RAINWATER RAINWATER HARVESTING DURING RECONSRUCTION
Many millions of people in the developing world do not have access to a clean, sufficient supply of water, a resource essential to almost all aspects of life. Poor people often lack the capital or technology to construct efficient water-supply systems, and make do with communal wells or rivers that are easily polluted. This situation is only compounded by disasters, which can displace populations away from infrastructure, and lead cramped, unhygienic conditions. The concept of rainwater harvesting technology (RWH) is being developed as an affordable alternative to traditional water supply systems, helping to give people access to a naturally clean resource. A PCR process attempts to improve peoples’ resilience, offering the opportunity to increase independence and participation; the planning and use of RWH within the reconstruction process from an early stage could allow people to develop and manage their own water supplies. Whilst it is unlikely that RWH can provide all necessary water in many cases (particularly in arid areas that receive little rainfall), its use can at least alleviate the harshest effects of insufficient water supply given the right conditions. This brief will look at the basic components and functions of a RWH system, and give details of real-world examples in practice. The relation of these different RWH technologies to the various stages of the PCR process will be developed, assessing what solutions are practical in which context. Please see Practical Action’s technical brief Rainwater Harvesting for original information. RWH technologies can generally be divided into two categories: Domestic Agricultural, erosion control, flood control (larger-scale)
Domestic RWH Domestic
Domestic systems can be household or community based; almost all consist of a collection surface, a channelling device (gutter) and a storage facility. Additional filtration and first flush devices can also be incorporated, along with post-storage water treatments described in Water Treatment during Reconstruction. Collection Collection Surface Guttering Guttering Storage Storage Facility PostPost Post-storage Treatment
Filtration Filtration and first-flush systems
Figure 1: Schematic of RWH System, Illustration: Martin Bounds
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Rainwater Harvesting for Reconstruction
Collection Surfaces Collection The most common form of collection surface is the corrugated iron roof; other surfaces such as tiles or thatch are also used, but are more difficult to clean and maintain. Regular cleaning of the collection surface can prevent biological contamination or pollution of the rainwater as it lands, and also reduces the workload on any filtration devices that are fitted. Other collection surfaces can include paved walking areas and rock surfaces, particularly where underground storage facilities are used.
Figure 2: Typical Corrugated Iron Roof, Photo: Practical Action
Guttering In most cases, factory-made gutters are too expensive to be considered in developing scenarios and especially in PCR contexts where local materials are preferable. Local timber can be used, but it is likely to degrade quickly, so should only be considered as a short-term measure. More durable is to use corrugated iron or steel sheet. A common technique is to use a clamp and two lengths of straight timber to hold a steel or iron sheet, which can then be bent into a V-shape. Alternatively, plastic piping can be cut in half lengthways where it is available. It is often difficult to attach these gutters to downpipes, so they may be run straight into storage facilities if this is the case. The figures below demonstrate the technique described and some pictures of common gutter options.
Figure 3: Simple gutter-making process Illustration: Martin Bounds (based on Practical Action technical brief ‘Rainwater Harvesting’ Ahmed, S. (2008) Figure 4: Simple gutter fixing options Illustration: Martin Bounds (based on Practical Action technical brief ‘Rainwater Harvesting’ Ahmed, S. (2008)
Folded steel sheet
Half plastic piping
Warwick University’s Development Technology Unit (DTU) has produced a comprehensive guide to gutter design for RWH applications available here.
Rainwater Harvesting for Reconstruction
Filtration Systems and First-Flush Filtration FirstIt is often necessary to include filtration and ‘first-flush’ components in a RWH system, where sporadic rainfall washes debris that has collected on a roof into the guttering system. If this debris is allowed into the storage facility, it may contaminate the water in there. Filtration may consist of a simple sand/charcoal/stone bed, which will filter efficiently but slowly due to the percolation rate through the sand. A simpler method may use a cloth or mesh to catch larger debris but prevent overflow. More complex systems have been developed by manufacturers and are in common use in developed countries; an example is WISY, a German manufacturer that has created a downpipe filter that is approximately 90% efficient. The website can be found here. A similar solution is provided by Guttermate in the UK, which is demonstrated in Figure 5. Many of these solutions are too expensive to be considered in the PCR context, and simple techniques as described may have to be used.
Figure 5: ‘Guttermate’ downpipe filter, Source: www.guttermate.co.uk
A first flush system can be incorporated to ensure that large amounts of debris do not block a filter or enter a storage tank after an initial rainfall following a dry period. An example is shown below, whereby a floating ball in a pre-storage tank will rise to the surface and eventually seal, diverting water directly into the storage facility. It may be more practical in a PCR context to choose a more simple option, as it will be easier to maintain and duplicate for large displaced populations. Post-storage treatment can provide additional filtration to the stored water and ensure it is of a safe level for consumption.
Figure 6: Ball First-Flush concept Illustration: Ahmed, S. (from Practical Action technical brief ‘Rainwater Harvesting’ (2008)
Rainwater Harvesting for Reconstruction
Storage Facility Storage The storage facility is usually the most expensive part of the RWH system, requiring the most construction and capital input. There are many types of storage, such as small clay jars or buckets, but larger vessels can generally be classified into tanks (above ground) and cisterns (below ground). Table 1 below describes some of the advantages and disadvantages of tanks and cisterns.
Pros: - Above ground structure allows easy inspection for leakages. - Easy manufacture from a wide variety of materials. - Simple water extraction through gravity, and natural water pressure. Cons: - Generally more expensive. - Requires space above ground. - Can be damaged more easily.
Table 1: Tank & Cistern Comparison
Pros: - Generally cheaper, lower material requirements. - Less vulnerable to water loss. - Lower requirement for space above ground. - Unobtrusive.
Cons: - Water extraction often requires pumping, - Leaks are more difficult to detect when they do occur.
The choice of solution will normally depend on the following considerations in a PCR context: Space availability Materials & skills available locally – does a market exist or can one be created? Local traditions for water storage Cost of purchase/construction – investment vs. payback. Ground conditions Use of RWH – whether the system will provide total or partial water supply Sizing of the solution must also be considered, based either on a demand or supply based approach. Details of this are covered in the original technical brief. Additionally, further information is available from the DTU here. Basic Basic Solutions In situations where local resources are limited, and the population may not be able to afford materials to construct advanced gutters or storage facilities, an innovative approach is often required: a common material obtained by transient populations is plastic sheeting (Burt, M. and Keiru, B. 2009), which can be used to divert rainwater into buckets or pots. The sheeting is commonly used for shelter protection as well, providing a transportable, adaptable and ultra-low cost solution. An example is shown below: More information on ultra-low cost rainwater harvesting techniques is available from the WEDC published paper Innovative rainwater harvesting techniques for emergencies: Lessons from the field.
Figure 7: Basic RWH technqiue Source: Burt, M. & Keiru, B. (2009)
Rainwater Harvesting for Reconstruction
Rainwater Quality & Health Rainwater There are two main issues when looking at the quality and health aspects of domestic RWH; firstly, there is the issue of bacteriological water quality. Rainwater can become contaminated by faeces entering the tank from the catchment area. It is advised that the catchment surface always be kept clean. Rainwater tanks should be designed to protect the water from contamination by leaves, dust, insects, vermin, and other industrial or agricultural pollutants. Tanks should be sited away from trees, with good-fitting lids and kept in good condition. Incoming water should be filtered or screened, or allowed to settle to take out foreign matter (as described in a previous section). Water which is relatively clean on entry to the tank will usually improve in quality if allowed to sit for some time inside the tank. Bacteria entering the tank will die off rapidly if the water is clean. Algae will grow inside a tank if sufficient sunlight is available for photosynthesis. Keeping a tank dark and sited in a shady spot will prevent algae growth and also keep the water cool. The area surrounding a RWH should be kept in good sanitary condition, fenced off to prevent animals fouling the area or children playing around the tank. Any pools of water gathering around the tank should be drained. Secondly, there is a need to prevent insect vectors from breeding inside the tank. In areas where malaria is present, mosquito breeding in the storage tank can cause a major problem. All tanks should be sealed to prevent insects from entering. Mosquito proof screens should be fitted to all openings. Some practitioners recommend the use of 1 to 2 teaspoons of household kerosene in a tank of water which provides a film to prevent mosquitoes settling on the water.
Figure 8: Covered water storage in Uganda Photo: Practical Action
Domestic RWH in a PCR Context How does RWH relate to the three stages of the reconstruction process and the principles of PCR? Rainwater harvesting cannot be relied upon in all emergency situations, but in cases where rainfall is present every effort should be made to capture it, as it provides a clean, safe source at low cost (UNHCR, 2007). Methods in an emergency stage are usually basic, most commonly consisting of plastic sheeting directing water to buckets; the resources and time required to implement more advanced systems are generally not available until later on in the reconstruction process. Transitional shelters afford more opportunity to capture rainfall, generally being occupied for longer periods. The construction of simple pots with sheeting as a catchment is a mobile solution that doesn’t require much capital or labour investment. However, the ability to capture fresh rainwater and to manage its use over periods of time can be developed to a much greater extent in the permanent housing stage of reconstruction; a key aim of PCR is to decentralise infrastructure and to give people more independence over controlling their water supply. Individual housing units that are permanent constructions have much more opportunity to invest in more complex harvest systems, with more time to experience the benefits and to understand and adapt to changing rainfall patterns. The greater space that comes with individual dwellings allows for the construction of tanks or cisterns, and guttering can be added to well-constructed permanent houses. The construction of harvest systems has potential to open up local markets, improving living conditions and livelihoods at the same time, whilst also reducing vulnerability to potential future droughts.
By Martin Bounds, Published by Practical Action on 11/18/11
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