We hypothesize that activated-sludge processes having stable and complete nitrification have significant and similar diversity and functional redundancy among its ammonia- and nitrite-oxidizing ...bacteria, despite differences in temperature, solids retention time (SRT), and other operating conditions. To evaluate this hypothesis, we examined the diversity of nitrifying bacterial communities in all seven water-reclamation plants (WRPs) operated by Metropolitan Water Reclamation District of Greater Chicago (MWRDGC). These plants vary in types of influent waste stream, plant size, water temperature, and SRT. We used terminal restriction fragment length polymorphism (T-RFLP) targeting the 16S rRNA gene and group-specific ammonia-monooxygenase functional gene (
amoA) to investigate these hard-to-culture nitrifying bacteria in the full-scale WRPs. We demonstrate that nitrifying bacteria carrying out the same metabolism coexist in all WRPs studied. We found ammonia-oxidizing bacteria (AOB) belonging to the
Nitrosomonas europaea/eutropha,
Nitrosomonas oligotropha,
Nitrosomonas communis, and
Nitrosospira lineages in all plants. We also observed coexisting
Nitrobacter and
Nitrospira genera for nitrite-oxidizing bacteria (NOB). Among the factors that varied among the WRPs, only the seasonal temperature variation seemed to change the nitrifying community, especially the balance between
Nitrosospira and
Nitrosomonas, although both coexisted in winter and summer samples. The coexistence of various nitrifiers in all WRPs is evidence of functional redundancy, a feature that may help maintain the stability of the system for nitrification.
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 (H
2
) or methane (CH
4
...), a wide range of oxidized contaminants can be reduced as electron acceptors, e.g., nitrate, perchlorate, selenate, and trichloroethene. When O
2
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.
A H2-based, denitrifying and sulfate-reducing membrane-biofilm reactor (MBfR) was shown to be effective for removing selenate (Se(VI)) from water or wastewater by reducing it to insoluble Se0. When ...Se(VI) was first added to the MBfR, Se(VI) reductionfirst to selenite (Se(IV)) and then mostly to Se0took place immediately and then increased over three weeks, suggesting enrichment for dissimilatory selenium-reducing bacteria. Increasing the H2 pressure improved the Se(VI) reduction rate and total-Se removal, and lowering the influent Se(VI) concentration from 1000 to 260 μg Se/L increased the average Se removal to 94%, which corresponded to an effluent Se concentration of less than 12 μg Se/L, a value well below the standard of 50 μg Se/L. The fact that the effluent suspended solids contained reduced Se suggests that Se0 was retained in the biofilm, which detached to form the effluent suspended solids. A series of short-term experiments elaborated on how decreased influent selenate loading and increased H2 pressure could systematically improve the reduction of Se(VI) and removal of total Se. Short-term experiments also demonstrated that selenate reduction improved with lower influent nitrate concentration, suggesting that H2 was more available for selenate reduction when the H2 demand for denitrification was smaller. Complete sulfate reduction, which occurred in parallel to nitrate reduction, dominated the electron-equivalent flux. Like selenate reduction, but unlike nitrate reduction, sulfate reduction was sensitive to H2 pressure and appeared to be inhibited by selenate. Finally, selenate reduction was relatively insensitive to pH in the range of 7.0 to 9.0. This research shows that the MBfR can be effective for removing Se(VI) in water or wastewater to below the 50 μg Se/L standard.
We present a critical review of the relationships among three microbial products: extracellular polymeric substances (EPS), soluble microbial products (SMP), and inert biomass. Up to now, two ...different “schools” of researchers have treated these products separately. The “EPS school” has considered active biomass and EPS, while the “SMP school” has considered active biomass, SMP, and inert biomass. Here, we provide a critical review of each of the microbial products. Then, we develop a unified theory that couples them and reconciles apparent contradictions. In our unified theory, cells use electrons from the electron-donor substrate to build active biomass, and they also produce bound EPS and utilization-associated products (UAP) at the same time and in proportion to substrate utilization. Bound EPS are hydrolyzed to biomass-associated products (BAP), while active biomass undergoes endogenous decay to form residual dead cells. Finally, UAP and BAP, being biodegradable, are utilized by active biomass as recycled electron-donors substrates. Our unified theory shows that the apparently distinct products from the SMP and EPS schools overlap each other. Soluble EPS is actually SMP, or the sum of UAP and BAP. Furthermore, active biomass, as defined by the SMP school, includes bound EPS, while inert biomass includes bound EPS and the residual dead cells.
We conducted a series of pseudo-steady-state experiments on a novel hollow-fiber membrane biofilm reactor used for denitrification of oligotrophic waters, such as drinking water. We applied a range ...of nitrate loadings and hydrogen pressures to establish under what conditions the system could attain three goodness-of-performance criteria: partial nitrate removal, minimization of hydrogen wasting, and low nitrite accumulation. The hollow-fiber membrane biofilm reactor could meet drinking-water standards for nitrate and nitrite while minimizing the amount of hydrogen wasted in the effluent when it was operated under hydrogen-limited conditions. For example, the system could achieve partial nitrate removals between 39% and 92%, effluent nitrate between 0.4 and 9.1
mg N/l, effluent nitrite less than 1
mg N/l, and effluent hydrogen below 0.1
mg H
2/l. High fluxes of nitrate and hydrogen made it possible to have a short liquid retention time (42
min), compared with 1–13
h in other studies with hydrogen used as the electron donor for denitrification. The fluxes and concentrations for hydrogen, nitrate, and nitrite obtained in this study can be used as practical guidelines for system design.
Experiments carried out in a hollow-fiber, membrane-biofilm reactor (HFMBR) showed that the optimum pH for autotrophic denitrification was in the range 7.7–8.6, with the maximum efficiency at 8.4. ...Increasing the pH above 8.6 caused a significant decrease in nitrate removal rate and a dramatic increase in nitrite accumulation. The pH rose by 1.2 units when a large buffer was not added, suggesting that some field applications may require pH control. Precipitation of Ca
2+ occurred in every experiment. Precipitation was the largest sink for carbonate, and it also offset alkalinity production by denitrification. Although the alkalinity increased in most cases, systems with a high carbonate buffer and high pH accentuated precipitation, and the net change in alkalinity was negative. The long-term success of field applications of the HFMBR may depend upon the interactions among calcium concentration, total carbonate concentration, pH, and alkalinity changes.
We present a modeling approach that quantifies the unified theory presented in the companion paper. In this approach, we use mathematical modeling to quantify the relationships among three solid ...species—bacteria, extracellular polymeric substances (EPS), and inert residual biomass—two soluble microbial products (SMP), original substrate, and an electron acceptor. According to the model, donor electrons are used for the synthesis of biomass, EPS, and utilization-associated products. Residual inert biomass and biomass-associated products are produced from the decay of active biomass and the hydrolysis of EPS, respectively. The model includes mass balance equations that consistently describe the flow of electrons among the components. It is solved with a set of parameters appropriate to the experimental study of Hsieh et al. (Biotech. Bioeng. 44 (1994) 219). Model outputs capture all trends observed in steady-state CSTR experiments and transient batch experiments. This agreement supports that the unified theory correctly captures the interconnections among SMP, EPS, and active and inert biomass.
We present the unified multi-component cellular automaton (UMCCA) model, which predicts quantitatively the development of the biofilm's composite density for three biofilm components: active ...bacteria, inert or dead biomass, and extracellular polymeric substances. The model also describes the concentrations of three soluble organic components (soluble substrate and two types of soluble microbial products) and oxygen. The UMCCA model is a hybrid discrete-differential mathematical model and introduces the novel feature of biofilm consolidation. Our hypothesis is that the fluid over the biofilm creates pressures and vibrations that cause the biofilm to consolidate, or pack itself to a higher density over time. Each biofilm compartment in the model output consolidates to a different degree that depends on the age of its biomass. The UMCCA model also adds a cellular automaton algorithm that identifies the path of least resistance and directly moves excess biomass along that path, thereby ensuring that the excess biomass is distributed efficiently. A companion paper illustrates the trends that the UMCCA model is able to represent and shows a comparison with experimental results.
We established the first complete electron-equivalent balances in microbial fuel cells (MFCs) fed with non-fermentable (acetate) and fermentable (glucose) electron donors by experimentally ...quantifying current, biomass, residual organic compounds, H
2, and CH
4 gas. The comparison of the two donors allowed us to objectively evaluate the diversion of electron flow to non-electricity sinks for fermentable donors, leading to different behaviors in energy-conversion efficiency (ECE) and potential efficiency (PE). Electrical current was the most significant electron sink in both MFCs, being 71% and 49%, respectively, of the initial COD applied. Biomass and residual organic compounds, the second and third greatest sinks, respectively, were greater in the glucose-fed MFC than in the acetate-fed MFC. We detected methane gas only in the glucose-fed MFC, and this means that anode-respiring bacteria (ARB) could out-compete acetoclastic methanogens. The ECE was 42% with acetate, but was only 3% with glucose. The very low ECE for glucose was mostly due to a large increase of the anode potential, giving a PE of only 6%. Although the glucose-fed MFC had the higher biomass density on its anode, it had a very low current density, which supports the fact that the density of ARB was very low. This led to slow kinetics for electron transfer to the anode and accentuated loss due to the substrate-concentration gradient in the anode-biofilm. The large drop of PE with low current, probably caused by a low ARB density and electron (e
−) donor concentration, resulted in a poor maximum power density (9.8
mW/m
2) with glucose. In contrast, PE reached 59% along with high current for acetate and the maximum power density was 360
mW/m
2.
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