What is rainwater harvesting (RWH)?
Rainwater harvesting is a technique of collection and storage of rainwater into natural reservoirs or tanks or the infiltration of surface water into subsurface aquifers (before it is lost as surface runoff). RWH is a simple, easy and cost effective technique of aggregating water and delivering it to water resources like bore wells. RWH has the potential to overcome problems caused by drought and water scarcity.
Water and Rain
As much as 97% of the world’s total water is in oceans, 2% is locked in the polar caps and only the balance 1% is fresh water. Of this 1%, India’s share is 4% whereas our population share is 18% of the world’s population.
As much as 85% of drinking water schemes in rural areas in India depend on groundwater. Due to the drought-like conditions over the last few years, groundwater extraction rate has exceeded the recharge rate. Desperate farmers are using tankers to irrigate farmlands. Rains help to maintain the ground water table through natural percolation, besides maintaining surface water levels. RWH is expected to play a major role in bringing depleted water levels up.
Potential of rainwater harvesting
The total amount of water received in the form of rainfall over an area is called the rainwater endowment of that area. Out of this, the amount that can be effectively harvested is called the water harvesting potential.
Rain water harvesting methods and principle:
The principle of Rainwater Harvesting is simple and most cases the same despite the large variety of systems on the market.
The Working Principle:
- Rainwater is captured via your existing guttering and drain-pipes.
- The rain water is then filtered.
- The rain water is stored in single, or multiple connected tanks, above or below ground.
- When needed, the rain water is pumped either directly to the points of demand, or to an internal holding tank.
- If the system runs low on rainwater the mains water back-up pump unit is activated and mains water supplies the systems. Equally if there is too much rainwater, it overflows to your existing soak away or drainage system.
- Once it rains again, the rainwater is collected and the system automatically changes back to the rainwater supply.
Key System components typically are:
Filter Units – remove silt and leaves.
Storage tanks – these can be above or below ground and come in a variety of shapes and sizes and are made of a variety of different materials i.e. plastic, GRP, or even concrete.
Pumps & Controls – manage demand i.e. pump water to your point of use and controls the level of mains supply to your system.
Key Considerations for installation
Existing guttering and downpipes from your roof may need to be modified to bring the water to a central point.
If you prefer the storage tanks to be underground then excavation work will be required.
Some internal plumbing is usually required to separate the drinking and bathing water from the non-drinking rain water for your toilets, washing machine & outside taps.
Types of Rainwater Harvesting systems
There are three main types of system:
- Gravity Fed Systems
- Direct Fed Systems
- Commercial Systems
Gravity Fed Systems
Gravity Fed Rainwater Harvesting systems simply collect rainwater from the roof of the property and passes it through a filter which removes any leaves and twigs. Thereafter, the water is pumped into a sub-surface storage tank.
As the water settles in the storage tank, the heavier particles drop to the bottom of the tank.
The stored water is pumped to a Header Tank located in the loft. The Header Tank then uses its own gravity pressure to feed the water demands. The pressure provided by a Gravity Fed system is not enough to supply washing machines or dishwashers.
However the advantage of this system is that it is easier to install and requires less pump power, giving the pump a longer life expectancy.
Direct Systems
Direct Rainwater Harvesting systems have a key advantage over gravity fed systems as they can be used for dishwashers and washing machines as well as toilets and gardens; this is because they work as a pressurized system.
The rainwater is collected through your existing drainpipe system and then flows through the first filter process which removes larger particles of leaves and twigs. It is then collected and stored in the enters the storage tank
In the tank, the rainwater settles and smaller particles such silt float to the bottom of the tank.
When the water is needed in the house, the water from the tank is directed through a smaller filter to thoroughly clean it before it enters the house.
The storage tank is connected to the existing mains water supply to ensure that if there is insufficient stored rain water it can then be topped up.
The key benefit of a Direct System is that it can be used for washing machines and dishwashers, as well as toilets, and gardens. In order to use rainwater for to be used as potable water it would have to go through a much stricter and more expensive filter and cleaning process.
Commercial Systems
Commercial Rainwater Harvesting systems work exactly the same way as domestic rainwater harvesting systems, with the exception that larger roof space means that they can collect more rainwater therefore can supply larger water needs.
Commercial rainwater harvesting systems are used principally for toilets and are typically only gravity fed systems. For those with large water needs, such as nurseries or farms gravity fed rainwater systems can offer significant savings.
Please note that underground storage of rainwater eliminates the growth of algae as the water can easily be filtered before entry thus also making it much cleaner.
Another advantage of underground storage is that you can have large amounts of storage without taking up room in your garden.