← Field Notes Water No. 28

Rainwater arrives nearly distilled

Then people ruin it. Here's the fix.

Rainwater starts as some of the cleanest water available anywhere — essentially distilled when it leaves the cloud. Everything after that determines whether you can drink it.

What contaminates it: the roof. Bird droppings, dust, pollen, roofing material, whatever blew up there. Rainwater is a great source; it is not automatically safe.

The first-flush rule: the first portion of any rain event carries the roof's accumulated filth. A first-flush diverter dumps that initial volume before the rest goes to your tank. Simple mechanical device, and the single highest-value part of a catchment system.

The chain that makes it drinkable: 1. Screen at the gutter — leaves and insects 2. First-flush diverter — dumps the dirty opening volume 3. Sediment filter — fine particles 4. Storage — food-grade, opaque, sealed (light grows algae) 5. Treatment before drinking — same rules as any surface water

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