Components of rainwater harvesting

Rainwater harvesting (RWH) is the collection and storage of rain falling on roofs or the ground for later use, or for recharging groundwater, instead of letting it run off.

Components of rooftop RWH

1. Catchment: the roof (RCC, tiles, GI sheet) that receives the rain; it should be a clean, non-toxic surface.
2. Gutters: half-round channels at the roof edge (GI, PVC, aluminium) with a slope of about 1 in 100, leading water to the down pipe; fitted with mesh to stop leaves.
3. Down-take (conveyance) pipes: PVC pipes (75-100 mm) taking water from the gutters to storage.
4. First-flush device: diverts the first dirty rain (first 10-15 minutes, or about 0.5-1 mm of rain), which carries dust, bird droppings and leaves, to waste.
5. Filter unit: layers of coarse sand, gravel and charcoal (or a mesh/sand filter) to remove suspended matter.
6. Storage tank: underground or overhead tank (RCC, ferrocement, plastic) sized for demand and rainfall, with inlet, overflow, outlet, vent and provision for cleaning.
7. Recharge structure (optional): recharge pit, trench or well for the overflow.
8. Disinfection / delivery: chlorination or UV before drinking use; pump and distribution.

   \   roof (catchment)   /
  ===+====================+===
     |gutter              |
     v                    |
   down pipe              |
     |                    |
 [first flush]--> waste   |
     |
 [ filter: gravel/sand/charcoal ]
     |
 [ storage tank ]--overflow--> recharge pit
     |
   pump -> use

Sizing: annual harvestable volume = annual rainfall (m) x catchment area (m^2) x runoff coefficient (about 0.8-0.9 for roofs). Example: a 100 m^2 roof with 1.5 m of rain and a coefficient of 0.85 gives 127.5 m^3 per year. The tank itself is sized from the daily demand and the length of the dry period.