Green Plumbing, Water Conservation and Rainwater Harvesting
This domain tests water efficiency, leak control, rainwater harvesting, greywater reuse and the environmental choices a plumber can influence. Expect calculations of harvestable rainwater and questions on low-flow fixtures and safe reuse.
Efficient systems
Specify dual-flush WCs, aerators, low-flow showers, sensor or waterless urinals, pressure management and sub-metering. Repair leaks promptly: a steady drip can waste more than a low-flow fixture saves. Insulate hot-water pipes and locate heaters near outlets to reduce standing losses. Choose lead-free materials and recycle offcuts where possible.
Rainwater harvesting
Collect roof runoff, divert the first flush, filter leaves and grit, store in a covered tank with mosquito screening, and provide overflow to a recharge pit or garden. Harvestable volume ≈ catchment area × rainfall × runoff coefficient × filter efficiency. Runoff coefficients are roughly 0.8–0.9 for pitched roofs and lower for flat or gravelled roofs. Recharge pits need silt traps and should not receive first-flush water.
Worked example
A 100 m² roof receives 800 mm annual rainfall. With a runoff coefficient of 0.8 and filter efficiency of 0.9: volume = 100 × 0.8 × 0.8 × 0.9 = 57.6 m³ per year, or 57,600 L. Size storage for dry-season demand, not the whole annual yield, and send overflow to recharge.
Traps that cost marks
Do not ignore the first flush; do not leave the tank open to mosquitoes; do not use untreated rainwater for drinking without a treatment train; do not oversize storage and let water stagnate; and do not claim a low-flow fixture saves water if the leak is not fixed.