Short note: 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.

Reasons

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

Methods

• Rooftop harvesting: roof water is led through gutters and down pipes, a first-flush device and a filter to a storage tank for use.
• Surface runoff harvesting: collection in ponds or tanks.
• Groundwater recharge: through pits, trenches and wells.

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

Benefits

• Supplements the water supply and reduces the water bill.
• Raises the groundwater table and improves its quality (dilutes fluoride and salts).
• Reduces urban flooding and erosion.
• An easy, low-cost technology that can be used in any building; the water is suitable for toilets, gardening, washing and, after treatment, drinking.

Design: volume that can be harvested per year = annual rainfall (m) x roof area (m^2) x runoff coefficient (0.8-0.9 for roofs).