Anaerobic digestion and integration at urban scale: Feedback and comparative case study

Abstract

Background: The anaerobic digestion process is well studied and developed since the last 50 years, notably for farming implementation. Furthermore, the international energy agenda has shifted in favor of renewable resources. In this context, anaerobic digestion has been highlighted as a valuable source of green energy. At urban scale, since the 2000s, some eco-friendly districts and cities tried to integrate this process as valorization treatment, with different approaches and results. The objective of this study is to provide a feedback from these anaerobic digestion projects at urban scale. Methods: To construct this paper, 15 international projects of urban waste valorization were selected and studied. Three of these projects are developed to describe the different elements and processes implied in these valorization systems. Thus, in situ and ex situ anaerobic digestions are described with functional implementation and units in development or abandoned. Once the leading steps and main parameters on waste management system identified, the discussion is performed to compare the operational systems implemented in the 15 cases studies. The analysis takes processes and biological parameters into account in addition to social, financial, and ecological elements available. The achievements and limits of case studied linked to their context, bringing advice and recommendations. Results: From these real cases and projects, we highlighted relevant information for further urban waste valorization system. Among others, the most frequent organic deposits produced at urban scale are identified, as well as the typical quantities generated. Collection, storage, and pretreatment processes were investigated to provide information about how they are conducted in situ and ex situ. Concerning some technical points, CSTR and UASB configurations are also identified as predominant reactors used to lead wet AD process at urban scale, due to practical aspects and characteristics of feedstock. Concerning the digestate, nutrients contents make it an interesting fertilizer for agricultural valorization. To its stabilization, the composting process is mainly deployed after anaerobic digestion of urban waste. The valorization of biogas produced with in situ urban AD is generally performed with CHP unit, due to the small quantity of methane. When the valorization is operated ex situ, processes are in link with local users’ needs and energy transition policy. Conclusion: In conclusion, we identified among others that available deposits, collection and storage processes, local grid, and scale mechanisms influence the urban integration of AD. Thus, anaerobic digestion in situ is emphasized for new district buildings with low urban density, while ex situ process could be more accommodated to high urban densities or projects including preexistent waste management. From the 15 case studies, co-digestion process is highlighted to perform global waste management and produce valuable AD reactions. Nevertheless, the use of blackwater, for in situ units, involves specific processes to limit amount of water and reduce the reactor sizing. Further knowledge is also currently needed about the development of collection systems for urban waste and the impact of pretreatment on AD. The involvement of scientific, economic, social, and political communities is also highlighted as essential for the long-term success of valorization system integrated at urban scale.