Anaerobic co-digestion of alkali-pretreated submerged macrophytes and acidified food waste for reduction of neutralizing agents

Alkaline pretreatment for lignocellulosic biomass is beneficial for enhancing the anaerobic digestibility, but the rise of pretreatment cost and the dilution of substrate by the addition of a neutralizing agent and an alkaline solution are significant drawbacks. To solve these problems, utilization of acidified food waste as a co-substrate during anaerobic digestion of alkali-pretreated submerged macrophytes was attempted under various macrophyte to food waste (MF) ratio, and the biochemical interactions between the co-substrates were investigated. The addition of acidified food waste dropped the pH of alkali-pretreated Potamogeton maackianus, which reduced the dosage of neutralizing agent by 50% at MF 1:1. In co-digestion experiment, more CH4 was produced as the dosage of food waste increased. Volatile-solid-based and wet-weight-based CH4 yields for MF 1:1 were 37% and 118% higher than those for the alkali-pretreated macrophyte, respectively, because of the high degradability and organic matter content of the food waste. Comparison between the measured and theoretical COD balance of the co-digestion revealed that there is no positive synergistic effect in the CH4 recovery. Instead, the hydrolysis of the co-substrates is inhibited possibly due to the existence of dissolved lignin. Overall, the study demonstrates that anaerobic co-digestion of alkali-pretreated macrophytes and acidified food waste could be a feasible option for reducing the neutralizing agent cost and enhancing the volumetric CH4 yield.