Evaluation of process parameters and treatments of different raw materials for biogas production

Sammanfattning: The anaerobic digestion process results in the production of biogas from a large diversity of organic residues, which can be used as an alternate source of energy generation and for waste management. However, the biogas production technology is not exploited efficiently due to the characteristics of raw materials, poor methane yields, process instability, limited raw materials and lack of knowledge about the process. Effects of various parameters and different treatments on the performance and stability of the anaerobic digestion process were investigated in this thesis work. Arrays of substrates were subjected to the biochemical methane potential (BMP) batch assay to evaluate the impact of source of inoculum, inoculum to substrate ratio, characteristics of substrate and presence of toxic chemicals on the methane yield and kinetics of methane production during anaerobic digestion. Biogas production from sugarcane bagasse and pulp mill sludges was studied after different pretreatments. Biogas production from rectified methanol condensate from pulp and paper mill was investigated at different operating conditions. Inoculum was found to be an important factor that affects the anaerobic digestion. Different parameters related to inoculum studied were: source of inoculum, inoculum to substrate ratio and adaptation of inoculum. It was found that inoculum to substrate ratio has impact on the reduction of volatile solids and the rate of methanogenesis during anaerobic digestion. Increasing the inoculum to substrate ratio enhanced the reduction in volatile solids and methane production rate during anaerobic digestion of sodium acetate. The source of inoculum significantly affected the methane production rate but had no marked influence on the ultimate methane yield. Long term exposure of anaerobic consortia to methanol condensate from pulp mill in an upflow anaerobic sludge bed reactor caused adaptation of the microbial consortia to the feed and hence it became more tolerant to toxic material in methanol condensate. The adaptation resulted in an increase in the methane yield and reduced the recovery time after failure of the process. The treatment with diluted acid of sugarcane bagasse followed by enzymatic hydrolysis resulted in the release of considerable amounts of monomeric sugars that afterwards were subjected to anaerobic digestion. These treatments led to significant improvements in yields as compared to when raw sugarcane bagasse was digested. Additionally, higher amount of methane was produced from anaerobic digestion of sugarcane bagasse treated with acid alone compared to when raw sugarcane was subjected to anaerobic digestion but the yields were lower than that of treatment with diluted acid followed by enzymatic hydrolysis. Furaldehydes i.e. furfural and hydroxymethylfurfural at concentrations formed during the pretreatments of lignocellulosic biomass were found to be non-inhibitory and degradable during anaerobic digestion. However, high concentrations of furaldehydes affected the biogas production. The effect of the higher concentration of furaldehydes was evident on biogas production kinetics. The amount and rate of biogas production from a substrate is highly dependent on the composition and characteristics of the substrate. To compare the extent of degradability among different substrates, the methane yield based on the COD basis provides a better option than the methane yield based on volatile solids.