A Household Greywater Treatment and Reuse Bed for Water-Short Settlements: A Sizing Rule and Computed Treatment Budget for a Gravel-and-Reed Subsurface-Flow Design

SIMULATED, NOT MEASURED. A household greywater treatment and reuse bed (gravel-and-reed subsurface flow) for water-short settlements, sized by a computed budget. For five people producing 250 L/day, organic loading governs: design area 6.5 m2 (1.30 m2 per person), retention 5.93 days. Computed effluent BOD falls from 150 mg/L to 41.1 mg/L at 10 C and 6.7 mg/L at 30 C; indicator reduction ranges 1.18 to 4.56 log10. Treated water is restricted to subsurface irrigation and is never drinking water. Clogging horizon 3.52 years without a pre-filter, 7.05 years with one. All numbers come from the deposited computation script.