USING THE US EPA SURFACE EMISSION ISOLATION FLUX CHAMBER TO ASSESS EMISSIONS FROM AN AREA SOURCE
Assessing the emission rate of volatile organic compounds (VOCs) from any an source presents a variety of challenges for environmental scientists. Area sources include waste treatment and disposal facilities (landfills, land farms, compost facilities, municipal wastewater treatment, industrial solvent treatment), surface spills, industrial processes, and subsurface contamination such as contaminants in groundwater (plume). Emission rate data are preferred over concentration data (mass/volume) for most applications because emissions rate data can be used for many purposes including: process compliance, health risk assessment, and designing emission control technologies. The most common usage of emission rate data is to determine yearly emissions of VOCs (mass/year) per emission source. Emission rate data are thus useful for assessing long-term air quality impacts.
There are many acceptable approaches for assessing emissions from area sources. The US EPA has published technical guidance manuals and formalized the approach for conducting air pathway analysis (APA) at hazardous sites. This effort is in support of site restoration activities including conducting exposure assessments for undisturbed and disturbed (i.e., during remediation) waste sites (EPA Volumes 1-4, 1989). Although written for assessing emissions from uncontrolled hazardous waste sources (i.e., Superfund hazardous waste landfills and lagoons), the technologies described are applicable to any area or fugitive source. Volume II of this four volume guidance series focuses on estimating emission rate from area sources such as hazardous waste landfills and lagoons (EPA Volume 2, 1989). The emission assessment approaches and their respective technologies described in Volume II describes: direct emission measurement, indirect emission measurement, fenceline monitoring and modeling, and predictive modeling. These three measurement approaches for measuring or estimating emission rate as described in APA Volume II and their respective assessment technologies, are paraphrased from Volume II and are discussed below.
Several factors will determine which technology or combination of technologies will be appropriate for evaluating a given site. And some sources are a combination of sources (i.e., point and area sources or multiple area/fugitive sources) and require several assessment technologies in order to generate representative emission rate data. These factors include an a priori estimate of compounds emitted and emission rate; the level of effort available to determine these emission rates; the degree of accuracy required; and the complexity of the source.
An important distinction to make in understanding the emission rate (mass/time) measurement technologies is whether the technology directly measures the emission flux (rate/area or mass/time, area) of a VOC at the source or the effect of the emission event (i.e., ambient concentration of the VOC). Technologies that directly measure the emission event must measure all of those parameters that are necessary to calculate an emission rate. An emission rate (mass/time) can be calculated by multiplying the emission flux rate (mass/time,area) by the surface area responsible for the emission event. Most technologies that measure the ambient concentration must use an interpretation (such as dispersion modeling) to determine the associated emission rate. Other methods calculate the pollutant concentration in a manner where the emission is not directly related to a given area or time interval. These technologies typically use air dispersion modeling to infer the emission rate responsible for the emission concentration measured given the unique source, dispersion, and transport conditions of the testing.
One last concept needs introduction prior to a discussion of assessment technologies. This concept includes the definitions of emission assessment approach or technologies, sample collection techniques, and analytical methodologies. The emission assessment approach or technology is the technical approach or scheme used to estimate emissions which incorporates sample collection and the analysis of samples. There are three emission measurement approaches and one modeling approach: direct measurement, indirect measurement, air monitoring (or fenceline monitoring) and dispersion modeling, and predictive emissions modeling. These approaches, which are composed of several technologies within these categories, are very different. Each type of technology has unique advantages and disadvantages providing for one best approach or technology per application. The three measurement techniques rely on assessing flux or the effect of flux using a measured parameter. Predictive modeling relies on estimating the release of VOCs from the source and the migration of the VOCs to the land surface.
Direct emission measurement approach is often the preferred approach for assessing VOC emissions from waste treatment processes and other area sources. This is because these technologies are relatively easy to use, are source-specific which allows for the assessment of single processes at sites with multiple VOC emission sources (i.e., no upwind interferences), they are not significantly influenced by meteorological conditions, and they do not require modeling in order to estimate emissions.
Direct emission measurement of covered and vented processes is straight-forward and the assessment is made by measuring gas velocity, vent cross-sectional area, and concentration of VOCs in the vent gas. This approach is standard vent or "stack sampling" technology and provides accurate and reliable emission assessment. Direct emissions measurement for area sources or fugitive (process vent leaks) sources consists of measuring the gas concentration, flow rate, and surface emissions area for the emission event at the emitting surface prior to dispersion into the atmosphere. Air is typically purged into the enclosed chamber as a carrier gas or sweep air. Technologies applicable to volatile emission rate measurements from surfaces are the surface emissions isolation flux chamber, (Radian, 1989; Eklund et al, 1985; Dupont, 1987; Eklund et al, 1987; Schmidt and Clark, 1983), head space samplers (Kapling et al, 1986), and wind tunnels (Castle, 1982; Cowherd, 1985). Cracks in surface covers also may be sampled by these technologies (Schmidt, 1990). Vents at uncontrolled hazardous waste sites typically have minimal or no flow and may be sampled by the above technologies or by head space emissions concentration measurements (Wood and Porter, 1986). Where measurable gas flow velocities are present, vent emission rates can be sampled by standard stack sampling methods.
Surface Emission Isolation Flux Chamber
The surface emissions isolation flux chamber is one of the most promising technologies for the direct measurement of VOC emissions. Guidelines have been developed by the EPA for application of this methodology to land surface (Radian, 1988). The technology is also applicable to liquid surfaces (Eklund et al, 1985; Eklund et al, 1987). The technology uses a chamber to isolate a known surface area for emissions measurement. Clean, dry sweep air is added to the chamber at a metered rate. Within the chamber, the sweep air is mixed with emitted vapors and gases by the physical design of the sweep air inlet and/or an impeller. The concentration of the exhaust gas is measured at the chamber outlet for specific VOCs by real-time instruments and/or is usually collected as a sample for laboratory analysis. The emission flux can be calculated as:
Ei = CiQ / A
where Ei = emission rate of component i (ug/m2,min); Ci = concentration of component i (ug/m3); Q = sweep air flow rate into chamber (m3/min); and A = surface area enclosed by chamber (m2).
Statistical methods are used to determine the number of measurements required to characterize the emissions from an area source. These methods are based on the source surface area and the variability of the measured emission rate at randomly selected locations across the site. The principal advantages of this technology are that an emission rate can be measured in the field without modeling, and the field personnel can control the testing conditions. The area source measurement is made at the emitting surface, whether the surface is a solid, a liquid, an opening (crack), or a vent. The principal disadvantage is that the measurement is made over a relatively small area and numerous measurements may be necessary to characterize an emission source. Also, the emission flux may be enhanced or suppressed in the process of performing the measurement thus altering the emission event.
The EPA recommended flux chamber technology as described in the literature can be applied to a variety of area sources with only minor adaption and equipment modification. The following sub-sections describe the applications of the technology to land surfaces, non-aerated, non-mixed liquid surfaces, aerated and mixed liquid surfaces, and fugitive process emission sources.
Land Surfaces. The flux chamber was designed and tested for land surfaces where the volatile/semi-volatile source (waste material) is on the surface or is subsurface. The difference regarding surface versus subsurface applications is that for direct contact with the waste, emission rates are generally greater and those rates are often subject to influences such as solar heating, ambient temperature, surface moisture, and physical disturbance of the waste material. Where the waste material is subsurface and the volatile species must migrate through a layer of soil, the emission rates can be lower and often are not affected by surface conditions or activity.
Assessing VOC species emissions from surface waste sources such as landfills, stock piled waste, and land farms is a straight-forward and common application of the technology. Since the chamber will be in contact with the waste, contamination of equipment and cross-contamination is a concern. This concern can be addressed by cleaning the chamber walls where contact with waste has been made and back-flushing the chamber exhaust or sample line. One approach is to avoid contact by wrapping the chamber lip with wide, disposable teflon tape per test location and preventing contact with solid waste material. Care must be taken to not introduce materials into the chamber like adhesive tape that may off-gas contaminate species. All wrapping must be secured outside the chamber. In addition to cleaning and preventing limiting contact with waste, more frequent (10-to-20%) blank testing is recommended for testing with waste contact.
Non-Aerated, Non-Mixed Liquid Surfaces. Application of the technology to liquid surfaces requires floating or suspending the chamber on/over the liquid surface. For non-aerated and non-mixed liquid surfaces such as abandoned lagoons and ponds, quality assurance testing has demonstrated that lower emission rates will result if the chamber lip significantly penetrates the liquid surface and prevents communication of the isolated surface with the waste body (Gholson et al, 1988). In non-mixed liquid systems, natural mixing occurs by heating and cooling of the surface layer and the resulting connective mixing. If the chamber contains a layer of trapped liquid, the volatile species may diffuse and the resulting liquid layer may retard emissions. This could result in a bias in test data (in this case, a negative bias). This bias can be prevented by suspending the chamber and keeping the chamber lip floating just under the liquid surface. Flotation/suspension systems have been designed and used successfully (Eklund et al, 1987). Aside from this concern, liquid testing can be accomplished following the testing protocol as described for land surfaces including spatial and temporal test strategies.
Aerated, Mixed Liquid Surfaces. Aerated and/or mixed surfaces, usually associated with a treatment process like municipal sewage or industrial waste water treatment, are tested using the flux chamber as per quiescent surfaces (Schmidt et al, 1991; Schmidt and Faught, 1990). Mixed liquid surfaces usually prevent a low bias in the emission estimate by continually renewing the liquid surface. For vigorous mixing, it may be necessary to attach flotation devises to the chamber to improve the stability of the equipment preventing chamber upset.
Aerated surfaces are unique in that bulk air flow from the system strips volatile species and carries contaminants into the chamber. This air flow must be measured and used in the calculation for determining emission rate. Aeration flow can be measured by attaching a volume-calibrated, deflated plastic bag to the chamber pressure port, sealing all other ports, and timing the bag filling (Schmidt and Faught, 1990). Dividing bag volume by filling rate affords the aeration flow rate assessment. Aeration rate can also be measured using a pump and a manometer to match flow at zero pressure difference and a mass flow measuring device such as a rotometer.
In assessing emissions from an aerated treatment process, both aerated and non-aerated zones must be assessed independently. Average unit emission rate from each zone and estimated surface areas are needed to assess emissions from each zone. Soil biofilters with forced aeration, which are aerated systems similar to aeration basins, have also been tested using this technology (Berry et al, 1991).
Process Fugitive Seam/Leak Assessment. The flux chamber technology can also be used to assess emissions from passive vents, seams, leaking valves, ports, and cracks in control devices ranging from fixed and floating roofs to clay caps on landfills (Schmidt and Clark, 1990). These applications require modifications to the standard protocol: 1) the chamber must be adapted to the fugitive source; and 2) the process or source must be well understood in order to properly design a testing strategy and assess the area source. Adapting the chamber to these fugitive emission sources can be as simple as placing the chamber on a flat seam or as involved as constructing an adaptor to interface between the port, valve, or process opening. When possible, these adapters should be made from inert materials and the entire system should be blank tested. Operating conditions like flow rate and residence time parameters may need to be changed due to increased enclosure volume.
The requirement of representative testing, however, is a bigger challenge for this application. Typically, process seam/fugitive emissions are first surveyed with real-time analyzers and all fugitive emissions are identified, organized into zones or ranges of similar emission potential, and tested as zones of emissions potential as described for land surfaces. An estimate of emissions can be obtained by averaging emission rate per zone and calculating emissions per zone by knowing the number of sources, area of source, or lineal feet of source. Unit emission rate data for this type of source can have units like mass per time per vent or foot of seam leak (Schmidt and Clark, 1990).
When selecting an emission assessment approach, use of an accurate and precise technology is desirable to allow the generation of representative emission estimates. The selection of assessment approach, assessment technology, and then sample collection technique and analytical methodology will depend on many factors, most important of which include: type of emission source (point or area source); type and level of VOCs emitted from the source; other source characteristics such as heterogeneity and dynamic properties, accessibility, surrounding interfering sources, and meteorological conditions; and capabilities available to conduct the emission assessment like expertise, equipment, and laboratory facilities.
When possible, direct emission measurement technologies, such as the emission isolation flux chamber, should be used to measure emission rates from waste bodies because they offer inherently greater sensitivity and lower variability. As such, the flux chamber is the ideal tool for collecting accurate data from low-level sources such as volatilization of compounds from contaminated groundwater. This application provides data that is useful in providing input to an exposure assessment when contaminated groundwater leaves the site and impacts a neighboring community.
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