Reuse Water vs. Stormwater Drainage — a Key Distinction
Reuse systems are frequently confused with, but functionally distinct from, the roof and site storm drainage systems covered elsewhere in plumbing design. Storm drainage exists to remove rainwater from the building and site as quickly and reliably as possible to prevent flooding and structural loading (see the roof and site storm drainage guide for gravity and siphonic drain sizing). Rainwater harvesting, by contrast, deliberately captures a portion of that same roof runoff, treats it to a level appropriate to its intended use, and stores it for beneficial reuse rather than discharging it to the storm system. Greywater reuse is a related but separate practice: it captures and treats wastewater from specific plumbing fixtures — not roof runoff at all — for a second beneficial use before it would otherwise be discharged to the sanitary sewer or septic system. Both practices reduce potable water demand and, in many jurisdictions, qualify for green building credit (LEED, WELL, local water-neutral development ordinances), but they draw from different sources, require different treatment trains, and are governed by different subsections of the plumbing code.