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RWH: reasons and basic components

Short answer

Rainwater harvesting (RWH) collects roof rain for use or recharge, addresses water scarcity, irregular supply, runoff, and provides soft water cheaply. Basic components: roof catchment, gutters, down pipes, first-flush diverter, filter, storage tank, recharge pit. A rooftop system includes a clean roof, meshed gutters, PVC down pipes, first-flush device, gravel/sand/charcoal filter, and a tank sized for demand with overflow to recharge.

Explain rainwater harvesting, write down its reasons behind it and list out its basic components. Illustrate components of the rooftop rain water harvesting system.

Rainwater harvesting

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

Reasons

  • Water scarcity and falling groundwater levels, especially in urban areas like the Kathmandu valley.
  • Poor or irregular piped water supply; reduces dependence on tankers and groundwater pumping.
  • Reduces surface runoff, urban flooding and soil erosion.
  • Rain water is soft and nearly free of salts; cheap and simple to collect.
  • Saves cost of water and energy; helps groundwater recharge.

Basic components

Catchment (roof), gutters, down pipes, first-flush device, filter, storage tank (and recharge pit for overflow).

Components of the rooftop system

  1. Catchment: the roof (RCC, tiles, GI sheet) that receives rain; clean and non-toxic surface.
  2. Gutters: half-round channels at roof edge (GI, PVC, aluminium) with slope 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 gutters to storage.
  4. First-flush device: diverts the first dirty rain (first 10-15 minutes or about 0.5-1 mm of rain) containing dust, bird droppings and leaves to waste.
  5. Filter unit: layers of coarse sand, gravel and charcoal (or 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 cleaning provision.
  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

✓ Answer checked by Claude Opus 5.5 (high effort)

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