TECHNOLOGY > RAIN WATER HARVESTING

Rain Water Harvesting

Introduction

Rain water harvesting (RWH) systems can, to some extent, help improve water provision where required and encourage water conservation, thereby reducing the demand on existing water sources.
Another benefit of RWH systems is that the surface runoff during storms, which causes floods and erosion, is reduced, since the rainwater is retained in the storage tanks. Additionally, the delayed release leads to accumulation of ground water.

The technology of RWH can be as complex or as simple as required. Many groups and individuals throughout the world have taken the initiative and developed a wide variety of different solutions. Due to the flexibility and adaptability to a very wide range of conditions, these systems are now being used in the wettest and driest regions and in the richest and poorest societies of the world.
The industrialised world has developed sophisticated RWH systems with the aim of reducing water bills or to meet the needs of remote communities or individual households in arid regions. These countries have recognised RWH as a solution to the problems of overexploitation of water resources and are working on user friendly, reliable and high quality systems in a cost-effective manner.

RWH is also able to address the developing world’s need for an inexpensive and suitable water supply technology, but the adapting and spreading happens more slowly than in the industrialised world.

Technique

A rainwater harvesting system contains three basic sub-systems: a collection area, usually as a roof, a guttering system to convey the water and a storage system. Additional peripheral equipment is or can be included, like a water gauge, filters and first flush devices.

Figure 1: Typical Domestic Roofwater Harvesting System in Kenya

- Catchment area: Most roofs are covered with impervious materials such as corrugated galvanised iron (GI) sheets. The size of the catchment area determines the quantity of the collected rainwater. There is also a close connection to the quality of the water, since the major contamination of rainwater occurs after contact with the catchment surface.

- Conveyance: The guttering system is used to convey the precipitation from the roof to the storage tank. Gutters are often the weak link in the rainwater harvesting system, which not only causes functional failure and less yield, but can also become a health hazard. Since poor installations allow water, leaves and other debris to enter and remain in the gutters, good workmanship and proper maintenance are crucial. This helps to safeguard the water quality and reduces the risk of creating mosquito-breeding places. The cost of the gutters and down-pipes mount up to 5-15% of the total system cost.

- Storage tanks: Storage is necessary, since rain does not always fall when it is required. The water demand and the amount of precipitation determine the required storage capacity for a given reliability. Since the tank is generally the most expensive part of the system, special attention has to be turned on its design and construction. However, rationing schedules and management strategies are essential to save capacity volume and cost of the system.

Construction of a ferro cement water tank.

A complete RWH system where the CGI roof was build especially for rain water harvesting

Downloads

Thesis about Rainwater Harwesting in Maseno by Andreas Krenn (.pdf, 12MB)

Description of project areas where RWH is implemented: