La captación de aguas lluvias de los techos

A rainwater capture system from a small household. Photo: Jalvardhini Pratishthan.

Rainwater harvesting refers to structures like homes or schools, which catch rainwater and store it in underground or above-ground tanks for later use. One way to collect water is rooftop rainwater harvesting, where any suitable roof surface — tiles, metal sheets, plastics, but not grass or palm leaf — can be used to intercept the flow of rainwater in combination with gutters and downpipes (made from wood, bamboo, galvanized iron, or PVC) to provide a household with high-quality drinking water. A rooftop 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. Rainwater harvesting systems have been used since antiquity, and examples abound in all the great civilizations throughout history.

Introduction

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.

The good thing about rainwater is that it falls on your own roof, and is almost always of excellent quality. Several studies have shown that water from well-maintained and covered rooftop tanks generally meets drinking water quality standards. 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. 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. This convenience is available at every house on which rain falls, whether on a mountaintop or an island in a salty sea.

Another option is to use water from different sources. Water that 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.

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.

Advantages	Disadvantages
- 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

Resilience to changes in the environment

Drought

Effects of drought: Water storage used up.
Underlying causes of effects: Lack of rainfall; Leaking linings due to bad construction; Storage not sufficient for demand – tanks are too expensive for volumes of water to outlast extended dry periods.
To increase resiliency of WASH system: Promote smaller tank structures so they are more manageable to construct and cover, while being more affordable to families; Reduce seepage due to poor construction & siting; Follow proper concreting guidelines (see drought effects on cement, below); Make tanks from cheaper lower quality materials and repair more often; Design the outlet of the tank so that there is no dead storage; Ensure the catchment itself is efficient (e.g. gutters); Improve access to micro-finance; Support the capacity of the government or private sector to be able to provide (for payment) a tankering scheme.

Drought effects on cement tanks

Effects of drought: Badly made concrete and cracked linings (e.g. in tanks, dams, waterways, wells, and other structures).
Underlying causes of effects: Less water used for curing; Impure water used for mixing.
To increase resiliency of WASH system: Ensure adequate mixing, ratios, purity of ingredients; Minimize water content in mixture; Ensure adequate curing.

More information on managing drought: Resilient WASH systems in drought-prone areas.
Making cement in regards to drought: Concrete production and drought.

Construction, operations & maintenance

Rooftop catchment. Drawing: WHO.

Catchment & storage tanks

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. 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.

If using storage tanks, structures made with ferrocement or brick-cement are the best and cheapest options, 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, ferrocement 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. It is advisable to first construct a small tank before attempting a large one. Storage tanks can additionally be filled up using pumps. 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.

Keeping the water clean

Roof rainwater is usually of good quality and does not require treatment before consumption. If the house has a chimney, however, it is 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. A concrete lid protects the tank from pollution. Small fishes can be kept in the tank to keep it free from insects.

A foul-flush device or detachable down-pipe can be fitted that allows the first 20 litres of runoff from a storm to be diverted from the storage tanks. This is because runoff is contaminated with dust, leaves, insects and bird droppings. To prevent the use of dirty water, the runoff is then led through a small filter of gravel, sand and charcoal before entering the storage tank, or a filter is placed between the catchment structure and the storage tank. Where there is no foul-flush device, the user or caretaker has to divert away the first 20 litres at the start of every rainstorm.

The EMAS filtration system

Two houses are connected to a rainwater capture unit, then a spout provided from the tank. Photo: Insieme Si Puo' in Africa

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

The system should be also checked and cleaned after every dry period of more than one month. The outsides of metal tanks may need to be painted about once a year. Leaks have to be repaired throughout the year, especially from leaking tanks and taps, as they present health risks. Chlorination of the water may be necessary.

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 rightfully.

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.

Shared roofs

Operation and maintenance (O&M) of shared roofs have more challenges. Rooftop-harvesting systems at schools, for instance, may lose water from taps left dripping. Padlocks are often needed to ensure careful control over the water supply. Ideally, one person should be responsible for overseeing the regular cleaning and occasional repair of the system, control of water use, etc. One option is to sell the water, which ensures income for O&M and for organizing water use. Where households have installed a communal system (e.g. where several roofs are connected to one tank), the users may want to establish a water committee to manage O&M activities. The activities may include collecting fees, and controlling the caretaker’s work and the water used by each family. External agents can play a role in the following O&M areas:
— monitoring the condition of the system and the water quality;
— providing access to credit facilities for buying or replacing a system;
— training users/caretakers for management and O&M;
— training local craftsmen to carry out larger repairs.

Chart: WHO. ¹

Potential problems

corrosion of metal roofs, gutters, etc.;
the foul-flush diverter fails because maintenance was neglected;
taps leak at the reservoir and there are problems with the handpumps;
contamination of uncovered tanks, especially where water is abstracted with a rope and bucket;
unprotected tanks may provide a breeding place for mosquitoes, which may increase the danger of vector-borne disease;
system may not fulfill drinking-water needs, during certain periods of the year, making it necessary to develop other sources or to go back to traditional sources temporarily;
financial investment needed is not affordable - households or communities cannot afford to construct a suitable tank and adequate roofing.

Costs

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.

In Southern Africa, US$ 320 for a system with 11 m of galvanized iron gutter; a 1.3 m3 galvanized iron tank; downpiping; tap and filters; cost does not include transportation. Where roofs are not suitable for water harvesting, the cost of roof improvement and gutters will have to be added to the cost of a tank. Such costs varied from US$ 4 per m2 (Kenya, subsidized) to US$ 12 per m2. ¹

Field 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.

Akvo RSR projects

The following projects utilize rooftop rainwater harvesting.


RSR Project 790 WaSH program in Rural Bangladesh	RSR Project 440 Raising awareness on rainwater harvesting	RSR Project 2618 Rainwater for Green Schools Initiative	RSR Project 107 Rainwater harvesting in Guinee Bissau