As awareness of water scarcity, water stress and environmental sustainability grows around the world, the concept of "water footprint" is becoming increasingly relevant. Unlike its more popular cousin, "carbon footprint", which focuses on greenhouse gas emissions, the water footprint (WF) provides a holistic view of water used throughout the life cycle of a product, process, or activity. It measures the amount of water consumed (directly and indirectly) and polluted –taking into account different types of water resources– and serves as a valuable tool for companies, policymakers, and individuals to understand and address their water-related impacts. There are even online calculators that measure our individual footprints through simple questions about our homes, appliances and even eating habits.
It is estimated that mankind uses 9 trillion tons of water annually. That's almost 300,000 tons of water per second, which is equivalent to 120,000 Olympic-sized swimming pools. And according to the OECD, global water demand is expected to increase by about 55% by 2050, due to increasing demand from manufacturing (+400%), thermal electricity generation (+140%), and domestic use (+130%). Unfortunately, the construction sector is also a voracious consumer, accounting for approximately 30% of global freshwater consumption, according to the United Nations Environment Programme. Contributing to this high consumption is the production of materials and activities during construction, such as concrete mixing, cleaning, and soil compaction, as well as activities that can result in water contamination and waste, impacting ecosystems and local water scarcity.
For a building, quantification of the water footprint in construction is usually performed through life cycle analysis (LCA) and environmental impact assessment methods. These approaches consider water use at all stages of the construction process, from the extraction of materials to the building's demolition, and provide a comprehensive view of the water consumption associated with construction. The water footprint of building materials varies significantly, depending on the type of material and the specific production methods used. Some of the main elements considered in the quantification are:
- Direct water use: This involves measuring the amount of water consumed at the construction site for activities such as mixing concrete, cleaning, irrigation, and providing drinking water for workers.
- Indirect water use: Also called Virtual Water, this covers the amount of water incorporated into building materials, from the extraction of raw materials to the manufacture, transport and assembly of building elements.
- Building life cycle: The water footprint also takes into account water consumption during a building's use and maintenance phase, including the use of potable water, wastewater treatment systems, and the operation of related equipment.
Another important concept to mention is the Blue or Gray Water Footprint. The first refers to the volume of freshwater consumed from surface or ground sources, which includes water used in the extraction of raw materials, manufacturing processes, and construction activities. The gray water footprint, on the other hand, represents the volume of freshwater required to dilute and assimilate the pollutants generated during the production of these building materials, which are responsible for water pollution and potential negative environmental impacts caused by the release of contaminants. To get a rough idea, here are some general water footprint estimates for common building materials, mostly based on the comprehensive study by researchers P.W. Gerbens-Leenes, A.Y. Hoekstra and R. Bosman.
Steel: Its production involves water-intensive processes such as cooling, cleaning, and processing raw materials. Chromium-nickel alloyed steel (18/8) has a blue water footprint of 77 liters/kilogram (l/kg) and a gray water footprint of 1500 l/kg, cadmium being the critical pollutant. Unalloyed steel has a blue water footprint of 11 l/kg and a gray water footprint of 2300 l/kg, also with cadmium as the critical pollutant.
Cement: Portland cement (CEM I) has a blue water footprint of 2.0 to 2.6 liters/kilogram (l/kg), depending on the source of gypsum used. The gray water footprint is 210 l/kg, determined by mercury if gypsum from flue gas desulfurization is used in production. Without the use of gypsum, the gray water footprint is 0.63 l/kg, with cadmium as the critical pollutant. Portland composite cement (CEM II/B) has a blue water footprint of 1.7 to 2.1 l/kg, with the same gray water footprint as above. The production of clinker by pyrometry mainly contributes to the blue water footprint due to the high energy consumption.
Glass: Soda-lime float glass, which is commonly used, has a water consumption of 5.8 liters per kilogram of glass produced. This is for glass using soda ash from the Solvay process. In addition, the production process for this type of glass generates a graywater amount of 1,300 liters per kilogram of glass, suspended solids being the critical pollutant found in this water. It is important to highlight that the Solvay process, responsible for the production of sodium carbonate, requires a large amount of water and the effluent from this process has high concentrations of heavy metals (and suspended solids and can have a high pH).
Brick: According to this study, the total water consumption footprint of a brick is estimated to be 2.02 liters, of which 1.71 liters (84.8%) correspond to blue water and 0.31 liters (15.2%) to green water. The theoretical gray water footprint of one brick was calculated to be 1.3 liters, a value that would be even higher if no on-site wastewater treatment was performed before the effluent was disposed of.
Wood: The water impact of wood in construction can vary depending on several factors, including the type of wood, the geographic location of the wood source, and the specific production processes involved. However, on average, wood has a relatively smaller water footprint compared to other building materials such as concrete, glass, or steel.
By quantifying the water footprint of different products, it is possible to identify water-intensive processes, inefficient use, and areas for improvement. This information guides the development of strategies and technologies to reduce water consumption, increase water efficiency, and mitigate water-related risks. Specifically for buildings, it is necessary to combine design strategies, technology, and changes related to behaviour, such as the use of lower water-use devices like toilets, faucets, and low-flow showerheads. In addition, it is vital to take a holistic approach that integrates water-efficient practices at all stages of a building's life cycle.
It is important to note that these estimates are general and can vary according to regional factors, production techniques, and specific material sources. In any case, have you ever stopped to think about the water footprint of your home or your latest architectural project?
This article is part of the ArchDaily Topics: Water in Architecture, proudly presented by Hansgrohe.
“Water is life and our passion. And water conservation is climate protection. We at Hansgrohe are committed to making a difference in how water is considered in Architecture, with products that save water while maintaining the same showering experience.”
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