Scaled decentralization for sustainable wastewater treatment in high-density cities

Sustainable urban wastewater management requires systems that effectively remove pollutants while enabling resource recovery and diminishing environmental impacts. In high-density cities, conventional centralized treatment plants often entail long conveyance distances, high pumping energy, and substantial methane emissions from widespread communal septic tanks, compounded by stringent effluent standards. Scaled decentralized systems (SDSs)-distributed facilities integrated into existing sewer networks-offer potential advantages through reduced transport and localized recovery, yet their performance across scales, technologies, and recovery strategies remains poorly quantified. Here we show that SDSs featuring 20,000 m³ d^-1 moving-bed biofilm reactor plants with combined water reuse and additional heat-pump energy recovery achieve the lowest life-cycle environmental impacts and costs. We applied whole-system life-cycle assessment and cost analysis of 29 scenarios treating 100,000 m³ d^-1 of wastewater in a city in China. These configurations reduce global warming potential by up to 52.5 % relative to an optimized centralized benchmark, owing to shorter sewers that preserve influent carbon for biological denitrification, elimination of external carbon dosing, and efficient dual recovery; upstream septic tanks, however, contribute 24-47 % of total warming potential, highlighting a key trade-off. These findings demonstrate that carefully scaled decentralization, paired with robust biofilm technology and integrated recovery, provides a superior pathway for sustainable wastewater infrastructure in dense urban settings.