Foreward

4.3. Site Visit

During a site visit health assessors could see, smell, or feel blowing fugitives. Observing the site characteristics and proximity of the community will aid them in evaluating the potential for the public to be affected by reasonable worst-case fugitive emissions.

ATSDR recommends health assessors conduct a site visit of the thermal treatment facility to observe waste and residuals handling practices, facility operation, and environmental sampling. A site visit is necessary to understand the configuration and operation of the facility, its relationship to the community, susceptible populations, and any topographic features in the area that could affect the dispersion of facility emissions. Health assessors will find the site visit most productive if they review in advance the facility design and operating conditions, the environmental monitoring plan, and the health and safety plan.

During site visits to thermal treatment facilities, health assessors should also look for such things as:

• How close are residences to the facility or site?
  
  
• Are residential areas likely to be contaminated from site runoff?
  
  
• Are residences in the predominant wind direction from the site?
  
  
• Are day care facilities, schools, hospitals, nursing homes, or other institutions that are occupied 24 hours a day, or other sensitive populations, etc. near the site?
  
• Are large or tall buildings nearby that could affect down-wash or could be fumigated by stack emissions?
  
  
• Are hills, mountains, valleys, canyons, large lakes, oceans, or other topographic features in the area affecting the wind currents that must be considered when modeling?
  
  
• Does the area experience frequent inversions or other weather conditions that should be considered when evaluating the facility emissions?
  
  
• Are food or water supplies such as home gardens, diaries, cattle ranches, farms, etc. likely to be affected by fallout from the facility?
  
  
• Does a potential for worker exposure exist, and does the site have an adequate worker protection program? Health assessors should also note any worker safety issues they observe while on site.

If health assessors identify worker safety problems or are unsure whether the facility's program is adequate, they should discuss their findings with their supervisor before contacting the National Institute for Occupational Safety and Health (NIOSH) or the Occupational Safety and Health Administration (OSHA).

When visiting RCRA or CERCLA sites, ATSDR Health assessors must comply with OSHA training and medical monitoring regulations and wear the appropriate personal protective equipment (PPE) for the site conditions (ATSDR 1991).

To evaluate the potential public health effects of a thermal treatment technology, health assessors must be familiar with the design of incinerators and desorbers. This chapter discusses the common subsystems, explains the significant features of various types of equipment that could be used, and describes the emissions that could be of public health concern.

5.1. Thermal Treatment Facility Designs

An incinerator removes and destroys organic constituents in the waste. A desorber removes and captures or treats organic constituents.

Thermal treatment units should be designed to handle the specific type(s) of waste to be treated. Properly designed thermal desorption units can effectively remove volatile organic compounds (VOCs), semi-volatile organic compounds (SVOCs), PCBs, pesticides, and petrochemicals from solid wastes such as contaminated soils. Some desorber designs can also decontaminate small amounts of sediment or liquid waste in conjunction with solid waste. Incinerators can be designed to treat all types of wastes simultaneously or a single type. Table 2 lists desorbers, the major incinerator types, and the various physical wastes they can treat.

The four major subsystems in a thermal treatment facility are (1) waste preparation and feed systems, (also known as pretreatment), (2) combustion or desorption chambers, (3) air pollution control equipment, (also known as gas post treatment), and (4) liquids and ash handling, (also known as solids post treatment), and residuals management systems. Figure 1, in Appendix C, shows the general orientation of the subsystems and typical process component options for incinerators. Figure 2, in Appendix C, shows the general orientation of thermal desorption systems. The desorber unit could be a rotary kiln/dryer, thermal screw, fluidized bed, distillation chamber, or belt conveyor system (EPA 1993). A desorption system and an incinerator have some design differences. A desorption system has a chamber operating at temperatures just high enough to effectively vaporize, but not combust, the organic compounds from the contaminated material. It also has condensers, carbon adsorption units or both in addition to or instead of any of the other air pollution control equipment (APCE) shown in Figure 1. An incinerator has one or two refractory lined combustion chambers operating at high enough temperatures to vaporize any organic compounds and destroy them (i.e., convert them to carbon dioxide, water, and acid gases such as hydrochloric acid vapors).

The so-called three T's - time, temperature, and turbulence - are important for complete incineration of organic chemicals. For complete combustion to occur, the volatilized or partially destructed organic chemicals need sufficient time at a high enough temperature and turbulence to mix thoroughly the off-gases with excess oxygen.

5.1.1. Pretreatment - Waste Preparation and Feed Systems

The type of waste preparation and feed system is determined by the physical form of the wastes, the type of contaminants, and the type of primary combustion chamber (PCC) or desorption chamber (DC). The physical forms of the wastes (gas, liquid, sludge, or solid) and the type of contaminants (flammable, VOCs, etc.) to be treated can also affect the design of the other three subsystems.

Gaseous wastes are piped into the combustion or desorption chambers. If the gases are flammable or ignitable they can be fed to the DC, PCC, or SCC. But non-flammable gases are usually fed only to the DC or PCC.

Liquid wastes are usually blended agitated or both, then pumped through nozzles or atomizing burners into either one or both of the incinerator combustion chambers. Liquid wastes should be screened to avoid clogging the small nozzle or atomizer openings. Small quantities of liquids can be treated in desorbers by spraying the liquids on solid wastes during pretreatment or injecting them into the chamber while feeding solid wastes.

Sludges are usually fed into the DC or the incinerator PCC through water-cooled lances. In desorbers, the percentage of liquids in waste feeds must often be limited because some desorption chamber designs cannot accommodate sludges or slurries. In those cases in which limitation is required, sludges or slurries should be dewatered or mixed in small amounts with the solid wastes, then fed to the DC.

Solid wastes are fed into the DC or PCC. Containerized wastes can only be incinerated, and are typically fed through an airlock ram feeder into the PCC. But they can also be gravity fed through a chute into the PCC. Special solid wastes, such as debris or PCB capacitors, are usually shredded before they are fed into the PCC; they are not normally treated in desorbers. Bulk solid wastes, such as contaminated soil, are typically fed to the desorber or incinerator via vibratory or screw feeders or conveyor belts. If the solid wastes are fed to a fluidized bed incinerator or desorber, all solids generally must be screened, crushed, or shredded to less than 2 to 2.5 inches in diameter. Some desorber designs (e.g., 4-inch screw conveyors) can accept waste only 0.5 inches in diameter or even smaller. If the screw conveyor is 12 inches in diameter or larger, the materials can be 2 inches in diameter or greater (Kulweic 1985). In this regard, it is important to note that screening, crushing, pulverizing, or shredding wastes can increase the potential for fugitive emissions.

Flammable or ignitable wastes require special nonsparking equipment to prepare and feed the waste. If the waste feed or preparation areas are enclosed, fire codes could require an automatic fire suppression system in addition to good ventilation to prevent vapors from accumulating and approaching the lower explosive limit (LEL). Good ventilation is also necessary to prevent oxygen displacement and worker asphyxiation hazard-especially if the pretreatment area is enclosed and the wastes contain VOCs. The air from enclosed pretreatment areas should be vented to the thermal treatment chamber or a carbon filter system. To prevent volatilization of the VOCs, waste piles containing VOCs should be covered.

Waste analysis is another important part of waste preparation. Waste analysis ensures that waste does not contain any chemicals e.g., PCBs, dioxins, certain metals, etc., that should not be treated in a particular treatment facility. Analysis also provides information, e.g., pH, viscosity, water content, concentrations of various metals and certain organics, solids, compatibility with other wastes stored on site, etc., necessary to protect the equipment and to properly manage the waste. The parameters for analysis at each facility is determined on a site-specific basis, depending on the type of equipment in use and permit conditions.

5.1.2. Combustion and Desorption Chambers

Incinerators operating with PCB and hazardous waste permits typically have two combustion chambers. This is to meet the EPA regulatory requirement to destroy or remove 99.9999% of the PCBs and dioxins present in the wastes, and 99.99% of the other designated hazardous organic constituents. If sludges or solids are to be incinerated, the PCC might be a rotary kiln, fixed hearth, infrared, or fluidized bed, which can handle solids and sludges as well as liquid and gaseous wastes. If only liquid wastes are to be treated, a liquid injection incinerator is typically used. Most PCB and hazardous waste incinerators also have a fired secondary combustion chamber (SCC) or afterburner to provide the temperatures (1800F - 2200°F) and residence time (2 seconds) necessary to destroy effectively the wastes. Because of the high temperatures, incinerator combustion chambers are refractory lined. The lining can be firebrick, cast-in-place refractory, or a sprayed-on insulation. Under the CERCLA program, the chambers' design can depend on whether TSCA or RCRA regulations are ARARs for the site.

Desorbers are used to clean up contaminated soils, including PCBs, petroleum, hazardous waste, and other Superfund wastes. EPA has not specified performance standards for desorbers. If they treat wastes classified as hazardous, they are regulated under 40 CFR Part 264 Subpart X, Miscellaneous Units. Site-specific operating conditions are set for each desorption facility based on the contaminants present in the waste and the design of the unit (see Chapter 2 for further discussion). Currently, no fixed commercial desorber facilities treating hazardous wastes are operating under a RCRA permit. The most commonly used desorption chambers (DCs) are rotary dryers (rotary kilns), thermal screws, indirect calciners, and belt desorbers.

Typical desorber operating temperatures are 200°F - 1000°F (EPA 1993). They could, however, operate as high as 1500°F, depending on the contaminant, waste matrix, and desorber construction materials. The air pollution control equipment train immediately follows the DC. DCs are not usually refractory lined because the flame is usually not in contact with the metals or wastes in the chamber. If, however, the DC must be run at higher operating temperatures to decontaminate effectively the soil being treated, it could require refractory lining.

5.1.3. Air Pollution Control Equipment

The types of wastes to be treated will also dictate the type of APCE needed to protect the public from exposure to harmful stack emission concentrations. All incinerators treating wastes containing halogens, such as chlorinated solvents, PCBs, or metal halides, need APCE designed to remove or neutralize acid gases. Acid gases, such as hydrogen chloride (HCl) could be generated when these wastes are combusted. Also, desorbers sometimes operate at such high temperatures that acid gases are produced from decomposition of halogenated compounds.

Acid gases are removed by either wet or dry scrubbing systems.

A typical wet scrubbing system consists of a quench (to reduce rapidly the gas temperature, which reduces recombinant reactions that could otherwise form dioxins, furans, etc., and to remove some particulate and acid gases), a venturi scrubber (to control particulate and to remove acid gases), a packed bed or tray tower (to
remove acid gas and additional particulate), and a demister (to reduce the visibility of the vapor plume). Generally, for acid absorption and neutralization, caustic soda or lime is added to the water circulating through the venturi scrubber and packed bed or tray tower.

A typical dry scrubbing system uses a waste heat boiler after SCC or thermal oxidization to cool the off-gas to 400°F - 600°F before it flows into an absorber. Lime, in the form of calcium hydroxide slurry, is injected into the absorber as a finely atomized spray, in the 5% - 50% slurry to water range. The temperature of the gases exiting the absorber are normally maintained in the 250°F - 300°F range. The flue gases next flow into a particulate collection device, such as an electrostatic precipitator (ESP) or a baghouse (also called a fabric filter).

To detect organic breakthrough as quickly as possible, it is very important to have a hydrocarbon monitor after a carbon adsorption unit.

To remove the organic contaminants volatilized from the wastes, desorbers typically have a wet or dry scrubbing system for particulate removal, followed by a series of condensers, then activated carbon adsorption units. If the condensers are not operated at temperatures well below the lowest-boiling organic expected in the flue gas, the carbon adsorption units could quickly become saturated and ineffective. Also, carbon adsorption units do not efficiently remove very low-boiling VOCs. Another problem with these units is that channeling can occur, which allows some of the flue gas to pass straight through the unit without filtration.

In order to detect organic breakthrough as quickly as possible, a CEM to detect total hydrocarbons (THC) should be installed after a carbon adsorption unit. The facility should have procedures in place to change out the carbon units when breakthrough occurs. The CEM flue gas monitors, such as for carbon monoxide (CO), total hydrocarbon (THC), radioactivity, or monitors for specific extremely toxic chemicals, are typically installed in the stack after the APCE.

High efficiency particulate air (HEPA) filters are usually only used on TT systems treating radioactive or extremely toxic wastes. To avoid some of the problems with condensers, some of the newer desorption facilities are using catalytic oxidizers to destroy the organic compounds in the off-gases. Many TT facilities could be adding catalytic oxidizers, carbon injection, carbon adsorption beds-or a combination of these-to achieve, as the regulatory standards require, lower dioxin and furan emissions.

The final piece of equipment for all TT systems is a stack or exhaust pipe, which should be designed to meet or exceed the EPA Air Program's Good Engineering Practice (GEP) standards to disperse adequately the stack emissions and to minimize public exposure. Minimum GEP is defined as follows:

GEP = H + 1.5L H = height of a nearby structure, and L = the lesser dimension of the height or projected width of the nearby structure.

The maximum GEP stack height that EPA will credit when it conducts stack emissions modeling is defined as the greater of 65 meters or H + 1.5L. If the stack is taller than the maximum GEP stack height, the facility will have better dispersion of its plume; thus ATSDR should not discourage tall stacks or exhaust pipes. That said, however, this discussion must NOT be taken to imply that ATSDR supports "dilution is the solution to pollution!" ATSDR supports proper design and operation of all technologies to minimize all emissions. Nevertheless, because zero emissions cannot be achieved, good public health practice recommends the use of distance, such as a buffer zone or a sufficiently tall stack, to prevent harmful exposures.

5.1.4. Residuals Management

The types of residuals generated by a thermal treatment facility depend on the types of waste feeds and APCE. The quality of the residuals depends on the treatment temperature, moisture content, residence time, and whether additional chemicals are added to the treatment chamber or APCE. Under RCRA, any process residual from treating a listed hazardous waste is still a listed hazardous waste and must be handled accordingly.

5.1.4.1. Liquids

Employees at thermal treatment facilities generate liquid waste streams when they wash the equipment and the waste processing and unloading areas. If it is included in the APCE design, a wet scrubber generates a liquid waste. The facility can dispose of the waste waters by

• Injecting them into the incinerator combustion chamber; injecting them into a desorber is, however, generally not economical.
  
  
• Treating and discharging them through the sewer system to the publicly owned treatment works (POTW).
  
  
• Treating and discharging them to a nearby body of water, such as a stream, lake, or surface impoundment.
  
  
• Spraying them on the hot soil or ash to cool it as it is discharged from the PCC or DC.

A permit is required to discharge waste waters. Depending on the facility location and the constituents present in the waste waters, the local POTW might require the facility to pretreat the waste waters. When evaluating a facility that discharges to a POTW, health assessors should also evaluate the impact of the thermal treatment facility on the POTW's sludge-especially if the facility's waste water stream has significant concentrations of metals or organics. Contaminants of concern can concentrate in POTW sludge. Sludges from POTWs are often used as fertilizers by farmers and homeowners, and could therefore be a completed exposure pathway requiring evaluation.

EPA regulations require all except CERCLA facilities (see section 3.2.) to obtain a National Pollution Discharge Elimination System (NPDES) permit for all discharges into certain bodies of water. The permit specifies parameters such as pH, biochemical oxygen demand (BOD), and chemical concentrations that can be discharged, and, possibly, treatment prior to waste water discharge. Nevertheless, because not all toxic chemicals in hazardous waste are regulated under the NPDES program, the discharge could still contaminate the local surface water, sediments, and food chain pathways.

The waste water generated from facility and equipment washing could contain small concentrations of any of the hazardous substances present in the wastes being processed. Although the waste waters from the wet scrubbing system could contain higher concentrations of some of the metals than were in the original wastes, they usually contain low concentrations of only a few organic chemicals. Waste waters (also known as process waters) need to be analyzed before they are sprayed onto hot soil or ash. This will ensure no recontamination of the treated materials. In that regard, TSCA requires the process water to contain less than 3 parts per billion (ppb) PCBs.

Desorbers having condensers as a part of their APCE generate liquid waste containing the same organic chemicals desorbed from the waste and liquified in the condensers. Because organic contaminants will be concentrated in this waste stream, employees need training in how to manage this waste safely.

5.1.4.2. Ash

High-ash-content liquid wastes, sludges, or solid wastes fed to a thermal treatment facility generate bottom ash and fly ash. Bottom ash is generated in and discharged from the PCC or DC. If the unit is used to decontaminate soil, the bottom ash is usually referred to as the decontaminated soil rather than the ash. Fly ash is the solid particles entrained in the flue gas and metals, or the organics that are volatilized and later, when the flue gases are cooled, condense in the air pollution control equipment. When the flue gas in the unit is fast and turbulent, dry friable soils and small granular materials can cause an increased particulate loading on the APCE. The bulk of the fly ash is captured and removed by the APCE to keep the facility in compliance with its particulate emission limitation.

5.1.4.2.1. Bottom Ash - Decontaminated Soil

Bottom ash or decontaminated soil (if soil is being treated) discharged from the PCC or DC should contain only very low concentrations of organic contaminants. This ash or soil could be a fine material with metals concentrated in it that is easily wind-dispersed. Some facilities discharge the ash or soil into a water quench for cooling and to prevent it from blowing. Some facilities have a water spray directed on the ash or soil conveyor, while others have a shroud around the conveyor where water sprays onto the hot ash or soil. Sometimes, however, the bottom ash from a high-temperature incinerator is a molten slag material that does not contribute to fugitive emissions.

5.1.4.2.2. Fly Ash - Particulate Matter

Particles vary in diameter, but those 2.5 microns or less have the greatest respiratory effect on humans. But even particles up to 10 microns are respirable and thus can cause respiratory problems.

Thermal treatment facilities using a dry scrubber system, or that treat contaminated soil or high-solids waste, generate fly ash-also known as particulate matter (PM)-that is entrained in the flue gas. The fly ash can be removed from the flue gas by cyclones, scrubbers, bag houses, or ESPs. Some facilities discharge the fly ash removed by the APCE directly into drums or other containers for disposal. This is usually effective in preventing fugitive dust emissions. Some facilities move the fly ash and the bottom ash to a residuals management area where they are analyzed before being mixed or discarded. To prevent fugitive dust emissions, these conveyors should be enclosed or shrouded and kept moist or under negative pressure.

5.1.5. Other Design Features - Thermal Relief Ventsa

The activation of the TRV must be tied into the AWFCO system to prevent additional wastes from being fed to the unit while the TRV is open.

Some desorbers-and most incinerators that treat solid wastes, sludges, or thick liquids with a high solids content-have a thermal relief vent (TRV) immediately after the combustion or desorption chamber(s). The TRV is also known as an emergency relief vent or dump stack. The hot gases can be diverted through the TRV if downstream equipment malfunctions. TRVs are necessary on certain facility designs to prevent downstream equipment fires and to prevent hot combustion gases from venting at ground level-which would be a greater hazard to facility workers and nearby residents. A TRV is not necessary if the desorber operates at temperatures low enough for the APCE to withstand in an emergency situation. But because the emergency vent allows the flue gases to bypasses the APCE when the stack is opened, the public could be exposed to metals, particulates, acid gases, and possibly organic chemicals if the emergency relief vent is opened while waste is still in the combustion or desorption chamber(s).

Today, TRV openings are infrequent; however, prior to RCRA permitting, TRV openings occurred frequently. According to a recent EPA survey of RCRA permit writers, a facility might not have a TRV opening for several months, then could have an equipment malfunction and experience several openings in a short time. Thirty TRV openings a year were once common. The bottom line is that facilities should work toward minimizing TRV openings, or eliminating them altogether.

5.2. Emissions of Public Health Concern

The two categories of emissions of potential public health concern are stack emissions and fugitive emissions.

5.2.1. Stack Emissions

5.2.1.1. Organics

Although thermal treatment units can efficiently destroy or remove organic chemicals in wastes, they can also emit low concentrations of some organics. The organics in incinerator stack emissions are known as products of incomplete combustion (PICs). Various PIC definitions exist, but this document will use the term generally to refer to any organic compound found in incinerator stack emissions. This section discusses PICs found in most waste incinerator emissions. EPA has sampled and analyzed the stack emissions of a number of hazardous waste incinerators for PICs. Table 3 shows the emission rates of some chemicals EPA found in stack emissions of hazardous waste incinerators (EPA 1990). Table 4 is a compilation of chemicals that have been detected in full-scale incinerators and research reactors (Costner and Thornton 1990).

Thermal desorption stacks can also emit small concentrations of organic chemicals and volatile metals present in the wastes being treated. Partial degradation or pyrolytic breakdown byproducts (carbon monoxide and hydrocarbons) can also be emitted. It should be noted, however, that thermal desorber stack emissions have not been characterized as extensively as incinerator emissions.

5.2.1.2. Dioxins and Furans

Test data from some TT facilities show low concentrations of polychlorinated dibenzo dioxins (dioxins or PCDDs) and polychlorinated dibenzo furans (furans or PCDFs) in the stack emissions of PCB and RCRA incinerators and thermal desorption facilities. The test data suggest that incomplete destruction of organic material in the combustion zone and adsorption of this material on entrained fly ash increases the possibility of PCDD and PCDF formation in the APCE. This is particularly true if the gas temperature or downstream surfaces are in the 400°F-650°F (200°C-340°C) range (EPA 1989). PCDDs and PCDFs are formed where particulates are held up while in the 400°F-650°F temperature range, such as when thermal treatment units treating soils have (1) a bag house after the primary or desorption chamber; or (2) when heat transfer surfaces (such as boilers or heat exchangers) are present, thus allowing the deposition of particulates. PCDD and PCDF emissions can be reduced by a rapid quench and by reducing the dioxin or furan precursors. A rapid quench stops recombinant reactions which could generate dioxins and furans. The most common method of cooling the gases quickly is water quenching; injecting air is another common method. Research has shown that good operating practices can also minimize the formation of chlorinated dioxins and furans.

Good operating practices to minimize dioxin and furan formation include: Maintaining low carbon monoxide (less than 100 ppm), Maintaining low total hydrocarbon (less than 10 ppm) levels in the stack gas, and Quickly cooling post combustion/desorption gases to below 400°F (200°C).

The data EPA used to establish the maximum achievable control technology (MACT) standards for hazardous waste incinerators establish that when the particulate matter control device is operating at or below 400°F, dioxin and furan emissions are below 0.4 nanograms toxicity equivalency quotient per dry standard cubic meter (ng TEQ/dscm), unless the incinerator is equipped with a waste heat recovery boiler.

Chlorinated dioxins and furans can be controlled through using APCE for the removal of the dioxins and furans from flue gases. Facilities can use activated carbon injection, catalytic oxidizers (Maaskant 2001), catalytic membrane filter systems (Pranghofer 2001), or a wet carbon adsorption/oxidation scrubber system (Siret and Bessy 2001) to lower the dioxin and furan emissions below levels expected to cause adverse health effects.

Research on the formation of dioxins and furans has primarily been conducted on incinerators. Still, using the same good operating practices at desorbers helped reduce dioxin and furan formation at some desorption facilities.

5.2.1.3. Metals and Halogens

Concentrations of metals in stack gases can be affected by: Solids temperatures, Chlorine Volatility of metals, and Type of APCE.

If activated carbon is used for effective mercury, dioxin, and furan control, the flue gas temperature must be below 400°F. The temperature should also be maintained below 400°F to avoid carbon fires. Arsenic, beryllium, cadmium, and chromium are metals sometimes found in wastes and stack emissions that because of their carcinogenicity could be of health concern. Other metals possibly present are antimony, barium, lead, mercury, nickel, selenium, silver, and thallium. Because of its volatility, mercury is a particularly difficult metal to capture in the APCE. It is important to control mercury emissions because mercury bioaccumulates in several animal species. Injecting carbon into the APCE is effective in adsorbing mercury, dioxin, and furan.

Because metals are elements, they cannot be destroyed by incineration or any other treatment technology. They could therefore remain in the bottom ash, be carried into the APCE, and removed as fly ash or in the scrubber liquor, or be emitted in the stack gases. The concentration of metals present in stack gases can be affected by the following:

• Solids temperatures-as the temperature of solids increases in the primary or desorption chamber, the concentration of some metals (e.g., barium and silver) in the flue gases also increases (see Table 6).
  
  
• Chlorine-with a few exceptions, chlorine generally increases the volatility of metals (see Table 7).
  
  
• Volatility of metals-Table 7 contains the volatility temperatures of 12 metals with and without chlorine. These metals volatilize at normal incineration temperatures; however, a few might not volatilize in desorbers, depending on their operating temperature.
  
  
• Type of APCE-Table 8 shows conservative metals removal efficiencies of different types of APCE. Performance burns almost always demonstrate that a facility's APCE has better removal efficiencies than those shown in the table, so the values in Table 8 could be used to estimate potential health impacts when health assessors do not have facility stack emission data.

Little data are available for desorber stack emissions. Stack sampling and monitoring requirements for desorbers have not been as stringent as those for incinerators, except under the PCB program. EPA requires identical monitoring for PCB desorber and incinerators during performance tests.

5.2.2. Fugitive Emissions

The two primary sources of fugitive emissions at any thermal treatment facility are the waste processing/feed area and the residuals management area.

Fugitive emissions from the waste processing/feed area can be volatilized organics or contaminated particulate. Particulates contain organics and metals blown off waste piles or waste transfer equipment or emitted through cracks in conveyor systems and storage and waste processing buildings. Fugitive emissions from the waste processing/feed area can be effectively prevented by a number of methods or combinations of methods, such as (1) covering the waste with tarps or plastic, (2) spraying it with water, (3) spraying it with foam or other coating materials, or (4) unloading and processing the waste inside a building maintained under negative pressure. The air drawn from the building should be piped directly to the incinerator or desorber for disposal, or vented through a carbon filter system to remove the organics before the air is released to the atmosphere. While water spray can be a very common and cost effective way to control metals and particulate matter, it is not very effective for VOC control. Other disadvantages include the need to manage contaminated water runoff and process problems, particularly if the waste is too wet or there is a broad variation in the moisture content of the waste feed (EPA 1992). If bulk liquids are received and stored in tanks, the tanks should be vented through pipes to a carbon filter system or piped directly to the incinerator combustion chamber.

To ensure effective fugitive emissions management, ambient air monitoring is recommended at TT facilities. If the facility does not have fixed monitoring stations, hand-held monitors should be used to screen during spills or other releases and to periodically screen the area and potential release points, e.g., flanges, valves, etc.

Fugitive emissions from the solid residuals (fly ash, bottom ash, or decontaminated soil) handling area are generally fine particulate matter that can, if not managed properly, easily become airborne. At most facilities the fly ash and bottom ash are stored separately, at least until they are analyzed, but some CERCLA facilities combine them within the treatment system.

5.3. Design and Operating Considerations Important to Public Health

The following thermal treatment facility design considerations are important in minimizing or preventing public exposure (see also Table 9):

• Controlling fugitive emissions can be done by physical or mechanical means, operating conditions, or enclosures or buildings. A facility could also be designed with a large buffer zone so fugitive emissions do not migrate off site. In this latter case, health assessors should determine whether access to the site is restricted to prevent exposure of people and pets, or animals that could become a part of the food chain.
  
  
• Venting waste feed tanks to the combustion chamber(s) when the incinerator is operating or through a carbon filter system.
  
  
• Installing an AWFCO system connected to the key desorption, combustion, and APCE operating conditions to prevent continued operation under poor operating conditions that could cause excess stack emissions.
  
  
• Triggering the opening of a TRV-if one exists-but only under extremely critical operating conditions. If health assessors are not sufficiently experienced with the design of incinerators to evaluate what parameters trigger the opening of the TRV, they should contact ATSDR combustion specialists. They should also ask how often the TRV is used, how long it stays open, and if any stack or ambient monitoring has occurred during its use.
  
  
• If the facility generates solid residuals, designing and operating the ash handling equipment to prevent blowing fugitive particulates.
  
  
• Installing CEMs that monitor stack emissions. ATSDR recommends that all TT facilities have a total hydrocarbon (THC) monitor. But if a carbon monoxide (CO) monitor at a combustion facility has historically run well below 100 ppm on volume basis (ppmv), then an THC monitor might not be needed. Desorbers including condensers, carbon adsorption units or both need an THC monitor to monitor VOC emissions.
  
  
• Locating the facility and stack high enough in relation to the surrounding terrain and buildings to comply with GEP will provide good dispersion of the plume. This will also minimize the likelihood of a plume down draft or fumigation, and minimize exposure of the public and workers from the stack emissions.

The following operating limits are key to preventing poor combustion, desorption, or APCE operation (see section 6.1.1.4. for more on recommended operating conditions). As appropriate on a site-specific basis, these operating limits should trigger the facility's AWFCO system (see Table 10).

• Minimum temperature in the desorption and each combustion chamber, and in a catalytic oxidizer.
  
  
• Maximum pressure in the desorption or primary combustion chamber.
  
  
• Maximum feed rate of each waste type, i.e., liquids, sludges, solids, etc., to the desorption chamber and each combustion chamber or maximum feed rate for each feed location.
  
  
• Maximum chlorine and metals feed rates.
  
  
• Maximum size of batches or containerized wastes.
  
  
• Maximum carbon monoxide and/or hydrocarbon stack emissions.
  
  
• APCE inlet gas temperature.
  
  
• Critical APCE operating parameters specific for each type of equipment at the facility.
  
  
• Maximum flue gas flowrate or velocity.
  
  
• Maximum temperature in the desorber or each combustion chamber if metals or NOx are a health concern at the site.

To thoroughly evaluate a thermal treatment facility's potential impact on the community, a team approach is recommended. Team members familiar with the design and operation of thermal treatment technologies should evaluate the following:

• Design and operating conditions,
  
  
• Trial burn or performance test plan,
  
  
• Test report,
  
  
• Inspection schedules,
  
  
• Operator training, and
  
  
• Contingency plans.

A site visit is necessary to understand the configuration and operation of the facility, its relationship to the community, susceptible populations, and any topographic features in the area that may affect the dispersion of the facility emissions.

The stack and fugitive emissions should be modeled using an EPA-approved dispersion model and reviewed by a team member experienced in air modeling. A toxicologist, physician, industrial hygienist, or other health specialist should evaluate ambient air and stack emissions for potential for health effects. An industrial hygienist or environmental scientist should evaluate the ambient air sampling and monitoring plans for stack and fugitive emissions, action levels and associated response actions, health, safety, and contingency plans, and community demographics. Experienced health assessors could do several or all of these tasks. After the thermal treatment facility is constructed, the person reviewing the thermal treatment design-if not all the members of the site team-should tour the facility, the community, and the locations of any on- and off-site ambient air monitoring and sampling stations. See section 4.3 for further discussion on site visits. As discussed in Chapter 4, referencing anywhere in this document items such as modeling, ambient monitoring stations, risk assessments, etc., does not imply that every facility will or should have all of these items. This chapter discusses how to evaluate different items or information if they are available for the site being evaluated.

If ATSDR is involved early in the planning stage of the project, the following 3-phase review can be conducted: the pre-operational phase, the testing phase, and the operational phase. This chapter discusses the types of information normally available in each of these phases and how to evaluate the information. This chapter also explains terms and acronyms normally used in the thermal treatment industry. If ATSDR becomes involved late in the process, such as after the facility is operational, staff should still obtain and review the information discussed in each phase below.

6.1. Pre-operational Phase - Information to Review for Health Implications

The pre-operational phase covers design through the construction of the thermal treatment facility.

Although several stages of design drawings could be available, ATSDR might not have the resources to review and comment on each level of design before plans are finalized. This could be the case despite the fact that EPA and the facility might want the agency's input early in the process. Yet, if the agency becomes involved only after the facility is already constructed, public health officials should still at least review the final design or as built drawings and descriptions. Also, if EPA and the facility staff understand ATSDR's public health concerns as outlined in this document, they should be able to address all of the agency's technical concerns. Thus ATSDR's involvement even late in the process should not be too disruptive.

6.1.1. Design and Operating Considerations Pertinent to Protecting Public Health

6.1.1.1. Effectiveness of the Technology

If the technology does not effectively destroy or decontaminate the waste on the first pass through the unit, public exposure to contaminants could increase.

If the technology does not effectively decontaminate the solid waste on the first pass through the unit, worker exposure to contaminants could be increased. This is especially the case if workers handle the partially treated waste as if it is clean prior to receiving the treated waste analysis. Because of the greater potential for fugitive emissions, reprocessing and additional handling of partially treated solid wastes can also increase off-site exposure. Although the partially treated waste might not be an acute hazard, the cumulative dose should be considered. While the cost of waste treatment is not a major consideration for ATSDR, it could be more economical to run the unit a few degrees hotter, increase the solids retention time, or enhance the agitation or tumbling of the solid waste. Any or all of these measures could ensure that wastes do not have to be reprocessed, rather than attempting to run the unit at a lower temperature or shorter solids retention time, and then having to reprocess a substantial portion ( i.e., more than 10%) of the batches. The key to effective decontamination is sufficient solids time at the required temperature.

At a pre-operational facility, data from treatability tests and waste analysis, or data from previous sites where the unit was operated, will help address these issues. For example, has the unit (or a similarly designed unit) treated a similar waste? If so, what were the concentrations of the contaminants, the operating conditions, and performance test and environmental monitoring results? Do current plans indicate the design will remain the same and that the facility will operate under the same conditions? If the only data available are from treatability tests, do the plans indicate that the proposed operating conditions are at least as conservative as, if not more conservative than, the conditions demonstrated in a successful treatability test? Have problems identified during the treatability test been addressed?

To ensure effective treatment of the waste, a minimum solids retention time should be set in conjunction with a minimum flue gas exit temperature in the desorption or primary combustion chamber. Or, in the alternative, a minimum bottom ash discharge temperature should be specified. These parameters should be a part of the AWFCO system.

To ensure effective treatment of the flue gas, the state or EPA should establish operating conditions for the APCE and for monitoring of the stack gas. The APCE operating conditions should be specified for each air pollution control device (APCD) based on the operating conditions during a successfully completed performance test. See section 6.1.1.4. for recommendations on APCE operating conditions.

6.1.1.2. Fate of Contaminants of Health Concern

ATSDR staff should evaluate the potential for public exposure to both process residuals and effluents. Therefore, health assessors need to know the fate of any contaminants of concern that are either present in the waste or created by the treatment process. For example, VOCs in desorber stack gas could include vinyl chloride or trichloroethylene, depending on temperatures or whether base-catalyzed dechlorination is conducted simultaneously in the desorber.

The design estimate of the fate of the contaminants should be verified by analyzing all residuals during the performance test.

If metals are present in the wastes, where will they ultimately end up? Tables 6 and 7 show the boiling points of some of the metals found in hazardous wastes, as well as the affect chlorine (if present in the waste streams) could have on the metals. Table 8 shows the contaminant removal efficiency of different air pollution control systems. Using these tables and the proposed operating conditions, health assessors should be able to estimate whether the metals will be left in the treated waste, captured in the air pollution control system effluent(s), or exit through the stack. Mercury, for example, is a highly volatile metal that is difficult to capture in APCE, so the amount of mercury in the total waste feed to thermal treatment units should be limited, if it is present at all in wastes at the site.

Any halogens present in the wastes could be converted to acid gases (e.g., hydrogen chloride, hydrogen bromide) in incinerators or high temperature desorbers, but might or might not be decomposed in low-temperature thermal desorbers (LTTDs). Acid gases can be removed, neutralized, or both in the APCE (see section 5.1.3.). Acid gases in the concentrations typically found in uncontrolled flue gases before the APCE of incinerators burning halogenated compounds can cause acute respiratory problems and initiate asthma attacks (see sections 8.1.1. and 8.1.2.). Nevertheless, a well-designed and operated thermal treatment facility can easily remove acid gases and particulates.

The fate of organic chemicals differs for incinerators and for desorbers.

Incinerators can destroy or remove > 99.99% to > 99.9999% of the organic chemicals present in waste streams. The fate of organic chemicals in desorbers depends on the type of air pollution control equipment employed. Most desorbers are designed to vaporize (volatilize)--but not destroy--the organic chemicals present in the waste(s). The primary chamber is usually operated at low temperatures (i.e., relative to incinerators). Most desorbers use condensers to liquefy the organics volatilized in the primary chamber and remove them from the flue gas. The flue gas then passes through carbon filters to remove any remaining organics not removed by the condensers/chillers. Catalytic oxidizers can also be used to destroy the organics in the flue gases. Desorber stack emissions should be analyzed to determine whether decomposition products such as vinyl chloride are blowing through (not being adsorbed by) the carbon filter. Depending on the concentration of organic solvents present in the original waste(s), a sufficient quantity of organics can be removed in the condensers, making it possible to recycle the condensed solvent(s).

The operating conditions for the APCE for desorbers are extremely important for protecting the public. If not captured in the air pollution control system, the organic contaminants present in the waste or soil are transferred to the flue gases and emitted from the stack. Most desorbers have carbon filters as the last air pollution control device to remove any organics in the flue gas after the condenser. To ensure the organics are removed from the flue gases, ATSDR strongly recommends that all the stack effluent of all desorbers be continuously monitored for THCs. And if the THC reading exceeds 10 ppm (see the hazardous waste incinerator and industrial furnace standard [64 Federal Register [FR] 52869-52870]) an interlock system should shut off the waste feed to the desorber. On a site-specific basis, other CEMs could be used to monitor for hydrocarbon breakthrough. Health assessors should evaluate carefully the CEM's ability to detect small changes in hydrocarbon emissions.

An increase of THC in the stack gases indicates (1) organics are breaking through the desorber's carbon filter and it must be changed, (2) the catalytic oxidizer's catalyst has been sintered or deactivated (blinded or poisoned), or (3) poor combustion is occurring in an incinerator. The recommended 10 ppm THC limit is based on what EPA deems technically feasible for incinerators; it is not a health-based standard per se. According to the EPA preamble to the Final Standards for Hazardous Air Pollutants for Hazardous Waste Combustors (64 FR 52869), "More than 85 percent of test conditions in our data base have hydrocarbon levels below 10 ppmv, and nearly 75 percent have levels below 5 ppmv." Although desorbers are not likely to have been included in the EPA combustors data base, ATSDR recommends that any alternative thermal technology used in lieu of an incinerator be at least as protective of public health as would be an incinerator.

One of ATSDR's missions is to prevent or minimize harmful exposures. If, based on a site-specific toxicological and modeling review, a THC limit lower than 10 ppm is deemed necessary, ATSDR staff should discuss with the facility operators and EPA or the state regulatory agency the need for tighter operating controls. These tighter controls should result in reduced emissions, the setting of a lower THC emission rate, or the monitoring of stack gas for the specific organic contaminant to be limited. But monitoring the stack for specific organic chemicals will rarely be appropriate, unless an extremely toxic chemical is being treated, such as nerve agents. The toxicological review should be based on the organics and metals detected in fugitives and stack gas samples during the performance test, and any other compounds on the approved waste list that were not in the performance test waste feed or were not analyzed for during the performance test. Nondetect results should be included on a compound and site-specific basis (see ATSDR 1992 for further guidance on this issue). If a site-specific evaluation determines that the facility needs a higher than 10 ppm THC limit to be able to operate continuously, then a toxicological evaluation of the hydrocarbons being emitted during the performance test should be conducted.

6.1.1.3. Engineering Design Considerations Affecting Stack Emissions

The only way to prevent the release of stack emissions from any thermal treatment technology at levels that would be a public health hazard is to design properly the unit for the waste to be treated and to set operating controls. The control conditions should ensure that the facility is operated in the same manner or more conservatively than during the performance or risk burns or both that were passed (see section 6.1.1.5. for a discussion on performance test burns). Some facility operators might argue that operating controls are not necessary, and that their staff would not operate the facility improperly because staff members are concerned about their own safety. Other operators argue that if public health officials feel they need assurances, they should rely on the CEMs to ensure that the emissions are not a health hazard; CEMs measure what is actually being released out of the stack. ATSDR agrees that CEMs are an important part of monitoring and controlling the system. Nonetheless, problems arise when relying only on stack CEMs.

CEMs usually only measure indicators of stack emission quality. For example, opacity or PM monitors are indicators of respirable particulates and metal emissions, CO monitors are indicators of good combustion in incinerators, THC monitors are indicators of good combustion and low PICs in incinerators and effective operation of the air pollution control or treatment system in desorbers, and oxygen (O2) and stack gas flow meters are indicators of favorable combustion conditions. But none of these monitors measure specific chemicals or metals of health concern. A flame ionization detector (FID) THC monitor does not report the full mass of chlorinated compounds (e.g., a FID might report the mass of a highly chlorinated compound as a negative value) and thus could be a poor indicator of organic emissions at a particular site. Stack gas monitors might be available in the near future to measure specific metals. Some facilities can measure on a fairly frequent, but not continuous, basis a few organic chemicals in stack gases using a gas chromatograph with a mass spectrometer detector (GC/MS). High moisture, particulate loadings or both could render chemical-specific measurements in the stack gases impossible at some facilities. The point is, controlling the waste treatment facility using only CEMs is not likely to prevent harmful exposure from occurring--operating controls are also needed.

EPA regulations do not set specific standards for desorbers as they do for incinerators. Instead, EPA relies on the permit writer to set whatever conditions are necessary to protect human health and the environment. Even though EPA guidance recommends that incinerator standards be considered when permitting a desorber, many facility operators argue against having to analyze their stack emissions and also having CEMs.

Most health assessors lack the engineering expertise to review design calculations and detailed engineering specifications to ensure that a thermal treatment system can achieve the projected operating conditions and performance standards. Still, when reviewing the information available during the pre-operational phase, health assessors should look for several key design and operational features. The following sections discuss the major equipment subsystems and suggest specific items affecting emissions and, ultimately, the level of public exposure to site contaminants and byproducts.

6.1.1.3.1. Waste Feed Handling

Fugitive emissions from the waste feed handling area could be the source of the highest public exposure to contaminants.

The more homogeneous the waste feed material is, the easier it is to ensure consistent operation of the treatment unit as a whole and consistent levels of stack emissions. Consequently, facility operators often have a waste feed preparation area where front-end loaders or other equipment mix contaminated soils or other wastes with high organic, moisture, or metals content with less-contaminated wastes. The wastes could also be crushed or sorted by size; other debris might have to be removed for further pretreatment or treatment by another method. The solid waste materials to be thermally treated are then placed on conveyor(s) or in augers or ram feeders and fed into the PCC or DC. If the waste material is relatively dry and blows easily or if some of the contaminants are fairly volatile, fugitive emissions from the waste feed handling area could be the source of the highest public exposure to contaminants from the site.

Some sites conduct air monitoring with hand-held monitors to ensure their controls are effective and to alert staff to take corrective action if fugitive emissions exceed a predetermined level.

Liquid wastes should be stored in tanks equipped with agitators, mixers, or recirculating pumps to keep the liquid waste feed homogeneous. Open top tanks should not be used if wastes contain VOCs. Feed lines should have screens to remove objects that might clog injection nozzle(s). Leaks of hazardous wastes from pipes, pumps, and valves can also be sources of fugitive emissions.

If the community is close to the waste handling area or excavation areas at a CERCLA site, the operator could erect a waste feed handling building, tent, or wind screen to control airborne emissions. The operator could install water runoff control devices such as berms, fabric fencing, hay bales or all of these around the excavation area. In cold or wet climates, waste feed handling enclosures might also be needed to ensure a more consistent waste feed and economical operation. Feed conveyors should be enclosed in a building or by shrouds or other enclosures to minimize worker and community exposure to fugitive emissions. During site visits, health assessors should evaluate whether fugitive emissions from the waste excavation, handling, and storage areas are under control.

6.1.1.3.2. Combustion/Desorber Chambers

Numerous designs of thermal treatment chambers (e.g., rotary kilns, fixed or moving hearths, hollow augers, conveyor belts, cylinders) exist. Incinerators normally have two combustion chambers, referred to as primary and secondary combustion chambers (PCC and SCC). Desorbers usually have only one chamber, the desorption chamber. The function of the PCC and the desorption chamber is essentially to volatilize the contaminants in the waste. Agitation or tumbling of solid wastes can increase contaminant volatilization by exposing the wastes to the hot flue gasses or heat source. In an incinerator, the combustion or decomposition of the organic chemicals present in the waste also begins in the PCC. The amount of time a waste needs to remain in the desorber or PCC depends on the size of waste particles, the volatility and concentration of the contaminants, the operating temperature, and the effectiveness of the heat transfer. Typically, solid wastes remain in this chamber between 15 and 45 minutes. The treatment time and minimum bottom ash temperature actually needed to ensure the waste consistently meets the site-specific treatment standards are determined during the system testing phase (see discussion in sections 6.1.1.1. and 6.2.).

Desorbers--The externally heated carrier gas, air or an inert gas such as nitrogen, flows through the desorption system, to convey through the post-desorption treatment system the contaminants removed from the waste. The volume of carrier gas used in a desorption system is generally much less than the volume used in an incinerator. If air is the carrier gas in a desorber, to prevent an explosion from occurring, the operator must ensure sufficient dilution air is present to, in turn, ensure that 25% of the lower explosion limit (LEL) is not exceeded. The American Academy of Environmental Engineers also warns that, "In gas post treatment, a potential fire hazard exists in the baghouse if hydrocarbons or other combustible materials are allowed to collect on the filters. This presents a potential problem especially in the countercurrent rotary desorber configuration when used to treat material contaminated with heavier organics."(AAEE 1993). The carrier gas flow can be co-current (i.e., flow in the same direction as the waste), or counter-current (i.e., flow in the opposite direction). The carrier gas is also known as flue gas.

Incinerators--To supply oxygen for the flame in the burners and to support combustion of the VOCs, an incinerator uses air as the carrier gas. Some incinerators also add oxygen gas through the burner block to improve the oxidation of the volatilized organics while maintaining a lower air flow through the system.

In most thermal treatment systems, the carrier gas is drawn through the entire system by an induced draft fan between the APCE and the stack. The flow should be sufficient to create a negative pressure at the so-called face of the PCC/DC where the waste is fed into the system, or the system should be sealed--both to prevent the escape of the contaminants being volatilized in the chamber, and to carry them through the air pollution control equipment.

6.1.1.3.3. Treated Waste Handling

Because all the moisture and organic chemicals have been volatilized from the waste, the treated waste (bottom ash or decontaminated soil) exiting the PCC/DC is hot (typically 400°F-1000°F), and usually dusty. The bottom ash can be cooled by (1) dropping it into a water chamber from which it is usually removed via an ash drag type conveyor or (2) spraying it with water through nozzles in the shroud/conveyor cover as it is conveyed to the treated waste storage area. Some facilities recycle water by using the water from their wet scrubber to cool the bottom ash. If VOCs have condensed in the wet scrubber they can be re-volatilized when the scrubber water is sprayed on the hot treated waste. Because the conveyor shrouds/covers are not typically sealed, the VOCs can be emitted as fugitive emissions through cracks and openings in the conveyor system. For example, at one desorber facility this practice caused acute health problems for people off site (ATSDR 1997a). Also, an explosion can occur if the LEL is reached in the conveyor equipment. But this is only a problem with desorbers, not incinerators, because the VOCs are destroyed in the incinerator combustion chambers.

If the waste is not treated sufficiently, i.e., if the temperature is not hot enough in the PCC/DC to volatilize all the organic chemicals, the steam generated by the hot ash falling into the water chamber or by spraying water on the conveyor could strip additional organic chemicals out of the waste. Those fugitive organics will then be emitted with the steam, and could pose a hazard to workers or nearby residents. Because this can be a problem with both incinerators and desorbers, a minimum bottom ash exit temperature should be specified to ensure that the waste is properly decontaminated on the first pass through the treatment system.

6.1.1.3.4. Flue Gas Treatment/Air Pollution Control System

A major difference between desorbers and incinerators is that desorbers' flue gas contain all the organic chemicals volatilized from the waste.

Different types of equipment exist for treating flue gases. Incinerators almost always have a SCC, sometimes called a thermal oxidizer or afterburner, in which additional burners complete the combustion of the organic chemicals volatilized from the waste. The flue gases from the SCC and the desorption chamber contain entrained particulate matter and volatilized metals (if metals are present in the waste).

If halogens are in the waste, desorber flue gases might contain acid gases. Incinerator flue gases contain acid gases if halogens are present in the waste, except when a fluidized-bed incinerator is used. If halogens are present in the waste, caustic material is often used in the fluidized-bed material to neutralize the acid as soon as it is generated.

The flue gas treatment system is known as the air pollution control system. It should contain a series of APCE that will remove the contaminants to below levels of health concern before the gas is vented through the stack. Health assessors should review site documents and the information in Tables 6-8 to determine whether the facility has appropriate APCE to treat the flue gases. See also sections 5.1.3. and 6.1.1.4.

6.1.1.3.5. Process Monitoring Equipment

Process monitoring equipment examples: thermocouples, manometers, pressure drop indicators, flow rate meters, pH meters, continuous emission monitors [CEMs].

EPA or state regulatory staff should conduct an in-depth review of the monitoring equipment specifications to ensure that the equipment is appropriate for the operating conditions it is intended to monitor. The facility's inspection schedule should include calibration of the various monitors, with National Institute of Standards and Technology (NIST) traceable gases on an appropriate schedule. Facility operators also often install duplicate monitors on some of the key operating conditions (e.g. thermocouples).

All the key operating conditions should have monitoring equipment connected to a data logger that continuously logs those operating conditions.

Health assessors might not have the expertise to do an in-depth review; however, if they do, they should review the equipment descriptions for the items below.

• Does the operating condition to be monitored occur in the middle of the monitor's operating range? The monitor's range should not be too large or it will not be very accurate in the range the facility will be operating. Conversely, if the monitor has a narrow operating range, it will not register the true high or low value reached by the facility. This could cause a lower rolling average, and the number to recover or come back into range more quickly.
  
  
• When calculating so-called rolling averages, what number does the equipment use in the calculation when the monitor is pegged out at the bottom or top of the range? Typically, the monitor uses the value where the unit is pegged-out (the maximum or minimum reading available). The rolling average was introduced to not force an AWFCO for instantaneous transient spikes, consequently allowing for greater overall operation approaching a steady state.
  
  
• Does the equipment have a dual operating range that automatically expands if the readings exceed the normal operating range? A narrow range should only be used if the monitor has a dual operating range that automatically expands when the narrow range is exceeded. This is particularly important on CO monitors since CO concentrations are typically less then 10 ppm, but could spike into the thousands (2,000-5,000 ppm).

During the operating phase, health assessors should review the following items:

• A drift test and relative accuracy testing audit (RATA) should be passed before full production begins. These tests should be done during shakedown or the performance test.
  
  
• Monitoring equipment should be calibrated in accordance with manufacturer's instructions and a written calibration protocol. Calibration should occur at a minimum of two points that bracket the normal facility operating range; however, three-point calibrations are preferred (top, middle, and bottom of monitor's range).
  
  
• CEMs should be calibrated daily with National Institute of Standards and Technology traceable gases. Thermocouples, flowrate meters, pressure drop indicators, and other monitoring equipment that are either sealed or cannot readily be calibrated can be calibrated monthly, quarterly, or annually. During the operational phase, health assessors should verify that the equipment is installed, inspected, and calibrated on schedule.
  
  
• If a CEM that is tied to an AWFCO cannot be properly calibrated within 15-20 minutes, the waste feed should be shut off until the CEM calibration is completed.

6.1.1.3.6. Stack Height

The taller the stack, the better the plume dispersion.

Proper stack height lowers the potential for harmful exposure to the public. The stack should be taller than any nearby buildings, so the plume will not directly affect the building's windows or air intakes. Some facility operators prefer a short stack so their facility will not be noticed. But this potentially increases the exposure of on-site workers and nearby residents or businesses. The EPA Air Program requires that stacks meet GEP standards, thus ensuring good plume dispersion and minimizing plume downwash or fumigation of nearby buildings and personnel. See section 5.1.3. for further discussion of GEP and how to calculate GEP stack height. The EPA CERCLA program commented that it does not consider GEP an ARAR. If the stack height is below GEP, or site documents do not address this issue, health assessors should pay particular attention to the stack height, and the distance from and height of, nearby buildings. This data will assist them to evaluate the potential for downwash or fumigation of nearby buildings and personnel.

If the area has frequent inversions lasting several hours and the stack height is below the typical inversion height, the modeling of the stack emissions could indicate that administrative controls, such as limiting the operating hours, are necessary during prolonged inversions. This is not a recommendation that should be suggested unless there is good justification. Starting and stopping the operation can sometimes cause more emissions and equipment problems then steady state operation.

6.1.1.3.7. Handling of Process Residuals

Health assessors should observe the handling of process residuals (e.g., fly ash, condensate, scrubber water, spent carbon, spent filters, etc.) during site visits to determine whether fugitive emissions from these areas could impact the public. They should also investigate whether the ultimate disposal of the residuals and any off-site transportation of process residuals could result in additional public exposure. If the facility operator is required to reprocess a substantial portion of the batches, health assessors should note the potential exposure routes that will be impacted. See sections 5.1.4., 5.2.2., and 6.1.1.1. for further discussion.

6.1.1.4. Operation and Maintenance Plan (O&M Plan)

The O&M plan, or permit application, should specify the operating conditions for each piece of equipment at the facility. It should also state that, based on the operating conditions actually demonstrated during the fully successful test(s), the operating conditions will be modified after the performance test. Health officials need to understand that the facility and regulatory agencies must carefully set the range of operating conditions to assure safe operations consistent with the performance test conditions, and to allow some flexibility in operations so that AWFCOs do not cause frequent upset conditions.

Key conditions to be continuously monitored should be interlocked with the AWFCO system. Waste feed should automatically be cut off when the conditions starred (*) below are exceeded. The O&M plan should specify the following conditions to ensure effective treatment of the waste:

• Minimum PCC flue gas exit temperature for incinerators, and minimum bottom ash temperature for desorbers and incinerators treating contaminated soil.*
  
  
• Minimum SCC flue gas exit temperature for incinerators.*
  
  
• A measure of solids residence time in the PCC or desorption chamber such as kiln rotation speed.
  
  
• Maximum size of solid materials to be fed to the unit (usually 2 inches or less).
  
  
• Maximum waste feed rate for each waste type to each chamber.*
  
  
• Maximum viscosity of pumpable wastes.
  
  
• Maximum burner turndown.
  
  
• Minimum atomization fluid pressure.
  
  
• Maximum size and/or btu content, and frequency of addition of containerized waste.
  
  
• Maximum quantity of waste and minimum treatment cycle for batch operated facilities.

The plan should specify the following conditions for APCE, as applicable to the facility, to ensure the effective flue gas treatment:

• Maximum inlet temperature to APCE.
  
  
• Minimum quench flow rate.
  
  
• Minimum pressure differential across a venturi scrubber.*
  
  
• Minimum/maximum nozzle pressure to scrubber.
  
  
• Minimum pressure differential across a baghouse.*
  
  
• Maximum pressure differential across a baghouse to activate the bag cleaning system.
  
  
• Minimum liquid-to-gas ratio and pH to a wet scrubber.*
  
  
• Minimum caustic feed to dry scrubber.*
  
  
• Minimum kilovolt-amperes (kVA) settings for electrostatic precipitators (wet/dry).*
  
  
• Carbon injection rate* and location.
  
  
• Minimum kilovolt (kV) and minimum liquid flowrate to ionized wet scrubber (IWS).*
  
  
• Maximum inlet temperature to carbon bed or carbon injection system.
  
  
• Maximum exit temperature from condensers.*
  
  
• Maximum temperature to catalytic oxidizer.*

The plan should specify the following operating conditions to ensure that stack and fugitive emissions are maintained below levels of health concern:

• Maximum total halides and ash feed rate to the system.
  
  
• Maximum CO emissions or maximum total hydrocarbon emissions measured at the stack or other appropriate location.*
  
  
• Flue gas temperature to APCE below 400°F to prevent dioxin production.*
  
  
• Maximum flue gas flow rate or velocity measured at the stack or appropriate location.*
  
  
• Maximum pressure in the desorption chamber or PCC.*
  
  
• Maximum metals feed rate of each metal of concern.
  
  
• Maximum of 25% of LEL in desorber gases using a combustible gas monitor in the stack or APCE (if air is the carrier gas).*
  
  
• Maximum gas exit temperature from PCC and desorption chamber, if the wastes contain metals that could be volatilized at temperatures higher than those used during the performance test.*
  
  
• Maximum air monitoring values allowable at facility fence line. More than one number should be set, i.e., one set of numbers for particulate matter and contaminants of concern (or VOCs) that trigger on-site measures to control fugitive emissions, and a higher number for each to trigger shut down of excavation, facility operations, or both. See section 6.1.2.1. for further discussion on ambient air sampling and monitoring.

6.1.1.5. Performance Test Plans

Performance tests could be labeled compliance tests, MACT tests, or trial burns for RCRA facilities. At CERCLA facilities they are also known as trial burns, performance tests, or proof of process (POP) tests. In this document the term "performance test" refers to any of these tests. The operating conditions during the performance test are used to set the facility operating conditions. When setting the operating conditions one should allow maximum operating range consistent with the performance test operating conditions. If the operating range is narrow, the frequency of AWFCOs will increase. This could cause unstable operations and increase the potential for PIC formation.

If a risk assessment is conducted, the thermal treatment facility could also do a separate risk burn. The risk burn measures the stack emissions during normal operation of the facility. By contrast, the performance test emissions should be the worst stack emissions that could occur during routine fluctuations in operating conditions of the thermal treatment system. Rather than doing two separate tests, some facility operators combine the risk burn with the performance test.

Performance tests should be conducted at worst-case operating conditions, i.e., at the minimum temperatures in the desorption and combustion chambers for organic chemicals, but at the maximum temperatures during normal operations for metals.

To provide the facility maximum operating flexibility, performance tests for thermal treatment facilities are usually conducted under worst-case operating conditions. At some sites, however, the operator's contract could require the performance test to be run at the facility's proposed operating conditions (normal conditions rather than worst-case). Because the operating conditions are set from the performance test, the performance test is the de facto worst-case operating condition. The stack emissions during normal operation should not be worse than those measured during the test because the AWFCOs are set at the operating conditions used during the performance test.

Worst-case operating conditions for organic chemical emissions include maximum feed and flue gas flow rate; minimum temperature in the desorption and combustion chambers; maximum halides, metals, ash, and water feed rates, as well as the minimum or maximum (as appropriate) operating conditions for all the starred operating conditions in the previous section. Unless the operator is willing to accept a narrow range of operating temperatures, it is impossible to do worst-case operating conditions for organic chemicals and metals at the same time. As a result, if metals are a health concern at that site, two performance tests are usually conducted. Worst-case operating conditions for metals emissions include maximum feed rate, maximum halides and metals feed rates, maximum PCC or desorber exit gas temperatures, but at the same minimum or maximum APCE operating conditions.

RCRA stack sampling requirements differ for incinerators and for desorbers. EPA does not have specific regulations for desorbers. RCRA permit writers usually use the incinerator regulations as guidelines for desorbers. The desorber performance test plan should include stack emissions sampling and analysis (S&A) for destruction and removal efficiency (DRE), PICs, TICs, and metals (if they are a health concern at the site). Some desorber operators might object to the DRE and PIC requirements, arguing that the standards are not applicable because they are not combusting or destroying the waste. Nevertheless, at the temperatures used in desorbers some decomposition of organics will occur. Because the condensers or carbon units-or both-might not capture all the organics in the flue gas, the identity and concentration of the organics in the stack emissions must be evaluated. The DRE test primarily measures the removal efficiency of the desorption facility, and can thus be called the removal efficiency test. To provide equivalent protection of the public, the removal efficiency for a RCRA desorber should be numerically the same as the RCRA incinerator DRE, i.e., 99.99% for the POHCs in the waste and 99.9999% for wastes classified as dioxin, furan, or PCB wastes (if over 50 ppm PCBs).

At CERCLA sites, site-specific concentrations are established for stack emissions and site clean-up concentrations, and are based on ARARs or a risk assessment (see section 3.2 or Appendix E for additional discussion of ARARs). If the stack emissions and clean-up criteria are protective of public health, health assessors do not need to consider removal efficiencies for CERCLA incinerators or desorbers. The stack emissions, however, do need to be characterized during the performance test to make sure they meet the approved site-specific values.

To ensure normal-to-worst case emissions during desorber testing, if the facility has a carbon adsorption bed, the performance tests should be conducted during the middle-to-end of the carbon adsorption units change-out cycle rather than immediately after changing the carbon bed. For more information on carbon adsorption systems see section 5.1 of EPA 1992. At CERCLA sites, EPA often requires the performance test to be conducted as soon as possible. If the unit has few operating problems during shakedown and the site-specific carbon bed life expectancy is calculated to be many months or years, the performance test should not be delayed solely to accommodate this recommendation. Condensers should operate at the maximum temperature allowed during normal operations.

Incinerators do not normally have carbon adsorption beds, but can have carbon injection systems to control dioxin, furan, and mercury emissions. Carbon injection systems should be operated at the minimum carbon injection feed rate for worst case operating conditions. For further information on carbon injection systems see the Federal Register for September 30, 1999, Part 63, Subpart EEE and the preamble thereto.

To show compliance with RCRA incinerator emissions regulations under their preferred operating conditions, operators often spike the waste feed with organic or metal compounds or both. The organic spikes are selected from lists of difficult-to-destroy compounds and are called POHCs, or target compounds. If POHCs or metals are spiked into the waste feed to demonstrate worst-case feed rates and compliance with the performance standards, the spiked waste feed should be fed to the thermal treatment unit before stack sampling begins. The minimum amount of time in advance of stack sampling that the spiked waste feed should be fed equals the solids retention time. Thus when sampling begins, the thermal treatment unit is operating at steady conditions.

Each performance test should consist of three runs under the same operating conditions. Some facilities do an additional run so they have backup data in the event some of the samples are lost or broken. Performance test plans should cover the following topics:

• Operating conditions for each piece of equipment,
  
  
• Waste feed sampling and analysis (S&A),
  
  
• Stack emissions S&A - POHCs/target compounds, PICs, products of contaminant degradation or reformation (i.e., organics not removed in the APCE), TICs, and metals,
  
  
• CEMs,
  
  
• Residuals (including treated waste) S&A - to characterize them and to identify and quantify the fate of contaminants,
  
  
• Treated waste S&A at CERCLA sites - to verify decontamination, and
  
  
• Quality assurance and quality control (QA/QC) - chain of custody for samples, field and method blanks, and laboratory spikes.

If EPA approved stack sampling, analytical, and QA/QC procedures are followed, the data can be assumed to be adequate for making public health determinations. That is, unless the QC data indicate the method was not sensitive or accurate enough for determining whether public exposure will be potentially harmful (i.e., below levels of health concern). EPA approved methods are in the EPA Publication SW-846, Test Methods for Evaluating Solid Wastes, Physical/Chemical Methods and the EPA Air Program regulations in 40 CFR Part 60 (see "http://www.epa.gov.") Other sampling and analytical methods or modifications of the EPA QA/QC procedures could provide data of adequate quality for making health determinations, but they must be evaluated based on the available QC data (i.e., demonstrated sensitivity and accuracy).

6.1.2. Additional Considerations Important to Public Health

ATSDR staff should also consider the items addressed below.

6.1.2.1. Ambient Air Sampling and Monitoring Plan

Health assessors should recommend that thermal treatment facilities have an ambient air sampling and monitoring plan. This is especially true if the stack data or risk assessment indicates the potential for air releases of contaminants at concentra