Comparison of rod- versus filament-type methanogenic granules: microbial population and reactor performance

Acetate-triggered granular sludge floatation in methanogenic bioreactors

Methanogenic granular sludge from anaerobic bioreactors plays a primary role in treatment of various high-strength industrial wastewaters. The common problem of sludge floatation can lead to washout of granules from the reactor and severely affect reactor performance. However, an understanding of the specific key trigger-factors and appropriate control strategies for granular sludge floatation remains elusive. In this study, the concentration of acetate, rather than that of other volatile fatty acids (VFAs, i.e. propionate and butyrate) and granular sludge properties, was identified to be positively, linearly correlated with the amount of floating granules. The number of floating granules on propionate (18 ± 6) or butyrate-containing (34 ± 13) wastewater was comparable with that of non-VFA control wastewater (30.5 ± 7.5), and much lower than that of acetate-containing wastewater (80.5 ± 10.5). A scenario of excessive acetate-triggered granular sludge floatation is proposed based on these results as well as on the microbial community profile and spatial distribution, porous structure of granules, and impacts of operational parameters. Two new strategies, acetate-depletion and co-substrate addition, effectively reduced the number of floating granules by 28.5% and 51.6%, respectively. These results deepen our understanding of granular sludge floatation in methanogenic bioreactors and provide effective strategies to control sludge floatation.

Ecophysiology of uncultured filamentous anaerobes belonging to the phylum KSB3 that cause bulking in methanogenic granular sludge

A filamentous bulking of a methanogenic granular sludge caused by uncultured filamentous bacteria of the candidate phylum KSB3 in an upflow anaerobic sludge blanket (UASB) system has been reported. To characterize the physiological traits of the filaments, a polyphasic approach consisting of rRNA-based activity monitoring of the KSB3 filaments using the RNase H method and substrate uptake profiling using microautoradiography combined with fluorescence in situ hybridization (MAR-FISH) was conducted. On the basis of rRNA-based activity, the monitoring of a full-scale UASB reactor operated continuously revealed that KSB3 cells became active and predominant (up to 54% of the total 16S rRNA) in the sludge when the carbohydrate loading to the system increased. Batch experiments with a short incubation of the sludge with maltose, glucose, fructose, and maltotriose at relatively low concentrations (approximately 0.1 mM) in the presence of yeast extract also showed an increase in KSB3 rRNA levels under anaerobic conditions. MAR-FISH confirmed that the KSB3 cells took up radioisotopic carbons from [(14)C]maltose and [(14)C]glucose under the same incubation conditions in the batch experiments. These results suggest that one of the important ecophysiological characteristics of KSB3 cells in the sludge is carbohydrate degradation in wastewater and that high carbohydrate loadings may trigger an outbreak of KSB3 bacteria, causing sludge bulking in UASB systems.

Methanogenesis under acidic pH conditions in a semi-continuous reactor system

Operating an anaerobic digester at low pH could offer several advantages over operation at neutral pH. Most wastewater streams targeted for anaerobic digestion are inherently acidic, requiring alkalinity supplementation (at added expense) to buffer the pH at neutral. Additionally, previously published work completed by the authors using batch systems suggested that lowering the system pH could increase methane production by as much as 30%. The goal of this research was to evaluate the feasibility of sustaining methanogenesis at low pH in a semi-continuous laboratory-scale fermentor. Significant methane production was achieved in a system ranging in pH from approximately 4.0-5.3. Results show that, if the consortium is allowed to sufficiently acclimate to acidic conditions, methanogenesis can be maintained under acidic pH conditions, resulting in overall chemical oxygen demand (COD) reduction and methane production comparable to that achieved in a neutral pH system.

Characterization of filamentous bacteria, belonging to candidate phylum KSB3, that are associated with bulking in methanogenic granular sludges

A fatal bulking phenomenon was found to occur occasionally in the methanogenic granular sludge of a mesophilic (35-40 degrees C), full-scale upflow anaerobic sludge blanket (UASB) reactor treating organic wastewater discharged from a sugar manufacturing factory. A vast number of filamentous cells were observed in the bulking sludge that were morphologically distinct from the previously recognized anaerobic bulking agent Anaerolinea thermophila. 16S rRNA gene-based analyses of the microbial populations in the bulking sludge revealed that the dominant filamentous organisms were members of proposed candidate bacterial phylum, KSB3. Fluorescence in situ hybridization (FISH) analysis of the healthy sludge granules showed that the KSB3 filaments were the dominant granule surface population suggesting that they are fundamental constituents of the sludge granules and that they occasionally overgrow in the reactor, possibly triggering the filamentous bulking. We surveyed 10 additional mesophilic and thermophilic anaerobic sludges for the presence and diversity of KSB3 populations. Bacteria closely related to the characterized KSB3 filaments were present in two types of mesophilically grown UASB sludge granules treating actual wastewater discharged from sugar-processing industries.

Monitoring granule formation in anaerobic upflow bioreactors using oligonucleotide hybridization probes

The process of granule formation in upflow anaerobic sludge blanket (UASB) reactors was studied using oligonucleotide hybridization probes. Two laboratory-scale UASB reactors were inoculated with sieved primary anaerobic digester sludge from a municipal wastewater treatment plant and operated similarly except that reactor G was fed glucose, while reactor GP was fed glucose and propionate. Size measurements of cell aggregates and quantification of different populations of methanogens with membrane hybridization targeting the small-subunit ribosomal RNA demonstrated that the increase in aggregate size was associated with an increase in the abundance of Methanosaeta concilii in both reactors. In addition, fluorescence in situ hybridization showed that the major cell components of small aggregates collected during the early stages of reactor startup were M. concilii cells. These results indicate that M. concilii filaments act as nuclei for granular development. The increase in aggregate size was greater in reactor GP than in reactor G during the early stages of startup, suggesting that the presence of propionate-oxidizing syntrophic consortia assisted the formation of granules. The mature granules formed in both reactors exhibited a layered structure with M. concilii dominant in the core, syntrophic consortia adjacent to the core, and filamentous bacteria in the surface layer. The excess of filamentous bacteria caused delay of granulation, which was corrected by increasing shear through an increase of the recycling rate.

In situ detection, isolation, and physiological properties of a thin filamentous microorganism abundant in methanogenic granular sludges: a novel isolate affiliated with a clone cluster, the green non-sulfur bacteria, subdivision I

We previously showed that very thin filamentous bacteria affiliated with the division green non-sulfur bacteria were abundant in the outermost layer of thermophilic methanogenic sludge granules fed with sucrose and several low-molecular-weight fatty acids (Y. Sekiguchi, Y. Kamagata, K. Nakamura, A. Ohashi, H. Harada, Appl. Environ. Microbiol. 65:1280-1288, 1999). Further 16S ribosomal DNA (rDNA) cloning-based analysis revealed that the microbes were classified within a unique clade, green non-sulfur bacteria (GNSB) subdivision I, which contains a number of 16S rDNA clone sequences from various environmental samples but no cultured representatives. To investigate their function in the community and physiological traits, we attempted to isolate the yet-to-be-cultured microbes from the original granular sludge. The first attempt at isolation from the granules was, however, not successful. In the other thermophilic reactor that had been treating fried soybean curd-manufacturing wastewater, we found filamentous microorganisms to outgrow, resulting in the formation of projection-like structures on the surface of granules, making the granules look like sea urchins. 16S rDNA-cloning analysis combined with fluorescent in situ hybridization revealed that the projections were comprised of the uncultured filamentous cells affiliated with the GNSB subdivision I and Methanothermobacter-like cells and the very ends of the projections were comprised solely of the filamentous cells. By using the tip of the projection as the inoculum for primary enrichment, a thermophilic, strictly anaerobic, filamentous bacterium, designated strain UNI-1, was successfully isolated with a medium supplemented with sucrose and yeast extract. The strain was a very slow growing bacterium which is capable of utilizing only a limited range of carbohydrates in the presence of yeast extract and produced hydrogen from these substrates. The growth was found to be significantly stimulated when the strain was cocultured with a hydrogen-utilizing methanogen, Methanothermobacter thermautotrophicus, suggesting that the strain is a sugar-fermenting bacterium, the growth of which is dependent on hydrogen consumers in the granules.

Effect of a water extract of Moringa oleifera seeds on the hydrolytic microbial species diversity of a UASB reactor treating domestic wastewater

The effect of a continuous supply of a water extract of Moringa oleifera seeds (WEMOS) on the hydrolytic microbial population of biomass grown in mesophilic upflow anaerobic sludge blanket reactors treating domestic wastewater was investigated. The WEMOS-treated sludge had seemingly a wider diversity, with enterobacter and klebsiella as dominant hydrolytic bacteria, compared with the control sludge. Additional tests indicated that various hydrolytic bacteria could degrade WEMOS. It appeared that a continuous supply of WEMOS to an anaerobic digester, treating domestic wastewater, increased the diversity of hydrolytic bacteria and therefore enhanced the biological start-up of the reactor.

Microbial utilization of electrically reduced neutral red as the sole electron donor for growth and metabolite production

Electrically reduced neutral red (NR) served as the sole source of reducing power for growth and metabolism of pure and mixed cultures of H2-consuming bacteria in a novel electrochemical bioreactor system. NR was continuously reduced by the cathodic potential (-1.5 V) generated from an electric current (0.3 to 1.0 mA), and it was subsequently oxidized by Actinobacillus succinogenes or by mixed methanogenic cultures. The A. succinogenes mutant strain FZ-6 did not grow on fumarate alone unless electrically reduced NR or hydrogen was present as the electron donor for succinate production. The mutant strain, unlike the wild type, lacked pyruvate formate lyase and formate dehydrogenase. Electrically reduced NR also replaced hydrogen as the sole electron donor source for growth and production of methane from CO2. These results show that both pure and mixed cultures can function as electrochemical devices when electrically generated reducing power can be used to drive metabolism. The potential utility of utilizing electrical reducing power in enhancing industrial fermentations or biotransformation processes is discussed.