Introduction to DRWH Technology
Rainwater harvesting has been a traditional practice in some cultures for centuries and as such many technologies are the result of a long evolutionary process. Even the current resurgence of interest in rainwater harvesting as a source of water supply has been in existence for over twenty-five years (more in some countries such as Australia). As a result a number of technologies have become accepted into the norms of rainwater harvesting practice. Some of these are modifications of designs that go back into antiquity while others have been developed more recently.
There is also a drive to improve and innovate. Developed countries such as Germany have recognised domestic rainwater harvesting as a solution to the problems of overexploitation of water resources and have been working apace to implement user-friendly, reliable and high quality systems in a cost-effective manner. The developing world’s need for inexpensive and sustainable water supply technologies is also being addressed (albeit more slowly), however, most research has been within the "institutional" sector which has resulted in most innovation being confined to large-scale/high-reliability systems rather than types poor householders can afford.
This section will outline current practice in rainwater harvesting technology, breaking the system down into components, describing the theoretical requirements of each component and the types of solutions currently employed. Techniques that have application in very low cost (VLC) systems have been given particular emphasis and those that have not as yet been so applied are discussed in terms of their shortcomings or potential for this application.
Rainwater harvesting systems can roughly be broken down into 4 primary processes and 3 treatment processes. These are outlined in Figure 1:
Figure 1: Process diagram of domestic rainwater harvesting systems
There is a considerable range in complexity of domestic rainwater harvesting systems, from simple opportunistic practice where the catchment may be a tree, the conveyance a banana leaf and the storage and delivery an earthenware pot, to highly sophisticated systems with automatic treatment at each stage of the process, electronic monitoring and dual reticulation systems.
A typical VLC rainwater harvesting system in developing countries is illustrated in Figure 2. The catchment is a roof, usually made of an impervious material such as corrugated galvanised iron sheet, the conveyance is by guttering and downpipe, the storage is a tank, and delivery is by a tap connected to the tank. Installed treatment includes a manual "first flush" system and a before-tank filter. There are also a number of processes that occur within the tank itself such as settlement, floatation and pathogen die-off. Finally, the household may employ some technique of post-storage disinfection such as chlorination, solar disinfection or a candle filter.
Figure 2 Typical Very Low Cost Roofwater Harvesting System in a Developing Country