This week, New York City Mayor Michael Bloomberg announced a plan to increase composting of food scraps generated by the city's eight million inhabitants. In a few years, separation of food waste from other refuse could be required of residents, the mayor said.

A number of other cities around the country already require food scrap recycling, including San Francisco and Seattle, but the idea has been slower to catch on in New York, where critics worried that the urban density may make it more difficult—and possibly smellier.

But city officials told reporters that a composting pilot program fared better than expected. (The city has yet to respond to a request for comment from National Geographic.)

In his State of the City address in February, Bloomberg had called food waste "New York City's final recycling frontier." The mayor said, "We bury 1.2 million tons of food waste in landfills every year at a cost of nearly $80 per ton. That waste can be used as fertilizer or converted to energy at a much lower price. That's good for the environment and for taxpayers."

The administration says it will soon be looking to pay a local composting plant to process 100,000 tons of food scraps a year, or about 10 percent of the city's residential food waste. In the Big Apple, only residential refuse is handled directly by the city, since businesses must hire private disposal service providers.

A few businesses have already been diverting food scraps for composting on the private market, especially from high-profile "green buildings" like the Hearst Tower and Bank of America Tower.

The city says it also intends to hire a company to build a plant that will turn food waste into biogas—methane that can be burned to generate electricity just like natural gas. The food waste program is expected to ramp up over the next few years, starting with volunteers, until it reaches full deployment around 2015 or 2016.

New York will also likely be able to tap into an existing network of composters, since private groups have been sowing the seeds for some time. The Lower East Side Ecology Center, for example, has operated a popular composting program for city residents since 1990. Some people also report saving up their kitchen scraps and bringing them to drop-off locations at farmers markets and other locations.

Christine Datz-Romero, the co-founder and executive director of the Lower East Side Ecology Center, said interest in composting has been building over the years. "I would venture to say it's because people are concerned about climate change and are looking for small changes in their lifestyle that will be part of the solution," said Datz-Romero. She notes that when placed in landfills, food waste releases methane, a potent greenhouse gas.

Datz-Romero added that the success of the program will depend on the details. She pointed out that the city did a pilot study of composting about three decades ago, but didn't have the "right trucks" or a facility that was located in a convenient location, so the experiment was deemed too expensive. She said frequent pick-ups are likely to be required in high-density areas to avoid odors.

"What I'm concerned about is our capacity to handle the material locally, because that's a big part of making composting really sustainable; otherwise you are spending a lot of fuel transporting it, and that doesn't make sense," she said.

Under the mayor's new program, participants will get picnic-basket-size containers, which they can fill with everything from used coffee filters to broccoli stalks. Those bins will then be emptied into bigger brown containers at the curb for pickup. Those who live in apartment buildings, as many Manhattanites do, will drop the waste off at centralized bins.

Administration officials told reporters that the city can save $100 million a year composting food waste instead of sending it to landfills, most of which are in other states. Bloomberg has said he expects the program may become mandatory in the coming years, although that will be up to his successors, since his term is winding down.

San Francisco's Zero Waste Example

To better understand how food composting can be handled on a large scale, National Geographic reached out to Guillermo Rodriguez, the communications director for San Francisco's Department of the Environment. In 2009, San Francisco passed a city ordinance that made composting food waste mandatory, making it the first U.S. city to tackle the issue on such a large scale.

Rodriguez said the mandate was part of the city's goal of becoming "zero waste" by 2020, meaning no material would be sent to landfills. Today, the city diverts about 80 percent of all its waste to recycling and composting.

"That's looking at all forms of waste, from residential to commercial, and it includes folks who commute here, tourists, and so on," said Rodriguez.

In contrast, New York City currently diverts only about 15 percent of its residential waste to recycling.

Rodriguez said his city's composting program has swelled, from collecting 400 tons a day three years ago to more than 600 tons a day today. Food waste and yard clippings are collected by the city's private waste contractor, Recology, which handles all waste, regardless of origin.

Much of the food waste is processed at a compost facility called Jepson Prairie Organics, which is 55 miles (89 kilometers) east of San Francisco in Vacaville. The orange rinds and pizza boxes are then feasted on by microbes, until they turn into rich compost, a natural fertilizer that is in demand by the region's agricultural producers.

"A lot of wineries in Napa and Sonoma are big buyers of the compost [because] it has [a] high nutrient value, so that's a nice way to close out the loop from what we put in our green bins," said Rodriguez. The compost is also sold to individuals, landscapers, and the highway department. It is approved for use with certified organic soil.

Robert Reed, a spokesperson for Recology, said New York City is "definitely on the right track." He added, "Food scraps are one of the most important types of resfuse because they are full of nutrients and carbon, critical resources for the environment and human health."

According to Recology's website, at Jepson Prairie, the owner-employees mix food waste and yard material into an "industrial sized grinder ... to attain a recipe of physical and chemical characteristics that are ideal for microbial decomposition." That blended material is then loaded into a composter called the ECS System, which has controls for temperature and oxygen levels, to stimulate breakdown by beneficial organisms and kill potentially harmful microbes.

When it is sufficiently broken down, finer material is screened out of the mix. It is then moved into outdoor piles called windrows, where it is "cured." The material is systematically wetted and turned (to provide oxygen), which keeps the beneficial microbes happily digesting away.

"We all want a healthy, red-leaf lettuce salad for lunch," said Reed. "Sending food scraps back to the farm helps make that happen." He added that such compost helps farmers rely less on chemical fertilizers.

Rodriguez added that the city is also looking into turning some of its waste organic material into usable biogas.

The "Fantastic Three"

About the composting program, Rodriguez said, "Our biggest success is making it easy for people." He added that his department and Recology teamed up to offer the "Fantastic Three": black, green, and blue bins. Trash goes in black, compost and organic material goes in green, and other recyclables like paper, glass, and metal go in blue.

"We do a lot to educate our residents about it," said Rodriguez. He said there was some resistance to the mandatory program, but said "a lot of that came down to education."

Rodriguez says his agency prefers not to issue fines for noncompliance, though it does have such authority. "We have elected not to choose the path of the stick," he said.

Instead, the agency spends a lot of energy on outreach by sending people into neighborhoods to explain the program and posting information.

"Do you put a used pizza box in your blue bin or the compost bin?" he asked as one example of a typical resident question. "If there are stains and a little cheese on it, it goes to compost, because you can't recycle oils off the cardboard," he said.

As for those who fear urban composting may get smelly, Rodriguez again pointed to education, and said there are easy steps people can take to reduce the problem. Some people opt to freeze their compost, he said, and the city provides compostable bags to make it easier. Others commit to emptying their free compost pails frequently, and some sprinkle them with baking soda.

Rodriguez added that achieving zero waste requires more than just composting food scraps. He said the city also passed a law that banned polystyrene food containers, inducing many businesses to switch to compostable or recyclable materials. Further, commercial accounts are charged more for heavier black bins, which encourages higher diversion rates. If people want to get rid of large items, they can call to arrange a pickup by the city.

"It's the combination of all the pieces coming together to achieve zero waste," he said.

So far, the composting plan is making an impact, he said. As of mid-2012, San Francisco's composting had reduced city greenhouse gas emissions to nearly 12 percent below 1990 levels.

Reed said, "Researchers have calculated that if every city in the U.S. replicated San Francisco’s compost collection program we could offset 20 percent of the nation's carbon emissions." He added that his city's success proves that composting can work on a large scale, in a dense urban environment.

"It's very exciting and gratifying to see composting finally getting its due in terms of public discussion," said Datz-Romero.