How it can lead to long-term savings on water bills and create a more sustainable way of life.

What is it?

Rainwater harvesting is the collection and storage of rainwater that would otherwise have gone down the drain and then re-using it within the home and garden. For example, flushing the toilet, supplying the washing machine and watering outside plants. These activities account for up to half of mains water used in the UK.

The average UK household uses around 330 litres of water per day, according to the Energy Savings Trust. In 1960, the average was just 165 litres per day. What’s more, the water we use to flush toilets and wash laundry has been treated with chlorine and other chemicals to make it safe to drink. Purifying mains water to drinking quality and then using it for cleaning purposes is wasteful – and we get charged for it. With the amount of rain that flows freely from the skies over the UK, why not use harvested rainwater instead?  

What can the saved rainwater be used for?

Pure rainwater is perfect for watering plants, flowers and grass. It’s a much better choice than tap water because it is soft. According to the Environment Agency, research in Germany shows rainwater is also preferable for clothes washing and may extend the lifecycle of a washing machine because there is no limescale build-up. Harvested rainwater can be used for general cleaning both inside and out. With no calcium or chlorine content, it’s perfect for washing windows and floors as it won’t leave spots or streaks. Saved rainwater can also be used for dishwashers, however, you may need to filter and purify the water before it can be used for this purpose. Collected and stored rainwater isn’t safe to drink or use for bathing due to bacterial and other possible contaminants.

Water butts and storage tanks

The simplest form of rainwater harvesting is the water butt which is usually used for irrigating gardens. Water flows from the roof down a downpipe (drainpipe) to the water butt where it is stored. A small fitting, known as a diverter, connects the downpipe to the water butt. When the butt is full, the water will flow directly into the drain. Those with larger gardens might want a butt on each corner of the house or to divert excess water to larger tanks to give sufficient storage for drier months.

A basic water butt costs about £75 and holds around 100 to 200 litres. But there is a variety of sizes, styles and colours available. If you like the idea of an unusual shape, you might consider a 300-litre Greek style imitation terracotta urn. Other options include bell-shaped, wood-effect, mock wine barrels and a plastic butt that looks like a giant tree trunk.  Perhaps not surprisingly, most people opt for a standard butt in green or black recycled plastic.

If you want to store more water outside, about £300 will buy a 400-litre underground water tank. There are also shallow profile tanks available that are easier to install as less digging is required but with similar capacity. Meanwhile £1,500 can get a 1,800-litre tank and pump with sufficient power to service a hose. This is likely to provide all the water you need for the garden, washing windows and the car. 

Whole house systems

If the rainwater is to be used for washing laundry and flushing toilets, the system needs to be plumbed into your property’s existing pipework. The rainwater will need to be finely filtered to remove leaves and other debris. Sterilisation may also be needed using chlorine or UV light depending on how the water is going to be used, for example a dishwasher.

You can either buy the components of a whole house system separately or together in a complete kit. Parts include the storage tank, pumps, filters, pipes, mains water back-up and a control system. Tanks can either be above or below ground and the cost of a whole house system is broadly £2,000 to £5,000, excluding installation.  

Aboveground tanks are cheaper to install but are more affected by sunlight and weather conditions, including freezing that may affect the system. They also take up a lot of space and can be unsightly. Meanwhile buried tanks have the advantage of storing water at a relatively constant temperature year-round, so it is less likely to go green and smelly. Plus, the buried tank can be hidden under a lawn or driveway.

Whole house systems can reduce household water consumption by up to 30% but the payback period can be lengthy. That’s how long it will take for the investment to be repaid through savings in your water bill.

What size tank is best?

A useful starting point is to work out the amount of water that could be collected from your house. Multiply the rainfall (mm) in your location by the flat area of your roof (m2) - which is similar to the footprint - and deduct 20% for evaporation and overflow. The result represents how many litres of water you can expect to harvest. Helpfully, the average rainfall for all areas of the UK can be found on the Met Office website (www. Metoffice.gov). So, for a house with a roof area of 100m2 in Southampton which has a rainfall of 779 mm per year, the calculation will be 100m2 x 779mm x 0.8 which equals 62,320 litres of potentially collectable rainwater a year.  As a rule of thumb, 5% of the available annual rainfall is a starting point for estimating the best size tank.

Specialist suppliers will be able to advise you. Some have online calculators to help estimate the capacity of tank you might need. Variables include the number of people or bedrooms in the house and average number of clothes washing cycles and toilet flushes per day. Direct Water Tanks range from 1,800-litres suitable for a two-bedroom house to 10,000-litre for a five-bedroom property.

Pump and gravity-fed

With whole house systems, rainwater is pumped from the storage tank, either directly to the points of use or up into a secondary header tank in the loft for gravity-fed supply. Direct Water Tanks systems work by supplying water directly to the point of use, for example when a toilet is flushed the pump starts and rainwater from the tank is pumped to fill the cistern. The pump automatically stops when the pressure in the pipework has been restored. If the rainwater in the tank reaches a low level, a limited amount of mains water is sent to the rainwater tank, leaving capacity free for the next rainfall. 

Freeflush rainwater harvesting systems use the alternative method. Rainwater is first pumped to a high-level header tank where it is this gravity-fed to the toilets and washing machine. This type of pump only needs to work when the header unit needs filling.

While rainwater harvesting cannot replace mains water completely, it can significantly reduce water consumption which if you have a water meter, will lead to long term savings - and create a more sustainable way of life. With climate change experts warning water shortages will become increasingly common, installing a system could make your home more attractive to potential buyers.

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