Scaled decentralization for sustainable wastewater treatment in highdensity citiesOA

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 m^(3)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 m^(3)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.