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From BioCycle Journal of Composting &Organics Recycling March 2002, Page 42 |
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RESEARCH REPORT
CONTROLLING ODORS USING HIGH CARBON WOOD ASH
A pilot study at a yard trimmings composting facility found that wood ash can be an effective odor management tool.
Paul Rosenfeld, Mark Grey and Mel Suffet
A PILOT study on the feasibility of using high carbon wood ash to control composting odor emissions was conducted at a yard trimmings composting facility in Sacramento, California. The physiochemical sorption of odorants using activated carbon has proven to be a very successful approach for odor control, however, its cost is often prohibitively expensive to use in municipal composting. Wood ash incorporation has been shown to reduce odor emissions from biosolids composting (see “Wood Ash Finds Niche In Biosolids Composting,” January 1997), making it a possible, less costly alternative to activated carbon for odor control. Wood ash is a by-product of cogeneration facilities as well as the pulp and paper industry. It has been found to have properties similar to activated carbon. The similarity is thought to result from incomplete combustion of wood residuals at temperatures greater than 700°C. Approximately 358,720 tons of high carbon ash are produced in California every year, according to Chris Trott, cogeneration procurement manager for Covanta Energy in Susanville, California.
The purpose of this study, funded by the California Integrated Waste Management Board, was to: Determine if high carbon wood ash can control compost odor under field conditions; Investigate difficulties of incorporating wood ash into compost under field conditions; Determine the effect of wood ash on specified compost product quality criteria; Estimate the traveling distance of odor emissions from a hypothetical site using a dispersion model; and Analyze the cost of odor control using wood ash versus other methods.
MATERIALS AND METHODS
Green material compost feedstocks from the city of Sacramento were used in this experiment. They were composed of ground wood chips, foliage, and grass. High carbon wood ash was produced at the Covanta Energy wood boiler cogeneration facility in Susanville. Whole tree chips are incinerated at greater than 1200°C. Compost and wood ash were mixed using a front end loader. Treatments consisted of adding zero percent, 12.5 percent, and 25 percent wood ash by volume to green materials in three separate windrows. The windrow dimensions were about 50 feet long, 20 feet wide at the base, and ten feet high.
Odor emissions (as perceived by human odor panels) and chemical odorant emissions were measured on days one and seven of composting from each of the three windrows. Emissions were sampled for sulfur compounds, ammonia, ketones and aldehydes, volatile fatty acids and dilution-to-thresholds. Gaseous emissions from surface migration were collected from an isolated surface area with an isolation flux chamber. The flux chamber was sunk into the compost to a depth of 2.5 cm to create a seal between the chamber and the surface.
Odor samples for evaluation by an odor panel were collected in Tedlar bags and shipped overnight to Odor Science and Engineering, Inc. in Bloomfield, Connecticut. Odor concentration was defined as the dilution with odor-free air at which 50 percent of an odor panel detected the odor. This point represents the odor threshold and is expressed in terms of “dilution-to-threshold” (D/T). The odor panel consisted of seven trained individuals screened for their olfactory sensitivity and ability to match odor intensities.
Odor traveling distances for odorant emissions were estimated for each treatment using SCREEN3, an area-source air pollution dispersion model developed by the U.S Environmental Protection Agency. The model predicts the concentration of pollutants at an array of distances using a time weighted one-hour average during all meteorological conditions. Assumptions used to run the SCREEN3 include area source emission data from compost and compost/ash treatments, assuming a 2.5 acre composting area, wind speed of 3.3 feet/sec, zero feet application elevation, 21°C ambient temperature, five feet receptor height, and moderately stable atmospheric conditions. The lowest reported human detection limits for odorants were used to predict the “worst case scenario” odor traveling distance for each treatment. (These worst case scenarios are based on a paper by J.H. Ruth – “Odor Thresholds and Irritation Levels of Several Chemical Substances” – published in the American Industrial Hygiene Association Journal in 1986.)
PILOT STUDY FINDINGS
Chemical and Physical Properties: Selected chemical and physical properties of the untreated compost, wood ash-compost mixture, and wood ash collected on day seven of composting are shown in Table 1. Wood ash had little affect on compost quality from an agronomic perspective given these results. However, more study needs to be done using compost with wood ash in its relation to seed germination assays, growth studies and nutrient release characteristics.
The wood ash was strongly alkaline, with a mean pH of 10.3. The combustion process forms carbonate, bicarbonate and hydroxide, which results in the alkalinity in the ash. The relative proportion of these compounds varies with combustion temperature. Carbonates and bicarbonates predominate when wood is combusted below 500°C, whereas oxides become more prevalent when combustion temperatures exceed 850°C. Because wood ash materials used in this experiment were combusted at temperatures greater than 1200°C, it is likely that much of the calcium in the ash was initially in the form of calcium oxide. The high pH of wood ash can be reduced to a pH of 8.6 exposing the ash to water and carbon dioxide from the atmosphere, which dissolves into solution forming calcium carbonate. Hence, it is possible to reduce the pH of the wood ash if desirable to control ammonia emissions.
The wood ash was found to have a surface area of 105 square meters per gram on a dry weight basis. This is a large surface area considering that commercial activated carbon often has a surface area of approximately 500 square meters per gram. The surface area of wood ash can vary from between five to more than 105 square meters per gram and is directly proportional to carbon content and in ash and incineration temperature.
The compost treated with 12.5 percent and 25 percent wood ash by volume slightly increased the pH and electrical conductivity of the compost. Important agronomic parameters such as total N and C, organic matter content, CEC, and soluble micronutrients were not affected by the addition of wood ash.
Emission Rates: Odorant concentration divided by the lowest reported human detection limit for each chemical odorant are presented in Table 2. Emissions from the control compost treatment included formic acid, acetic acid, propionic acid, isobutul and butyl acid, isovaleric acid, valeric acid, acetaldehyde and propionaldyhyde, which were estimated to be 9, 3.5, 3.4, 450, 110, 240, 14,000, and 7.7 times the lowest human detection limit on day one and 0.4, 0, 0.1, 45, 0, 490, 680, and 0.6 times the lowest human detection limit on day seven. Emissions from the 12.5 percent ash treatment included acetaldehyde, ammonia and ethyl mercaptan, which were estimated to be 386, 4 and 11 times the lowest human detection limit on day one and 160, one and zero times the lowest human detection limit on day seven. Emissions from the 25 percent ash treatment included acetaldehyde and ammonia, which were estimated to be 100 and two times the lowest reported human detection limit on day one, and 470 and 0.3 times the lowest reported human detection limit on day seven. All compounds not listed were below analytical or human detection limits.
Reductions in emission rates from the wood ash treatment were noted for all volatile fatty acids and most ketones and aldehydes. The emission rates from the 12.5 percent wood ash treatment reduced acetaldehyde, propionaldyhyde, crotonaldehyde, and butanaldehyde emissions, but not as sharply as the 25 percent wood ash treatment. In some instances, the formaldehyde, acetone, methyl ethyl ketone and valeraldehyde emission rates were higher with the wood ash amended treatments but these compounds were not attributed to objectionable odors as measured by the odor panel.
Dilution-To-Threshold Values: Mean dilution-to-threshold values for the 25 percent wood ash treatment were reduced by 88 percent and 89 percent on days one and seven, respectively compared to the control treatment. Similarly, the 12.5 percent wood ash treatment reduced the mean dilution-to-threshold values by 73 percent and 25 percent on days one and seven, respectively, compared to the control treatment. These data suggest that wood ash can reduce odor emissions from yard trimmings composting facilities. The 25 percent by volume wood ash treatment provided longer periods of active adsorption of odorants and hence greater reduction in odor emissions.
The odor panels used for dilution-to-threshold olfactometry also provided qualitative descriptors of compost odor. The term “medicinal” was used for all treatments during day one, while some individuals noted a “burnt wood” odor in the wood ash treatments. For the control treatment, panelists noted a “moldy” and “mildew” odor that was not detected in the wood ash treatments. Medicinal is indicative of ammonia, while moldy and mildew odors are often associated with incomplete decomposition or anaerobic conditions.
Ammonia (NH3) emission rates were generally found to be higher in the compost treated with wood ash. This effect likely occurred because the wood ash had a strongly alkaline pH. Under high pH conditions, NH4 ions in the compost are continually converted into NH3, which can be an odorant depending on concentration.
Sulfur emissions were only detected in one of the treatments on day one. Ethyl mercaptan was detected on day one at a low emission rate in the 12.5 percent ash treatment; no ethyl mercaptan was detected in the 25 percent treatment. The results suggest that sulfur emissions from this particular green material feedstock were not important in generating odor that could affect composting operations.
Odorant Dispersion: The SCREEN3 model for estimating dispersion of odorant emissions predicts the concentration of pollutants at an array of distances, and assumes stable night time conditions when air currents move parallel to the surface of the earth and can be detected by humans. The lowest reported human detection limits for odorants were then used to predict the “worst case scenario” odor traveling distance for each treatment. The model suggests that the highest wood ash application rate can dramatically reduce the traveling distance of compost odor.
Mean Emission Rates: The mean emission rates were calculated for each chemical odorant using two data points from each of the three treatments (one data point for day one and one for day seven). Increasing dilution-to-thresholds were found to be statistically correlated to formic acid, acetic acid, propionic acid, isobutyl and butyl acid, isovaleric acid, isocaprinic acid, caprinic acid, acetaldehyde, propionaldehyde, crotonaldyhyde, butanaldehyde and valeraldehyde. Hence, aldehydes and volatiles fatty acids appear to be the major odorants responsible for odor emissions.
COST ANALYSIS AND ASH HANDLING
In California, wood ash can be provided free of cost (except for hauling costs) from several cogeneration and paper facilities throughout the state. The ash was found to be easy to handle, did not create dust, and blended quickly into the compost with a front-end loader, although it could be better incorporated with a windrow turner. It is most cost-effective when the ash is put on top of the windrows and then incorporated later during normal turning.
The costs of hauling wood ash typically run about $1.25 to $1.35/mile for a 24-ton load. These economics can be improved if “back hauling” is incorporated into a program, for example, by trucking wood fuel back to a cogeneration facility after transporting the ash. In addition, wood ash increased the amount of compost, increasing the tonnage of marketable material .
Assuming a 50,000 ton/ year facility incorporating 25 percent wood ash (12,500 tons) and a hauling distance of 50 miles, the annual cost for ash transportation would be approximately $34,000, using the hauling figures cited above. However, this cost can be offset by the additional revenue from the 12,500 additional tons of compost (i.e., the increased volume resulting from incorporation of the wood ash).
Paul Rosenfeld is with Komex H2O Science, Inc., Mark Grey is with Synagro Composting Co. of California, and Mel Suffet is in the UCLA Environmental Science and Engineering Program at the School of Public Health.
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