Preserving sugarcane trash for year-round biogas production: effects of additives on ensiling properties and methane potential

Sugarcane trash (SCT) composed of tops, and dry and green leaves represents the non-millable fractions of cane that is usually left to decay on the fields for soil protection after mechanized harvesting. The fractional recovery of this material for biogas production can potentially result in generation of renewable energy without negatively influencing soil quality, since digestate from the anaerobic digestion process can be used as a soil conditioner. However, as the length of the sugarcane harvesting season is around 6 months per year, strategies for biomass preservation that allow SCT to be used as a substrate throughout the entire year are required. Therefore, the present study aimed to optimize the ensiling of SCT using a commercial lactic acid bacterial (LAB) inoculant to promote lactic acid (LA) fermentation with the addition of molasses to increase the content of biodegradable sugars. The ensiling experiments were conducted in laboratory-scale silo-bags under anoxic conditions with the following additives applied to the sugarcane biomass: SCTctr (no additives); SCTmol (addition of molasses); SCTmol+lab (addition of molasses and lactic acid bacteria); and SCSctr (sugarcane stalks without additives). The experiment was run for 70 days and the following parameters were analyzed at days 0, 5, 15 and 70: total and volatile solids, total volatile fatty acids, LA, ammonium-nitrogen, pH and solubilized chemical oxygen demand (COD). Biochemical methane potential (BMP) tests were performed with fresh and ensiled material at day 70 to assess energy losses/gains due to ensiling. In general, the results showed a decrease in pH from 5.7-5.9 to 3.8-4.2 for all SCT treatments during the ensiling period. The rationale behind pH reduction lies in the increase in lactic acid concentration from 1.0-1.4 % DM (dry matter) up to 3.0 % DM in the SCTmol + lab treatment. SCSctr showed a marked increase in lactic acid (from zero to 3.7 % DM) resulting in the lowest observed pH of 3.7 among all treatments. The ensiling process increased methane yields by 17%, 39%, 17% and 79% for SCSctr, SCTctr, SCTmol and SCTmol+lab, respectively. Thus, ensiling of sugarcane is not only a suitable way for wet biomass preservation but also an effective method to improve the digestibility of sugarcane residues for biogas production