Comparative performance of biofilm reactor types

Development of a unified model of biofilm-reactor kinetics is based on substrate-utilization kinetics, mass transport, biofilm growth, and reactor analysis. The model is applied to steady-state conditions for complete-mix, fixed-bed, and fluidized-bed reactors with and without recycle. The results of modeling experiments demonstrate that simple loading factors and kinetic relationships are insufficient to describe the performance of a variety of biofilm processes. Instead, the interactions among utilization kinetics, biofilm growth, and reactor configuration determine the performance. For example, fluidized-bed reactors can achieve superior performance to complete-mix and fixed-bed reactors because the biofilm is evenly distributed throughout the reactor while the liquid regime has plug-flow characteristics. When it is possible, experimental results which demonstrate key concepts are presented.     

A unified model describing the role of hydrogen in the growth of desulfovibrio vulgaris under different environmental conditions

A unified model for the growth of Desulfovibrio vulgaris under different environmental conditions is presented. The model assumes the existence of two electron transport mechanisms functioning simultaneously. One mechanism results in the evolution and consumption of hydrogen, as in the hydrogen-cycling model. The second mechanism assumes a direct transport of electrons from the donor to the acceptor, without the participation of H2. A combination of kinetic and thermodynamic conditions control the flow of electrons through each pathway. The model was calibrated using batch experiments with D. vulgaris grown on lactate, in the presence and absence of sulfate, and was verified using additional batch experiments under different conditions. The model captured the general trends of consumption of substrates and accumulation of products, including the transient accumulation and consumption of H2. Furthermore, the model estimated that 48% of the electrons transported from lactate to sulfate involved H2 production, indicating that hydrogen cycling is a fundamental process in D. vulgaris. The presence of simultaneous electron transport mechanisms might provide D. vulgaris with important ecological advantages, because it facilitates a rapid response to changes in environmental conditions. This model increases our ability to study the microbial ecology of anaerobic environments and the role of Desulfovibrio species in a variety of environments. Copyright 1998 John Wiley & Sons, Inc.     

Modeling biofilm kinetics for a low-loaded expanded-bed anaerobic reactor

The effect of surface coverage of biomass on biofilm kinetics in an expanded-bed, granular activated carbon an aerobic reactor was evaluated. Acetate was used as the sole organic carbon source. An assumption of 10% surface coverage of biofilm was examined and compared to 100% coverage. Best estimated values of k(a) and K(sa) did not differ significantly from one case to the other. The confidence region analysis also showed that the biofilm was fully penetrated in the expanded-bed reactor for the case of 10% coverage, as well as when 100% coverage was assumed. Because the biofilm was fully penetrated, a model having no internal diffusion resistance and using the best estimates of k(a) and K(sa) obtained from the 10 and 100% coverage assumptions was capable of giving good predictions of effluent acetate concentrations for an in dependent experiment having a reduced liquid detention time. Consideration of biofilm surface-loading criteria demonstrated how the results can be applied to other reactors for the purpose of predicting when the extent of surface coverage and internal diffusion resistance are not significant factors in biofilm modeling.     

Kinetics of anaerobic methane oxidation coupled to denitrification in the membrane biofilm reactor

Anaerobic oxidation of methane coupled to denitrification (AOM-D) in a membrane biofilm reactor (MBfR), a platform used for efficiently coupling gas delivery and biofilm development, has attracted attention in recent years due to the low cost and high availability of methane. However, experimental studies have shown that the nitrate-removal flux in the CH₄-based MBfR (<1.0 g N/m²-day) is about one order of magnitude smaller than that in the H₂-based MBfR (1.1–6.7 g N/m²-day). A one-dimensional multispecies biofilm model predicts that the nitrate-removal flux in the CH₄-based MBfR is limited to <1.7 g N/m²-day, consistent with the experimental studies reported in the literature. The model also determines the two major limiting factors for the nitrate-removal flux: The methane half-maximum-rate concentration (K₂) and the specific maximum methane utilization rate of the AOM-D syntrophic consortium (k_max2), with k_max2 being more important. Model simulations show that increasing k_max2 to >3 g chemical oxygen demand (COD)/g cell-day (from its current 1.8 g COD/g cell-day) and developing a new membrane with doubled methane-delivery capacity (D_m) could bring the nitrate-removal flux to ≥4.0 g N/m²-day, which is close to the nitrate-removal flux for the H₂-based MBfR. Further increase of the maximum nitrate-removal flux can be achieved when D_m and k_max2 increase together.     

Enhanced dimethyl phthalate biodegradation by accelerating phthalic acid di-oxygenation

The aerobic biodegradation of dimethyl phthalate (DMP) is initiated with two hydrolysis reactions that generate an intermediate, phthalic acid (PA), that is further biodegraded through a two-step di-oxygenation reaction. DMP biodegradation is inhibited when PA accumulates, but DMP’s biodegradation can be enhanced by adding an exogenous electron donor. We evaluated the effect of adding succinate, acetate, or formate as an exogenous electron donor. PA removal rates were increased by 15 and 30% for initial PA concentrations of 0.3 and 0.6 mM when 0.15 and 0.30 mM succinate, respectively, were added as exogenous electron donor. The same electron-equivalent additions of acetate and formate had the same acceleration impacts on PA removal. Consequently, the DMP-removal rate, even PA coexisting with DMP simultaneously, was accelerated by 37% by simultaneous addition of 0.3 mM succinate. Thus, lowering the accumulation of PA by addition of an electron increased the rate of DMP biodegradation.     

Metabolic reconstruction and experimental verification of glucose utilization in <Emphasis Type="Italic">Desulfurococcus amylolyticus</Emphasis> DSM 16532

Desulfurococcus amylolyticus DSM 16532 is an anaerobic and hyperthermophilic crenarchaeon known to grow on a variety of different carbon sources, including monosaccharides and polysaccharides. Furthermore, D. amylolyticus is one of the few archaea that are known to be able to grow on cellulose. Here, we present the metabolic reconstruction of D. amylolyticus’ central carbon metabolism. Based on the published genome, the metabolic reconstruction was completed by integrating complementary information available from the KEGG, BRENDA, UniProt, NCBI, and PFAM databases, as well as from available literature. The genomic analysis of D. amylolyticus revealed genes for both the classical and the archaeal version of the Embden-Meyerhof pathway. The metabolic reconstruction highlighted gaps in carbon dioxide-fixation pathways. No complete carbon dioxide-fixation pathway such as the reductive citrate cycle or the dicarboxylate-4-hydroxybutyrate cycle could be identified. However, the metabolic reconstruction indicated that D. amylolyticus harbors all genes necessary for glucose metabolization. Closed batch experimental verification of glucose utilization by D. amylolyticus was performed in chemically defined medium. The findings from in silico analyses and from growth experiments are discussed with respect to physiological features of hyperthermophilic organisms.     

Managing microbial communities in membrane biofilm reactors

Membrane biofilm reactors (MBfRs) deliver gaseous substrates to biofilms that develop on the outside of gas-transfer membranes. When an MBfR delivers electron donors hydrogen (H₂) or methane (CH₄), a wide range of oxidized contaminants can be reduced as electron acceptors, e.g., nitrate, perchlorate, selenate, and trichloroethene. When O₂ is delivered as an electron acceptor, reduced contaminants can be oxidized, e.g., benzene, toluene, and surfactants. The MBfR’s biofilm often harbors a complex microbial community; failure to control the growth of undesirable microorganisms can result in poor performance. Fortunately, the community’s structure and function can be managed using a set of design and operation features as follows: gas pressure, membrane type, and surface loadings. Proper selection of these features ensures that the best microbial community is selected and sustained. Successful design and operation of an MBfR depends on a holistic understanding of the microbial community’s structure and function. This involves integrating performance data with omics results, such as with stoichiometric and kinetic modeling.