Rainwater recovery

photo Rainwater recovery


Rainwater recovery saves money and preserves water resources. It also has the advantage of mitigating the impacts of water runoff in urban areas, where the ground is increasingly paved over and sealed, exacerbating flood risk.
Rainwater can be employed for professional and industrial applications, excepting those that require the use of potable water fit for human consumption, as defined by article R.1321-1 of the French Code of Public Health, provided that specific regulations in effect are complied with, in particular EC regulation 852/2004 of 29 April 2004 of the European Parliament and Council pertaining to hygiene of food handling.


Within tourist accommodations, rainwater use is authorised only for floor mopping and scrubbing, and, experimentally and under certain conditions, for clothes washing. Rainwater recovered and used within the building is subject to the sanitation tax. It is more advantageous to recover rainwater for outdoor uses, notably garden watering, in order to save water. And even when using recycled water it is preferable to water in the evening, to limit evaporation losses.
Different recovery systems are available on the market
==> Downspout directly connected to roof gutter, with round or rectangular collection tank; 500-600 L of water can be recovered per m² of roof area, annually.
==> Underground or above-ground rainwater recovery tank, with filter system if connect to existing water supply lines
==> Capacity 200 to 2,000 L, 3,000 L, 4,000 L, 7,500 L
==> Cement or polyethylene models: cement tanks are more expensive but require less maintenance, last longer, and the water is of better quality.

Choose tank size according to needs and site constraints:
- assess needs (having answered the question of projected use for recovered water)
For gardening, needs are an estimated 17 L/m².
Annual needs for toilet flushing are estimated at about 8,000 L per person.
- determine the amount of rainwater that can be recovered.
As a rule of thumb, 600 L of rainwater can be recovered per m² of roof.
Tank capacity will depend on average annual rainfall in your region (in mm/m²) and roof surface area (in m²).
Remark: do not recover rainwater from roofs covered with tarpaper or asphalt sealing materials that release hydrocarbons. You must wait about one year before using rainwater from wood-shingled roofs, until it has become clear and tannins have been eliminated.
- choose the place where the rainwater recovery tank will be installed:
underground, above ground, in a basement, in a sanitary crawl space, etc.
An above-ground tank is ideal for small capacity recovery, and dispenses with the need to dig a hole.

Rainwater runoff from roofs can contain pathogenic micro-organisms. In order to prevent contamination of public water supply, water regulations stipulate that when a water supply other than the public supply network is used, water department employees or agents must have access to private property in order to inspect internal drinking water supply lines and water wells and drill holes (Law pertaining to water and aquatic environments of 30 December 2006, art. 57, and General Code of Local Authorities, art. L.2224-12). Inspection costs are borne by the subscriber.
If a risk of contamination of the public water supply is observed, the water department will order the subscriber to implement the necessary protective measures. If these measures are not implemented, the water department may cut off the supply line (see decree no. 2008-652 of 2 July 2008).


1- In-depth preliminary study to assess needs, depending on projected uses, the number of users, and type of building occupancy, and rainwater resources (local rainfall information obtained from the nearest weather station, monthly statistics or more frequent periodicity).
2- Determine tank size (see detailed explanations)
3- Evaluate payback time for all options
4- Installation is rapid, except for underground tanks, which require intervention by a specialist
5- Recovered water that is used inside the building and discharged to the sewer system is subject to the sanitation tax. The property owner is required to file a statement of use with the municipality (General Code of Local Authorities, art. R.2224-19-4).
6- Rainwater recovery conduits and outlets must be marked as "Not drinking water" (in writing or pictogram).


A rainwater recovery system costs between €4,000 and €6,000 (equipment and labour; all taxes included) for a complete system with 5,000 L tank (ADEME estimation).


Environmental benefits
 - Lower impact on water resources
 - Reduced treatment of wastewater discharged to public sewer system
Economic benefits
 - lower water consumption, therefore lower water bills
 - tax credits (up to 31 December 2015), applied to 25% of costs for eligible equipment purchased between 1 January 2007 and 31 December 2015.


1- Very simple installation for above-ground tanks, no special knowledge required or need to call in a specialist
2- For underground tanks, verify feasibility of connection to your present system, the necessary modifications, the period of work that will entail noise and visual inconvenience
3- If recovered water is injected into your water supply line, regular maintenance carried out and validated by a professional to avoid all risk of contamination.