Effects of pig slurry acidification on methane emissions during storage and subsequent biogas production

In addition to undesirable odorous gases, substantial amounts of greenhouse gases (GHG), particularly methane (CH₄), are generated during the storage of livestock manure. To reduce the CH₄ emissions, first, pig slurry (PS) was stored for 40 d at 30 °C after adjusting the pH at 5.0-7.0 using H₂SO₄ solution. In the control (non-acidified PS), 3.7 kg CO₂ eq./ton PS of CH₄ emissions was detected, which was reduced to 1.8, 0.9, 0.4, 0.2, and 0.1 kg CO₂ eq./ton PS at pH 7.0, 6.5, 6.0, 5.5, and 5.0, respectively. Methanosarcina was found to be the dominant genus (67% of the total archaeal sequence) in the control, whose dominance was reduced as storage pH decreased. The results of ribonucleic acid analysis and specific methanogenic activity test further confirmed the inhibition of indigenous methanogens by acidification. Later, the biochemical CH₄ potential of stored PS was tested. Compared to the control (10.6 L CH₄/L PS), the acidified PS showed higher CH₄ yields of 12.7-14.6 L CH₄/L PS, presumably by keeping degradable organic matters in PS under acidic condition. Among different acidification pHs tested, the maximum amount of GHG reduction was achieved at pH 6.0 by reducing CH₄ emission to +0.4 kg CO₂ eq./ton PS during storage while increasing biogas production potential equivalent to 48.3 kWh/ton PS (-22.5 kg CO₂ eq./ton PS), resulting in a further reduction of (-)9.6 kg CO₂ eq./ton PS compared to the control.