Co-Digestion of Food Waste: A Triple Greenhouse Gas Solution

Thursday, April 1, 2021

An estimated 35% of food that is produced is uneaten, with losses occurring along the supply chain from farms to consumers. The majority from non-industrial sources ends up decomposing in landfills, where it releases methane, a powerful greenhouse gas (GHG). Recycling food waste through anaerobic digestion (AD), in which bacteria break down organic material in the absence of oxygen and create biogas, can create a triple-win for GHG mitigation. It reduces landfill methane emissions, creates renewable energy from the biogas, and sequesters carbon in soils through land application of biosolids-based soil nutrients and amendments. The wastewater sector, with substantial excess capacity in the anaerobic digesters it uses to process biosolids, can contribute substantially to this climate change solution.

To address food waste, the top priorities are preventing wasted food and recovering edible food to feed the nearly 14 million food-insecure households in the United States. But for the waste that unavoidably remains, the goal is to recycle it through composting, or anaerobic digestion in farm, wastewater, or standalone digesters. However, the lack of recycling facilities poses a major challenge for this solution.

The most suitable choice of facility type will depend upon the context. But AD provides an environmental advantage to composting, when renewable energy is generated from the biogas and biosolids are land-applied. Wastewater-sector AD facilities, which have a lot of excess installed capacity, tend to have a locational advantage over other AD types because they are widely distributed across the landscape, and food waste is generated by the same sources as wastewater.

Water resource recovery facility (WRRF) adoption of co-digestion can bring financial benefits to the utility, including energy cost savings and/or revenues, and revenues from accepting food waste and nutrient and soil amendment products. Though WRRFs of different sizes and locations have adopted co-digestion, the rate of adoption has been limited to date.

Aerial view of co-digestersTo stimulate and inform further evaluation and adoption of co-digestion, ELI published a report, Food Waste Co-Digestion at Water Resource Recovery Facilities: Business Case Analysis, that provides insights about the strategies WRRFs have used to create value and manage the risks and impediments associated with adopting co-digestion. The report also outlines a diagnostic framework that individual WRRFs can use to analyze opportunities and potential business strategies for co-digestion at their own facilities.

One issue highlighted is that the wastewater sector has been slow to incorporate food scraps, relative to fats, oils, and greases (FOG) and food processing residuals, as feedstock for co-digestion, due to several impediments. For one, food scraps are typically managed through the solid waste system and thus are less familiar to wastewater facilities than other food waste feedstocks, which are managed through the wastewater system. Food scraps also need further preprocessing—removing contaminants, reducing particle size, and adding liquid to create a slurry—to be suitable as an AD feedstock. Finally, it is more costly for municipal solid waste (MSW) agencies to collect them separately and preprocess them.

However, demand for recycling food scraps is increasing, with more cities and states issuing organic waste landfill bans or recycling mandates, and more food-sector companies making sustainability commitments, which create demand independent of regulations. As a result, innovations in food scrap sourcing arrangements are beginning to emerge.

A set of new and updated ELI case studies of food scrap co-digestion at WRRFs highlight two emerging business models for sourcing preprocessed food scraps—onsite sourcing through WRRF vertical integration and external sourcing through regional solid waste partnerships.

Vertical integration

A relatively new, and to date limited, development is WRRF vertical integration into this new product line of food scrap acquisition, depackaging, and preprocessing. The City of Hermitage Municipal Authority (processing an average of 3.5 million gallons per day, or 3.5 mgd) and the City of Muscatine WRRF (5.5 mgd) are the two current applications. In both cases, their organics processing facilities are the first in their regions to accept packaged food wastes for recycling, filling a market gap for food-sector companies with zero-waste goals in their region. Adding a new organics recovery product line can create synergies with co-digestion by providing a WRRF with improved control over food waste feedstocks and information to guide management of digester feeds. On the other hand, adding an organics recovery product line takes a WRRF further from the wastewater sector’s core mission—wastewater treatment—and may trigger more solid waste permitting requirements.

Regional solid waste partnerships

The predominant business model WRRFs used to source food scraps is to form regional partnerships with the solid waste sector. Partners may be public solid waste utilities, or private resource recovery companies. Some of these private-sector partners are companies that have developed turnkey pumpable food-scrap slurry products marketed in different locations throughout the country. These businesses include Waste Management, those from the traditional private solid waste sector, and Divert, a small company that uses IoT tools and prescriptive analytics to reduce food waste and enhance donation by grocery stores.

Large WWRFs with substantial AD capacity for co-digestion—such as East Bay Municipal Utility District (EBMUD) WRRF in Oakland, California (processing an average of 60 mgd dry weather flow) and Los Angeles County Sanitation Districts (LACSD) WRRF in Carson, California (280 mgd)—are more likely to employ multiple partnership strategies, as diversifying sources becomes increasingly important. Following a successful pilot project with Waste Management’s Engineered BioSlurry (EBS ®), LACSD—which houses both solid waste and wastewater departments—has been developing a diversified set of sources, including a food scrap preprocessing facility it built at one of its solid waste facilities, and private-sector suppliers.

In contrast, Central Marin Sanitation Agency (CMSA) WRRF (7.5 mgd), located in San Rafael, California, has a close partnership with one nearby private solid waste hauling company, Marin Sanitary Services, to implement its Food2Energy program.

Currently, many WRRF/solid waste partnerships—including EBMUD, LACSD, and CMSA—occur in places with organics landfill bans or recycling mandates. With these policies, the solid waste sector is incentivized (and sometimes directly mandated) to provide source-separated organics (SSO) collection and preprocessing.

Some WRRFs have forged partnerships in states without mandatory requirements. ELI case study examples include the Oneida-Herkimer Food2Energy program (with a 48 mgd facility) in upstate New York, initiated by neighboring solid waste and wastewater agencies prior to the New York State food donation and food recycling mandate (effective January 2022), and the Derry Township (PA) Municipal Authority (3.9 mgd), which co-digests preprocessed food scrap slurry from Divert’s nearby preprocessing facility.

Ultimately, to achieve broad-scale food scrap recycling, it will be essential to recycle organics currently in the MSW stream. Solid waste-sector partners will supply the required SSO collection and preprocessing services if the public policies are in place and tip fees are set so that the service providers can cover their costs and generators have incentives to participate in SSO collections.

Carol Adaire Jones is Co-Lead of ELI’s Food Waste Initiative.