Environmental Concerns Elliott Goldberg and Maung K. Min 1.0 ENVIRONMENTAL REGULATIONS AND TECHNOLOGY 1.1 Regulatory Concerns Environmental laws and regulations including permits are reviewed in this chapter. Included are the Federal Clean Air Act Amendment (CAAA), the Federal Clean Water Act (CWA) regulations, the Resource Conservation and Recovery Act (RCRA) or, as it is also known, the Solid Waste Disposal Act. Also discussed along with the regulations under OSHAare the National Institute for Occupational Safety and Health (NIOSH) and the Hazardous Waste Operations and Emergency Response (HAZWOPER). The environmental regulations covered here are not intended to be all- inclusive but to provide a basic understanding ofthe important environmental laws and regulations. 1.2 Technology The environmental technology section includes reviews of waste water treatment and air and waste minimization/pollution prevention. Waste water treatment procedures discussed include biological treatment, activated car- bon adsorption, air and steam stripping, chemical precipitation, ion ex- change, and membrane separation. 635 636 Fermentation and Biochemical Engineering Handbook Air pollution control technology includes thermal incineration, cata- lytic incineration, carbon adsorption, absorption, condensation, baghouse filtration, wet scrubbing, and electrostatic precipitation. The range of technology will provide the engineer with a sufficient background to understand the important air control measures. 2.0 LAWS, REGULATIONS AND PERMITS 2.1 Air The Federal Clean Air Act Amendments (CAA) were initially enacted in 1963 and modified in 1970 and 1977. The Clean Air Act Amendments of 1990 involved major changes to environmental regulations. These included a national permitting system to regulate air pollution emissions. Its purpose was to protect the public health and environment by indicating how and when the various industries involved must control a list of air toxics. The regulatory authority was given to the states and local governments. Congress, through the CAA, authorized the EPA to develop the necessary regulations to carry out the provisions of the act. The EPA established the National Ambient Air Quality Standards (NAAQS), which included allowable ceilings for specific pollutants. How- ever, the states have the option to make any or all parts of the Clean Air Act requirements more stringent than the minimums set by EPA. The EPA is required to regularly evaluate the compliance status of all geographic areas with respect to pollutants, that is, whether the NAAQS is being met for each criteria pollutant. An area where NAAQS is not met is designated as a non- attainment area (N.A.) for that pollutant. Areas where the Federal Ambient Air Quality Standards are being met are designated attainment and are subject to Prevention of Significant Deterioration (PSD) requirements and are required to identify those areas that are attaining or not attaining the standards. Compliance and noncompliance can be costly. It has been estimated that the installed cost of equipment and systems to control emissions could range from $20 to $50 billion or higher. The technologies expected to be used include wet scrubbing, thermal incineration, catalytic incineration, carbon absorption, and solvent recovery. New sources and modifications ofexisting sources of air pollution in an attainment area are regulated under the Environmental Concerns 637 Prevention of Significant Deterioration Program (PSD). PSD review is required if the new source or modifications result in a net emission increase above specified levels. The specific pollutants referred to include carbon monoxide, nitrogen dioxide, lead, ozone, inhalable particulates, and sulfur dioxide. Primary and secondary standards also were set by EPA, with second- ary standards reflecting levels necessary to protect welfare in addition to health. An area may be in an attainment status for one pollutant and in a non- attainment status for another pollutant. In most areas, PSD authority has been assigned to either the state or local jurisdiction. The use of the Best Available Control Technology (BACT) is required for each pollutant and is based on the emission level and capital and operating costs. Regulations in non-attainment areas are required to meet the EPA’s New Source Review (NSR) regulations. The Clean Air Act of 1990 included a list of 189 toxic chemicals to be controlled and such emissions are to be reduced 90% by theyear 2000. It also included the phasing out of chlorinated fluorocarbons (CFC’s) and carbon tetrachloroflurocarbons (HCFC’s) by 2030. All new and modified emission sources must meet the New Source Performance Standards (NSPS). These standards are generally less strin- gent than either the Best Available Control Technology (BACT) or the Lowest Available Emission Rate (LAER). The National Emission Standards for Hazardous Air Pollutants (NESHAPS) specie emission standards for various hazardous air pollut- ants and cover asbestos, arsenic, benzene, beryllium, mercury, vinyl chlo- ride, PVC, etc. The CAAA was promulgated to strengthen the federal air protection program and concerns about air toxics by including an expanded National Emission Standards for Hazardous Air Pollutants (NESHAP) program. Concerns over the effects of hazardous air pollutants (HAP) or air toxics resulted in the Title Voperating permit program. The relationship ofthe Title V program to other CA4 titles is shown in Fig. 1. 638 Fermentation and Biochemical Engineering Handbook Air Toxics Acid Rain i-xq Permit Program Program - I Figure 1. The relationship of the Title V program to other CAA titles. 2.2 Water In 1972 Congress enacted the Federal Water Pollution Control Act known as the Clean Water Act (CWA). In 1977 further amendments were enacted which strengthened the provisions of the Clean Water Act. Further refinements were enacted by Congress with the Water Quality Act Amend- ments of 1987. The purpose of the CWA was to restore and maintain the chemical, physical, and biological integrity ofour country's waters. It set up specific effluent guidelines for SIC industry categories and BOD and suspended solids continued to serve as the primary parameters. The National Pollutant Discharge Elimination System (NPDES) was set up and authorized EPA to establish and enforce effluent limitations on waste water discharges. Designated priority pollutants were introduced with a permit program and aquatic toxicity also became a permit requirement. Volatile Organic Compounds (VOC) emissions from waste water treatment plants were severely restricted and control of nutrients such as nitrogen and phosphorous were required. The Clean Water Act established standards that area-wide waste water treatment plants be developed and implemented to assume adequate control Environmental Concerns 639 of the quality of the effluent for industrial discharges of toxic pollutants into Publicly Owned Treatment Works (POTW). It also stated that federal financial assistance would be provided to construct publicly owned waste water treatment works. It also established that the federal agencies, the state water pollution control agencies, interstate agencies, and the municipalities and industries involved, prepare or develop comprehensive programs for preventing, reduc- ing or eliminating the pollution of navigable waters and ground waters and improving the sanitary condition of surface and underground waters. Due regard shall be given improvements which are necessary to conserve such waters for the protection and propagation of fish and aquatic life. The Clean Water Act lays the basis for technology based effluent standards of conventional pollutants such as Biochemical Oxygen Demand (BOD), Total Suspended Solids (TSS), fecal coliform, oil and grease, pH, toxic pollutants, and non-conventional pollutants such as active pesticides, ingredients used in the pesticide manufacturing industry, etc. A complete list of toxic pollutants can be found in the Code of Federal Regulations, 40 CFR, Part 40 1.15. The CWA established requirements for setting standards for dis- charges from new sources for specific industries. It also lists requirements for preventing and responding to accidental discharges of oil or hazardous substances into navigable waters with notification requirements for releases, removal requirements, liability standards and civil penalties. Furthermore, the CWA established permitting programs to control discharges and severe civil and criminal enforcement provisions for failure to comply with the law. Compliance with the CWA must be incorporated into the design and operation of every chemical process plant. To summarize, the focus of the CWA is the control of pollutants in effluent discharged from a facility through any conveyance to virtually any stream or significant body of water. These discharges are primarily controlled through the National Pollution Discharge Elimination System (NPDES). If the discharge is to a Publicly Owned Treatment Works, the plant needs to meet pretreatment standards to limit pollutants that cannot be readily removed by the POTW. Discharges from the POTW are required to be in accordance with the effluent limitations contained in the NPDES permit for the POW. If the facility discharges directly into receiving waters, the facility must file for and obtain its own NPDES permit. 640 Fermentation and Biochemical Engineering Handbook 2.3 Solid Waste Resource Conservation and Recovery Act (RCRA) (Solid Waste Disposal Act) was originally enacted by Congress in 1976 and amended several times subsequently. The 1984 amendments set deadlines for enforc- ing the regulations. They also placed restrictions on disposal of wastes on land and forced tighter regulation of hazardous wastes. In effect, Congress gave EPA the authority to control hazardous wastes from their generation to their ultimate disposal. Congress also sought to encourage the recycling of recoverable material. The RCRA included the statements that: Millions oftons of recoverablematerial which could be used are needlessly buried each year. Methods are available to separate usable materials from solid waste. The recovery and conservation of such materials can reduce the dependence ofthe United States on foreign resources and reduce the deficit in its balance of payments. Principally, however, Congress aimed at the environment and Disposal of solid waste and hazardous waste in or on the land without carehl planning and management can present a danger to human health and the environment. As aresult ofthe Clean Air Act, the Water Pollution Control Act, and other federal and state laws respecting public health and the environment, greater amounts of solid waste, in the form of sludge and other pollution treatment residues, have been created. Similarly, inadequate and environmen- tally unsound practices for the disposal or use of solid waste have created increased amounts of air and water pollution and other problems for the environment and health. Open dumping is particularly harmful to health since it can contaminate drinking water from underground and surface supplies and pollutes the air and land. health: Environmental Concerns 641 Alternatives to existing methods of land disposal must be developed, since many of the cities in the United States will be running out of suitable solid waste disposal sites within five years, unless immediate action is taken. Objectives of the Act are to promote the protection of health and the environment and to conserve valuable material and energy resources by providing technical and financial support to state and local governments and interstate agencies for the development of solid waste management plans, including resource recovery and resource conservation systems. Furthermore the act proposed to prohibit future open dumping on the land and required the conversion of existing open dumps to facilities which do not pose a danger to the environment or health. The Act requires that hazardous waste be properly managed, thereby reducing the need for corrective action at a future date. An important consideration of RCRA was that it required the promulgation of guidelines for solid waste collection, transport, separation, recovery, and disposal practices and systems. The Act set up specific procedures for establishing standards. Enforce- ment of job safety and health standards were also written into the Act. 2.4 Occupational Safety and Health Act (OSHA) In 1970, Congress enacted the Occupational Safety and Health Act, which requires employers to provide safe and healthful working conditions for their employees. It authorized the Secretary of Labor to set mandatory occupational safety and health standards to protect employees. As a result, the Occupational Safety and Health Review Commission was created to review the enforcement actions taken by OSHA. The National Institute for Occupational Safety and Health (NIOSH) was set up to research work place safety and health and to recommend standards to OSHA for controlling exposure to harmful and toxic substances. The OSHA act of 1970 is comprehensive in scope and covers enforce- ment of standards, penalties, research activities, state programs, financial assistance, employees duties and rights, and OSHA’s effect on other laws. The Hazardous Waste Operations and Emergency Response Standard (HAZWOPER; 29 CFR 1910.120) was issued by OSHA in March 1990. These regulations serve as a guide to a safety and health plan for hazardous waste operations. 642 Fermentation and Biochemical Engineering Handbook The HAZWOPER regulations includes the following: RCRA Corrective Actions Clean up operations for uncontrolled hazardous waste sites, including voluntary operations, and routine operations Emergency responses where there is a release of hazardous substances or a potential release of hazardous substances exists The formulation of safety and health plans for hazardous waste A preliminary site characterization analysis and hazard as- signment before entering a site known to be contaminated A site specific safety and health program to control safety and health hazards A training program for all employees and contractors em- ployed on the site who may be involved in hazardous waste activities operations, which include the following: A medical surveillance program Proper work practices, including appropriate personnel A site control system to prevent contamination of personnel A monitoring program to establish the appropriate levels of Decontamination procedures before entering a site Observation of applicable regulations issued by the Depart- ment of Transportation, Environmental Protection Agency, and the Occupational Safety and Health Act, in handling, labelling, moving and disposing of containers containing contaminated material An emergency response plan for emergencies which may occur on site The preliminary site plan characteristic analysis and hazard assess- ment should be performed by an experienced and trained technician before entering the site. A more detailed site evaluation and analysis must be done to establish the necessary engineering controls and personnel protective equipment. All potential hazards must be identified and evaluated and an air protective equipment. and equipment personnel protective equipment Environmental Concerns 643 monitoring program must be set up to ascertain that it is safe for work to begin and proceed. In summary, the hazards must be identified, a health risk assessment performed, a medical surveillance program instituted, potential sources of fire and explosion identified, and other possible risks and hazards evaluated. Site specific safety and health rules must be set up, distributed and posted. Such a plan should include the applicable items from the following list. Workers at the site must be informed of the potential hazards and must be cognizant of the site specific safety and health plan. Safe@ and Health Plan: Safety and Health Procedures Personnel Responsibilities Decontamination Procedures Required Monitoring Training Requirements Hazards Identification Personnel Protective Equipment Emergency Procedures Hazardous and Toxic Materials On Site Medical Surveillance Hospital Location Emergency Response Personnel 2.5 Environmental Auditing Environmental auditing can cover a wide range of objectives. The approach can focus on how well a manufacturing facility is complying with the various environmental regulations, such as the Clean Air Act Amend- ments (CAAA), the Clean Water Act (CWA), the Resource Conservation and Recovery Act (RCRA), Title I11 of the Supehnd Amendments and Reautho- rization Act (SARA), the Comprehensive Environmental Response, Com- pensation and Liability Act (CERCLA), the Toxic Substances Control Act (TSCA), various aspects ofthe Occupational Safety andHealth Act (OSHA), and can also cover property acquisition. It can also cover the various State regulations, for instance, in New Jersey an environmental audit can cover the Toxic Catastrophe Prevention Act (TCPA), the Spill Act, and the State 644 Fermentation and Biochemical Engineering Handbook Permit Compliance. If desired, it can also cover waste minimization and pollution prevention. The environmental audit offers a comprehensive assessment of a facility’s compliance with applicable federal, state and local regulations. It also can identify problems before the local or state regulator can be made aware of them and allows time to correct the inadequacies. Other advantages of the environmental audit is to allow time to properly assess the problem, plan its solution and allow for funding the capital cost required. There are potential problems in having an environmental audit performed. The results of recent court decisions indicate that the environmen- tal audit results may not be able to be kept in confidence and furthermore, they may be used as evidence of noncompliance in civil or criminal court actions. It is conceivable that an audit can increase the potential liabilities. Conse- quently, management should be prepared to commit to satisfy any negative findings before the audit is undertaken. The Department of Justice has developed guidelines for compliance and reporting that will be taken into consideration before assessing penalties for environmental regulations. Consequently, the scope and purpose of the audit should be filly understood and agreed to by both management and the consultant prior to undertaking the audit, and questions such as who does the consultant report, who will be given copies of the environmental audit, who will be in charge of document control, who should meet with the consultant, etc., should be decided. Audit-Environmental auditing consultants have developed forms for collectingldeveloping information on various aspects of the site. The more important of these will include the following: Site information data Types of adjacent land use Primary site use Site configuration Buildings on site, number and size For manufacturing on site: - Products - Intermediates - Waste materials disposal Solid Liquid Gaseous Environmental Concerns 645 Chemical storage on site Solid Liquid Gas Are underground storage tanks used Number How long in service Volume Material stored Material of construction Leak tested Above ground storage tanks Number How long in service Volume Material stored Material of construction Leak tested Waste piles Size/volume How long used Material How contained Hazardous/toxic wastes generated on site How are wastes handled on site/off site treatment Disposal-How Site ownership history Regulatory/environmental history Spills history Accident history Environmental problems 646 Fermentation and Biochemical Engineering Handbook List of permits Air emissions Waste water Solid wastesihazardous wastes 2.6 National Environmental Policy Act The purpose of the Act as stated in a Congressional Declaration of National Environmental Policy is: To declare a National Policy which will encourage produc- tive and enjoyable harmony between man and his environ- ment; to promote efforts which will prevent or eliminate damage to the environment and biosphere and stimulate the health and welfare of man; to enrich the understanding of the ecological systems and natural resources to the Nation; and to establish a Council on Environmental Quality. Congress further agreed that to carry out the policy set forth in this Act; it is the continuing responsibility of the federal government to use all practicable means to improve and coordinate federal plans, function pro- grams, and resources that the nation may: Fulfill the responsibilities ofeach generation as trustee ofthe To assure for all Americans safe, healthful, productive area, Attain the widest range of beneficial uses ofthe environment . undesirable and unintended consequences. Preserve important historic cultural and natural aspects of our national heritage and maintain, whenever possible, an environment which supports diversity and variety of indi- vidual choice which will permit high standards of living and a wide sharing of life’s amenities Enhance the quality of renewable resources and approach the maximum attainable recycling of depletable resources environment for succeeding generations aesthetically and culturally pleasing surroundings without degradation, risk to health or safety, or other Achieve a balance between population, and resource use Environmental Concerns 647 2.7 Storm Water Regulations An overview of storm water regulations is included in this section. As a result of the 1987 amendments to the Federal Clean Water Act, the United States Environmental Protection Agency (EPA) adopted rules in 1990 which require permit applications for a number of storm water discharges. The intent of storm water regulations is to reduce and prevent pollution due to storm water. A primary approach is source reduction and pollution minimi- zation. A number of different regulatory programs cover storm water, which may be treated as either a point or a non-point source discharge. The new federal storm water permitting regulations require permit applications to be submitted for all large and medium municipal separate storm sewer systems. Storm water discharges from residential or commercial sites, except for construction activities, are not subject to current federal storm water permit application regulations; however, such storm water discharges may be subject to existing state regulations and may be subject to future federal regulations. The discharge of contaminated storm water to surface water or ground water, including discharges through separate storm sewers, requires an NJPDES permit in the State of New Jersey and other states. Traditionally, discharges of storm water in ground water have not been controlled by the NPDES program. 3.0 TECHNOLOGY (WASTE WATER) 3.1 NPDES Under the NPDES program, all industrial and municipal facilities that discharge waste water directly into Unites States waters must obtain a permit. Specifically, the water act requires NPDES permits for discharges from point sources such as municipal waste water treatment plants, industries, animal feed lots, aquatic animal production facilities, and mining operations. NPDES permits specifjl effluent limitations for each individual industrial and municipal discharge, a compliance schedule, monitoring and reporting requirements, and other terms and conditions necessary to protect water quality. NPDES permits are valid for five years, although EPA may issue them for shorter terms. NPDES permits may be revoked, transferred, or modified. 648 Fermentation and Biochemical Engineering Handbook NPDES permits are available from the EPA or from a state authorized to issueNPDES permits. Upon authorization of a state NPDES program, the state is primarily responsible for issuing permits and administrating the NPDES permit program. State NPDES programs must be consistent with minimum federal requirements. Under the Federal Clean Water Act’s National Pollutant Discharge Elimination System (NPDES) permitting program, two approaches exist for controlling pollutant discharges from individual and municipal waste water treatment facilities: the technology-based approach and the water-quality based approach Technology-based controls consist of uniform EPA established stan- dards of treatment that apply to direct industrial dischargers and publicly owned waste water treatment works. These uniform standards, known as effluent limitations, generally are in the heart of NPDES permits and place numeric limits on the amount of effluent pollutant concentrations permitted at the point of discharge (end-of-pipe). Industrial effluent limitations are derived from technologies that are available for treating the effluent and removing pollutants, and also are based on considerations of cost and economic achievability. The water quality based approach is used to develop stricter effluent limitations where technol- ogy based controls will not be stringent enough to ensure that waters can support their intended uses. 3.2 Emuent Limitations EPA and the states issue waste water discharge permits to individual factories, power plants, refineries, and other private companies, based on national effluent limitation guidelines. These are based on chemical, physical and biological characteristics of effluent that industry may dump into water ways. An effluent limitation guideline sets the degree of reduction of a pollutant that can be achieved through the application of various levels of technology. An effluent limitation is a restriction on the amount of a pollutant that can be released from a point source into a water body. The discharge of waste water containing metals has effluent limitations, standards, or prohi- bitions, expressed in terms of the total metal, that is, the sum of the dissolved and suspended fractions of the metal. Environmental Concerns 649 3.3 Continuous Discharger All permit effluent limitations, standards, and prohibitions, including those necessary to achieve water quality standards, will be stated as maxi- mum daily and average monthly discharge limitations for all dischargers. 3.4 Non-Continuous Discharger A permittee’s noncontinuous discharge is limited and described as Frequency Total mass Maximum rate of discharge of pollutants during the dis- Prohibition or limitation of specified pollutants by mass, follows: charge concentration, or other appropriate measure 3.5 Mass Limitations All pollutants limited in a discharger’s permit will have limitations, standards, or prohibitions expressed in terms of mass except: pH, temperature, radiation, or other pollutants which cannot be expressed by mass when standards and limitations are expressed in terms of other units of measurement ifthe permit limitations were issued on a case-by-case basis, limitations expressed in terms ofmass are infeasible because the mass of the pollutant discharged cannot be related to a measure of operation permit conditions to ensure that dilution will not be used as a substitute for treatment A permittee must comply with pollutants limited in terms of mass. Additionally, pollutants may be limited in terms of other units of measure- ment, in which case a permittee must comply with both limitations. 650 Fermentation and Biochemical Engineering Handbook 3.6 Waste Water Characterization An understanding of the nature of the physical, chemical, and biologi- cal characteristics of waste water is essential in the design and operation of collection, treatment, and disposal facilities, and in the engineering manage- ment of environmental quality. The analyses performed on waste waters may be classified as physical, chemical, and biological. These analyses vary from precise quantitative chemical determinations to the more qualitative biological and physical determinations. Physical Characteristics. The most important physical characteristic ofwaste water is its total solids content, which is composed offloating matter, matter in suspension, colloidal matter, and matter in solution. Other physical characteristics include temperature, color, and odor. These consist of organic matter, the measurement of organic content, the inorganic matter, and the gases found in waste water. The measurement of organic content is very important because of its importance in both the design and operation of waste water treatment plants and the management of water quality. BioIogicaI Characteristics. Biological aspects with which the sani- tary engineer must be familiar include knowledge of the principal groups of microorganisms found in surface and waste waters, as well as those respon- sible for biological treatment, knowledge of the organisms used as indicators of pollution and their significance, and knowledge of the methods used to evaluate the toxicity of treated waste waters. Chemical Characteristics. 3.7 Common Pollutants Generally, under NPDES program, the following pollutants are re- Oxygen Demand quired to be monitored and reported. - Biochemical Oxygen Demand - Total Oxygen Demand - Total Organic Carbon - Total Suspended Solids - Total dissolved Solids Solids Environmental Concerns 651 Nutrients - Inorganic Phosphorus Compounds - Inorganic Nitrogen Compounds Detergents and Grease - MBAS (Methylene Blue Active Substances) - OilandGrease - Calcium - Chloride - Fluoride - Magnesium - Sodium - Potassium - Sulfur - Sulfate - Total Alkalinity - Total Hardness - Aluminum - Cobalt - Iron - Vanadium - Cyanide - Total Residual Chlorine Minerals Metals Inorganics 4.0 WASTE WATER TREATMENT STRATEGY Different types of waste water streams are generated from various processes in pharmaceutical and biotechnology industries. Treatment of these streams is not only ethically required for not polluting the waters of the nation, but USEPA and the states have mandated strict discharge standards that their plants must meet with their NPDES (or equivalent) permitting requirements. Waste water treatment can no longer be considered as a secondary issue in the plant’s scheme of things, but must be considered an integral issue. The cost of operation and the capital cost are considerable and, therefore, should attract management’s attention. 652 Fermentation and Biochemical Engineering Handbook Waste water treatment process is generally divided into two types: Biological Treatment Physical Treatment Biological treatment utilizes microbial organisms to reduce pollutant loadings of process waste streams to EPA (andor state) acceptable limits. Physical treatment involves reduction of pollutant of process waste stream utilizing physical procedures, such as stripping, ion exchange, membrane separation, etc. 4.1 Activated Carbon Activated carbon is a recommended and established process used in separating organic and certain inorganic species from aqueous waste streams. Normally, concentration of the waste species should be 1% or less so that carbon regeneration is less frequent. Since this process is generally cost- effective, it has been applied in numerous industrial municipal and pharma- ceutical waste water treatment facilities. Adsorption is based on physicaVchemica1 interaction between the organic pollutant and the carbon surface. In this process, a filter bed of activated carbon is placed in a vessel and used to adsorb certain components. A large adsorptive surface area is generally used for the process. Different raw materials, including coal, wood, coconut shells, peat, and coke, are used to produce activated carbon. These products may impact carbon effective- ness for a given application. Carbons for waste water adsorption generally havea large adsorptive surface area, about 2.5 to 7.5 million ft./lb. Pore sizes range within 50 8, and 1000 A. Activated carbon is available in granular and powdered form. Granular carbons are used in the treatment of continuous and semi-continuous waste water streams. It can be packed in canisters or beds which then constitute a process unit. Powdered carbon is used for batch adsorption applications and in conjunction with biological treatment. Favorable results are achieved when the pollutant is slightly soluble in water, has a high molecular weight, high polarity, and low ionization capability, and when the concentration of any suspended solids is less than 50 ppm. Once the carbon has become saturated, regeneration of the carbon is required. Regeneration methods include the following: Environmental Concerns 653 Thermal reactivation in a multiple hearth furnace or rotary Steam stripping Ozone medicated on-site oxidation The method used will vary according to characteristics of the plant site and properties of agiven system. Energy cost from carbon adsorption ranges from 5 to 25% ofthe total operating cost. If carbon is thermally regenerated, equipment cost increases since a furnace, after burner, and a scrubber are needed. If carbon is not regenerated, a means of carbon disposal is needed. kiln at temperatures of 1200’F to 1700’F 4.2 Air Stripping For pharmaceutical waste water streams containing volatile organic compounds (VOC’s), air stripping can be used to reduce containment discharge. Air stripping can be used for treating waste water containing less than 100 ppm VOC and insoluble organics, such as methylene chloride and toluene. Treatment efficiency using air stripping is dependent on Henry’s law, which is as follows: where, PA = Vapor pressure of compound A (atm) HA = Henry’s law constant of compound A (atm) X, = Liquid phase mole fraction of compound A Henry’s constant is a function of temperature and a weak function of composition and pressure, however, the air stripping process is normally run at ambient conditions and published data within 20’ and 3OoC is available. Maximum pollutant removal by air stripping can be predicted by the following: % Removal = ‘Ai -‘A0 100 = 1- XlOO ‘Ai l+H,R,,RT 654 Fermentation and Biochemical Engineering Handbook where: CAi = initial concentration C,, = final concentration HA = Henry’s constant atm m3 / mole R, = volumetric air / water ratio = V / L V = airfedinch L = water fed inch R = gas constant = 8.206 x lom5 atm m3/mol K T = temperatureK The packed tower is compact and efficient for air stripping. The waste water, in a lime slurry (for phosphate removal and pH control), is first sent to a mixing tank and then to a shell settling tank to settle out calcium phosphate and calcium carbonate. The clarified waste water enters near the top ofthe packed tower, while air is introduced countercurrently at the bottom ofthe tower. Waste water product is then sent to a recarbonation basin so that calcium carbonate may be precipitated, removed, and reused. Air strippers are typically 15 to 60 feet in height, and have diameters ranging from 1 to 10 feet. Water flow rates are typically in the range of 40 to 250 lb/ch air and aidwater volumetric ratios are typically in the range of 10 to 300. Generally, contaminated air from the stripper may be routed to an incubator or vapor phase carbon adsorption. 4.3 Steam Stripping Steam stripping is used to remove dilute concentrations of ammonia, hydrogen sulfide, and other volatile components from pharmaceutical waste stream. Steam stripping can typically achieve contaminant removal of 99% or better and is effective for the removal of organics having boiling points of less than 150°C. The steam stripping process is carried out in a distillation column, which may be either a packed or tray tower. Steam enters at the bottom ofthe column while waste water is countercurrently supplied from the top of the distillation column. The product stream, rich in volatile compo- nents, may further be treated to recover these components. Steam strippers are generally designed by the use of computer simula- tion programs, although preliminary estimates can be prepared by modifica- Environmental Concerns 655 tions of McCabe-Thiele or Fenske-Underwood-Gilliland methods. The development of reliable equilibrium data is critical to the completion of a successful design. In general, such data should be obtained through pilot scale testing on the actual waste water stream to be treated. 4.4 Heavy Metals Removal While heavy metals (i,e., chromium, copper, nickel) are not typical pollutants in apharmaceutical waste water stream, removal becomes an issue in some segments of the industry, namely chemical intermediates. These streams are generally treated at the process source in order to minimize the waste water volume. Also, heavy metal streams must be treated prior to any biological treatment that the waste water also requires. Since heavy metals are toxic to microorganisms (even at very low concentrations), their presence reduces biological treatment efficiency. 4.5 Chemical Precipitation Waste stream metal loading can be reduced by hydroxide precipitation. Hydroxide precipitation uses lime or liquid sodium hydroxide as reactants to form insoluble metal hydroxide. Solids are settled, filtered, and removed as sludge. Liquid sodium hydroxide or quick lime are commonly used as precipitation reagents. Generally, lime is cheaper than other reagents, however, it has a higher operating cost because it is difficult to handle. Chrome bearing waste water requires pretreatment, since hexavalent chromium will not react with hydroxide. Hexavalent chrome must be reduced to the trivalent form by reaction with ferrous sulfate, sodium meta-bisulfite or sulfur dioxide. The reduction must be carried out at a pH below 3.0. Achieving complete reduction is important, since any remaining hexavalent chromium will remain in solution in the effluent. The pH is then raised to pH 4.5 to 8.0 and hydroxide precipitation is carried out. Sludge generated from this process requires carefil handling as this waste is considered hazardous. Water content of the sludge can be reduced, generally using plate and frame presses or clarifiers. Plate and frame presses, generally require more operation and attention, however, they can achieve higher solids concentration in the cake, typically 50% to 60%. 656 Fermentation and Biochemical Engineering Handbook 4.6 Electrolysis Electrolysis is the reaction of either oxidation or reduction taking place at the surface of conductive electrodes immersed in an electrolyte, under the influence of an applied potential. This process is used for reclaiming heavy metals from concentrated aqueous solutions. Application to waste water treatment may be limited because of cost factors. A frequent application is the recovery for recycle or reuse of metals, like copper, from waste streams. Pilot applications include oxidation of cyanide waste and separation of oil- water mixtures. Gaseous emissions may occur and, ifthey are hazardous and cannot be vented to the atmosphere, firther treatment, such as scrubbing, is required. Waste water from the process may also require further treatment. The most common waste water treatment application of electrolysis is the partial removal of heavy metals from spent pickling solutions. When the typical concentration of the spent pickling solution is a 2 to 7% copper, the system design is similar to that of a conventional electroplating bath. When recovering from more dilute streams, mixing and stirring are necessary to increase the rate of diffusion; it may also be necessary to use a large electrode surface area and a short distance between electrodes. Another concern with this method is the removal of collected ions from the electrodes. This removal may or may not be difficult, but must be addressed. The material collected must be ultimately disposed of if it is not suitable for reuse. 4.7 Ion Exchange Ion exchange can be useful for heavy metal removal, particularly for nickel, zinc, copper, or chrome, where the metals can be recovered from the regenerating solution and recycled to the process or sold. Ion exchange has also been applied to treatment of streams containing complexing agents or their compounds, that would interfere with a precipitation process. Ion exchange is a two-step process. First, a solid material, the ion exchanger, collects specific ions after coming into contact with the aqueous waste stream. The exchanger is then exposed to another aqueous solution of a different composition, that picks up the ions originally removed by the exchanger. The process is usually accomplished by sending the two aqueous streams through one or more fixed beds of exchangers. The ion-rich product stream may be recovered or disposed and the ion-poor stream is usually dilute enough for discharge to sewers. Environmental Concerns 657 The chemistry of the ion exchange process may be represented by the following equilibrium equations: Reaction: RH+Na+ f RNa+H+ RNa, + Ca" +; RCa + 2Na' Regeneration: RNa+HCl S RH+NaCl RCa + 2NaCI S RNa, + CaCl, where R represents the resin. These equations represent the reactions involved in the removal of sodium and calcium ions from water, using a synthetic cationic-exchange resin. The extent of completion of the removal reactions shown depends on the equilibrium that is established between the ions in the aqueous phase and those in the solid phase. For the removal of sodium, this equilibrium is defined by the following expression: where KH + Na = selectivity coefficient [ 3 X, XRNa = concentration in solution phase = mole fraction of hydrogen on exchange resin = mole fraction of sodium on exchange resin The selectivity coefficient depends primarily on the nature and valence of the ion, the type of resin and its saturation, and the ion concentration in waste water. Although both natural and synthetic ion exchange resins are available, synthetic resins are used more widely because of their durability. To make ion exchange economical for advanced waste water treat- ment, it would be desirable to use regenerants and restorants that would remove both the inorganic anions and the organic material from the spent resin. Chemicals successfbl in the removal of organic material from resins include sodium hydroxide, hydrochloric acid, and methanol. 658 Fermentation and Biochemical Engineering Handbook 4.8 Membrane Technology Reverse osmosis uses semipermeable membranes and high pressure to produce a clean permeate and a retentate solution containing salts and ions, including heavy metals. The technique is effective ifthe retentate solution can be reused in the process. The equipment tends to be expensive, and fouling of the membranes has been a common problem. Considerable research effort is being carried out on membrane processes, however, and they are likely to be more commonly applied in the future. Concentrations of dissolved components are usually about 34,000 ppm or less. Reverse osmosis employs a semipermeable membrane that allows passage of the solvent molecules, but not those of the dissolved organic and inorganic material. A pressure gradient is applied to cause separation of the solvent and solute. Any components that may damage or restrict the function of the membrane must be removed before the process is performed. Capital investment and operating costs depend on the waste stream composition. 4.9 Organic Removal Sequencing Batch Reactor (SBR). SBRs are used in batch modes. They consist of one or more reactors used for equalization, aeration and clarification in sequence. Operation cycle of atypical SBR system consists of five steps, (A) fill, (B) react, (C) settle, (0) draw and (a idle. A. Fill-the waste water is pumped into reactor tank, con- trolled either by volume or time. B. React-in this step, the aeration equipment is energized to supply air to the batch, where oxidation of the organic occurs. C. Settle-solids are separated from the liquid in this step and the clarified liquid is then discharged from the system. D. Draw-in this step, clarified water is discharged from the reactor. E. Idle-this step is used where two or more reactors are connected in an SBR system. One reactor remains idle while others (one or more) are filled. Environmental Concerns 659 SBR systems are generally advantageous to use over conventional flow One SBR tank can serve as an equalization tank, reactor tank and clarifier. SBR can tolerate high peak hourly flows, since it serves as an equalization tank. Effluent quality is not compromised. Operating level in SBR tank is adjustable. Hence, during low flow conditions, a smaller batch can be collected and treated as desired. Solids in tank do not get washed out by hydraulic surges as solids can be held in the tank as long as necessary. systems. They include: 4.10 Activated Sludge Systems Different types of activated sludge systems are used in treating pharmaceutical waste water. Some sludge systems include conventional, complete mix, contact stabilization, extended aeration, and step aeration. Conventional-This type includes an aeration basin, a clarifier, and a sludge recycle system. The recycle sludge (RAS) and influent enter the aeration basin at the inlet and leave the basin in the opposite or outlet end. The solids in the mixed liquor get separated out in the clarifier and are recycled back to the aeration basin. Aeration in the basin can be achieved by diffused air or mechanical aerations. CompleteMix-The influent and the RAS are introduced at several points in the center of the aeration basin from a header or central channel. Effluent from the aeration basin is collected from both sides of the basin by means of two effluent channels. A clarifier is used to separate the solids in the basin effluent before the sludge is recycled back to the head of the system. Excess sludge is sent to the sludge handling facility of the treatment plant. Aeration can be accomplished by diffused air or mechanical aeration. 660 Fermentation and Biochemical Engineering Handbook Contact Stabilization-The influent is first mixed with the return activated sludge for approximately 30 minutes or long enough for the organics to be absorbed in a contact basin. Effluent from the contact basin is fed to a clarifier, where the activated sludge is settled out and returned to another aeratiodstabilization basin. Inside the stabilization basin the activated sludge is aerated for 3-6 hours before it is mixed with the influent in the contact basin. The absorbed organics are converted to carbon dioxide, water, and new cells, by means of biochemical oxidation. Excess sludge, generated from the process, can be wasted from the bottom of the clarifier or from the stabilization basin outlet. Extended Aeration-In this process, aeration time is about 24 hours. The activated sludge system operates in an endogenous respiration phase in which there is inadequate organic material in the system to support all the microorgan- isms present, due to a low BOD loading. Under this type of operating condition, extended aeration process can produce low sludge and a highly treated effluent. Step Aeration-Influent to the aeration basin is split up into four or more equal streams and are then fed into four parallel channels separated by baffles. Each channel is equivalent to a separate step and these steps are connected together in series, very similar to having four small plug-flow systems arranged in series. The first step is commonly used to reaerate the return activated sludge, assuming the sludge is not oxygen starved when it comes in contact with the influent. With the return activated sludge aerated, the organic material in the influent can be readily absorbed and broken down within a relatively short contact time. 5.0 AIR (EMISSIONS OF CONCERN) A major concern associated with chemical processing is the emission of air pollutants. The greatest mass of air contaminants consists primarily of the following pollutants: Environmental Concerns 661 Volatile Organic Compounds (VOC) Inorganics Particulates 5.1 Volatile Organic Compounds (VOC) VOC's are emitted from chemical processes either controlled or uncontrolled. Control techniques are provided in the following sections. Emission can also result from the incomplete combination of organic constituents and conversion of certain constituents present in the raw material, auxiliary fuel, andor combustion air. 5.2 Inorganics Inorganic pollutants, such as HCL, HF, NO,, SO,, are also formed as Hydrogen chloride and small amounts of chlorine from the combustion of chlorinated hydrocarbons Sulfur oxides, mostly as sulfur dioxide (but also SO,) formed from sulfur or sulfur compounds present in the products andor fuel mixture Nitrogen oxides from the nitrogen in the combustion air and or from organic nitrogen present in the product. a result of incomplete combustion. Inorganics include: 5.3 Particulates Particulates emissions are strongly influenced by the chemical composition of the raw material and the auxiliary fuel, type of combustion process, the operating parameters, and the air pollution control system. Most of the pollutants of concern, other than VOC and inorganics, are collected as particulates. 6.0 SELECTING A CONTROL TECHNOLOGY There are a number of control options for air toxics, particulates, SO, and NO,. It is up to the chemical engineer to choose the most cost-effective control equipment for the source application. 662 Fermentation and Biochemical Engineering Handbook Selection of control equipment begins with gathering relevant data on the emissions and key process parameters. The properties ofthe exhaust-gas stream and the pollutants need to be characterized. These include: 6.1 Exhaust Stream Pollutant concentration Flowrate Temperature Pressure Moisture content Oxygen content Heat content Corrosiveness Explosivity 6.2 Pollutant Particle size distribution Molecular weight Vapor pressure Solubility Adsorptive properties Lower explosive limits Reactivity This infomation can be obtained from vent testing, a mass balance, engineering calculations, or simple engineering estimates (using AP-42). Vent testing, though expensive, is the most accurate method. Other important required information is as follows: The level of control required by the regulatory agency must be known. This will allow the minimum control efficiency to be established. Environmental Concerns 663 Site-specific issues impacting the selection of the control equipment, must be quantified. These include availability of utilities, pace constraints, disposal options, and cost of residue generated by emissions control. Cost effectiveness has to be judged. Cost effectiveness is defined as the annual operating expense required to control each zone of emissions. The cost effectiveness calculation provides a gauge for ranking various control combinations within a facility so that the greatest emission reduction can be selected for the least cost. Secondary environmental impacts, as well as energy im- pact, also must be considered. Consideration must be given to potential for waste minimi- zation at the facility. If feasible, this could reduce or even eliminate the need for emission control equipment. 7.0 VOLATILE ORGANIC COMPOUND (VOC) EMISSIONS CONTROL The control of VOC’s is the single largest environmental challenge facing CPI companies, especially with the enactment of CAAA of 1990. About 80% of the annual air toxic emissions (per SARA Title 111, Sec. 3 13, Form R Reporting) are VOC’s. Most commonly employed control technology for VOC emissions control are as follows: Thermal Incineration Catalytic Incineration Carbon Adsorption Condensation Absorption The choice is often dependent on VOC concentration of the stream being controlled, because control efficiency depends on VOC content. 664 Fermentation and Biochemical Engineering Handbook 7.1 Thermal Incineration In incineration, gaseous organic-vapor emissions are converted to carbon dioxide and water through combustion. There are two types of thermal incinerator, based on heat recovery employed, regenerative and recuperative. Thermal incinerators depend upon contact between the contaminant and the high-temperature combustion flame to oxidize the pollutants. The incinerator, generally consists of refractory-lined chamber, one or more burners, a temperaturecontrol system, and heat-recovery equipment. Contaminated gases are collected by a capture system and delivered to the preheater inlet, where they are heated by indirect contact with the hot incinerator exhaust. Gases are mixed thoroughly with the burner flame in the upstream portion of the unit, and then passed through the combustion zone, where combustion process is completed. An efficient thermal incinerator design must provide: Adequate residence time for complete combustion Sufficiently high temperature for VOC destruction Adequate velocities to ensure proper mixing The residence times for incinerators are on the order of 0.5 to 1 seconds, at a temperature ranging from 1200°F to 1600°F. Destruction efficiencies in excess of 95% can be commonly achieved. Advantages: Simple operating concept Nearly complete (>95%) destruction of VOC's No liquid or solid residual waste generation Low maintenance requirements Low initial capital costs Disadvantages: High fuel cost The fuel costs can be minimized by utilizing air pre-heaters incorpo- rated into the incinerator design. With a recuperative heat exchanger, efficiency of 60% is typical. Thermal incinerators with regenerative heat exchangers can recover 80-95% of the systems energy demands. Regenera- tive incinerators can initially cost roughly 80% more than recuperative Environmental Concerns 665 designs. With annual fuel costs of about 10-30% those of recuperative units, the savings can be significant if contaminated air has a low VOC concentra- tion (<25 ppmv) and auxiliary fuel costs are high (>$YmmBTU). 7.2 Catalytic Incineration The operation of catalytic incineration is similar to thermal incinera- tion in that heat is used to convert VOC to carbon dioxide and water. The presence of a catalyst lowers the oxidation activation energy, allowing the combustion to occur at about 600°F. Operation. fie preheatedgas stream is passedthrough a catalyst bed, where the catalyst initiates and promotes the oxidation of the organic without being permanently altered. The catalyst is normally an active material, such as platinum, copper chromite, chromium, or nickel, on an inert substrate, such as honeycomb-shaped ceramic. For the catalyst to be effective, the active sites upon which the organic gas molecules react must be accessible. The buildup of polymerized material or reaction with certain metal particu- lates will prevent contact between active sites and the gas. A catalyst can be reactivated by removing such a coating. Catalyst cleaning methods are as follows: Air blowing Steam blowing Operating at elevated temperature (about 100°F above Advantages: Nearly complete destruction of VOC (>95%) No residual waste generation 0 Low maintenance cost Disadvantages : High capital costs Catalyst deactivation over time Inability to handle halogenated organics Supplement fuel cost operating temperature) 666 Fermentation and Biochemical Engineering Handbook 7.3 Carbon Adsorption Adsorption is a process by which organics are retained on the surface ofgranulated solids. The solid adsorbent particles are highly porous and have very large surface-to-volume ratios. Gas molecules penetrate the pores ofthe adsorbent and contact the large surface area available for adsorption. Activated carbon is the most common adsorbent for organic removals. The amount of VOC retained on the carbon may be represented by adsorption isotherm, which relate the amount of VOC adsorbed to the equilibrium pressure (or VOC concentration) at a constant temperature. The adsorptive capacity of the carbon (expressed as VOClb/Clb) depends not only on properties on the carbon, but also on the properties of the organic. Generally, the adsorptive capacity increases with: Increased molecular weight of the VOC Polarity Degree of cyclization (ringed compound more easily ad- Regenerative carbon adsorption systems operate in two modes- adsorption and desorption. Adsorption is rapid and removes essentially all the VOCs in the stream. Eventually, the adsorbent becomes saturated with the VOC and system efficiency drops. At this breakthrough point, the contaminated stream is directed to another bed containing regenerated adsorbent and the saturated bed is then regenerated. The adsorption cycle typically lasts two hours to many days, depending on the inlet VOC concentration, the variability of organic loading, and the design parameters of the carbon bed. The regenerative cycle typically lasts from one to two hours, including the time needed for drying and cooling the bed. One important consideration of this system is the operating tempera- ture of the process gas stream. Operating temperature must be less than 100OF. This is because the adsorption capacity decreases with the increase in temperature. The efficiency of carbon adsorption depends on both the concentration of VOC in the gas stream and its composition. Generally, efficiencies of over 95% can be achieved when the organic concentrations are greater than 1,000 ppmv. Advantages: Recovery of relatively pure product for recycle High removal efficiency (>95%) Low fuel costs sorbed than straight chain hydrocarbons) Environmental Concerns 667 Disadvantages: Potential generation of hazardous organic wastes Generation of potentially contaminated waste water Higher operating and maintenance cost for disposal of these waters 7.4 Adsorption and Incineration This process involves a combination of activated carbon adsorption with incineration. The adsorber concentrates the organic laden air before treatment by incineration. This approach is particularly useful for organic streams with a low concentration and higher volumes (e100 ppmv and flowrates over 20,000 ch), such as paint spray booths. This process has many advantages. These include: High destruction efficiency Little or no generation of liquid or solid waste of incineration Low fuel consumption 7.5 Condensation Condensation is a basic separation technique where a contaminatedgas stream is first brought to saturation and then the contaminants are condensed to a liquid. The conversion of vapor to liquid phase can be accomplished either by increasing the pressure at constant temperature, or reducing the temperature, keeping the pressure constant. Generally, condensation systems are operated at a constant pressure. The design and operation of the system is affected by the concentration and type of VOC’s in the emission stream. Before condensation can occur, the dew point of the system (where the partial pressure of the organic is the same as the system pressure) must be reached. As condensation continues, VOC concentration in the vapor decreases, and the temperature must be lowered even further. The removal efficiency of the condenser ranges from 50% to 95% or more and depends on the partial pressure of the organic in the gas stream, which is a function of the concentration of the organic and the condenser temperature. For a given temperature, the greatest potential removal efficiencies are achieved with the largest initial concentrations. VOC removal efficiencies via condensation may reach 95% or more for concentra- tions in excess of 5,000 ppmv. 668 Fermentation and Biochemical Engineering Handbook Plots of vapor pressure versus temperature (Cox charts) are used to determine the temperature required to achieve the desired removal efficiency. Generally, the condenser outlet organic concentration will be greater than 10,000 ppmv for a water-cooled system. For higher removal efficiencies, other coolants, such as a brine solution (-30°F to 40°F) may be used. Condensation offers the advantage of: Product recovery No disposal problems Modest space requirements Disadvantages include: LimitedapplicabilitytostreamswithhighVOCconcentrations Limited applicability to streams with single components if the product is to be recycled and reused. 7.6 Absorption Absorption is the mass transfer of selected components from a gas stream into a nonvolatile liquid. Such systems are typically classified by the absorbent used. The choice of absorbent depends on the solubility of the gaseous VOC and the cost of the absorbent. Absorption is a fbnction of both the physical properties of the system and the operating parameters of the absorber. The best absorption systems are characterized by low operating temperatures, large contacting surface areas, high liquid/gas (L/G) ratios, and high VOC concentration in the gas stream, For inlet concentration of 5,000 ppmv, removal efficiencies of greater than 98% may be achieved. Absorption may also be efficient for dilute streams provided the organic is highly soluble in the absorbent, removals of 90% may be attained for concentrations as low as 300 ppmv. Packed towers, venturi scrubbers, and spray chambers may be used for absorption. The efficiency of an absorber depends on: Solubility of VOC in solvent Concentration of VOC in the gas stream Temperature L/G ratio Contact surface area Environmental Concerns 669 Higher gas solubilities and inlet gas concentrations provide a greater driving force and hence, a higher efficiency. Also, lower temperature causes higher solubility, absorption as enhanced at lower temperatures. Generally, the most economical absorption factor is 1.25 to 2 times the minimum L/G. Absorption increases with contact surface are, thereby, removal efficiency, however, this also raises overall pressure drop through the packed bed, hence increasing energy costs. 8.0 PARTICULATE CONTROL Most commonly used particulate control technologies are: Fabric filters (baghouses) Cyclones/mechanical collectors Electrostatic precipitators 8.1 Fabric Filters (Baghouses) The basic components of a baghouse are: Filter medium in form of fabric bags Tube sheet to support the bags Gas-tight enclosures Mechanism to dislodge accumulated dust from the bags The particulates laden gas normally enters the lower portion of the baghouse near the collection hoppers, then passing upward through the device, either on the outside or the inside of the bags, depending on the specific design. Commercially available baghouses employ either felted or woven fabric. A fabric is selected based on its mechanical, chemical, and thermal characteristics. Some fabrics (like nomex) are better suited than others (like polyester) for high temperature operations, some perform well in the presence of acid gases, while others are especially good at collecting sticky particulates because of good release characteristics. The two design and operational parameters that determine fabric filter performance are air-to-cloth ratio and pressure drop. The air-to-cloth ratio is the volumetric flowrate of the gas stream divided by the surface area of the fabric. The higher the ratio, the smaller the baghouse and higher the pressure 670 Fermentation and Biochemical Engineering Handbook drop. Shaker and reverse-air baghouses with woven fabrics, generally have alowerair-toclothratic+rangingfiom2.0 to3.5, dependingonthedusttype being collected. Pulse-jet collectors with felted fabrics have higher air-to- cloth ratios, ranging from 5 to 12. The pressure drop across the filter medium is a fknction of the velocity of the gas stream through the filter and the combined resistance of the fabric and accumulateddust layer. Pressure drop across the filter medium is usually limited to 6-8 in. H,O. Advantages: Performance Uniform collection efficiency independent of particle resistivity Collection efficiencies exceeding 99 wt% Disadvantages: Clogging ofthe filter medium, due to condensation in the gas stream Cementation of the filter cake in humid, low-temperature gases (especially in the presence of lime from a scrubber) Excursions of high particulate concentrations during bag breaks 8.2 CyclonedMechanical Collectors Cyclones are seldom used as the primary means of particulate collec- tion, but often serve asfirst-stage air cleaning devices that are followed by other methods of particulate collection. Cyclone collection efficiency is probably more susceptible to changes in particulate characteristics than are other types of devices, therefore, care should be taken if used. Cyclone operation is dependant, generally, on physical parameters such as particle size, density, and velocity, as opposed to the chemical nature or properties of the material being collected. 8.3 Electrostatic Precipitators ESP's are generally used to remove particulates from gas streams that can be easily ionized. A typical ESP consists of charged wires or grids and positively grounded collection plates. A high voltage is applied between the Environmental Concerns 671 negative electrodes and the positive collection plates, producing an electro- static field between the two elements. In the space between the electrodes, a corona is established around the negatively charged electrode. As the particulate-laden gas passes through this space, the corona ionizes molecules of the electronegativegases present in the stream. These particles get charged and migrate to the oppositely polarized collection plates. ESP's can be designed for virtually any control efficiency, with most units operating in the 9599% range. ESP control factors include the Specific Collection Area (SCA), which is the area ofthe collecting electrodes or plates divided by the volumetric flowrate of the gas, The higher the SCA, the greater the collection efficiency. Particles with resistivities in the range of 104-1010 ohm-cm are the most suitable for control by ESP's and most common industrial particulates will exhibit resistivities in this range. Less resistive particles will give up their charge too easily when they contact the collection plates and may be re- entrained in the flue gas. More resistive particles will adhere to the collection plates and be difficult to dislodge, acting as an insulator and reducing the ability of the electrode to hrther collect particulate matter. Since resistivity changes with temperature, efficient particulate collection requires selection of an optimum ESP operating temperature. Entrained water droplets in the flue-gas can encapsulate the particles, thus lowering resistivity. High flowrates decrease the residence time of the particles in the ESP, reducing the number of charged particles that migrate to the collection plates. The particle migration velocity, the rates at which charged particles travel toward the collection plates, also affects ESP efficiency and the unit's design specifications. A slow migration velocity indicates less particle capture per unit of collection plate area. The surface area of the collecting electrode would, therefore, have to be increased for applications involving large quantities of small particles. Advantages: Reliability and low maintenance requirements Relatively low power requirements, due to low pressure High collection efficiencies over a wide ranges of particle Ability to treat relatively humid gas streams drops sizes 672 Fermentation and Biochemical Engineering Handbook A 50,000 ach ESP operating on a coal fired boiler, with an estimated mean particle diameter of 7 micrometers, could achieve 99.9% control with a total ESP pressure drop of 0.38 in H,O. Disadvantages: Collection efficiency unreliable if gas property or particle Inorganic particulates difficult to collect Requires heating during start-up and shutdown to avoid size distribution changes. corrosion because of condensation of acid gases 9.0 INORGANICS With the passage of the CAAA, emissions control for numerous inorgaincs have become mandatory. Technologies that can be used effec- tively to control emissions include: Wet scrubbing Adsorption Incineration Adsorption and incineration are discussed in Sec. 7. 9.1 Wet Scrubbing The ionic nature of acids, bases and salts are removed from flue gases by wet scrubbing because the ionic separation that occurs in water creates advantageous equilibrium conditions. Removal may often be enhanced by manipulation of the chemistry of the scrubbing solution. Both spray towers and packed-bed towers operate based on common principles of absorption. Pollutants in the form ofgases are transferred from the gas stream to the scrubbing liquid as long as the gas is not equilibrium in the liquid stream. An important consideration in design of the spray or packed-bed towers isflooding. Flooding, where liquid is carried back up the column by the gas stream, occurs when the gas stream velocity approaches the flooding velocity. Tower diameter is established based on superficial gas velocity ranging from 50% to 75% of the flooding velocity. Spray towers operate by delivering liquid droplets through a spray- distribution system. Generally, the droplets fall through countercurrent gas Environmental Concerns 673 stream by gravity. A mist eliminator removes liquid entrained in the gas stream prior to its discharge to the exhaust stack. Typical pressure drops in a spray tower are 1-2 in H20, and design L/G ratios are, generally, about 20- 100 gaVl000 ft3. Spray towers have relatively low energy requirements (about 3 x 104kW/actm airflow), however, water usage is high. Economics of spray tower operation is influenced by waste water disposal costs. Capital costs consists mainly of cost of the vessel, chemical treatment system, and waste water treatment system. In packed-bed scrubbers, liquid is flown from the top of the tower and it flows over a random or structured packing. Generally, in the industry, countercurrent flow with high LIG ratio packed-bed scrubber are prevalent, when particulate loadings are higher. These provide the highest theoretical removal efficiencies, because gas with the lowest pollutant concentration contacts liquid with the lowest pollutant concentration, thus maximizing the absorption driving force. Pressure drops of 1-8 in H20 are typical, while an L/G ratio range of 10-20 gaVlOOO ft3 is generally employed. Packed-bed towers can achieve removal efficiencies of over 99% and have relatively lower water consumption requirements. They also offer design and retrofit flexibility. Disadvantages include high system pressure drops, relatively high clogging and fouling potential, potentially high main- tenance costs, and waste water disposal requirements. Packed-bed scrubbers are also more expensive to install and operate than spray towers. REFERENCES 1. 2. 3. 4. 5. 6. 7. 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