Having worked with biofilms since the 1970s, I know that they are ubiquitous in nature, of great value in water technology, and scientifically fascinating. Biofilms are naturally able to remove BOD, ...transform N, generate methane, and biodegrade micropollutants. What I also discovered is that biofilms can do a lot more for us in terms of providing environmental services if we give them a bit of help. Here, I explore how we can use active substrata to enable our biofilm partners to provide particularly challenging environmental services. In particular, I delve into three examples in which an active substratum makes it possible for a biofilm to accomplish a task that otherwise seems impossible. The first example is the delivery of hydrogen gas (H2) as an electron donor to drive the reduction and detoxification of the rising number of oxidized contaminant: e.g., perchlorate, selenate, chromate, chlorinated solvents, and more. The active substratum is a gas-transfer membrane that delivers H2 directly to the biofilm in a membrane biofilm reactor (MBfR), which makes it possible to deliver a low-solubility gaseous substrate with 100% efficiency. The second example is the biofilm anode of a microbial electrochemical cell (MxC). Here, the anode is the electron acceptor for anode-respiring bacteria, which “liberate” electrons from organic compounds and send them ultimately to a cathode, where we can harvest valuable products or services. The anode's potential is a sensitive tool for managing the microbial ecology and reaction kinetics of the biofilm anode. The third example is intimately coupled photobiocatalysis (ICPB), in which we use photocatalysis to enable the biodegradation of intrinsically recalcitrant organic pollutants. Photocatalysis transforms the recalcitrant organics just enough so that the products are rapidly biodegradable substrates for bacteria in a nearby biofilm. The macroporous substratum, which houses the photocatalyst on its exterior, actively provides donor substrate and protects the biofilm from UV light and free radicals in its interior. These three well-developed topics illustrate how and why an active substratum expands the scope of what biofilms can do to enhance water sustainability.
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•Biofilms are ubiquitous in Nature and widely useful in environmental biotechnologies.•Active substrata can allow biofilms to achieve important, but very difficult tasks.•Hydrogen gas can be delivered as an electron donor to reduce oxidized contaminants.•The anode of a microbial electrochemical cell can be the electron acceptor.•A macroporous carrier can allow intimate coupling of photocatalysis and biodegradation of recalcitrant organics.
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.
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•Tetracycline (TCH) is bio-recalcitrant, but photocatalytically degradable.•VPCB involved macroporus carriers coated Ag/TiO2 on surface and biofilm in core.•VPCB accelerated TCH ...removal by ∼23% over photocatalysis.•Mineralization was enhanced by ∼26%, as photocatalytic products were biodegraded.•The biofilm in VPCB was acclimated to utilizing the photocatalytic products.
Intimately coupled photocatalysis and biodegradation (ICPB) has been studied for treating bio-recalcitrant pollutants. The key principle of ICPB is that photocatalysis occurs on a porous carrier’s outer surface, and biofilms are present in the interior, where they are protected from inhibition. Little is known regarding the ability of ICPB to degrade antibiotics, and a primary issue is whether or not the bacteria in the carriers’ interior can acclimate to biodegrade the products from photocatalysis of antibiotics. This study evaluated the removal and mineralization of tetracycline hydrochloride (TCH) using visible-light-induced ICPB (called VPCB) with Ag-doped TiO2 as the photocatalyst. The biofilms inside the VPCB carriers could mineralize the photocatalytic products, which led to more than 20% enhanced removal of chemical oxygen demand (COD). By biodegrading the products of TCH photocatalysis, the biofilms lowered the competition for free radicals between TCH and its photocatalysis products; thus, VPCB accelerated TCH removal by ∼11% in the first 2h of operation, compared with photocatalysis alone. The biofilms in the VPCB carriers evolved to being enriched in Methylibium, Runella, Comamonas, and Pseudomonas, which are known for biodegrading aromatics and being resistant to TCH. In conclusion, VPCB enhanced degradation and mineralization of TCH.
A multispecies biofilm model is developed for simultaneous reduction of nitrate and perchlorate in the H2-based membrane biofilm reactor. The one-dimension model includes dual-substrate Monod ...kinetics for a steady-state biofilm with five solid and five dissolved components. The solid components are autotrophic denitrifying bacteria, autotrophic perchlorate-reducing bacteria, heterotrophic bacteria, inert biomass, and extracellular polymeric substances (EPS). The dissolved components are nitrate, perchlorate, hydrogen (H2), substrate-utilization-associated products, and biomass-associated products (BAP). The model explicitly considers four mechanisms involved in how three important operating conditions (H2 pressure, nitrate loading, and perchlorate loading) affect nitrate and perchlorate removals: (1) competition for H2, (2) promotion of PRB growth due to having two electron acceptors (nitrate and perchlorate), (3) competition between nitrate and perchlorate reduction for the same resources in the PRB: electrons and possibly reductase enzymes, and (4) competition for space in the biofilm. Two other special features are having H2 delivered from the membrane substratum and solving directly for steady state using a novel three-step approach: finite-difference for approximating partial differential and/or integral equations, Newton–Raphson for solving nonlinear equations, and an iterative scheme to obtain the steady-state biofilm thickness. An example result illustrates the model’s features.
Intimately coupled photocatalysis and biodegradation (ICPB) was realized in a macroporous carrier in which a photocatalyst was present on the outer surface, while a biofilm accumulated inside the ...carrier. In ICPB, photocatalysis products are rapidly biodegraded by a protected biofilm, leading to mineralization of the refractory organics, such as antibiotics. However, mineralization in ICPB could be compromised if the photocatalysis products remain refractory or are inhibitory. To address this, we attempted to increase metabolic activity by providing a readily biodegradable co-substrate (acetate) that could act as a source of energy and electrons to improve biotransformation and mineralization of the refractory antibiotic tetracycline (TCH). When we added acetate during ICPB of TCH, TCH removal increased by ∼5%, mineralization increased by ∼20%, and almost all photocatalysis products disappeared. Acetate addition also led to an increase in active biomass, an increase in the biomass's respiratory activity, and evolution of the microbial community to having more members able to biodegrade photocatalysis and biotransformation intermediates. Thus, providing an easily biodegradable co-substrate was an effective means for enhancing TCH removal and mineralization with the ICPB technology.
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•Intimately coupled photocatalysis and biodegradation (ICPB) is a means to biodegrade and mineralize refractory organics.•Biodegradation and mineralization of the antibiotic tetracycline (TCH) was accelerated by ICPB.•Adding a readily biodegradable substrate enhanced the acceleration effect of ICPB.•Acetate addition enriched the biofilm in bacterial genera able to utilize TCH photocatalysis products.
Part 1 of this work developed a steady-state, multispecies biofilm model for simultaneous reduction of nitrate and perchlorate in the H2-based membrane biofilm reactor (MBfR) and presented a novel ...method to solve it. In Part 2, the half-maximum-rate concentrations and inhibition coefficients of nitrate and perchlorate are optimized by fitting data from experiments with different combinations of influent nitrate and perchlorate concentrations. The model with optimized parameters is used to quantitatively and systematically explain how three important operating conditions (nitrate loading, perchlorate loading, and H2 pressure) affect nitrate and perchlorate reduction and biomass distribution in these reducing biofilms. Perchlorate reduction and accumulation of perchlorate-reducing bacteria (PRB) in the biofilm are affected by four promotion or inhibition mechanisms: simultaneous use of nitrate and perchlorate by PRB and competition for H2, the same resources in PRB, and space in a biofilm. For the hydrogen pressure evaluated experimentally, a low nitrate loading (<0.1 g N/m2-d) slightly promotes perchlorate removal, because of the beneficial effect from PRB using both acceptors. However, a nitrate loading of >0.6 g N/m2-d begins to inhibit perchlorate removal, as the competition effects become dominant.
Complete biodegradation and mineralization of pentachlorophenol (PCP), a priority pollutant in water, is challenging for water treatment. In this study, a hydrogen (H2)-based membrane biofilm reactor ...(MBfR) was applied to treat PCP, along with nitrate and sulfate, which often coexist in contaminated groundwater. Throughout 120-days of continuous operation, almost 100% of up to 10 mg/L PCP was removed with minimal intermediate accumulation and in parallel with complete denitrification of 20 mg-N/L nitrate. PCP initially was reductively dechlorinated to phenol, which was then mineralized to CO2 through pathways that began with aerobic activation via monooxygenation by Xanthobacter and anaerobic activation via carboxylation by Azospira and Thauera. Sulfur cycling induced by SO42− reduction affected the microbial community: The dominant bacteria became sulfate-reducers Desulfomicrobium, sulfur-oxidizers Sulfuritalea and Flavobacterium. This study provides insights and a promising technology for bioremediation of water contaminated with PCP, nitrate, and sulfate.
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•Complete dechlorination and mineralization of pentachlorophenol (PCP).•PCP, NO3− and SO42− reduction were fueled by exogenous H2 or endogenous organics.•Sulfur cycling provided extra pathways of electron flow to PCP and NO3− reduction.•Phenol can be mineralized through monooxygenative and/or carboxylative activation.
Fully understanding the metabolism of SRB provides fundamental guidelines for allowing the microorganisms to provide more beneficial services in water treatment and resource recovery. The ...electron-transfer pathway of sulfate respiration by Desulfovibrio vulgaris is well studied, but still partly unresolved. Here we provide deeper insight by comprehensively monitoring metabolite changes during D. vulgaris metabolism with two electron donors, lactate and pyruvate, in presence or absence of citrate-chelated soluble FeIII as an additional competing electron acceptor. H2 was produced from lactate oxidation to pyruvate, but pyruvate oxidation produced mostly formate. Accumulation of lactate-originated H2 during lag phases inhibited pyruvate transformation to acetate. Sulfate reduction was initiated by lactate-originated H2, but MQ-mediated e− flow initiated sulfate reduction without delay when pyruvate was the donor. When H2-induced electron flow gave priority to FeIII reduction over sulfate reduction, the long lag phase before sulfate reduction shortened the time for iron-sulfide crystallite growth and led to smaller mackinawite (Fe1+xS) nanocrystallites. Synthesizing all the results, we propose that electron flow from lactate or pyruvate towards SO42− reduction to H2S are through at least three routes that are regulated by the e− donor (lactate or pyruvate) and the presence or absence of another e− acceptor (FeIII here). These routes are not competing, but complementary: e.g., H2 or formate production and oxidation were necessary for sulfite and disulfide/trisulfide reduction to sulfide. Our study suggests that the e− donor provides a practical tool to regulate and optimize SRB-predominant bioremediation systems.
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•D. vulgaris transfers e− for SO42− reduction to H2S through multi-routes regulated by e− donor.•H2/formate as e− carriers were necessary for SO32− and disulfide/trisulfide reduction to H2S.•H2 cycling through cytochrome c3 gave priority to FeIII reduction over SO42− reduction.•Pyruvate-induced concomitant FeIII/SO42− reduction extended FeS crystallization duration.•e− donor can be a practical tool to optimize SRB-predominant bioremediation systems.
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