Anaerobic digestion and digestate use: accounting of greenhouse gases and global warming contribution

Greenhouse gas (GHG) emissions related to composting of organic waste and the use of compost were assessed from a waste management perspective. The GHG accounting for composting includes use of electricity and fuels, emissions of methane and nitrous oxide from the composting process, and savings obtained by the use of the compost. The GHG account depends on waste type and composition (kitchen organics, garden waste), technology type (open systems, closed systems, home composting), the efficiency of off-gas cleaning at enclosed composting systems, and the use of the compost. The latter is an important issue and is related to the long-term binding of carbon in the soil, to related effects in terms of soil improvement and to what the compost substitutes; this could be fertilizer and peat for soil improvement or for growth media production. The overall global warming factor (GWF) for composting therefore varies between significant savings (-900 kg CO(2)-equivalents tonne(-1) wet waste (ww)) and a net load (300 kg CO(2)-equivalents tonne( -1) ww). The major savings are obtained by use of compost as a substitute for peat in the production of growth media. However, it may be difficult for a specific composting plant to document how the compost is used and what it actually substitutes for. Two cases representing various technologies were assessed showing how GHG accounting can be done when specific information and data are available.

The anaerobic digestion (AD) of Municipal Solid Waste (MSW) is a subject approached in different ways across Europe, as a result of the various national waste management and energy policies and other issues such as the typology of the waste. The Southern European countries, like Portugal, are among those that bet the most on AD for treating MSW from undifferentiated collection, instead of biowaste, the selective collection of the organic fraction of MSW, typical of Northern European countries.

Managing food waste is key to tackling climate change

Waste-to-energy is one effective waste management approach for a sustainable society. The purpose of this study was to clarify the potential for energy recovery and greenhouse gas (GHG) reduction that could be achieved by introducing anaerobic digestion (AD) facilities in the process of reconstructing aging incineration facilities in Japan. Using statistical data from 1068 incineration facilities, four future scenarios were considered and compared with the current situation. As results, compared with the current situation the amount of electricity generated could increase by 60 % in 2030, by combining AD facilities for food waste with new, high-efficiency incineration facilities for remaining municipal solid waste (MSW). From a life cycle perspective, net energy recovery in 2030 was approximately three times greater than in 2011, and GHG emission could be reduced by 27 %. The introduction of AD facilities is attractive for small authorities, which currently treat <100 t/day of MSW through incineration facilities without energy recovery. An AD facility is also beneficial for large authorities. On the contrary, in middle-scale authorities that treat 100–299 t/day of MSW, the reconstruction of incineration facilities to include electricity production capabilities requires careful consideration, because it will significantly influence energy recovery and GHG reduction effects.

In response to the need for renewable energy resources, the replacement of fuel gas with methane produced from the anaerobic digestion of sewerage, agricultural and municipal solid wastes is considered. The utilization of methane for power generation offsets the energy requirements of the digester facility. We discuss the optimization of methane output for a model digester. The model uses Monod based kinetics of methane fermentation and does not include spatial effects. The model assumes that the solid waste acts as a substrate for acid forming bacteria which produce volatile fatty acids, which is converted to methane by a second type of bacteria. It is found that the initial concentrations of the two bacteria and biodegradable volatile solids that maximize the total methane output are independent of the temperature. However, the optimal hydraulic residence time and initial concentration of volatile fatty acids are temperature dependent. This suggests that flow rates should be adjusted, depending on the temperature, to maximize methane output. References D. T. Hill, Simplified Monod Kinetics of Methane Fermentation of Animal Wastes, Agricultural Wastes, 5:1–16 (1983). doi:10.1016/0141-4607(83)90009-4 D. T. Hill, E. W. Tollner and R. D. Holmberg, The Kinetics of Inhibition in Methane Fermentation of Swine Manure, Agricultural Wastes, 5:105–123 (1983). doi:10.1016/0141-4607(83)90089-6 D. T. Hill, Design Parameters and Operating Characteristics of Animal Waste Anaerobic Digestion Systems–-Swine and Poultry, Agricultural Wastes, 5:157–178 (1983). doi:10.1016/0141-4607(83)90081-1 Y. R. Chen and A. G. Hashimoto, The Kinetics of Methane Fermentation, Biotechnology and Bioengineering Symposium, 8:269–282 (1978). A. Husain, Mathematical Models of the Kinetics of Anaerobic Digestion, Biomass and Bioenergy, 14:561–571 (1998). doi:10.1016/S0961-9534(97)10047-2 D. J. Batstone, J. Keller, I. Angelidaki, S. V. Kalyuzzhni, S. G. Pavlostathis, A. Rozzi, W. T. M. Sanders, H. Siegrist and V. A Vavilin, Anaerobic Digestion Model No. 1 (ADM1), Water Science and Technology, 45:65–73 (2002). http://www.iwaponline.com/wst/04510/wst045100065.htm O. Levenspiel, Chemical Reaction Engineering, 3rd Ed., Wiley, New York (1999). J. F. Andrews, A Mathematical Model for the Continuous Culture of Microorganisms Utilizing Inhibitory Substrates, Biotechnology and Bioengineering, 10:707–723 (1968). doi:10.1002/bit.260100602 K. J. Beers, Numerical Methods for Chemical Engineering: Applications in Matlab, Cambridge, New York, USA (2007).

Anaerobic digestion of municipal solid waste: A modern waste disposal option on the verge of breakthrough

Greenhouse gas (GHG) emissions related to recycling of glass waste were assessed from a waste management perspective. Focus was on the material recovery facility (MRF) where the initial sorting of glass waste takes place. The MRF delivers products like cullet and whole bottles to other industries. Two possible uses of reprocessed glass waste were considered: (i) remelting of cullet added to glass production; and (ii) re-use of whole bottles. The GHG emission accounting included indirect upstream emissions (provision of energy, fuels and auxiliaries), direct activities at the MRF and bottle-wash facility (combustion of fuels) as well as indirect downstream activities in terms of using the recovered glass waste in other industries and, thereby, avoiding emissions from conventional production. The GHG accounting was presented as aggregated global warming factors (GWFs) for the direct and indirect upstream and downstream processes, respectively. The range of GWFs was estimated to 0-70 kg CO(2)eq. tonne( -1) of glass waste for the upstream activities and the direct emissions from the waste management system. The GWF for the downstream effect showed some significant variation between the two cases. It was estimated to approximately -500 kg CO(2)-eq. tonne(- 1) of glass waste for the remelting technology and -1500 to -600 kg CO(2)-eq. tonne(-1) of glass waste for bottle re-use. Including the downstream process, large savings of GHG emissions can be attributed to the waste management system. The results showed that, in GHG emission accounting, attention should be drawn to thorough analysis of energy sources, especially electricity, and the downstream savings caused by material substitution.

Elucidation of the thermophilic phenol biodegradation pathway via benzoate during the anaerobic digestion of municipal solid waste

Biomethane production as a green energy source from anaerobic digestion of municipal solid waste: A state-of-the-art review

Anaerobic digestion of municipal solid waste (MSW) is a controlled process of microbial decomposition where a consortium of microorganisms convert organic matter into methane, carbon dioxide, inorganic nutrients, and humus. In a generalized scheme for anaerobic digestion, feedstock is harvested or collected, coarsely shredded, and placed into a reactor which has an active inoculum of microorganisms required for the methane fermentation. This chapter reviews the status of anaerobic digestion as applied to MSW. It discusses principles of the microbiology of biomethanogenesis and their application in the design, operation, and evaluation of the anaerobic digestion process. The predominant hydrolytic microorganisms in rumen differ from anaerobic digestion systems. Anaerobic digestion models can be used for optimizing process design and operation and for process control. The starting point for developing a process model for anaerobic digestion is developing mass balance equations which account for the changes in concentration of the substrates, microbial populations, and products during the course of the digestion process.

Ireland produces two million tonnes per annum of biodegradable municipal solid waste. The implementation of the Landfill Directive (1999/31/EC) will lead to the construction of centralised biological facilities; these facilities may be either composting facilities or anaerobic digestion facilities. A technical, economic and environmental analysis of composting and anaerobic digestion is undertaken in this paper. The results of the analysis suggest that composting is economically preferable to anaerobic digestion at scales at or below 50 kt/a of biowaste treated. However when CH4-enriched biogas is produced for use as a transport fuel and excise duty is reduced, as allowed by the Biofuels Directive (2003/30/EC), then the economics of anaerobic digestion improve greatly. If 100% of excise duty is removed then anaerobic digestion is economically preferable to composting above 20 kt/a of biowaste treated. From an environment perspective anaerobic digestion saves more greenhouse gas due to displacement of fossil fuel powered energy. Anaerobic digestion with CH4-enriched biogas has the potential to save 1,451 kgCO2/t of biowaste treated as opposed to composting, which has the potential to save 1,190 kgCO2/t.

The present study was designed to investigate the capability of algae biomass to increase methane biogas production from anaerobic digestion of municipal solid waste. Batch anaerobic digester was used for digesting the mixture of algae and organic fraction of municipal solid waste (OFMSW). A variety condition of algae to organic fraction municipal solid waste mixing ratio, pH, temperature, and total solid are studied for a period of 12 days. It was observed that maximum methane biogas production was found to be 946.0012 mL/gm v.s at optimum condition of mixing ratio of algae to OFMSW were, 1:2, temperature, total solid and pH of 32 oC, 8 % and 7.5 respectively. Multiple correlation methodology optimized the methane production with a correlation coefficient (R2) to be 0.925. The first order kinetic model was used to assess the dynamics of the biodegradation process. The obtained negative value of (k = - 0.2543), indicates that the solid waste biodegradation was quick with a correlation coefficient (R2) of 0.9906. The Gompertz model was used to adequately describe the experimental cumulative methane biogas production from lab scale anaerobic digesters. The theoretical methane biogas yield was found to be 1016.76 mL/gm v.s which is very close to experimental value 946.0012 mL/gm v.s. with high correlation coefficient R2 of 0.998.

Recently, there are increased efforts by municipals and researchers to investigate the potential of utilizing municipal solid wastes (MSW) for resources recovery. In many parts of developing countries, MSW is mostly collected for disposal with little emphasis on resources recovery. However, the MSW has high organic and moisture contents, and are suitable substrates for anaerobic digestion (AD) process to recover biogas for energy and digestate which can be used as fertilizers or for soil amendments. Resources recovery from the AD process consists of four metabolic stages; hydrolysis, acidogenesis, acetogenesis, and methanogenesis. These metabolic stages can be affected by several factors such as the nature of substrates, accumulation of volatile fatty acids, and ammonia inhibition. In this review, different optimization strategies towards resources recoveries such as pre-treatment, co-digestion, trace elements supplementation, optimization of key parameters and the use of granular activated carbon are discussed. The review reveals that the currently employed optimization strategies fall short in several ways and proposes the need for improvements.

The Gulf Cooperation Council (GCC) countries are on the verge of a significant shift in waste management, with governments increasingly focused on reforming the waste sector. In Qatar, which currently landfills 90% of its waste, efforts are underway to establish one of the region’s most advanced waste treatment centers, including an anaerobic digestion (AD) facility. However, the question arises whether AD solutions are justified in a fossil fuel-rich nation like Qatar. To this end, this study is among the very few ones that aim to evaluate “concomitantly” the benefits of green energy generation and fertilizer substitution– and the first to consider the specificities of GCC countries. Three organic waste streams were considered: organic fraction of municipal solid waste (OFMSW), livestock manure (LMW) and sewage sludge waste (SSW). The results indicate the potential of AD to generate 850 million m³ of biogas and 2.5 million metric tons of digestate. The biogas could produce up to 3.5 million MWh of surplus energy, equivalent to a reduction of 642 million kg CO2-eq. Substituting traditional fertilizers with digestate could further save 49 to 788 million kg CO₂-eq annually– reaching a total of 691-1,430 million kg CO₂-eq mitigated annually. Overall, AD of the three studied organic waste streams can potentially offset 0.7–1.4% of Qatar’s GHG emissions. The findings also highlight the importance of selecting appropriate feedstock sources to maximize GHG savings. For instance, the complementarity between OFMSW and LMW boosts both clean energy production (by OFMSW’s high biogas yield) and fertilizer replacement (by the high LMW nutrient content). Graphical Abstract This paper showed that three of the major organic waste streams in Qatar may be valorized, through AD, to generate two useful products: (1) biogas that can be used for combined generation of electricity and heat, and (2) digestate that can be used as soil amendment and to replace synthetic fertilizers. The outcome is 691-1,430 million kg CO₂-eq mitigated annually.

Two-phase anaerobic digestion of municipal solid wastes enhanced by hydrothermal pretreatment: Viability, performance and microbial community evaluation