環境工学

In recent years, air-bubble plumes have been applied as a hydraulic measure for water quality improvement to some Japanese water supply reservoirs. In order to optimize the hydraulic design of air-bubble systems, the relationship between mixing efficiency and system parameters must be clarified. Based on a review of previous studies, this paper discusses (1) analytical methods of air-bubble plumes in both homogeneous and nonhomogeneous environments, and (2) analytical methods of flow in lakes or reservoirs induced by air-bubble plumes. Examples of prototype designs for destratification and hypolimnetic aeration are presented. Future studies needed are summarized finally.

In recent years, as people have become more and more health conscious, they have taken a greater interest in bathroom amenities. As a result, whirllpool baths have become increasingly popular, because consumers believe that whirlpool baths are effective for relieving fatigue. This report introduces TOTO's applied technique, which controls the jet stream mixed air (gas-liquid two-phase flow) according to somatic sensations, and in addition, submits subjects for further research.

The drought of 1994 in northern Shikoku was very severe, but Shikoku island as a whole has large amount of rainfall which causes flooding in the southern part. Regional unbalance of water resources is the main cause of the water problems in Shikoku island: flooding in the South and drought in the North. The circumstances of water in Sikoku island which cause the above mentioned problems are described from the view points of geography, regional rainfall distribution, discharge and water resources development of main rivers. The reasons for the recent tendency of increasing water shortage in Matsuyama are insufficient water resources development of groundwater or surface water, increase of water demand due to the growth of population and the change of life style to use more water. Besides these reasons, extremely small amount of rainfall in summer made the shortage of water disastrous in Matsuyama in 1994.

Carbon dioxide is regarded as one of the greenhouse gases. The methods of recovering, fixing and sequestering carbon dioxide are being developed in many parts of the world. Concerning the carbon dioxide recovering method, in Japan, the electric power industry initiated research on carbon dioxide recovery methods well ahead of other industries. Tokyo Electric Power Company (TEPCO) has been tackling the environmental problems such as air pollution caused by SOx and NOx in view of their managerial importance. Also, we are implementing effective countermeasures for carbon dioxide mitigation. In this paper, we will introduce mainly TEPCO research on the carbon dioxide removal technology.

CO2sequestration in ocean posseses a characteristic of multi-phase flow where CO2 clathrate-hydrate is formed at the interface between liquid CO2 and seawater in high pressure condition in ocean. The present report describes following three items of multi-phase flow in CO2 sequestration in ocean which are dissolution of liquid CO2 droplets at intermediate sea depth and formation of liquid CO2 pool at a seabed deeper than 3000m. The first one is an experiment of liquid CO2 dynamic behavior affected by hydrate-clathrate, which are CO2 injection from a nozzle, formation of CO2 pool and propagation of hydrate-clathrate film induced by the nucleation. The second one is measurement of liquid CO2 droplet dissolution rate in the pressurized water flow with clathrate on the surface, which is important in dissolution of liquid CO2 droplets at intermediate sea depth. Effects of flow velocity, temperature and pressure on the dissolution rate were clarified. Further, the dissolution behavior of liquid CO2 droplets dispersed in the sea water has been numerically simulated, where the effects of released droplet size and ambient CO2 concentration have large effect on the dissolution behavior due to the CO2 clathrate-hydrate. The last one is an experiment of dissolution of liquid CO2 pool which simulates liquid CO2 stored at a seabed and dissolves into an undercurrent flow. The measured surface concentration was applied to the estimation of time-scale for dissolution, where the mass transfer coefficient was obtained from the actual conditions of deep ocean data.

The increasing of the CO2 concentration in the atmosphere might be induced by consumption of fossil fuel and destruction of tropical forests. In the research field of global warming, clarification of the global circulation mechanism of CO2 is one of the important targets. Now, many kinds of option to reduce global warming have been proposed and researched. CO2 sequestration into the ocean is one of hopeful methods from a view point of feasibility. CO2 transfer mechanism between the atmosphere and the ocean in the North Pacific Ocean and the CO2 circulation in the ocean are discussed, which has been surveyed in NOPACCSproject of MITI. Still more, the new method which can dissolve CO2 in shallow sea-water and transport the solution to the deep sea-water is discussed. The method is an utilization of a gas lift effect by CO2 bubbles in the pipeline inverse J shape.

Theoretical studies of processes for dissolution of CO2at 500-2000m depth in the ocean have revealed a potential of this method as a way to reduce the CO2emissions to the atmosphere, and thus reduce the impacts of such emissions on the global climate and environment. However, complexities associated with the mass transfer of liquid CO2from droplets via hydrate films to dissolution in seawater, as well as the dynamics of buoyant dropletplumes, influenced also by the increase in seawater density due to dissolution of CO2, precludes firm conclusions to be drawn about the vertical distribution of CO2in the water column following injection. Numerical model results indicate that the fate of disposed CO2is strongly dependent on droplet size. If small droplets can be created and maintained, open ocean outlets may become a practical disposal technique. However, the pH of water peeling off from the plume is so low that marine life outside the plume would be adversely affected.

Groundwater contamination due to chlorinated hydrocarbons like trichloroethylene and tetrachloroethylene has been a great environmental issue in Japan. Because these substances are volatile and little soluble in water, and can exist in unsaturated zone as vaporized, dissolved and undiluted non-aqueous phases, they migrate as gas-water-liquid three phase flow in subsurface. Recently, the air sparging coupled with the soil vapor extraction as the physical remediation technologies has been applied to a site contaminated with chlorinated hydrocarbons. The air sparging technology facilitates the contaminant volatilization with air injected into saturated zone and the soil vapor extraction technology extracts the contaminant in soil and injected gas. Therefore, in order to establish these technologies, it is indispensable to understand the mechanism of gas-water two phase flow in subsurface.

Possibility of CO2 underground disposal in Japan was studied by some authors. Summaries of the results were introduced first. Then, capacity of the disposal amount was estimated for geologically classified reservoir types. The result indicated that enough capacity existed in Japan. Finally an example simulation study to investigate distribution of CO2 injected from surface into the reservoir was explained.

In order to remove polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs) in exhaust from refuse incineration plants, the treatment of the gases by using a fabric filter with activated carbon injection is available as one of the applicable methods. This method using a fabric filter with activated carbon enabled efficient removal of PCDDs/PCDFs in operating plant. In particular, the concentration of PCDDs/PCDFs was successfully reduced to 0.1ng/Nm3-TEQ.

The new law regulation for dioxins has been published in Dec. 1997, and it gives a special attention to municipal solid waste incinerators. In order to observe the strict regulation for dioxins, the greater part of existing municipal solid waste incinerators must be carried out some reduction of dioxins. This paper presents the removal systems for dioxins using Activated Coke (AC). These systems are effective for reduction of dioxins at existing municipal solid waste incinerators.

For the reduction of PCDDs/DFs emission, catalytic decomposition method is one of the useful high technological waste gas treatment system. In this paper, on the flue gas of MSW incineration plant, study as to decomposition of PCDDs/PCDFs by catalyst is described.

In Japan, mainstream municipal waste processing is by incineration, and this has been established from the viewpoint that it offers stable, sanitary processing for large quantities and that it enables a great reduction in the ultimate disposal volume.However, the annual volume of municipal waste has a tendency to increase year by year.With the decreasing capacity of ultimate disposal sites, further reduction for volume of incineration ash both in municipal solid waste (MSW) and sewage sludge is required.In addition, secondary pollution by the harmful components of heavy metals or dioxins eluded from landslide ash is feared, and the inactive and stabilization of incineration ash has become an increasingly important issue.The melting and solidification process, in which ash is heated up to a temperature higher than the fusion point and melted, then cooled to solidify and recovered as slag, is considered an effective technique that will meet the expectations mentioned above.And, several processes that differ from heat source for ash melting for each other have been appeared in these days, and are divided into two groups equipped with combustion devices or electrical heated up devices.Reported in this paper are the characteristics of typical incineration ash first.And next, ash melting performance, such as behavior of heavy metals contained in ash, the characteristics of the melt and solidification slag for sample ashes taken from different types of incineration are described.These report are based on operation of our established two types of melting furnace, “swirling flow melting furnace” and “plasma melting furnace”. The first one is included in combustion melting methods, and other is in electrical melting methods.Both processes enable the melting furnace to melt a fly ash individually that contains harmful components.As a swirling flow melting furnace, we have operated commercial plants, with the maximum capacity of 120ton/day.On the other, we have operated commercial plant of plasma melting furnace with the capacity of 5ton/day.This commercial plant has the advantage to enables the plasma furnace to melt fly ash of the fluidized-bed incinerator individually.Based on the performance of these plants, we are now going to optimize the design for more large size melting furnace with stable operation.

Hydrates have peculiar crystal structure that one guest molecule is surrounded with a cage composed by water molecules.These water molecules are connected by hydrogen bonding each other, but there is no chemical bonding between the guest molecule and the water molecules.In this short monograph I will mention the possibility of hydrate's application to solve environmental problems.The cage-like structure of hydrates can be utilized as an environmental pollutant gas separation method since in mixture hydrates the mole fraction of components in a hydrate phase is different from that in a gas phase.The outline and principle of this technique is considered to be similar to the separation technology using phase equilibrium.Thus firstly the separation technology using phase equilibrium is briefed, then properties of hydrates concerning with the separation technology are summarized concisely.After that the separation technology with hydrates is introduced.

When a shock wave propagates over a dust layer composed of many small solid particles, lifted and dispersed particles form a dust cloud in the shock-induced flow. The dust cloud has a possibility of causing a propagating dust explosion in galleries of the coal mine and factories which treat flammable powder, so that the analysis and prediction of this phenomenon are very important to forestall the dangerous explosion. In this paper, two results of numerical simulations about the initial process of this phenomenon are introduced. The first result shows that a particle on a wall is lifted by the shock-induced flow, and a lift force on the particle changes with the clearance between the particle and the wall. The second result shows that many particles are lifted and dispersed in the shock-induced flow, and the upward velocities of lifted particles were given by particle-particle interactions rather than by fluid lift forces such as Saffman force and Magnus force. Furthermore, the computed dust cloud was almost same as observed in experiments.

It is an increasing concern that the depletion of dissolved oxygen (DO) concentration at the bottom layer of the northern-part of Lake Biwa with 100m in maximum depth has been worsen in recent years not only due to eutrophication in water but also the effects of global warming such as the raise of air temperature during winter. Many research have been carried out by various organizations to clarify the mechanism of DO depletion near the lake bottom and to propose the countermeasures. This article firstly describes the seasonal variations of the vertical water temperature and water quality distributions to explain the fundamental features of heat and mass transfer occurring in the northern part of Lake Biwa throughout the year. Since it is well known that the thermal convection during the cooling period is the main mechanism of DO transfer from the surface to the lake bottom, the mixing processes caused by thermal convection are shown using the results of a 3D CFD simulation. Then, the electrolysis of water at the lake bottom for the restoration and DO recovery is introduced as one of the countermeasures proposed by Lake Biwa Environmental Research Institute, Shiga Prefecture. The simulation results of O2 bubble plumes with the dissolution into lake water are also shown under the conditions of laboratory and field experiments.

As semiconductor-devices become more highly integrated and their geometry continues to shrink, conventional aqueous (or liquid-phase) cleaning/drying tends to collapse high-aspect-ratio fragile LSI (large-scale integrated circuit) structures due to the high surface tension of aqueous chemicals and water. The use of physical cleaning aids such as ultrasonic agitation can also cause damage to the fragile nano-structures. The process window for damage-free cleaning is becoming narrower as the device geometry shrinks every year. This makes the development of novel damage-free cleaning methods a high priority, with preferably less chemical/water consumption. In this paper, the problems of aqueous cleaning of the semiconductor surfaces are briefly reviewed and then various alternative cleaning techniques to employ multiphase flows are described and discussed which include gas-injected megasonic cleaning, high-pressure atomizing jet spray cleaning, and cryogenic aerosol cleaning. There will be more research challenges and business opportunities in the environmentally-benign multi-phase flow technologies in the near future.

With a life style of healthy and sustainable (LOHAS) intention, superheated steam cooking ovens have attracted attention recently. According to the main issue of this review, the establishment of a resource saving society, we introduce an outline of the principle of superheated steam including its characteristics; the generating apparatus itself, the application of food processing including the sterilization, drying and environmental fields. Superheated steam is steam at a temperature higher than water's boiling point. If saturated steam is heated at constant pressure, its temperature will rise, producing superheated steam. To produce superheated steam in a power plant or for processes (such as cooking), the saturated steam, from the steam drum is passed through a super heater. Superheated steam has three heat characteristics; condensation heat transfer, convection heat transfer, and emission heat transfer, respectively. In the present review, we describe the induction heating of a carbon element for a generator. By using these merits, we applied them for food processing, including household cooking. Besides energy saving and environmental benefits, the oxygen-free atmosphere appears to improve product quality by retarding some deterioration reactions in food materials. Under-atmospheric pressures are often required with foodstuffs, which are sensitive to high temperatures due to denaturalization reactions. These benefits increase the added value of foodstuff, including unused or discarded natural resources by changing them into a powder. Superheated steam characteristics are useful for environmental remediation, especially recycle and reuse of active-charcoal and the washing and drying processes of the drum tube recycling industry and the same time we show not only the total cost saving, but also decreased dramatically energy and water exhaustion. From our evidence, we propose that superheated steam technology is useful in the food processing and environmental industries, but have a high potential for other industrial applications in the near future as well.

JAEA has been conducting diversified activities on the environmental remediation for Fukushima after the Great East Japan Earthquake, including radiation monitoring, decontamination demonstration, public communications, and related R&D. The decontamination pilot project was carried out in the off-limit and scheduled evacuation zones, where existing and/or improved clean-up techniques were tested their applicability and efficiency, waste storage was established, and how to secure workers’ safety for radiation was investigated. Related R&D has also been continued, such as computer simulation for decontamination planning, and aircraft radiation monitoring.

Development of closed-loop air revitalization producing oxygen (O2) from carbon dioxide (CO2) exhaled by astronauts is required for future manned space missions. This paper shows recent research on two of three subsystems of a closed-loop air revitalization system: water generation form CO2 reduction and water electrolysis to provide O2. Practical large-scale tests of CO2 reduction and water electrolysis showed that various gas-liquid two-phase conditions in a reactor would be important to establish systems operated under microgravity.

Over 30 years, we have accumulated technologies of heat exchange, dehumidification, and VOC concentration by TSA method using functional honeycomb rotors. By combining these technologies and newly developed CO2 concentration technology, we have developed CO2 separation and recovery apparatus anew. A part of the flue gas from the stack of an incineration plant was introduced into the CO2 separation and recovery apparatus. After removing moisture and hazardous chemical from the flue gas, CO2 was separated and recovered. By utilizing waste heat from incineration plant as a heat source, reducing CO2 recovery cost was suggested.

PSA system for the recovery of carbon dioxide from blast furnace is examined. From the results of bench plant experimental operation, it was revealed that recovery cost of carbon dioxide was below 2,000 yen per ton. Further experimental operation to assure the commercial PSA system is ongoing.

Catalytic hydrothermal oxidation of 4-chlorophenol (CP) has been examined using the slurry flow reactor system. The mixed suspension containing CP, hydrogen peroxide and Cu2O catalyst was reacted with pressure, temperature and flow rate controlled at 10 MPa, 100-200°C and 1.0-16.0 cm3/min, respectively. The remarkable enhancement of CP decomposition by Cu2O catalyst was observed at 133-166°C. The semi-quantitative analysis of hydroxyl radicals generated by catalytic hydrothermal oxidation using salicylic acid as a molecular probe revealed that the production of hydroxyl radical was substantially enhanced by Cu2O catalyst at the same temperature as the decomposition of CP was enhanced. It was thus indicated that hydroxyl radicals were formed by Fenton-like reaction with Cu+ ions. It was also found that the degradation of CP was complete in less than 10 sec and that oxalic acid was produced as an intermediate.

Several types of air pollution control technologies about waste combustion system were developed on the last quarter century. If those new technologies have come into wide use, we ought to explain several new technology convincingly to customers or users. Therefore I will take a few examples of explanations about those new technologies.

Microbubbles are the bubbles of one to several hundred micrometers in diameter. Depending on the work field, their sizes are less than 100 μm for bioactivity. The microbubble technology has become more and more popular in many fields of science and engineering since its birth in “The Japan Kosen Association for College of Technology” in 1997. Recently, a considerable number of studies have been conducted on the effect of microbubbles on the environmental, industrial, food and medical fields. However, the basic characteristics and efficient application methods of microbubbles remain unclear. In this paper, the basic knowledge for the use of microbubbles in a variety of practical applications is indicated.

In recent years, it has been defined by the International Organization for Standardization (ISO) that fine bubble of 100 μm or less be referred to as fine bubble. Fine bubbles will have specific properties different from bubbles that are seen in everyday life such as millimeter size and centimeter size by refining the gas. For example, the total surface area of a plurality of fine bubbles having the same volume as a single bubble of centimeter size is significantly large, and the chemical reaction, physical adsorption and mass transport at the gas-liquid interface are dramatically improved. Moreover, the fine bubbles have such property that dissolution efficiency of internal gas is improved from a huge total surface area acquired. In this study, cleaning and purification were performed utilizing the specific properties of these fine bubbles. In cleaning, the research was conducted on the removal of oils adhered to polypropylene and the removal of salt fixed to aluminum foil. Regarding the oil removal, the removal effect that relies on the total surface area of fine bubbles was verified. Regarding the removal of fixed salt, the result that by efficiency improvement of the removal of fixed salts is attained from phenomena such as fine bubble dissolution and bubble formation again. On the other hand, in the purification, fine bubbles were supplied to a river model tank containing the actual river sediment, and the changes in the water quality were investigated. As a result, the oxide layer of the sediment was formed from the high dissolution efficiency of the fine bubbles, and the suppression of elution of phosphorus from the sediment was verified.

Transport of fine bubbles (FBs) in porous media has drawn increasing attention, as a promising technology for soil and groundwater remediation. Understanding the transport characteristics of FBs in soils is essential to optimize FB-based remediation techniques. FB transport is highly influenced by flow rate, gas species, chemical properties of FB water such as pH and ionic strength. Transport models for colloidal particles including colloid filtration theory are applicable for characterizing FB transport in the porous media. In addition, DLVO theory representing interaction energy between colloidal particles and collector was helpful for understanding attachment mechanism of FBs in the porous media.

Non-aqueous phase liquids (NAPL) are immiscible materials and their flow behavior in the soil is multi-phase flow. In general, NAPL is categorized as contaminant fluid in the ground so it is important to study the contamination and remediation mechanism of NAPL in the ground. Micro focused X-ray computed tomography (CT) scanner is a powerful tool to observe inner condition of soil. The objective of this study was to understand the flow behavior of NAPL in soil. In this paper, experimental study was introduced to visualize the behavior of NAPL. In particular, it was explained that the study using X-ray CT to visualize the pore structure of sandy soil contaminated by light-NAPL (LNAPL) and remediated by flushing water and air. The result suggested that increase of the air flow rate did not attribute to reduce the volume of LNAPL blob in the range of capillary number which was defined as fingering phenomenon. However，different conditions of residual LNAPL blobs were found out significantly with increase of injection rates. The result of further image analysis, which could determine the number and size of blobs, indicated that it was caused by interaction of entrapment by wetting phase liquid and small pore.

A chemical transport model (CTM) is a type of numerical model that represents atmospheric behavior of various trace substances. This article describes a regional CTM and its application to numerical simulations of particulate matter with a diameter of 2.5 μm or less (PM2.5). A regional CTM is driven by meteorological fields produced by a meteorological model with emission data derived from various emission inventories and boundary concentrations calculated by a global CTM. Two sets of PM2.5 simulations are presented: one is an application of a regional CTM with a tracer method to an estimate of the contribution of long-range transport (LRT) from the Asian Continent to PM2.5 concentration in Japan, and the other is an application of an online-type regional CTM with meteorology-chemistry feedbacks to an evaluation of the impacts of the aerosol direct effect on simulated PM2.5 fields in East Asia.

Membrane bioreactor (MBR) is a promising technology for future wastewater management. However, occurrence of membrane fouling in the process hinders widespread application of this technology. There are two types of membrane fouling: physically reversible fouling and physically irreversible fouling. The former is the fouling that can be cancelled by physical cleaning such as hydraulic backwashing, whereas the latter is the fouling that needs chemical cleaning to be cancelled. In the operation of MBRs, physically reversible fouling is usually dominant. Therefore, control of physically reversible fouling with a low energy consumption is important. In most of commercialized MBRs, control of physically reversible fouling is attempted by intensive aeration through aerators placed beneath the membrane modules. Various design factors including bubble size, air flow rate and arrangement of aerators and membranes would affect the efficiency of cleaning by aeration. Currently, however, it is totally unclear what is occurring in MBR tanks: multiphase flow in MBRs composed of biomass suspension with high viscosities, bubbles and membranes is too complex to monitor. As a result, no rational method for design of aeration for MBRs is available, which leads to considerable underestimation of the performance of MBRs. Application of advanced research methods of multiphase flow could change this situation: rational design of MBRs is enabled by the aid of cutting-edge knowledge on multiphase flow.

Membrane separation technologies has been widely applied to water treatment and importance of fouling mechanism of membrane and development of fouling mitigation methods is now one of the key issued in water treatment technologies using membrane separation. Outline of membrane bioreactor (MBR) technology, which uses membrane separation as solid-liquid separation is here explained and two examples of experimental research to investigate influence of air-water multiple phase flow characteristics on the performance of the MBR performance are shown.

Although water is a continuous phase, water treatment tanks often contain solid particles and bubbles, thereby adding a dispersed phase. Thus, water treatment tanks will often contain a multi-phase fluid. The solid particles and bubbles contribute significantly to the formation, promotion, and suppression of circulating flows that have a significant effect on water treatment performance. Even with just a small number of bubbles present in the dispersed phase, a density current is generated and a circulating flow is formed－nd a similar outcome is seen, when solid particles exist in the dispersed phase, due to the high bulk density. However, when the concentration of a solid is very high, friction between the solid particles increases, thereby greatly affecting the sludge viscosity. The increased viscosity suppresses the formation of circulating flows, which reduces the mixing performance of the solution. The circulating flow in an aeration tank or dissolved air flotation (DAF) tank and settling tank then have low oxygen transfer and solid separation efficiencies, respectively. This undesirable situation is due to the suppression of a circulating flow, which prevents deposition of the solid particles in the aeration tank and reduces the mixing efficiency of the anaerobic digester. Computational fluid dynamics (CFD) helps to investigate the effects of multi-phase flow on the performance of water treatment tanks. In this study, thermal convection in settling tanks, sponge-media deposition in an aeration tank, mixing performance in an anaerobic digester, and the rheology of dense anaerobically-digested sludge will be studied, and activated sludge model (ASM) and CFD coupled simulation for simultaneous nitrification and denitrification in shallow aeration tanks will be performed.

The objective of this study is to elucidate the formation and growth behavior of CO2 hydrate. Based on hydrate film thickness measurements, the process of hydrate film development was classified into "formation" and "growth". A macro-scale thickness prediction model based on mass transport was constructed for these processes. Molecular dynamics calculations were also introduced and combined with the macro-scale prediction model to construct a model capable of predicting film thickness.

This review is concerned with a physical cleaning method based on ultrasound-superposed water jets that collide with cleaning targets and spread over their surface; this method is called ultrasonic water flow cleaning and can be used in semiconductor manufacturing processes. Visualization of the acoustic wave and cavitation bubbles in ultrasonic water flow is presented to examine the role of bubbles oscillating under ultrasound irradiation in the process of removing small silica particles from glass surfaces. The visualization results suggest that hydrodynamic force from single-phase water flow does not suffice for particle removal and that acoustic cavitation bubbles play a major role in particle removal as in the case of ultrasonic bath immersion cleaning.