Exploring transition scenarios towards more circularity of organic matters in urban areas : an agent-based model

The green transition cannot be achieved without profound transformations in cities, where nearly 70% of the global population is expected to reside by 2050. Urban metabolism is currently predominantly linear, characterised by high levels of resource imports and the export of most waste. The development of a circular urban bioeconomy can improve both environmental performance and urban selfsufficiency. Human biowaste and excreta are nutrient-rich resources that could partially meet the fertilisation requirements of agricultural areas within the urban hinterland. The use of wastewater sludge on agricultural soils is already practiced, but remains subject to sanitary regulations that may become more restrictive, while source separation of biowaste is improving too slowly, despite legislation mandating accessible source-separation solutions for French households as from January 2024. The transition towards greater circularity of organic matter in urban areas and their hinterland is far from straightforward and requires coordinated changes across a wide range of actors and sectors, from agriculture and food consumption, to waste-treatment infrastructures, as well as consideration of interactions between social, technical, and regulatory drivers. To explore these dynamics, we developed an agent-based model of a virtual city surrounded by peri-urban and agricultural land. The model represents the production and flows of biomass and biowaste: production of biomass (for food, energy, green space), its transformation, its use and consumption, and the sorting and partial recycling and transformation of remaining biowaste, as well as the return to soil of nutriments. It models interactions among 13 stylized agents, namely: agriculture, transformation unit, retailing, urban population, green space, 3 types of waste collectors (for biowaste, greenwaste, or mixed waste), wastewater treatment plank, composter, anaerobic digester, incinerator and landfill.We compare four scenarios varying along two axes inspired by the BETTER prospective 1 , whith low vs. high level of technology, and low or high level of environmental awareness. Our results show that even a moderate surface (approximately 15%) devoted to agriculture within the city's hinterland can meet the demand for locally produced fresh food, provided that such products are available (i.e. fruit and vegetable production). In contrast, organic matter generated within the city remains far from sufficient to supply local agriculture, even in the most virtuous scenarios, potentially constraining yields in organic farming systems which are not allowed to use mineral fertilizers. These findings suggest that, even under favourable social and regulatory conditions, nitrogen recycling major technical innovations will be required to achieve a substantially higher level of circularity between urban organic matter flows and surrounding agricultural areas.