How Rainwater Harvesting Works
The principle of rainwater harvesting is simple:
Capture → Filter → Store → Distribute
It is both financially and environmentally attractive. Around 27% of the domestic mains water we pay for is used simply to flush toilets.
The success of a rainwater harvesting system depends on the quality of its components and the expert advice provided during the design stage. Water treatment, storage and distribution technologies continue to develop, with modern systems offering minimal maintenance, long service life and consistently clean water.
What Determines the System Design?
The design and components of a rainwater harvesting system will depend on:
- What the harvested water will be used for
- How excess rainwater and tank overflow will be managed
- Whether the storage tank will be installed above or below ground
- The number of users and the expected water demand
Core Components of a Rainwater Harvesting System
An office block requires different equipment from a house, but every rainwater harvesting system contains the same basic elements.
The size, capacity and complexity of these components will vary according to the scale and requirements of the installation.
Rainwater Storage Tank
The system may use an above-ground or below-ground tank, depending on the amount of water that needs to be stored, the available space, aesthetic considerations and any planning restrictions.
Above-ground tanks may be manufactured as one-piece or sectional units and are often insulated. Underground tanks are normally made from plastic or fibreglass, although concrete and steel tanks may also be used.
A storage capacity of around 1,500 to 3,000 litres is often sufficient for a domestic property. Large commercial rainwater harvesting systems may require capacities of 100,000 litres or more.
A well-designed tank installation may include:
- A pre-tank rainwater filter
- A calmed inlet to prevent sediment on the tank floor from being disturbed
- An overflow unit or trap with backflow protection
- An outlet connected to the building’s water distribution system
The harvested water can then be supplied to a house, commercial building, farm or specialist application such as commercial vehicle washing.
Rainwater Filters
The initial rainwater filter is one of the most important parts of the system. Selecting a reliable, high-quality filter can significantly reduce maintenance and improve water quality over the life of the installation.
The best rainwater filters are designed to resist blockages. This can remove the need for a first-flush diverter, which discards the first portion of rainfall.
Common filter types include:
- Downpipe filters
- Pre-tank filters
- In-tank filters
- Self-cleaning filters
Where a higher level of water quality is required, the system may also include fine particulate filtration and ultraviolet treatment. UV treatment can control microorganisms and, when combined with the correct filtration and testing, may allow the water to be used for bathing or other higher-quality applications.
Additional System Components
Storage tanks and filters form the basic structure of a rainwater harvesting system. A simple gravity-fed domestic installation with a header tank in the roof may need little more than these core components.
Larger buildings and more demanding applications may require additional equipment.
Pumps and Pump Controllers
Water stored underground must normally be pumped to the points of use, such as toilets, washing machines, irrigation systems or washdown outlets.
This can be achieved using:
- Submersible pumps installed inside the storage tank
- External suction pumps
- Pump controllers that regulate pressure and automatically start or stop the pump
Advanced Water Treatment
Additional filtration may be required where the roof or surrounding environment presents a greater risk of contamination.
This may include:
- Fine particulate filters
- Carbon filters
- UV disinfection
- Specialist treatment equipment
Monitoring and Control Equipment
Monitoring equipment can provide information about:
- Tank water levels
- Water pressure
- Pump operation
- Water quality
- System faults
- Mains water backup
These functions may be incorporated into a local control panel or connected to a building management system.
Other Components
Depending on the installation, the system may also include:
- Booster pumps
- Floating suction filters
- Mains water backup units
- Header tanks
- Break tanks
- Secondary storage tanks
The Main Types of Rainwater Harvesting System
There are three main types of rainwater harvesting system:
Direct Pumped Systems
Water is pumped directly from the storage tank to the appliances or outlets where it is required.
Indirect Pumped Systems
Water is pumped from the main storage tank to a smaller header or break tank before being distributed around the building.
Indirect Gravity Systems
Water is pumped to a high-level header tank and then supplied to the points of use by gravity.
In some situations, a completely gravity-fed system may be possible. However, these installations are relatively uncommon because the storage tank must be positioned high enough to provide sufficient water pressure.