By Paul Davis
Each day the average American sends three pounds of solid waste to the landfill, out of which nearly six ounces is energy rich food scraps. While a small portion of our food scraps are currently recaptured for composting, the EPA estimates that nearly 97 plus percent ends up in landfills where it produces prodigious amounts of CH4 gas – contributing to global warming while wasting a precious form of potentially renewable energy. However, as recognition has grown over landfill wastes as a substantial source of greenhouse gas emissions, researchers and businessman have began to dedicate increased amounts of time and money to developing advanced digester technologies that can effectively capture food waste gas emissions and utilize them to produce green energy.
One such example is that of researcher Dr Ruihong Zhang who has designed a cutting edge Anaerobic Phased Solid Digester that can harvest the energy stored in food scraps to produce two valuable commodities – methane gas and nutrient rich compost (Onsite 2006, Keith 2006). In the following paper, the potential greenhouse gas reduction that could be achieved by digesting instead of landfilling all of the US’s food waste is calculated. This study finds that anaerobic digestion could reduce total greenhouse gas emissions by 44.9 million MTCO¬2e, which is equivalent to the GHG emissions produced by the combustion of 5.1 billion gallons of gasoline. In addition to producing such voluminous carbon savings, anaerobic digestion would also produce enough green energy to power 470,000 homes and enough natural fertilizer to reduce the need to fix 800,000 tons of nitrogen for synthetic fertilizers.
Each day the average American sends three pounds of solid waste to the landfill, out of which nearly six ounces is energy rich food scraps. While a small portion of our food scraps are currently recaptured for composting, the EPA estimates that nearly 97 plus percent ends up in landfills where it produces prodigious amounts of CH4 gas – contributing to global warming while wasting a precious form of potentially renewable energy.
However, as recognition has grown over landfill wastes as a substantial source of greenhouse gas emissions, national and local governments, businesses and researchers have began to dedicate increased amounts of time and money to developing advanced digester technologies that can more effectively capture food waste gas emissions and utilize them to produce green energy. One such example is that of researcher Dr Ruihong Zhang who has designed a cutting edge Anaerobic Phased Solid Digester that can harvest the energy stored in food scraps to produce two valuable commodities – methane gas and nutrient rich compost (Onsite 2006, Keith 2006).In the following paper, the volume of food waste currently produced by America will be discussed and the amount of greenhouse gases currently produced by this voluminous waste stream will be calculated. I will then calculate the potential greenhouse gas reduction that could be achieved if all food waste was anaerobic digested instead of landfilled. In particular I will calculate:
- the potential GHG reduction associated with increased methane gas capture.
- the potential GHG reduction associated with generating electricity using landfill gas in lieu of grid electricity.
- the potential GHG reduction associated with replacing synthetic nitrogen fertilizers with the nitrogen rich soil amendment produced by anaerobic digestion.
WASTED ENERGY: FOOD SCRAPS IN AMERICA
Disposal of municipal solid waste is a massive industry in the United States. In 2007, the EPA estimates that Americans produced 254 million tons of post consumer waste (EPA MSW 2007). While 85 million tons of this waste was eventually recovered and recycled, a total of 169 million tons (or roughly 3 pounds of garbage per person per day) were sent to national landfills.
According to the EPA 12.5 percent of the current waste stream is composed of food scraps. Food scraps are herein defined as uneaten meals or preparation waste resulting from residential homes, commercial establishments (e.g. grocery stores, restaurants), and institutional/industry cafeterias (e.g. hospital and workplace lunchrooms). Since solid data regarding food scrap production is virtually nonexistent, the EPA generates annual food scrap estimates based on a combination of regional garbage samples, demographic data (population, student/patient numbers) and economic indicators (grocery/restaurant sales figures). Based on the aforementioned methodology – the EPA estimated that in 2007 the US produced 31.7 million tons of food waste.
Of the 31.7 million tons of food waste produced in ‘07, only around two and a half percent of food waste was recovered via recycling (composting) (EPA 2008). As a result, 30.9 million tons of food waste generated in 2007 found it’s final resting place in US landfills (Figure 1.). In the following section, we will discuss the predicted amount of greenhouse gases generated by this 30.9 million tons of food scraps.
Landfilling of Foodscraps and Greenhouse Gas Generation
When deposited in landfills, food scraps are stored in a low-oxygen anaerobic environment. Under these anaerobic conditions, food scraps decompose to produce relatively equal amounts of carbon dioxide and methane gas emissions. However, since carbon dioxide would be produced as a result of natural degradation processes, the CO2 emissions are considered to be biogenic in nature, and thus not counted as anthropogenic greenhouse gas emissions (EPA 2006). On the other hand, CH4 off-gassing from landfills is considered to be anthropogenic in origin since methane would not be produced through natural decomposition.
Extensive tests regarding the amount of CH4 produced per wet ton of food waste have been conducted by the laboratories of Dr. Morton Barlaz and his colleagues at North Carolina State University. In accord with Dr. Barlaz’s test results (Barlaz 1998) the EPA estimates that 0.445 Metric Tons Carbon Equivalent (MTCE) are produced per wet ton of food scrap material (EPA 2006). According to these estimates, America’s 30.9 million tons of annually discarded food waste would generate over 50.4 million metric tons of carbon dioxide equivalents (See Figure 2).
Of course not all of the methane gas that is produced at US landfills is off-gassed into the atmosphere. Currently, 59 percent of all solid waste is discarded at sites with landfill-gas (LFG) capture systems (LFGCs). LFG capture systems are composed of gas wells and pipelines, which capture a portion of the generated methane and use it to produce electricity or as a natural gas supply. These LFGC facilities have the dual benefit of simultaneously reducing methane emissions while producing a renewable form of electricity. Unfortunately, according to a 2005 study conducted by the Earth Engineering Center at Columbia University, these current LFGC facilities are on average only 34% effective at capturing landfill gases (Themelis 2006).
BIODIGESTION: UNLOCKING THE ENERGY OF FOOD WASTE.
Rather than ship our urban waste off to landfills where energy is needlessly lost and greenhouse gases unnecessary released, a number of US businesses and universities have begun exploring how to efficiently capture the energy produced by food waste decomposition to produce two marketable products – methane gas and compost. One such project and the focus of this investigation is the Anaerobic Phased Solid Digester (APSD) developed by the University of California-Davis Biogas Energy Project.
UC Davis Agricultural Engineer Ruihong Zhang in coordination with Onsite Energy company opened the first commercial APSD in 2006. The four tank power plant which is connected to a 22 kilowatt generator is currently collecting and processing waste from 300 restaurants, 50 grocery stores and 150 hotels and businesses (Zhang 2007, Keith 2006). The technology functions as follow: The food waste is collected and grinded before it is dumped into a sealed tank with bacteria that rapidly breaks down the food waste into water and various organic acids. The organic acids are then pumped into a second tank where a different bacteria converts the organic acids into methane gas. The methane gas is then captured and sold or burned onsite to generate electricity.
For the purposes of this paper we will assume that the methane gases captured through biodigestion are burned onsite to generate electricity. Finally, in addition to methane gas, the biodigestion process also produces a nutrient rich compost which can be utilized as a soil amendment in lieu of traditional fertilizers. Below we will discuss the potential for such APSD technology to reduce the annual GHG emissions produced by the US’s food waste stream through methane gas capture, green power generation, and the production of natural fertilizers.
Reducing Emissions through Biodigestion.
Reducing Landfill Emissions through APSD Build-out.
As outlined above only 59% of all existing landfills have LFG capture facilities; and even those that have such facilities suffer from rather low LFG capture rates. Assuming that the percentage food waste is a uniform 12.5% at all U.S. landfills, then we can assume that 41% or 12.9 million food scraps are discarded in landfills without LFG capture facilities, while 59% of total food scraps are processed at LFG-equipped facilities. Using Barlaz’s GHG emission coefficient, it can be estimated that the 12.9 million tons of food scraps at non-LFGC facilities will produce a total of 20.6 million metric tons of CO2e annually. Meanwhile at those facilities with LFGC facilities, it is estimated by Themelis etal. that 66% of the emissions produced are uncaptured, leading to the offgassing of a additional 19.6 million tons of CO2e. In sum, a total of 40.3 million tons of CO2e are annually emitted into the atmosphere from landfilled food waste.
According to EPA Agricultural Specialist Cara Peck, the greenhouse gas impact of food wastes would be dramatically altered by the construction of APSD facilities, which are close to 100% effective at capturing the CH4 produced as the result of decomposition (Peck Interview 2009). Therefore, institution of APSD facilities across the states would lead to annual reduction of over 40.3 million tons of CO2e.
Reducing Energy Emissions through APSD Gas-Energy Generation.
According to reports produced by Onsite Power System’s and Zhang’s laboratory, Onsite’s Anaerobic Phased Digester can produce up to 198 kWh of electricity for every wet ton (wt) of food scrap processed (Zhang 2009, Konwinski Interview 2009). Given this level of production, America’s annual waste stream of 30.9 million wt food waste could be captured and converted into 6,118 million kWh per year. However, some methane is currently being captured by LFGC facilities and converted into electricity. Given that 41 percent of current landfills have LFGC facilities which operate with a methane capture rate of 34%, it can be inferred that about 14% of landfill methane is currently being captured and utilized to generate electricity. If this current capture rate is accounted for and subtracted from total potential production, then rollout of APSD systems nationwide could potentially net 5,260 additional kWh of production, or roughly enough energy to meet to the annual electricity demand of 470,000 homes.
In addition, utilization of this 5,260 million additional kWh would led to a reduction in the emissions produced by power generation from the traditional grid. While it is true that the burning of CH4 gas produces CO2 emissions, these CO2 emissions are considered to be a natural product of food waste decomposition and are thus considered to be biogenic in-origin. Therefore, the capture and combustion of food waste gas should be credited for the reduction in grid-based electricity that it replaces. Based on a report produced by the EPA in 2000, the US grid emits roughly 1.35 lbs. of CO2e per kWh (or 612.5 MTCO2e per million kWh). Assuming a current emission factor of 1.35 lbs. CO2e/kWh for the current grid, the generation of 5,260 million kWh of electricity would offset 3.2 million metric tons of carbon dioxide (See Fig. 4).
Sludge as a resource.
In addition to methane gas, anaerobic digestion also produces a nutrient rich soil amendment as a digestion byproduct. This byproduct could potentially be utilized by farmers and gardeners as a replacement to synthetic nitrogen based fertilizers. As discussed in previous articles on SNRE biofuels blog, the generation of synthetic fertilizers is an energy intensive process. The mere stoichiometry of the Haber reaction releases 0.375 mol of C for every 1 mole of Nitrogen converted to N fertilizer (Schlesinger 2000). Furthermore, when accounting for the inefficiencies inherent in the fertilizer fabrication process, the IPCC estimates that a total of 0.58 moles of C are released through generation of one mole of N. or roughly 1.82 g of CO2 for every g of N fertilizer generated (IPCC 1996).
To determine the amount of nitrogen fertilizers that could be displaced by the APSD generated soil amendment, the simplifying assumption was made that 1g of N in the digester’s compost was functionally equivalent to 1g of fixed N in synthetic fertilizer.
The Earth Engineering Center at Columbia estimates that average US food waste has a composite molecular formula of C6H9.6O3.5N0.28S0.2, which equates to approximately 2.65% N by mass. Therefore the 30.9 million tons of food waste discarded every year would contain about 0.8 million tons of nitrogen. Studies conducted by Zhang indicate that nitrogen content of the food waste will not change significantly as a result of biodigestion as the reaction is occurring in an anaerobic environment (Zhang 2006, Levine 2009). As a result, the APSD’s soil amendment byproduct would contain the same amount of nitrogen as the food waste input – or 0.8 million tons of N. Assuming that this nitrogen product could be used to replace synthetic nitrogen fertilizer, and the IPCC emissions rate per g N fixed, then the APSD by-product compost could offset 1.4 million MT CO2e (See Fig 5).
In conclusion, food waste streams are an energy and nutrient rich waste stream that is currently underexploited as an “industrial resource”. Rollout of Anaerobic biodigestors across the country could lead to substantive reductions in greenhouse gas emissions through enhanced capture of landfill methane emissions, green energy generation, and natural fertilizer production. The rough approximations and calculations made in this paper suggest that by exploiting APSD as a tool to enhance food-waste recycling could reduce total greenhouse gas emissions by 44.9 million MT CO¬2e. To put these GHG reductions in more commonly understood units, 44.9 million MTCO2e is equivalent to the emissions produced by 5.1 billion gallons of gasoline or roughly enough gas to run the US economy for 13 days. Finally, in addition to producing such voluminous carbon savings, anaerobic digestion would produce enough green energy to power 470,000 homes and enough natural fertilizer to reduce the need to fix 800,000 tons of nitrogen for synthetic fertilizers.
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