Sustainable Wastewater Management in Gotland: A System Dynamics Model for Urine Diversion and Resource Recovery

Sustainable Wastewater Management in Gotland: A System Dynamics Model for Urine Diversion and Resource Recovery E. C. Francisco*, J. McConville* * Department of Energy and Technology, University of Agricultural Sciences (SLU), Lennart Hjelms väg 9, 75651 Uppsala, Sweden (E-mail: [email protected]; jennifer.mccon[email protected]) Abstract Sanitation systems are integral to urban infrastructure but face critical challenges, including inadequate coverage, sustainability issues, and environmental impacts exacerbated by population growth and urbanization. Innovative approaches, such as urine diversion, have emerged as promising solutions for resource recovery and sustainable nutrient management in wastewater treatment. This study develops a system dynamics model to evaluate the potential of urine diversion in enhancing the sustainability of urban wastewater management, with a case focus on Visby, Gotland, Sweden. Gotland’s seasonal tourism, coupled with the Baltic Sea’s status as a highly eutrophic and vulnerable water body, intensifies the region’s water and wastewater management challenges. Using the STELLA software, the system dynamics model incorporates two modules: (1) the Visby WWTP module, which simulates the operations of Visby’s wastewater treatment plant, accounting for seasonal tourism fluctuations, and (2) the Urine Diversion module, which examines urine collection during peak tourist seasons for fertilizer production. Data were derived from environmental reports and secondary sources to simulate environmental indicators, including marine eutrophication potential, carbon footprint, and water footprint. The results will provide valuable insights into how urine diversion can enhance long-term sustainability in urban wastewater management by reducing environmental impacts and promoting resource efficiency. This study underscores the importance of integrating circular economy principles into sanitation systems to address global environmental challenges, particularly in regions experiencing seasonal population dynamics. Keywords Fertilizer, resource recovery, sanitation, system dynamics modelling, urine, wastewater INTRODUCTION Sanitation systems represent a critical component of urban infrastructure, ensuring the safe collection, treatment, and disposal or reuse of human waste. However, existing sanitation systems face substantial challenges, including inadequate service coverage, limited sustainability, and significant environmental impacts (Spuhler & Lüthi, 2020). These challenges are further exacerbated by population growth, urbanization, and increased chemical loads, which intensify the pressure on wastewater treatment systems and compromise their capacity to function efficiently and sustainably (Spuhler et al., 2020). In response, researchers and practitioners are developing innovative sanitation strategies that incorporate resource recovery and circular economy principles (Scott & Cotton, 2020; Spuhler & Lüthi, 2020). Among these strategies, urine diversion has emerged as a promising decentralized technology to address the dual challenges of resource recovery and sustainable nutrient management in domestic wastewater treatment (Aliahmad et al., 2025a; Simha et al., 2023). To support the planning and implementation of resource recovery technologies in sanitation systems, decision-makers require robust decision-support tools, such as modelling and simulation methodologies. These tools enable a comprehensive analysis of complex systems and facilitate the identification of optimal solutions (Diaz-Elsayed et al., 2019; Trimmer et al., 2019). System dynamics modelling, in particular, is well-suited for capturing feedback loops, delays, and the intricate interactions between factors such as population growth, infrastructure, resource flows, and economic variables (Mohammadifardi et al., 2019; Prouty et al., 2020). The objective of this study is to develop a system dynamics model to provide insights into how urine diversion can contribute to long-term sustainability or urban wastewater management. MATERIALS AND METHODS Case of study The Baltic Sea, located in Northern Europe, is one of the most eutrophic bodies of water globally. This semi-enclosed, brackish sea is highly vulnerable to anthropogenic pressures, which have significantly contributed to the expansion of hypoxic and anoxic zones (Almroth-Rosell et al., 2021). Gotland, Sweden's largest island, lies within the Baltic Sea approximately 100 km from the mainland. Despite its relatively small permanent population of approximately 61,173 residents, Gotland experiences a substantial influx of visitors. In 2023, over 2 million people travelled to the island (Region Gotland, 2023). Tourism peaks during the summer months, resulting in pronounced seasonal fluctuations in both water demand and wastewater production. This seasonal variability, coupled with the island's environmentally sensitive context, exacerbates the risk of eutrophication in the Baltic Sea. To address these challenges, the present study focuses on wastewater management practices at Visby the main island’s city. System Dynamics Modelling (SDM) A model is a formal and simplified representation of reality, designed to balance simplicity and complexity to ensure accurate analysis without becoming intractable. The development of simulation models follows a structured series of steps (Figure 1) that support the understanding and analysis of complex problems: Figure 1. System dynamics modelling development process. In this study, the methodology specifically focuses on the initial four steps, highlighted in red square. RESULTS AND DISCUSSION The WWTP-UrDiv model (Figure 2) was developed using the STELLA software (2023) and is configured in 2 modules: WWTP Visby and Urine diversion. The first, considering the structure of Visby’s WWTP and tourism’s seasonality. Additionally, a stock named “Urine” was added to the module as a connector with the second module. The Urine diversion module represent the urine collection during the summer season, considering the urine collect during festivals and touristic events in Visby. The aim of this technology is to improve the resource recovery and circular economy principles through fertilizer production as the final product. Figure 2. WWTP-UrDiv model representing the sewage treatment and urine diversion technology. The stock-flow and parameters from both modules represent the components from the Visby’s WWTP and fertilizer production by urine diversion. The data referring to the modules were collected from the Environmental Report (Gotland, 2023) for the WWTP module and, from secondary references for urine diversion (Aliahmad et al., 2025b; Simha et al., 2020). As output for the simulations and future respective scenarios, on the next stage, the environmental indicators selected were marine eutrophication potential, carbon and water footprints aimed to access the traditional sanitation system impacts and, the insights into how urine diversion can contribute to long-term sustainability or urban wastewater management. REFERENCES Aliahmad, A., Lima, P. de M., Kjerstadius, H., Simha, P., Vinnerås, B., & McConville, J. (2025a). Consequential life cycle assessment of urban source-separating sanitation systems complementing centralized wastewater treatment in Lund, Sweden. 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