The Carbon Footprint of Tap Water Is a Lot Higher Than You Think

Living on the shore of a Great Lake, I never worried too much about how much water I used, knowing that the world's largest supply of freshwater was just down the street. But according to a study by researchers at the University of Florida, it takes about 1.1 kilowatt-hours to treat and distribute 100 gallons of water, the average amount used per person per day in the United States. Paula Melton of BuildingGreen explains that much of this is due to the energy required for pumping, and points to a report from the Lawrence Berkeley National Laboratory:

Water systems are different across the continent, depending on the source. The University of Florida study looked at Tampa, Florida which got surface water from a river, and Kalamazoo, Michigan, which got groundwater from wells.

"The two systems evaluated have comparable total energy embodiments based on unit water production. However, the onsite energy use of the groundwater supply system is approximately 27% greater than the surface water supply system," write the authors of the study. "This was primarily due to more extensive pumping requirements. On the other hand, the groundwater system uses approximately 31% less indirect energy than the surface water system, mainly because of fewer chemicals used for treatment."

They also listed the lifecycle energy associated with water supplies based on different technologies and sources, which vary wildly. These are taken from different studies and were listed in megajoules, so I have done a conversion to kilowatt-hours: A cubic meter is 264 gallons.

That doesn't seem like much, but it is before distribution. The intent is to show how much it can vary, with desalinated water having 14 times the footprint of surface water.

Melton also reminds us the water then goes back to the utility for treatment, and we have to account for the energy used cleaning up the water before we use it and the cleaning it again after.

"According to the U.S. Environmental Protection Agency (EPA), water and wastewater utilities are among the largest individual energy users in a city, and they account for about a third of a typical municipal government’s energy use. Some cities use as much as 60% of their energy on these utilities. The energy consumed for water and wastewater treatment is around 3% to 5% of total global energy consumption."

That's an extraordinary number, higher than the energy consumption of aviation or ammonia which have a far higher profile.

A Look at a City by a Lake

Melton's comment about cities using as much as 60% of their energy on water and wastewater shocked me, and I wondered what it was where I live, in Toronto, Canada, sitting on the shore of Lake Ontario. The city has a remarkable water system designed after the first World War. R. C. Harris, the commissioner of Public Works, was worried it might be bombed in the next war and made it three times as big as was needed at the time to have redundancy, and it is still supplying the entire city.

The giant art deco plant in all the photos and that bears his name supplies a third of the water for the city. According to the city:

"The water pumping infrastructure distributes potable water from treatment plants and throughout the City. Since water treatment plants are located near Lake Ontario, water pumping involves moving water uphill towards the north end of the City. Pumping uphill uses more energy and requires high-level pumps. In contrast, sewage pumping facilities move sewage to sewage treatment plants. Since most sewage is flowing downhill, gravity assists with this process, reducing the amount of pumping energy required. Thus, sewage pumping is less energy-intensive than potable water pumping."

Toronto gets its water from the lake, cleans and filters it, and then pumps it uphill to reservoirs and water towers. It then runs back down by gravity to the water treatment plant a few miles to the east, which then dumps the treated water back into the lake. This has always seemed like a bad idea to me, given that the treatment plant can't remove hormones and antibiotics, relying on the classic "solution to pollution is dilution."

But they do a good job: I once fell out of my rowing shell and the coach who came to rescue me, who worked for the city water department, yelled out, "Don't worry Lloyd, the coliform count is low and we check the water 15 times an hour!"

Even though surface water is the cheapest and most efficient source of all municipal water, the amount of energy used is astonishing; water and sewer treatment together use 700 million kilowatt-hours per year and put out 50,086 tonnes of greenhouse gases, mostly from burning natural gas since Ontario electricity is so clean. It is the single biggest user of energy in the city, bigger even than the transit system (TTC). It's fully 32.8% of the city's electricity consumption and 30.35% of its greenhouse gas emissions.

However, every few years someone raises the issue that we are getting our drinking water from the same place that we dump our waste, and that maybe this isn't such a good idea. They then float the idea of a giant pipe from Georgian Bay on Lake Huron, upstream from most of the major cities on the Great Lakes. If this ever happens, one can expect that the carbon footprint and the cost of our water will go way up.

It's hard to convert the energy per gallon to a carbon footprint without knowing the energy mix. But Toronto gives the data, with the water system totaling 50,086 tonnes of carbon dioxide (CO2) emissions.

Given the volume of water, about a billion liters a day, it doesn't amount to much per liter, about 0.13 grams, giving the footprint of my personal water consumption about 21 grams of CO2 per day. Not the biggest item on my list, and a good time to remind readers that according to Mike Berners-Lee in How Bad are the Bananas, a one-liter bottle of water has a carbon footprint of about 400 grams, about three thousand times as much.

This post has been updated to correct mathematical errors.