Rainwater harvesting systems can provide a variety of economic benefits to businesses. Alongside the standard irrigation and toilet flushing, it can also be used for cooling systems. In some areas it also reduces the risk of flooding.

Generally, the systems with the fastest payback periods use 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. To find out if a rainwater harvesting system will  benefit your business follow these simple steps:

• Work out how much water you currently use by checking recent water bills.
• Work out the maximum amount of water you can harvest in a year using the following equation: Annual rainwater yield (Y) in m3 = P x A x 0.8 (P = annual precipitation (in metres); and, A = collection area (in square metres); typically, you should expect to collect approximately 80 per cent of this water each year, due to small losses in filtering and small rainfalls that do not generate enough runoff – hence the 0.8.
• Work out the cost by check your water bills to find out how much you pay for water and how much you could save by using rainwater instead. Don't forget to include not only supplied water costs, but also waste water costs that would be reduced. Typically, your waste water charge is calculated at 95 per cent of the supplied water, but the price per m3 is higher.

Annual rainfall data for your area can be obtained from the Met Office.

Once you know how much water you need, you can look at where to put the rainwater storage and what modifications of existing drainpipes you need to ensure maximum collection.

Example: With an average rainfall of 0.6 metres in the south east of England, and a roof area of 25 x 20 metres, you could save almost £500 each year in water and waste water charges.

The following case study gives an idea of the size of the prize:

Leicester City Council has reduced the volume of mains water used at Humberstone Golf Course through rainwater harvesting for irrigation. Rainwater from the clubhouse is collected and stored, before being pumped onto greens - mains water use has been reduced by 20 per cent and the expected payback of the system is 5 years.

To make things more attractive, financially the Enhanced Capital Allowance (ECA) scheme1 enables businesses to claim 100 per cent first year capital allowances for investment in rainwater harvesting equipment named in the Water Technology List.

The following equipment is supported:

• monitoring and control equipment.
• rainwater filtration equipment.
• rainwater storage vessels.
• rainwater treatment equipment.

• Each cu metre of mains water uses 5.5KWh of electricity.
• Each cu metre of harvested water uses 0.25KWh of electricity.

photo by diongillard

About the author: Barry Nutley is co-founder and director of Viridis Energie Consultants

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