Why Co-digest Food Waste and Sludge? Part 2

Some municipalities are co-digesting food waste and sludge at wastewater treatment plants, finding that adding food to the mix significantly increases biogas production without affecting quality. 

Still, once food is added to these plants’ digesters, the biogas receives less RINs credit (Renewable identification numbers) under the federal Renewable Fuel Standard (RFS) program—driving its value down by 50 to upward of 70 percent.  This is preventing projects from moving forward and has struck a chord with industry proponents who argue that de-valuing the product once food waste is mixed with biosolids makes no sense from either an environmental or economic perspective. 

They are trying to convince the U.S. Environmental Protection Agency (EPA) to assign the higher RINs to fuel from co-digested feedstock. 

RINs are assigned to feedstock by how much that feedstock reduces greenhouse gas emissions over fossil fuel.  Manure and biosolids cut emissions by at least 60 percent while food waste reduces them by 50 percent, according to EPA.  But the industry asserts that digesting food cuts emissions by more than 60 percent.

“Every California low-carbon fuel standard project is evaluated for its carbon emissions, and they have actually shown reductions by 80 to over 100 percent with food compared to gasoline,” says Patrick Serfass, executive director, American Biogas Council.

Further, he points out, adding food scraps and or fats, oil and grease (FOG)  to wastewater digesters boosts methane production by at least two to three times over biosolids and manure alone, while diverting these materials from landfills and sewer lines, and enables jurisdictions to leverage existing infrastructure.  

 A few municipalities are moving forward with co-digestion projects. The city of Mesa, Ariz. just finished a feasibility study involving turning commercial food waste into a bioslurry and delivering it to one of its wastewater treatment plants where it was mixed with biosolids and anaerobically digested. The food waste did boost biogas production and the end product was very similar to what was produced from sludge alone, according to Scott Bouchie, environmental management and sustainability director for the city of Mesa.

But co-digestion projects come with high costs.

The biggest expense is the capital investment to include a receiving station for material; equipment to de-package and remove contaminants; storage tanks and aeration equipment to stir the slurry; and piping that runs from the tanks into wastewater treatment plants’ digesters. 

“So, you have to spend more money and do something more complex that requires additional labor and risk. And at the end of the day, you produce a commodity that is less valuable,” says Bouchie.

He is among industry stakeholders talking to the EPA and the Department of Energy, including the National Renewable Energy Laboratory (NREL), about maintaining D3 RINs value of the material once food is added to municipal sludge. Currently food waste alone, as well as a mix of food waste and sludge, receive the lower D5 RINs.

“Regulators understand it is an issue and preventing projects from moving forward. And we are trying to work with them to find a solution that makes it more economically feasible for someone like the city of Mesa to do this project,” says Bouchie, adding decreasing the value of captured gas from food waste does not seem logical from either an environmental or economic perspective.

“From an environmental standpoint, you can control food waste a lot better when you send it to an anaerobic digester. And from an economic standpoint it makes more sense to me to put it in a closed system and leave landfill space for other materials besides food waste,” he says. 

There is history around how food waste as feedstock became valued as D5 RINs rather than D3 RINs. It has to do with language in the RFS and interpretation of that language. EPA determined that feedstocks must be at least 75 percent cellulosic to qualify for D3 RINs. 

The agency values cellulose because at one time it was trying to encourage corn ethanol production to reduce U.S. dependence on imported oil. Then Congress wanted to incentivize other renewable fuels; it updated the RFS to include non-corn ethanol and called it cellulosic biofuel, explains Serfass.

“EPA lawyers think they can better protect the agency from liability by interpreting the D3 category as including only cellulosic. But we think the language can be interpreted that D3 can be assigned for any renewable fuel with 60 percent or greater greenhouse gas emission reduction,” says Serfass. 

Shayla Allen, water resources engineer at Arcadis U.S., a global design firm, who worked on the Mesa, Ariz. co-digestion project, also does not understand why food waste devalues the biogas.

“Cellulosic and food digest to create the same biogas. Meanwhile, there is so much energy value in food waste. It can produce more biogas than biowaste because it has more nutrients since it has not been digested yet. That you can create more biogas from food waste is a huge reason plants want to digest it,” she says. 

But Allen has seen plants look into these projects with interest and end up dissuaded due to minimal or no monetary benefits. So while municipalities, particularly at wastewater treatment plants, are realizing the capacity to handle food waste, it comes down to economic balancing. 

A few jurisdictions are saying they care more about long-term sustainability, getting materials handled locally, and diverting from landfill. 

“They will take the lower return on their investment because they think it’s the right thing to do,” says Matt Tomich, president of Energy Vision.

“But more often municipalities take the position, we will see if RINs works out and for now will stick with what we know. So, they don’t bring in outside food waste. It’s cheaper to just treat wastewater and biosolids,” he says.

Even if the co-digestion ruling does not change, the market dynamics are starting to change, namely the voluntary market whereby corporations and utilities want renewable natural gas. 

“They are willing to pay a premium similar to what the D5 RINs yields, but over a longer term. So these voluntary markets might make product from co-digestion more competitive,” surmises Tomich.