Due to the high global warming potential (GWP) in a short time scale (GWP100 = 28 vs. GWP20 = 86), mitigating CH4emissions could have an early impact on reducing current global warming effects. The manure storage tank emits a significant amount of CH4, which can diminish the environmental benefit resulting from the anaerobic digestion of manure that can generate renewable energy. In the present study, we added the reverse osmosis concentrate (ROC) rich in salt to the pig slurry (PS) storage tank to reduce CH4emissions. Simultaneously, pure NaCl was tested at the same concentration to compare and verify the performance of ROC addition. During 40 days of storage, 1.83 kg CH4/ton PS was emitted, which was reduced by 7–75% by the addition of ROC at 1–9 g Na+/L. This decrease was found to be more intensive than that found upon adding pure sodium, which was caused by the presence of sulfate rich in ROC, resulting in synergistic inhibition. The results of the microbial community and activity test showed that sodium directly inhibited methanogenic activity rather than acidogenic activity. In the subsequent biogas production from the stored PS, more CH4was obtained by ROC addition due to the preservation of organic matter during storage. Overall, 51.2 kg CO2eq./ton PS was emitted during the storage, while 8 kg CO2eq./ton PS was reduced by biogas production in the case of control, resulting in a total of 43.2 kg CO2eq./ton PS. This amount of greenhouse gas emissions was reduced by ROC addition at 5 g Na+/L by 22 and 65 kg CO2eq./ton PS, considering GWP100 and GWP20 of CH4, respectively, where most of the reduction was achieved during the storage process. To the best of our knowledge, this was the first report using salty waste to reduce GHG emissions in a proper place, e.g., a manure storage tank.