Environmental Risk Assessment - Term Paper Example

1. 2. Volatile organic compounds (VOCs) are the most common pollutants emitted by the chemical process industries. VOCs are carbon compounds that react with nitrogen oxides and other airborne chemicals in the presence of sunlight (photo chemically) to form ozone which is the primary component of smog(Wang, Perrira and Hung, 2004) . This makes air pollution control a pressing concern. Depending on the physic-chemical properties of VOCs and on the requirements for the degree of treatment, there exist different methods for destruction or removal of VOCs: thermal destruction, adsorption and catalytic oxidation (Wang, Perrira and Hung, 2004). Thermal oxidation systems also known as fume incinerators are no longer simple flares or afterburners. The modern thermal oxidizers are designed to accomplish 95-99 per cent destruction of virtually all VOCs. These systems can be designed to handle a capacity of 200-80000 m3/h and VOC concentrations from 0.2 to 4 g/m3 (Vigneron, Hermia and Chaouki, 1999). Thermal oxidations systems combust VOCs at temperatures of around 900-1000 degree centigrade. However, operating temperatures near 1000 degrees can produce elevated levels of nitrogen oxides, a secondary pollutant that may, in turn, require further treatment, such as selective catalytic reduction. This method requires greater heat input when compounds are either difficult to combust or present greater low inlet concentration (Vigneron, Hermia and Chaouki, 1999). Among adsorption technologies, the activated carbon has been widely used in adsorption processes due to higher micropore volume, easy operation, low cost and efficient recovery of most VOCs. Carbon adsorption is a very common method of VOC emission control. VOCs are removed from the inlet air by physical adsorption onto the surface of the carbon. Variable flow rates and VOC concentrations are not disruptive to carbon adsorbsers. These systems can be designed to handle a capacity of 170-100000 m3/h (Vigneron, Hermia and Chaouki, 1999). The main disadvantages of the methods are the periodic operations of the process and the high cost of sorbent regeneration. In addition, carbon adsorption is not recommended or VOC streams containing ketones. The method can be used to remove VOCs from gas streams by contacting the contaminated air with a liquid solvent. Any soluble VOCs will transfer to the liquid phase. In effect the air stream is scrubbed. This takes place in an absorber tower designed to provide the liquid vapour contact area necessary to facilitate mass transfer (Vigneron, Hermia and Chaouki, 1999). Biofiltration is an emerging technology for controlling volatile organic compounds (VOCs) emission in waste gas streams. Biofiltration has been extensively used in Europe, especially for odour control and it has been demonstrated at full scale in the US (Wang, Perrira and Hung, 2004). In the biofiltration process, the waste gas is vent through a biologically active material where the biodegradable VOCs are oxidized into carbon dioxide and water. Physical sorption and chemical degradation may also occur and contribute to the overall removal efficiency (Wang, Perrira and Hung, 2004). The applicability of biofiltration is dependent on the characteristics of the waste gas emitted from a (VSD) or other treatment processes. Typical biodegradable contaminants include alcohols, ethers, aldehydes, ketones, amines, sulfides and certain monocyclic aromatics (xylene, benzene, toluene and phenol). Waste streams containing chlorinated solvents are not readily biodegradable and are not appropriate for emissions control by biofiltration. The process itself is cost effective for large volumes gas streams with relatively low concentrations (Wang, Perrira and Hung, 2004). 2. 1. Guidance in the UK [DOE 1995a] defines risk management as the process of implementing decisions about tolerating or altering risks (Calow, 1998). Research has found that the assessment of risk as a component o risk management in that should provide for a structured understanding of the nature of potential adverse outcomes in the context of taking decisions (Choppin, 1999). Ideally, the assessment should consider risks without accepted mitigation in place when address the residual risk with appropriate mitigation. Only by this admittedly more time consuming approach can the relative effectiveness of the different mitigation measures be understood (Choppin, 1999). Risk management for waster treatement and disposal can include one or a combination of the following strategies (Calow, 1998). First, the management of risk through the strategic consideration of the BPEO for specific waste streams at the national or regional level (Calow, 1998). Second, the prevention of risk through the siting process (Calow, 1998). Third, the incorporation of appropriate engineering, monitoring and management controls during the design, operational and decommissioning and post closure stages of a project (Calow, 1998). Fourth, the interruption in a complete exposure pathway thereby breaking the link between source and receptor, often possible in extreme cases as it may involve facility closure. It is clear that risk management for waste treatment and disposal has become one of the priorities in the more developed world, but there is the need to integrate the technique more and more in the third world countries (Calow, 1998). This will result from a combination of public pressures for control over facilities which can present inequitable risks (Calow, 1998). This means that local communities need to bear risks arising from the management of waste not generated by themselves; expert and public recognition of the fact that waste management in the past has not always been conducted with a full appreciation of potential adverse impacts and deregulatory pressures encouraging non prescriptive approaches to design and operation (Calow, 1998). Difficult areas of resolution remain in relation to the use of conservative assumptions and the understanding of the impact of different types of uncertainty and variability upon assessment outputs. Further work on probabilistic uncertainty analysis seems likely, including detailed development of distribution functions and ranges for inputs (Calow, 1998). 3. 1 Gaussian dispersion model that will handle continuous or instantaneous liquid or gas elevated or surface releases from point or area sources. Output consists of concentration contour plots, concentration at a specified location, and maximum concentration at a given elevation and time (Schery, 2002). The Gaussian plume model plays a role that is in contrast to outdoor box models. It tends to be applied over shorter time scales and smaller regions for situations where the air is far from well mixed. Specifically, the Gaussian plume model provides a prediction of the concentration of an atmospheric emission such as a gaseous pollutant or aerosol, downwind of a point source of release (Greenway, 2002). A common application for the case of radioactive atoms is description of the downwind transport of releases from the exhaust stack of a reactor from an incinerator treating medical or biological wastes containing radioactive materials, or from airborne activity from a transportation accident. However, the model applies to any continuous point source release (whether gaseous or aerosol) in the atmosphere where the height above ground is specified. In particular, it applies to release of smoke from an exhaust stack (Greenway, 2002). The Gaussian plume model gets its name from the use of Gaussian (or normal) functions that provide a characteristic bell-shaped curve for the concentration of an emission at a given distance downwind of the release point as a function of the perpendicular distance from the centerline trajectory of the emission (Greenway, 2002). This spreading cone of a pollutant or emission downwind of the point of release is called a plume. An exhaust stack or other point source releases a radionuclide at a constant rate (atoms per unit time) at a constant height h above the ground. The wind carries the radionuclide downwind of the source. As the atoms of the radionuclide move downwind (x direction), they also spread horizontally (y direction) and vertically (z direction) perpendicular to the direction of the wind due to the turbulent motion in the air. As a result, the concentration of the radionuclide becomes progressively less, since the same number of atoms of a radionuclide is being spread over a larger volume of air. If decay is significant over the time period of transport, the concentration will be reduced further due to radioactive decay (Greenway, 2002). A frequently used version of the Gaussian plume model for a radionuclide predicts its concentration at ground level at a distance downwind of the stack. Since the radionuclide is mixed horizontally and vertically in the lateral directions as it moves downwind, another important variable is the distance along the ground perpendicular to and the vertical plane containing the centerline of the plume (Schery, 2002).. The model is appropriate and recommended for applications which involve industrial source complexes in either rural or urban areas with flat or rolling terrain, it is recommended that the model only be used when the transport distances are less than 50 km. it is appropriate to use the model for 1-h to annual averaging times and to model continuous toxic air emissions (Schery, 2002).. 4. 1. Fabric filters: In fabric filtration, the gas flows through a number of filter bags placed in parallel, leaving the dust captured by the fabric. Extended operation of a fabric filter requires periodic cleaning of the cloth surface (Wang, Perrira and Hung, 2004. The process itself is an accepted technology for the removal of particles from industrial gas streams and is widely used in a number of industries. Fabric filters have advantages over competing technologies for many relatively small particle laden gas steams, for flammable or explosive gas streams, and efficiently collect valuable products or noxious materials. For the electric power industry, fabric filters are especially attractive because they provide a clear stack (Wang, Perrira and Hung, 2004. Most fabric filters used on industrial and utility boiler installations have been highly efficient and fairly successful. However, many utility fabric filters operate at average pressure drops greater than expected. Most of the time the pressure drop continues to increase with time during the life of a filter bag (Greenway, 2002). The process itself is one the most common techniques to collect PM from industrial waste gases. The use of the process itself is based on the principle of filtration which is reliable, efficient and economic method to remove PM from the gases. The air pollution control equipment using fabric filters are known as baghouses. A baghouse or a bag filter consists of numerous long vertically hanging bags approximately 4-18 inches in diameter and 10-40 feet long. The number of bags can vary from a few hundred to a thousand or more depending on the size of the baghouse (Greenway, 2002). They are used widely by industrial and waste incinerators for particle removal. The recommended filtering velocity varies from 1.5 to 4 ft/min (Greenway, 2002). The second method is that of electrostatic precipitation, which is today considered by many experts in this field as the most promising method for getting rid of small particles from exhaust streams of large power generation plant (Wang, Perrira and Hung, 2004. The process involves the application of an electric field to remove electrically charged particles from their gaseous carrying media. An electrostatic precipitator has a discharge electrode (negative) of small area, such as wire and a collection electrode (positive) for large surface area such as a plate or a tube. There are essentially four steps in electrostatic precipitation of particles (Raber, Johnston and Schroder, 1986): 1. Placing an electrical charge on the particles 2. Migration of the charged particles in the electrical field to the collecting surface 3. Neutralisation of the electrical charge of the particle after impacting on the collection electrode. 4. Removal of the precipitated particles from the collection electrode The particles get charged by the process of colliding with air molecules ionized at the discharge electrode (Raber, Johnston and Schroder, 1986). The neutralized particles are removed from the collecting surface by rapping, scarping, brushing or washing (Srinivasan, 2009). There are two general types of electrostatic precipitators, low voltage (two stages) and high voltage (single stage). The process is beneficial given the fact that gas pressure and vacuum operating conditions can be used along with the fact that there is essentially no limit to usage of solids, liquids and corrosive chemicals. Also important is the fact that the units handle a wide range of gas velocities operation or convenience in cleaning (Srinivasan, 2009). The disadvantages are that installations are high, along with the fact that’s space requirements are high for cold precipitators and even greater for hot precipitators (Srinivasan, 2009). 7.2 Environmental risk assessment has become a fundamental tool for the environmental decision making process, especially for chemical risk control. Several complimentary factors led to the definition of this fundamental role. The most important of these was increased public concern about pollution and environmental risks, which increased demand for prevention and protection. As a consequence the development of environmental regulations and policies were accelerated, in order to define stringent environmental benchmarks (i.e.) environmental quality standards and innovative assessment approaches to support environmental management processes. Risk is the potential for harm. It may more formally be defined as the potential or realization of unwanted, negative consequences of an event. The word potential refers to the probabilistic nature of risk. It is negative because risk is only one part of a decision process. One only takes a risk to obtain some benefit i.e. one gamble. Carrying out an evaluation of such gambles is an innate element of the risk assessment process. A major concern innate to the process of risk assessment is involuntary risk imposed inequitably upon those at risk (Society for Testing and Materials, 1982). The term risk assessment would be used loosely for the description of the total process of risk analysis, which would in turn embrace both the determination of levels of risk and the social evaluation of these risks.   Risk Determination: Risk determination is inclusive of the concepts of the factors of both risk acceptance and risk aversion-the former relates to the acceptable levels of societal risks and the latter to the methods that could be used for avoiding risks, as an alternative to the involuntary opposition of risks. Risk identification and elimination it is to be understood are purely parts of risk assessment-both would in the long run involve uncertainty and the interpretation of impact of uncertainties relying on scientific consideration more than technical and value judgments (Marcomini, Suter and Critto, 2009). With respect to risk identification again, changes in the levels of risk are identifiable in three circumstances: when a new risk is created, when the magnitude of an existing risk changes and when perception of an existing risk changes. All three may occur simultaneously. It has to be understood that the correct management of fire safety is necessitated by the need of ensuring that poluution itself is brought under control, or at least the risk of it occurring is reduced to the minimum. It is also necessary that there are steps taken that would ensure the timely control or containment of the issue. Finally, it is also necessary that when it occurs grows; there need to be apt measures in place to control the problem (Marcomini, Suter and Critto, 2009). Risk assessment and management techniques are used more and more as decision making tools for designing regulations, providing a basis for site specific decisions, ranking environmental risks and finally in comparing these risks. Because environmental risk management has been used by regulators, it is also increasingly used by the industry. In fact, companies use environmental risk assessment to determine the levels of risk associated wth certain processes or plants and for industrial financial planning (Marcomini, Suter and Critto, 2009). Finally risk assessment and management can be important decision making tool in the prioritization and evaluation of industrial risk reduction measures. Innovation can be understood as the remodeling of novel information and the transfer of this knowledge into the betterment of products and services. (Marcomini, Suter and Critto, 2009) It is all about the creation of value and creating impetus for productivity, and therefore for growth. Innovations have often been equated to the concept of risk taking but it has been demonstrated time and time again that it is the ones that look to innovate and take these risks that grow and expand. The basic formula that underlies innovation is the acceptance of the risk factor to accept risk and the measurement performance (Marcomini, Suter and Critto, 2009). 8.1 Whilst there is a range of source-receptor pathways that may potentially result in exposures to pollutant emissions from landfill sites depending on the special circumstances, at a given landfill site, inhalation of atmosphere landfill gas emissions is the only exposure pathway considered to be common to the majority of the landfills. Inhalation of landfill gas is also the pathway most consistent with epidemiological studies (Haukohl and Marxen, 2000). The exposure risk assessment would appear to indicate the fact that emissions of trace gas constituents in landfill gas are not sufficiently high to represent a theoretical base for adverse health effects in the vicinity of landfill sites (and certainly not at the distances indicated by the epidemiological studies (Haukohl and Marxen, 2000). The dilutions that would occur between the landfill surface and off site receptors would reduce concentrations below health based criteria with margins of safety of several orders of magnitude (Haukohl and Marxen, 2000). Even without engineered controls the margins of safety appear to be sufficiently wide such that the overall conclusion is unlikely to be affected by the uncertainties associated with the approaches (like variations of emission characteristics within and between landfills, limitations in the toxicological data, synergistic effects, indirect pathways such as food chains uptake of gaseous compounds etc) except under exceptional circumstances (Haukohl and Marxen, 2000). Another method is incineration, a method becoming scarce, therefore making it more expensive (Haukohl and Marxen, 2000). One advantage of incineration is that energy can be produced from the burning waste and there are now some 'energy from waste' schemes in operation (Haukohl and Marxen, 2000). With this method, some large landfills generate enough electricity for 10,000 homes! One disadvantage of incineration is that even more gases are produced, contributing further to global warming. Incineration is usually found at the most advanced level of the waste disposal treatment hierarchy. Additional environmental control is introduced at each level and the disposal costs in this case tend to increase substantially (Haukohl and Marxen, 2000). The process does not complete eliminate but does to a certain extent reduce the volume of waste that is to be land filled. The process itself serves two purposes in the advanced waste management system. First, it reduced the amount of waste for sanitary landfilling and it uses waste for energy production (power or district heating) (Haukohl and Marxen, 2000). References: Vigneron, S., Hermi, J., and Chaouki, J., (1999). Characterization and control of odours and VOC in the process industries: proceedings of the Second International Symposium on Characterization and Control of Odours and VOC in the Process Industries, Louvain-la-Neuve, Belgium, 3-5 November 1993. Elseiver. P479-480 Wang, L. K., Perrira, N. C., and Hung, Y. T., (2004). Handbook of Environmental Engineering: Advanced biological treatment processes, Volume 4. Springer Books. P459 Choppin., G. R., (1999). Chemical Separation Technologies and Related Methods of Nuclear Waste Management: Applications, Problems, and Research Needs. Springer Books. Pp21-26 Calow, P., (1999). Handbook of environmental risk assessment and management. Wiley Blackwell Publishing. p448 Schery, S. D., (2002). Understanding radioactive aerosols and their measurement. Springer Books. P281 Greenway, A. R., (2002). How to Obtain Air Quality Permits. McGraw-Hill Professional. P81 Srinivasan, (2009). Environmental Engineering. PHI Learning Pvt. Ltd. P119 Raber, R. R., Johnston, P. R., and Schroder, H. G., (1986). Fluid Filtration: Gas. Springer Books. P316 Haukohl, T., and Marxen, H. U., (2000). Municipal solid waste incineration: requirements for a successful project, Volumes 23-462. World Bank Publications. Pp4-6 Read More

Biofiltration is an emerging technology for controlling volatile organic compounds (VOCs) emission in waste gas streams. Biofiltration has been extensively used in Europe, especially for odour control and it has been demonstrated at full scale in the US (Wang, Perrira and Hung, 2004). In the biofiltration process, the waste gas is vent through a biologically active material where the biodegradable VOCs are oxidized into carbon dioxide and water. Physical sorption and chemical degradation may also occur and contribute to the overall removal efficiency (Wang, Perrira and Hung, 2004).

The applicability of biofiltration is dependent on the characteristics of the waste gas emitted from a (VSD) or other treatment processes. Typical biodegradable contaminants include alcohols, ethers, aldehydes, ketones, amines, sulfides and certain monocyclic aromatics (xylene, benzene, toluene and phenol). Waste streams containing chlorinated solvents are not readily biodegradable and are not appropriate for emissions control by biofiltration. The process itself is cost effective for large volumes gas streams with relatively low concentrations (Wang, Perrira and Hung, 2004). 2. 1.

Guidance in the UK [DOE 1995a] defines risk management as the process of implementing decisions about tolerating or altering risks (Calow, 1998). Research has found that the assessment of risk as a component o risk management in that should provide for a structured understanding of the nature of potential adverse outcomes in the context of taking decisions (Choppin, 1999). Ideally, the assessment should consider risks without accepted mitigation in place when address the residual risk with appropriate mitigation.

Only by this admittedly more time consuming approach can the relative effectiveness of the different mitigation measures be understood (Choppin, 1999). Risk management for waster treatement and disposal can include one or a combination of the following strategies (Calow, 1998). First, the management of risk through the strategic consideration of the BPEO for specific waste streams at the national or regional level (Calow, 1998). Second, the prevention of risk through the siting process (Calow, 1998).

Third, the incorporation of appropriate engineering, monitoring and management controls during the design, operational and decommissioning and post closure stages of a project (Calow, 1998). Fourth, the interruption in a complete exposure pathway thereby breaking the link between source and receptor, often possible in extreme cases as it may involve facility closure. It is clear that risk management for waste treatment and disposal has become one of the priorities in the more developed world, but there is the need to integrate the technique more and more in the third world countries (Calow, 1998).

This will result from a combination of public pressures for control over facilities which can present inequitable risks (Calow, 1998). This means that local communities need to bear risks arising from the management of waste not generated by themselves; expert and public recognition of the fact that waste management in the past has not always been conducted with a full appreciation of potential adverse impacts and deregulatory pressures encouraging non prescriptive approaches to design and operation (Calow, 1998).

Difficult areas of resolution remain in relation to the use of conservative assumptions and the understanding of the impact of different types of uncertainty and variability upon assessment outputs. Further work on probabilistic uncertainty analysis seems likely, including detailed development of distribution functions and ranges for inputs (Calow, 1998). 3. 1 Gaussian dispersion model that will handle continuous or instantaneous liquid or gas elevated or surface releases from point or area sources.

Output consists of concentration contour plots, concentration at a specified location, and maximum concentration at a given elevation and time (Schery, 2002). The Gaussian plume model plays a role that is in contrast to outdoor box models.

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