A Subsurface-Flow Constructed Wetland for a Household Cluster's Greywater, Sized by Removal Kinetics: The Bed Area, the Pathogen Reduction, and the Arid Salt Budget That Decide Its Reuse Class

SIMULATED, NOT MEASURED. A horizontal subsurface-flow constructed wetland for a stated 10-person cluster producing 600 L/day of greywater (150 mg/L BOD, 1,000,000 E. coli per 100 mL) is sized by closing kinetics, hydraulics and salt on one bed. Areal first-order kinetics (k20 = 0.10 m/day, temperature factor 1.06) require 12.92 m2 to reach 30 mg/L BOD at 15 C (7.22 m2 at 25 C, 4.03 m2 at 35 C). The bed passes its flow subsurface with a Darcy capacity margin of 20.75 and delivers an effective residence time of 3.88 days after a 0.75 dispersion derate, giving a 1.68 log E. coli reduction to 20,717 per 100 mL: inside the stated restricted-irrigation class but 20.72 times the unrestricted limit of 1,000 per 100 mL, which would need 23.03 m2 on bacterial arithmetic alone and is independently barred by a virus ceiling of at most 2.0 log against a stated 4.0 log requirement. In an arid case (6.0 mm/day evapotranspiration), recovery is 522.46 L/day at a salinity concentration factor of 1.148 (1.00 to 1.15 dS/m), with 69.12 kg of salt per 180-day season and leaching requirements of 29.82% (sensitive crops), 10.12% (moderately sensitive) and 2.96% (tolerant). All computations are reproducible from the deposited compute.py and results.json.