Down, or out?

The riddle of the banana peel: You're standing in the kitchen, with a banana in one hand and the peel in the other, and before you can eat the banana you have to decide what to do with the peel. The sink disposal, or the garbage can?

One is easier than the other, one is quieter. But which one is better? Which puts the least strain on the environment?

(So, OK, composting would be better than either, but put that aside for now; a lot of people lack the yard space for compost, and others lack the will.)

The people at Baltimore's Department of Public Works aren't going to tell you what to do; they're fine with both methods. But they're happy to describe what each entails.

You talk to Robert Mohr, division chief for waste water facilities, and he tells you that after your chopped up banana peel has been carried through the city's sewers to either the Back River or Patapsco wastewater treatment plants, it passes through a filter and a sedimentation tank before it comes to what he calls the activated sludge process.

Microbes feast on it (and in doing so they take nitrogen, which comes from other sources, out of the mixture). Clumping together, they begin settling out as sludge. This goes to an anaerobic digester, which extracts methane; some is used to heat the building, and the rest is flared off, though the city has plans to use it to generate electricity.

The rest of the sludge goes to a dryer, where its moisture content is reduced from 80 percent to 5 percent, and then it is converted to pellets by a private company called Synagro. The pellets are used as fertilizer and are being experimented with as a fuel source for a Lehigh Cement plant in Carroll County. The biggest use of energy in this whole process is in the 900-degree, natural gas-fired dryer.

Then you talk to Joe Odziejski, head of the bureau of solid waste, and he tells you what happens when you put your banana peel out back for the garbage pickup. More than 90 percent of the city's household trash (and a lot from Baltimore County, too) goes to the BRESCO plant off Russell Street. Every week, the city's garbage trucks go about 6,000 miles, at four miles to the gallon. That's about 75,000 gallons of fuel annually.

At BRESCO, which gets $33 a ton from the city, everything flammable (and that includes banana peels) is incinerated to make steam - about 4 million pounds a year. (You realize you didn't know that steam was measured in pounds.) Some goes to heat buildings downtown, and the rest is used to crank out electricity - nearly 300,000 megawatts.

Any household trash that doesn't go to BRESCO ends up at the Quarantine Road landfill, where rotting banana peels and other garbage produce methane; the city will soon be capturing it and selling it to the Coast Guard shipyard nearby, for $200,000 a year.

So which is better? Both methods give something back - fertilizer or heat or power. The wastewater system is cheaper to run, with 522 employees against solid waste's 1,008, and with no plastic bags or trucking involved (though there is that little jot of electricity to run the disposal). The emissions controls on the BRESCO plant are complex, expensive and imperfect.

But the sewage treatment plants have drawbacks, too. Back River removes just 70 percent of the nitrogen, a major source of bay pollution, and Patapsco gets only 31 percent. Kim Coble, Maryland executive director of the Chesapeake Bay Foundation, says the issue isn't the banana peel itself, which is mostly carbon and oxygen, but the extra load it puts on the plants: The more sewage there is, the faster it moves through the process and the more polluted the discharge water will be when it is released into the river.

At the same time, a study by New York City, which once banned disposals, found that they typically increase household water usage by about a gallon a day; that's not much.

In the end, it's your call. It would be interesting to know how other people come down on this. Here's a vote for garbage collection: At least it gives you a chance to walk to the alley and back, get some exercise and maybe even meet a neighbor. Just don't slip on the banana peel on your way out there.