The Carbon Cost of Water
The focus of Lifecycle Carbon Assessments is typically embodied carbon of materials. Although the carbon emissions associated with manufacturing, construction, and transportation of materials are crucial to understanding the overall impact of a project, the water consumed to make these products is not currently included in the embodied carbon value of these materials. In the context of a sustainable site development or building project, we are concerned with water availability and flood risk reduction but rarely look at the embodied carbon of water of a project’s lifecycle.
Water usage is tied to everything we do. We use water for our domestic needs, food production, livestock husbandry, landscape irrigation, waste management, materials production, natural resource extraction, and construction among others. Most importantly, water is part of a feedback loop with energy, known as the Water-Energy Nexus, in which water is needed to provide energy (alternative energy generation and fossil fuel extraction) and energy is needed to provide water.
Energy, primarily in the form of electricity, is required for water distribution, treatment, and heating. The United States uses about 521 million MWh/yr on water supply alone, which accounts for about 13% of the total U.S. electricity consumption. This in turn translates to about 290 million metric tons of CO2 emissions per year, which make up 5% of all U.S. carbon emissions and is equivalent to the annual emissions from over 62 coal fired power plants.
In terms of embodied carbon, it can be estimated that water usage contributes about 4,900 pounds CO2/Mgal, or 720 pounds CO2 per year per household, from water alone.
Unlike many other trends in systems efficiency and technology, the water-carbon footprint is growing without garnering much attention. As climate change continues to make freshwater sources less reliable, we must resort to energy-intensive means of potable water production more frequently, such as desalination. In addition, global population growth not only increases water and energy demand but will continue to stress our limited freshwater resources and require additional treatment and distribution infrastructure to keep up with demand. Large-scale treatment facilities used to supply potable water still largely rely on energy-intensive treatment processes.
Luckily, we already have the means to begin reducing the carbon cost of water and it starts with awareness. Water conservation and water efficiency measures can have a huge impact by reducing unnecessary demand on our water supply systems. Simple-to-implement methods of stormwater management and water capture reuse—such as rainwater harvesting, cooling tower blowdown recovery, building condensate capture, etc.—also provide on-site reuse and recirculation of water. Water reuse is not only smart economically but also reduces the energy demand of water, and thereby carbon footprint, by reducing distribution distance, treatment volume (restricting potable water from non-potable uses), and even heating and cooling through innovating heat recovery systems.