Water management: sustainable solutions that flow naturally
DOSSIER C21 - The heatwave of 2023, which resulted in the drying out of soils and the weakening of vegetation and biodiversity, has made it abundantly clear that it is high time to rethink the way we use water resources and adapt recovery and treatment systems, at the level of buildings, towns and regions.
In the past, stormwater management based on the "all-pipe" approach has long since shown its limitations, with saturated networks, increased run-off and flooding, and greater pollution of natural environments. From now on, this management must be integrated, maintaining as much as possible the natural cycle of water at the scale of the project, so that it can be infiltrated and reused as close as possible to where it falls.
Managing rainfall at source requires knowledge of the local rainfall context, and in particular its seasonal dynamics, topography, landscape, geotechnics and outlets. There are major disparities between regions and even within them, depending on topography, maritime influences and wind regimes. It should be remembered that 80% of the rain that falls in France is small rainfall that infiltrates the soil to a high degree. As a result, whatever the region and its specific characteristics, the aim of integrated rainwater management is to maintain current rainfall at plot level using alternative techniques.
No artificialization, renaturation
With this in mind, one of the first measures is to avoid artificialising the land, which means emphasising the sobriety of development. As far as possible, areas of natural soil should be restored or maintained, and hydraulic functions can be combined with a variety of uses, such as paths, leisure areas or car parks. It also seems appropriate to proceed with soil renaturation, a new term that reflects the paradigm shift in the way we take into account soil, how it functions and all the services it provides.
This is one of the objectives of the "Cours oasis" programme, which aims to replace tarmac surfaces in schools with permeable, vegetated surfaces. The permeability of surfaces must also be optimised. If for technical reasons (load-bearing capacity, pull-out, PRM accessibility, etc.) a constructed surface is necessary, there are nevertheless permeable surfaces (paving, flooring, slabs, porous concrete, etc.). Pavements with the highest possible coefficients of permeability and the brightest possible light (albedo) should be favoured to reduce the storage of heat. Pedestrian walkways and parking areas lend themselves easily to this exercise.
One way of managing rainwater more effectively is to defer - or even avoid - sending it to a collection system. Most of the time, collected rainwater is sent back into the networks, which increases the volumes of water to be treated, and can cause saturation downstream. Solutions can be found to slow down the flow of rainwater into the pipes (particularly useful during torrential downpours), or even prevent it from being discharged into the network altogether.
For example, planting vegetation reduces the amount of rain sent to the network, firstly by buffering (sponging) and then by evapotranspiration (reducing volumes). The plant cover provides other benefits (improved thermal comfort in summer, refuges for wildlife, revitalisation of streets, etc.). When planting, it is important to ensure that the plants have access to water, even in dry periods (hydraulic studies showing the presence of reserves, access to the water table), and if necessary, to create reserves (reservoir, pond, etc.). The planting period must also be studied and planned so as not to subject the plants to water stress during the recovery period. It also seems necessary to limit steep slopes, which increase run-off and erosion, to encourage the planting of walls (roofs and façades) and to landscape the water circuit within the project.
Finally, recovering rainwater and reusing it will help preserve drinking water resources, thanks to above-ground or underground storage systems. Collection bins can be installed, or flexible tanks that are easy to install and conceal. There is also the concept of a rain garden adapted to the scale of the building, whose role is to collect rainwater with the aim of reusing it for any useful purpose, or sponge plots, which have the dual role of water retention zones and new planting opportunities on the renewed site.
Minimising, reusing and treating wastewater
One of the sources of damage to surface water (rivers, lakes) and groundwater (aquifers) is chemical or physico-chemical pollution, some of which comes from domestic wastewater. This pollution can have harmful effects on aquatic ecosystems and human health. The challenge is therefore to minimise the production of wastewater upstream, and to reuse it as much as possible by recovering it on site before sending it on to the wastewater treatment cycle.
It is possible to reduce the quantities of water to be treated by separating rainwater from other wastewater via disconnected circuits. Next, it is necessary to act on the proper use of buildings to reduce water consumption, limit grey water and/or reduce the pollutant load to be treated. Some schools, for example, offer guides for staff and pupils, as well as displays on good practice in the use of drinking water. To reduce pollution, healthy cleaning products with a low health impact should be prioritised. To replace products that are harmful to the environment and human health (bleach, ammonia).
Until now, the reuse of grey water was mainly for outdoor uses (watering, cleaning). From now on, it will include indoor uses (supplying toilets, cleaning floors), with an additional precaution in the context of an experiment or a prefectoral agreement (since 2015, regulatory work has been in progress on this issue). The part of the grey water that can be used is that from the bathrooms (baths, showers, washbasins and washing machines). Grey water from the kitchen, which contains more organic matter, is generally discarded. The grey water collected cannot necessarily be reused as it is, and has to undergo a more or less extensive treatment process, such as a micro filtration plant. With a capacity of 100 litres, the plant can supply a toilet with up to 14 flushes a day. Larger systems exist, such as the membrane bioreactor, which can save 2,000 litres of drinking water a day.
Once all possible solutions for limiting the use of drinking water and reusing grey water as much as possible have been put in place, polluted residual water must be treated before being released into the natural environment. According to the regulations, any building used for domestic purposes must be connected to a public sewerage system if one exists. Connection can be conventional, which requires civil engineering work and highly technical equipment maintenance.
It requires significant energy consumption and has a health impact linked to the physico-chemical products used, particularly in waste water treatment plants (WWTPs). Purification can, however, be achieved by phytodepuration, or purification by living organisms. This type of treatment is attractive because it is easy to implement and maintain, consumes little or no energy, requires no chemical inputs, produces little sewage sludge and uses plants that promote biodiversity. This process can treat wastewater volumes ranging from single-family homes to several thousand population equivalent (p.e.).
Designing to limit the use of water in buildings
In order to minimise the use of water and maximise the use of non-networked water, the first step is to identify in detail all the uses, their intensity and their timing. Alternative techniques and/or more water-efficient solutions can be considered at the design stage of the project, but also at the construction stage, through the choice of materials and the use of water during the works. Once the water (usually potabilised) has entered the building, its use must be limited, it must circulate as little as possible, its use must be maximised before it is evacuated, leaks must be eliminated, and it must be replaced as far as possible by rainwater/running water. This involves reducing flow points, such as toilets without water, including in communal establishments. In addition, pooling water use in collective housing seems to be an avenue worth pursuing.
A common practice abroad (Switzerland, Sweden, Australia, Singapore, etc.) is to create communal laundry rooms. This avoids taking up floor space in an insulated, heated area, adding heat to the living space during the summer months, and minimising the number of inlet and outlet ducts. There are also advantages in terms of user-friendliness and the sharing of more efficient equipment, with a view to saving on functionality. Water circulation also needs to be optimised, with the aim of using materials sparingly, and limiting the number of bends and welds. This approach also reduces the risk of leaks and loss of pressure. The circuit must include features for monitoring and facilitating maintenance. In addition to the physical components, it is important to designate people to monitor, control and intervene on the systems.
Water during the construction phase
On average, 42% of the water used on a building site is wasted. A new build consumes 50% more water than the renovation of an existing building. The main challenges are therefore to reduce the use of drinking water, avoid polluting it and recycle waste water on site. To optimise its use, dry assembly and workshop prefabrication are to be preferred, as well as limiting the cleaning of equipment with clean water and recycling collected water. For example, some dry assembly materials (wood, straw, metal, earth and stone) can be used almost entirely without water.
In addition, this choice considerably reduces the problems associated with drying times on site and the pathologies that can result. Dry foundations (cyclopean foundations, tyre foundations, driven or screwed piles) also reduce water requirements. To limit the use of water on site, it is important to optimise cleaning with water, install dry toilets and collect wash water, in particular by recovering water from concrete mixing plants, vehicles and mixer chutes after decanting.
Ultimately, there are easy-to-implement solutions for sustainable use of water resources on projects. The professional associations that are members of the Collectif des démarches quartiers et bâtiments durables have a role to play in gathering expertise and disseminating it in an educational way to professionals in the sector. Those involved are encouraged to implement simple principles, from the design to the maintenance of the building, based on concrete examples and inspiring feedback.
- This article was written by the Collectif des Démarches Quartiers et Bâtiments Durables (Envirobat Occitanie, Ekopolis (IDF), Batylab (Bretagne), Terragilis (Bourgogne-France-Comté), Odéys (Nouvelle-Aquitaine) and EnvirobatBDM (PACA).
Read all the articles in french in the adaptation and resilience of buildings section
Ekkopolis