Rainwater Harvesting RWH

Rainwater Harvesting RWH

Millions of people throughout the world do not have access to clean water for domestic purposes.In many parts of the world conventional piped water is either absent,unreliable or too expensive.One of the biggest challenges of the 21st century is to overcome the growing water shortage.Rainwater harvesting (RWH) has thus regained its importance as a valuable alternative or supplementary water resource, along with more conventional water supply technologies.

Water harvesting in its broadest sense can be defined as the collection of runoff rainwater for domestic water supply,agriculture and environmental management.Water harvesting systems,which harvest runoff from roofs or ground surfaces fall under the term rainwater harvesting.This focuses on rainwater harvesting from roof surfaces at household or community level for domestic purposes,such as drinking,cooking and washing.

Each rainwater harvesting system consists of three basic components:

1 catchment or roof surface to collect rainwater 


2 delivery system to transport the water from the roof to the storage reservoir (gutters and drainpipes)  


3 storage reservoir or tank to store the water until it is used.The storage reservoir has an extraction device that depending on the location of the tank may be a tap, rope and bucket, or a pump.

Catchment surface 
The catchment of a water harvesting system is the surface that receives rainfall directly and drains the water to the system.This focuses on rooftop RWH, but surface runoff RWH is also possible.Surface water is,however,in most cases not suitable for drinking purposes since the water quality is not good enough.

Any roofing material is acceptable for collecting water.However, water to be used for drinking should not be collected from thatched roofs or roofs covered with asphalt.Also lead should not be used in these systems.Galvanised,corrugated iron sheets,corrugated plastic and tiles make good roof catchment surfaces.Flat cement or felt covered roof can also be used provided they are clean. Undamaged asbestos cement sheets do not have a negative effect on the water quality.Small damages may, however, cause health problems!

Delivery system 
The delivery system from the rooftop catchment usually consists of gutters hanging from the sides of the roof sloping towards a downpipe and tank.This delivery system or guttering is used to transport the rainwater from the roof to the storage reservoir.For the effective operation of a rainwater harvesting system,a well designed and carefully constructed gutter system is crucial because the guttering is often the weakest link in a rainwater harvesting system.As much as 90% or more of the rainwater collected on the roof will be drained to the storage tank if the gutter and downpipe system is properly fitted and maintained.Common material for gutters and downpipes are metal and PVC.With high intensity rains in the tropics,rainwater may shoot over the (conventional) gutter,resulting in rainwater loss and low harvesting production; splash guards can prevent this spillage.

Storage reservoirs 
The water storage tank usually represents the biggest capital investment element of a domestic RWH system.It therefore usually requires the most careful design to provide optimal storage capacity and structural strength while keeping the costs as low as possible. Common vessels used for very small scale water storage in developing countries include plastic bowls and buckets,jerry cans,clay or ceramic jars,old oil drums or empty food containers.

For storing larger quantities of water the system will usually require a tank above or below the ground.Tanks can vary in size from a cubic metre (1,000 litres) up to hundreds of cubic metres for large reservoirs.In general the size varies from 10 up to a maximum of 30 cubic metres for a domestic system at household level and 50 to 100 cubic metres for a system at community or school level, of course very much dependent on the local rain pattern throughout the year.Round shaped tanks are generally stronger than square-shaped tanks.Furthermore, round-shaped tanks require less material compared to the water storage capacity of square tanks.

There are two categories of storage reservoirs: surface tanks and subsurface tanks. Surface tanks are most common for roof collection. Materials for surface tanks include metal, wood, plastic, fibreglass, brick, interlocking blocks, compressed soil or rubble stone blocks, ferro cement and reinforced concrete.The choice of material depends on local availability and affordability.In most countries, plastic tanks in various volumes are commonly available on the market.Surface tanks are generally more expensive than underground tanks, but also more durable.A tap is required to extract the water from the surface tank.

The material and design for the walls of sub-surface tanks or cisterns must be able to resist the soil and soil water pressures from outside when the tank is empty.Tree roots can damage the structure below ground. Careful location of the tank is therefore important. Keeping it partly above the ground level and largely above the groundwater table will prevent problems with rising groundwater tables and passing trucks, which may damage the construction below the surface. Local materials such as wood, bamboo and basket work can be used as alternatives to steel for reinforcing concrete tanks. A sub surface tank or cistern requires a water lifting device, such as a pump or bucket rope system. To prevent contamination of the stored water, a safe water lifting device and regular maintenance and cleaning are important.