Improved anaerobic digester stability to organic loading rate shocks with the use of an environmentally derived inoculum.

摘要

Based on previous findings, there were two hypotheses driving this study. The first was that the methanogens present in an acidic wetland would be distinctly different than methanogens found in a constructed anaerobic reactor. The second hypothesis was that a reactor inoculated from an acidic peatland would be more tolerant of sudden increases in feeding rate, and the resultant drop in pH, than a reactor inoculated from a traditional anaerobic digester.;To investigate the first hypothesis, the methanogens in a local oligotrophic, acidic fen known as Bear Meadows Bog and a mesophilic anaerobic digester treating municipal wastewater sludge at the Pennsylvania State University Wastewater Treatment Plant were analyzed with clone libraries. Two clone libraries were created for each environment, one for the 16S rDNA and a second for the methyl coenzyme M reductase alpha subunit gene (mcrA). The clone libraries of the bog and digester environments showed no overlap at the species and genus levels, and almost no overlap at the family level. Both libraries were dominated by clones related to uncultured methanogen groups within the Methanomicrobiales, specifically the Fen Cluster in the bog and MCR-7 in the digester. Despite differences in the sequences detected, diversity was similar in both environments.;Based on the results of the clone libraries, real-time quantitative PCR (qPCR) methods were developed to distinguish and quantify members of different methanogen clades. These methods were further tested by determining total mcrA and mcrA of different methanogen groups from six samples: four samples from anaerobic digesters treating either primarily cow or pig manure, and two aliquots from an acidic peat sample stored at 4°C or 20°C. The three samples obtained from cow manure digesters were dominated by members of the genus Methanosarcina, whereas the sample from the pig manure digester contained detectable levels of only members of the Methanobacteriaceae. The acidic peat samples were dominated by both Methanosarcina and members of the Fen cluster. In two of the manure digester samples only one methanogen group was detected, but in both of the acidic peat samples and two of the manure digester samples, multiple methanogen groups were detected.;To investigate the second hypothesis of this study, lab-scale reactors were inoculated with either bog sediments (Bog reactors,) wasted sludge from the anaerobic digester (Digester reactors), or with a mixture of these two inocula (Hybrid reactors).;A total of three sets of organic shocks were delivered. Glucose pulses were equal to 1--10 times the normal amount of volatile carbon in a feeding, and these resulted in the production of large amounts of volatile fatty acids which reduced reactor pH to less than 5. The first shock was 10 g of glucose, and this caused the Digester- and Hybrid-inoculated test reactors to fail, but the Bog-inoculated reactor recovered. After re-inoculating the Digester- and Hybrid-test reactors, three successive glucose pulses of 1, 5, and 10 g of glucose were delivered. The Hybrid-test reactor failed after receiving the 5 g glucose pulse, but the Bog- and Digester-test reactors only failed after receiving the 10 g glucose pulse. All reactors were recovered by the addition of NaOH to raise reactor pH to neutral levels. A second set of test reactors was established from the first set, and a third glucose pulse of 10 g was delivered. One set of reactors received mineral nutrients with the glucose pulse, but the second set did not. All reactors failed, but the Bog- and Digester-inoculated reactors recovered after pH was raised to only 4.8--5. The Hybrid-inoculated reactor did not show signs of recovery until the end of the testing period. Reactors with the same inoculum acted similarly, suggesting mineral nutrients did not prevent reactor failure.;Although distinct differences existed in the methanogen communities from the bog and digesters, these differences could not be maintained in constructed reactors. The methanogen communities in control reactors were less diverse than those in test reactors, and the acidophilic Fen Cluster was detected when reactor pH was acidic. The test reactors showed greater resilience to low pH conditions than has been previously reported in the literature, and reactors were able to recover activity after a glucose pulse from a pH as low as 4.8--5. Further research is needed to understand the role that different methanogen groups, specifically the Fen Cluster and Methanosarcina , play during periods of acidic operation, and how their presence may be maintained and their activity enhanced in order to provide greater stability to anaerobic digesters. (Abstract shortened by UMI.)