Water, just like electricity or heating is an essential service within a building. Without stable water supplies, the construction of new buildings cannot continue at the current rate, especially as our supplies are already considered seriously stressed in many regions of England. There is a misconception that the UK is a relatively water-rich country, but population growth and increasingly unpredictable weather patterns have resulted in rising water scarcity. The lack of understanding of water supply issues has led to little action in many areas and, still to this day, buildings are being constructed without vital rainwater harvesting or water reuse infrastructure.

Many properties in the UK, including newly constructed ones, use water in an outdated way. The traditional approach to supplying water sees a single mains supply entering the building and distributed to all water-using outlets. In a water-scarce country, relying solely on a mains water supply is wasteful, especially because mains water is treated to drinking quality and involves many carbon-intensive treatment processes. Water treated to drinking quality is not necessary for non-potable water-using outlets, including toilets, irrigations, vehicle washing, and many other commercial processes.

The preferred approach is to have a secondary sustainable water supply produced through an on-site rainwater harvesting or water reuse system. The supply generated from the harvested rainwater can be used exclusively for non-potable outlets and reduces the transportation and filtration requirements when compared to mains water supplies. Of course, the rainwater is safe for end users through filtration technologies in the rainwater harvesting system.

Now you have heard why rainwater harvesting is so important for every new building going forward, let us take a look at the different types of rainwater harvesting systems available for non-household and commercial properties…

 non-pressurised rainwater harvesting system in a commercial building
Non-pressurised Arrangement

The easiest way to recall this style of system is through the header tank! Non-pressurised approaches to rainwater harvesting include a header tank within the building. This header tank is where the mains water backup is located for periods of drought. Once in the header tank, rainwater can then be distributed to usage points via gravity, or a booster set for low-level header tanks.

As can be seen in the system schematic, the header tank is supplied with rainwater from the main storage tank via submersible pumps. We hear you thinking that the header tank is just an extra cost, but the header tank plays a vital role in protecting end users from disruption. Without a header tank, the submersible pump will operate frequently to cover pressure drops within the system. This is a problem! Why? Well, the submersible pumps will be overused shortening their lifespan and making them more prone to failure.

Submersible pumps are also fixed-speed pumps meaning they are not as energy efficient when compared to a variable speed booster set (so we want to minimise their usage)! Even if the submersible pumps fail within the main storage tank or if someone accidentally digs up the rainwater pipework between the two tanks (yes, it has happened), then the header tank can supply outlets with mains water to minimise disruption for building users.  

Where to use

  • Where the demand for water within the building exceeds 620 litres within an hour.
  • Large water-using buildings.
  • Typically installed within sporting stadiums, schools, hospitals, and leisure venues.
 Pressurised rainwater harvesting system in a commercial building
Pressurised Arrangement

Replace that header tank capacity with a pressure vessel and you will have a pressurised system! Pressurised approaches to rainwater harvesting do serve a good purpose, especially for smaller buildings requiring a rainwater harvesting system that takes up a small footprint and is a cheap solution. As a large commercial solution to rainwater harvesting though, the pressurised approach carries a warning. 

Instead of a header tank after the main storage tank, the pressurised approach utilises a pressure vessel that provides a boosted rainwater supply for the building. The mains water backup facility within pressurised rainwater harvesting systems is supplied directly into the main storage tank. This is problematic because the rainwater harvesting system will be reliant on more mains water as the main storage tank has a larger area to fill when compared to a header tank. Now, the second word of caution for this approach. If submersible pumps or the pressure vessel fails, both the mains water and rainwater supplies will be unable to reach the points of use as both supplies go through these components.

Where to use

  • Small buildings with low usage
  • Keep the accessible WC on mains water
 combi rainwater harvesting system in a commercial building
Combi Arrangement

A hybrid of the above two approaches! Combi rainwater harvesting systems have the excellent reliability credentials of a non-pressurised system all in the footprint of a pressurised system. Combi Processor Units contain an internal header tank (where the mains water backup is located), a variable speed booster set to deliver rainwater to points of use, and an electrical control panel. The beauty of this approach to rainwater harvesting is that Combi Units are pre-wired, pre-built, and pre-tested meaning an installer can simply wheel the Combi into its final location.

Combi Processor Units come in a range of capacities, including 225, 400, and 620. The capacity refers to the volume of water that can be held in the integrated header tank. Of course, the correct size capacity should be chosen during the design stage to ensure the rainwater harvesting system is not undersized.

Where to use

  • Where the demand for water within the building is below or equivalent to 620 litres within an hour.
  • Schools, distribution centres, supermarkets, and many other larger commercial buildings.
Learn more

We know that rainwater harvesting is just a small part of your overall project but selecting the best type of system is key. If you are interested in learning more about rainwater harvesting, visit here or contact our friendly team of experts.

Constructionline Gold Member CHAS Accredited UK Rainwater Harvesting Association Member Safecontractor Approved Waterwise Affiliate UK Business Awards Winner 2022