Development of an on-site method to measure fugitive greenhouse gas (GHG) emissions from covered pig manure storage pits

摘要

With a production of about 7 million pigs annually in Quebec over the last decade, manure storage becomes a considerable source of greenhouse gases (GHG). For instance, manure management (all types of manure combined) generated emissions of 2,15 Mt eq. CO_2 in 2015. To reduce this carbon footprint, storage tanks can be covered and equipped with a biogas treatment system. However, there is a concern that some management operations, such as opening the door and mixing the manure, could lead to leakage of GHG in the environment. The relative importance of these GHG losses is still unknown due to little study on the subject and the challenge of measuring them. Thus, the objectives of this study were to 1) characterize GHG emissions from covered manure storage tanks, and 2) evaluate the importance of the gas losses during management operations prior to manure spreading, by developing a strong and comparative on-site measurement method for GHG fugitive emissions. Three manure storage tanks, each covered by a concrete or plastic structure and equipped with a methane (CH_4) biofiltration system, were studied on two different swine fattening facilities (< 50 km south of Quebec City, Canada). The emissions at the exhaust air of the tanks were monitored (airflow, temperature, pressure, humidity) and characterized (CO_2, CH_4, N_2O and NH_3) weekly during the warm season over 2 years (from May to November 2017 and 2018). Also, manure samples were collected biweekly and analyzed (pH, N, P, K, Ca, Mg, Na, total solids and volatiles). To ensure comparison between the storage tanks and over time, the manure biomethanisation potential (BMP) was measured periodically. For this purpose, a proposed method to determine the BMP of swine manure was developed during the present study based on major literature references as no standard methodology is defined yet. To measure fugitive emissions during operations prior to manure spreading, a method was first developed at laboratory-scale and then applied directly on-site (spring and fall 2018). In this method, the opened door was sealed over the manure pump and mixing device to prevent losses in the environment. The manure was mixed for about 4 hours and the emission monitoring frequency at the entrance of the biofilter was increased to 15 minute intervals (for a few days before and after the operation as well). The procedure was then repeated with a free opened door. Each storage tank showed a different global footprint (16 to 80 t. CO_2eq in 2018) according to their specific characteristics such as manure content. The BMP analysis could be an interesting way to estimate emissions from a storage tank and to evaluate the relevance of installing an air treatment system. The on-site evaluation of fugitive emissions during manure mixing operation indicates that losses are greater in fall than spring but negligible compare to the total emissions measured.