From Is a Dishwasher a Green Machine?:
To really green up your automatic dishwashing, you should always use the air-drying function, avoid the profligate “rinse hold” setting, wash only full loads, and install the machine far away from your refrigerator.
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Just promise that you’ll scrape your dishes instead of pre-rinsing, use the shortest wash cycles possible, and buy phosphate-free detergents – or, if you’re handy with a blender, make your own.