Many homes and businesses now separate food waste so that it can be collected and centrally processed. It is then used to produce biogas (methane) and the remaining solid waste is used as biofertilizer. This prevents the rotting waste from releasing the methane as a powerful greenhouse gas (GHG) and reduces reliance on fossil fuels. Additional environmental benefits are also gained through the production of biofertilizer. This article explores how food waste processing works and how greenhouse gasses are reduced.
What Happens to Unprocessed Food Waste?
The organic materials in food waste are mostly made up of carbon and hydrogen, as well as a significant amount of nitrogen. In an uncontrolled landfill, food waste is digested by microorganisms. This converts most of the carbon and hydrogen into methane gas which is released into the environment.
At the same time, because of a lack of biofertilizer to produce crops, fertilizers are synthesized using fossil fuels. Because production is concentrated in large industrial facilities, they are often transported large distances to farms, causing further transport related emissions. The synthetic fertilizers are then spread over the land, where they leech through the soil and break down much more quickly than organic fertilizers, releasing GHG’s such as N20, NOx and NO3.
Global Warming Potential – Comparing Different Greenhouse Gasses
The global warming potential (GWP) measures how much heat a gas traps in the atmosphere over a specified period of time, relative to carbon dioxide (CO2). Different gases absorb different amounts of infrared radiation and last for different amounts of time in the atmosphere. GWP therefore depends on the period of time being considered. GWP is often calculated for 20, 100 or 500 years. It is most commonly calculated for a 100-year period and this convention will be used for all subsequent figures in this article.
Although water vapor is the most significant atmospheric GHG, its levels rise and fall rapidly, according to the temperature. It therefore acts to magnify the impact of other GHG’s, in a positive feedback loop, but it is not considered a direct cause of warming. There are four significant GHG’s that account for most of the warming caused by human activities:
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Carbon dioxide (CO2) accounts for 76% of warming due to burning fossil fuels and deforestation.
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Methane (CH4) accounts for 14% of warming due to agriculture, fossil fuel extraction and decay of organic waste. It has a GWP of 34.
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Nitrous oxide (N2O) account for 8% of warming due to synthetic fertilizer, livestock and industrial processes. It has a GWP of 298.
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Fluorinated gases account for 1% of warming due to industrial processes and refrigerants. They are only released in small quantities but have extremely high GWP’s that can be over 10,000.
Multiplying the GWP of a gas by the amount of the gas released into the atmosphere, gives the equivalent carbon dioxide (CO2e). This allows all emissions to be expressed in the same units. For example, if 1 ton of methane with a GWP of 34 is released, this is expressed as 34 tons CO2e.
Food Waste Processing
The main method of processing food waste is anaerobic digestion (AD), which involves microorganisms breaking down organic matter in the absence of oxygen. This is also how sewage and many other waste products are processed. Anaerobic digestion produces methane which may be either cleaned up for injection into the natural gas grid, or generate electricity in a reciprocating gas engine, potentially producing combined heat and power (CHP). The remaining organic matter can then be used as a biofertilizer.
Before digestion, the waste is first pre-processed, this may involve sorting to remove packaging, shredding or, in the latest plants, thermo-pressure hydrolysis (TPH). When TPH is used, organic packaging materials such as cardboard can be effectively digested. These woody materials cannot normally be digested easily but TPH breaks lignin down into cellulose, allowing digestion and increasing biogas production. The resulting slurry can then be easily screened to remove contaminants such as glass and metal. TPH also greatly increases digester throughput produced by reducing the production of ammonia during digestion, which can kill off microorganisms and therefore reduce the rate of digestion. This also improves the quality of biofertilizer by retaining more nitrogen.
Environmental Benefits
The most obvious environmental benefit is methane capture. When it is burnt, the methane is converted to carbon dioxide and water vapor. Carbon dioxide has a much smaller GWP than methane and—in any case, since it replaces fossil fuel use—it may be regarded as completely carbon neutral. Surprisingly, the production of biofertilizer may actually result in a more significant reduction in GHG emissions.
The biofertilizer produced as a waste produce of methane generation contains valuable nutrients. These would normally be synthesized using fossil fuels and then transported over large distances causing further carbon emissions. The use of nitrogen containing fertilizers results in the release of the powerful GHG nitrous oxide. Biofertilizer does not require fossil fuels for its production and is normally produced locally, minimizing the impact of transport. As an added benefit, biofertilizer contains microorganisms that enable plants to obtain additional nutrients through natural processes such as nitrogen fixation and solubilizing phosphorus. This further reduces the quantity of synthetic fertilizers required. Because these processes generate nitrogen more gradually and closer to the roots, it may also reduce the amount of nitrous oxide released to the environment.
The actual reductions in GHG emissions vary considerably since the composition of food waste is not consistent. Based on typical characteristics, the processing of one ton of food waste would prevent 79kg of methane from being released into the atmosphere, which equates to 2.38 tons CO2eq. If this gas is used to produce energy that would otherwise have been produced using natural gas, this will prevent 0.22 tons of fossil fuel CO2 from being released while also reducing further GHG emissions due to the gas leaks that accompany natural gas use. These leaks normally occur as the natural gas, which is primarily methane, is extracted and also during its transmission over large distances. The reduced natural gas leakage equates to a further GHG reduction of 0.13 tons CO2eq. One ton of food waste would also yield biofertilizer containing 12kg nitrogen (N), 1.7kg phosphorus (P) and 4.7kg of potassium (K), saving a further 0.033 tons CO2eq due to the reduced production and transport of NPK fertilizer.
These GHG emissions are summarized below. Amazingly, processing one ton of food waste reduces emissions by the equivalent of 2.8 tons of CO2. For the average American, processing all of their food waste for a year would be a saving equivalent of about half a ton of CO2.