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Rainwater Harvesting: A Beginner's Guide

Rainwater Harvesting - Eco friendly home
(Image credit: Getty Images)

Rainwater harvesting is a great way to collect rainwater, reuse it and reduce your need for using excess water from the mains. 

This is becoming more important due to extreme weather caused by climate change. Very dry and very hot weather is making water shortages an increasingly urgent issue, especially when combined with population growth.

The Energy Saving Trust says that the average UK household uses around 330 litres of water per day (for two-person households the figure is 276 litres. With four people it rises to 450 litres). In 1960, the average was just 165 litres per day.

Below we set out what you can do in order to harvest rainwater at home.

(MORE: Inspiring Eco Homes)

What is Rainwater Harvesting?

Rainwater harvesting is an eco-friendly way of collecting and reusing rainwater in our daily lives which would otherwise just wash away.

Some people use it for outside purposes only such as garden irrigation and other people invest more to get it set up to use the water inside for laundry and toilet flushing.

How Much Water Can I Collect?

When you start to think about rainwater harvesting it’s important to know what kind of rainfall you can expect and what you may be able to collect.

The average annual rainfall for each region can be found at Current Results and this is a good starting point for finding out.

Then you need to do the following in order to determine how much water you can collect:

  • Calculate the ‘flat area’ of the roof, which is usually the same as the footprint of the house below
  • Then multiply the rainfall by the roof area
  • Finally deduct 20% for evaporation and overflow will give us the figure we need

So as an example: 

A house with a roof area of 100m2, near Oxford, which has a rainfall of 660mm per year, the calculation will be 100m2 x 660mm x 0.8, which equates to 52,800 litres of potentially collectable rainwater each year.

House Designed by Ellis Williams Architects

(Image credit: Ellis Williams Architects)

How Much Does a Rainwater Harvesting System Cost? 

Rainwater harvesting can cost anything from £30 for basic garden irrigation to £4,000 for a whole house water pump and tank.

If you only want to simply irrigate your garden, then properly organised gutters and downpipes emptying into a couple of water butts costing from £30 and storing 100 litres each, should provide all the water you need.

Intense horticulturists might need a butt on each corner of the house to give sufficient storage for the drier months.

If you want to do more outside, then £750 will buy a 700 litres storage tank with a pump. This is likely to provide enough water for the garden, car washing and any other external activities.

Uses for Harvested Rainwater

To decide how much rainwater to collect, you need to know how you will use it. 

Typical domestic uses are:

  • Garden irrigation
  • Toilet flushing
  • Car washing
  • Laundry

Rainwater isn’t usually used to drink or bathe in (as it will need treating accordingly), but it is an option. 

Rainwater Harvesting - Water storage collection

(Image credit: Rainwater Harvesting Ltd)

Can You Drink Harvested Rainwater?

Regulations specify that rainwater is ‘not wholesome’, meaning not drinkable. It is possible to make rainwater wholesome by getting it tested so the appropriate filters can be added. 

There are testing companies all over the country, so it is usually easy to get this done at a reasonable price.

Any pollutants found are likely to be either chemical, which a carbon filter will clean up, or heavy metals, which a redox alloy filter will deal with. Both of these are relatively rare, unless the property is close to an industrial area. 

In all cases, bacterial disinfection will be needed and this is done with an ultraviolet filter. All of the filters, including the UV lamp, will need to be changed at least annually, with an associated cost implication.

The effect of this is that the water produced will be clean and additive free. The government decided many years ago that we need additives in the water – fluoride and chlorine being the main ones – to keep us healthy. 

If you want to drink harvested rainwater, then we are in the realm of whole-house systems.

What is a Whole-House Rainwater Harvesting System?

A whole-house rainwater harvesting system involves piping rainwater from your downpipe to a storage tank and filtering it on the way to remove coarse material like leaves. Depending on the use the water is going to be put to, finer filtering and disinfection may be needed as well. 

The components needed for a whole house rainwater harvesting system include the following:

  • A storage tank
  • Pumps
  • Filters
  • Pipes
  • Connectors
  • Mains water back-up
  • A control system

Tanks can be either above or below ground, and the cost of these elements is broadly the same for above-ground and below-ground systems: in the region of £3,000 to £4,000, including VAT. 

Above-ground systems are cheaper to install, but are seldom pretty, while below-ground tanks have the advantage of storing water at a relatively constant temperature, inhibiting bacteria growth and making the water safer to use for watering fruit and vegetables. 

There are now shallow-dig tanks available that are intended to reduce installation costs. These are slimmer, but still retain the same storage capacity.

Harvesting Rainwater - water butts

(Image credit: Getty Images)

Is Rainwater Harvesting Worth It?

Today homeowners are looking to make eco-friendly changes with solar panels and renewable energy installations on the rise 

When it comes to water waste, Building Regulations already require that no more water leaves a plot after a new house is built than left previously, a rule aimed at protecting the increasingly overloaded drainage system. 

We therefore have to deal with the rain that is falling onto our plots in one way or another. The difference in cost between land drains and a rainwater harvesting system is not so great as to break any new build budget, especially for people who enjoy a bit of horticulture on a weekend.