Unveiling in-pipe carbon-sulfur transformation and microbial function during urine transport for centralized...

Source-separated urine collection and centralized nutrient recovery at city-scale hold great potential for advancing sustainable resource management. As the critical link between urine collection systems and nutrient recovery facilities, urine-transporting sewer systems have recently been incorporated into life cycle assessments (LCA), yet their potential for biochemical transformations has not been explored. Here, for the first time, we experimentally unveil key pollutant transformations and microbial functions in a urine-fed bioreactor (representing urine transport), with a sewage-fed bioreactor serving as a control. Major urine nutrients (N, P, and K) remained largely stable during transport, whereas organic carbon and sulfate decreased markedly. Methane production was negligible over 160 days, while sulfide production initially declined but fully recovered by day 80, accompanied by elevated microbial activity and substantial sulfide accumulation in sediments. Microbial community analyses revealed that urine exposure reduced community richness and led to a pronounced community, with methanogenic archaea strongly inhibited and sulfate-reducing bacteria (SRB) becoming dominant under prolonged urine stress. A Desulfomicrobium-like SRB species was progressively enriched (∼35% of total metagenome-assembled genomes (MAGs)) and likely responsible for the sulfide rebound. Spatial heterogeneity of microbial communities in sediments further explains depth-specific sulfide accumulation. Overall, this study provides important insights into carbon-sulfur transformations and microbial adaptation in urine transport systems, informing improved system design, operation, and further LCA.