Importance of maintaining the freshness of cattle manure for enhanced biogas production

Cattle manure (CM) is rich in organics, nutrients, and effective microorganisms for biogas production. However, its energy potential can decrease during storage due to indigenous microbial activity. This study investigated the effects of storage period on the CH₄ yield (MY) of CM and elucidated the underlying mechanisms through batch and semi-continuous experiments. In batch experiment, the MY of fresh CM (storage period 1-3 days) was 0.207 L CH₄/g VS, 1.66-fold higher than that of old CM (storage period 2-3 months). The lowest MY of 0.06 L CH₄/g VS was obtained in a reactor fed with old CM at an OLR of 2.6 g VS/L/d. Ammonia stripping was applied to alleviate the ammonia inhibition, resulting in an initial increase in MY, but CH₄ production nearly ceased as free ammonia (FAN) levels subsequently rose. In contrast, when fresh CM replaced old CM, the MY increased to 0.13-0.15 L CH₄/g VS at an OLR of 1.9-3.17 g VS/L/d and this performance was sustained over a prolonged period. Moreover, fresh CM enhanced the relative abundance of protein-degrading bacteria and hydrogenotrophic methanogens in the digestate and led to substantially higher specific methanogenic activity (SMA) values for both acetate and H₂/CO₂ utilization. These results suggest that the low MY of old CM is primarily due to the depletion of readily degradable proteinaceous organics during storage, leaving mainly degradation byproducts and recalcitrant compounds. Therefore, minimizing the storage period is essential to improve the MY of CM.