Where most people see a pile of leftovers or yard waste, Paul Sewell sees a revenue stream, delivering energy and valuable nutrients.

Sewell is the CEO of 3-year-old Harvest Power, a company formed to take recycling to the next level. Paper, metals, and plastics account for about 60 percent of municipal solid waste. The next hill to climb is waste that originally came from the ground: wood, yard trimmings, and now food scraps, which altogether are more than 30 percent of that waste stream.

"We look at it as next-generation solar," said Sewell. "We're capturing sunlight in the form of biomass in a stable form. And we have the ability to use biological systems to get the maximum benefit out of it."

There are a growing number of experimental technologies being developed for turning organic waste back into something valuable, driven both by environmental and financial reasons. One method Harvest Power is pursuing, called anaerobic digestion, has been around for decades and is already used at wastewater treatment plants and farms, more so in Europe.

Anaerobic digesters use microorganisms in an oxygen-starved tank to convert organic material into compost fertilizer and biogas. That gas, made mostly of methane, can be burned to make heat or electricity. Fuel cells can also use biogas instead of natural gas to generate electricity, something already being done at wastewater plants in California.

At the consumer level, organic collection is largely limited to yard waste, which is converted into compost soil fertilizer. Now, a handful of municipalities, including San Francisco, are starting to expand into collecting food waste, diverting more waste from landfills or incinerators.

"Recycling has kind of peaked. The only way we can reach our recycling goals is by adding more organics, so we're promoting it and working with industry," said Sumner Martinson, the composting program director at the Massachusetts Department of Environmental Protection.

The ability to generate a second product--renewable energy--from waste in addition to a soil fertilizer is another reason there's more interest. "If you can generate some energy and have some energy to sell to the grid, then it's economically more cost effective," Martinson said, adding that municipal composters can be big energy consumers.

Tipping fees and sustainability It's not hard to understand why energy entrepreneurs and municipalities are looking to turn more garbage into gold. In the U.S., there's a lot of it and it's expensive to remove, with many locations, particularly in dense population centers, trucking trash to other states. And decomposing material in landfills gives off methane, a potent greenhouse gas.

In 2009, there were 243 million tons of municipal solid waste in the U.S., translating into an average of more than four pounds of waste per person per day, which is just a fraction of the waste from commercial and industrial sources, according to the EPA.

Recyclers will pay for plastics, metals, and paper because they can sell that material, once separated, to mills which use it as raw material. The diversion of 82 million tons from landfills for recycling in 2009 is the equivalent of taking 33 million cars off the road for a year, according to the EPA.

But after growing rapidly in the 1980s and 1990s, recycling rates have tapered off, standing now at about 33 percent. Services such as single-stream recycling can help boost those rates. But there is still a sizeable portion of household garbage, about 35 percent of the total pile, which comes in the form of wood, yard trimmings, and food scraps.

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The region around Vancouver, British Columbia, was looking at adding more incinerators or landfills, but met public opposition on both plans. In response, the 18 municipalities in the area formed an aggressive plan to reduce their waste disposal by 70 percent in 2015 and 80 percent by 2020 to avoid siting problems.

They expanded to collecting food scraps, which includes pretty much everything from the kitchen--soiled paper, napkins, bones, meat, fruit, vegetables. Food scraps can be included with yard waste, which has been collected since the 1990s, in garbage bags with a special decal or paper yard waste bags. Now, they plan to go beyond composting, too.

In a $12 million project, Harvest Power will start construction of an anaerobic digester that will produce both compost and capture usable energy. The gas from the decomposing material will be captured and used to make electricity to power the plant or piped into the natural gas pipeline, said Suzanne Bycraft, the manager of fleet and environmental programs from the City of Richmond in British Columbia which will host the facility. The digester speeds up natural decomposition, taking weeks rather than months.

Reducing greenhouse gases from landfills for incinerators was a big reason for the investment in the anaerobic digester, but there's a financial incentive as well. Right now, the tipping fee rates are almost $100 per ton of municipal solid waste. Although cities need to pay a contractor to collect more material, Richmond saved about $350 million on disposal fees last year, said Bycraft.

"As the tipping fees increase, it becomes more and more economical to divert that material into a composting facility at a lower rate than a landfill," she said. "It's certainly a more sustainable approach and sustainability is becoming a priority in our region."

Digging deeper still For decades, landfills have captured methane from decomposing organic material or operated incinerators, which Bycraft said are less polluting than they used to be. But a handful of waste-to-energy companies are eyeing municipal solid waste, and other forms of biomass, as cheap feedstock.

For an idea of the possibilities, consider the activities of hauling giant Waste Management. The company has been operating dozens of waste-to-energy facilities for years. In Southern California, captured landfill gas is being converted into natural gas fuel for 300 of its collection vehicles.

Over the past few years, it has invested in technology companies which promise better energy extraction and a more benign environmental footprint. Montreal-based Enerkem, for example, is in the process of building two commercial-scale facilities that use gasification to convert municipal solid waste or wood residue into a synthetic gas, which can then be turned into ethanol or other chemicals.

Waste Management, along with oil refiner Valero Energy, last year invested in Terrabon, a company which also uses gasification but its end product is a gasoline replacement. Another waste company with another thermo-conversion technology planning a pilot project is S4 Solutions, a joint venture of Waste Management and InEnTec, which has a gasification process for making different gaseous fuels and chemicals.

Whether these alternative waste-to-energy processes take hold remains to be seen. One of the big challenge is making them economically compelling and scaling them up, said Waste Management company representative Wes Muir.

Harvest Power's Sewell says just getting energy through landfill gas capture and incineration is selling the value of the organic waste stream short. In addition to the embedded energy, people should also consider how the carbon and nutrients are being repurposed to improve soil quality.

"Call it my farming background, but we need soil. We forget that it supports our agricultural and food production system," he said.