Biodigesters: One Solution to the Food Waste Problem

Last updated on April 29th, 2024 at 05:12 pm


Biodigesters: One Solution to the Food Waste Problem

Why do we need biodigesters?

Before we get to what a biodigester is and how it works, we need to talk about why it’s needed in the first place.

Say you add a head of lettuce to a home compost pile, occasionally turning the pile to ensure the contents have access to oxygen, which it needs to break down. It would take about two to six weeks to fully decompose, emitting carbon dioxide into the atmosphere during this natural process.

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 heat 84 times more than carbon dioxide. Ammonia is also released, which can have negative effects on air quality, such as contributing to Salt Lake City’s winter inversions.

How much food is wasted in America?

Now back to the head of lettuce. Let’s say it weighs one pound. Can you imagine a pile…no, a mountain…of 133 billion green leafy spheres? That’s how much food is wasted in America every year. This is enough to fill the Rose Bowl in Pasadena, CA (which seats around 90,000 people and occupies 10 acres) to the brim EVERY SINGLE DAY.

It is estimated that 30-40% of all food produced is thrown away, accounting for 25% of landfill waste, making it the largest category of all waste.

Food waste also accounts for 10% of all greenhouse gas emissions. The UN states that if global food waste was a country, it would be the third largest contributor to greenhouse gas emissions in the world, after the US and China.

Now that you understand the magnitude of this problem, it’s clear why we need solutions such as the food biodigester.

Read more about food waste and prevention strategies here.

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. It harnesses the resulting gases and converts them into renewable fuel used for energy.

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 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, and some are used to process animal manure or wastewater biosolids, like sewer sludge.

Like composting, the process results in decomposed, nutrient-rich material, which in this case is referred to as digestate.

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 2.5 million gallon digesters with room for 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.

roosters standing next to bales of cardboard

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. 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

If you don’t have access to a facility like this, which is most likely the case, you can and 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 on my food waste page here.

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 and I can’t wait until my neighborhood is included in the curbside pickup destined for Wasatch Resource Recovery.

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