Biological methods for increasing the biogas yield of livestock waste

Danish full-scale biogas plants operate with manure as a primary substrate, whereas cattle manure represents a significant fraction. Conversion of lignocellulosic fiber fraction of manure was shown to be the rate-degrading step of the anaerobic digestion. The aim of this project was to investigate if the biological methods, such as bioaugmentation of reactor with specific anaerobic, hydrolytic bacteria will improve hydrolysis of fibers and result in an increase of the methane yield of cattle manure. Results of this study showed, that there is a potential for increase the methane yield of manure by inoculating the fiber-containing substrate with cellulose- and alkaliphilic xylane-hydrolyzing bacterial cultures. The highest increase of methane yield in batch experiment was obtained at 37 deg. C and 68 deg. C, while the effect at 55 deg. C was poor. Direct inoculation of a mesophilic reactor with Clostridium cellulovorans DSM 3052 was not successful. The fate of the organism, followed by 16S rRNA probing, proved that DSM 3052 was not a member of the indigenous microflora, and that the active population of DSM 3052 could not be established within the reactor. On the other hand, inoculation of a reactor with our own isolate, a new clostridial strain, Clostridium sp. SA 14, resulted in a significant increase of the methane yield from 220 ml CH4/g VS up to 330 ml CH4/g VS. However, during the continuous reactor operation in the period of one retention time, the effect was reduced and finally disappeared. Therefore, it would be necessary in the future to develop a new strategy for establishment of this strain within the reactor environment. Anaerobic digestion of manure was also studied in a two-step process, where manure was first hydrolyzed at 68 deg. C, and thereafter digested in a conventional methanogenic step at 55 deg. C. Investigation from batch experiment resulted in a 24%- increase in methane yield for the two-step digestion comparing to the single stage digestion at 55 deg. C. Moreover, the extra yield of 55% for the fraction of digested fibers, which have been pretreated at 68 deg., indicates a new possibility for improvement of the overall biogas production by recirculating this fraction in biogas plants. Furthermore, the batch experiment demonstrated that inoculation of manure with Caldicellulosiruptor lactoaceticus 6A, improved the methane yield additionally with more than 20%. A lab-scale reactor experiment with combined extremely thermophilic/moderate thermophilic treatment of manure resulted in a stable methane yield of 260 ml CH4/g VS, comparing to the yield of 240 ml CH4/g VS for the single stage operating reactor. The economical estimate of implementation and operation of the two-step reactor systems looks to be favorable for the medium- and large size biogas plants. Regarding the bioaugmentation at the extremely thermophilic temperature, it would be still interesting to investigate if inoculation of the pretreatment reactor with Caldicellulosiruptor lactoaceticus 6A will improve the methane yield of the two-step reactor system further