Drainage Velocity Constraints Limit Greywater Reuse in Building Water Systems: An Integrated One Water Framework with Design Guidelines

Abstract

This study investigates an often-overlooked constraint on greywater reuse feasibility: drainage self-cleansing velocity requirements. An integrated simulation framework coupling WNTR (potable hydraulics), PySWMM (drainage modeling), and NSGA-II (multi-objective optimization) evaluated 72 building configurations varying in height (3-20 floors), demand intensity (0.3-0.6 L/s per floor), and drain diameter (100-200 mm). Results demonstrate that drainage velocity constraints (≥0.6 m/s), not water availability, impose the binding limit on reuse potential. Buildings with 3-5 floors cannot achieve feasible reuse regardless of pipe diameter due to insufficient baseline velocity. Counter-intuitively, smaller pipes enable greater reuse: 100 mm drains allow 47% reuse at 10 floors versus 0% for 150–200 mm drains at the same height. Only buildings with 7+ floors (100 mm drain) or 20+ floors (150 mm drain) achieve meaningful reuse. These findings yield a key design recommendation: drain sizing should be based on post-reuse flows, not pre-reuse peaks. The resulting guidelines enable practitioners to assess greywater feasibility early in planning before committing to hydraulically infeasible targets.