The figures speak for themselves: according to the Energy Saving Trust (EST) the average family uses 150 litres of mains water a day. As much of this is to flush the loo, run the washing machine or clean the car, that’s a big waste of the energy that’s been expended on making the water fit for human consumption. It’s also a waste of money: by using some of the 85,000 litres of water that falls on your roof each year for such purposes instead, you could greatly reduce your metered water bills.
So, how do you go about it? Rainwater harvesting can be anything from a simple water butt – available from about £20 from DIY sheds, and even from local councils – to underground systems that pump water to header tanks, or straight to your toilet, washing machine or dishwasher.
A butt, or even an old barrel, placed under a gutter downpipe can collect water for dousing the garden or washing your car. To prevent overflowing, use a diverter system to redirect water back to the downpipe when the butt gets full, or attach an overflow to drain away any excess water. Remember to put a filter over the incoming pipe to prevent leaves and debris getting in, and ensure the butt is installed high enough to slip a watering can or bucket underneath.
It’s not essential for the butt to fit directly under the downpipe – by cutting a notch out of the pipe and fitting a rain trap and connecting or diverting pipe, you can put your butt in the most convenient place.
A more sophisticated technology, this enables collected water to be used inside the house, directing rainwater from the roof (first check your roof is clean and not made from toxic metal or asbestos) into an underground storage tank installed close to the house. From here, a submersible pump supplies the toilets, washing machine and dishwasher on demand.
There are two types of system: a gravity-fed version that needs a header tank, or a direct system that pumps filtered rainwater straight to WCs and appliances. The latter will deliver water at mains pressure, but has the disadvantage that if your electricity supply is cut for any reason, you’ll be left without a rainwater supply, too. With this sort of system it might be wise to leave one toilet on mains water, just in case.
There are three sizes of domestic tank – 3,500 litre, 4,700 litre and 6,500 litre – your selection will depend on the size of your roof, average local rainfall and the household’s estimated consumption.
Before entering the tank, the water first passes through a filter to remove any surface debris, then a water-smoothing inlet so it doesn’t disturb the sediment at the bottom of the tank.
The tanks are designed to overflow two or three times a year into a soakaway or storm drain to sweep and scum from the surface, which also cleans the filter. And to avoid drawing surface scum into the system, water is drawn off via a floating filter from just below the surface. As the water is kept dark, cool – below 18°C – and well oxygenated, no harmful organisms can grow.
In remote areas, rainwater can be upgraded to fully drinkable standard using non-chemical ultra-violet sterilisation.
During dry spells, when the rainwater in the tank gets low, a sensor is activated and either mains water is drawn in to top up the tank, or the system switches over automatically to mains water supply.
A one-way valve ensures that the liquid in the tank cannot back up into the mains water system.
Building Regulations cover the installation itself, siting of the tank and pipe runs. You are required to use separate pipework for the rainwater-fed appliances, using a different colour or otherwise making them easily identifiable, and you must inform your water supplier if your system uses back-up from the mains.
Planning permission isn’t normally required. In fact, some councils suggest that including a system as part of a new home scheme can help with planning applications.
Systems can be retrofitted in existing buildings, but it’s far easier to fit them when building a new property or extension, as the installation of the tank and piping won’t cause too much disruption.
According to Ecozi, fitting a system in a new build is pretty low cost, as most of the tasks required to install a rainwater harvesting system – digging drains, installing plumbing, etc – have to be done anyway. The largest additional task is excavating the tank hole, the cost of which will depend on what you plan to do with the spoil.
The tank itself is made of very strong recycled polyethylene and is typically guaranteed for 15 years, but should last indefinitely. Being underground, it is not affected by frost. The electronic control system and pump are typically guaranteed for two years.
As for maintenance, the only regular chore you’ll need to stay on top of is to check the filter every quarter, and flush it through with water from your garden hose.
The outlay for the initial system will be between £2,000 and £3,000 for a good quality domestic version, depending on the size of the tank.
As pumping costs are estimated to be less than 10p a week, the UK Rainwater Harvesting Association claims that the payback period on domestic systems is between 10 and 15 years. The good news for those looking to install a system is that, as water bills are expected to rise 10% a year for the next five years, the payback period could be much shorter than that.
Main image: The Envireau rainwater harvesting system by Kingspan Water
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