Anaerobic digestion of crop residues at low temperatures
Abstract: Biogas is a renewable source of energy. Residues from agriculture contain considerable energy potential and are suitable substrates for biogas production, but in Sweden the utilization of these residues is low. The economic feasibility of biogas production in general is low, and only limited subsidies are provided by the state. In the present work, the feasibility of low-temperature digestion of crop residues was studied, since such a process may be a cost-efficient alternative for the digestion of this substrate on farm scale.One aspect studied was that of the microbiology of the low-temperature process. Fluorescence in situ hybridization followed by image analysis was evaluated as a method of quantifying methanogenic populations. The method proved to be useful for monitoring population changes. On the other hand, the method has poor precision in general, and results may be difficult to reproduce between laboratories. Changes in the microbial population in reactors operating at low temperatures were shown to be reflected by the reactor performance. Furthermore, it was shown that the microbial biomass adapted to low temperatures by changing the population of methanogens to a higher percentage of Methanomicrobiales and Methanosarcinaceae. Lake sediment was tested as a source of low-temperature-adapted microbial biomass, and this showed potential for providing a culture well-adapted to low temperatures. However, it was probably slow growing and the methanogenic population of the culture appeared to be sensitive to ammonia.The other aspects studied were those of energy balance and reactor configuration. The results of these studies showed that hydrolysis was the limiting step in low-temperature degradation of crop residues. Due to the lower yield obtained in a continuously stirred reactor at 15°C, this operation temperature resulted in a poorer energy balance than for a process operated at 30°C. On the contrary, digestion of an acidified substrate in methane filters was shown to be efficient at 10 and 15°C. A two-stage system may be suitable for low-temperature digestion of solid substrates.
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