Difference between revisions of "Water Portal / Rainwater Harvesting / Rooftop rainwater harvesting"
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* [http://www.unep.org/depi/rainwater Rainwater Partnership ] | * [http://www.unep.org/depi/rainwater Rainwater Partnership ] | ||
* [http://www.rainwater-toolkit.net/ Rainwater Toolkit] | * [http://www.rainwater-toolkit.net/ Rainwater Toolkit] | ||
| + | * [[Solution_of_the_week_5|Akvo solution of the week 5]] | ||
Revision as of 10:39, 6 September 2010
Rainwater harvesting refers to structures which catch rainwater and store it in underground or above-ground tanks for later use. Any suitable roof surface — tiles, metal sheets, plastics, but not grass or palm leaf — can be used to intercept the flow of rainwater and provide a household with high-quality drinking water. Rainwater harvesting systems have been used since antiquity, and examples abound in all the great civilizations throughout history.
In many cases, groundwater or surface water may be unavailable for drinking water. The groundwater level may be too deep, groundwater may be contaminated with minerals and chemicals such as arsenic or salt, surface water may be contaminated with faeces or chemicals. In these cases, rainwater harvesting can be an effective and low-cost solution. Several studies have shown that water from well-maintained and covered rooftop tanks generally meets drinking water quality standards.
Another option is to use water from different sources. Water which is salty or has arsenic might still be good enough for washing and sanitary purposes. High-quality rainwater, caught and stored in a tank can then be used for drinking and cooking.
History and social context
Rainwater falls on your own roof, and is almost always of excellent quality. It enables households as well as community buildings, schools and clinics to manage their own water supply for drinking water, domestic use, and income generating activities. It provides the luxury of “water without walking”, relieving the burden of water carrying, particularly for women and children. This convenience is available at every house on which rain falls, whether on a mountain top or an island in a salt sea.
A rainwater harvesting system might be a 500 cubic meter underground storage tank, serving a whole community, or it might be just a bucket, standing underneath a roof without a gutter. Each 20 litre container of clean water might save a kilometers long walk to the nearest source of clean water, and as fetching water on cold, wet and slippery days is particularly unpleasant, even this small yield is highly valued. In Uganda and Sri Lanka, rainwater is traditionally collected from trees, using banana leaves or stems as temporary gutters.
Suitable conditions
Rainwater harvesting requires at least an annual rainfall of 100-200 mm. Many places in Latin America have rainfalls of about 500 millimeters per year.
It is suitable even when the roof is small. For example a 5 x 6 meters (that is to say 30 square meters) house, with 500 mm annual precipitation, receives a rainfall of 15.000 liters on its roof; this is a sufficient amount for a family formed by 5 members.
Storage tanks can additionally be filled up by pumps.
| Advantages | Disadvantages/limitations |
|---|---|
| - Possible in almost any climate - Rainwater generally meets drinking water quality standards, if system is well-designed and maintained |
- Storage is needed to bridge dry periods |
Technical specification
Storage tanks
Generally, structures made with ferrocement, or brick-cement are the best and cheapest option, and they can be made locally. When a water tank is below ground, it is called a cistern. Among the different storage types are the underground tank, ferro-cement tank, plastic-lined tank, etc. The size of the tank is a compromise between cost, the volume of water used, the length of the dry season, etc.
The cheapest storage of all is to use the ground as storage area, a technique called groundwater recharge. It is accomplished by letting rainwater infiltrate in the ground. The recharge will locally lead to a higher water table, from which water can be pumped up when needed. Whether the infiltrated water raises the water table in a local area or is spread across a wider area depends on soil conditions.
Operation
The flow of water can be intercepted in different ways. Different catchment types are used, such as roof catchment, paved surface catchment, surface catchment and riverbed catchment. Once the water has been intercepted, it is led to a storage tank. Usually, a filter is placed between the catchment structure and the storage tank.
Roof rainwater is usually of good quality and does not require treatment before consumption. If the house has a chimney, it is however possible that the water becomes smoky. High chimneys are therefore preferred. Water is collected through roof gutters made of PVC, bamboo, etc. and stored. The most important thing to ensure water quality is a good lid, keeping out light and insects, and a filter, keeping out all kinds of dirt.
Basic water quality testing is recommended during the first year, with further testing when water quality is in doubt. A low cost water test is the ‘HACH’ test, about US$1 per test. If contamination is suspected or when water quality needs to be guaranteed, the water can be treated in several ways.
Several pump systems can be used to lift the water from underground tanks, for example with a rope pump or with a deep well pump, which can elevate water up to a height of 30 m.
EMAS system
The EMAS system for rainfall collection uses various EMAS technologies as well as simple tools to convert rainwater into usable drinking water. If roof rainwater is being used, it is collected through a regular gutter. To filter the water, at the bottom of the gutter, a pitcher or ferrocement tank is placed, with an outlet pipe. A synthetic cloth bag is attached to the rim of the pitcher using an iron ring or wire which fits around the edge. The bag should be cleaned every 3 months.
As water begins to collect, to avoid too much garbage collecting here, first some amount of water is deflected, along with most of the garbage. Hereafter, water can be directly sent to an EMAS Cistern . It is advisable for multiple cisterns to be available for storage, depending on the size of the roof. Connect one cistern at a time to the outlet pipe. From here water can be pumped and distributed using a regular EMAS pump. The pump can also be connected to faucets and tanks around the house.
Maintenance
Removal of debris and overhanging vegetation from gutters and the roof is important to prevent the gutter being clogged. Tank maintenance consists of physical inspection and repairing cracks with cement.
Several studies have shown that water from well-maintained and covered rooftop tanks generally meets drinking water quality standards if maintained rightfuly.
Manufacture
It is advisable to first construct a small tank before attempting a large one. A concrete lid protects the tank from pollution. Small fishes can be kept in the tank to keep it free from insects.
Cost
Comparison of costs
- Brick cement tank of 6 m3: 3 bags of cement, 300 bricks, 3 kg of wire US$ 40
- Brick cement tank of 1 m3: 1 bag of cement, 100 bricks, 1 kg of wire US$ 20
- Plastic-lined tank of 5 m3: US$ 50
- Sub-surface ferro-cement tank of 60 m3: US$ 1,900
The bigger the volume of the storage tank, the lower the material demand (and thus costs) for construction per m3 of tank volume.
Country experiences
Rainwater harvesting is a technology which is extremely flexible and adaptable to a wide variety of settings, it is used in the richest and poorest societies on the planet, and in the wettest and driest regions of the world.
In Ocara, Brazil, rainwater tanks have been constructed of concrete blocks.
A low-cost option is the brick cement tank, used in for example Nicaragua and Ghana.
Manuals
- Download the book "Roofwater Harvesting: A Handbook for Practitioners" from the IRC website
- Booklet Smart Water Harvesting Solutions
Movies
- Rainwater Harvesting Nepal, by BSP-Nepal
- Rooftop Rainwater harvesting India, by Zenrainman, [1], documenting the Sachetana programme of the government of Karnataka, India.
- Indian movie by CSE (www.cseindia.org) promoting use of rainwater harvesting
External links
- Rainwater Harvesting Implementation Network (RAIN)
- Rainwater Harvesting information on Practical Action
- www.rainwaterharvesting.org, Indian website on rainwater harvesting
- Wikipedia article on rainwater harvesting
- www.eng.warwick.ac.uk/dtu/rwh Rainwater Harvesting info on the DTU unit of University of Warwick
- Rainwater Partnership
- Rainwater Toolkit
- Akvo solution of the week 5
