Advancements In Waste-to-Energy Technology: Anaerobic Digestion to Gasification

As novelist Jim Butcher once said, if you have one problem, all you have is a problem. But if you have two problems, each one may provide a solution to the other. In the U.S., one big problem is disposing of the 292 million tons of garbage we produce each year. Another is finding renewable energy sources to replace dirty, planet-warming fossil fuels. Waste-to-energy technologies have the potential to solve both these problems at once by turning trash into fuel.

There are many ways to convert waste to energy. The best known is incineration: burning trash to produce heat. But there are many other methods, including anaerobic digestion, gasification, and new thermal technologies. These techniques can be more effective for dealing with specific types of waste.

However, all these methods come with problems of their own—financial, technological, and environmental. We need to find ways around these challenges to make waste-to-energy technologies truly sustainable.

Waste-to-energy basics

The U.S. government uses the term municipal solid waste, or MSW, to refer to all the stuff Americans throw away. This category covers a wide range of materials, including:

• Food and yard waste
• Paper
• Plastic
• Textiles
• Metals
• Wood

Many of these materials are organic matter, or biomass. They contain the embodied energy of the plants and animals they’re made from. Others, such as plastic, are not organic but still contain stored energy. The term waste-to-energy, or WtE, refers to any process for releasing this stored energy in a form we can use. This can mean burning the waste directly or converting it to liquids and gases that can be burned for fuel.

Environmental benefits of waste-to-energy technologies

Waste-to-energy kills two birds with one stone. It gets rid of MSW we don’t want and produces energy we need. Its benefits include:

• Waste management. The U.S. currently disposes of 12% of its MSW—over 34,000 tons per year—by burning it for energy. Other nations process even more of their waste this way—as much as 75% in Japan and the Scandinavian countries. Burning trash reduces the need for landfills, which is a big advantage in smaller countries.
• Renewable energy. MSW doesn’t come from nature like solar or wind energy. But it is something modern society has a steady supply of. That makes it a renewable energy source—something we can keep using for fuel without running out.
• Reduced landfill emissions. Organic waste in landfills breaks down to produce methane, a powerful greenhouse gas. Unless the landfill traps and burns this gas, it enters the atmosphere and contributes to climate change. By reducing the need for landfills, WtE reduces these planet-warming emissions.
• Improved water quality. Another problem with landfills is that as waste breaks down, pollutants can seep into the ground. Liquid seeping from landfills, called leachate, can pollute nearby water sources. Limiting landfill use reduces this source of pollution.
• Resource recovery. MSW often contain valuable materials such as metals. Some of these materials can’t easily be recovered through recycling. Burning or otherwise transforming waste may allow these materials to be sifted out of the ash or other residue.

Challenges and sustainability considerations with waste-to-energy technologies

Despite these advantages, waste-to-energy isn’t a perfect solution to our waste woes. The various forms of WtE technology pose a variety of challenges, including:

• Resource availability. Our society produces a lot of waste. However, the places it’s produced aren’t always the same places where it can most easily be processed. Transporting waste to processing sites is an option, but it adds to the cost and the environmental burden.
• Technological limitations. Some waste-to-energy techniques are fairly new. Scientists can demonstrate that that they work, but they haven’t perfected the technology. These technologies need more refining before they’re practical and cost-effective to use on a large scale in the real world.
• Pollution. Although waste-to-energy eliminates landfill emissions, it can also create emissions of its own. Incineration is particularly harmful. It releases a variety of harmful air pollutants, including fine particles, heavy metals, and toxins like PFAS and dioxins. It doesn’t always reduce greenhouse gas emissions, either. One British study found that by 2035, burning trash in the U.K. will be even more carbon-intensive than landfilling it.
• Public perception. There are ways to reduce (though not eliminate) pollution from waste-to-energy sites. However, even when WtE is relatively clean, many people still perceive it as dirty. Communities often oppose the construction of incinerators and WtE plants because of concerns about odors, traffic, and pollution.
• Discourages waste reduction. The most fundamental problem with using waste as an energy source is that it requires society to keep producing waste. There’s less incentive to reduce, reuse, and recycle, which would be better for the environment.

These problems don’t apply equally to all waste-to-energy methods. Some techniques are cleaner or more cost-effective than others. The way waste is handled also makes a difference. For instance, MSW that’s shipped overseas is often burned without sifting out hazardous waste first. Sorting waste before burning or processing it can remove hazardous materials and recover usable resources.

Incineration

The simplest way to turn waste to energy is to burn it. In the U.S., this usually means mass burning: putting large amounts of waste into an incinerator. The heat from burning the waste turns water to steam, which turns a turbine to generate electricity. This leaves only about 13% of the original volume of waste behind as ash to go into landfills. It also generates about 550 kilowatt-hours (kWh) of electricity—about $25 worth—per ton of waste.

There are two big problems with waste incineration. The first is pollution. Incinerators pollute the air with foul odors, particulates, and other harmful chemicals. One 2019 study found that trash incinerators release higher levels of mercury, lead, and nitrogen oxides than coal power plants. These pollutants can sicken nearby residents—usually members of environmental justice communities (low-income people and people of color). The ash left behind by incineration is also toxic. And burning trash doesn’t always reduce its carbon emissions compared to landfilling. Because of these problems, fewer than half of U.S. states consider burning waste to be renewable energy.

The other major problem is cost. Building a new waste incinerator typically costs at least $100 million. The fuel these plants burn is free, but it costs money to transport. Maintenance costs are also high. Plus, waste incineration produces far less energy than burning fossil fuels. According to the 2019 report, these problems make waste burning one of the costliest forms of energy in the U.S. Worse, it produces more greenhouse gas emissions per unit of energy than fossil fuels.

Modern incinerator designs do a lot to address these problems. Advanced emission controls significantly reduce toxic pollution. At the same time, better energy recovery methods—such as cogeneration, or using leftover steam to heat buildings--improve efficiency. However, older incinerators don’t have these features, and adding them can cost tens of millions.

In short, while incineration is the oldest and simplest waste-to-energy method, it’s probably the least useful. Newer techniques offer greater benefits.

Anaerobic digestion

In anaerobic digestion, bacteria break down food waste, manure, and other organic wastes in absence of oxygen. This converts the waste into biogas, a mixture of methane and other gases. Biogas can be burned as a fuel source or filtered to create renewable natural gas. This more concentrated fuel source can be used just like regular natural gas for heating or power generation.

Anaerobic digestion is the same process responsible for the creation of methane in landfills. It also occurs naturally in sewage ponds. Thus, one way to use anaerobic digestion to convert waste to energy is to capture landfill or sewage gas. Burning this gas for fuel converts the methane to carbon dioxide (CO2), a weaker greenhouse gas, while also providing energy. The U.S. currently burns about 232 billion cubic feet of landfill gas each year, producing 9.4 billion kWh of electricity. That sounds like a lot, but it’s only 0.2% of U.S. electricity production. It’s also possible to produce biogas under controlled conditions in tanks called anaerobic digesters. These can be used in sewage treatment plants and in industrial plants like paper mills. In 2021, there were 57 such facilities in the U.S. providing 1 billion kWh of electricity. Farmers can also add anerobic digesters to manure pools to capture biogas. They can then sell the methane or burn it to heat water and buildings on the farm. However, digesters are expensive, so they’re only cost-effective for large farms.

As a bonus, anaerobic digestion leaves behind a nutrient-rich mix of solids and liquids called digestate. This substance can be broken down to produce nutrient-rich fertilizer or animal bedding. It can also serve as the basis for bioplastics: forms of plastic made from plant-based material rather than petroleum. In this way, waste can become part of a circular economy in which materials are reused again and again.

Gasification

Gasification is another way to turn waste into fuel gas. It involves heating carbon-rich waste to superhot temperatures with controlled amounts of oxygen and/or steam. This process synthesis gas, or syngas: a mixture of carbon monoxide, hydrogen, and CO2 with smaller amounts of other gases.

Syngas is a versatile fuel source. It can be used for heating, burned for electricity, converted into liquid fuels, or broken apart to produce pure hydrogen. It’s relatively easy to capture carbon from syngas to reduce its climate impact.

Gasification works on MSW, animal waste, and plant waste from farming and forestry. It produces much less pollution than traditional incineration and is more cost-effective. A gasification plant in Vermont has been converting forest waste to energy for over 25 years. It’s capable of processing up to 76 tons of wood chips per hour and producing 50 megawatts (MW) of electricity. An even bigger gasification plant at a Chinese chicken farm processes 220 tons of chicken manure each day. Its power output is 14,600 megawatt-hours (MWh) of electricity per year—enough to power over 1,000 U.S. homes.

Advanced thermal technologies

Advanced thermal technologies are similar to gasification, but with some variation in heat source, oxygen level, or temperature. These changes make them better at handling specific types of waste materials. However, these newer methods are not yet well developed enough to be cost-effective.

Pyrolysis

Pyrolysis involves heating waste to high temperatures in low-oxygen environment. It works at lower temperatures than traditional gasification—as low as 300°C (600°F). Pyrolysis produces a mixture of syngas, a solid called biochar, and a liquid called pyrolysis oil or bio-oil. Biochar is useful as a soil amendment for farms, but the main product of pyrolysis is bio-oil. This dense liquid contains about half as much energy as petroleum. In theory, it could be refined to produce renewable gasoline, diesel, or jet fuel.

Unfortunately, bio-oil is unstable and difficult to refine. The U.S. Department of Agriculture (USDA) is currently working on improving pyrolysis methods to produce more usable bio-oil. Scientists in the Middle East are also testing a pyrolysis method for mixed plastic waste. In 2019, they announced that they had used it to produce a bio-oil with a higher energy value comparable to diesel fuel.

Supercritical water gasification

Supercritical water gasification, also called hydrothermal gasification, transforms liquid waste to gas at high temperature and pressure. It produces a gas rich in hydrogen, methane, or both. This process can handle waste with too much water for normal gasification, such as sewage sludge. Swiss company TreaTech has used it to convert sludge to a mix of biogas, clean water, and nutrient-rich mineral salts. TreaTech is also working on ways to use this method on chemical waste.

How individuals can be part of the solution

As an individual, you can’t easily burn or digest your own trash for energy. But you can still be a part of the solution to the twin problems of waste and energy demand. For instance, you can:

• Reduce waste. The less waste we produce, the less we need to dispose of. Two types of waste you can reduce at home are food waste and packaging waste. To reduce food waste, plan meals, eat up leftovers, and take care to use food before it spoils. To reduce packaging waste, seek out products with less packaging or, better yet, with none. For instance, drinking tap water instead of bottled water saves hundreds of plastic bottles each year and costs less, too. And of course, bring your own bags to the store.
• Recycle. When you can’t eliminate waste, recycling is the next best thing. If your community has a curbside recycling program, use it. Your town or county website should have a page explaining the rules and how to sort your waste properly. For materials you can’t recycle at the curb, search Earth911 to find a recycling center near you.
• Compost. You don’t need a digester to turn waste into useful material. All you need is a simple compost bin. Just toss in food and yard waste and turn the pile occasionally. Over time, that waste will turn into rich fertilizer for your garden. Even if you don’t have a yard, can probably make room in your home for a small worm compost bin. Or check to see if your town offers a municipal composting program.
• Use clean energy. The easiest way to use clean energy at home is to sign up for community solar. If that’s not available in your area, switch to a clean electricity supplier instead. To eliminate gasoline use, consider an electric vehicle for your next car. And when it’s time to replace your home heating system, look into energy-efficient electric heat pumps.
• Be an advocate. Support clean energy policies at all levels of government—federal, state, and national. Write and call legislators and talk about environmental issues with friends and family and on social media. Environmental advocacy groups like Climate Changemakers or Green America can keep you informed about policies to support.

The future of waste-to-energy technology

Waste-to-energy technology can address both the problem of waste problems and the problem of energy supply. However, as currently practiced in the U.S., it’s not a major part of the solution to either. The main method in use, incineration, accounts for a fraction of waste disposal and an even smaller fraction of energy production. And right now, incineration isn’t very clean or cost-effective. Advanced incinerators help address these problems but don’t eliminate them.

Newer WtE methods have greater potential. Anaerobic digestion is particularly promising because it produces valuable digestate, adding to its financial and environmental benefits. But gasification and advanced thermal techniques could eventually be part of the puzzle. These technologies have the potential to process hard-to-manage types of waste like plastics and sewage sludge. With further research, they could eventually be a major source of new, sustainable alternatives to fossil fuels.

In short, waste-to-energy isn’t that important to our energy system yet. But in time, it could become a key part of a circular economy in which nothing—even “waste”—is ever truly wasted.