The first-flush diverter
The cheapest water-quality improvement in rainwater catchment, and it has no moving parts worth mentioning.
What it is
A vertical standpipe plumbed between the downspout and the tank. The first water off the roof — carrying the dust, bird droppings, pollen, and grit that collected since the last rain — fills the standpipe. Once it's full, clean water flows past it into the tank. A small drip hole empties the standpipe between storms.
The numbers
- Rule of thumb: divert about 10 gallons per 1,000 sq ft of roof, more if the roof is under trees.
- A 1-inch rain on 1,000 sq ft of roof delivers roughly 623 gallons, so you're giving up well under 2% to get most of the contamination.
- Total parts: a length of 4-inch PVC, a cap, a ball, and a drilled drip hole — about $40.
Why it's here
It removes the worst water in every storm for the price of a pipe. Nothing else in a catchment system gives that ratio.
The automation angle
Tie it to the tank level sensor and the weather station: you'll see each storm as a step in the tank curve, and a storm that *doesn't* produce a step means a blocked gutter, a stuck diverter, or a leak.
The catch
The drip hole clogs, and a diverter full of old water does nothing — it just sits there full while the dirty first flush goes straight past it into your tank. Check it twice a year. Also: first flush is not treatment. Roof water still needs filtration and disinfection before anybody drinks it.