|dc.description.abstract||Organic solid wastes resulted from agricultural activities including livestock droppings (such as cow and pig manures) and lignocellulosic materials (such as rice straws) are no longer considered as “wastes”, because they are stable biomass sources that can be anaerobically treated and converted to useful fuels (such as methane, hydrogen, and ethanol) as renewable energy for a variety of applications. However, conventional anaerobic treatment processes have been conducted with “wet-state” digestion, instead of “dry-state” (or “solid-state”) digestion. As a result, there is a need to develop a new technology that not only can efficiently treat the agricultural wastes but can also be operated in a more sustainable fashion. In this study, a percolating solid-state anaerobic digestion system was established, and co-digestion approaches were employed to improve the biogas production. Specifically, fermentation of cow manure using the optimal design methodology was conducted to produce hydrogen, and the key factors that would influence co-digestion of pig manure (PM) mixed with paper & pulp sludge (PPS) or rice stalks (RS) to generate methane in the percolating system were evaluated and identified.
For the hydrogen production, the Taguchi method with level settings and an L18 orthogonal array to identify the primary factors affecting thermophilic anaerobic fermentation (temperature and pH value) was employed, followed by the central composite design of response surface methodology to predict the hydrogen content and production and to identify the optimal temperature and pH conditions. Results show that controlling the system at the ideal temperature and pH of 60C and 5.20±0.21 respectively resulted in the optimal hydrogen concentration of 54.64±11.45%, hydrogen generation rate of 405.54±93.61 mL-H2/L/d, and specific hydrogen yield of 10.25±1.96 mL-H2/g-VS.
For the methane production, after determining that the optimal solid retention time was 6 days, semi-continuous experiments were conducted to co-digest PM and PPS in hopes of increasing the quality and quantity of methane generation. Results indicated that a wet weight ratio of 50:50 for PPS and PM produced the highest methane concentration of 57.53%; further, co-digestion of PPS and PM increased methane concentration, methane yield, and specific methane production rate by 5.8%, 35.61%, and 49.22%, respectively, in comparison with those from the digestion of PM alone. Apart from PPS, RS were used as the feedstock substrate for methane production in the percolating system. Results show that when both pre-acidified and un-acidified rice straw were homogenized to the same particle size, similar methane-production levels were obtained, suggesting that pre-acidification of RS is not a prerequisite for the “biomass to methane” conversion in this system; moreover, results from permeability (or hydraulic conductivity) tests showed that porosity of the feedstock may be the key factor in modulating the overall methane production. In addition, metagenomic analysis revealed that the highest methane production rate and methane concentration were obtained from the systems where Mechanosarcinal and Methanobacterial dominated the archaean communities, in accord with elevated levels of acetate detected in these systems. Surprisingly, Thermoplasmatales, the thermophiles, only occupied a lower proportion of the archaean community.