Addressing Practical Barriers to Large-Scale Co-Digestion to Improve Feasibility

APPLICABILITY The desire to reduce greenhouse gas emissions related to food waste disposal in landfills has driven great interest in co-digestion as one way to avoid methane emissions (see Figure 1). Utilities and regulators in New York, California, and other states with climate change mitigation goals understand the promise that co-digestion holds and the opportunity to leverage excess digester capacity in furtherance of those goals (see Figure 2). However, large-scale implementation of food waste co-digestion has been hampered by practical barriers that must be addressed before the process can achieve its promise and deliver on its benefits. DEMONSTRATED RESULTS AND OUTCOMES Our identification and understanding of both benefits and barriers has grown through several co-digestion projects including: - California Co-Digestion Capacity Analysis completed in 2019 for the State Water Resources Control Board - Characterization and Contamination Testing of Source Separated Organic Feedstocks and Slurries for Co-digestion at Resource Recovery Facilities completed in 2023 for the Water Research Foundation - Food Waste Pre-Processing and Co-Digestion Evaluation completed in 2023 for the City of Oxnard, California The benefits of large-scale food waste co-digestion paired with beneficial use of biogas include GHG emissions reduction from landfills, offsets of fossil-fuel based natural gas, reduction of purchased electrical power and/or natural gas for water resource recovery facilities (WRRFs), and potential revenue generation at WRRFs through tipping fees. The environmental benefits just in California exemplify the promise of these processes. If the state were to successfully implement co-digestion and biogas utilization for 35-50% of its 6.8 million wet tons of food waste projected for disposal in 2030, it could reduce landfill emissions by approximately 2 million metric tons of carbon dioxide equivalent (MT CO2e) - half of its legislated reduction goal of 4 million MT CO2e in 2030. The scale of these benefits drove the passage of California's Senate Bill (SB) 1383, which legislated organics diversion from landfills and went into effect in 2022. However, large scale co-digestion has not yet been realized in California, in large part because of remaining barriers to successful implementation and sustained operations. So what are these barriers and how can they be overcome? The projects listed above included identification of the physical and logistical elements required to implement co-digestion, determination of operational impacts and unintended consequences, environmental justice considerations such as truck traffic and odors, documentation of conflicting regulatory drivers, and estimates of capital and operational costs. Having identified these barriers and determined their scale, we suggest strategies to overcome the obstacles which have hampered large-scale implementation thus far. A few such strategies developed from these and other projects include: - Matching pre-processing technology to current municipal solid waste collection system (i.e. source separated vs combined waste) to avoid added costs for different collection schemes. - Regional facilities (combined or separate pre-processing and digestion facilities) that can distribute the burden of capital and operating costs broadly and offer opportunity to offset some costs through tipping fees charged by the entity operating the associated systems. - Public/private partnerships that reduce capital burden and risk for public utilities. - Collaboration with disparate regulatory agencies to develop cohesive strategies towards common goals, especially related to production and use of biomethane and biosolids. - Bench scale studies with expected feedstock to better assess and mitigate operational impacts, including biomethane production, nutrient loads in recycle streams, foaming potential, and dewaterability of digested sludge. (See Figure 3 for foaming potential and volume expansion in digesters with varying amounts of food waste.) We will present eight ways to implement these strategies to address practical barriers for consideration by utilities and regulators as they seek to facilitate co-digestion to meet their climate change mitigation objectives . By incorporating such guidance, utilities can incorporate sustainable co-digestion into their WRRFs as the Los Angeles County Sanitation Districts (LACSD) has done through it's regional food waste processing system. RELEVANCE TO AUDIENCE This presentation will identify practical barriers to large-scale implementation of co-digestion and offer strategies to overcoming these obstacles such that co-digestion becomes more feasible in the short-term and remains viable in the long-term.