Anaerobic digester installation significantly reduces liquid manure management CH4 emissions at a California dairy farm

Authors: RODRIGUEZ, MICHAEL - University Of California, Riverside; ROJAS ROBLES, NIDIA - University Of California, Riverside; CARRANZA, VALERIE - University Of California, Riverside; THIRUVENKATACHARI, RANGA - University Of California, Riverside; REYES, MARIANA - University Of California, Riverside; PREBLE, CHELSEA - Lawrence Berkeley National Laboratory; PEXTON, JOYCE - University Of California, Davis; MEYER, DEANNE - University Of California, Davis; Anderson, Raymond; VENKATRAM, AKULA - University Of California, Riverside; HOPKINS, FRANCESCA - University Of California, Riverside

Submitted to: Global Change Biology Bioenergy
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/30/2025
Publication Date: 7/1/2025
Citation: Rodriguez, M.V., Rojas Robles, N., Carranza, V., Thiruvenkatachari, R., Reyes, M., Preble, C., Pexton, J., Meyer, D., Anderson, R.G., Venkatram, A., Hopkins, F.M. 2025. Anaerobic digester installation significantly reduces liquid manure management CH4 emissions at a California dairy farm. Global Change Biology Bioenergy. 17:Article e70047. https://doi.org/10.1111/gcbb.70047.
DOI: https://doi.org/10.1111/gcbb.70047

Interpretive Summary: Methane emissions occur from dairy animal operations. Methane emissions contribute both to climate warming and smog formation, thus many policy makers and governments (including the State of California) seek to reduce emissions from dairies. Anerobic digesters have been increasingly incorporated in dairy operations to capture methane emissions from manure and to provide renewable methane for other energy purposes. However, these emission reductions have not been well quantified in major dairy production regions of California. In this study, we measured methane emissions around the manure handling infrastructure of a commercial dairy in the San Joaquin Valley, California before and after implementation of an anerobic digester. We found significant reductions in methane emissions after implementation of the anerobic digester, but there were significant differences depending on meteorology and farm practices. This work is of interest to farmers, policy makers, and energy professionals who seek to reduce methane emissions and to increase a source of renewable biogas for residential and commercial applications.

Technical Abstract: Anaerobic digesters are expected to significantly reduce CH4 emissions from dairy manure management by capturing them for use as biogas. Anaerobic digestion is the current major mitigation strategy for agricultural CH4 emissions in California's climate policy. However, verification of the effectiveness of anaerobic digesters to reduce CH4 emissions has not been conducted at scale in California. We made atmospheric measurements from a mobile platform and used dispersion modeling to estimate CH4 emissions from a liquid manure storage complex at a typical California dairy before and after digester installation across nine field campaigns. The anaerobic digester reduced CH4 emissions by an average of 82%±16%, comparing paired months to predigester values. Prior to the digester, atmospheric CH4 mole fractions showed a persistent hotspot near the manure settling basin cells of 28.6±8.9 ppm. After the digester, atmospheric CH4 mole fractions from manure storage were greatly reduced. We observed strong temporal variability across measurement campaigns due to weather, on-farm management practices, and digester operations. Estimated emissions greatly exceeded those based on inventory calculations used by the California Air Resources Board (CARB) but were in line with expected relative emissions reduction from digester installation. Scaling these results to 139 dairies with digester projects statewide suggests that similarly operating digesters would reduce CH4 emissions by 1.6±0.3 MMT CO2e (65±12 Gg CH4), 39% of the emissions reduction goal for livestock manure management set by California law. This work demonstrates the effectiveness of anaerobic digesters to reduce dairy manure management CH4 emissions in practice, along with the importance of understanding operations and management for interpreting on-farm CH4 emissions studies.