There are many problems with landfills — even the biodegradable trash that ends up there produces a harmful greenhouse gas, methane, as it breaks down.  Fortunately, many cities now have recycling programs and even composting programs to help reduce landfill waste.  But household composting is still far from ubiquitous.  In the absence of having a compost bin, is it better to put food waste down the garbage disposal or in the trash?

Putting food down the garbage disposal keeps it out of the landfill, but putting it into the water treatment system is not necessarily a good thing.  Grease and fats cause lots of problems, including burst pipes and sewage leaks.  Non-greasy food waste particles can also end up in freshwater and harm aquatic life.  Water treatment to remove food waste requires chemicals, and the process releases methane (rather than the CO2 that would be released during composting).  However, some water treatment plants make use of the methane and/or solid waste for green purposes.  So the best answer depends on your local circumstances.  However, there are a few consistent rules to follow:

The research is unambiguous about one point, though: Under normal circumstances, you should always compost if you can. Otherwise, go ahead and use your garbage disposal if the following conditions are met: First, make sure that your community isn’t running low on water. (To check your local status, click here.) Don’t put anything that is greasy or fatty in the disposal. And find out whether your local water-treatment plant captures methane to produce energy. If it doesn’t—and your local landfill does—you may be better off tossing those mashed potatoes in the trash.

If you live in Cambridge, MA, someone has already done your legwork for you, and explains how to do this research in your own region:

The MWRA [Massachusetts Water Resource Authority] focuses on green methods. The methane is used and the solid waste is used. The water is treated, de-chlorinated and diffused into the water system. The only things that go to the landfill are large non-organic items that weren’t supposed to make it into the water system….

Here’s the final count [for Cambridge, MA]:
Best option: Compost
Next best: Sink disposal
Not-so-good: Trash/Landfill

Also, if your city doesn’t offer compost, but does give you yard waste bins, you can give them a call to find out if they’ll accept fruit and vegetable waste as well.