March 24, 2009 | General

Estimating Greenhouse Gas Emissions Of Biosolids Management

BioCycle March 2009, Vol. 50, No. 3, p. 26
After a separate economic analysis of biosolids disposal options, Association studies energy consumption and GHG emissions for the town of Merrimack, New Hampshire.
Ned Beecher

DECISIVE national actions on climate change are likely to happen soon, especially given the new administration in Washington. Like other professions in the U.S., the organics management field is learning quickly about greenhouse gas (GHG) accounting. Many developed countries are ahead of the U.S. in calculating emissions, driven by requirements of the Kyoto Protocol. However, the U.S. is 1 of 192 countries that signed the U.N. Framework Convention on Climate Change (UNFCCC), which requires regular reporting of estimated GHG emissions. The U.S. Environmental Protection Agency therefore has some experience in calculating GHG emissions, such as with a 2005 report that provides details and trends on U.S. GHG emissions from 1990 through 2005.
Addressing climate change includes a great deal of mundane, arcane work – including figuring out how to calculate the total GHG emissions (expressed as carbon dioxide equivalents) from a given jurisdiction or facility. The International Panel on Climate Change (IPCC) developed protocols for these calculations on a gross scale, providing rough default values that could be applied across nations. Other organizations have refined such protocols and applied them to smaller jurisdictions. For example, the so-called “Corporate Protocol” has, for years, provided guidance to companies for reporting their total GHG emissions (their “carbon footprint”).
Individual U.S. states, notably California, have taken significant steps as well. The State of California created the California Climate Action Registry (CCAR) in 2001 in order to have a central location to which companies could report their emissions and reductions.
In 2007, CCAR helped spawn The Climate Registry (TCR), an entity destined to set the North American standard on reporting GHG emissions. Its General Reporting Protocol, consistent with and built upon the IPCC, CCAR, and other protocols, was finalized in the spring of 2008. Forty U.S. states, 12 Canadian provinces, and 6 Mexican states are participating in TCR.
However, the TCR General Reporting Protocol only provides overall guidelines on how to calculate GHG emissions, and it focuses mostly on emissions from fossil fuel use. It, and the protocols on which it is built, yield only rough estimates – which are adequate on the scale of a nation or state. More precise and detailed methods have been developed in some sectors of the economy, especially electricity generation, which allows for individual companies or utilities to make more precise estimates of their carbon footprints.
Based on the work of the IPCC, U.S. EPA, and other organizations around the world, some specifics regarding GHG emissions from organics management have become clear. For example, it is clear that placing organics in landfills leads to increased methane production and, if not captured, release to the atmosphere, where methane has 21 times the greenhouse impact as carbon dioxide (CO2) over a 100 year timeframe (according to TCR protocol).
Because of the significance of such emissions, methods have been developed to calculate, using the best current data available, landfill methane emissions in comparison to, for example, composting of organics. The Chicago Climate Exchange (CCX), a market for the sale of carbon emission reduction instruments (offsets), has recently created a methodology for calculating the net reduction in GHG emissions when organics (food wastes, soiled paper, biosolids and yard and landscaping wastes) are diverted from a landfill and composted. (An article on the protocol is being prepared and will appear in a spring issue of BioCycle.) Obviously, when markets and money begin to get involved, the methodology by which GHG emissions are calculated becomes critical and requires a greater level of specificity than a national inventory.
With the probability of increasing regulation on greenhouse gases in the U. S., which are likely to include market incentives for reductions of emissions (e.g., cap and trade), the need for specific methodologies for calculating emissions is here and now. In the wastewater treatment field, professionals are scrambling to develop more refined methods for understanding emissions from their activities. Treatment facilities not only consume considerable electricity and fossil fuels (thus contributing CO2,), they are also sources of methane and nitrous oxide (N2O has 310 times the global warming potential of CO2). In the past few years, the leading wastewater treatment professional groups in England and Australia have developed detailed methodologies for estimating GHG emissions from the entire wastewater treatment process. In the U.S., some California cities formed a working group that contributed to the development of the TCR Local Government Operating Protocol, the first iteration of which was released last fall.
Work also is progressing on more specific components of wastewater treatment, such as the management of biosolids. In 2008, for the first time, numerous presentations on GHG emissions associated with biosolids management began showing up at national and regional conferences. This is new territory, and the various participants are employing a plethora of approaches and assumptions.
In an effort to create a systematic and consistent methodology, the Canadian Council of Ministers of the Environment is funding a project developing calculators for GHG emissions from various biosolids management options in use in Canada. The first iteration of that methodology is due out this spring. It is one of many recent and current efforts to use more precise data inputs, based on published research, to refine calculations and estimates.
The beauty of the protocols that have been developed, from the IPCC through The Climate Registry, is that they allow a wide range of precision. National estimates of GHG emissions can be made based on gross assumptions – and, because they are consistent from country to country and year to year, they are useful. These are called Tier C estimates by TCR. At the same time, the same basic protocols and methodologies can be used to estimate emissions from single organizations, using actual, measured, local data. This yields relatively precise estimates, called Tier A estimates.
Swept up by the trend, the North East Biosolids and Residuals Association (NEBRA) entered “the Wild West” of GHG accounting last year. With support from IPS/Siemens and Agresource, NEBRA calculated estimates of the energy consumption and GHG emissions associated with two biosolids management options – composting and landfill disposal – under consideration by the Town of Merrimack, New Hampshire.
The NEBRA study focused on Merrimack, which has had a long-running, successful composting program, including the last 14 years with an enclosed agitated bed process (the IPS Composting System). Merrimack’s compost, marketed by Agresource, has been used in a number of high profile landscape projects, such as New York City’s Central Park and Boston’s Rose Kennedy Greenway. In 2002, Merrimack received a U.S. EPA first place beneficial use award.
At the time of the study, the town had completed a separate economic analysis of the options for either landfill disposal of biosolids or renovation of the aging composting facility to continue the existing program. The GHG study used the proposed plant upgrade components from that economic analysis, and local facility data, in developing its calculations. The general methodologies put forth by IPCC and others were followed, but many assumptions about the scope and other details of the analysis were made based on the local situation.
At this local scale, some GHG emissions calculations can be quite precise – especially those having to do with consumption of electricity and burning of fossil fuels. However, there was only minimal research and no local data on methane (CH4) and N2O emissions, so these estimates had to be much less refined. The overall results, therefore, are rough estimates with considerable uncertainty (Tier B).
Wastewater, wastewater solids (sewage sludge) and other organic wastes are of interest in carbon emissions accounting primarily because of their potential contribution to generating CH4 and N2O. None of these wastes contain significant amounts of fossil carbon, i.e., carbon from petroleum or coal. Wastewater solids and the amendments in compost are composed of actively cycling, or biogenic, carbon. Carbon that is actively cycling in the biosphere has no net impact on overall long-term levels of carbon in the atmosphere. Therefore, no matter how these materials are managed, any CO2 released over the short term is not added to GHG emissions calculations.
This study of Merrimack biosolids management options estimated the net emissions of CO2, CH4 and N2O, and the amounts of sequestered carbon (C) associated with each option. Total GHG emissions are expressed in megagrams (Mg), or metric tons CO2 equivalents.
Each biosolids management option was charged with CO2 released when fossil fuels are burned during operations and with estimated CH4 or N2O emissions to the atmosphere. Credit was given for any significant amount of carbon sequestered (maintained in the soil or landfill for 100 years) that would otherwise have been released to the atmosphere as CO2, and whenever the option results in displacing fossil fuel (e.g. using biosolids-generated methane) or displacing the use of other resources that, when mined or produced, cause carbon emissions (e.g. peat, synthetic nitrogen fertilizer).
Although the study focused on the current composting operations, it also predicted GHG emissions impacts for a future operation that includes centrifuge dewatering prior to composting. Increases in electricity use for dewatering, and the resulting decreases in the uses of electricity, fuel and sawdust amendment (due to transporting and composting drier material) will be the most significant impacts of installing a centrifuge, and are incorporated in the calculations. (Sawdust is becoming more difficult and costly to obtain.)
For the purposes of the study – and to ensure an apples-to-apples comparison of the two options – the process point at which they diverge is after the assumed upgraded dewatering, at the point when the wastewater solids are deposited into a truck and conveyed either to the composting facility or to the landfill. The study took into account use of any equipment and the actions of any person required to manage the biosolids from dewatering to the final use or disposal. Also included are all ancillary GHG emissions costs and benefits associated with each option believed to be significant. In carbon accounting terms, this included both direct (Scope 1 and 2) and indirect (Scope 3) emissions.
When assumptions had to be made, they were based on carefully developed rationales. Because of significant uncertainty regarding the likely CH4 and/or N2O emissions from a few steps in each of the biosolids management options considered, two or more independent calculations were used to arrive at average GHG emissions estimate subtotals for each of these steps. For example, in estimating methane emissions from landfill disposal, five different calculations were made using data from independent research on bioreactor landfills, anaerobic digestion and decay of food discards. The range of results was from 27 to 579 Mg methane emissions; the mean of 175 Mg was used in the final calculations. Even if the lower, more conservative median number had been used, landfill disposal would have been found to generate twice (rather than 3.4 times) the amount of CO2 equivalent emissions.
One particularly unusual assumption included in the calculations of GHG emissions from the Merrimack composting operation was if composting did not occur, 30 percent of the sawdust that would have been used would instead go to the robust biomass fuel market to create electricity that offsets fossil fuel use (a current reality in New Hampshire). The GHG emissions avoided by using that amount of sawdust for fuel is charged as a debit to the composting option. (This is not standard in GHG accounting, but was included to ensure that the landfill disposal option was given the benefit of the doubt and any bias in favor of composting was minimized.)
In the final analysis, this “sawdust” factor proved to the single largest GHG emissions from the composting option. Excluding this factor would make composting even less of a GHG emitter. However, in the real-life situation at Merrimack, where the composting operation can control what amendments are used, the sawdust factor is worth taking into account, if, for no other reason, than to highlight the potential issue of what the most sustainable use of sawdust might be. Having recognized the significance of the sawdust factor, the study authors made the recommendation that yard and leaf waste, which is a poor biomass fuel, could be processed and used in the composting operation instead (although additional calculations regarding the GHG emissions from that processing would need to be done to understand the net impact of such a change).
Reviewing the literature to help frame assumptions used in the calculations led to the following:
Potential methane release during composting: While CH4 may be generated in some composting operations, the extensive aeration and agitation in the Merrimack operation reduces any chance that anaerobic conditions required to generate CH4 will occur. The Merrimack facility is enclosed and the air from the composting area is treated through a biofilter that presumably breaks down methane. The only part of the composting operation that may produce CH4 is during outside curing and storage. A moderate value for CH4 emissions from curing and storage was included in the GHG emissions total.
Nitrous oxide release during composting: Nitrous oxide has been measured from some composting operations; however, as with methane, the high level of aeration in the enclosed composting area reduces the risk of N2O generation (which requires low oxygen or anaerobic conditions). Some N2O may be released from the biofilter and during compost curing and storage. A moderate value for N2O emissions from curing and storage was included in the GHG emissions total.
Carbon sequestered in soil: It is widely agreed that this is a significant positive impact of compost use. However, estimating how much carbon is sequestered for the long term (100 years) is difficult and depends on many factors, such as tilling practices, climate and soil type.
Methane generation from landfill disposal of wastewater solids: Wastewater solids are likely to produce CH4 in significant volumes in a short time after they are landfilled and before significant landfill gas (including CH4) capture can begin. For this reason, CH4 generation from solids in the landfill had the single largest impact on the carbon emissions analysis in this study. However, of the small portion of Merrimack wastewater solids’ CH4 that is predicted to be captured at the landfill, 100 percent is utilized for electricity (this particular landfill has begun providing biogas for heat and power to the University of New Hampshire). Utilizing biogas for energy gives the landfill option a credit, because of the reduced need for fossil fuel combustion.
Carbon sequestration in landfill: Five percent of the carbon in landfilled Merrimack wastewater solids was considered sequestered.
From conducting this study, the following became clear: Assumptions used are critical to the outcome; the most significant potential and real GHG impacts from managing organics are emissions of CH4 and N2O; there is a need for many more field measurements of emissions of these gases (or lack thereof); and more research is needed to better understand the factors that affect carbon sequestration in soils and landfills.
Table 1 details the estimated energy use and GHG emissions of the various stages of the two biosolids management options, landfill disposal and composting. Table 2 details the estimated GHG emissions of the various stages of the two management options. Figure 1 graphically depicts the GHG emissions comparisons.
Energy Consumption: The current composting operation requires more energy than either of the future options. The basic operations at Merrimack use approximately 181 kWh equivalent energy for each wet ton of compost output, not including solids dewatering, curing, loading, screening, marketing, delivery and biofilter maintenance. If all energy costs associated with all of the composting, transportation and delivery steps are included, the energy use is about 375 kWh equivalent per wet ton of compost output. Switching to centrifuge dewatering of the wastewater solids will considerably reduce energy consumption, from the current 735 kWh equivalent per dry ton solids processed to 568 kWh. Landfilling after centrifuge dewatering would likely reduce energy consumption further by about 46 percent.
GHG Emissions: In contrast to energy use, this study’s estimates indicate that landfill disposal will lead to larger greenhouse gas emissions than continued composting. Current composting operations at Merrimack emit an estimated 1,529 Mg CO2 equivalent emissions; future composting operations with centrifuge dewatering would emit an estimated 1,094 Mg CO2 equivalent emissions – a decrease of about 29 percent. Future landfill disposal would emit an estimated 3,754 Mg CO2 equivalent emissions, 2.5 times more than the current composting operation and 3.4 times more than the future composting option.
How do these carbon emissions numbers compare to other known sources? The 1,529 Mg of CO2 equivalent emissions is equal to the estimated annual CO2 emissions from 283 average U.S. passenger cars or the estimated annual emissions from the electricity used by 203 average U.S. homes. In comparison, this study’s estimated carbon emissions from the proposed future landfill disposal of Merrimack wastewater solids would be the equivalent of about 700 cars.
It takes measuring something to understand it. This study brought into focus the highest priorities for further optimizing the composting or landfill disposal systems to reduce energy use and GHG emissions.
For example, the enclosed, in-vessel system utilized at Merrimack allows for careful control of the composting process, an advantage over other forms of composting. This level of control is also significant in comparison to landfill disposal. By optimizing the composting process -proper C:N ratios, dry feedstocks and adequate, consistent aeration – it is possible to eliminate CH4 and N2O emissions or reduce them to very low levels. Biofiltration of the air from the active composting operations helps; however, further analysis and understanding of the potential for N2O emissions from the biofilter would be needed to ensure minimal emissions of this powerful greenhouse gas. If N2O emissions are found, it may be relatively easy to reduce them; for example, a scrubber that reduces the levels of ammonia entering the biofilter could be effective.
The downside of the active, enclosed, in-vessel composting process is its large demand for electricity. However, it could be possible to reduce energy consumption by replacing blowers with more energy efficient units. Ultimately, it may also become possible to purchase or produce renewable energy, which, while not reducing energy consumption, would reduce total anthropogenic GHG emissions.
Composting operations at Merrimack also could be optimized by using the facility’s full capacity of 60 wet tons/day. Currently, Merrimack processes only about 31 wet tons/day. The energy required to operate at full capacity would be more than that used in current operations, but the per-ton energy costs would be lower due to efficiencies of scale. With the upgraded composting option, the percentage of the facility’s capacity taken up by Merrimack solids would be reduced by almost one-third, resulting in additional excess capacity that could be filled with other compostables (e.g., food waste).
If the capacity cannot be filled with outside materials, Merrimack might consider utilizing it for further treating its own biosolids compost, i.e., conducting the curing phase in the enclosed facility. Compost could remain in the bays for as much as 45 to 50 days (based on current operations). After the initial 21 days of active composting, agitation could be reduced in frequency, so additional energy costs (from that and additional ventilation run time) could be moderate. In return, keeping the material indoors longer may reduce the fugitive CH4 and N2O emissions possible with outdoor curing. Measurements of actual emissions from the current outdoor curing piles and the biofilter would have to be conducted so as to determine the true value of this change in operations.
Significant GHG emissions from landfill disposal of wastewater solids may be difficult to avoid, because the material is highly and quickly putrescible and prone to emitting methane before gas collection systems are in place and functioning. Nonetheless, research on improving landfill methane capture is ongoing. As with the composting option, landfill operations – which already use less fossil fuel energy – will likely be able to reduce GHG emissions in the future by utilizing renewable energy sources such as biofuels or electricity from renewable sources to run trucks, dozers and compactors.
Avoiding production of methane from highly putrescible materials like wastewater solids may be an opportunity for composting operations to generate carbon credits. This will depend on how carbon accounting is formalized over the coming years. If landfill disposal is considered the status quo for the management of wastewater solids, then composting can be considered an option that reduces methane generation, resulting in carbon credits for the owner of the composting facility.
The price of carbon offsets on the Chicago Climate Exchange has ranged from about $2 to $5/Mg CO2 equivalent over the past two years If the March 2008 relatively high price of $5.70 (CCX, 2008) were used, the market value of upgraded composting over landfill disposal would be worth about $15,000. While there is plenty of uncertainty, it is expected that, at some point, the market price will rise as more states – and the U.S. government – increase requirements for emissions reductions. However, in the current economy, the offset price plunged to $1 late last year and, at the end of February, was about $2.
However, a critical aspect of the developing carbon offset markets is the idea that offsets will likely only be allowed for new, verifiable, reductions in GHG emissions. By composting its solids, Merrimack has already been “doing the right thing” in minimizing GHG emissions. Therefore, if the rules develop as expected, it is unlikely that Merrimack would be able to benefit monetarily from marketable carbon offsets if the Town continues with composting, since this represents no change from “business as usual.”
Ned Beecher is Executive Director of the North East Biosolids and Residuals Association. The complete report on which this article is based, including references, is available at For additional supporting documentation, contact the author,

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