March 23, 2007 | General

Composting And Greenhouse Gas Emissions: A Producer's Perspective

BioCycle March 2007, Vol. 48, No. 3, p. 37
West Coast Conference Preview
How do the three core elements of composting – feedstocks, composting process and compost use – fare in analyses of climate change impacts?
Sally Brown and Scott Subler

EVERYONE who has ever tended a compost pile or used compost in their gardens is sure that what they are doing is environmentally beneficial. Transforming lemon rinds, old spaghetti and sundry other wastes into a rich compost has to be a good thing. The growth response of plants grown in compost tells us that they certainly think so. However, with the recognition that our every day activities such as mowing the lawn (petroleum or electricity), washing and drying clothes (electricity) and taking the car to go shopping (petroleum) have a negative impact on greenhouse gas emissions, we are faced with the task of reexamining a range of practices to determine their impact on our atmosphere. We are also coming to recognize that relatively minor changes in how we do things can turn an every day practice into a carbon friendly practice.
Reexamining environmentally friendly practices such as composting – using their impact on global warming as a gauge – can potentially provide additional evidence that these are solid, beneficial practices. It may also show how these operations can be improved to limit emissions of greenhouse gasses. In some cases, for larger scale operations, there are potentially economic benefits to be gained by appropriate understanding of greenhouse gas balances associated with the operation. An accounting of compost operations was undertaken to see how compost adds up with regards to global warming.
Global warming occurs as the concentration of gasses that effectively trap heat released into the atmosphere increases. The main culprit here is carbon dioxide (CO2). However, other gasses such as methane (CH4) and nitrous oxide (N2O) also can be released into the environment. Both of these gasses are much more efficient than CO2 (23 times for CH4 and 296 times for N2O) at holding heat. Therefore their impact on global warming is many times that of CO2.
Methane is formed when organics decompose in an anaerobic environment. It is an inefficient decomposition reaction from a microbial perspective and will only occur when all oxygen is depleted. Nitrous oxide is formed as fixed nitrogen is oxidized. This reaction generally occurs in a mildly oxygen deficient environment where nitrogen isn’t limiting. As organic material decomposes or transforms in a compost pile, the primary gas given off is CO2. This is not considered to have an effect on global warming, as the plant matter and other feedstocks that are decomposing are from what is called the short-term carbon cycle. This cycle consists of what is grown, what we eat and the associated wastes from these processes.
Composting “counts” (i.e., net benefit or detriment) from a greenhouse gas (GHG) perspective based on the following factors: 1) If by composting different wastes, we prevent gasses other than CO2 from being released into the environment; 2) If composting operations give off gasses other than CO2; 3) If the use of compost results in a change of practice with net reductions in GHGs. In carbon parlance, the impacts that composting can have on GHG fall into one or both of two categories: avoidance or sequestration. Avoidance refers to situations where a new practice is undertaken that results in reduced emissions of GHGs into the atmosphere. Sequestration covers practices that result in carbon being put back into storage in the earth, such as increasing soil carbon concentrations.
In order to add up the gas balance for composting, you have to divide the compost operation into three separate stages. The first stage involves what goes into the compost pile and where it would have gone if there were no pile to go to. The second stage revolves around the composting process itself, how much energy is used in composting and if gasses other than CO2 are emitted from the compost pile. The final stage refers to what happens to the finished compost. How the compost is used also can have an impact on GHG accounting.
There are two primary elements to consider with this accounting. The first is to evaluate whether or not you are doing a good thing for the atmosphere by composting – and how you can alter your operation to maximize the benefits associated with composting from a GHG perspective. The second is that there are potential financial incentives associated with practices that result in either avoidance or sequestration of GHGs. The Chicago Climate Exchange (CCX) is the first functioning market in the United States that trades carbon for dollars. Public and private entities can join the CCX. When they join, they enter into a legally binding agreement to reduce their emissions by a certain percentage over a specific time period.
Current members of the Exchange include the State of New Mexico and the Ford Motor Corporation. Because these members may not be able to meet their required reductions in the time allotted, there is a demand for “offset” projects. These are projects that are new and innovative and result in reductions of GHG emissions or increases in the quantity of carbon stored. Examples of these projects include anaerobic digesters for animal manures and no till farming operations. For each of these operations, an accounting is done to determine how much CO2 is saved each year. This number is then traded for the current value of metric tons (1,000 kg) of CO2 on the CCX. Compost operations may qualify as offset projects and if so there will be a dollar value associated with each compost operation.
To assess the climate change impacts, three primary components of composting need to be evaluated. The first part of an evaluation on the GHG balance of composting focuses on the feedstocks used to make the compost. If, instead of going into a compost pile, the feedstocks would have been disposed in a way that resulted in emissions of CH4 or N2O, composting results in avoidance of these emissions.
To quantify these credits, it is necessary to figure out the methane generation potential of each type of feedstock. General categories for feedstocks commonly used for compost production include manures and municipal biosolids, food residuals, yard trimmings and paper waste. When left to decompose in anaerobic environments without any mechanisms for gas capture, these materials will generate and release GHGs. Examples include manure stored in an uncovered lagoon, or other residuals disposed in landfills. When these residuals are composted instead, it is possible to qualify for methane avoidance credits if the material is used or stabilized in a way that releases significantly less methane into the atmosphere.
The two greenhouse gasses of concern for material decomposing in landfills are CH4 (methane) and N2O (nitrous oxide). While there is a fair amount of literature on methane emissions from decomposition of organics, there is very little literature on nitrous oxide emissions. To quantify the amount of methane, we did a review of the literature. For municipal biosolids and animal manures, there is extensive information on CH4 generation potential as these materials are frequently stabilized in anaerobic digesters. Methane generation potential can be related to the volatile solids content of these materials (Metcalf and Eddy, 2003; IPCC, 2006a). Volatile solids (VS) are defined as the portion of the solids that will turn into gas when heated at 500° C. The compounds that will volatilize are often carbon-based and so, under anaerobic conditions, will generate biogas. Biogas consists of a mixture of volatile organic compounds, and depending on the source as well as the conditions for decomposition, can contain about 60 percent methane.
However, total VS are not the same as the portion that is likely to decompose under anaerobic conditions. Here a certain percentage of the VS is resistant to decomposition and is termed refractory. The refractory VS will likely take decades to decompose and although it is carbon-based, is unlikely to produce any GHG in a relevant time frame. For example, a 455 kg cow will normally excrete 2.7 kg/day of VS. Of this, 45 percent is likely to be easily decomposed. This has the potential to generate 0.85 m3/day of gas, of which 59 percent is likely to be methane. Per year, the excrement from this cow, if stored under the worst circumstances, will produce the equivalent of 7.4 Mg CO2 (Hansen, 2004).
For biosolids to meet federal land application requirements, a certain portion of the volatile solids must be destroyed. Anaerobic digestion is a common technique to reduce VS. In addition to meeting land application requirements, reduced VS also will mean reduced quantities of biosolids. If the biosolids are then routinely landfilled, diverting them to a compost pile can generate methane avoidance credits if the quantity of VS destroyed during anaerobic digestion is less than a realistic estimate of the portion of VS that can easily be volatilized. If a 100 day retention time is used as a maximum value, about 80 percent of the total VS can be destroyed in a digester (Metcalf and Eddy, 2003). So, if a municipality takes its biosolids out of the digester after 16 days and composts them instead of transporting them straight to the landfill, there is a potential to receive methane avoidance credits.
Animal manure in the United States is often stored in large, uncovered lagoons. These lagoons are the source of a number of environmental problems including ground and surface water contamination when improper linings are used or lagoons overflow. In addition, these lagoons can release large quantities of methane into the environment (DeSutter and Ham, 2005). There are a number of viable alternatives to the standard practice of lagoon storage. If selected, methane avoidance credits are merited. For example, covering lagoons for methane capture is becoming a viable alternative to standard operating procedures. Direct land application of manure is also an option. Composting also has potential to reduce the CH4 released by the lagoons, as well as provide a beneficial soil amendment. For each of these options, release of GHGs will be minimized in comparison to lagoon storage. Emissions during composting will be discussed. As with municipal biosolids, even when manures are anaerobically digested, depending on retention time and VS destruction, there is also a potential for credits if manures are composted rather than being returned to some type of lagoon storage following anaerobic digestion.
Food, yard and paper wastes are commonly sent directly to landfills. As with biosolids and manures, the VS content of these materials is related to their CH4 generation potential. Scientists have determined maximum methane yields for these materials by incubating them under laboratory conditions to simulate landfill decomposition (Eleazer et al., 1997). Table 1 summarizes the methane yields. Composting instead of landfilling these wastes would be a direct means to avoid CH4 generation.
Making compost requires energy. Machines are necessary to grind and mix feedstocks as well as to set up compost piles. These piles will generally require turning, forced aeration or some type of agitation to insure that aerobic conditions are maintained. Aerobic composting will produce a stable compost rapidly with the least amount of objectionable odors. All energy used for compost production will have an associated equivalent GHG cost. Liters of diesel fuel and kilowatts of electricity used each have an associated GHG debit. For example, combustion of one liter of fuel produces 2.75 kg of CO2 (ROU, 2003). This is minor in comparison to the methane avoidance credits gained by diverting material from landfills. However, if, for example, biosolids are composted rather than directly land applied, the energy costs of composting may compare unfavorably with those for direct land application. It should also be noted that if the energy is from renewable sources such as biodiesel fuel or wind power, energy costs would be further minimized.
Of greater concern than energy requirements is the potential for fugitive GHGs to be emitted directly from the compost piles as they are decomposing. Both CH4 and N2O emissions from composting feedstocks have been observed. Methane is formed under severely anaerobic conditions. The formation of N2O, which has 296 times the global warming potential of CO2, is not as well understood. Nitrous oxide can be formed during both nitrification and denitrification reactions although it is more commonly produced during denitrification Nitrification is the reaction that turns organic nitrogen into ammonia and nitrate. Denitrification is the reaction that returns nitrate to its gas form. Both reactions will occur during composting. While CH4 is normally detected at the bottom of a compost pile where oxygen is absent, N2O will evolve closer to the surface of the pile, where some oxygen may still be present (Hao, 2001). It will also tend to form in cases where N is not limiting. Where N is in short supply, the microbes that are actively decomposing organics in the pile will scavenge available N for their own uses and release of nitrogen gas will be minimal.
From the range of studies published on this topic, some general trends are clear. More CH4 is formed when the compost feedstocks are wet. More N2O is formed when feedstocks are wet and also when less carbonaceous material is present. For example, composting manure with high moisture and low straw will produce more GHGs than composting manure with more straw in a drier pile (Sommer and Moller, 2000). As methane is formed where conditions are anaerobic, it will be more abundant at the bottom of a pile (Hao et al., 2001). Turning the pile will release the methane. However, turning also will ensure better aeration and therefore reduce methane production overall. A study with static piles without forced aeration showed very high methane release (Lopez-Real and Baptista, 1996). A cover of finished compost will also limit methane release as microbes in the finished compost will oxidize the methane before it is released into the atmosphere.
These results suggest that careful management of composting operations can significantly reduce or eliminate GHG emissions from compost piles. Keeping a pile dryer and well aerated also will accelerate stabilization and reduce odors. In general, good composting practices are also good practices for GHG reduction. A pile that is managed to ensure rapid decomposition and low odors also will have low GHG. Values from the literature suggest that maximum emissions from piles would be 2.5 percent of the initial C and 1.5 percent of the initial N to volatilize as CH4 and N2O. If a pile contains 75 percent organic matter on a dry weight basis with a C:N ratio of 30:1, this would translate to about one ton of C evolving as methane for each 100 dry tons. That same pile would have 0.02 tons of N that would evolve as N2O. It should be noted that these are conservative estimates and actual GHG release can easily be an order of magnitude lower.
Because using compost is good for so many things, it seems intuitively obvious that using compost should also reduce GHG emissions. However, reductions are difficult to quantify. A very detailed estimate of potential GHG related benefits from very specific uses of compost was laid out in a life cycle analysis of composting prepared by the Recycled Organics Unit at the University of New South Wales in Australia (Recycled Organics Unit, 2003). Different aspects of compost use, including potential savings in irrigation water and fertilizer, pesticide and herbicide usage, were included in the estimate. Increases in soil organic matter also were taken into consideration. Credits for each category were quantified for two specific end uses of compost products. Changes in water use as a result of compost application are detailed below.
When compost is added to soil, the water holding capacity of the soil increases. Infiltration also increases while runoff simultaneously decreases. A secondary effect of compost addition is reduced bulk density. This allows for more extensive root growth, which in turn suggests that plants will have access to a larger area of soil and the water contained in this expanded area. Finally, surface application of compost can act as a mulch, reducing evapotranspiration – effectively increasing soil water. Taking all of these factors into account, the quantity of water required to raise a crop is reduced. Reductions in water use of between 30 and 70 percent as a result of mulching are noted. Increases in water holding capacity also show a wide variation, based both on rate of compost applied as well as soil type.
Although it is clear that using compost will reduce the amount of water required to grow a crop, developing guidelines on how to quantify these reductions is difficult. From a GHG perspective, the credits associated with reduced water use would be related to the reduced power required to pump water to the field. Actual reductions would depend on how much compost is applied to the soil and how it is applied, i.e., incorporated or as a mulch. Different soil types, different crops and different climates also will effect how much water is required. Finally, management of individual farms will play a role. Different farmers will be more or less diligent when it comes to monitoring soil moisture status. Because of this high potential variability, it would seem that credits for use of compost would need to be granted to the individual users rather than the compost producer. Compost producers have little to no control over the use of their products unless it is used on their own farms.
Similar problems exist for quantifying credits for other potential GHG-saving uses of compost. Even if the compost has a nutrient value, it is not clear if the individual user will take this into account and reduce application of conventional fertilizers accordingly. The situation is similar for herbicide and pesticide use. In each case, it makes sense to look at the practices of the end user and grant credits based on individual practices.
The United Nations (through the Kyoto Protocol) has developed approved methods for quantifying and crediting composting projects that reduce methane emissions. The U.S. is still in the early stages of developing a generally accepted GHG accounting system that composters could use to assess their operations.
Composting can be a greenhouse gas conserving practice. Savings in GHG emissions as a result of composting operations are most dramatic when potential methane-emitting feedstocks are diverted from landfills or open storage lagoons into composting operations. The way that the current international GHG accounting system functions, if these potential methane-emitting feedstocks are diverted from direct land application to composting, they are not eligible for methane avoidance credits. Unlike landfilling or storage lagoons, land application is not a significant source of methane (so methane avoidance credits would not be valid).
The composting process itself is a source of emissions, coming from both use of equipment to set up and maintain compost piles as well as from gas emissions from piles during composting. In both cases, these emissions can be minimized by use of renewable energy sources like biodiesel fuel and by careful control of the compost process to eliminate anaerobic conditions. Although use of compost is a GHG friendly practice on many levels, difficulties in dictating how the compost is used and the resultant decrease in other inputs such as water and fertilizer suggest that these credits need to be determined on a farm-by-farm basis rather than granted to the compost generator.
Sally Brown is a Research Associate Professor at the University of Washington in Seattle. She will be expanding on this topic at the BioCycle West Coast Conference. She can be reached at slb@u.washington. edu. Scott Subler is President of Environmental Credit Corporation in State College, Pennsylvania. He can be reached at
DeSutter, T.M., and J. M. Ham. 2005. Lagoon-biogas emissions and carbon balance estimates of a swine production facility. J. Environ. Qual. 34:1:198-206.
Eleazer, W.E., W.S. Odle, Y.Wang, and M.A. Barlaz. 1997. Biodegradability of municipal solid waste components in laboratory-scale landfills. Environ. Sci. & Tech. 31:3: 911-918.
Hansen, R.W. 2004. Methane generation from livestock wastes Colorado State University Cooperative extension bulletin #5.002 http://www.ext.colostate. edu/pubs/farmmgt/05002.html
Hao, X., C.Chang, F. Larney, and G.R. Travis. 2001. Greenhouse gas emissions during cattle feedlot manure composting. J. Environ. Qual. 30:2:376-386.
Haug, R. T. The Practical Handbook of Compost Engineering. Lewis Publishers 2003. Boca Raton, FL 717 pp.
Lopez-Real, J. and M. Baptista. 1996. A preliminary comparative study of three manure composting systems and their influence on process parameters and methane emissions. Compost Sci. & Util. 4:71-82.
Metcalf & Eddy. 2003. Wastewater Engineering Treatment and Reuse. McGraw Hill, Boston 1819pp.
Recycled Organics Unit (2003). Life Cycle Inventory and Life Cycle Assessment for Windrow Composting Systems. Report prepared for NSW Department of Environment and Conservation (Sustainability Programs Division), Published by Recycled Organics Unit, The University of New South Wales, Sydney
Sommer, S.G. and H.B. Moller. 2000. Emission of greenhouse gases during composting of deep litter from pig production- effect of straw content. J. Agric. Sci. 134:327-335.
IN ORDER to qualify for carbon credits there are some basic rules that have to be followed:
o A practice must result in avoidance or reduction in releases of greenhouse gasses or sequestration of carbon into the soil.
o A practice must be new and different from conventional, business as usual.
o The practice has to be recognized by an official organization such as the Chicago Climate Exchange.
o Appropriate monitoring and accounting methods must be followed.
o No “cherry picking” allowed: a large company or municipality can’t just list their carbon friendly activity. The activity has to be part of a larger carbon accounting that covers the company or municipalities carbon footprint (this is not the case for smaller organizations or businesses).

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