Assessing the environmental and financial impacts of treating municipal food waste using anaerobic digestion and co-digestion technologies
Creators: name: Edwards, Joel
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 401101
subject: Air pollution modelling and control
classificationCode: 401103
subject: Global and planetary environmental engineering
classificationCode: 410402
subject: Environmental assessment and monitoring

description: Australia has a successful history of recycling kerbside collected municipal metal, plastic and paper. The progressive implementation of many garden waste collection and diversion programs has also furthered a reputation for high municipal waste recycling rates. Yet, municipal food waste (FW) has largely remained untouched by efforts in recycling kerbside waste. FW instead is most commonly disposed of to landfill along with inert and non-recyclable metals, plastics and other municipal waste. In landfill, FW is a significant contributor to environmental pollution. Government, at all levels have therefore begun focusing on the diversion of FW away from landfill, using more stringent policy measures including landfill levies, source separation incentives and renewable energy incentives, to promote alternative collection and treatment methods. Whilst, many FW management and treatment alternatives are technologically feasible, the environmental and human health impact of treatment technologies have yet to be comprehensively assessed, especially in the context of Australia.<br><br>Environmental and human health impact needs to be measured in a manner that can fairly compare FW treatment technologies. This is inherently difficult in waste management as treatment technologies often have different pre-requisite conditions to their treatment method, for example, household source separation of FW. Moreover, by-products from a treatment technology may have impacts or benefits to the environment. This necessitates a treatment technology to be considered as part of a wider waste management and treatment system. It is crucial, therefore, that the methodology for comparing the environmental impact be considered in the context of a waste service, ensuring the same quantity of waste collected and quality of treatment is maintained across systems. Not comparing systems in this way may lead to misleading outcomes, with the potential to shift the burden of environmental impact from one process to another.<br><br>To ensure a more complete environmental evaluation, life cycle assessment (LCA) has become a widely used method for comparing waste management systems. LCA has been used in over 200 solid waste management studies worldwide, and over 22 papers focus explicitly on FW management. Many studies however have not compared systems but instead used LCA to investigate weak points of high environmental impact within systems, or have used the method to focus on the treatment process exclusively. Other studies use theoretical waste flows in their analysis and may often unfairly compare systems.<br><br>This research focused on the whole waste service provision and used two Australian case studies in order to provide real life examples. The investigation focused on seven contemporary waste management systems. Landfilling of FW along with gas capture represents the business-as-usual waste management system. The composting of FW and garden waste, either at a centralised composting facility or a small-scale home composter, were the second and third alternative systems assessed, whilst the remaining four systems were variations of waste management systems that utilise anaerobic digestion (AD) as the key FW treatment technology.<br><br>AD is a technology that can generate energy and a valuable compost product from organic wastes. AD is most commonly used in Australia to treat sewage sludge (SS) generated at wastewater treatment plants (WWTPs). Yet, within the past decade many AD facilities in Europe and east Asia have been successfully treating FW. A small number of WWTPs have also begun simultaneously treating FW and SS known as anaerobic co-digestion (AcoD). In these cases AcoD has been shown consistently to have a synergistic effect; making AD more efficient, boosting energy production, as well as stabilising and increasing the amount of key nutrients in the compost product. As WWTPs are the largest users of AD in Australia, with existing expertise, and often underutilised infrastructure. AcoD provides a unique opportunity for WWTPs as a practical means to optimise nutrient and energy recovery from sludge. Additionally, AcoD offers a new and localised treatment alternative of FW management. Therefore, AcoD formed the fourth FW management system assessed in this research using LCA. The fifth system was also based on AcoD but included a different FW collection mechanism – via in-sink maceration and sewer transportation. The sixth and seventh systems assessed were based on the AD of FW as a single susbtrate, with these systems being differentiated by either digesting source separated FW or mechanically separated FW.<br><br>The use of LCA in this study for systems comparison aims to enable the ranking of FW management systems according to key environmental impact categories including global warming potential, acidification, eutrophication, ozone depletion, photochemical oxidation and human toxicity. The research also provided an insight into the effect of crucial parameters and assumptions through sensitivity analysis, as well as examining the effect inventory data uncertainty has on the results by using Monte Carlo analysis coupled with lognormal uncertainty distributions according to the pedigree matrix. This research also provided a comprehensive mass balance across the entire waste service, depicting where and how much a certain type of waste stream or by-product comes from and ends up. Moreover, a life cycle costing (LCC) of all waste management systems was produced in order to determine the expected cost for all stakeholders involved in a waste service including local governments, contractors, and residents. <br><br>Waste management systems were then ranked from best to worst in terms of cost and environmental impact for all categories. In ranking seven of the most likely FW management systems this study provides waste managers and decision makers with the analysis and data to determine the most suitable waste management system for their waste catchments. The research also provides an analytical framework where decision makers are able to use sensitivity analysis to explore key parameters that can significantly alter the performance of systems. Moreover, the research provides uncertainty analysis for a probabilistic representation of performance that quantifies and compares the environmental risk associated with systems.