Determining the Carbon Footprint of Biogas Production from Sewage Sludge

Abstract: Anaerobic digestion of sewage has been practiced over 150 years and results in the production of a methane enriched biogas which is considered a renewable energy source. After processing, biogas can be used directly to produce electricity via a co-generation plant, or following removal of the carbon dioxide within it, be used as a substitute to natural gas, or after compression, vehicle fuel. By displacing fossil-fuels, use of this biogas can help Water Utilities with their quest to become carbon-neutral. However, to make biogas, sludge needs to be anaerobically digested, the biogas cleaned, and the nutrients released to the digestate need to be destroyed. Furthermore, there are direct methane emissions from storage, leaks and incomplete processing. This paper looks to quantify these to determine the carbon impact of using biogas produced from municipal digestion of sewage sludge. KEYWORDS Anaerobic digestion; biogas; biomethane; carbon footprint INTRODUCTION There is increasing concern over the impacts of anthropogenic activities on the climate. This is leading to growing interest in renewable energy as an alternative to fossil-fuels. Biogas, a by-product generated from sewage treatment, is a valuable source of renewable energy. It can be used to generate electricity from engines which displaces energy made elsewhere at a power station resulting in a reduction in carbon footprint. However, to make biogas, sludge has to be anaerobically digested, and both the gas and liquid streams need to be processed (summarized in Figure 1). This processing generates a carbon impact in the form of electricity consumption (for example to mix digesters and to run conveying systems), chemical use (polymers for dewatering and carbon for nutrient removal) and direct methane losses (from storage, incomplete use or processing of the gas). However, the benefits of producing and using biogas are potentially becoming less clear. Globally, a shift towards cleaner forms of energy is making the displacement of energy using electricity derived from biogas less beneficial. Greater reductions of carbon were possible before when 'dirtier' forms of energy, such as coal were being displaced. Now, with cleaner energy mixes, these benefits are eroding. Despite this, direct losses of methane are still being seen. The wastewater sector is a major source of methane emission, contributing to 5--8% of global anthropogenic emissions (Ocko et al, 2021) and traditionally in the UK, open secondary digestion was common to provide additional destruction of pathogens from the wastewater. Recent measurements in the USA showing far greater losses of methane than previously believed (Vasquez, 2023). Methane has a global warming potential of 25 times that of carbon dioxide over a hundred-year period, but there has been emphasis in the industry to use a more realistic 20-year horizon, in which case it is 86 times worse. This means that leaks do not have to be large to offset the benefit of using the biogas in the first instance. For example, the digestion of sewage sludge can typically yield 350 m3 biogas/tonne solids fed to a digester. This is on average 65% methane, and based on its inherent energy, the biogas will have an energy content of 2,400 kWhr. After losses, a third of this energy could be used in an engine to offset fossil-fuel, i.e. 720 kWhr. This would displace 570 kg carbon equivalents of coal. A 1% loss of biogas, i.e. 3.5 m3 would contain 2.3 m3 of methane with a weight of approximately 4.6 kg. Using a 20-year horizon for methane, this has a carbon footprint of 396 kg. For a displacement of 570 kg carbon, therefore a loss of (576/396 =) 1.45% of the biogas would cancel the benefit of using the biogas. Figure 2 shows the contributing factors towards carbon footprint for the various stages on the carbon footprint of biogas used in co-generation, on the grid or as a displacement fuel for gasoline or diesel. This is only referring to the loss of methane. As mentioned, the sludge, biogas and liquid streams derived from biogas generation need processing and these also carry an environmental impact. This paper will go into detail into the processing impacts of biogas production and use, and compare that to the alternatives where digestion is absent, or organic materials are not digested but rather diverted to landfills, in order to provide a greater insight into the influence of anaerobic digestion on reducing carbon pollution. REFERENCES Ocko I. B.; Sun T.; Shindell D.; Oppenheimer M.; Hristov A. N.; Pacala S. W.; Mauzerall D. L.; Xu Y.; Hamburg S. P. 2021 Acting rapidly to deploy readily available methane mitigation measures by sector can immediately slow global warming. Environmental Research Letters, 16 (5). Vasquez, K. (2023) Wastewater treatment releases nearly double the methane that was previously thought, Chemical & Engineering News.