Last updated on April 18th, 2025 at 12:41 am
Biodigesters: One Solution to the Food Waste Problem
What do you think is the single largest waste stream being diverted to landfills?
Whenever I ask people this question, the most common answer is “plastic.” While this is a reasonable guess, it is not correct.
You might be surprised to learn that the largest category of waste is actually food. An estimated 30% to 40% of all food produced is thrown away, accounting for 25% of landfill waste–more than any other type of waste.
To illustrate the magnitude of this waste, imagine a pile—no, a mountain—of 133 billion heads of lettuce. If each weighs about one pound, that’s how much food is wasted in the U.S. every year. It’s enough to fill the Rose Bowl—the iconic stadium in Pasadena, California that seats nearly 90,000 and spans 10 acres—to the brim EVERY SINGLE DAY.
Why do we need biodigesters?
Food waste accounts for 10% of all greenhouse gas emissions (GHG), and according to the United Nations, if global food waste were a country, it would be the third largest contributor to GHG in the world (after the US and China).
You might be wondering how food waste leads to such substantial emissions, especially since it is a biodegradable material.
Say you add a head of lettuce to a backyard compost pile, occasionally turning the contents to aerate them and provide the oxygen needed for efficient decomposition. It would take about two to six weeks to fully break down, emitting carbon dioxide into the atmosphere during this natural process.
Now take that same head of lettuce, only this time toss it in your garbage can. From there, it goes into the landfill and is quickly buried by subsequent loads of trash. It no longer has access to oxygen, so its breakdown will be much slower. So slow, in fact, that its full decomposition can take up to 25 years. In addition, the anaerobic process (from the lack of oxygen) results in a different gas being created: methane, which traps 28 times more heat than carbon dioxide, contributing much more to global warming in both intensity and duration. Ammonia is also released, which can have negative effects on air quality, such as contributing to Salt Lake City’s winter inversions.
A biodigester harnesses these emissions and converts them into biogas.
What is a biodigester and how does it work?
A biodigester, or anaerobic digester, is a facility that essentially acts as a huge stomach that digests organic wastes. The methane and other resulting gases are harnessed into renewable fuel used for energy. The solid waste becomes a rich compost that can be returned to the earth to enrich the soil for crop production.
Biodigesters can be utilized to process other organic waste in addition to food, such as animal manure from farms or sewer sludge at wastewater treatment facilities.
How is biodigestion different than composting? Is it better?
Composting is limited to plant-based materials, while a biodigester can accept just about any organic materials. Foods that cannot go into a compost pile include processed foods, meats and bones, dairy, fats, oils, and grease. Hence, most cooked foods (even plant-based or vegetarian ones) are excluded since they were likely prepared with one of these ingredients. A biodigester can process all of these–quicker, and on a larger scale.
Like composting, the anaerobic digestion process results in decomposed, nutrient-rich material, which in this case is referred to as digestate.
Composting vs. Biodigestion
| Feature | Composting | Biodigestion |
|---|---|---|
| Process Type | Aerobic (uses oxygen) | Anaerobic (no oxygen) |
| End Product | Nutrient-rich compost | Biogas (energy) + digestate |
| Speed | Slower (weeks to months) | Faster in controlled systems |
| Handles Meat/Dairy/Fats? | No | Can process any food type |
| Energy Production | None | Produces renewable biogas |
| Greenhouse Gas Benefit | Prevents methane by keeping food out of landfills | Captures methane for energy use |
| Best For | Gardeners, small-scale home use | Energy recovery, broader waste streams |
| Cost/Complexity | Low-tech and low-cost | Higher upfront cost and maintenance |
To summarize, composting is simple, no-cost, and accessible to anyone, but materials that can be added are limited, and decomposition is slower. Biodigestion can process a wider variety of materials that may have ended up in a landfill and turn it into renewable energy. The downside is that fewer people have access to these facilities and there may be a fee associated with use.
Visiting a biodigester
I toured Wasatch Resource Recovery in North Salt Lake, a food biodigester which is the first and only facility of its kind in Utah. They have two digesters that can accommodate 2.5 million gallons of waste, with land space for the addition of two more. Four types of materials are currently accepted here.
1. Solid Food Waste
The facility’s ‘tip floor’ is where collection trucks come and deposit their food waste. The food comes largely from grocery stores, restaurants, and some residential waste. This area is definitely the most fragrant part of the grounds. There wasn’t a huge pile the day I visited, but frankly, this photo is as close as I wanted to get.
From here, the food waste is scooped up by a front loader and deposited into the system, where it is ground up and liquified with secondary water that is recirculated throughout the system. Packaging and other contaminants are filtered out of the mix before it enters the actual digester.
Not far from the food pile, the facility’s resident roosters were keeping things in check.
2. Packaged foods
Trucks from stores like Walmart and Costco pull up to the loading dock to empty out pallets of expired packaged foods. The packages enter a computerized machine that can discern packaging from food, cutting open the package, and separating the two. The food enters the system and the packages end up in a pile destined for the landfill.
3. Liquids
At this facility, liquid waste makes up the majority of what is processed, around 70%. The most common liquids are dairy products (such as whey) and distillery and brewery byproducts. (Surprise, Utah actually has a fair amount of craft breweries!)
4. Fats, oils, and grease
These include wastes such as cooking oils and grease trap contents from restaurants. We didn’t see this in action so, sorry, no pics.
The anaerobic food digestion process
After the waste materials are liquified and contaminant-free, they enter the biodigester for 21 days. Inside the oxygen-free (anaerobic) environment, everything is broken down, or digested, by microbes similar to those in our own digestive systems.
As in a landfill, this process creates methane, but in this controlled environment, the methane is captured. The second byproduct is the “digested” material, which is essentially compost.
The methane is sent directly into the local gas pipeline, where it is used to power nearby homes. Other gases such as ammonia are separated and sold.
At the end of the process, the water is pressed out of the remaining biosolids and recirculated. The solids are dropped into a pile for drying. This material is given to local farmers to fertilize their fields, which can increase crop yields by 10-30%.
The advantages of biodigesters
Biodigesters are enclosed systems, therefore they contain the odors, pathogens, and gases that are emitted by decomposing organic waste. This protects surrounding waterways, as well as harnessing the greenhouse gases that would otherwise be entering our atmosphere if the waste was allowed to decompose in a landfill. And diversion of these materials conserves space in the landfills.
Dependence on fossil fuels is decreased, as this energy is derived from natural sources and can decrease transportation emissions, as with Wasatch Resource Recovery, since the gas is directly added to existing pipelines. Compared to fossil fuels, biogas systems can cut total carbon emissions in half.
The compost is a great natural alternative to fertilizer for farmers, which can also decrease costs. There are also many economic benefits, including jobs.
For more details, head over to the American Biogas Council.
The immense potential for biodigester growth
There are currently about 2,300 biodigester sites spread across every state in the country. Most of these biodigesters are at water resource recovery facilities where municipal wastewater is processed. The rest are utilized as landfill gas projects and on farms, where they process materials like manure and dairy waste. Food waste anaerobic digesters like the one I visited are few; less than 100 exist.
By contrast, Germany has 10,000 facilities in use, which enable some communities to be nearly independent of fossil fuels.
If you’re lucky, you have an anerobic digesting facility in your area. It may allow you to drop off their food waste for free. Or your city might have a curbside food waste collection program for residential, businesses, or both.
Wasatch Resource Recovery is still in its infancy, having only started operations in 2019, which were of course set back by the pandemic. Their customers are are primarily businesses; there is a small pilot program for residential collection that will hopefully expand in the coming months.
Final thoughts on biodigesters
Regardless of whether you have access to a facility like this, which most likely you do not, you should still work to decrease your food waste, which is, of course, the root of the problem. Make sure you place compostable food scraps in your yard/green waste bin, if you have one, or start your own compost pile. If you don’t have a yard, consider a countertop food composter or learning how to vermicompost with worms (it’s not as gross as it sounds). More information about all of these things can be found in my guide on food waste.
I think the anaerobic digestion process is fascinating and a promising way to deal with the astronomical amounts of food waste, as well as a great alternative to fossil fuels. I really hope to see these types of facilities expanding. My neighborhood is now included in the curbside pickup program destined for Wasatch Resource Recovery, and I am grateful to be able to send the food waste we create to this fantastic facility.
Read more about food waste and prevention strategies here.
