Quantification of greenhouse gas emissions from a biological waste treatment facility

Whole-site emissions of methane and nitrous oxide, from a combined dry anaerobic digestion and composting facility treating biowaste, were quantified using a tracer dispersion technique that combines a controlled tracer gas release from the treatment facility with time-resolved concentration measurements downwind of the facility. Emission measurements were conducted over a period of three days, and in total, 80 plume traverses were obtained. On-site screening showed that important processes resulting in methane emissions were aerobic composting reactors, anaerobic digester reactors, composting windrows and the site's biofilter. Average whole-site methane emissions measured during the three days were 27.5 ± 7.4, 28.5 ± 6.1 and 30.1 ± 11.4 kg CH4 h−1, respectively. Turning the windrows resulted in an increase in methane emission from about 26.3-35.9 kg CH4 h−1. Lower emissions (21.5 kg CH4 h−1) were measured after work hours ended, in comparison to emissions measured during the facility's opening hours (30.2 kg CH4 h−1). Nitrous oxide emission was too small for a downwind quantification. Direct on-site measurements, however, suggested that the main part of the emitted nitrous oxide came from the biofilter (about 1.4 kg N2O h−1). Whole-site emissions were compared to emissions previously measured at different point sources on-site. Whole-site fugitive emissions were three to eight times higher than the sum of emissions measured at on-site sources. The magnitude of the emissions had a significant influence on the overall environmental impact of the treatment facility, assessed by consequential life cycle assessment. Including the higher whole-site fugitive emissions led to an increase in global warming potential, from a saving of 97 kg CO2-eq. tonne−1 of treated waste (wet weight) to a loading of 71 kg CO2-eq. tonne−1, ultimately flipping the environmental profile of the treatment facility.