Treatment Trains vs. Single-stage Systems: Nutrient removal and economic tradeoffs of biochar-enhanced woodchip bioreactor systems

Biochar has been widely used as a soil amendment and also as a cost-effective adsorbent for pollutant treatment. Herein, we reexamined the system design and interactions between biochar and woodchip bioreactors (WBRs) to achieve synergies and minimize trade-offs, providing the first integrated assessment of treatment configuration, mechanistic interactions, and techno-economic performance in biochar-enhanced WBR systems. Three types of biochar-enhanced WBRs were evaluated: a single-stage system (biochar-woodchip mixture, W+B) and two treatment trains (WBRs followed by biochar systems, W-B, and biochar systems equipped with secondary WBRs, B-W). The results indicated that all types of biochar-enhanced WBRs exhibited simultaneous removal of nitrate and dissolved reactive phosphorus (DRP). Single-stage W + B systems achieved the highest reductions in DRP and nitrate but resulted in the highest amount of dissolved organic carbon (DOC) leaching. Meanwhile, the treatment train design with a downstream biochar system effectively mitigated byproduct leaching from upstream WBRs while maintaining stable nitrate removal. DRP removal showed similar trends across the three types of biochar-enhanced WBRs, with a gradual decrease in removal efficiency over time. Microbial analysis revealed that the interaction between biochar and woodchip amendments selectively supported the growth of microbes that facilitate denitrification and organic matter decomposition, while reducing microbial diversity in W+B systems, which likely reflects system functional specialization towards denitrification. Furthermore, techno-economic analysis identified W-B systems as most cost-effective, achieving $187.9 ± 160.0/kg DRP and $5.7 ± 1.7/kg N per year. The relatively higher uncertainty associated with DRP removal costs was primarily attributed to variability in influent DRP loading and removal efficiency. In contrast, the W+B system, despite higher removal performance, exhibited higher unit costs due to decreased treatment capacity. Overall, our findings highlight the importance of infrastructure design and media interaction for advancing biochar application into WBRs to enhance their feasibility, effectiveness, and sustainability in water quality management.