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23 April 2019
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Rainwater harvesting

Recycling rainwater has become an important aspect of many new building projects. A bespoke harvesting system that uses recycled rainwater can cut water bills while protecting natural resources.

 10 February 2011

1⁄ Establish use

Rainwater can be collected from roofs and other outdoor surfaces, stored, and then used in place of mains supply for several key building functions:

- Flushing of lavatories
- Washing vehicles
- Fire fighting
- Topping up ponds or fountains
- Washing of textiles
- Topping up heating or cooling systems and industrial process tanks (subject to possible requirements for additional filtration or treatment).

2⁄  Harvesting
Rainwater harvesting reduces the demand for treated and purified mains water in buildings. The Environment Agency estimates that as much as 80 per cent of the mains water used by commercial and industrial buildings could be replaced by harvested rainwater. Since the price of mains water is expected to rise, installing rainwater harvesting systems could result 
in significant savings.

Installing rainwater harvesting may enable organisations to negotiate a reduction in local authority charges for surface water drainage. There are also tax incentives for the use of rainwater harvesting – the Enhanced Capital Allowance (ECA) scheme enables UK businesses to claim a 100% first-year capital allowance on investments in rainwater harvesting systems. This enables companies to write off the whole cost of the rainwater harvesting system against taxable profits.

3⁄  Legislation

Inevitably, certain red tape has to be negotiated, but if you apply the guidance set out in the HVCA TR/36: Installation of Rainwater Harvesting in non-Residential Buildings, arriving at an effective and compliant installation will 
be much easier.
Below is a summary of the key pieces of guidance and legislation that installers must adhere to:

l Risk Assessment to BS 31100: This has to be done at the 
design stage. Harvested rainwater systems should be designed in detail in accordance with BS 8515 Code of Practice for the Installation of Rainwater Harvesting Systems (2009).

- Water Supply (Water Fittings) Regulations 1999 (England 
and Wales)
- Scottish Water By-laws 2004
- Water Supply (Water Fittings) Regulations (Northern Ireland) (2009)
- The Building Regulations (England and Wales) 2000 (Part G)
- Building (Scotland) Regulations 2004
- The Health and Safety at Work Act 1974
- The Control of Substances Hazardous to Health (COSHH) Regulations 2002.

In addition, before any system is installed, the design must be submitted to and approved by the local water supplier, the sewerage undertaker, the local building control officer.
Any external built structure to hold tanks or water storage may also require local government planning permission.

4⁄  Techniques

Most rainwater harvesting systems, particularly on non-domestic buildings, use the roof to collect the rain. Most commercial buildings have more than enough roof area to collect the water needed

- Rainfall is conveyed to the storage facility by gutters and pipes to the storage facility at ground level

- Filtration is necessary at the point where the rainwater enters the storage tank so that vegetable matter, such as twigs or leaves, does not enter the storage facility to rot and cause bacterial build-up

- If, for some reason, a storage tank has to be installed below sewer level, more detailed consideration has to given 
to risks of overflow from the 
sewer during heavy rainfall.

Porous paving collection:
An alternative to collecting from 
a roof is porous paving collection:

- Permeable or porous paving, either at ground level or on an outdoor mezzanine, allows rainwater to pass through or between paving

- There are paving materials available that mimic grass or gravel surfaces, some of which are able to support the weight of heavy vehicles

- Filtration to BS8515 is likely to be necessary at the entrance to the rainwater storage facility when water comes from porous paving collection.

Mains water top-up:

When low rainfall means that the rainwater harvesting system does not yield enough water for the building’s need, there will need to be a mains water top-up system. This simply operates like a lavatory cistern, topping up the storage tank when the stored water falls to a certain level.

Paul Hancock is chairman of the HVCA’s Technical Committee