First-Flush Diverter

Between one shower and the next a roof collects dust, leaves and bird and rodent droppings. When rain starts, the first water off the roof carries most of it. A first-flush diverter is a chamber in the downspout that fills with that first runoff and holds it back, so only the cleaner water behind reaches the tank. What it catches is thrown away, not treated.

How much to throw away is a rate, not a fixed volume: about 0.4 to 0.8 litres per square metre of roof (1 to 2 gallons per 100 square feet), taking the upper end where trees overhang or dry spells run long, since what matters is how long the dirt has sat there, not how big the roof is. Discarding it deals with the debris and the bacteria and parasites in droppings. Metals are another matter: they seep from the roofing and flashing continuously rather than washing off in one go, so the answer is material choice - no lead or copper flashing and no copper gutters - not a bigger chamber. This is the first stage of a treatment train, not a substitute for one: screens at the gutter and downspout come before it, one at the tank inlet after it, and drinking water is filtered and disinfected downstream.

The failure that matters is quiet. It works only if it is empty when the next rain begins; a chamber still full from the last storm passes the dirty flush straight through while looking as installed. The usual answer is a slow drain, a tap cracked just far enough to empty it between storms without losing clean water; where nobody will see to it each storm, an automatic drain replaces the hand valve. Upkeep is frequent and small, and the screens on either side clog: clear the diverter after each storm, the standpipe at least monthly through the wet season, the tank inlet screen after every rain, and the whole train yearly. Any roof feeding a tank deserves one, not just the house. No approved source gives a service life.