Greywater and rainwater reuse explained

07-04-2025

What do you think of when you see the word ‘wastewater’?

For some people, it might conjure up unpleasant images of the brown, smelly sewage water that flows through drainage systems.

But not all wastewater is created equal; because while toilets produce contaminated wastewater known as ‘blackwater’, other domestic water fittings (such as sinks, baths and washing machines) create slightly cleaner ‘greywater’.

Greywater contains far less pathogens than blackwater, which means it can be reused and recycled for certain applications.

Where does greywater come from?

In homes and offices, greywater is generated by:

Sinks and wash hand basins
Baths and showers
Washing machines
Dishwashers

Kitchen sink water – if you’ll excuse the pun
is something of a grey area.

It’s sometimes considered to be greywater, but it’s often too contaminated with grease, fats, food waste and detergent chemicals to be useful.

Instead of trying to reuse it, you’re usually best off just pouring it down the drain.

Other commercial settings may produce greywater from industrial processes; such as food and drink production, machinery cooling, equipment/product washing and more. The level of contamination and potential reusability of this water depends on the kinds of processes involved.

What is rainwater harvesting?

Like greywater, rainwater can also be collected and reused for different purposes.

Rainwater can be ‘harvested’ from rooftops and collected in tanks or reservoirs. These can range from simple domestic garden water butts to large underground tank & pump systems suitable for commercial applications.

You’ll sometimes see rainwater (and water from other natural sources such as rivers and streams) referred to as ‘whitewater’.

However, this term can also be used to refer to the drinking water that comes from the tap; which of course comes from natural sources too but undergoes treatment before it reaches you.

Even more confusingly, rainwater is sometimes viewed as greywater; despite the two being quite different.

In comparison, rainwater is much more pure than greywater, and can be stored for longer without requiring treatment.

How can greywater and rainwater be reused?

In commercial settings, both Greywater and rainwater reuse for irrigation of certain crops, leisure facilities (such as football pitches and golf courses), and office yard/garden spaces.

It can also be utilised for washing vehicles and machinery, toilet flushing, and often in some types of industrial processes.

Why bother reusing greywater and rainwater?

With changing climate conditions leading to less rainfall here in the UK, reducing your organisation’s reliance on new water can help preserve water supplies and reduce the carbon footprint from water treatment plants.

Greywater and Rainwater Reuse isn’t just for the environmentally conscious. Making the most of the water available to your business can help you minimise your energy bills, and reduce the impact of droughts on your day-to-day operations.

That said, the costs and environmental impacts of installing greywater and rainwater systems may outweigh the benefits for your business.

Before going ahead with an installation, we’d recommend taking a close look at the costs versus savings, and investigating whether other means of saving water might be a better option.

Is it really safe to reuse greywater and rainwater?

Absolutely.

With appropriate filtration, rainwater can be stored and used indefinitely. Greywater usually requires more extensive purification and disinfection treatment, but otherwise it’s perfectly safe for the different reuse purposes mentioned above.

That said, it’s important to remember the potential hazards associated with water facilities, and your organisation’s responsibility to keep your employees and customers safe.

All greywater and Rainwater reuse systems should comply with the Water Supply (Water Fittings) Regulations 1999, and regular legionella testing and appropriate treatment to prevent the spread of Legionnaire’s disease.

For irrigation and plant watering, it’s also recommended to apply the water at soil level, rather than using sprinklers or any other method which creates aerosolisation.

And don’t forget; if you have a water safety plan, or you’re planning on implementing one in the future, any greywater and/or rainwater recycling systems will need to be incorporated into the plan.

Need help with installing or maintaining a new water recycling system safely?

Dynamika can provide dedicated legionella risk assessments, microbiological testing and disinfection treatments for a wide range of commercial greywater and rainwater system technologies. Call us on 03333 22 0800 or get in touch online to find out more.