Hydraulic Structures in Water Aeration Processes

The primary purpose of water aeration is to increase the oxygen saturation of the water. This can be achieved by using hydraulic structures because of substantial air bubble entrainment at these structures. This paper reviewed the literature on hydraulic structures used in water aeration processes. The hydraulic structures were divided into two groups as the high-head flow systems and the free-surface flow systems. The high-head flow systems were circular and venturi nozzles, pipe with venturi tube, and high-head conduit, and the free-surface flow systems were weir, stepped cascade, and free-surface conduit. Air/water flow ratio and aeration efficiency in circular nozzles with air holes and venturi nozzles were significantly high. Pipes with venturi tubes showed high aeration efficiency although they had low air/water flow ratio. In high-head and free-surface conduits, almost full oxygen transfer, up to the saturation value, occurred. Forty-five degrees triangular sharp-crested weir had significantly better air/water flow ratio and aeration efficiency than other sharp-crested weir shapes. Stepped cascades, in particular nappe flow regime, were very efficient means of aeration.     

A gaussian-box modeling approach for urban air quality management in a northern chinese city—I. model development

A Gaussian-box modeling approach was presented in this paper to examine the urban air quality due to multiple point- and area-source emissions in the northern Chinese city of Fengnan, which is associated with a deteriorated air quality as a consequence of industrialization and rapid urban growth. A 3-D multi-box (3DMB) air quality model was developed to predict air quality due to area-source emissions. It improved upon the conventional box models by allowing consideration of more details in spatial variations of emission sources and meteorological conditions. The modeling domain was divided into various layers within the mixing height, while each layer was associated with a number of sub-boxes. A multi-source and multi-grid Gaussian modeling approach was then applied to predict the air quality in different sub-boxes that are associated with multiple point-source emissions. Thus the Gaussian-box modeling approach could effectively simulate impacts of both area- and point-source emissions but also reflect more details of the spatial variations in source distributions and meteorological conditions. This modeling approach was employed to predict daily average SO₂, TSP and PM₁₀ concentrations for each sub-box during the heating and non-heating seasons, respectively. The analysis of the mean normalized error of the modeling results demonstrates the feasibility and applicability of the developed method, and the presented method could provide more useful and scientific bases for developing effective urban air quality control and management strategies.     

Efficacy of nitrogen oxides and hydrocarbons emissions controls in ozone attainment strategies as predicted by the Urban Airshed Model

The primary object of this paper is to provide a preliminary assessment of the effectiveness of NO _x vs Volatile Organic Compounds (VOC) emissions control options in improving O₃ air quality over the New York metropolitan area. To this end, we have applied the Urban Airshed Model (UAM) with the Carbon Bond IV (CB-IV) chemical mechanism utilizing the results of the Regional Oxidant Model (ROM) for the specification of initial/boundary concentrations and wind fields to the UAM. After examining the sensitivity of the predicted O₃ concentrations to initial/boundary conditions and biogenic emissions, we have evaluated the impact of various hypothetical emissions reduction options on O₃ air quality. Nested ROM/UAM simulations with an across-the-board reduction of 75% in the NO _x and VOC emissions from sources located within the New York metropolitan area indicate that the option of VOC-only control is superior to the NO _x -only control in reducing not only peak O₃ levels over the entire modeling domain but also population exposure to unhealthy O₃ levels. The model predicts that the combined 75% NO _x and VOC control option also reduces the peak O₃ concentration, but the improvement in O₃ air quality is less than that predicted for the 75% VOC-only control strategy. Additional modeling analyses with different mix and levels of emissions control and meteorological conditions are needed to confirm these preliminary findings.     

Comparing the Effects of Aquatic Stressors on Model Temperate Freshwater Aquatic Communities

Nuisance odors generation from waste and wastewater treatment plants are a cause of public discomfort and complaints. This situation impairs the air quality and represents a growing social and public health problem, especially in developing countries. Several modeling approaches have been developed and successfully implemented in the frame of a wastewater treatment plant for both the biological treatment and physicochemical processes. The mathematical modeling of the odor generation process is still considered a quite complex issue, mainly due to the fact that olfactory nuisance can be caused by many different chemical compounds and the perception of odors is influenced by subjective thresholds. Moreover, the impact of odor sources on air quality is highly conditioned by complex atmospheric dispersion processes. This review presents a critical state-of-art and assessment where information related to odor emissions impact studies as well as modeling applications are compiled and discussed.     

An Approach to Temporally Disaggregate Benzo(a)pyrene Emissions and Their Application to a 3D Eulerian Atmospheric Chemistry Transport Model

To simulate the atmospheric fate of air pollutants, it is first necessary to know the emission rates that describe the release of pollutants into ambient air. For benzo(a)pyrene emission data are currently only available as yearly bulk emissions while the simulation models typically require temporally resolved emissions (e.g. hourly). Because residential heating is by far the most important source for benzo(a)pyrene, we developed a method to temporally disaggregate these bulk emission data using the linear dependency of benzo(a)pyrene emission rates stemming from residential combustion on ambient temperature. The resulting time-dependent hourly emission rates have been used in a chemical transport model to simulate concentrations and deposition fluxes of benzo(a)pyrene in the year 2000. The same simulations were repeated with constant emission rates and emission rates that varied only seasonally. By comparing the modeling results of the three emission cases with monthly measurements of air concentrations, the characteristic and the benefit of our disaggregation approach is illustrated. The simulations with disaggregated emissions fitted best to the measurements. At the same time the spatial distribution as well as the yearly total deposition was notably different with each emission case even though the yearly total emissions were kept constant.     

Volatile Organic Contaminant Emissions from Wastewater Treatment Plants During Secondary Treatment

Volatile organic contaminant (VOC) emissions from wastewater treatment plants may occur through a variety of mechanisms which mainly include volatilization, evaporation, biodegradation, and photodecomposition. A mathematical model incorporating the significant factors affecting VOC emissions from a secondary treatment facilities has been developed and verified with the operational data from wastewater treatment plant in south Florida. The model includes mass transfer considerations for volatilization from clarifier surfaces and weirs, in addition to volatilization from aeration basins. A simple model such as this one can be a useful tool to estimate VOC emissions from municipal wastewater treatment plants and to evaluate the present and future state of compliance with applicable air quality standards.     

Adaptive Grid Modeling with Direct Sensitivity Method for Predicting the Air Quality Impacts of Biomass Burning

The objective of this study was to improve the ability to model the air quality impacts of biomass burning on the surrounding environment. The focus is on prescribed burning emissions from a military reservation, Fort Benning in Georgia, and their impact on local and regional air quality. The approach taken in this study is to utilize two new techniques recently developed: (1) adaptive grid modeling and (2) direct sensitivity analysis. An advanced air quality model was equipped with these techniques, and regional-scale air quality simulations were conducted. Grid adaptation reduces the grid sizes in areas that have rapid changes in concentration gradients; consequently, the results are much more accurate than those of traditional static grid models. Direct sensitivity analysis calculates the rate of change of concentrations with respect to emissions. The adaptive grid simulation estimated large variations in O₃ concentrations within 4?×?4-km² cells for which the static grid estimates a single average concentration. The differences between adaptive average and static grid values of O₃ sensitivities were more pronounced. The sensitivity of O₃ to fire is difficult to estimate using the brute-force method with coarse scale (4?×?4 km²) static grid models.     

A Geochemical Equilibrium Modeling Approach to Assessing Soil Acidification Impacts Due to Depositions of Industrial Air Emissions

Soil acidification impacts arising from depositions of industrial air emissions may become a serious environmental concern. Currently, in the literature quantitative mechanistic modeling and the experimental acid neutralizing capacity (ANC) approach and a qualitative evaluation approach classifying soils into various levels of sensitivity to acid additions have been reported to assess the long-term soil acidification impacts due to industrial air emissions. Another alternative quantitative approach proposed by this study is the geochemical modeling approach that can be used to similate an ANC curve based on relevant soil chemistry data by calculating the equilibrium distributions of chemical species in the soil solution according to the specified geochemical processes. The purpose of this syudy was essentially to illustrate the potential applications and practical utility of the proposed geochemical modeling approach to assessing soil acidification impacts due to industrial air emissions. The application of the geochemical modeling apprach was illustrated by comparisons of the experimental and simulated ANC curves for a calcareous and a noncalcareous soil representing insensitive and sensitive soil cases, respectively. Results obtained from these comparisons reveal that, in terms of producing the ANC curve for the soil solution, the geochemical modeling approach seems to perform well and produce more reliable results for calcareous soil than for noncalcareous soil. However, the approach can also be used for noncalcareous soils when the air emission rates are low and may need further testing with additional measured data for a wide range of soils other than those presented in this study.     

Modeling of first plume encounters with precipitation

A regional air pollutant transport model was used to simulate the fate of S emissions in the northeast United States. Hourly calculations were analyzed to describe trajectory characteristics and mass balances as pollutant trajectories first encounter precipitation during transport away from emission sources. Model results include air concentrations of SO₂ and sulfate, dry deposition values, and sulfate concentrations scavenged by precipitation, along with trajectory statistics. Results of sensitivity tests are compared to base case simulations which consider all precipitation events as a means of suggesting priorities for future regional transport model development.     

5HCCX-1.6型多单元带式干燥机干燥过程的计算机模拟

多单元带式干燥机干燥过程的计算机模拟研究,有利于优化其干燥工艺参数,提高干燥效率,减少能耗,保证物料的干燥品质。该文利用面向对象的高级程序语言Visual C++建立了玉米多单元带式干燥的深床干燥数学模拟程序,并编制人机交互界面,预测干燥设备内的物料和干燥介质状态参数。同时用Matlab软件对数据进行处理,分析了热风温度、相对湿度以及风速等干燥工艺参数对干燥过程的影响,结果表明热风温度对干燥速率影响最大,其次是风速,热风相对湿度影响最小。为验证模型的准确性,选用5HCCX-1.6型多单元带式干燥设备进行试验研究,结果表明该模拟程序能较好的预测该类干燥设备的干燥过程,并对实际生产有一定的指导作用。     

Trends in Airborne Sulphur and Nitrogen Compounds in Norway during 1985–1996 in Relation to Air Mass Origin

Major reductions in emissions of sulphur dioxide and nitrogen dioxide in Europe have significantly reduced the ambient concentrations of both sulphur dioxide, particulate sulphate and nitrogen dioxide, as well as of sulphate in precipitation at Norwegian monitoring sites. In this study, trends in ambient air concentrations were studied in relation to air mass origin by sector analysis. Associated trends in ambient concentrations were derived by non-parametric statistical methods and evaluated on the basis of emission figures within the various sectors. The observed trends correspond well with reported trends in emissions.     

Simulations of ozone formation from different emission sources in sweden

An emission inventory concerning volatile organic compounds (VOC) and their emission profile linked to their sources in Sweden has been undertaken. The inventory has been used in model simulations to predict the ozone formation from different emission source categories in Sweden. The studies have been carried out using the IVL photochemical trajectory model for two types of air masses which describes clean and polluted air. In Sweden mobile sources contribute to 45 % by mass of the total national VOC emissions, 58 % of the NOx emissions and to at least 43 % of the ozone formation from national sources. In general, the ozone formation in Sweden is more dependent and sensitive to emissions of NOx rather than VOC.     

A multi-media approach to permitting mercury releases from coal-fired power plants

The current approach taken by many regulators in protecting the environment involves limiting the emission of pollutants to specific media (land, air, and water) through a permit. These “single media” permits do not take into account the mobility of pollutants, such as Hg, once they enter the environment, nor do they provide the regulated community with incentives to adopt pollution prevention practices. This paper explores multi-media permitting to limit coal-fired power plant releases of Hg; a pollutant that poses significant risk to human health and the environment. Power plant Hg emissions are a function of fuel, combustion regime and emission treatment. The approach presented is based on consideration of the Hg concentrations in various power plant emissions and the regulations governing these emissions. Multi-media permit development involves conducting a mass balance for Hg across the facility. Where pollution prevention is not an option, the permit should act to transfer the Hg to the waste stream in which it presents the least risk to human health and ecosystems. Development of cross-program risk based regulations is also stressed.     

Carbon Monoxide and Nitrogen Oxides Pollution in Moscow

Major factors (emissions and meteorology) controlling pollution patterns in Moscow are discussed in the context of different types of urban land use. Nitrogen oxide pollution is one of the main air quality problems in the city. Power generation is the major source of nitrogen oxides: in 1994, it accounted for 63% of the total NOx emissions with transport contributing 30%. CO emissions are produced almost entirely by road transport. An increase in CO levels has been observed since 1990 in line with growing car ownership. Analyses of seasonal and diurnal variations in CO, NO and NO2 concentrations are presented. Meteorological conditions during an intense pollution episode are analysed in the context of the characteristics of the main sources of pollution. The occurrence of high levels of CO concentrations is associated with high pressure systems, surface-based inversions and low wind speeds. High concentrations of NO2 are caused by fumigation of the surface with pollutants emitted by power plants. The passage of warm weather fronts, the decay of elevated temperature inversions, and stronger winds are identified as meteorological condition leading to NO2 build-up.     

Simulation Model for Composting Cellulosic (Bagasse) Substrates

A dynamic model of aerobic composting with solid substrates was developed as a first step to predict the rate of decomposition of cellulosic materials. The model was based on heat and mass transfer principles and reaction kinetics of biological processes. The relations describing the rates of reaction and the degradability potential of the substrates were determined based on laboratory-scale experiments. The model was developed using a graphical software package, Stella II® (which was able to solve feedback loops encountered in actual composting processes.

The model was validated with temperature, oxygen distribution and mass degradation accompanying the composting of bagasse. Experimental setups consisted of 30 gallon drum reactors, continuously aerated. Temperature and oxygen in exhaust gases were monitored on an hourly basis and measurements of moisture content, pH and volatile solids were done on a weekly basis.

The data fit with the model was excellent for mass degraded and temperatures for the first 10 days. From day 10 to 14, the deviations between model predictions (as solved with Stella II®) and actual measurements did not exceed 2.6 percent. The concentrations of oxygen in outlet gases showed fair agreement between predicted and experimental values. Simulating the model with varying airflow rates showed that the maximum mass breakdown rates occurred for airflow rates of 0.12 kg dry air/(kg dry compost.hr) while maintaining the temperatures below 50°C, through a temperature feedback control strategy.     

An Integrated Assessment Model for Fine Particulate Matter in Europe

Exposure to fine particles in the ambient air is recognized as a significant threat to human health. Two pathways contribute to the particle burden in the atmosphere: Fine particles originate from primary emissions, and secondary organic and inorganic particles are formed from the gas phase from the emissions of 'conventional' pollutants such as SO₂, NO_x, VOC and NH₃. Both types of particulate matter can be transported over long distances in the atmosphere. An integrated assessment model for particulate matter developed at IIASA addresses the relative importance of the different types of particulates, distinguishing primary and secondary particles and two size fractions. The model projects these emissions into the future and seeks cost-effective strategies for reducing health risks to population. The model integrates the control of primary emissions of fine particles with strategies to reduce the precursor emissions for the secondary aerosols. Preliminary results addressing the PM_2.5 fraction of both primary and secondary particulate matter indicate that in Europe the exposure to particulates will be significantly reduced as a side effect of the emission controls for conventional air pollutants (SO₂, NO_x, NH₃).     

Measurement of Dynamic Dough Density and Effect of Surfactants and Flour Type on Aeration During Mixing and Gas Retention During Proofing

A new method for measuring dough densities is presented, based on weighing small dough samples in air and immersed in xylene. The method can be used to evaluate the air content of low‐density doughs and to follow the changing density of a proofing dough sample. The method is applied to evaluate the effect of flour strength and surfactant addition on dough aeration and subsequent proofing. Doughs were mixed in a high‐speed mixer from two flours, a strong breadmaking flour and a weak flour. Surfactants sodium stearoyl lactylate (SSL) and diacetyl tartrate esters of monoglyceride (DATEM) were added at three levels, and the air content, proofing dynamics, and baked loaf quality were evaluated. The air content of dough was proportional to headspace pressure in the mixer, while the strong flour occluded less air than the weak flour. Surfactants greatly improved the volume of baked loaves but appeared to have no significant effect on air incorporation during mixing. The addition of surfactants appeared to increase the rate of growth of the dough piece during proofing, possibly due to increased bubble breakup during mixing or to increased rates of mass transfer of CO₂ into bubbles during proofing.     

ACID DEPOSITION DURING TWO CONTRASTING FRONTAL RAINFALL EVENTS

Analysis of the chemical composition of rain at high temporal resolution provides additional information on wet deposition processes. High resolution data was obtained using a microprocessor-based acid rain monitor at two sites in SW Scotland and SE England. Meteorological details of the transport and wet deposition processes during two frontal rain events were examined and related to rainfall composition. Rapid depletions of ion concentrations during heavy rainfall in the first event were interpreted using a rainfall scavenging model. The sub-event data for the second event showed the influence of frontal discontinuities. Increasing ionic concentrations during this second event were attributed both to the change in air mass, and to diminished upwind precipitation scavenging.     

On Morphometric Properties of DNAPL Sources: Relating Architecture to Mass Reduction

The fundamental step in the identification of the most appropriate strategy for the remediation of sites contaminated with dense nonaqueous phase liquids (DNAPLs) is a comprehensive characterization of the contaminated source region as the morphology of DNAPL strongly governs the mass transfer processes. The influence of DNAPL distribution geometry and groundwater flow velocity on mass reduction was explored through the evaluation of a series of laboratory studies conducted in a two-dimensional tank under different hydrodynamic conditions. An image analysis procedure was used to determine the distribution of DNAPL saturation and the morphology of the contaminated region. Experimental observations revealed a dependence of mass transfer rate on the aqueous phase velocity under high flow regimes, whereas the mass transfer rate was controlled mainly by morphometric indexes under low velocity flow conditions. Experimental results indicate that higher mass reduction and contaminant fluxes are obtained at low saturation values. The mass flux emanating from an elongated source aligned perpendicularly to the direction of water flow is greater due to a higher DNAPL?Cwater contact surface in comparison to a lower mass flux from horizontal pools with high saturation. These aspects should be considered in an inverse modeling technique for locating the source zone and also in all remediation approaches based on an increase in water circulation through a contaminated zone (i.e., pump and treat).