Digestate and biochar can be land applied to sequester carbon and improve net primary productivity, but the achievable scale is tied to expected growth in bioenergy production and land available for application. We use an attributional life-cycle assessment approach to estimate the greenhouse gas (GHG) emissions and carbon storage potential of biochar, digested solids, and composted digested solids generated from organic waste in California as a test case. Our scenarios characterize changes in organic waste production, bioenergy facility build-out, bioenergy byproduct quality, and soil response. Moderate to upper bound growth in the bioenergy sector with annual byproduct disposal over 100 years could provide a cumulative GHG offset of 50-400 MMTCO2 equiv, with an additional 80-300 MMTC sequestered in soils. This corresponds to net GHG mitigation over 100 years equivalent to 340-1500 MMTCO2 equiv (80-350% of California's annual emissions). In most scenarios, there is sufficient working land to apply all available biochar and digestate, although land becomes a constraint if the soil's rest time between applications increases from 5 to 15 years.

The U.S. places approximately 53% of its total municipal solid waste (MSW) in landfills, but state and local governments across the country are now setting ambitious environmental and waste diversion policies requiring, among other things, diversion and utilization of organics. Municipalities across the U.S. are employing anaerobic digestion (AD) as part of their strategy to divert organic MSW from landfills, produce biogas, and yield other beneficial coproducts such as compost and fertilizer. However, AD faces many technical, regulatory, and economic barriers to greater deployment, including upstream waste contamination, local odor and air pollution concerns, lengthy siting and permitting processes, and requirements and sizable costs for interconnecting to the electric grid. We identify a combination of scientific, operational, and policy advancements that are needed to address these barriers.

Waste-to-energy systems can play an important role in diverting organic waste from landfills. However, real-world waste management can differ from idealized practices, and emissions driven by microbial communities and complex chemical processes are poorly understood. This study presents a comprehensive life-cycle assessment, using reported and measured data, of competing management alternatives for organic municipal solid waste including landfilling, composting, dry anaerobic digestion (AD) for the production of renewable natural gas (RNG), and dry AD with electricity generation. Landfilling is the most greenhouse gas (GHG)-intensive option, emitting nearly 400 kg CO_2e per tonne of organic waste. Composting raw organics resulted in the lowest GHG emissions, at -41 kg CO_2e per tonne of waste, while upgrading biogas to RNG after dry AD resulted in -36 to -2 kg CO_2e per tonne. Monetizing the results based on social costs of carbon and other air pollutant emissions highlights the importance of ground-level NH₃ emissions from composting nitrogen-rich organic waste or post-AD solids. However, better characterization of material-specific NH₃ emissions from landfills and land-application of digestate is essential to fully understand the trade-offs between alternatives.