BioCycle March 2011, Vol. 52, No. 3, p. 54
While air emissions continue to pose challenges, the current knowledge base makes testing and the path to their minimization less painful and more advantageous to composting site owners and operators.
T.R. Card and C.E. Schmidt
AIR emissions from composting facilities have been and will continue to be an issue for composting site owners and operators. However, with the current knowledge base regarding what is emitted from operations, and the available and regulatory approved testing technologies, air emissions measurement is less painful and more advantageous to site owners. Knowing what the air emissions are, and how to improve operations to minimize them, is a clear advantage for industry sustainability. In many cases, existing emissions can be “grandfathered” when regulations are changed or first implemented – if they are quantified using regulatory approved techniques. This is known in the air emissions regulatory trade as “baselining.” In addition, voluntary emission reductions, if quantified, can be sold as emission offsets for other industries in some jurisdictions. The value of these offsets can be worth millions of dollars for some facilities.
The South Coast Air Quality Management District (SCAQMD), in the Los Angeles area, has developed measurement methods, based on the United States Environmental Protection Agency (USEPA) flux chamber technology, specific for the composting industry. This is a direct measurement technology that has been shown to be cost-effective, producing data that meet the expectations of regulatory agencies and other more stringent demands such as litigation support.
This article presents the process and technologies used in assessing air emissions from composting sites with the intent and purpose of generating a list of quantitative emissions factors that, when combined with operating information and area surface estimates, can be used to calculate (not model) measured air emissions from each unit process. Adding together the air emissions from each unit process provides the total site emissions estimate of study compounds. These emission factor or unit process emission data are also useful for engineering purposes, including identifying “hot spot” emissions, providing useful information for process engineers, and evaluating air pollution control measures necessary to achieve compliance with site permit requirements.
Composting site air emissions characterization typically involves determining the air emissions from the site in order to meet permit requirements, and often includes efficiency testing of air pollution control equipment (scrubbers and biofilters). The site emissions are estimated from the life-cycle of the compost. This is accomplished by directly measuring the “flux” (mass of compound emitted per time unit per surface area unit, like milligram per square meter per minute) from each unit process. These data are summed and expressed as pounds of emissions per ton of stock material received for composting.
The compost unit processes can include: Material receiving or feedstock pile; Preparation of material for composting (sorting piles); Blending of waste material with a bulking agent; Composting the blended mix in windrows or in a static pile; Windrow or pile maintenance including watering, turning or covering; Odor/air pollution control devices (scrubbers or biofilters); Windrow/pile breakdown; Curing; Screening,; and Product stockpiling.
The air emissions from any and/or all of these various composting processes and control devices can be measured using the SCAQMD methods from compost surfaces or other regulatory approved assessment methodologies for the point (stack) sources. The technical approach using these direct measurements is the route to take.
FLUX CHAMBER SAMPLING
The USEPA flux chamber technology designed for compost surfaces (and used by SCAQMD) is a direct source measurement technique designed and validated for assessing air emissions from area sources that are considered “diffusion flow,” meaning that the gases diffuse from the test surface. Since most area sources at composting sites have a “volumetric” or “advective flow” (either natural or forced aeration), this direct measurement technique needed to be modified to address this additional flow in the assessment. Therefore, the SCAQMD modified and validated the technology to accommodate this characteristic of compost sources.
Key enhancements to the USEPA technology included:
1) Measurement of the total gas flow rate in the flux chamber by using a tracer compound in the flux chamber sweep air; and 2) Modifying the flux chamber to handle higher gas flow rates. Figure 1 shows an example of this equipment. This sampling technology is well established and has since been used on many sites for the purpose of meeting regulatory compliance to air quality standards.
Other options for making these assessments include indirect measurement technologies (measuring at the fence line) or predictive modeling technologies. However we have found that the direct measurement (flux chamber) approach offers the advantage of providing for an assessment that is not affected by upwind sources, generates emission factors from each component of the compost operations and produces data that is accepted by regulatory agencies.
SAMPLING METHODS FOR SPECIFIC COMPOUNDS
The other component of the assessment task includes selecting the analytical method for the species (pollutants) of interest emitted from the site. For many sites, the most important pollutant from a regulatory standpoint are volatile organic compounds (VOCs), which are measured differently in almost all jurisdictions. Since VOCs are very important in the Los Angeles area, the SCAQMD developed a specific method for its jurisdiction. The District officially calls VOCs total non-methane non-ethane organic carbon (TNMNEOC) and has it measured at composting facilities using its Method 25.3. This method (Figure 2) uses a glass vial filled with ultra pure water followed by a Summa® canister. The sample gas stream is drawn through the water and into the evacuated canister. The glass vial is placed in an ice bath to keep the ultra pure water as cold as possible during the sampling.
VOCs are considered a pollutant because most are photo-reactive, forming smog when exposed to sunlight. However, not all VOCs are photo-reactive. As implied by the TNMNEOC acronym, methane and ethane are not photo-reactive, and don’t count as VOC for smog purposes. These types of compounds are called “exempt” compounds and are subtracted from the VOC total. Other important exempt compounds at composting facilities are acetone and siloxanes. These can be quantified and subtracted just like the methane and ethane is. The preferred method to measure these exempt compounds is USEPA Method TO-15 using a Summa® canister analyzed by GC/MS (gas chromatography/mass spectrometry).
An emerging consideration at composting facilities is adding to the list of non-photoreactive compounds that can be subtracted from the total VOC number. University of California Davis researcher Peter Green has developed a field photo-reaction chamber to study the overall reactivity of composting facility off-gas (see “Are Compost Emissions A Strong Ozone Source?,” March 2010). Preliminary findings indicate that the overall photo-reactivity of compost off-gas is relatively low, suggesting that the air emissions impact of composting may currently be overstated.
The next most critical compound from a regulatory perspective is ammonia. This is measured using a impinger to dissolve the ammonia in a mild acid solution, per SCAQMD Method 207.1. Figure 3 shows the Teflon® impinger for ammonia sample collection in the same ice bath as the Method 25.3 sample. Air is drawn into the impinger using specially calibrated battery powered personal sampling pumps.
Odor quantification also can sometimes be of interest. Odor samples can be taken using the same flux chamber set up and are taken in a Tedlar® bag using an evacuated chamber. They are analyzed by an odor panel for thresholds, intensity, and/or character within 24 hours of sampling.
Another emerging issue at composting sites are greenhouse gas emissions. Carbon dioxide, carbon monoxide and methane are measured as part of the SCAQMD Method 25.3 process; the data are available whenever that method is used. In addition, nitrogen oxide species can be measured by using either field or laboratory instruments from the same flux chamber apparatus.
DAILY COMPOSTING PROCESS EMISSIONS
Most composting facilities operate in a batch process where one pile/windrow takes 20 to 80 days to fully compost. The emissions from this batch process will be different every day. Our team has found that measuring on key process days will allow us to construct the likely daily emissions for each of the process days.
Figures 4 and 5 show the daily emissions for a windrow composting operation for VOC and ammonia respectively. This curve was generated by sampling on process days 1, 3, 7, 15, 29 and 63. In addition, intense sampling was undertaken during a windrow turning event. The spikes on the graph represent the increase in emissions from row turning. The total area under the curve represents the emissions from that batch. By dividing out the total mass of mix in the batch, the emissions can be expressed as pound of emissions per ton of mix. The ratio of VOC to ammonia emissions can be a strong function of the compost carbon to nitrogen ratio, so the relative emissions may be managed somewhat by feedstock adjustments. The same approach of measuring key process days can be taken for the assessment of long term storage emissions of either feedstock or product.
The San Joaquin Valley Air Pollution Control District (SJVAPCD) recently published emission values for composting facilities in California. The following tables are provided from their report. Table 1 presents the most relevant green waste data, Table 2 has the most relevant food waste data, and Table 3 includes the most relevant biosolids data.
PUTTING IT ALL TOGETHER
The following example provides insight as to how all this information can be used together.
Agri Service, Inc. operates a yard trimmings composting facility in Vista, California. The company wanted to complete a quick evaluation of air emissions benefits of possibly purchasing a GORE™ fabric covering system for improving green waste composting and/or biosolids composting. Table 4 provides a summary of the data collected.
Using this data, the published emission data, and our knowledge base of how emissions vary during the composting cycle, we were able to estimate that the Gore covering system will likely offer 98 percent VOC control and 99 percent ammonia control for green waste, and 94 percent VOC control and 80 percent ammonia control for biosolids, as compared to the SCAQMD existing emission factors. This data is unique to the Agri Service site and micropore cover technology may not offer this level of control for all cases.
Tom Card, a civil engineer with extensive experience in performing air emission estimates from complex sites, is with Environmental Management Consulting in Enumclaw, Washington (email@example.com). Chuck Schmidt, Ph.D., an analytical chemist with expertise in performing air emission assessments, is with CE Schmidt Environmental Consulting in Red Bluff, California (firstname.lastname@example.org; www.ceschmidt.com).
March 23, 2011 | General
Air Emissions Assessment Technologies
BioCycle March 2011, Vol. 52, No. 3, p. 54