A sufficient, safe drinking water supply is essential to life. However, millions of people throughout the world still do not have access to this basic necessity. After decades of work by governments and organisations t o bring potable water to the poorer people of the world, the situation is still dire. The reasons are many and varied , but generally speaking, the poor of the world cannot afford the capital intensive and technically complex traditional water supply systems. Unfortunately these technologies are widely promoted by governments and agencies throughout the world. Rainwater harvesting (RWH) is an alternative to these unaffordable options. It has been adopted in many areas of the world where conventional water s upply systems have not been provided, too expensive or failed to meet people’s needs. RWH is a proven technology that has been in use since ancient times. Examples of RWH systems can be found throughout history. In industrialised countries, sophisticated RWH systems have been developed to reduce water bills or to meet the needs of remote communities or individual households in arid regions. RWH is also used in developing countries. In Uganda and Sri Lanka, for example, rainwater is traditionally collected from trees, using banana leaves or stems as temporary gutters . Up to 200 litres may be collected in this way from a large tree in a single storm. Many individuals and groups have taken the initiative and developed a wide variety of RWH systems throughou t the world. Many kinds of rainwater harvesting are practised throughout the world. Basically RWH may be divided into two types:
Domestic RWH RWH for agriculture, erosion control, flood control and aquifer replenishment
Domestic RWH is a simple mechanism to collect and store rainwater mainly for drinking and cooking. It may be household based or community based. The system uses a collection surface such as a roof, gutters to guide the rainwater , and a container to store the water. Larger RWH systems are used for water resource management. These systems use vast catchment areas to collect rainwater and store it in reservoirs. The water is then used for irrigation or to recharge aquifers. These systems may also help in flood control and erosion prevention by holding storm water into reservoirs and discharging at a controlled rate. This paper involves domestic RWH only. We must remember that rainwater harvesting is not the ultimate answer to household water problems. Many factors have to be considered when selecting the appropriate water source. These include cost, climate, hydrology, social and political elements, as well as technology . All of these play a role in making the final choice of a suitable water supply scheme. RWH is only one of many possible choices. But RWH is often overlooked by planners, engineers and builders. The reason that RWH is rarely considered is often due to barriers such as lack of technical and other information. In many areas where RWH has been introduced as a part of drinking water supply options, it was at first un-popular. This was simply because little was known about the technology by the beneficiaries. In most of these cases, the technology has quickly gained popularity. The users soon realised the benefits of a clean, reliable water source at the home. This is especially true in areas where the town supply is unreliable or where local water sources dry up for a part of the year. In many cases RWH has also been introduced as a part
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of an integrated water supply system. It is a technology that is flexible and adaptable to a very wide variety of conditions. RWH is used in the richest and the poorest societies on our planet, and in the wettest and the driest regions of the world.
Components of a domestic RWH system
Domestic RWH systems (DRWH) vary in complexity. Some of the traditional Sri Lankan systems are no more than a pot situated under a piece of cloth or a plastic sheet tied to four poles. The cloth captures the water and diverts it through a hole in its centre into the pot. In contrast, some of the sophisticated systems manufactured in Germany incorporate clever computer management systems, submersible pumps, and links to grey water and domestic plumbing system mains. Somewhere between these two extremes , we find the typical DRWH system in use in developing countries. Such a system will usually comprise a collection surface (a clean roof or ground area), a storage tank, and guttering to transport the water from the roof to the storage tank. Other peripheral equip ment is sometimes incorporated, for example: first-flush systems to divert the dirty water which contains roof debris after prolonged dry periods; filtration equipment and settling chambers to remove debris and contaminants before water enters the storage tank or cistern; handpumps for water extraction; water level indicators, etc.
Typical domestic RWH systems
A typical domestic RWH consists of a collection surface, gutters and a storage container. In addition, there are options for diverting first -flush water and filtration.
For domestic rainwater harvesting the most common surface for collection is the roof of the dwelling. Many other surfaces can be, and are, used: courtyards, threshing areas, paved walking areas, plastic sheeting, trees, etc. In some cases, as in Gibraltar, large rock surfaces are used to collect water which is then stored in large tanks at the base of the rock slopes. The style, construction and material of the roof affect its suitability as a Figure 1: A typical corrugated iron sheet roof, collection surface for water. Typical showing guttering. Photo: Practical Action materials for roofing include corrugated iron sheet (also known as tin roof), asbestos sheet; tiles (a wide variety is found), slate, and thatch (from a variety of organic materials). Most thatch are suitable for collection of rainwater, but only certain types of grasses e.g. coconut and anahaw palm (Gould and Nissen Peterson, 1999), thatched tightly, provide a surface adequate for high quality water collection. The rapid move towards the use of corrugated iron sheets in many developing countries favours the promotion of RWH.
Guttering is used to transport rainwater from the roof to the storage vessel. Guttering comes in a wide variety of shapes and forms, ranging from the factory made PVC type to home -made guttering using bamboo or folded metal sheet (Figure 1). Guttering is usually fixed to the building just below the roof and catches the water as it falls from the roof. Some common types of guttering and fixings are shown in Figure 2:
Manufacture of low-cost gutters
Factory-made gutters are usually expensive and beyond the reach of the poor people of developing countries, if indeed available at all in the local marketplace. They are seldom used for very low-cost systems. The alternative is to make gutters from materials that can be found cheaply in the locality. There are a number of techniques that have been developed to help meet this demand; one such technique is described below. V- shaped gutters from galvanised steel sheet can be made simply by cutting and folding flat galvanised steel sheet (Figure 3a). Such sheet is readily available in most market centres (otherwise corrugated iron sheet can be beaten flat) and can be worked with tools that are commonly found in a modestly equipped workshop. One simple technique is to clamp the cut sheet between two lengths of straight timber and then to fold the sheet along the edge of the wood. A strengthening edge can be added by folding the sheet through 90o and then completing the edge with a hammer on a hard flat surface. The better the grade of steel sheet that is used, the more durable and durable the product. Fitting a downpipe to V-shaped guttering can be problematic and the V-shaped guttering will often be continued to the tank rather than changing to the customary circular pipe section downpipe. Methods for fixing gutters are shown in Figure 3a. Plastic pipes may be cut into half to make gutters (Figure 3b). This requires only a saw and some clamps to fix the half-pipes to roofs. It may be made quickly and cheaply in areas where plastic pipes are available.
Figure 2: a variety of guttering types showing possible fixings
Figure 3a: folding galvanised steel sheet to make V-shaped guttering
Storage tanks and cisterns
The water storage tank usually represents the biggest capital investment element of a domestic RWH system. It therefore requires careful design to provide optimal stor age capacity while keeping the cost as low as possible. The catchment area is usually the existing rooftop or occasionally a cleaned area of ground, as seen in the courtyard collection systems in China. The guttering for the system can often be obtained relatively cheaply, or can be manufactured locally. There are an almost unlimited number of options for storing water. Common vessels used for very small-scale water storage in developing countries include plastic bowls and buckets, jerrycans, clay or ceramic jars, cement jars, old oil drums, empty food containers, etc. For storing larger quantities of water, the system will require a tank or a cistern. For the purpose of this document, we will classify the tank as an above-ground storage vessel and the cistern as a below-ground storage vessel. These can vary in size from a cubic metre or so (1000 litres) up to hundreds of cubic metres for large projects . The typical maximum size for a domestic system is 20 or 30 cubic metres. The choice of system will depend on a number of technical and economic considerations listed below. Space availability Options available locally Local traditions for water storage Cost of purchasing new tank Cost of materials and labour for construction Materials and skills available locally Ground conditions Use of RWH – whether the system will provide Figure 3b: Cutting plastic pipe into half to make gutter total or partial water supply One of the main choices will be whether to use a tank or a cistern. Both tanks and cisterns have their advantages and disadvantages. Table 1 summarises the pros and cons of each:
Above ground structure allows easy inspection for leakages Many existing designs to choose from Can be easily purchased ‘off-the-shelf’ Can be manufactured from a wide variety of materials Easy to construct from traditional materials Water extraction can be by gravity in many cases Can be raised above ground level to increase water pressure Require space Generally more expensive More easily damaged
Generally cheaper due to lower material requirements Not vulnerable to water loss by tap left open Require little or no space above ground Unobtrusive Surrounding ground gives support allowing lower wall thickness, and thus lower costs
Water extraction is more problematic, often requiring a pump Leaks are more difficult to detect
Prone to attack from weather Failure can be dangerous
Contamination of the cistern from groundwater is more common Tree roots can damage the structure There is danger to children and small animals if the cistern is left uncovered Flotation of the cistern may occur if groundwater level is high and the cistern is empty. Heavy vehicles driving over a cistern can cause damage
Figure 4a: An owner-built brick tank in Sri Lanka
Figure 4b: A corrugated iron RWH tank in Uganda
Much work has been carried out to develop the ideal domestic RWH tank. The photographs (Figures 4-6) illustrate the variety of tanks that have been built in different parts of the world.
Figure 5 : Ferrocement tank in Ruganzu Village, Tanzania. Photo credit: DTU
Figure 6 : Small jars used in Cambodia as part of a multi-sourced water supply.
Photo Credit: DTU
Debris, dirt, dust and droppings will collect on the roof of a building or other collection area. When the first rains arrive, this unwanted matter will be washed into the tank. This will cause contamination of the water and the quality will be reduced. Many RWH systems therefore incorporate a system for diverting this ‘first flush’ water so that it does not enter the storage tank. The simpler ideas are based on a manually operated arrangement whereby the inlet pipe is moved away from the
5 Figure 7a: Tipping gutter mechanism
By Dia Rees, updated by Shafiul Ahmed, Published by Practical Action on 05/02/08
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