Adapting sanitation systems to demographic transitions: Optimizing hybrid configurations of sewered and non-sewered solutions

Non-sewered sanitation systems play a vital role in achieving global sanitation goals by providing resource-efficient and flexible solutions, particularly in resource-constrained settings. When effectively integrated with infrastructure of centralized systems, they offer multiple advantages, including reduced asset redundancy, improved economic feasibility, intergenerational equity, and better public health outcomes-ultimately maximizing overall system performance. This study presents a novel multi-objective optimization framework that identifies the optimal spatial configuration of sewer-based system and the Johkasou system as a representative fully road-transported, non-sewered system, explicitly accounting for population decline and diverse performance criteria, including capital and operating costs, greenhouse gas (GHG) emissions, and biogas recovery potential. The optimization model integrates generalized objective functions for these performance criteria, all linked to spatial population dynamics. A case study showed that maintaining a centralized system alone increases both costs and GHG emissions over a 50-year horizon. In contrast, transitioning to an optimally hybridized sewered and non-sewered system configuration reduces expenditures and environmental impacts. Additional benefits are achieved through optimized onsite treatment technologies that reduce operational GHG emissions by 33 %, and through efficient sludge collection. The results demonstrate the superior performance of hybrid systems over single-technology approaches and emphasize their potential as smarter, more sustainable sanitation solutions in regions facing aging infrastructure and depopulation. Owing to its generic structure and interpretable outputs, the model is broadly applicable and adaptable to varying policy contexts, offering decision-makers a robust basis for designing sustainable, context-sensitive sanitation strategies.