Effects of floc and bubble size on the efficiency of the dissolved air flotation (DAF) process.

Dissolved air flotation (DAF) is a method for removing particles from water using micro bubbles instead of settlement. The process has proved to be successful and, since the 1960s, accepted as an alternative to the conventional sedimentation process for water and wastewater treatment. However, limited research into the process, especially the fundamental characteristics of bubbles and particles, has been carried out. The single collector collision model is not capable of determining the effects of particular characteristics, such as the size and surface charge of bubbles and particles. Han has published a set of modeling results after calculating the collision efficiency between bubbles and particles by trajectory analysis. His major conclusion was that collision efficiency is maximum when the bubbles and particles are nearly the same size but have opposite charge. However, experimental verification of this conclusion has not been carried out yet. This paper describes a new method for measuring the size of particles and bubbles developed using computational image analysis. DAF efficiency is influenced by the effect of the recycle ratio on various average floc sizes. The larger the recycle ratio, the higher the DAF efficiency at the same pressure and particle size. The treatment efficiency is also affected by the saturation pressure, because the bubble size and bubble volume concentration are controlled by the pressure. The highest efficiency is obtained when the floc size is larger than the bubble size. These results, namely that the highest collision efficiency occurs when the particles and bubbles are about the same size, are more in accordance with the trajectory model than with the white water collector model, which implies that the larger the particles, the higher is the collision efficiency.     

Fundamental characteristics of bubbles and ramifications for the flotation process.

Flotation processes involve the use of very small bubbles (micro-bubbles) to separate particles from water. The process has become a good alternative to sedimentation, especially where the particles are small or of low density. Although the flotation process commences with a collision between particles and bubbles, most research has been focused only on the characteristics of the particles. In this paper, recent theoretical and experimental research on the characteristics of bubbles is summarized. The effect on the collision efficiency of the size and charge of bubbles is calculated through trajectory analysis. The size and charge of bubbles are measured under different conditions and the ramifications of the results are discussed. The results may lead to a better understanding and optimization of the existing process. In particular, we discuss an idea that a new advanced flotation process might be possible by the modification of the characteristics of the bubble alone or of both bubble and particle.     

Particle visualisation--a tool for determination of rise velocities.

Particles from a post denitrifying Kaldnes Moving Bed Process were studied using an optical borescope, a visualisation technique that provides opportunities for both qualitative and quantitative studies of the separation mechanism in Dissolved Air Flotation (DAF). Rise rates for particle/bubble aggregates were estimated showing great variability. Two groups of aggregates were distinguished; relatively small flocs (<100 microm) with single bubbles attached rising comparatively slowly and large flocs (>100 microm) with several bubbles attached rising very fast. The high rise rates for large aggregates are discussed, possibly explained and suggested as the reason for the effective separation of large particles noticed in previous studies. Removal efficiency of different size categories of particles in DAF were investigated on the basis of particle size analysis indicating increased separation efficiency with increasing particle size.     

Investigation of multimodal zeta potential and size distribution in chemical pulp process water.

Multimodal zeta potential distribution in chemical birch pulp process water was studied by filtrating the water into fractions and subsequently measuring zeta potential, charge quantity, turbidity and organic substances. Filtrations were made using 12 microm, 1.6 microm, 1.2 microm, 0.45 microm and 0.1 microm membranes. The number of populations with different zeta potentials diminished with filtration. With the unfiltrated water, 12 microm and 1.6 microm filtrates, three or four different zeta potentials were observed. When the filtration was performed with a 1.2 microm membrane or smaller, only two populations of different zeta potentials were detected. The charge quantity steadily approached zero from unfiltrated water towards a 1.6 microm fraction filtrate. After that, it remained constant. Turbidity constantly decreased when using smaller membranes. The amount of wood extractives decreased to half with 0.1 microm filtration. No significant difference in the amount of carbohydrates and lignin between the filtrated fractions and the unfiltrated water was detected. A comparison dealing with particle size analysis between two different apparatuses was also made.     

絮团与颗粒不等速沉降碰撞研究

絮团和单个颗粒的碰撞、粘结规律对于深入理解粘性泥沙不等速沉降引起的絮凝具有重要意义。在暂不考虑絮团与颗粒间相互作用力的条件下,用不同密度球形颗粒近似描述絮团,利用格子Boltzmann方法模拟了不同密度絮团和单个颗粒静水沉降过程,分析了絮团和颗粒相对运动轨迹和碰撞效率。模拟结果表明,絮团和单个颗粒不等速沉降碰撞过程中,当较大粒径的絮团沉降速度快但密度较小时,单个颗粒相对于絮团运动可能形成封闭轨迹,絮团和颗粒间粘结概率几乎为零,絮团很难通过捕捉单个颗粒而形成更大絮团。     

自由液面附近空化泡的溃灭特性

研究自由液面附近空化泡的溃灭特性对认识水利水电工程中的空蚀破坏机理具有重要意义。为进一步认识自由液面附近空化泡的溃灭特性,开展了电火花激发空化泡试验。结果表明:(1)空化泡与自由液面距离较近时总是背离自由液面溃灭,背离的临界条件为空化泡与自由液面的距离约是空化泡最大半径的5倍。空化泡尺寸越大或空化泡与自由液面距离越近,其背离现象越明显。(2)自由液面附近双空化泡相向溃灭融合,且融合体背离自由液面移动。(3)在自由液面和空气泡的共同影响下,空化泡的溃灭方向是二者分别对空化泡作用的矢量和。通过本文研究,对自由液面附近空化泡的溃灭有了定量认识,可以在一定条件下预测空化泡的溃灭方向。     

High-speed experimental photography of collapsing cavitation bubble between a spherical particle and a rigid wall

The synergetic effects between cavitation bubbles and silt particles on the damages of materials are essential problems in fluid machineries. For studying the underlying microscopic mechanisms, in the present paper, the dynamic behaviors of a single cavitation bubble between a spherical particle and a rigid wall are experimentally investigated with a high-speed camera. The results indicate that the existence of the particle can affect the bubble shape during collapse and significantly accelerate the collapse velocity of the bubble. The influences of the particle size, the distance between the bubble and the particle and the distance between the bubble and the rigid wall on the phenomena are qualitatively and quantitatively analyzed. These parameters can prominently affect the collapse velocity of the bubble(especially its maximum value).     

Tertiary filtration in small wastewater treatment plants.

Tertiary filtration can be proposed in small wastewater treatment plants with impact on protected water bodies. Rotating disk filters may be adopted, in respect to conventional sand filters, when low availability of space and low investment costs are the prevailing conditions. The overall objective of this research was to evaluate the filtration efficiency of rotating disk filters; to compare effectiveness with traditional sand filters; to analyse thoroughly the importance of particle size distribution in wastewater tertiary filtration. In the experimental activity, conventional wastewater quality parameters were investigated and particle size distribution (PSD) was characterized to discuss the filter effectiveness. The effect of design and operation parameters of tertiary filters were discussed related to particle removal curves derived from particles counts. Analysis of particle size distribution can be very useful to help comprehension of filtration processes, design of filtration treatments and to decide the best measures to improve filter performance.     

Zeta potential measurement on the surface of blue-green algae particles for micro-bubble process.

Any kind of blue-green alga produces metabolites of musty substances and toxins. Therefore, it is necessary to remove the blue-green algae, and processing also including nutrient removal is desired for the water quality improvement of eutrophic lakes. The purpose of this study has been to investigate the possibility of a flotation system using a hybrid technique (chemical compounds and electrostatic bridge) applied to raw water containing phytoplankton with high pH of water, and to examine the zeta potential value of phytoplankton surface and the removal efficiency for phytoplankton, ammonia, nitrogen, and phosphoric acid.The results were as follows: firstly, zeta potential of M. aeruginosa particles was observed to achieve charge neutralization on their surface by adhesion of magnesium hydroxide precipitation with increasing pH. Secondly, maximum removal efficiency concerning chlorophyll-a was observed as 84%, and this efficiency was obtained in the condition of pH > 10, and magnesium hydroxide precipitation was observed. Thirdly, in the pH condition that the maximum removal efficiency of chlorophyll-a was obtained, the removal efficiency and the amount of decrease of NH(4)-N and PO(4)-P before and after the change of pH values were observed as 6.7% (0.04 mg-P/L) and 63.6% (0.07 mg-N/L), respectively.     

叶栅含沙水流中沙粒运动的数值模拟

利用Lagrangian颗粒运动方程，数值求解了叶栅含沙水流中沙粒的运动。定义了撞击效率，给出了来流雷诺数(或来流速度)，来流冲角和沙粒尺寸对撞击效率的影响。数值模拟了不同来流雷诺数，来流冲角和尺寸的沙粒在栅中的运动轨迹，考虑了壁面、边界层、尾迹及少粒与壁面碰撞对颗粒运动的影响。数值分析了沙粒对叶栅固壁的冲击情况，数值模拟结果与水涡轮机械中所表现的磨损情况较为一致。     

Particle count and size alteration for membrane fouling reduction in non-conventional water filtration.

If coagulation is not completely successful and produces aggregates which are too small, fouling may increase. In some cases, a deep-bed filter could perhaps provide a solution. The paper examines these effects using experimental results for different waters. Activated sludge effluents, stormy seawater containing microalgae and spent filter backwash water (SFBW) were coagulated by alum or ferric chloride. Sand filtration tests were carried out. Tests were performed in a membrane filtration stirred cell, filtration pilot plant equipped with SDI analyzer (seawater) and pilot UF plant (SFBW). For activated sludge effluent, alum residual ratio curves of turbidity and total particle count (TPC) followed one another. With ferric chloride, low coagulant dosage showed negative turbidity removal. Contact granular filtration reduced membrane fouling intensity. Increasing the dose resulted in higher improvement in membrane flux. For seawater, a filter run period under storm conditions reached 35 hours with satisfactory filtrate quality. An iron chloride dose of 0.3 mg/l during normal conditions and 0.5 mg/l for stormy condition should be injected, mixed well before the filters, while maintaining 10 m/hr filtration rate and pH 6.8 value. For SFBW, alum flocculation pretreatment of SFBW was effective in reducing turbidity, TPC, viruses and protozoa. SFBW settling prior to flocculation did not enhance turbidity and TPC removal. The largest remaining particle fraction after alum flocculation was 3-10 microm in size, both Cryptosporidium and Giardia are found in this size range. Coagulation enhanced the removal of small size particles, a positive impact on reducing membrane fouling potential.     

液固悬浮液中气泡生成的数值模拟

将Level Set方法与硬球离散颗粒方法相结合,建立了一个三相流混合模型。应用该模型对液固悬浮液中气泡从单孔及多孔中的形成过程进行了三维模拟,并与文献中已有的结果进行了定性比较。数值试验主要比较了在悬浮液与纯液体中的气泡生成与上升行为。模拟结果表明,颗粒的存在对气泡的形成与上升有重要影响;气泡诱导的流体流动对气泡与颗粒的运动均有显著地影响。     

Helminth eggs removal by microscreening for water reclamation and reuse.

Irrigation with reclaimed water is becoming a practical alternative to conventional irrigation in semi-arid areas of the Mediterranean like Spain, but it requires a reliable treatment process to provide a safe water supply. Helminth eggs are one of the main concerns for the safe use of reclaimed water, as they can survive adverse environmental conditions and they are highly infective. Spanish water quality criteria and International guidelines set a limit of 0.1 eggs/l for water uses with unrestricted human exposure. Two microscreening processes have been tested to determine their potential for helminth eggs removal, after a conventional physic-chemical reclamation process. Hydrotech Drum and Discfilters, provided with 10 microm pore size filter cloth, were tested to determine their efficiency for helminth eggs straining. An experimental test was conducted using 20 microm spherical latex particles, as surrogates for helminth eggs, to test the removal efficiency of a small full-scale drumfilter. In a subsequent laboratory test, actual Trichuris suis eggs were strained using a 10 microm pore size filter cloth from a discfilter. Results from both tests indicate that drum and discfilters are able to achieve 99% removal efficiency for spherical latex particles and a complete removal for helminth eggs in reclaimed water.     

Effects of surfactant on bubble hydrodynamic behavior under flotation-related conditions in wastewater

Bubble behavior is fundamental to the performance of froth flotation operations used in wastewater treatment processes. To fully understand and characterize bubble behavior under flotation-related conditions in wastewater, the high-speed photographic method has been employed to examine the motion of single bubbles and size distribution of bubble swarms with intermediate sizes ranging from 1 to 4 mm in the presence of surfactants in a laboratory scale flotation column. Both distilled water and synthetic municipal wastewater have been used to make solutions as well as two types of common surfactants. The instantaneous bubble motion has been recorded by a high speed camera. Subsequently, bubble trajectory, dimensions, velocity and distribution have been determined from the recorded frames using the image analysis software. The experimental results show that the addition of surfactant into wastewater has similar effects on bubble hydrodynamic behavior as in pure water (e.g., improving trajectory stabilization, dampening bubble deformation, slowing down terminal velocity, reducing bubble size and increasing the specific surface area of bubble swarm) due to the Marangoni effect. However, it is interesting to note that surfactant effects on single bubble hydrodynamics in wastewater are slightly stronger than those in pure water while surfactant effects on size parameters of bubble swarms in wastewater are significantly stronger than those in pure water. This finding suggests that besides surfactant, inorganic salts present in synthetic wastewater have an important influence on bubble dispersion.     

滨海含水介质胶体释放的粒度和动电性质表征

为了研究滨海地区含水介质胶体在不同水动力条件下的释放规律,在野外调查的基础上,采用室内土柱实验研究不同水力梯度下胶体释放的动态特征,同时表征了释放胶体的粒径分布、ζ电位和电泳淌度。结果表明:随着水力梯度的增大,释放胶体浓度逐渐增大,到水力梯度为10时突增达到峰值,随后浓度逐渐降低,变化幅度小;胶体累积释放量随着孔隙体积数的增大呈线性增长,不同水力梯度下增长幅度不同;在低水力梯度下粒径小的胶体优先释放出来,胶体的ζ电位、电泳淌度大多数为正,而在高水力梯度下粒径大的颗粒也会释放出来,胶体的ζ电位、电泳淌度大多数为负。     

不同孔径平板膜对景观水体水质控制效果的试验研究

为研究平板膜处理景观水体的特性,采用孔径为0.1 μm、0.2μm和0.3μm平板膜处理景观水体用水.结果表明,三种膜对悬浮物和藻类有很好的去除效果,去除率都在95％以上,对于溶解态的N、P去除效果不好;三种膜处理效果相比较,孔径最小的0.1μm膜对各种污染物去除效果最好;在过滤不同污染物配水时,0.1μm膜的膜通量下降速率最慢,稳定后较0.2μm和0.3μm膜的膜通量大;通过对污染物的粒径分析和膜表面污染物分析认为,景观水体中,主要污染物对膜通量的影响顺序为:腐殖酸＞藻＞悬浮物;150 nm是景观水体中污染物质粒径的分界点,污染物质粒径主要集中在150 nm以上;与0.2μm、0.3μm膜相比,0.1μm膜抗污染能力更强,更适合用来处理景观水体用水.     

Particle size distribution and property of bacteria attached to carbon fines in drinking water treatment

The quantitative change and size distribution of particles in the effluents from a sand filter and a granular activated carbon （GAC） filter in a drinking water treatment plant were investigated. The average total concentration of particles in the sand filter effluent during a filter cycle was 148 particles/mL, 27 of which were larger than 2 μm in size. The concentration in the GAC effluent （561 particles/mL） was significantly greater than that in the sand filter effluent. The concentration of particles larger than 2 μm in the GAC filter effluent reached 201 particles/mL, with the amount of particles with sizes between 2 μm and 15 μm increasing. The most probable number （MPN） of carbon fines reached 43 unit/L after six hours and fines between 0.45 μm and 8.0 μm accounted for more than 50%. The total concentration of outflowing bacteria in the GAC filter effluent, 350 CFU （colony-forming units） /mL, was greater than that in the sand filter effluent, 210 CFU/mL. The desorbed bacteria concentration reached an average of 310 CFU/mg fines. The disinfection efficiency of desorbed bacteria was lower than 40% with 1.5 mg/L of chlorine. The disinfection effect showed that the inactivation rate with 2.0 mg/L of chloramine （90%） was higher than that with chlorine （70%）. Experimental results indicated that the high particle concentration in raw water and sedimentation effluent led to high levels of outflowing particles in the sand filter effluent. The activated carbon fines in the effluent accounted for a small proportion of the total particle amount, but the existing bacteria attached to carbon fines may influence the drinking water safety. The disinfection efficiency of desorbed bacteria was lower than that of free bacteria with chlorine, and the disinfection effect on bacteria attached to carbon fines with chloramine was better than that with only chlorine.     

Influence of biofilm on removal of surrogate faecal microbes in a constructed wetland and maturation pond.

The effect of biofilm on the attenuation of pathogen-sized particles from wastewater was compared for biofilms cultivated in a surface flow constructed wetland (SFW) and maturation pond (MP) The fate of fluorescently labelled microspheres (FLM) as surrogates for viruses (0.1 microm), bacteria (1 microm) and parasitic protozoa (4.5 microm dia) was investigated in microcosms in the presence or absence of biofilms. Rates of FLM removal from suspension were higher in the presence of biofilms for all particle sizes (kd 0.02-0.11 h(-1)) in MP and SFW microcosms with removal efficiency related to particle size and biofilm thickness and structure. Greater removal of 0.1 microm (79-81%), 1 microm FLM (92-96%) and 4.5 microm FLM (up to 98%) from suspension were found for microcosms containing thicker (autotrophic) biofilms grown in the MP or open water zone of the SFW. Lower removal of 43% (0.1 microm), 59% (1 microm) and 84% (4.5 microm) occurred in microcosms containing thinner heterotrophic biofilms from SFW vegetated zones. Providing surfaces for attachment of photosynthetic biofilms offers potential to enhance pathogen removal in open water systems. In vegetated systems, linkage to more oxic openwater zones may allow thicker and 'stickier' epiphytic biofilms to develop, improving pathogen interception and removal.     

Numerical analysis of bubble dynamics in the diffuser of a jet pump under variable ambient pressure

Recent studies have shown that the collapse of cavitation bubbles in a jet pump can generate an extremely high pressure with many potential applications. The dynamics of the bubble is governed by the Rayleigh-Plesset equation. With the bubble dynamics equation and the heat and mass transfer model solved with the Runge-Kutta fourth order adaptive step size method, the oscillations of the bubble in the diffuser of the jet pump are assessed under varied conditions. To obtain the pressure variation along the diffuser, the Bernoulli equation and the pressure measured in experiment are coupled. The results of simulation show that a transient motion of the bubbles can be obtained in the diffuser quantitatively, to obtain the pressure and temperature shock in the bubble. Moreover, increasing the outlet pressure coefficient would result in a more intense bubble collapsing process, which can be used in the subsequent studies of the cavitation applications. The predictions are compared with experiments with good agreement.     

Influence of compost characteristics on heavy metal sorption from synthetic stormwater.

This paper has the aim to assess the ability of garden derived compost to remove dissolved heavy metal contaminants typically found in stormwater. Compost was found to have excellent chemical and physical properties for the sorption of dissolved metal ions (Cu2+, Pb2+ and Zn2+). Batch sorption data were used to determine the sorption efficiency of Cu (93%), Zn (88%) and Pb (97%) by compost. The relative sorption affinity of these metals by compost is found to be in the order of Pb2+ > Cu2+ Zn2+. The effect of different particle size fractions of compost upon the sorption of Cu was also investigated. Sorption conformed to the linear form of the Freundlich isotherm and can be considered favourable because the sorption intensity values obtained in this study are between 0.1 and 1. Compost with a smaller particle size fraction has larger surface areas and greater sorption than the larger particle size fraction. Compost derived from garden waste is efficient for removal of heavy metals from wastewater or treating water for industries.