Rainwater harvesting/Rainwater harvesting system

From Wikiversity
Jump to navigation Jump to search

A rainwater harvesting system is an integrated group of elements that work together to provide collected rainwater ready for utilization. A Rainwater Harvesting System collects, diverts and stores water from rain events for later reuse. A rainwater harvesting system utilizes building infrastructure surfaces, such as roofs, paving materials and exterior walls to divert atmospheric water and store the water in underground or above-ground tanks for later use. These systems require a cohesive design process in order to ensure that water can be supplied of the appropriate quantity and quality as anticipated. 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 stored water, that along with appropriate filtration and/or disinfection, can be used for potable and non-potable end-uses. 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.


[edit | edit source]

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 feces 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.

Suitable Conditions

[edit | edit source]

Rainwater harvesting can be accomplished in areas with remarkably little amounts of rainfall or environments with order of magnitude greater amounts of rainfall. Considering both the supply and demand will assist a designer in sizing certain elements of a rainwater harvesting system. Numerous rainwater tank/cistern sizing calculators exist to meet the desires of both commercial, educational and private interests.

Elements of a Rainwater Harvesting System

[edit | edit source]

Collection and Catchment Surface

[edit | edit source]

The surface for collecting water is typically the roof of a building, but other surfaces and areas can used as well. Other types of area may require a greater measure of treatment.

Depending on how the rainwater collection system is developed into a site or building, it may be difficult or impossible for a designer to choose a particular surface for the system. An important step in the process is assessing the benefits and risks of using different materials for the system.

In general, typical roofing surfaces include:

  • Metal (steel: galvanized, painted, stainless, terne-coated, aluminum and copper)
  • Membrane: modified bitumen, ethylene propylene diene monomer rubber (EPDM), polyvinyl chloride (PVC), polyester (occasionally with insulation and aggregate ballast)
  • Asphalt/fiberglass shingles (usually fine aggregate surface)
  • Wood shingles
  • Roll roofing
  • Builtup roofing
  • Slate
  • Tiles: clay and concrete
  • Glass/plastic/fiberglass panels
  • Green roof: typically membrane-covered with growing plants
  • Mixed material of an occupied roof area (common in dense urban areas)

All materials and subcomponents of a roofing material that come into contact with rainwater must be considered as well. For example "asphalt shingles," "clay tiles" and "painted steel" are used to describe the collection surface, supplementary material must be considered. Underlayments (water-resistant or waterproof barrier material installed directly on roofing deck), flashings (thin pieces of metal for weatherproofing joints and cracks) and even fasteners are important. Due to erosion, chemical decomposition, and other degradation can all cause roofing material, mineral grit from asphalt shingles, clay tile flakings and paint flecks to be washed off the roof and into the rainwater.

Other supplementary equipment and devices installed on collection surfaces, such as heating and ventilation air conditioning units (HVAC), enclosures, gas lines, exhaust fans and chimneys can all effect the quality of the rainwater. Different materials, local weather and pollution levels interact with and influence the physical and chemical reactions these materials can have with rainwater. Proximity to certain human activities such as agricultural land, manufacturing and heavy construction can play a role in the quality of incoming rainwater.

Depending on whether potable or nonpotable water is desired, and the availability of treatment, will effect the how much thought needs to be given to certain components. Many nonpotable uses do not require for water to be completely contaminant free.

The surface characteristics and the local ecological and environmental conditions influence the quality of collected rainwater in different ways. Among the different catchment types that can be used, such as roof catchment, paved surface catchment, surface catchment and riverbed catchment. Within roof catchment, one must consider the effects of different roofing surfaces on potential challenges year-round and depending on the frequency of rain events.

Water Conveyance

[edit | edit source]

If the collection surface is a roof, rainwater is usually directed one of two ways:

1. A sloped roof drains to gutters and downspouts at the outer edges of the building envelope. Scuppers (opening in sidewall of a building), oversized gutters and other methods are utilized for protection against overflow. 2. A roof (especially flat or nearly flat ones), may have area drains that connect to leaders (downspouts). The leader penetrate and enter the roof and flow either directly to the exterior or further down and under the roofing floor to the exterior. Scuppers and unobstructed roof edges can be used for overflow protection.

Either pipes that use gravity or siphonic action convey the water to the storage containers. Siphonic action roof drains utilize the siphonic pressure generated in a drainpipe running full of water to create suction to pull water into the storage area. Siphonic pipes allow two things; 1. Horizontal runs with smaller-diameter pipes 2.nearly level pipes until connecting to vertical pipes.

Internal or external roof drains and gutters can be used to direct the water from the collection surface to the storage container.


[edit | edit source]

The element of a rainwater harvesting system responsible for removing primary contaminant rainwater prior to storage is prefiltration, inlet filtration, pre-storage filtration.

Contaminants that can cause issues in a system include:

  • Leaf litter, branches and other plant matter
  • Bird, rodent, insect and other animal droppings
  • Bird, rodent, insect and other animal carcasses
  • Trash
  • Dirt, leftover construction materials and pollen
  • Natural and artificial pollution particulate
  • Degrading collection surface materials

Often, prefilters, usually gravity flow operated, are installed in a rainwater harvesting system to prevent the introduction and accumulation of large debris and organic material in the storage container. This step is critical for keeping a higher level of rainwater quality, while also reducing maintenance costs and efforts for tanks, equipment and other components to a minimum.

Calming Inlet

[edit | edit source]

A calming inlet is recommended for incoming water to minimize the disturbance of the water with previously accumulated sediment and particulate on the bottom of the tank or cistern. A calming inlet can be located anywhere within the tank and is most commonly a U-shaped fitting that directs water upwards.[1] If a calming inlet is not installed, water pumped or gravity fed out of the tank can contain greater concentrations of pollutants, and can become potentially hazardous both to further system components downstream and human health depending on application.

Storage Containers

[edit | edit source]

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 PVC pump, which can elevate water up to a height of 30 m.

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.

Filtration, Treatment and Disinfection

[edit | edit source]

Depending on the environmental and desired end-use, minimal to no treatment and disinfection is required in the rainwater harvesting system. 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

[edit | edit source]
Rainwater harvesting tank, capturing from roof. The tank is also connected to a piped water supply.

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.


[edit | edit source]

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

[edit | edit source]

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.

Potential problems

[edit | edit source]
  • 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.

Field experiences

[edit | edit source]
  • 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.
[edit | edit source]


[edit | edit source]
[edit | edit source]


[edit | edit source]
  1. Celeste Allen Novak, et al. Designing Rainwater Harvesting Systems : Integrating Rainwater into Building Systems. Hoboken, New Jersey, Wiley, 2014, p. 160.