Greenhouse Gas and Toxic Air Pollutant Emission Rates from Industrial-Scale Composting of Municipal Solid Waste

State and local governments across the U.S. are adopting ambitious policies that require greater landfill diversion and use of organic municipal solid waste (MSW). One promising process is the dry anaerobic digestion (AD) of organic MSW, which can produce biogas fuel for electricity generation and reduce overall greenhouse gas emissions and global warming potential by reducing net methane emissions from landfills. Composting of the resulting digestate material can also yield usable final products.

Due to the short operational history of dry AD in the U.S., emissions estimates for the process are limited. Additionally, emission rates from composting piles are uncertain, given differences in feedstocks and sampling methods. Research is needed to better quantify pollutant emission rates from dry AD of organic MSW to comprehensively evaluate their climate forcing and air quality impacts. This study quantifies emission rates of greenhouse gases, volatile organic compounds (VOCs), and hazardous species from composting of AD digestate at a large-scale industrial facility in California.

Determining emission rates from composting windrows at large-scale facilities can be difficult, given the scale of the operations. Windrows at this California facility are typically 100 m × 6 m × 3 m, covered with plastic sheeting, and aerated with two perforated pipes that run through the bottom of the pile and are connected to two blowers on timers at the windrow head. We measure concentrations of carbon dioxide (CO₂), methane (CH₄), nitrous oxide (N₂O), ammonia (NH₃), hydrogen sulfide (H₂S), and a range of speciated VOCs across each windrow pile and relate the concentrations to emission rates expressed in terms of pollutant mass emitted per kg of wet material. Measurements are made along the length of the windrow with VOC sorbent samplers and by collecting gas in sampling bags that are later analyzed in the lab. We evaluate how the emission rates evolve over the 14-week composting cycle. We also evaluate the spatial variability in emission rates across compost piles to develop an improved methodology for capturing emission rates from industrial-scale composting facilities.