About this Guide
Welcome to our Guide for Commercial Rainwater Harvesting Applications, a resource written to support design engineers specifying rainwater harvesting solutions for commercial projects.
As a design engineer, you will already know that water is an increasingly important consideration on today’s projects. It is no longer just about drainage and flood management, but also about improving water efficiency by collecting and reusing rainwater locally.
When specifying a commercial rainwater system, getting it right first time is critical to achieving a safe, compliant, and efficient solution that will perform reliably for years to come. That is why this guide has been created using over two decades of practical experience in water reuse technologies.
What we’ll cover in this guide…
- Where commercial rainwater harvesting works best across different building types
- What recycled rainwater can and cannot be used for
- What the regulations and standards actually require
- Best practice tips for specific rainwater applications
- Key design considerations for efficient, compliant systems
- Supporting evidence to assist with your designs.
Applications Checklist
Building Type Information
The biggest question when considering commercial rainwater harvesting applications is project viability. Independent research carried out by Ricardo in 2017 shows that it is financially beneficial to install a rainwater harvesting system where the roof area exceeds 500 m². This means that recovering rainwater for non-potable reuse makes both financial and environmental sense for most small-, medium- and large-scale non-household buildings.
We would also advise considering the future cost of water for non-household properties, particularly as we approach the projected daily deficit of 5 billion litres, as forecast by the Environment Agency.
In general, most commercial buildings are well suited to rainwater harvesting systems due to the scale of their roof areas, which allows for efficient rainwater collection. This is coupled with a high demand for water in non-household buildings, much of which is for non-drinking applications. As a result, this demand can often be easily met using recycled rainwater.
In many cases, commercial projects already include some form of attenuation or flood risk reduction capacity to manage rainwater on site. Where this capacity exists, rainwater harvesting can be easily integrated through the use of Smart SuDS.
Commercial rainwater harvesting applications checklist
- Attenuation is planned for the site.
- The roof area is 500 m² or greater.
- There is a high demand for water for non-drinking purposes.
- There is space for a collection tank, either above or below ground.
- Schools, leisure centres, distribution centres, universities, manufacturing plants, offices and community buildings are all examples of non-household buildings where systems are ideal and can lead to water savings of up to 75%
if one or more of these conditions exist on your project, then commercial rainwater harvesting is ideal. We can provide full specification support.
Uses of Recycled Rainwater
A key consideration when designing a commercial rainwater harvesting system is understanding where harvested rainwater can and cannot be used. The answer is straightforward: rainwater is suitable for non-potable uses only. Non-potable refers to any application not involving human consumption, such as drinking, food preparation, or personal hygiene.
Common non potable uses in commercial buildings include toilet flushing, laundry, and the irrigation of landscaped and green spaces. In this section of the guide, we break these applications down in more detail, highlighting specific design considerations and sharing best practice tips for each type of use.
It is also important to note that, in the UK, regulations and standards strictly prevent rainwater harvesting systems from being used for drinking water. Key legislation and standards includes the British Standard for non-potable water systems, BS 16941-1, and the Water Supply (Water Fittings) Regulations 1999. These requirements are in place to protect public water supplies and ensure drinking water remains safe and of the correct quality.
Toilets
Yes, toilet flushing is one of the most common and effective commercial rainwater harvesting applications for available. In modern commercial buildings, using harvested rainwater to flush toilets and urinals instead of high-quality drinking water makes both environmental and practical sense.
Toilets and washroom areas are major drivers of water use in commercial properties, with washroom areas accounting for around 90% of a building’s total water use and 50% of this being associated with toilet flushing, making it one of the biggest opportunities for water efficiency.
Information required for Sizing
- Number of toilets
- Litres used per flush
- Building occupancy
Design considerations
- For buildings with occupants who may have less developed or compromised immune systems, such as schools, hospitals, or care homes, it is recommended to specify a rainwater harvesting system with UV disinfection to reduce bacteria levels. This is because toilet flushing can generate airborne particles that disperse within the washroom environment.
- Where UV disinfection is used, an additional, self-cleaning stage of filtration is strongly recommended to maximise the effectiveness of the UV system and ensure reliable protection against bacteria.
Irrigation
Plants and green spaces do not need to be watered with high quality drinking water. In fact, the Royal Horticultural Society actively promotes the use of harvested rainwater for irrigation, highlighting its suitability for plants and its environmental benefits. This makes rainwater harvesting an excellent solution for a wide range of commercial irrigation applications.
- Football and rugby pitch irrigation at large stadiums
- Maintenance of living walls and green roof systems
- Irrigation of soft landscaping and planted areas
- Spray irrigation systems
- External bib taps, where clearly labelled as non potable
Information required for Sizing
- Irrigation run time per day, in minutes
- Water demand, typically measured in litres per second
Design considerations
- Where irrigation methods create airborne particles, such as spray irrigation, it is recommended to include UV disinfection along with additional self-cleaning filtration to reduce bacterial risk and protect end users.
- Projects with high irrigation demand may benefit from linking the rainwater harvesting system with on-site attenuation storage to increase the volume of usable rainwater available. This can be achieved through Smart SuDS solutions.
- For operation critical irrigation systems, such as those used at large sports stadia, additional reliability and resilience features may be considered to minimise the risk of downtime. Our technical team can provide further guidance on this.
Further Uses
In addition to toilets and irrigation, rainwater harvesting can be used across a wide range of other commercial and industrial applications, including:
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Vehicle washing, such as HGV and car wash facilities
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Train washing
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Cooling systems
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Manufacturing processes
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Commercial laundry facilities
Design considerations
Some of these applications may require enhanced levels of filtration and disinfection to meet performance, hygiene, or process specific requirements. Our experts can advise on the most appropriate treatment solutions to ensure systems are safe, compliant, and fit for purpose.
Summary
Commercial rainwater harvesting systems offer a wide range of applications, helping to improve water efficiency while also supporting sustainable drainage requirements. To achieve the best outcome, we always recommend involving an experienced rainwater harvesting specialist like Stormsaver as early as possible in the design process, ensuring the system is correctly designed from the outset. It is quick and easy to find out whether rainwater harvesting is right for your commercial project, simply get in touch with our team to discuss your requirements.
