How Does Rainwater Harvesting Work?

Posted by Callum Vallance-Poole, on June 27, 2022.

Rainwater harvesting is quickly increasing in popularity with homeowners across the UK who are discovering the many wonderful benefits it brings to their lives. But do you know how a rainwater harvesting system works? We explain how below.

How does a rainwater harvesting system work?

Step 1

The first and most important thing you need for harvesting system is rainfall. It doesn’t have to be a downpour that lasts for hours. Any type of rainfall is enough for it to filter down into the harvesting system where it can be processed. The start point of its journey is the roof, which offers a large surface area for water run-off to be collected.

Step 2

Rainwater then runs down the pipe and into the storage tank via the inlet pipe. Here it passes over the filter to remove any leaves or debris that may have been washed down with the rain. This is important, because if leaves or other debris is left inside the tank, they can start to rot and spread diseases that affect the quality of water, making it unusable around the home. If you intend to use it to water the garden, you do not want to spread diseases that could harm or even kill your plants.

Step 3

The water is then stored inside the tank for use later. There are lots of different sized tanks, depending on your budget and how much you are likely to use. Above the ground systems are popular as they can be installed without disturbing any soil, while larger storage systems installed underground provide even more storage space.

Step 4

The harvested water can be used in two ways: either pumped to an appliance or to a header tank. If it is pumped to an appliance, it will pass through another filter to remove any small particles that may still be present. This type of water is non-potable (not suitable for drinking) and can be used for washing machines, meaning you cut down on the amount of water you are using for domestic tasks around the house. If connected to a header tank, the water relies on a gravity feed from a high level where it can be sent down to WCs and other outlets positioned at a lower level in the property.

During the installation process, a separate pipe system for the harvested water supply will need to be installed so it can feed into the washing machine, WCs etc. The water you collect with a harvesting system will also not be enough or appropriate for the full water demands of your home, especially drinking water, so you will still need to remain connected to the mains supply. Backup piping will be included so it can cut in if the water tank runs dry. This tends to happen if you have a smaller installation and go through a period of low rainfall.

What should I consider before installing a rainwater harvesting system?

Before installing a rainwater harvesting system there ae a number of things that should be taken into consideration. For example, make sure the area size of the roof is large enough, as the smaller it is, the less you will harvest.

The geographical location of your home makes a difference, as someone living in a small, terraced house on the east coast will probably not harvest a lot of water to make much of a difference on household activity.

Consider usage levels as this will affect availability and storage capacity. So, if it will be used a lot the system needs to produce enough to meet demand. Likewise, if the system stores too much water for the number of people living in the property to use it all, it could mean you have too much water for your needs.

Can you install a rainwater harvesting system under hard-standing flooring?

The best place to install an underground rainwater harvesting system is in the garden, either in the front or back, whichever is more practical. It is not advised to install a tank under the driveway, as the work involved would increase the installation cost and would also require a heavy-duty access frame and cover and the tank would need to be protected by a concrete slab so it remains safe from vehicle loading. This is not something you have to be concerned about when purchasing an above ground rainwater harvesting system. Rather, you just need to ensure there is enough floor space for the tank to sit close by to the guttering downpipe without restricting access. Weight is something else to consider, as every 1,000 litres of water equates to one tonne.

Are there any water regulations I should be aware of?

If you are installing a rainwater harvesting system, you should ensure you are aware of the following:

Water Regulations Advisory Scheme (WRAS)

The main thing to be aware of here relates to backflow prevention. If the system is being connected to the mains supply, you should ensure there is a sufficient enough gap that stops non-potable water from entering the mains supply.

BS 8515:2009

This is more relevant for manufacturers of rainwater harvesting systems. The standard exists to ensure there is a code of practice in place to produce consistency of performance and that high standards are maintained. All systems sold and installed by Graf UK meeting BS 8515:2009.

Planning permission

In most cases planning permission is not needed by homeowners as rainwater harvesting systems fall under permitted development rights. However, you should always check with the local authority before starting the project to ensure you don’t fall foul of any unknown regulations.

Pipe marking

Any pipes that are being used for harvested rainwater have to be labelled as non-potable (non-drinkable). This applies whether the pipes are located below ground or inside the house. All outlets, valves and appliances must also be tagged in accordance with WRAS Guideline IGN 9-02-5. An example of this could be seen on a washing machine, which should have the appropriate label on the back.

Marketing Coordinator - Based at our UK HQ in Banbury, Oxfordshire, Callum is responsible for promoting Water Management Systems, Attenuation Tanks, Treatment Plants, Rainwater Harvesting Systems and more!