Busting the myths about water reuse

Most of the information on this page has been taken from the Ricardo (2020) Independent Review of Costs and Benefits of RWH and GWR, which can be found here. Scroll down to learn more about rainwater and greywater.    

What do we mean by water reuse?

Water reuse generally refers to the capture, treatment (if required) and use of alternative water supplies for non-potable purposes. It includes rainwater harvesting, greywater (typically the used water from baths, showers and hand basins) and black water (sewage) recycling and any other type of water recycling or reuse. 

What is rainwater harvesting?

The ancient practice of rainwater harvesting (RWH) can be traced back at least 4000 years. Rainwater harvesting is the term used to describe the collection of rainwater that falls onto surfaces, such as roofs, and is stored for future use. 

For more information, the British Standard on RWH systems is BS EN 16941-1:2018.

Is rainwater harvesting the same as stormwater or surface water harvesting?

Some people use the term RWH to include both the capture of rainfall directly from roof areas as well as the capture and reuse of surface water runoff (also known as stormwater or surface water harvesting). These two types of rainwater harvesting may vary quite a bit in terms of water quality, scale, application, cost etc. depending if it is rainwater from roof areas only or a surface water harvesting system.

What is greywater recycling?

Greywater, especially where it is limited to that from baths, showers and washbasins (sometimes termed ‘light grey water’) can be considered high volume, low strength wastewater with high potential for reuse. Most existing buildings plumbing directs all wastewater to the sewer. Separating out grey water from the more polluted wastewaters (e.g. from toilets, often termed ‘black water’) means it can be treated and used as an alternative source of water for non-potable purposes. Generally, greywater recycling (GWR) systems can supply water for toilet flushing, irrigation, outside water use and laundry.

For more information, the British Standard for greywater recycling is BS 8525-1:2010.

What is blackwater recycling?

Blackwater is any wastewater that is contaminated with water discharged from a toilet, so this can and usually does contain grey water. It can also be used to describe wastewater, process or cooling water from industry. Blackwater recycling requires treatment before it can be reused. This usually involves aerobic screening, biological treatment, ultrafiltration, ultraviolet disinfection, TDS (total dissolved solids) and nutrient removal and chlorination.

What is the link between water reuse and water neutrality?

Water neutrality is achieved when the footprint of a building is offset by making efficiencies somewhere else. But before you offset, you should aim to make the building as efficient as possible and utilise water reuse technologies to bring the overall footprint down.


Fact: The maintenance requirements for RWH systems vary depending on the system type, complexity and scale of operation. Maintenance should be undertaken in accordance to the manufacturer’s guidelines however it is recommended that you: keep gutters free of debris to prevent blocking the system; clean filters approximately twice a year, depending on tree cover over the collection area; visually inspect the tank at least once a year; check the mains water top-up once a year; carry out pump maintenance. Ensure you find a tradesperson specifically skilled in RWH systems, not all plumbers will be.

Fact: Harvested rainwater is not the same quality as tap water. It is not treated to a quality on par with drinking water standards. It is sometimes treated for specific uses, but is never safe to drink. Harvested water should not be used as drinking water unless it is treated to potable standards, to which there are strict regulations. Rooftop collection surfaces are relatively clean, however the runoff can contain heavy metals and unwanted nutrients and harmful pathogens which you would not want to drink for health reasons.

Water quality requirements for rainwater harvesting systems and how to manage them are stated in the building regulations BS EN 16941-1. 

Fact: In a Waterwise study, when asked how interested they would be in having a RWH system in their homes, 87% described themselves as being either “very interested” or “somewhat interested” (out of 194 participants). This study can be found here and includes a literature review of other research looking at the public perceptions of RWH. 

Fact: The level of water and financial savings are ranging and different for each building and system. The amount of water saved is determined by the quantity of rainfall, the surface area of the collection area, storage size, rainfall, and the demand for water in the building. Financial savings can only be seen if you have a metered water supply and savings differ greatly between properties, with the greatest savings being those where RWH is fitted upon construction, not retrofitted. 

Fact: Life cycle analyses (LCAs) show the main operational energy contribution for rainwater-harvesting (RWH) systems are generated from pumping rainwater from the tank to the building and ultraviolet UV disinfection, if used. The level of operational emissions will depend on the energy requirements of the system and the source of electricity – where renewable energy is used to power the system emissions will be much lower. The reduction in water from the mains infrastructure assumes a reduction in the CO2 embedded in that system. Recycling water onsite decreases the energy consumption needed to pump water over long distances between the treatment plant and the end user. According to Ricardo’s 2020 report (which can be found here) the majority of RWH systems do produce a carbon saving over their lifetime of 20 years. 

Fact: The British Standard for RWH systems (BS EN 16941-1) provides clear guidance on the minimum acceptable standards, this covers the design, installation, maintenance, water quality and risk management. Building regulations are also relevant when installing RWH systems and would need to be consulted before a system was installed. In terms of the water quality the standards are not the same as tap water as RWH water is non-potable and therefore not for drinking.

Fact: They can add costs to a development, but might increase the desirability of a building too. According to the UK Rainwater Management Association a high quality domestic system costs an average of £2,500-£6,000, this price differs greatly between property due to different amounts of piping, sized tanks and collection areas. For non domestic buildings the cost ranges even more due to different demands of companies (£8,000-£7,0000), however for large buildings installing larger RWH systems will provide a greater return on investment (figures taken from Ricardo’s 2020 report which can be found here).

Fact: Systems are easier to install and more cost effective if they are installed as the building is being built. Installing as a retrofit to a building that already exists costs about 50% more and can be difficult because a secondary pipe work needs to be installed for the non potable water. It is possible to install an entirely gravity fed system that is not plumbed back into the house (i.e above ground tanks used for gardens) which could be a lower cost option for a retrofit. 

Fact: Depending on the context it can be very beneficial to combine RWH and SuDS. A combined system would need to be specifically designed and built as an integrated system. This might include a smart system that has the ability to store both stormwater runoff/attenuation and rainwater which can manage and reuse the water.


Fact: Greywater means the water has been used before, this may have been from a shower or sink, so it will have some added bacteria in it, but it can still be used again for something like flushing a toilet. It could also be described as any domestic wastewater produced excluding sewage. The use of greywater might also include treatment of the water, depending on the system and end use.

Fact: GWR systems can be integrated with RWH systems, these can bring notable benefits when planned strategically for larger scale, especially mixed use, developments. However, at the individual building level, the benefits of an integrated GWR and RWH need to be considered as the added efficiency from the rainwater depends on the building use and demand for water.

Fact: Based on Waterwise research 86% of people surveyed said they would be happy to have a system that took shower water and used it to flush their toilet. The report can be found here and it includes a literature review of other studies looking at public perceptions of GWR. 

Fact: If water is used in a building then grey water is generated, and if enough water is generated they could benefit from some type of water reuse system. A system would be more cost beneficial in buildings such as hotels, student accommodation or blocks of flats which tend to have a higher supply of grey water. It is sometimes the case that RWH systems, that collect rain from the building’s roof, are not optimal for these buildings as they can have small roof surface areas.

Fact: GWR systems need slightly more maintenance compared to RWH systems, with checks on the pipes, filters (including membranes) and the tank needed around once a year. Due to the more complex treatment process of GWR systems extra checks are required periodically to ensure system performance and that the water is safe for reuse. Checks are also required to make sure there is no contamination of the mains supply. If systems operated with additives, these would need to be replenished on a regular basis. Ensure you find a tradesperson specifically skilled in GWR systems, not all plumbers will be. 

Fact: GWR systems tend to be more expensive than RWH systems on average, varying between £3,000 – £6,000 for domestic buildings and £40,000 – £270,000 for non domestic depending on the size. This is due to the system being more complex and the additional  treatment steps that RWH does not necessarily require.

Fact: In domestic buildings, where water demand is low to medium, there could be a net cost of GWR over their 20 year life, as the money saved on your water bill are lower than the cost of installation. However, when you consider the social impacts too, such as benefits to the environment, a medium water demand building could be cost effective. In larger buildings such as commercial offices or entire residential tower blocks the water demand is far higher and a system becomes very much cost effective. The larger the water demand the greater the benefit because of economies of scale. 

Fact: You need to consider the supply and demand balance for that individual building to choose the correct type of system. For example, how much water does it need, how much does it use, and for what, does that geographic area have good rainfall and is there a big enough rain collection area. A building that produces a lot of greywater and has a small roof surface area (like a hotel or a block of flats) might be better suited to a GWR system. But a building with a large rain collection area (whether than be on a roof or run off from the road) might be best suited for RWH.