Many cities of the world are faced with multiple sustainability challenges, for example related to food and energy supply, transportation, waste management, clean air, and more. Preferably, these challenges are addressed with broad and interconnected solutions with the ambition of addressing several challenges simultaneously, in this paper referred to as multi-functional urban solutions. Implementation of multi-functional urban solutions requires well informed decisions, supported by knowledge about the potential contributions that the solutions can make to a more sustainable city as well as on issues that may hinder or facilitate their implementation. Thus, in this paper, we suggest a soft multi-criteria decision analysis method that can be used to gather and structure this knowledge. This method acknowledges the importance of incorporating local knowledge, is based on life-cycle thinking, and is flexible and open-ended by design so that it can be tailored to specific needs and conditions. The method contributes to existing practices in sustainability assessment and feasibility studies, linking and integrating potential and performance assessment with issues affecting solutions’ feasibility of implementation. This method offers a way for local authorities, researchers and exporting companies to organize and structure the diverse range of knowledge to be considered for more informed decisions regarding the implementation of multi-functional urban solutions. While the main contributions of the paper are methodological, brief descriptions of two studies that have applied this method to assess biogas solutions are shown as clarifying examples. One of these studies was performed in Chisinau, Moldova and the other in Johannesburg, South Africa.

In this paper, we performed technology assessment and systems analysis of primary digestate processing techniques to provide a comprehensive analysis of their environmental and cost performance. We compiled more than 100 observations from large-scale biogas plants and considered digestate based on manure, crops and agro-wastes, and food waste under the geographical contexts of Sweden and Belgium. Centrifuge, screw press, and rotary drum were identified as suitable primary processing techniques. We analyzed the climate impact, energy use, and operational cost of digestate management under these scenarios: no processing, partial processing (solid-liquid separation) and full processing (solid-liquid separation followed by ammonia stripping). As expected, the suitable digestate processing varied with the context, transport was often the most critical cost factor, and emissions from storage reduced the climate savings from the use of biofertilizers. However, treating liquid fraction became a main contributor to cost and climate impact under the Belgian conditions. Consequently, the possibility for local application of liquid fraction as biofertilizer could prevent costs and impacts associated with its further treatment. The main novelty of this work is in its integrative and comprehensive approach toward the choices and impacts of primary processing of digestate. We tried to bridge many individual case studies, drew from experiences of biogas plants in different geographical contexts, assessed suitable processing techniques for different digestate types, and analyzed the environmental impacts and cost of digestate management from a life cycle perspective. We believe that such integrated approaches would help decision-making for increased sustainability of the biogas sector.

The transition toward a circular and biobased economy requires the biorefineries and bio-based industries to become more resource efficient with regards to their waste and by-product management. Organic by-products and waste streams can be an important source of value if used in feasible pathways that not only have a low environmental impact but also preserve or recover their energy, nutrients, and other potentially valuable components. Through development of a multi-criteria assessment framework and its application on a real case, this article provides methodological and practical insights on decision making for enhanced by-product management. Our framework includes 8 key areas and 18 well-defined indicators for assessing the environmental performance, feasibility, and long-term risk of each alternative. We studied six different management options for the stillage by-product of a Swedish wheat-based biorefinery and our results shows that the most suitable options for this biorefinery are to use the stillage either as animal fodder or as feedstock for local biogas production for vehicle fuel. This multi-criteria approach can be used by bio-based industrial actors to systematically investigate options for by-product management and valorisation for a circular and bio-based economy. Graphic