What's Better, Dishwasher or Sink?

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Yet another study weighs in on the most efficient way to clean dirty dishes.

The debate over washing dishes by hand vs. using a dishwasher has been raging on TreeHugger since its inception. In the earliest article I could find from 2005, the dishwasher came out the clear winner, with researchers from the University of Bonn saying it uses only half the energy and one-sixth the water.

Fifteen years later, we're still talking about it, and a new study published in the journal Environmental Research Communications shows not much has changed. Dishwashers still take the prize for efficiency, both in terms of energy and water used, but there are better and worse ways of using it, and of washing dishes by hand. The findings are interesting because kitchen cleanup is something we do every day, so why not learn the optimal way?

Forty participants were asked first to load and run a dishwasher and then to wash dishes by hand the way they would at home. They answered survey questions afterward about their dishwashing behaviors. Three other participants were then asked to load a dishwasher and wash dishes by hand following best practices. This meant not pre-rinsing dishes before loading in the dishwasher and using the recommended normal cycle with heated dry, rinse-aid, and high-quality detergent. The machines were assumed to be fully loaded, as 93 percent of participants reported being able to do this regularly. For washing dishes, this meant using the two-basin method "where dishes are soaked and scrubbed in hot water, rinsed in cold water, and are air-dried."

These 'best practices' differ from typical dishwashing behaviors. Most people use "sub-optimal loading patterns" and pre-rinse their dishes before loading into a dishwasher. They also run the tap while washing by hand, which wastes significant amounts of water, and rinse with hot water. The researchers found that these typical practices produce "5,620 and 2,090 kg of greenhouse gas emissions respectively based on washing 4 loads (8 place settings per load) a week for 10 years." So the dishwasher was less than half as bad as hand-washing, even when improper techniques were used.

When it comes to water usage, the benefits of dishwashers continue. Over the course of ten years, a dishwasher will use 16,300 gallons of water, 99.8 percent of which comes from daily use, not production; whereas, washing the same quantity of dishes by hand for ten years will use 34,200 gallons.

Learning proper techniques could go a long way toward improving one's footprint: "If manual dishwashers switched from typical to recommended practices, they could reduce emissions by 249 percent." The resulting greenhouse gas emissions from the recommended two-basin method were only 1,610 kg over 10 years. But that's not a whole lot less than a properly-run dishwasher at 2,090 kg, which suggests that using a dishwasher – especially if you're factoring in the cost of your time – really sounds like the way to go.

(It is important to note that the study was conducted with help from Whirlpool, a major dishwasher manufacturer, who contributed the research space at its headquarters in Michigan and provided the sample machines; and its employees were the ones asked to demonstrate loading machines – something at which they may be better than the average person. But the data analysis was carried out by independent researchers at the University of Michigan.)

You can read the full study here.