By Heather Miller
MN350 volunteer working on the Food Solutions Team

Editor’s Note: This blog post is part of a series on food waste. Read about the issues with food waste and curbside composting. You can also read our series on food choices and climate change: part 1, part 2 and part 3

What do you get when you combine garbage, food scraps, manure, sewage sludge, compostable dishes, yard waste, and other biodegradable items with microbes? The answer is energy in the form of heat and natural gas. The anaerobic digestion (AD) process uses up food scraps and other organics that would normally create greenhouse gases in a landfill or incinerator and instead turns them into bioenergy that can heat whole cities, power the process of AD, and even fuel vehicles. The AD process has the potential to make a significant positive impact on controlling waste, creating renewable energy, and helping to contribute to climate justice.

Anaerobic digestion—otherwise known as combined heat and power generation (CHP)—is the process of turning waste into energy. It’s not a new process. Many farms use the process to convert manure into heat for their buildings, but capturing methane on a large scale to power cities IS relatively new. There are fewer than 80 such facilities in the United States.

Food scraps take a long time to decompose anaerobically (without oxygen). We were stunned to learn that a head of cabbage may take as long as 25 years to decompose, all the time producing greenhouse gases. Landfills cannot be completely capped to capture the methane until they are full, which might take as long as 20 years, and by then much of the methane has already escaped. Methane’s effect as a greenhouse gas in the atmosphere is 34+ times greater over the course of a century than carbon dioxide.


In AD, the energy is produced by microorganisms, which consume organic trash and release methane. The methane can be fully captured and compressed to use as a fuel. The remaining residue (digestate) can be used as a rich fertilizer.

Some of that fuel and heat is used to power the digester and produce heat for the process, but there is leftover methane (natural gas) that is typically flared off to get rid of the methane. A much better use is to feed the gas back through the natural gas lines, after it is pasteurized to remove toxins. It can then be used to heat buildings or to power a turbine to create electricity. When the natural gas is burned, it releases CO2, which is in fact carbon neutral because it would have also been released if the compostable items had decomposed naturally.

This video explains the process and demonstrates the machinery.

Currently, Duluth has CHP, and about 25% of the gas they produce is flared off to dispose of the methane. If their latest funding appeal is approved, the city is poised to begin generating electricity with that surplus fuel and accepting waste from other businesses as well. The digesters can run on food waste and bio-solids, as well as industrial, farm, and residential/business organic waste.*

Trash is a real problem. We have lots of it, but we have few ways to sustainably dispose of it. We can place it in the landfill, which entails the release of lots of methane, or we can incinerate it, which releases lots of carbon dioxide and other toxic chemicals. Plus no one wants to live around an incinerator. Often they are located near low-income, black or indigenous neighborhoods because those populations generally lack the political clout to resist. AD/CHP creates a great renewable fuel that can be used for vehicle fuel, heat, or electricity. It’s also preferable to fossil fuels or nuclear energy when it comes to balancing wind and solar because it’s available when needed.

Hennepin County is gathering requests for proposals for CHP and should be ready to review the proposals and begin construction in 2024. They, as well as Western Lake Superior Sanitation District, are seeking funding.

* From an interview with Karen Anderson of Western Lake Superior Sanitation District, which processes waste for 17 communities.

Heather Miller is a MN350 volunteer working on the Food Solutions Team. She has been a climate activist for the last 15 years in Michigan and in Minnesota. Her family, husband, two daughters, two sons-in-law, and four grandchildren are her major motivators in supporting the climate movement. When she isn’t spending time with them, she likes to create pastel or watercolor landscapes of beautiful places.