Enhanced Hydrogen Production from Sewage Sludge by Co-fermentation with Forestry Wastes

Dark fermentation of sewage sludge (i.e., the byproduct of biological sewage treatment process) is a promising way for renewable hydrogen production. However, low carbohydrate content and low C/N ratio essentially limited hydrogen production efficiency from sewage sludge. In this study, three raw forestry wastes with higher biodegradable carbohydrate content (fallen poplar leaves, flower waste, and sheared ryegrass) were added into a batch sludge fermentation system, aiming to explore an effective and practically feasible method to enhance hydrogen production from sludge. The results showed that the hydrogen yield from sole sewage sludge increased from 11.2 to 20.8, 32, and 51.7 mL/g-volatile solids (VS)(added) with the addition of poplar leaves, flower waste, and ryegrass as co-substrates, respectively, with relevant increase ratios of 0.85, 1.85, and 3.60 times, respectively. Model simulation results indicated that the lag time of sludge fermentation was also shortened through the addition of the above three raw forestry wastes as co-substrates. Meanwhile, the VS removal for sole sludge fermentation increased from 3.4% to 7.2, 12.7, and 18.6% by co-fermentation with poplar leaves, flower waste, and ryegrass, respectively. The enhancement from co-fermentation might be due to the addition of more biodegradable carbohydrate, more suitable C/N ratio of substrate, and higher carbohydrate utilization. After fermentation, the acetate-type fermentation was predominant in all four groups. Co-fermentation with these forestry wastes did not change the dominant hydrogen fermentation type. This study demonstrated that it was feasible to enhance hydrogen production and sludge reduction by co-fermentation with poplar leaves, flower waste, and ryegrass.