Rainwater harvesting and greywater recycling systems often take a backseat to renewables, but minimising water use is just as pressing. Tim Pullen explains the pros and cons of such systems

According to Waterwise, the leading authority on water efficiency in the UK, London is drier than Istanbul, and the south-east of England has less water available than Sudan. They also state that the average person here uses around 150 litres of water a day — a figure that has been increasing at a rate of 1% per year since 1930. Add a rising population to this increasing individual demand and we can see why water conservation has become something of a hot topic.

Around 30% of the water we use in the home is quite literally flushed down the toilet, 24% is used for washing and bathing, while just 1.3% is used for drinking — although 100% is of drinking quality. We have a finite amount of water available. Clearly, at some point, something will have to give.

The Options

Rainwater harvesting and greywater recycling both generate water for use in the home. Recycled greywater and filtered rainwater are similar in that neither is toxic (black water), neither is drinkable, and they do the same job — they can be used for garden irrigation, washing the car, flushing toilets and for laundry.

Greywater recycling can deal with up to 60% – or around 140,000 litres – of the water entering the house (assuming a family of four). A rainwater harvesting system can provide 56,000-130,000 litres, depending on rainfall levels and roof size.

What both systems do is make more water available to the home. Hosepipe bans, consumption reduction for Code for Sustainable Homes or Building Regulations can be dealt with, with little inconvenience.

A borehole is another option in reducing demand on the water grid. They’re a well-established technology, but the practicality, depth and cost will be entirely dependent on the location. A borehole, capped with a manhole and a suitable pump installed, will cost upwards of £2,500 but could set you back over £20,000.

Greywater Recycling

A whole-house system which recycles and filters greywater from taps, baths, washing machines, and provides water for external use, flushing toilets and laundry, etc., can cost in the region of £4,000 to £6,000 installed, but there are cheaper alternatives.

The reAqua, for example, collects water from the shower and bath, filters and disinfects it and makes it available just for flushing toilets. It costs around £1,300 installed and is said to save 30% off a metered water bill. More interesting, perhaps, is the reAqua+ which costs an extra £1,000 but recovers some of the heat in the greywater and feeds it in to the central heating system.

Rainwater Harvesting

In basic terms, rainwater harvesting comes in two forms: systems that enable the collection of rainwater, which is then filtered, disinfected, stored and made available for use in the garden and house; or water butts. The former requires a storage tank, which is typically buried underground, but can also be installed above.

“An underground tank is preferable as it can’t be disturbed. For some, legionella is a concern too with stored water, but an underground tank keeps water below 11°C, meaning there’s no risk. The ground also insulates against frost,” explains Lisa Farnsworth of Stormsaver. ‘Shallow dig’ tanks, typically installed no deeper than 1-1.5m, can make installation an easier task, too.

installing a rainwater harvesting tank

‘Shallow dig’ tanks, such as F-Line Flat Tank available at Rainwater Harvesting, helps save on excavation, spoil and associated labour costs

We tend to think in terms of large, complex systems involving pumps, filters and a big tank — and a price tag north of £5,000. But rainwater butts are an often-overlooked option. They are cheap, easy to install, effective and can provide all the water the garden needs. With a pump fitted – such as the RainPerfect solar-powered rain butt pump kit< available at Rainwater Harvesting – they can even provide water for washing the car.

A couple of hundred pounds will provide all the water butts needed to meet the whole of a home’s external usage, and that can be up to 40% of the house’s total demand.

Slim-line Solution

Mike and Jane Fry self built their award-winning 72m² home on their former garden in Somerset. Keen to be as sustainable as possible, and to keep utility bills to a minimum during their retirement, the property is highly insulated and features solar thermal panels. There’s also a sedum roof, which reduces rain surface runoff, along with a rainwater harvesting system.

A low-cost sustainable home

“The rainwater harvesting is unlikely to pay for itself, but we included it for the feel-good factor,” says Mike.

The couple opted for Halsted’s Slim-Line Rainwater Harvesting System; a good solution for their tight plot. The narrow, above-ground tanks are now hidden on the house’s rear façade.

“Installation was a relatively straightforward DIY task, too. This system suits urban homes and smaller households, but you’d likely need a bigger system otherwise,” advises Mike.

Running Costs

Greywater and rainwater harvesting systems both use a small amount of electricity. In all cases there is a running cost to consider and a maintenance requirement. Running costs tend to be £30-£50 per year, but will depend on the system. While some of the gravity-fed systems are notionally running cost-free, there is still a need to change filters.

Greywater recycling requires a greater degree of filtration than rainwater and so maintenance costs can be higher too. It’s also worth noting that all systems include mechanical and electrical components, so there’s always the slight chance that they could be subject to breakdown.

Retrofit Solutions

Whole-house greywater recycling can be difficult in a retrofit as it will mean significant changes to the existing plumbing. Equally, digging a large hole for a rainwater harvesting tank may not be a good idea in an established garden; that said, an above-ground tank solves this.

There are also rainwater harvesting systems specifically designed for retrofit, like the Ifore and Astro Turf systems, which tend to be more compact and cheaper.


We ask Lisa Farnsworth, of rainwater harvesting manufacturer Stormsaver, what to consider:

“There are typically two stages of filtration with rainwater harvesting, and the filters will require cleaning/rinsing through every three to six months — sometimes just once a year, depending on the system. Most tanks are designed to overflow a couple of times a year too, meaning any scrum or debris at the top is removed. The main issue, however, is leaves. Clearing gutters regularly is important, and systems installed on homes in close proximity to trees could require more maintenance.

“Another consideration is the pump: ask your supplier/manufacturer how the water is pumped, and what the voltage is? This can have an impact on running costs and energy consumption. Our Monsoon system uses a 90W pump, compared to other systems which use a 800W submersible pump. This can reduce energy consumption by over 75%. It can be connected to a small solar panel too. There are limitations though; such pumps aren’t always possible with longer pipe runs (for example, if the tank is located at the end of a long garden).

“Also enquire whether the incoming supply is AC or DC. With a DC supply, the pump can be connected to a battery back-up so that if the power fails, the homeowner will always have a water supply for toilet flushing.”

In Conclusion

If the requirement is to make more water available to the house, then greywater or rainwater harvesting will both do it, and such systems come into their own in new builds where either system can be designed-in from the outset making them cheaper and easier to deal with. In terms of capital cost, running cost and effectiveness, there is little to choose between them; both have cheap and more expensive options.

The size, type and location of the property will be factors too, but ultimately the decision will be down to the homeowner’s preference. It’s also important to note that cutting down water in the first instance can make an impact. What’s more, water-saving options such as aerated taps, flow regulators, ultra-low flush toilets, etc., are all much cheaper to specify than whole-house rainwater harvesting and greywater recycling systems.

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