Sustainable Development

Rainwater harvesting situation

Water harvesting and use in France and in the world but also some solutions to the problems

Published by Développement Durable the 2008-03-26
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The CERTU (the French Research Center on Networks, Transport, Town Planning and Buildings) has published the guide for water sanitation. It suggests an approach on this. It makes the difference between the different rain levels. There is level 1 (light rain) which aim is to avoid any release in the natural environment. On the second level (moderate rain) releases would be limited and controlled. In case of heavy rain (level 3), overflows might be localized and limited but these would be heavy and non controlled so it would permit ensuring the absence of important damages. In case of exceptional rains, overflow would be general but everything would be done to avoid damages in persons.

The water-borne sewage system strategy and the growing urbanization have showed the limits related to terrifying consequences. River’s floods in case of thunderstorm are stronger due to the soil waterproof and due to the ride time. The problem is specifically at the valley floor so it might be conceived urbanizations compatible with these restraints.

Rainwater harvesting led to the risk of leaving pollution bruises. Rainwater wash atmosphere and it is in charge of the pollutants at the time of its soil runoff. It is for this reason that it is better to intercept and clean these waters before to flow them into the nature. All techniques must be used: green roofs, infiltration (eavesgutter, trenches…), waterproof soils… to harvest this water.

The whole planet water is about 1500 millions of km3 of which most essential is located in oceans (97, 4%). Freshwater is represented by way of ice (2%), subterranean waters (0, 58%), upper water masses (0, 02%) and by way of atmospheric water steam (0,001%). Water is bountiful in our blue planet but there is little usable water for being use by men in a specific moment. That is for this reason that water harvesting is important in areas where freshwater is insufficient.

However, water production from this resource needs a purification installation and a distribution system to outweigh comfort and security given by the water under pressure purification network. Nowadays, different systems are commercialized for one family house or apartment blocks. It must be emphasized that French health authorities are very restrictive; they forbid in the heart of the buildings double networks, for drinking water and non-drinkable water.

Paris experience on this subject is regularly outlined. At the beginning of the 20th century, buildings were connected to non-drinkable water network for the industrial or handcraft uses and, in various cases, a bad connection have been pulled out within the buildings. Similarly, the recent two networks real property transactions analyses showed that there are many connection errors and pollution risks. Water storage, transport and distribution can affect water quality.

France has a tax credit (25% of the cost) for the rainwater harvesting installations for outer uses (and they are looking for do the same with interior uses).

Drinking water production form water rain in a decentralized way can be energetically more expensive than water produced by a centralized way. That is why the pilot scheme in Fribourg has ruled out the rainwater harvesting due to the place was leaded to the drinking water network and the resource was bountiful.

A research done in Netherlands shows that harvested water distribution leads to a change in population’s behavior and to an increase, in some cases in a 20%, of the water consumption. The research estimates the gains in a family which has replaced the use of the 40% of the drinking water in a year for the use of water rain.

However, some European countries have improved this position (Germany) while in some developing countries it is necessary having a rain water tank.

Rain water is a resource complement for some uses. In BedZed ecoquarter, in London surroundings, they have done efforts to save water (round 100 litres/day/person against the average of 170 litres/day/person in London) and just round 15% f used water comes from harvested water (for the W.C).

Finally, rain water harvesting is not a solution to drainage problems. In fact, the recuperator would want to harvest more water than its capacity and, on the other hand, the clean up system would most part of its keeping capacity in case of strong rainstorms.