微细颗粒物在过滤介质中过滤特性的CFD-DEM模拟

基于CFD-DEM（离散单元法）方法模拟了微细颗粒物在纤维过滤介质中的气-固两相流动特性,模拟时,充分考虑了颗粒群组成、粒径分布、颗粒间及颗粒与纤维间的反弹作用以及颗粒团聚等因素,分析了纤维过滤中颗粒群的运动特性和微细颗粒的沉积形式。结果表明：采用CFD-DEM模拟过滤介质的过滤过程以及微细颗粒在介质表面沉积过程和形式的方法是方便且可行的,模拟结果与前人的实验观测结果基本吻合;在过滤过程中,表面过滤的贡献较大,大部分的颗粒在介质表面即被捕集,进入到介质内部的部分粒径较小的颗粒经深层过滤作用而被捕集;大量的颗粒捕集是由颗粒-颗粒捕集机制来实现的;不同颗粒体系的颗粒群其过滤效果也有所差别,对于本文所研究的过滤介质模型,多颗粒体系的过滤效率比单一的颗粒体系的过滤效率高20%左右。     

Prevalence of Cryptosporidium oocysts and giardia cysts in the drinking water supply in Japan

A one-year monitoring of Cryptosporidium oocysts and Giardia cysts was conducted at a water purification plant. A total of 13 samples of 50 L river source water and 26 samples of 2,000 L-filtered water, treated by coagulation flocculation, sedimentation and rapid filtration, were tested. Prior to conducting a survey of a water purification plant, we developed a method for concentrating Cryptosporidium oocysts from a large volume of raw or filtered water using a hollow fiber ultrafiltration (UF) membrane, and this procedure was adapted to survey a water purification plant. Cryptosporidium oocysts were detected in all of the 13 raw water samples. The geometric mean concentration was 40 oocysts 100 L. Giardia cysts were detected in 12 of 13 raw water samples (92%) and the geometric mean concentration was 17 cysts/100 L. Probability distributions of both Cryptosporidium oocyst and Giardia cyst concentration in raw water were nearly lognormal. In filtered water samples, Cryptosporidium oocysts were detected in 9 of the 26 samples (35%) with the geometric mean concentration of 1.2 oocysts /1,000 L and Giardia cysts in 3 samples (12%) with 0.8 cysts/1,000 L. The estimated log10 removal efficiency of Cryptosporidium oocysts and Giardia cysts by rapid-sand filtration was 2.47 and 2.53, respectively. Empty particles were removed at a higher log10 than intact oocysts and cysts. The efficiency of particle removal in the rapid sand filtration process tends to be reduced under cold-water conditions. Close management is necessary in the winter when the water temperature is low.     

A Practical Assessment of the Use of Particle Counters for Monitoring the Risk of Cryptosporidium and Giardia

The paper describes a project which was carried out to establish whether particle counts can be used to predict the risk of Cryptosporidium and Giardia breakthrough at a large water-treatment works. The project was based at a direct river-abstraction works which treats up to 120 MI/d. On-line particle counters were installed following three of the rapid-gravity filters, and a fourth counter was used to measure particles in spot samples of raw and clarified water.
There was a clear relationship between particle counts and the occasional low-level (oo)cyst (crypto oocysts and giardia cysts) occurrence in the filter samples, and no (oo)cysts were found in the final water. There appear to be complex inter-relationships between particle counts and other parameters such as raw-water conditions, environmental factors and treatment-plant variables.     

Fabrication of a multi-walled carbon nanotube-deposited glass fiber air filter for the enhancement of nano and submicron aerosol particle filtration and additional antibacterial efficacy

We grew multi-walled carbon nanotubes (MWCNTs) on a glass fiber air filter using thermal chemical vapor deposition (CVD) after the filter was catalytically activated with a spark discharge. After the CNT deposition, filtration and antibacterial tests were performed with the filters. Potassium chloride (KCl) particles (< 1 μm) were used as the test aerosol particles, and their number concentration was measured using a scanning mobility particle sizer. Antibacterial tests were performed using the colony counting method, and Escherichia coli (E. coli) was used as the test bacteria. The results showed that the CNT deposition increased the filtration efficiency of nano and submicron-sized particles, but did not increase the pressure drop across the filter. When a pristine glass fiber filter that had no CNTs was used, the particle filtration efficiencies at particle sizes under 30 nm and near 500 nm were 48.5% and 46.8%, respectively. However, the efficiencies increased to 64.3% and 60.2%, respectively, when the CNT-deposited filter was used. The reduction in the number of viable cells was determined by counting the colony forming units (CFU) of each test filter after contact with the cells. The pristine glass fiber filter was used as a control, and 83.7% of the E. coli were inactivated on the CNT-deposited filter.     

Filtration behaviors of Giardia and Cryptosporidium--ionic strength and pH effects

The laboratory-scale filtration tests of Giardia cysts and Cryptosporidium oocysts in both 2 mm-phi glass beads and 2 mm-phi polystyrene beads filters were conducted to investigate their filtration behaviors. The protozoan parasites were used as target particles, while the chemical system altered by changing the electrolyte concentration and pH. The results significantly indicate that ionic strength have a positive effect on the removal efficiencies for Giardia cysts and Cryptosporidium oocysts. The removal efficiency of two filters for Giardia cysts slightly decreased from pH 2.4 to 8.7 and decreased significantly in pH as pH up to 8.7, while that for Cryptosporidium slightly rippled beyond pH 8.7, and with the decrease in pH up to pH 8.7. The experimental collision efficiencies from the interactions between colloids and the filter media were calculated with a semi-empirical approach of the single sphere model and clean-bed filtration theory. The results also indicated that experimental collision efficiencies for (oo)cysts corresponded to the (oo)cysts removal efficiencies in all trials, and oocysts exhibits higher collision efficiencies than cysts.     

The effectiveness of stand alone air cleaners for shelter-in-place

Stand-alone air cleaners may be efficient for rapid removal of indoor fine particles and have potential use for shelter-in-place (SIP) strategies following acts of bioterrorism. A screening model was employed to ascertain the potential significance of size-resolved particle (0.1-2 microm) removal using portable high efficiency particle arresting (HEPA) air cleaners in residential buildings following an outdoor release of particles. The number of stand-alone air cleaners, air exchange rate, volumetric flow rate through the heating, ventilating and air-conditioning (HVAC) system, and size-resolved particle removal efficiency in the HVAC filter were varied. The effectiveness of air cleaners for SIP was evaluated in terms of the outdoor and the indoor particle concentration with air cleaner(s) relative to the indoor concentration without air cleaners. Through transient and steady-state analysis of the model it was determined that one to three portable HEPA air cleaners can be effective for SIP following outdoor bioaerosol releases, with maximum reductions in particle concentrations as high as 90% relative to conditions in which an air cleaner is not employed. The relative effectiveness of HEPA air cleaners vs. other removal mechanisms was predicted to decrease with increasing particle size, because of increasing competition by particle deposition with indoor surfaces and removal to HVAC filters. However, the effect of particle size was relatively small for most scenarios considered here. PRACTICAL IMPLICATIONS: The results of a screening analysis suggest that stand-alone (portable) air cleaners that contain high efficiency particle arresting (HEPA) filters can be effective for reducing indoor fine particle concentrations in residential dwellings during outdoor releases of biological warfare agents. The relative effectiveness of stand-alone air cleaners for reducing occupants' exposure to particles of outdoor origin depends on several factors, including the type of heating, ventilating and air-conditioning (HVAC) filter, HVAC operation, building air exchange rate, particle size, and duration of elevated outdoor particle concentration. Maximum particle reductions, relative to no stand-alone air cleaners, of 90% are predicted when three stand-alone air cleaners are employed.     

BAC滤池对浊度和颗粒数的控制研究

传统的贾第鞭毛虫和隐孢子虫（简称“两虫”）检测方法存在诸多不足，为此选用浊度和颗粒数作为“两虫”的替代指标，以对浊度和颗粒物的去除率来衡量生物活性炭（BAC）滤池对“两虫”的控制效果。试验结果表明：采用颗粒数表征滤后水水质比采用浊度更适宜。过滤初期颗粒数从峰值降到50个／mL以下所需的时间比浊度降到0．1NTU所需的时间多1h左右。正常过滤期间BAC滤池进水浊度一般在0．1NTU以下，经过BAC滤池处理后，浊度得到进一步降低，平均去除率为52．7％。炭层对浊度的去除率为56．4％，其出水浊度基本上都低于0．05NTU，而砂层对浊度不但没有去除能力，反而使出水浊度平均上升了约3．7％。炭层对颗粒物的平均去除率为33．3％，砂层对颗粒物的平均去除率为8．5％。     

活性炭滤池中颗粒物数量和粒径分布的研究

结合水厂水处理工艺及活性炭深度处理装置,探讨了生物活性炭滤池出水中颗粒物数量的变化及粒径分布规律。结果表明,在过滤周期内活性炭滤池出水颗粒物数量与浊度会出现相似的变化规律,二者之间相关性较好,但活性炭滤池出水浊度的变化滞后于颗粒物数量的变化。生物活性炭滤池过滤初期,滤后水中大于2μm的颗粒物数量可达到111个／mL,50min后数量降至50个／mL以下。生物活性炭滤池出水中大于2μm的颗粒物主要由粒径为2～7μm的颗粒物组成。     

Collection of biological and non-biological particles by new and used filters made from glass and electrostatically charged synthetic fibers

Synthetic filters made from fibers carrying electrostatic charges and fiberglass filters that do not carry electrostatic charges are both utilized commonly in heating, ventilating, and air-conditioning (HVAC) systems. The pressure drop and efficiency of a bank of fiberglass filters and a bank of electrostatically charged synthetic filters were measured repeatedly for 13 weeks in operating HVAC systems at a hospital. Additionally, the efficiency with which new and used fiberglass and synthetic filters collected culturable biological particles was measured in a test apparatus. Pressure drop measurements adjusted to equivalent flows indicated that the synthetic filters operated with a pressure drop less than half that of the fiberglass filters throughout the test. When measured using total ambient particles, synthetic filter efficiency decreased during the test period for all particle diameters. For particles 0.7-1.0 mum in diameter, efficiency decreased from 92% to 44%. It is hypothesized that this reduction in collection efficiency may be due to charge shielding. Efficiency did not change significantly for the fiberglass filters during the test period. However, when measured using culturable biological particles in the ambient air, efficiency was essentially the same for new filters and filters used for 13 weeks in the hospital for both the synthetic and fiberglass filters. It is hypothesized that the lack of efficiency reduction for culturable particles may be due to their having higher charge than non-biological particles, allowing them to overcome the effects of charge shielding. The type of particles requiring capture may be an important consideration when comparing the relative performance of electrostatically charged synthetic and fiberglass filters. PRACTICAL IMPLICATIONS: Electrostatically charged synthetic filters with high initial efficiency can frequently replace traditional fiberglass filters with lower efficiency in HVAC systems because properly designed synthetic filters offer less resistance to air flow. Although the efficiency of charged synthetic filters at collecting non-biological particles declined substantially with use, the efficiency of these filters at collecting biological particles remained steady. These findings suggest that the merits of electrostatically charged synthetic HVAC filters relative to fiberglass filters may be more pronounced if collection of biological particles is of primary concern.     

Fine and ultrafine particle removal efficiency of new residential HVAC filters

Particle air filters used in central residential forced‐air systems are most commonly evaluated for their size‐resolved removal efficiency for particles 0.3‐10 µm using laboratory tests. Little information exists on the removal efficiency of commercially available residential filters for particles smaller than 0.3 µm or for integral measures of mass‐based aerosol concentrations (eg, PM_2.5) or total number concentrations (eg, ultrafine particles, or UFPs) that are commonly used in regulatory monitoring and building measurements. Here, we measure the size‐resolved removal efficiency of 50 new commercially available residential HVAC filters installed in a recirculating central air‐handling unit in an unoccupied apartment unit using alternating upstream/downstream measurements with incense and NaCl as particle sources. Size‐resolved removal efficiencies are then used to estimate integral measures of PM_2.5 and total UFP removal efficiency for the filters assuming they are challenged by 201 residential indoor particle size distributions (PSDs) gathered from the literature. Total UFP and PM_2.5 removal efficiencies generally increased with manufacturer‐reported filter ratings and with filter thickness, albeit with numerous exceptions. PM_2.5 removal efficiencies were more influenced by the assumption for indoor PSD than total UFP removal efficiencies. Filters with the same ratings but from different manufacturers often had different removal efficiencies for PM_2.5 and total UFPs.     

Removal of viable and inactivated Cryptosporidium by dual- and tri-media filtration

The limited efficacy of disinfectants, other than ultraviolet irradiation and ozonation, as a barrier against Cryptosporidium parvum in drinking water treatment has underscored the increased importance of oocyst removal by filtration. Currently, no reliable surrogates have been identified for C. parvum removal by filtration. As a result, evaluations of the Cryptosporidium removal by treatment operations have been performed using oocysts. It has typically been assumed that chemically inactivated oocysts are suitable surrogates for viable oocysts. Measurements of electrophoretic mobility, however, have shown that chemical inactivation changes the surface charge of Cryptosporidium oocysts. The present bench-scale research indicated that formalin-inactivated oocysts are reliable surrogates for viable oocysts during both stable filter operation and periods where filtration processes are challenged, such as coagulation failure. This finding is important because of the practical difficulties associated with using viable oocysts in filtration investigations. Poor coagulation conditions severely compromised removal of viable and inactivated oocysts by dual- and tri-media filters compared to stable operating conditions and filter ripening, emphasizing the importance of optimized chemical pre-treatment (coagulation) for the successful removal of oocysts during filtration. The treatment optimization experiments also indicated that tri-media filters offered only marginally higher oocyst removals than dual-media filters.     

Removal of particulate matter and phosphorus in sand filters treating stormwater and drainage runoff: a case study

Runoff is often delayed and treated in wet ponds to retain particles and particulate substances. To increase the treatment efficiency, a sand filter can be placed before the outlet. The filter material is often renewed after 10–15 years due to presumed clogging by trapped particles, but often it clogs much earlier. Knowledge of how clogging develops over time is therefore important. This study has examined two filters, focusing on particle size, content and placement of particles, organic matter (OM) and phosphorus (P) retained in the filter. The study concludes that both particles and P are retained in the upper few cm’s of the filter, causing clogging after a few years, thus leaving the deeper filter material unused. Even small particles (<63 µm) are efficiently retained as long as clogging is avoided. This is preferable, as runoff is rich in small particles and as particulate P is associated with the small particles.     

The impact of increased loading rate on granular media, rapid depth filtration of wastewater

The impact of loading rate on tertiary filtration of wastewater was studied using a pilot-scale, dual-media, rapid depth filtration system. Loading rates of 12.2, 15.3, 18.3, 21.4, and 24.4m/h were tested on parallel filter columns treating the same coagulated secondary wastewater to determine the impact on removal of turbidity, particles (2-15 microm), total coliform bacteria, Escherichia coli, and MS2 bacteriophage, as well as on the particle deposition profile in the filter bed. Increasing the loading rate from 12.2 to 24.4m/h decreased the removal efficiencies for all metrics. The observed impact of loading rate on particle removal was similar to that predicted by a clean-bed filtration model, although the model significantly underestimated the removal efficiencies of the smaller particles. For two loading rates, 12.2 and 18.3m/h, the effect of coagulant dose was also studied; the negative impact of loading rate on removal efficiency was eliminated by increasing the coagulant dose for the higher loading rate, which also resulted in removal of particles deeper in the filter bed. For all conditions studied, loading rate had no observable impact on the ability to disinfect filter effluents with chloramines. The results of this research indicate that loading rates higher than those typically used in tertiary filtration can produce acceptable effluent quality, and support a regulatory approach based on filter effluent turbidity.     

Characteristics of particle removals in a compressible media filter system

The ability to adjust the properties of the filter medium by altering the degree of the medium compression is important because the different porosities in media affect the ideal filter performance according to the properties of influent wastewater. This study therefore focused on the particle size distribution (PSD) of compressible media filter (CMF) effluents depending on the compression ratio (0–30%) and the filter bed depth (Layers 1–5) in order to evaluate the CMF performance. The results showed that the removal efficiency for particles larger than 10 µm was more than 90% while that for particles smaller than 10 µm was less than 85% of removal efficiency. From the power law fit, substantial decreases in the number of large‐sized particles were observed in the PSD. Particle removal efficiencies increased along with the average compression ratio increase. Furthermore, the results demonstrated that each layer in the bed had a different role in the filter performance, and it was found that a filter bed of about 0.45–0.60 m would be needed for the production of the desired effluent quality in this study. The detachment phenomenon was observed in particles larger than 5 µm, especially in 5–15 µm particles.     

Particle removal efficiency of the portable HEPA air cleaner in a simulated hospital ward

Use of a HEPA (high efficiency particulate air) filter in a room is believed to assist in reducing the risk of transmission of infectious diseases through removing the particles or large droplets to which pathogens may be attached. Use of a portable HEPA filter(s) in hospital wards is hypothesized to increase the effective ventilation rate (for particles only). Use of a portable HEPA filter is also hypothesized to increase the effective airflow rate of the general ward to the standard of an isolation ward for emerging infection diseases. This may be a good solution for housing patients when the number of beds in an isolation ward is insufficient. An experiment was conducted in a full scale experimental ward with a dimension of 6.7 m × 6 m × 2.7 m and 6 beds to test these hypotheses for a portable HEPA filter. The removal efficiency for different size particles was measured at different locations. The influence of the portable HEPA air cleaner on the airflow pattern was also studied through smoke visualization and computational fluid dynamics (CFD) simulations. Results show that the HEPA filter can effectively decrease the particle concentration level. The effective air change rate achieved by the HEPA filter (for particle removal only) is from 2.7 to 5.6 ACH in the ward. The strong supply air jet from the portable HEPA filter interacted with the room airflow pattern and became dominate, introducing global airflow mixing in the room. Background noise levels were also measured and noise level in the room increased when the maximum airflow of the filter was used.     

Removal of viable bioaerosol particles with a low-efficiency HVAC filter enhanced by continuous emission of unipolar air ions

Continuous emission of unipolar ions has been shown to improve the performance of respirators and stationary filters challenged with non-biological particles. In this study, we investigated the ion-induced enhancement effect while challenging a low-efficiency heating, ventilation and air-conditioning (HVAC) filter with viable bacterial cells, bacterial and fungal spores, and viruses. The aerosol concentration was measured in real time. Samples were also collected with a bioaerosol sampler for viable microbial analysis. The removal efficiency of the filter was determined, respectively, with and without an ion emitter. The ionization was found to significantly enhance the filter efficiency in removing viable biological particles from the airflow. For example, when challenged with viable bacteria, the filter efficiency increased as much as four- to fivefold. For viable fungal spores, the ion-induced enhancement improved the efficiency by a factor of approximately 2. When testing with virus-carrying liquid droplets, the original removal efficiency provided by the filter was rather low: 9.09 +/- 4.84%. While the ion emission increased collection about fourfold, the efficiency did not reach 75-100% observed with bacteria and fungi. These findings, together with our previously published results for non-biological particles, demonstrate the feasibility of a new approach for reducing aerosol particles in HVAC systems used for indoor air quality control. PRACTICAL IMPLICATIONS: Recirculated air in HVAC systems used for indoor air quality control in buildings often contains considerable number of viable bioaerosol particles because of limited efficiency of the filters installed in these systems. In the present study, we investigated - using aerosolized bacterial cells, bacterial and fungal spores, and virus-carrying particles - a novel idea of enhancing the performance of a low-efficiency HVAC filter utilizing continuous emission of unipolar ions in the filter vicinity. The findings described in this paper, together with our previously published results for non-biological particles, demonstrate the feasibility of the newly developed approach.     

Study on dielectrophoretic deposition of airborne particles in a vertical micro channel

Airborne particles significantly contribute to the toxicity of environmental pollution. A mathematical model is developed and analyzed to study the dielectrophoretic deposition of airborne particles in a vertical micro channel. Finite element programs are utilized to calculate the air flow and potential distribution in the vertical micro channel. The electric field in the numerical solution space is obtained by employing a potential gradient in the x and y directions. The dielectrophoretic force and sedimentation motion of a spherical particle are achieved. The simulated results show that the dielectrophoretic force is maximal at the tips of the shorter electrode. The initial position of the particle, initial direction of the particle, radius of the particle, electrode dimension, applied voltage and air flow velocity are the main factors affecting the trajectories of airborne particles in a micro channel. Modification of the particle size distribution using the technology as a dielectrophoretic filter is more feasible in practice. Sedimentation micro channels in series with the shorter electrode on an alternating side will be beneficial to improve particle sedimentation efficiency.     

Optimization of the extended terminal subfluidization wash (ETSW) filter backwashing procedure

The increased passage of particles and microorganisms through granular media filters immediately following backwashing is a common problem known to the water treatment community as filter "ripening" or maturation. While several strategies have been developed over the years to reduce the impact of this vulnerable period of the filtration cycle on finished water quality, this research involves a recently developed filter backwashing strategy called the extended terminal subfluidization wash (ETSW). ETSW is a method of terminating the backwash cycle with a subfluidization wash for a period of time sufficient to pass one theoretical filter-volume of water upward through the filter. ETSW was shown to remove significantly greater quantities of backwash remnant particles thereby reducing the magnitude of filter ripening turbidity and particle count spikes. Optimum ETSW flow rates were determined for deep-bed anthracite and granular activated carbon filters herein by monitoring filter effluent turbidities and particle counts during the filter ripening period. Optimality of the coagulation process was also shown to influence the magnitude of filter ripening particle passage. ETSW was found to be equally effective for biological and conventional deep-bed anthracite filters.     

基于数字图像的粒状材料细观组构特征分析技术

针对由特殊截面形状的金属棒所组成的理想二维粒状材料,基于数字图像技术,提出了一种细观组构特征分析方法,并编制了程序IPFA,实现了颗粒识别、接触搜索与组构分析等功能。该方法首先对粒状材料试样的原始数字图像进行增强,以消除噪声干扰。在此基础上采用模板匹配技术进行颗粒识别,并将识别结果用于颗粒间的接触搜索,最后给出试样内颗粒长轴方向与接触法线方向等组构特征的统计分析结果。该方法特别适用于由规则颗粒组成的二维粒状材料,识别精度与效率均较高,可作为粒状材料的细观组构特征及其演化规律分析的有效工具。     

Removal of submicron particles using a carbon fiber ionizer-assisted medium air filter in a heating,ventilation, and air-conditioning (HVAC) system

Laboratory tests of particle removal were performed with a pair of carbon fiber ionizers installed upstream of a glass fiber air filter. For air flow face velocities of 0.4, 0.6, and 0.8 m/s, the overall particle removal efficiencies of the filter for all submicron particles were 17%, 16%, and 14%, respectively, when the ionizers were not turned on. These values increased to 27%, 23%, and 19%, respectively, when the ionizers were used to generate ions of 6.0 × 10⁹ ions/cm³ in concentration. The carbon fiber ionizers were then installed in front of a glass fiber air filter located in a heating, ventilation, and air-conditioning (HVAC) system. Field tests were performed in a test office room with a total indoor particle concentration of 2.2 × 10⁴ particles/cm³. When the flow rate was 75 cubic meters per hour (CMH), the steady-state values of the total indoor particle concentrations using the glass fiber air filter with and without ionizers decreased to 0.87 × 10⁴ particles/cm³ and 1.15 × 10⁴ particles/cm³, respectively, resulting in a 25% decrease of the ionizer effect. When the operation flow rate was increased to 115 and 150 CMH, the effect of the ionizer decreased to 19% and 17%, respectively. These experimental data match the results calculated using a mass-balance model whose parameters were determined from laboratory tests.