more pool disturbance from the under-accelerated feed results in smaller flocs, and higher polymer dosage (9 kg/t). Sixth, an important application of the model is to predict solids recovery using an estimated median floc size and this approach has predicted the solids recovery reasonably well on the fourth set of tests (9 tests). Seventh, the scale-up for predicting solids recovery of flocculated suspension from decanter centrifuges of different sizes, designs, and operating conditions has been demonstrated using the dimensionless Leung number and the minimum-to-median floc size ratio.

A two-parameter model has been developed to interpret the pressure excursion during cleaning of a nanofiber filter preloaded with nano-aerosols. The model can well predict the three stages of cleaning – an initial rapid cake discharge phase, and a transition and a final phase, both of which are related to cleaning aerosols trapped in the filter. One of the two parameters of the model affects the residual pressure drop while the other parameter affects the cake discharge. The filter cleaning test data based on variations from the five operating and geometric parameters have been well explained by the new model – the backpulse, backblow, and combined mode; pulse duration; applied over-pressure; diameter of nanofiber in filter; and the filter thickness. Furthermore, the model can project the residual pressure, which provides an indication of residual aerosols trapped in the nanofiber filter, using individual or combination of these operating and geometric parameters.

Highlights • Backpulse (BP) and backblow (BB) remove nano-aerosols in cake and in filter, BP more effective than BB. • BB following BP can further remove the detached aerosols preventing aerosols recaptured by the filter. • Higher tank pressure up to 6.5 bars can be used for air jets., Nanofibrous filter have been proven effective to remove nano-aerosols with size less than 100 nm. Cleaning is required after long-term use; however, very little has been published on the subject. An experimental investigation has been launched to determine backpulse, backblow and combined backpulse–backblow on cleaning of a loaded nanofiber filter. Nylon 6 nanofiber filters were loaded with polydispersed NaCl particles, 60%

Highlight • Loading single/multilayer, uncharged/charged nanofiber filter experiments. • Charged/electret multilayer nanofiber filter increases greatly aerosol capacity. • Electret 4-layer nanofiber filter >50% more capacity than 1-layer uncharged nanofiber filter. • Filter fully utilized from charged fibers, 4X aerosols captured in depth filtration. • Lowest possible pressure gradient 11 Pa/gsm deposit., Aerosol loading behavior of PVDF nanofiber electret filters using neutrally charged nano- and submicron aerosols was investigated experimentally for the first time. The loading behavior include variations of filtration efficiency and pressure drop and distribution of deposited aerosols in the filters all having the same fiber basis weight (3.060 gsm). Through the filtration efficiency variations of uncharged/charged, single-/multi-layer filters with aerosol loading, it was observed that mechanical PVDF filters had continuously increasing filtration efficiency, while PVDF electret filters had initially decreasing and subsequently increasing filtration efficiency until reaching 100% due to diminishing electrostatic effect and enhancing mechanical effect. By combining the pressure drop evolution of different filters during aerosol loading and detailed SEM images of the loaded filters, we have demonstrated that multilayer PVDF filters, especially the electret ones, could significantly slow down the pace of filter clogging (skin effect) and increase significantly the aerosol holding capacity during depth filtration. Generally, the multilayer nanofiber filters received the most aerosol deposit during depth filtration, whereas the single-layer nanofiber filters with the same basis weight of fibers received the most deposit during cake filtration. The multilayer nanofiber filters had approximately 70% aerosol deposit in the filter during depth filtration fully utilizing the full filter thickness, especially for the electret filters that had charged fibers, and only 30% deposit in the cake. On the contrary, the single-layer uncharged/charged nanofiber filters were exactly the reverse due to persistency of the skin effect with only 30% deposit in the filter mostly located in the upstream layer, yet 70% deposit in the cake. During depth filtration, the pressure drop per added mass deposit for the multilayer electret filter was very low at 11 Pa gsm−1, which was at least twice below any other nanofiber filters. This was all attributed to the uniform capture of aerosols by electrostatic effect across the entire filter depth from the upstream to downstream layers of the multilayer electret filter. The above conclusion was confirmed by the detailed SEM images taken across the different filter layers for the multilayer filter configuration. The 4-layer electret nanofiber filter with a 3.060-gsm basis weight has 4 times more aerosol holding capacity than the single uncharged/charged nanofiber filter with the same fiber basis weight in depth filtration. Based on the standpoint of highest efficiency and capacity with maximum pressure drop 800 Pa imposed on the filtration operation, the 4-layer nanofiber electret was the best among all 4 filters. It had 52% more aerosol holding capacity than the single layer uncharged nanofiber filter and 38% more capacity than the charged single-layer and the uncharged multilayer nanofiber filters. The multilayer PVDF electret filters have excellent filtration performance for long-term aerosol filtration and also great potential applications in the fields of personal health care and environmental protection.

Nanofiber filter, with fibers 98–300 nm, has been used to effectively remove sub-micron aerosol. Despite the effectiveness of filtration, the pressure drop of the filter is high and increases with solid aerosol loading. In this study, the pressure drop and filtration efficiency of nanofiber filter were investigated experimentally. In particular, the empirical model of Brown was modified to determine the distribution of aerosol deposit across filter thickness using the experimental parameters. It was found that there is a large pressure drop across the loaded thin nanofiber layer, and likewise aerosol deposit distributed largely non-uniformly across the filter thickness, with more aerosol deposited near the face region compared to the back side. This can be regarded as the skin effect that exists particularly in nanofiber filtration of sub-micron aerosol. This skin effect can be mitigated by installing a microfiber filter upstream of the nanofiber filter. This is confirmed by a lower measured pressure drop across the inhomogeneous dual-layer filter.

The objective of this study is to investigate the filtration of nano-aerosols (50–500 nm) using nanofiber filter for strong diffusion capture wherein Peclet number Pe drops to single digit and even below unity. The effect of strong slip flow or transitional flow, with the dimension of the flow domain comparable in order of magnitude to the distance of mean-free-path of air molecules (i.e. Knudsen number Kn f of order one), has also been investigated with nanofiber diameter reducing below 100 nm. Also, we examined implications due to use of very small diameter nanofibers. Nanofiber filters are made by electrospinning Nylon 6 (N6) nanofibers on microfiber substrate. The diameter of nanofibers electrospun from 12, 20, and 24% N6 solution are 94, 185, and 220 nm, respectively (corresponding Kn f of 1.44, 0.71, and 0.62). Previous studies showed that the conventional semi-empirical correlation as developed by Payet et al. [1] on single fiber efficiency due to diffusion mechanism ( η D ) as developed from moderate slip flow assumption offers good agreement with experimental results even when Kn f increases to 0.65 (i.e. transition flow regime) over Pe starting from 40 and dropping to 6. In this study, under large slip flow ( Kn f equals to 1.44), we show that Payet's correlation on η D deviating from measurements when Pe is below 10 and the deviation gets worse when Pe drops below unity. Therefore, Payet's correlation, pertaining only to moderate diffusion capture, underpredicts the single fiber capture efficiency for nano-particle or ultrafine particle (sized below 100 nm according to ISO Standard [2] ), which may lead to respiratory [3] and cardiovascular [4] problems or diseases. As such, an improved empirical correlation on η D should be established based on new measurement to accurately assess the filter performance on these important particles for selection and sizing of filters in exercising preventive measures. In the study of effect of fiber diameter on capture efficiency and pressure drop, reducing d f from 185 to 94 nm facilitates the filtration of 50–500 nm nano-aerosol, but unfortunately elevates the pressure drop (Δ P ). The benefit-to-cost effect is judged by the quality factor ( QF ) which should be higher for filter with better performance. The QF of a filter with 185-nm nanofibers is higher than the one with 94-nm nanofibers when filtering 50–90 nm nano-aerosol, and vice versa when filtering 100–380 nm nano-aerosol. Filtration efficiency increases when nanofiber basis weight ( W ) increases from 0.042 to 0.333 g m −2 . However, the QF drops rapidly when W increases from 0.042 to 0.085 g m −2 , and decreases slowly from W = 0.085 g m −2 onwards to W = 0.333 g m −2 . As such, for a high performance filter a multi-layer filter formed from stacking up of a series of low basis weight nanofiber layers, each independently supported, is more favorable over using a single-layer nanofiber filter with the same total basis weight.

The objective of this study is to evaluate the effect of face velocity, nanofiber packing density and nanofiber layer thickness on filtration efficiency and pressure drop of filters with nanofibers coated on a substrate. The substrate is a non-woven microfiber medium with negligible filtration efficiency and pressure drop as compared to nanofiber layer. The nanofiber layer is composed of electrospun polyethylene oxide nanofibers with mean diameter equals to 208 nm. Test aerosol is sodium chloride with size ranging from 50 to 480 nm. Experimental results show that the most penetrating particle size decreases from 140 to 90 nm when nanofiber packing density increases from 3.9 to 36 × 10−3. When face velocity is increased from 5 to 10 cm s−1, the filtration efficiency generally decreases, and the reduction becomes larger at smaller particle sizes especially for those below 100 nm. By maintaining nanofiber packing density at 8.7 × 10−2 and almost triple the nanofiber layer thickness, the most penetrating particle size decreases slightly from 140 to 120 nm. This suggests nanofiber layer thickness has less prominent effect on most penetrating particle size than nanofiber packing density. By adding more nanofibers on microfiber substrate in a single layer especially under high fiber packing, the pressure drop across the filter elevates without significantly improving the filtration efficiency exhibiting a diminishing return behavior. Hence, the method of “multi-layering” is proposed to fabricate nanofiber filter with greatly reduced pressure drop. This is advantageous for high efficiency applications with nanofibers of high packing density.

The aim of this study is to experimentally investigate pressure drop and filtration efficiency of fibrous filters operating in aerodynamic slip regime under continuous loading of sub-micron aerosols. Test aerosol is sodium chloride (NaCl) particulates with electrical mobility diameter (EMD) ranging from 41 to 514 nm generated from atomization. Two types of non-woven filters composed of fibers with diameter 1.8 μm (micro-fibrous) and 300 nm (nano-fibrous) are tested. Most penetrating particle size (MPPS) of clean micro- and nano-fibrous filters are 203 and 103 nm EMD, respectively. Both of them have clean filter filtration efficiencies closely agree with Payet's model. Under continuous aerosol loading, MPPS of both types of filters shift to smaller size. A semi-empirical model is developed to describe pressure drop across a fibrous filter operating in aerodynamic slip regime under continuous loading of sub-micron aerosols. Experimental results of filters with thickness ranging from 1 × 10−5 to 1 × 10−4 m generally agree with this model. It shows the potential of extending the model to describe pressure drop across aerosol-loaded filter of increased thickness with non-uniform particle deposition across filter depth being taken into account. Although clean micro- and nano-fibrous filters have comparable pressure drop while nano- offers higher filtration efficiency against sub-micron aerosols, which may suggest it as an “absolutely better” filter, experimental results show that the mass holding capacity of nano-fibrous filter is lower than its micro-fibrous counterpart. Hence, it is essential to analyze loading behavior through quantitative model during filter design stage.

An analytical model has been developed on separation, classification, and clarification of dispersed suspension with well-characterized particle size distribution using rotating tube centrifuge. The model is based on limiting trajectory analysis on individual particles of different sizes settling in the rotating tube under centrifugal acceleration with results expressed in a new dimensionless sedimentation Leung (Le) number, which corresponds with its counterpart for continuous-feed centrifuge. For sedimenting in a rotating tube the governing Le number depends on the effective viscosity of the suspension, the initial height of suspension, density difference between solid particles and suspension, and therefore operating a centrifuge centrifugal gravity, and the time duration that the suspension has been subject to centrifugation. Le is directly related to the cut size and at condition with small Le number leads to improved separation. In the first part of the model, centrifugal gravity G is assumed constant in the entire suspension domain wherein the effective radius for determining G is taken as the geometric mean of the smallest and largest radii of the suspension, and the sediment thickness is assumed negligible. The prediction from the model is compared with experimental measurements carried out in laboratory on four different suspensions. Laboratory measurements include (a) solids concentration, (b) particle size distribution, and (c) density, respectively, of feed and supernatant after subjecting test samples at different centrifugal gravities and time duration. Results of four laboratory test studies involving classification and separation of different suspension compared reasonably well with the model prediction. In two cases, the test results obtained from a small pilot continuous-feed centrifuge were compared with that of the rotating tube based on the same test sample. In the second part, a variable-G model is also developed to account more accurately that the G-force increases linearly with radius across the suspension but also with negligible sediment thickness. The results confirmed that the constant-G model using the effective radius is reasonably accurate for engineering calculations. In the third part, the effect on separation due to sediment accumulating over time at the tube bottom has been investigated for the constant-G model. The accumulating sediment shortens the particle settling distance; on the other hand it also undermines particles settling with clarified liquid flowing radially inward (opposing sedimentation) from displacement of liquid in sediment formation. The improved model with finite sediment thickness compares slightly more favorably with experimental results under condition of high-solids capture and high-rate of sedimentation at small Le. Not only the rotating tube can be used to determine whether a given suspension is separable by centrifugation and also the pertinent parameters involved in scale-up associated with such laboratory testing, the present study addresses the feasibility of projecting results of process separation using continuous-feed centrifuges in absence of dynamic effects associated with complicated flow and sediment/cake solids transport. Pilot study using continuous-feed centrifuges is needed for demonstration and production scale-up and to further assess the impact based on any unforeseen complications.