BioCycle January 2006, Vol. 47, No. 1, p. 37
Detailed report on quantifying air pollution emissions for new cocomposting facility in Virginia reviews state and federal criteria for calculating emissions and operating the site.
PLANNING a new solid waste cocomposting facility can mean dealing not only with odors, but with air quality and permitting issues as well. That’s the challenge at a proposed private 500 wet tons/day site in Virginia that will use the aerated static pile composting process enclosed in a building. It will be a multifeedstock location treating biosolids, industrial residuals, water treatment plant sludges, food processing wastes, agricultural residuals and wood wastes – just about anything that can be composted. A biofilter will be utilized for air exhaust treatment. The biofilter is sized for eight air changes per hour in the building, which is about 360,000 cubic feet per minute (cpm) of air to be treated by the biofilter.
As indicated in the schematic layout of the proposed composting facility (Figure 1), feedstocks enter at one end of the building and go through a mechanical mixing system. The compost bays are positive aeration – air is blown into the building and into the compost piles. After about a 14 to 18 day residence time in the bays, the compost is taken out and screened. The overs are returned back to the mixing system, and the screened product taken outdoors for windrowing and curing.
Emissions of criteria and hazardous air pollutants are regulated by the federal Clean Air Act (CAA). The first Clean Air Act was passed in 1963 and has been amended several times over the years. There are several titles within the legislation. The main titles that can affect composting operations are Title I – Air Pollution Prevention and Control, Title II – Standards for Mobile Sources, and Title V – Operating Permits for Major Sources. Traditionally focused on smokestack industries, the CAA is now becoming an issue in the world of composting.
KEY PROVISIONS OF CLEAN AIR ACT
The major elements that affect composters are Part A of Title I (Air Quality and Emissions Limitations) that include New Source Performance Standards and emissions standards for hazardous air pollutants (HAPs). Part C of Title I (Prevention of Significant Deterioration) is a program designed to minimize the impact of new air pollutants in sensitive areas. Part D (Plan Requirements for Nonattainment Areas) is probably the most significant aspect of air pollution emissions limitations potentially affecting composting operations.
Title II – Standards for Mobile Sources – can affect permitting decisions by virtue of emissions from mobile equipment on-site (i.e. front end loaders and diesel-powered screens). Under Title V, there is the potential for needing an operating permit for major sources, although few composting facilities would emit enough air pollutants to qualify as a major source.
In Virginia, the Clean Air Act is administered by the Department of Environmental Quality (DAEQ), and state regulations are patterned along federal legislative lines. They have ambient air quality standards for attainment and nonattainment areas. A nonattainment area is a declaration that the air quality in a particular area does not meet federal standards. There are new source review regulations for emissions of hazardous air pollutants and permitting programs both for construction and operation.
Nonattainment areas in Virginia are limited at this time to only those involving violations of the eight-hour ozone air quality standard. As indicated on the map (see page 37), nonattainment areas in the state tend to be along the major areas of development – in the northern Virginia, Richmond, and Tidewater areas. The site of the proposed composting facility is shown on the map in the southwest part of the state. During the site selection process, one of the issues affecting site selection was designation of nonattainment areas because of the difficulty of getting an air permit in a nonattainment area.
The site selected is not in a nonattainment area. Nonetheless, the Virginia DEQ requires sites with a “potential to emit” to conduct an air emissions analysis as part of the process to determine if an air emissions permit needs to be obtained. This article describes the methodologies used by our company to complete a potential to emit analysis. It is based on a talk I gave on this topic at the BioCycle Southeast Conference in Charlotte, North Carolina last November.
VOLATILE ORGANIC COMPOUNDS AND HAPS
The major air pollutant causing designation as an ozone nonattainment area is volatile organic compounds (VOCs), a frequent pollutant emitted by composting facilities. VOCs can contribute to ozone pollution problems, and if the site had been located in one of these nonattainment areas, the composter would have been forced to obtain emissions offsets. For every pound of VOCs emitted by a new facility, you have to ensure that more than one pound of existing air pollution emissions is reduced. That is accomplished through a program called emission offsets or emissions trading, where a deal is worked out with another generator – usually costing a fair amount of money – for them to reduce their emissions in order to “make room” for your new emissions of that particular pollutant.
You also have to have Lowest Achievable Emissions Rate (LAER) technology if you locate in a nonattainment area. This may require the use of chemical scrubbers for treating exhaust air (as was done at the former Montgomery County Regional Composting Facility in Calverton, Maryland). The permitting process is also more restrictive in nonattainment areas as to how much can be emitted before a permit is needed. The bottomline is, if a new composting facility is located in a nonattainment area, the permitting process is more complicated.
The criteria pollutants regulated in Virginia are presented in Table 1 and are typical for the types of smokestack industries – particulate matter, sulfur dioxide, etc. Virginia also regulates three different kinds of sulfur compounds: Hydrogen sulfide, Total Reduced Sulfur (defined as hydrogen sulfide, methyl mercapatan, dimethyl sulfide and dimethyl disulfide), and Reduced Sulfur Compounds (which are defined as hydrogen sulfide, carbon disulfide and carbonyl sulfide). Each one of these is a regulated pollutant in the state of Virginia.
Hazardous air pollutants in Virginia are regulated under the same standard as the federal regulations. Those hazardous air pollutants are listed in Section 112b of the Clean Air Act. There are 188 HAPs on EPA’s list, including at least 29 HAPs that have been quantified in composting air emissions. The kinds and quantities of HAPs tend to be a function of feedstocks, with biosolids composting having the most carefully and thoroughly studied air emission profiles. That is where most of these HAPs have been located. As this proposed facility will compost biosolids, that was a key issue to be researched.
The most prevalent HAPs found in the reported literature are methanol, methylene chloride, methyl ethyl ketone and triethylamine. New Source Review is a regulatory process in many states, and certainly in Virginia; it is a preconstruction review of new sources, potential air criteria pollutants and HAPs, and compliance with the requirements for Prevention of Significant Deterioration (PSD) if they apply. For example, if a facility were located on the boundary of the Shenandoah National Park in Virginia, it would be subject to PSD review.
Regulators also look at compliance of nonattainment area requirements, emission offsets issues, and Lowest Achievable Emission Rates standards. If the potential emissions exceed stated regulatory thresholds, a permit to construct is issued by the regulatory agency. You build your facility and then you have to do performance testing of the air pollutant control device that you’ve selected. It has to be the Best Available Control Technology (BACT) and then once you have demonstrated to the satisfaction of the regulators that the pollution control device works properly, you get a permit to operate.
“POTENTIAL TO EMIT” ANALYSIS
The way to tell the regulators how much air pollution your composting facility will emit is called a “Potential To Emit” analysis. It is defined as “the maximum capacity to emit a pollutant based on the physical and operational design of a source.” The regulators want it based on a worst case analysis. They want to look at uncontrolled emissions – what is the emission from the facility prior to the use of any kind of air pollution control device. They also want all of the numbers calculated based on a 24 hours a day, seven days a week operation. Even though the facility may only operate eight hours a day, five and one-half days a week, they want the numbers calculated based on a 24/7 operation.
The acceptable methods for determining potential to emit are source testing, materials balance, emissions factors (developed by EPA) or data from the literature. The composter proposing this plant wanted to try to get through this process with the least amount of cost. We elected to use data from the literature analysis and to try to convince the regulators that the data was high enough quality to support our conclusions that no permit was needed.
In the Potential To Emit analysis, emissions from mobile sources must be included. In the case of this proposed composting facility, that included air pollution from diesel-powered front-end loaders operating inside the building. The proposed screening equipment to be used in the plant would be electrically driven so that the only pollutant to be considered was Particulate Matter (PM).
Methods For Source Testing
Source testing is done in a couple of different ways – passive sampling is a frequent method of what is called qualitative source testing. For example, a carbon monoxide monitor in a home is considered a passive sampling technique. In some manufacturing facilities, you see people wearing what are called diffusion badges that register the presence of a pollutant and change color, thus warning the wearer of the badge that they are in an atmosphere that is potentially harmful.
Active sampling is the best way to do source testing. A known volume of air is captured, and sent to the lab for analysis. Laboratory analysis is done with a gas chromatograph and a mass spectrometer. One big challenge in active sampling is getting a representative sample of the emissions.
Another way of testing sources is with continuous emissions monitors. These are extremely expensive devices and certainly not used in the composting industry, but are used in many other industries. Active sampling is normally done by inserting a probe through a port in the flue or chimney and taking a sample of the exhaust air going up the chimney. Another method of active sampling is a flux chamber. This is often used in windrow composting measurements. The flux chamber captures a known volume of air being emitted from the surface of the windrow and the sample is drawn into a refrigerated cooler for transport to the lab. The big problem with flux chamber testing is ensuring that the bottom of the flux chamber is sealed tightly against the surface. If air leaks in under the flux chamber boundaries, it dilutes the air samples, therefore making the reading inaccurate. It is very difficult to do this accurately.
The costs of active sampling can be very high. For example, a single test for the presence of two HAPs, triethylamine and carbonyl sulfide, both inside the composting building and downstream of the biofiltration unit, would have cost $6,500 – one test of two pollutants. There are 29 HAPs and a dozen or so criteria pollutants and more than one test is needed to get an accurate data. You can’t just take one data point and say this represents the situation. The numbers mount up very quickly. You could easily spend $250,000 or more on source testing at a composting facility.
Another way to do a Potential to Emit analysis is with a materials balance. Materials balance is a process that accounts for the weights of compounds and elements that come through a composting process (Figure 2).
Assume you have 100 pounds/day of a pollutant in a feedstock coming to your composting facility. You have a compost pile: 10 lbs/day get emitted into the atmosphere, 10 lbs/day end up in the leachate, and 80 lbs/day go out in the product. The materials balance is used to mathematically balance what’s coming in with what’s going out and which way it is going. This concept was originally developed for conservative substances like metals. If you had 100 lbs/day of iron coming in, then you figure out how many pounds per day of iron is going in each direction – air, leachate and product. What makes it difficult in composting is the biochemical nature of the process, which changes compounds during the process. Take, for example, hydrogen sulfide. Hydrogen sulfide is oxidized to sulfate in the composting process. If you calculate 100 lbs/day of hydrogen sulfide coming in and you measure the amount going out in the atmosphere, the amount in the leachate and the amount in the product, those numbers are not going to add up. The materials balance won’t work because hydrogen sulfide has been converted to sulfate in the compost process. Materials balances can be very difficult to do and the lab analysis costs for calculating the pollutants in each of these different fractions can be extremely expensive.
Emissions factors have been developed by EPA to catalog air emissions from a wide variety of industrial activities. Unfortunately, very few of those relate to composting. They relate to quantities of air pollutants being emitted from a particular activity to the activity levels that produce that pollutant. The general equation for emissions estimation is:
E = A x EF x (1-ER/100)
E = emissions;
A = activity rate;
EF = emission factor, and
ER =overall emission reduction efficiency, %
For example, if you want to know how much particulate matter is being emitted by curing piles, you could put a flux chamber down and do an air sample and test. Alternatively, you could use an emission factor from an EPA document called AP-42 (a catalogue of air pollution emission factors). They have estimated that particulate matter is emitted from the active storage pile of sand and gravel operations at the rate of 13.2 pounds per acre/per day. That is an emission rate tied back to an activity level, and is a reasonable approximation. Particulate matter generated by sand and gravel operations is generally windblown erosive sand. That is the kind of particulate matter you’ll get from a curing pile, wind blown emissions, and that is also what will be suspended by the windrow turner as the turner passes through the windrow.
Virginia’s air quality regulatory program requires that you use one of these methodologies to conduct a Potential To Emit analysis. If you are considered a major source after completing this analysis, you have to then prove that the proposed air pollution control system that you are going to build is Best Available Control Technology (BACT). In our industry, there are two major methods of proving BACT. One is a biofilter; the other is a chemical scrubber. Chemical scrubbers are very expensive. So most composters use biofilters, and biofilters are accepted by regulators as BACT.
Calculating Loading Rates
A permit to construct can be issued after a Potential To Emit analysis is done. To get an operating permit, a performance test has to be conducted. So the key question you have to ask yourself in doing a Potential To Emit analysis is: Can we qualify for a permit exemption? The levels of emissions under which Virginia allows you to avoid permitting are shown in Table 1. The strategy we used was to see what the technical literature was reporting in other studies of air pollution, calculate the PTE numbers for this proposed composting facility in Virginia and try to convince the state that those numbers were low enough that a permit was not needed.
When you do a Potential To Emit analysis, you need to establish what are called loading rates. The loading rate is expressed in terms of tons per year, or pounds per hour of pollutants being emitted by the facility. But air quality data is often expressed as an ambient concentration, just like water quality data is. The units of measure is milligrams per cubic meter. Occasionally you will see it expressed as a mass flux rate, in terms of pounds per square foot per day.
To convert a concentration to a loading rate, you have to go through a calculation process to use the air pollutant concentration and the air flow rate to convert from milligrams per cubic meter of an ambient air concentration to pounds per hour or tons per year of a loading rate. For emissions reported as ambient air concentrations, the following formula (MAERS, 2004) was used:
Lbs/hr = C (mg/m3) x V (DSCFM) x 60 min/hr x 0.028 m3/ft3 / 453.6 gm/lb x 1000 mg/g
C = ambient air concentration
V = Volume flow of air (in dry standard cubic feet per minute)
For emissions reported as mass flux rates, the area of the corresponding portion of the process (i.e. square feet of active composting bays, square feet of curing windrow surfaces, etc.) was used as a multiplier to estimate emissions. For a windrow with a mass flux rate of two pounds of ammonia per square foot per day, you simply multiply the total square footage of the windrow surface by 365 days in the year to get the loading rate.
Analyzing Potential Pollutant Emission Rates
A literature review was helpful in establishing potential emission rates from the composting facility. Literature references are at the end of this article. Carbon monoxide is not normally thought of as a pollutant in composting, but one study found refers to a source of carbon monoxide as an intermediate product of the reductive acetyl-CoA pathway for carbon dioxide fixation by autotrophic bacteria. Emissions from diesel-powered engines are another source of carbon monoxide. A 1998 study by VanderGheynst measured CO emissions from food waste and biosolids composting. For diesel engines, we used the EPA Tier III emission limits for off-road diesel engines that go into effect in 2006. We felt that was valid as the plant won’t come on line until 2007. So running all the numbers, we came up with a calculated potential carbon monoxide emissions of 4.8 tons/year.
Nitrogen oxides represent a group of highly reactive gases that contain nitrogen and oxygen. Nitrogen dioxide is the chemical responsible for the reddish brown haze you see in urban areas. Only nitrous oxide emissions references were found in the composting literature. Czepiel (1996) measured N2O emissions from composting biosolids and livestock wastes. Emission rates were measured from active composting at 2.2 grams N2O per square meter per day and from curing at 0.9 g N2O/m2/d. We used the Tier III emissions limits for the mobile sources. EPA’s Tier III allowable emissions of NOx/NMHC from diesel engines are 3.0 g/bhp-hr. Calculations yielded a total estimated potential NOx emission of 24.1 tons/year.
Sulfur dioxide is produced by the combustion of fuels containing sulfur. There are no combustion sources proposed at the composting facility, so it was assumed there would be no SO2 emissions from composting itself. Diesel engines do have an emission of sulfur dioxide, and we used the emission factors out of EPA’s AP-42 emissions factor handbook to calculate total potential sulfur dioxide emissions at 1.7 tons/year. Last year, EPA announced that sulfur contents of fuels must be reduced to 15 ppm by 2010 but we elected not to use that because that is too far in the future given the plant will open in 2007.
PM-10 is particulate matter that is less than 10 microns in diameter. Because the building is enclosed and all air is exhausted to the biofilter, we felt there would be no PM or PM-10 emissions from composting. Biofilters do a very good job of capturing that kind of material. So, for emissions from curing, we used the AP-42 factor for sand and gravel operations as opposed to doing source testing on a windrow. There appears to be very little in the literature where others have done source testing to quantify particulate matter. For emissions from diesel engines, we used the Tier III limit. The EPA Tier III allowable particulate matter emissions from diesel engine off-road vehicles is 0.15 g/bhp-hr (EPA, 2001) and came up with an estimated potential to emit of 11.6 tons/year.
VOCs are clearly the most well researched and characterized air pollutant in our industry for obvious reasons. Four major studies that I found were VanDurme (1992) at Hampton Roads Sanitation District; Eitzer (1995) who looked at MSW composting facilities in Europe; Philadelphia (1995-1997) which did an enormous amount of work characterizing air pollution from the biosolids composting facility; and the Montgomery County Regional Composting Facility (1999). Philadelphia calculated a mass flux rate from its aerated static pile composting facility of 414 milligrams per square meter per day for both composting and curing. We assumed there were no VOCs coming out of diesel engines, and we came up with a total estimated VOC emissions of 12.6 tons/year.
Lead, fluoride and sulfuric acid mist are also regulated in Virginia as criteria pollutants. These are combustion process pollutants, and as there is no combustion process, we concluded that they would be zero. Sulfuric acid mist comes from the hydration of sulfur trioxide which is formed by the reaction of sulfur dioxide with water in air. We didn’t expect that to be present in the composting process either.
Virginia has three different categories of regulated sulfur compounds. They have been very well documented in the literature. Van Durme (1992) measured H2S concentrations at Hampton Roads Sanitation District’s composting facility. South Coast Air Quality Management District (SCAQMD) (1996) measured flux rates of H2S in curing piles. Van Durme (1992) recorded H2S concentration of 1.06 mg/m3 in the blower exhaust. Using the biofilter air flow expected at the new facility, the emissions of H2S from active composting are estimated to be 3.2 tons/year. SCAQMD estimated emissions of 0.08 lbs/hr of Total Sulfur Compounds from 57-day old windrows of biosolids and green waste. Gooden (2000) measured quite a few reduced sulfur compounds at the Rockland County, New York biosolids composting facility which were reported in BioCycle in 2000 (Volume 41, Number 1, January 2000). We didn’t expect any sulfur compounds from the diesel engines, and we came up with these estimates of potential to emit hydrogen sulfide – 3.5 tons/year; total reduced sulfur compounds – 7.4 tons/year; and reduced sulfur compounds – 4.1 tons/year.
Then we turned our attention to hazardous air pollutants. These are the 29 HAPs that have been detected in composting and reported in the literature. There may well be more but a lot of this data is not in the public literature domain.
In Virginia, any HAP emitted above 100 tons/year must be permitted. The state exempts low levels of HAPs based on percentages of threshold limit value concentrations. These are industrial hygiene exposure limits of time-weighted averages that represent an average over an eight hour period (TLV-TWA). Short-term exposure limits (TLV-STEL) and ceiling concentrations (TLV-C) also are used in evaluating HAPs. Virginia’s regulatory program is tied to emit a certain amount of HAPs as long as they are not more than 14.5 percent of the time-weighted average of that exposure limit (or 3.3% of the TLV-STEL or TLV-C). We concluded that none of the HAPs that this biosolids composting facility might emit would be generated in enough quantities to require an air emissions permit.
We then had to tell the state what we knew about the best available control technology – that is, the biofilter. We did some research on what others had reported in the literature about removal efficiencies of biofilters for various pollutants (see Table 2).
Biofilters are extremely efficient at removing a vast amount of air pollutants. The uncontrolled emission rates (based on the Potential To Emit analysis), multiplied by the removal efficiencies, gives the controlled emissions from the proposed facility.
The net result was that the Virginia DEQ accepted the Potential To Emit analysis and concluded that the composter did not need an air emissions permit.
Craig Coker is the Chief Engineer for McGill-Leprechaun, a private composter in North Carolina. He is also the immediate Past President of the Carolinas Composting Council and serves as a member of the Board of Directors of the U.S. Composting Council.
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January 30, 2006 | General
Emissions And Air Quality Compliance — It's Not Just About OdorsI
BioCycle January 2006, Vol. 47, No. 1, p. 37