Methane emission reduction by adding sulfate to liquid dairy manure

Abstract

Dairy farmers are interested in reducing the carbon footprint of milk. Reducing methane (CH4) emissions is a key part of this goal, and manure is a significant CH4 source. Technologies like anaerobic digesters for biogas production are effective; however, adoption rates are slowed by upfront costs and infrastructure needs. Achieving near-term emission reductions needs low-cost alternatives that can be quickly and widely adopted. Previous studies have shown that “acidification” of manure by adding sulfuric acid (H2SO4) suppressed CH4 emissions; however, widespread adoption may be hindered by the challenge of handling acid on farms. This laboratory study was performed for 157 days at 24°C, and compared the efficacy of a sulfate-based non-acidic fertilizer (CaSO4), and two rates of acidification, one at pH > 7 and one at pH < 7, for a sulfate-based acid (H2SO4) and a sulfate-free acid (H3PO4). Methane suppression by CaSO4 at multiple rates was also analyzed. Two mechanisms of suppression were observed: acidification had a demonstrable early effect, lowering cumulative CH4 emission within 40 days by up to 65% for H2SO4 and 54% for H3PO4, while sulfate-containing compounds showed increasing suppression after 50 days. Final cumulative CH4 suppression was up to 63% for CaSO4 and 91% for H2SO4, while H3PO4 was least effective. These results suggest H2SO4 is highly effective due to the combination of acidity and sulfate. Adding sulfate alone (CaSO4) was more effective than adding acid alone (H3PO4). Hence, sulfate-based additives—like gypsum—may hold promise as an alternative near-term solution for dairy farms to make large CH4 reductions.