In the United States and most of the rest of the industrialized world, energy and water have a symbiotic relationship; it’s sometimes called the “water-energy nexus.” A great deal of energy is expended to extract, move, use, and treat water. And fresh water is a key ingredient in most forms of energy production.
In 2010, approximately 3,605 terawatt-hours of energy—roughly 12.6% of U.S. total primary energy consumption—was consumed in water-related activities throughout all sectors of the economy. Most of that energy is in the form of electricity to run pumps (though some pumps operate on diesel or other fuels). Water is also treated with energy, again mostly in the form of electricity.
The heating of water is a significant energy use within buildings, and also in industrial processes—from food processing to textile manufacturing. Although solar water heaters are becoming more popular the overwhelming majority of water heating is done directly by fossil fuels (usually natural gas), or by electricity—which is often generated by fossil fuels.
Meanwhile, water is essential to our current energy industry, accounting for approximately 15 percent of global water use—second only to agriculture. It is a critical resource used in drilling or mining for natural gas, coal, oil, and uranium. Resource extraction often produces wastewater, which must be treated or injected deep underground. Oil, natural gas, and uranium require refining, which again uses substantial amounts of water. Water is also used to transport coal through pipelines as a slurry—finely ground coal mixed with water.
More than 90 percent of U.S. power plants are thermoelectric, using heat from coal, natural gas, or uranium to produce steam for generating electricity. Nearly all thermoelectric plants use water cooling. After water from rivers, lakes, or seas is used for power plant cooling, it is released back into its source at a higher temperature; this thermal pollution can impact aquatic organisms.
Renewable energy power plants sometimes also require water (though usually much less than fossil fuel power plants). The most obvious, of course, are hydroelectric dams—by far the largest current source of renewable energy in the world. But geothermal power relies on water to convey heat within the planet’s crust to the surface, while solar panels require periodic washing to remove dust or to cool concentrated solar power systems. (With geothermal power, the permeability of “hot rock” in some situations may be increased by fracking—which brings a host of additional water pollution concerns.) There is some concern about the potential stresses of renewable energy systems on global water resources, depending on energy demand and system (in)efficiencies.
This issue will be compounded by global climate change, which is expected to increase water scarcity and extreme weather around the world. Droughts and mega-storms not only cost lives and devastate communities’ infrastructure, food systems, and livelihoods; they can also threaten electricity sources.
Example: Household Water Use
A recent study of residential water use showed that U.S. households consume roughly 88,000 gallons per year on average. Indoor household water use comes to 137 gallons per household per day, 24% of which is used just for flushing toilets. Outdoors varies greatly across the country–in California, for example, more than 50% of household water use is for landscaping.
Energy is involved in every stage of household water use—sourcing, treatment, distribution, consumption, and disposal of wastewater.
Extracting water from rivers and streams, or from underground aquifers, and then transporting it to storage facilities (often at great distances and even over hills and mountains) requires a great deal of energy, usually in the form of electricity. The water is then treated at a municipal water facility through a multi-stage process to remove debris and sedimentation, and “purify” the water by the addition of chlorine and other chemicals. Energy is used throughout this process, usually in the form of electricity, but is also embodied in the construction of facilities and the manufacture of chemicals—many of which are derived from fossil fuels.
The water is then distributed to homes through a network of large and small pipes. Energy is used, usually in the form of electricity, to pump the water but is also embodied in the manufacturing and construction of the large network of concrete, copper, steel, iron, and PVC pipes (the last made from natural gas liquids).
On average, one-third of the water consumed indoors by households is heated. Most Americans heat their home water with a natural gas-fired tank, which keeps the water hot—usually at about 140 degrees—day and night, even when the family is away on vacation.
After water is used in the home—by flushing the toilet, washing hands, taking a shower, or running the dishwasher—the “waste water” is usually transported through a different set of pipes to a municipal waste water facility.
Wastewater goes through a complex, multi-stage process of treatment before being released back into a river or lake, or being reused. Sludge produced during these treatment steps is pumped to digesters, where it is dewatered and dried before being transported off-site for sludge disposal. Most of these stages use electricity, while the final sludge transport uses oil.