Water Portal / Rainwater Harvesting / Rooftop rainwater harvesting

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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.

A women using her water storage tank in Guinnee-Bissau. Photo: Paul Akkerman

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. Storage tanks can also 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

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 catchement structure and the storage tank.

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

Roof rainwater is usually of good quality and does not require treatment before consumption. 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.

Maintenance

Removal of debris and overhanging vegetation from gutters and the roof, and preventing water from stagnating in the gutters. 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

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

Movies


External links

References

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