School Latrine Ventilation: A Sizing Bound for Ventilated Improved Pit Latrines from Wind and Solar Airflow Physics

Ventilated improved pit (VIP) latrine vent-pipe airflow modelled from wind suction and solar stack pressure against pipe, drop-hole and fly-screen losses. A 150 mm black pipe, 3.2 m tall, with static cowl and clean 1.2 mm screen meets a 0.20 m/s down-hole odour criterion (32.4 m3/h, 10.5 air changes per hour) in 99.5% of 1,080 simulated school hours at a stated hot semi-arid site; a 110 mm pipe meets it in only 61.3%. Screen fouling cuts coverage to 69.1% (moderate) and 28.5% (severe); calm overcast hours and flooded pits fail at any diameter. Cost US$0.282 per pupil per year for a ten-cubicle block serving 480 pupils. SIMULATED, NOT MEASURED. All numbers are produced by the accompanying compute.py.