Ecology, physiology and performance in high-rate anaerobic digestion

The design demands on water and sanitation engineers are rapidly changing. The global population is set to rise from 7 billion to 10 billion by 2083. Urbanisation in developing regions is increasing at such a rate that a predicted 56% of the global population will live in an urban setting by 2025. Compounding these problems, the global water and energy crises are impacting the Global North and South alike. High-rate anaerobic digestion offers a low-cost, low-energy treatment alternative to the energy intensive aerobic technologies used today. Widespread implementation however is hindered by the lack of capacity to engineer high-rate anaerobic digestion for the treatment of complex wastes such as sewage. This thesis utilises the Expanded Granular Sludge Bed bioreactor (EGSB) as a model system in which to study the ecology, physiology and performance of high-rate anaerobic digestion of complex wastes. The impacts of a range of engineered parameters including reactor geometry, wastewater type, operating temperature and organic loading rate are systematically investigated using lab-scale EGSB bioreactors. Next generation sequencing of 16S amplicons is utilised as a means of monitoring microbial ecology. Microbial community physiology is monitored by means of specific methanogenic activity testing and a range of physical and chemical methods are applied to assess reactor performance. Finally, the limit state approach is trialled as a method for testing the EGSB and is proposed as a standard method for biotechnology testing enabling improved process control at full-scale. The arising data is assessed both qualitatively and quantitatively. Lab-scale reactor design is demonstrated to significantly influence the spatial distribution of the underlying ecology and community physiology in lab-scale reactors, a vital finding for both researchers and full-scale plant operators responsible for monitoring EGSB reactors. Recurrent trends in the data indicate that hydrogenotrophic methanogenesis dominates in high-rate anaerobic digestion at both full- and lab-scale when subject to engineered or operational stresses including low-temperature and variable feeding regimes. This is of relevance for those seeking to define new directions in fundamental understanding of syntrophic and competitive relations in methanogenic communities and also to design engineers in determining operating parameters for full-scale digesters. The adoption of the limit state approach enabled identification of biological indicators providing early warning of failure under high-solids loading, a vital insight for those currently working empirically towards the development of new biotechnologies at lab-scale.

Anaerobic oxidation of methane at a marine methane seep in a forearc sediment basin off Sumatra, Indian Ocean

A cold methane seep was discovered in a forearc sediment basin off the island Sumatra, exhibiting a methane-seep adapted microbial community. A defined seep center of activity, like in mud volcanoes, was not discovered. The seep area was rather characterized by a patchy distribution of active spots. The relevance of anaerobic oxidation of methane (AOM) was reflected by C-13-depleted isotopic signatures of dissolved inorganic carbon. The anaerobic conversion of methane to CO2 was confirmed in a C-13-labeling experiment. Methane fueled a vital microbial community with cell numbers of up to 4 x 10(9) cells cm(-3) sediment. The microbial community was analyzed by total cell counting, catalyzed reporter deposition fluorescence in situ hybridization (CARD FISH), quantitative real-time PCR (qPCR), and denaturing gradient gel electrophoresis (DGGE). CARD FISH cell counts and qPCR measurements showed the presence of Bacteria and Archaea, but only small numbers of Eukarya. The archaeal community comprised largely members of ANME-1 and ANME-2. Furthermore, members of the Crenarchaeota were frequently detected in the DGGE analysis. Three major bacterial phylogenetic groups (delta-Proteobacteria, candidate division OP9, and Anaerolineaceae) were abundant across the study area. Several of these sequences were closely related to the genus Desulfococcus of the family Desulfobacteraceae, which is in good agreement with previously described AOM sites. In conclusion, the majority of the microbial community at the seep consisted of AOM-related microorganisms, while the relevance of higher hydrocarbons as microbial substrates was negligible.

Biogeochemical Cycling in a Headwater Stream and Riparian Zone

Several teams of researchers at multiple universities are currently measuring annual and seasonal fluxes of carbon dioxide and other greenhouses gases (nitrous oxide and methane) in riparian wetlands and upland forests in the Tenderfoot Creek Experimental Forest (TCEF), a subalpine watershed in the Little Belt Mountains, Montana. In the current thesis, the author characterized the geochemistry and stable carbon isotope composition of shallow groundwater, soil water, and stream water in upper Stringer Creek, near sites that are being investigated for gas chemistry and microbial studies. It was hypothesized that if methanogenesis were a dominant process in the riparian wetlands of upper Stringer Creek, then this should impart a characteristic signal in the measured stable isotopic composition of dissolved inorganic carbon in shallow groundwater. For the most part, the major solute composition of shallow groundwater in upper Stringer Creek was similar to that of the stream. However, several wells completed in wetland soil had highly elevated concentrations of Fe2+ and Mn2+ which were absent in the well-oxygenated surface water. Use of sediment pore-water samplers (peepers) demonstrated a rapid increase in Fe2+ and Mn2+ with depth, most feasibly explained by microbial reduction of Fe- and Mn-oxide minerals. In general, the pH of shallow groundwater was lower than that of the stream. Since concentrations of CO2 in the groundwater samples were consistently greater than atmospheric pCO2, exchange of CO2 gas across the stream/air interface occurred in one direction, from stream to air. Evasion of CO2 partly explains the higher pH values in the stream. Microbial processes involving breakdown of organic carbon, including aerobic respiration, anaerobic respiration, and methanogenesis, explain the occurrence of excess CO2 in the groundwater. In general, the isotopic composition of total dissolved inorganic carbon (DIC) decreased with increasing DIC concentration, consistent with aerobic and/or anaerobic respiration being the dominant metabolic process in shallow groundwater. However, a minority of wells contained high DIC concentrations that were anomalously heavy in u13C, and these same wells had elevated concentrations of dissolved methane. It is concluded that the wells with isotopically-heavier DIC have likely been influenced by acetoclastic methanogenesis. Results from shallow groundwater wells and one of the peeper samplers suggest a possible link between methanogenesis and bacterial iron reduction.