Biomass is one of the renewable energies. One of the best ways to reduce methane emissions to the atmosphere and to utilize biomass energy is to use anaerobic digestion of wastes and biogas production. Among the huge sources of wastes are plant and animal wastes. If these wastes are not managed, irreparable damage will be done to the environment. By removing impurities from the produced biogas, it can be used as an energy source in the transportation industry, electricity generation, or transmission to the gas supply network. In this study, bio-methane production from animal waste (cattle manure) in single-stage and two-stage systems were investigated. First, biogas production was eva‎luated in a single-stage system. Hydrothermal pretreatment was used at 120, 150, and 180 °C and the effect of pretreatment on biogas production in a single-stage system was investigated. In the second phase of the study, biogas production was studied by a two-stage system in which the sample with the highest efficiency of methane production from solids in the single-stage system was selected as feed for the two-stage system. Then, the effect of three different organic loading rates on methane production in a two-stage system was investigated. Both single-stage and two-stage systems were in the mesophilic temperature range (37 °C). The experiment for the single-stage system lasted 45 days for which 23 pair reactors with different conditions were tested, i.e. 46 reactors were used in the single-stage system. The two-stage system was tested for 66 days while for this experiment, 3 pairs of two-stage systems were used, i.e. a total of 6 systems with 12 reactors were used. Each reactor contained a microbial mixture (refinery sludge mixed with sheep rumen fluid) and a substrate (cow manure). To measure the amount of gas produced, the liquid column displacement method was used. Also, the gas composition and methane percentage were obtained for each system by gas chromatography. In the single-stage system, the use of pretreatment increased the production of biogas compared to the untreated state. The highest production rate for the pretreatment conditions was 216 (ml methane/gram of volatile slid-VS) at 150 °C and particle size of 0.5-1 mm, whereas the lowest production rate for the pretreatment conditions was 136.8 (ml methane/g VS) at 180 °C for the particle size of 1-2 mm. In the use of pretreatment liquids, the highest production rate was 388.8 (ml methane/g VS) at 150 °C and particle size of 1-2 mm. The lowest production rate for pretreatment condition was 256.2 (ml methane/g VS) at 180 °C while the particle size was 0.5-25 mm. Although the pretreatment temperature affected the amount of methane produced, the different particle sizes did not have obvious effect on methane production. The sample with the highest efficiency of methane production from solids in the single-stage system was selected as feed for the two-stage system and the effect of organic loading rate in three different rates of 0.5, 1, 2 (g VS/L-day) on methane production rate was studied in a two-stage system which produced 252.2, 265.2 and 235.9 (ml methane/g VS), respectively. The production rate in the two-stage system was higher than the single-stage system and in the highest case, 23% more production was reported than the single-stage system.

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احمد سوهان كار، محمدرضا سليم پور