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Rainwater harvesting

Is rainwater harvesting suitable for you?

Installing rainwater harvesting systems can provide a variety of economic benefits, as well as reduce the risk of flooding in some areas. Rainwater is ideal for many uses, including cooling systems and appliances, as there is a lower concentration of salts to build up. If water requires treatment to a high standard, cost and annual maintenance will be higher than a system requiring only low quality water. Generally, the systems with the fastest payback periods utilise large collection areas to supply a constant demand of general quality water. In certain commercial installations, the project payback can be as short as 2-3 years.

Will a rainwater harvesting system provide benefits to your business? Follow these steps to find out:

Step 1: Quantify the amount of water you currently use 
This can most easily be done by checking recent water bills - it will probably be useful to develop a spreadsheet for your business with your water consumption for different seasons of the year. This will also allow you to see the effects of rainwater harvesting on your water costs.

Step 2: Quantify the maximum water you can harvest in a year 
Use the following equation:

Annual rainwater yield (Y) in m3 = P x A x 0.8

Where P = annual precipitation (in metres); and, A = collection area (in square metres); 0.8 = typically, you should expect to collect approximately 80% of this water each year, due to small losses in filtering and small rainfalls that do not generate enough runoff.

The table below illustrates how much water you could save based upon your collection area. Annual
rainfall data can be obtained from the Met Office website

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Step 3: Quantify the cost
Check your water bills to find out how much you pay for water and how much you could save by using rainwater instead.

What should also consider before installing rainwater harvesting systems?

Water quality
When considering rainwater harvesting system installation, water quality and its potential reuse must be considered. For example, a food processing plant may require large quantities of high quality water, whereas water for staff toilets and garden areas will require less pre-treatment.

Storage - tanks and pipework
Once volume and required water quality is known, you should determine where to locate the rainwater storage and consider modifications to your existing drainpipes.

TIP: If you are planning on connecting the rainwater system to existing plumbing, you need to prevent any collected water from accidentally pushing back into the mains system. Your plumber should be able to advise you on the best location to install sufficient backflow protection.

Also consider safety of others; use signs to indicate the quality of water if available through a tap. If appropriate, tap fittings that cannot be interchanged with standard town supply tap fittings could be used.

Running out of water need not be a concern - a mains connection will ensure that systems are topped up when they get very low. A large tank will give you more capacity to store the water from heavy downpours, increasing savings, but cost more to purchase. Also consider tank location carefully; available options include under or above ground.

Ideally, a balance should be found, where water from the wettest time of year is collected without tank
overflow. There are many companies that can help you with developing bespoke rainwater harvesting systems: details can be found on UK Rainwater Harvesting Association (UKRHA) website.

The UKRHA reports that annually, over 400 rainwater harvesting systems are installed in the UK. With increasing water prices, the feasibility of rainwater harvesting is becoming more favourable. The UK has fairly regular rainfall spread during the year, meaning unused space in storage tanks is reduced, making UK rainwater harvesting projects even more ideal. Further guidance can be found on UKRHA website.