Water Portal / Rainwater Harvesting / Fog and dew collection / Fog collection and storage
Fog water collection, Nepal, Peru and Chile Fog interception technology copies the function of trees and other natural features. It uses large polypropylene mesh nets on ridges, erected perpendicular to the prevailing wind, to capture water-loaded fog, which forms in humid months in mountainous regions or coastal areas. The mesh captures water droplets (1 to 40 μm), which trickle into a collection trough and drain into a series of tanks.
Large collectors are usually 12 m wide and >4 m high. The mesh covers the upper 4 m of the collector, giving a collecting surface of 48 m2, providing fresh drinking water for rural communities.
| Advantages | Disadvantages |
|---|---|
| Low project costs Simple technology and maintenance |
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Capacity
Typical water production rates from a fog collector range from 150 to 750 litres per day but some schemes are capable of producing 2,000 to 5,000 litres per day.
If sufficient water is collected, vegetation or crops can also be planted and sustained. Once vegetation is established, it can sustain itself by catching the fog droplets directly.
Costs
- Material: Polypropylene mesh per 1 m2 (Peru and Chile) US$ 0.25
- Labour: construction and installation of large fog collectors, reservoir tanks and taps:
- Skilled labour: 140 man days (Nepal) US$ 4 per day
- Unskilled labour: 400 man days (Nepal) US$ 2.75 per day
- All inclusive (materials, labour):
- Fog collectors including building materials: US$ 100 - 200
- 48 m2 fog collector providing 3 l/m2/day: US$ 378
- Cost per m2 (Nepal, including reservoir and tap): US$ 60
Applying conditions
- The technique is suitable for locations with frequent fog periods. Upland areas where fog is produced by the advection of clouds over the terrain or where clouds are forced to rise over mountains are most suitable. Fog formed on the ocean surface, or nocturnal radiation fogs in low-lying areas normally lack sufficient liquid water content or sufficient wind speeds for substantial water collection.
- A number of meteorological and geographic considerations are important in choosing a site: predominant wind direction, clouds forming below the maximum terrain height, sufficient space for the fog collectors, and no major terrain obstacles. In the case of coastal cloud decks, the mountain range should be within 5 or 10 km of the coast.
Considerations
- Frequent fogs are needed. Examine meteorological records and consult local people about their observations.
- Relatively small water quantities can be harvested.
- Efficiency of collection improves with larger fog droplets, higher wind speeds, and narrower collection fibers / mesh width. In addition, the mesh should have good drainage characteristics.
- A polypropylene mesh has a lifetime of about ten years. In Nepal, operation and maintenance is difficult due to the unavailability of spare parts (mainly polypropylene mesh). Hence keeping stock of mesh and other spare parts is highly recommended.
