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Is it better to wash the dishes by hand or in the dishwasher? There may be a discussion at the family table or in the office among colleagues: machine wash or hand washing, which has an advantage on the environment when it comes to cleaning up dirty plates, bowls and cups?
The answer is not so simple. It is necessary to calculate the water and energy consumption of the device, not only during its operation, but also during its initial production and end-of-life.
As for water consumption, the debate was quickly settled: a washing cycle in a dishwasher consumes from 10 to 20 liters of water, while running the dishes under the tap for five minutes requires 25, depending on Climate and Energy Transition Office Quebec Ministry of the Environment.
For a device to obtain Energy Star certification, water consumption is also the most stringent criterion: certified dishwashers must use 30% less water than “classic” models.
A team of American researchers decided to get to the bottom of this debate by comparing several methods of washing, taking into account the life cycle of the devices. She asked 38 people to fill a dishwasher as if they were at home. Another group of 37 participants had to wash dishes by hand, according to each person’s habits.
It should be noted that this study was funded in part by Whirlpool, but was conducted entirely by independent researchers at the University of Michigan.
Their findings were peer-reviewed Published in 2020 in the journal Environmental Research Communications. Overall verdict: A properly used dishwasher produces fewer greenhouse gas emissions on average in the United States, than does “typical” hand washing.
hot water power
says Gregory Kiulian, one of the researchers who conducted the study and director of the Center for Sustainable Systems. From the University of Michigan: “Consumers have a huge impact on their environmental footprint, through the products they consume.”
It should be noted, however, that these findings are more true for the United States, but less true for Quebec, since the calculated footprint depends largely on the type of energy used. However, hydroelectric power, which accounts for 97% of the total electrical energy consumed in the governorate, has little impact on the environment compared to the energy used in the US scenario considered in this study.
By comparison, hydroelectric power accounted for only 6% Of the national average sources of electric power in the United States, coal, approximately 22%, gas, 38.4%, and nuclear, 19%.
But no matter what kind of energy is used, “behaviours also matter a lot,” says Keulian. In this particular study, participants washed dishes by hand using the one or two bowl method. This involves first immersing the dishes in hot water, to scrape off stuck-on food or letting it soak. In a sink or a second basin, we rinse the dishes only in cold water, and then leave them to dry in the open air.
In fact, it is this method that produces the least greenhouse gas emissions in the American context. “But that is only if you are a good cleaner,” the researcher specifies, which is not really given to everyone. In the exercise conducted in the laboratory, only 16% of the participants actually used this way of doing things, compared to 41% who washed dishes under running water, and left the tap running, which is the method that shows the worst results.
Dishwasher usage habits are also very important. The expert explained that the habit of rinsing dishes before filling the device is “stubborn”, as 74% of the participants did so. However, it increases emissions by 3%.
According to this study, turning off the warm air drying option reduces the effect by 11%. The initial effect of factory dishwasher production was extinguished over a period of 10 years, according to a routine of four washes per week. “Dishwashers usually last between 10 and 13 years,” says Keulian.
He adds that performance standards are “getting stricter” in terms of energy and water consumption, which he hopes will continue to improve.
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