Life cycle assessment of anaerobic digestion for the organic fraction of municipal solid waste

Abstract

An increase in the quantity of generated municipal solid waste (MSW) compel societies to implement coordinated waste management plans. At the same time, municipal waste's composition diversifies in tandem with increased volumes. Plastic, paper, metal, glass, food and green waste are the main constituents of comingled waste generated by people on daily basis. Among all types, the biggest portion belongs to organics which are carbon-based materials. With a designed process, organic waste can generate energy through methanisation biologically. The process called anaerobic digestion, is one of commonly used methods to managing organic waste in a sustainable manner. On contrary, other alternative management ways such as landfilling or incineration offers more linear solutions that valuable parts of the organics (methane) are nearly wasted. In Türkiye; along 2022, according to Turkish Statistical Institute, 61% of collected municipal solid waste is mostly disposed in landfill sites. Due to commingled collection, organic fraction is mixed with other type of wastes and therefore disposed off without efficient recovery. However, a circular mindset necessitates considering waste to be handled as a source. Far from disposing organics as waste; partial recovery even like collecting landfill gas, is not adequate when the potential is considered. As a matter of fact, countries are setting targets about landfill diversion in recent years. Sustainable management for the organic fraction of municipal solid waste (OFMSW) need to be a top concern for relevant authorities. Anaerobic digestion can be considered one of the suitable methods. Principally, it imitates the natural decomposition procedure in an engineered systematic process. It produces energy with the help of microorganisms in controlled conditions. In Türkiye nearly half of the MSW collected comprise organics which have considerable potential for energy generation. The scope of this study comprises organic waste management in one of the biggest cities in Türkiye. With its more than one million populations, Sakarya generates 1500-ton municipal solid waste per day. The whole collected waste is transferred to Sakarya Integrated Solid Waste Management Plant (SEKAY). From mentioned mixed waste, every day nearly 300 tons of organics are separated through mechanical separation units in the facility. In addition to those, external organics such as waste from recreational, agricultural activities, chicken manure or waste sludge, expired foods etc. are fed to three anaerobic digesters of the plant. Mentioned organics are degraded by various microorganism groups then nutrient-rich digestate and energy-rich biogas are produced without elemental oxygen. Released methane gas is collected via pipelines and stored in the gas balloon. As the main target is producing electricity, whole process is ended in gas engines where electricity is generated and transferred to the city grid. Heat is also another outcome of the process. Due to certain temperature needs of biological activities, the digesters are externally heated with the heat produced at the facility. The study's entire set of data is obtained from SEKAY's one-year interval anaerobic digestion operations from April 2022 to April 2023. According to records, an average of 300 tons of organic is fed into digesters and 112 MW of electricity is produced per day. The objective of this study is to evaluate the environmental sustainability of the anaerobic digestion process with a life cycle approach. The life cycle assessment (LCA) can be defined as a software-based process modeling tool. The method uses all material and energy input-output data to compile possible environmental impacts of the system. In this study, all calculations and analyses are done corresponding to a functional unit which is 1 kWh of electricity produced in SEKAY. The environmental burdens of producing 1 kWh of electricity from OFMSW are analyzed with Gabi software which has the standard framework presented in ISO 14040/14044. Every resource and emission is allocated to one or more of the impact categories included in the assessment. Thus, by utilizing the CML (Centre of Environmental Science at Leiden University) 2001 method, this study focused on impact categories of global warming potential (GWP), ADP abiotic depletion potential-elements (ADP elements), abiotic depletion potential-fossil (ADP fossil), acidification potential (AP), eutrophication potential (EP), human toxicity potential (HTP), marine aquatic ecotoxicity potential (MAEP), ozone depletion potential (ODP), terrestrial ecotoxicity potential (TETP), freshwater aquatic ecotoxicity potential (FAETP), human toxicity potential (HTP), photochemical ozone creation potential (POCP). Eventually, ideas having waste to energy technologies circle around escalating obligatory waste management issues into another sustainability level. Not to have 'more trouble than it's worth' situation; precautions must be taken by authorities. Therefore, the process's environmental burdens should primarily be assessed and addressed appropriately. To conclude, the present study was designed to identify the impacts of anaerobic digestion of OFMSW with the aim of providing both environmental and technical opportunities to waste management sector. Results attained from LCA of anaerobic digestion (AD) study indicate that the biggest burden is attributed to the combined heat and power (CHP) unit. Whereas operational activities have higher impacts cumulatively, construction of the AD plant, CHP unit, composting facility has a significant effect which increases the total influence of organic fraction of municipal solid waste (OFMSW) management. Once the specifics are examined, electricity consumption on various points of the AD process has a noticeable negative environmental impact due to national grid content. In this manner, it is deemed necessary to conduct further sensitivity analysis which includes different electricity sources: national grid, bioenergy and solar. The results revealed that the least environmentally friendly source is the national grid due to its yet insufficient renewable source content. By taking into consideration of facility's real situation and future possibilities, storing liquid digestate in specially constructed lagoons and purifying (upgrading) biogas scenarios are also assessed.