Regulating microbial redox reactions towards enhanced removal of refractory organic nitrogen from wastewater.

Biotechnology for wastewater treatment is mainstream and effective depending upon microbial redox reactions to eliminate diverse contaminants and ensure aquatic ecological health. However, refractory organic nitrogen compounds (RONCs, e.g., nitro-, azo-, amide-, and N-heterocyclic compounds) with complex structures and high toxicity inhibit microbial metabolic activity and limit the transformation of organic nitrogen to inorganic nitrogen. This will eventually result in non-compliance with nitrogen discharge standards. Numerous efforts suggested that applying exogenous electron donors or acceptors, such as solid electrodes (electrostimulation) and limited oxygen (micro-aeration), could potentially regulate microbial redox reactions and catabolic pathways, and facilitate the biotransformation of RONCs. This review provides comprehensive insights into the microbial regulation mechanisms and applications of electrostimulation and micro-aeration strategies to accelerate the biotransformation of RONCs to organic amine (amination) and inorganic ammonia (ammonification), respectively. Furthermore, a promising approach involving in-situ hybrid anaerobic biological units, coupled with electrostimulation and micro-aeration, is proposed towards engineering applications. Finally, employing cutting-edge methods including multi-omics analysis, data science driven machine learning, technology-economic analysis, and life-cycle assessment would contribute to optimizing the process design and engineering implementation. This review offers a fundamental understanding and inspiration for novel research in the enhanced biotechnology towards RONCs elimination.

Improved efficiency and stability using a novel elemental sulfur-based moving-bed denitrification process.

Elemental sulfur-based denitrification (ESDeN) technology is known as a cost-saving alternative to its heterotrophic counterpart for nutrient removal from organic-deficient water. However, the traditional fixed-bed reactor (FixBR), as an extensively used process, suffers from a low denitrification rate and even performance deterioration during long-term operation. Herein, we proposed a novel elemental sulfur-based denitrifying moving-bed reactor (ESDeN-MovBR), in which a screw rotator was employed to drive the filled sulfur particles to be microfluidized vertically (a state of vertical-loop movement). Our results showed that the ESDeN-MovBR realized much superior and more stable denitrification performance compared to the ESDeN-FixBR, as indicated by 3.09-fold higher denitrification rate and over one order of magnitude lower intermediates (NO2- and N2O) yield, which could last for over 100 days. Further research revealed that the microfluidization of sulfur particles facilitated the expelling of nitrogen bubbles and excessive biomass, resulting in the prolongation of actual hydraulic retention time by over 80 % and could partially explain the higher denitrification rate in ESDeN-MovBR. The remaining contribution to the improvement of denitrification rate was suggested to be result from changes in biofilm properties, in which the biofilm thickness of ESDeN-MovBR was found to be 3.29 times thinner yet enriched with 2.52 times more autotrophic denitrifiers. This study offered a completely new solution to boost up the denitrification performance of ESDeN technology and provided in-depth evidence for the necessity of biofilm thickness control in such technology.

Thermodynamic Inhibition of Microbial Sulfur Disproportionation in a Multisubunit Designed Sulfur-Siderite Packed Bioreactor.

Sulfur disproportionation (S0DP) poses a challenge to the robust application of sulfur autotrophic denitrification due to unpredictable sulfide production, which risks the safety of downstream ecosystems. This study explored the S0DP occurrence boundaries with nitrate loading and temperature effects. The boundary values increased with the increase in temperature, exhibiting below 0.15 and 0.53 kg-N/m3/d of nitrate loading at 20 and 30 °C, respectively. A pilot-scale sulfur-siderite packed bioreactor (150 m3/d treatment capacity) was optimally designed with multiple subunits to dynamically distribute the loading of sulfur-heterologous electron acceptors. Operating two active and one standby subunit achieved an effective denitrification rate of 0.31 kg-N/m3/d at 20 °C. For the standby subunit, involving oxygen by aeration effectively transformed the facultative S0DP functional community from S0DP metabolism to aerobic respiration, but with enormous sulfur consumption resulting in ongoing sulfate production of over 3000 mg/L. Meanwhile, acidification by the sulfur oxidation process could reduce the pH to as low as 2.5, which evaluated the Gibbs free energy (ΔG) of the S0DP reaction to +2.56 kJ, thermodynamically suppressing the S0DP occurrence. Therefore, a multisubunit design along with S0DP inhibition strategies of short-term aeration and long-term acidification is suggested for managing S0DP in various practical sulfur-packed bioreactors.

Insights into the response of nitrogen metabolism to sulfamethoxazole contamination in constructed wetlands with varied substrates.

This study conducted an analysis of the variations in nitrogen metabolism pathways within constructed wetlands (CWs) using zeolite (CW-Z), ceramsite (CW-C), and lava (CW-L) under high concentration sulfamethoxazole (SMX) stress. The introduction of SMX hindered the formation of hydrogen bonds on the substrate surfaces; however, these surfaces still maintained a dense and thick biofilm. CW-Z exhibited superior removal efficiencies for ammonium nitrogen (NH4+-N) and nitrate nitrogen (NO3--N) compared to CW-C and CW-L, with removal rates of 92.54 ± 2.88 % and 89.39 ± 6.74 %, respectively. Interestingly, the proportion of genes involved in nitrification, denitrification and nitrate reduction genes in CW-C (36.05 %) were higher than that in CW-C (29.81 %) and CW-L (29.70 %) but the interactions among nitrogen functional bacteria in CW-Z were much more complex. Further analysis of the nitrogen metabolism pathway indicated that under CW-Z enhanced dissimilatory nitrate reduction SMX stress, while CW-L enhanced assimilatory nitrate reduction process compared to CW-C.

Inducement mechanism and control of self-acidification in elemental sulfur fluidizing bioreactor.

The sulfur fluidizing bioreactor (S0FB) has significant superiorities in treating nitrate-rich wastewater. However, substantial self-acidification has been observed in engineering applications, resulting in frequent start-up failures. In this study, self-acidification was reproduced in a lab-scale S0FB. It was demonstrated that self-acidification was mainly induced by sulfur disproportionation process, accounting for 93.4 % of proton generation. Supplying sufficient alkalinity to both the influent (3000 mg/L) and the bulk (2000 mg/L) of S0FB was essential for achieving a successful start-up. Furthermore, the S0FB reached 10.3 kg-N/m3/d of nitrogen removal rate and 0.13 kg-PO43-/m3/d of phosphate removal rate, respectively, surpassing those of the documented sulfur packing bioreactors by 7-129 times and 26-65 times. This study offers a feasible and practical method to avoid self-acidification during restart of S0FB and highlights the considerable potential of S0FB in the treatment of nitrate-rich wastewater.

Highly Efficient Coremoval of Nitrate and Phosphate Driven by a Sulfur-Siderite Composite Reactive Filler toward Secondary Effluent Polishing.

Reactive fillers consisting of reduced sulfur and iron species (SFe-ReFs) have received increasing attention in tertiary wastewater treatment for nitrate and phosphate coremoval. However, the existing SFe-ReFs suffer from either low performance (e.g., pyrrhotite and pyrite) or unsatisfactory use in terms of combustible risk and residual nonreactive impurities (e.g., sulfur mixing with natural iron ores). Here, we developed a new type of sulfur-siderite composite ReF (SSCReF) with a structure of natural siderite powders eventually embedded into sulfur. SSCReFs exhibited many excellent properties, including higher mechanical strengths and hardness and especially much poorer ignitability compared to pure sulfur. By using SSCReF to construct packed-bed reactors, the highest denitrification and dephosphorization rates reached 829.70 gN/m3/d (25 wt % siderite) and 36.70 gP/m3/d (75 wt % siderite), respectively. Dephosphorization was demonstrated to be dependent on sulfur-driven denitrification, in which the acid produced from the later process promoted Fe(II) dissolution, which then directly combined with phosphate to form vivianite or further converted into phosphate adsorbents (ferrihydrite, a green rust-like compound). Water flush was an effective way to finally wash out these surface deposited Fe-P compounds, as well as those nonreactive impurities (Si and Al-bearing compounds) detached from SSCReF. Such a highly efficient and safe SSCReF holds considerable application potential in secondary effluent polishing.

Managing microbial sulfur disproportionation for optimal sulfur autotrophic denitrification in a pilot-scale elemental sulfur packed-bed bioreactor.

Elemental sulfur packed-bed (S0PB) bioreactors for autotrophic denitrification have gained more attention in wastewater treatment due to their organic carbon-free operation, low operating cost, and minimal carbon emissions. However, the rapid development of microbial S0-disproportionation (MS0D) in S0PB reactor during deep denitrification poses a significant drawback to this new technology. MS0D, the process in which sulfur is used as both an electron donor and acceptor by bacteria, plays a crucial role in the microbial-driven sulfur cycle but remains poorly understood in wastewater treatment setups. In this study, we induced MS0D in a pilot-scale S0PB reactor capable of denitrifying over 1000 m3/d nitrate-containing wastewater. Initially, the S0PB reactor stably removed 6.6 mg-NO3--N/L nitrate at an empty bed contact time (EBCT) of 20 mins, which was designated the S0-denitrification stage. To induce MS0D, we reduced the influent nitrate concentrations to allow deep nitrate removal, resulted in the production of large quantities of sulfate and sulfide (SO42-:S2- 3.2 w/w). Meanwhile, other sulfur-heterologous electron acceptors (SHEAs), e.g., nitrite and DO, were also kept at trace levels. The negative correlations between the SHEAs concentrations and the sulfide productions indicated that the absence of SHEAs was a primary inducing factor to MS0D. The microbial community drastically diverged in response to the depletion of SHEAs during the switch from S0-denitrification to S0-disproportionation. An evident enrichment of sulfur-disproportionating bacteria (SDBs) was found at the S0-disproportionation stage, accompanied by the decline of sulfur-oxidizing bacteria (SOBs). In the end, we discovered that shortening the EBCT and increasing the reflux ratio could inhibit sulfide production by reducing it from 43.9 mg/L to 3.2 mg/L or 25.5 mg/L. In conclusion, our study highlights the importance of considering MS0D when designing and optimizing S0PB reactors for sustainable autotrophic sulfur denitrification in real-life applications.

Long-distance responses of ginger to soil sulfamethoxazole and chromium: Growth, co-occurrence with antibiotic resistance genes, and consumption risk.

The coexistence of antibiotics and heavy metals in agroecosystems is nonnegligible, which permits the promotion of antibiotic resistance genes (ARGs) in crops, thus posing a potential threat to humans along the food chain. In this study, we investigated the bottom-up (rhizosphere→rhizome→root→leaf) long-distance responses and bio-enrichment characteristics of ginger to different sulfamethoxazole (SMX) and chromium (Cr) contamination patterns. The results showed that ginger root systems adapted to SMX- and/or Cr-stress by increasing humic-like exudates, which may help to maintain the rhizosphere indigenous bacterial phyla (i.e., Proteobacteria, Chloroflexi, Acidobacteria and Actinobacteria). The root activity, leaf photosynthesis and fluorescence, and antioxidant enzymes (SOD, POD, CAT) of ginger were significantly decreased under high-dose Cr and SMX co-contamination, while a "hormesis effect" was observed under single low-dose SMX contamination. For example, CS100 (co-contamination of 100 mg/L SMX and 100 mg/L Cr) caused the most severe inhibition to leaf photosynthetic function by reducing photochemical efficiency (reflected on PAR-ETR, φPSII and qP). Meanwhile, CS100 induced the highest ROS production, in which H2O2 and O2·- increased by 328.82% and 238.00% compared with CK (the blank control without contamination). Moreover, co-selective stress by Cr and SMX induced the increase of ARG bacterial hosts and bacterial phenotypes containing mobile elements, contributing to the high detected abundance of target ARGs (sul1, sul2) up to 10-2∼10-1 copies/16S rRNA in rhizomes intended for consumption.

Response of sulfur-metabolizing biofilm to external sulfide in element sulfur-based denitrification packed-bed reactor.

Dosing sulfide into the sulfur-packed-bed (S0PB) has great potential to enhance the denitrification efficiency by providing compensatory electron donors, however, the response of sulfur-metabolizing biofilm to various sulfide dosages has never been investigated. In this study, the S0PB reactor was carried out with increasing sulfide dosages by 3.6 kg/m3/d, presenting a decreasing effluent nitrate from 14.2 to 2.7 mg N/L with accelerated denitrification efficiency (k: 0.04 to 0.27). However, 6.5 mg N/L of nitrite accumulated when the sulfide dosage exceeded 0.9 kg/m3/d (optimum value). The increasing electron export contribution of sulfide a maximum of 85.5% illustrated its competition with the in-situ sulfur. Meanwhile, over-dosing sulfide caused serious biofilm expulsion with significant decreases in the total biomass, live cell population, and ATP by 90.2%, 86.7%, and 54.8%, respectively. This study verified the capacity of dosing sulfide to improve the denitrification efficiency in S0PB but alerted the negative effect of exceeded dosing.

Synergistic between autotrophic and heterotrophic microorganisms for denitrification using bio-S as electron donor.

In recent years, biological sulfur (bio-S) was employed in sulfur autotrophic denitrification (SAD) in which autotrophic Thiobacillus denitrificans and heterotrophic Stenotrophomonas maltophilia played a key role. The growth pattern of T.denitrificans and S.maltophilia exhibited a linear relationship between OD600 and CFU when OD600 < 0.06 and <0.1, respectively. When S.maltophilia has applied alone, the NorBC and NosZ were undetected, and denitrification was incomplete. The DsrA of S.maltophilia could produce sulfide as an alternative electron donor for T.denitrificans. Even though T.denitrificans had complete denitrification genes, its efficiency was low when used alone. The interaction of T.denitrificans and S.maltophilia reduced nitrite accumulation, leading to complete denitrification. A sufficient quantity of S.maltophilia may trigger the autotrophic denitrification activity of T.denitrificans. When the colony-forming units (CFU) ratio of S.maltophilia to T.denitrificans was reached at 2:1, the highest denitrification performance was achieved at 2.56 and 12.59 times higher than applied alone. This research provides a good understanding of the optimal microbial matching for the future application of bio-S.

Effect of sulfur particle morphology on the performance of element sulfur-based denitrification packed-bed reactor.

The effect of particle morphology on denitrification performance in element sulfur-based denitrification (ESDeN) packed-bed process is a gap. In this study, three different types of commercial sulfur particles were selected to build the ESDeN reactors. The results showed the reactors filled with rougher sulfur particles took shorter time to reach stable denitrification performance in the start-up stage. The reactors filled with cap-shape sulfur particles received the maximum nitrate removal rate of 849.49 ± 79.29 g N m-3 d-1 at empty bed contact time of 0.50 h, which was 2.34 times higher than that with ball-shape sulfur particles in the steady stage. The superior denitrification performance in the cap-shape particles set linked to its larger effective volumetric surface area (ωe, 1.67 times larger) and to the longer actual hydraulic retention time (AHRT, 1.80 times longer). This study extends the knowledge of the dependency of sulfur particle properties on denitrification performance in ESDeN packed-bed reactor.

Element sulfur-based autotrophic denitrification constructed wetland as an efficient approach for nitrogen removal from low C/N wastewater.

Constructed wetlands (CWs) integrated with sulfur autotrophic denitrification to stimulate high-rate nitrogen removal from carbon-limited wastewater holds particular application prospect due to no excessive carbon source addition, high efficiency, and good stability. In this study, we conducted elemental sulfur-based constructed wetland (SCW) and traditional constructed wetland (CW) under different C/N (2, 1, and 0.5) to explore the feasibility and mechanisms for nitrogen removal from low C/N wastewater. Compared with CW, SCW was demonstrated more robust in nitrogen removal in the case of low C/N influent. When the influent C/N control was at 0.5, SCW observed total nitrogen (TN) and nitrate removal efficiency of 69.36 ± 3.96% and 81.71 ± 3.96%, with the corresponding removal rate of 1.18 ± 0.66 and 1.70 ± 0.92 g-N·m-2·d-1, which were 2.11 and 10.03 times of CW, respectively. The nitrate removal rate constant k in the SCW was 1.05, 3.83, and 10.33 times higher than the CW with C/N of 2, 1 and 0.5. Furthermore, 14.40, 54.51, and 79.82% of nitrogen were removed by the sulfur autotrophic denitrification (SAD) in SCW, which also contributed 43.89, 73.68, and 71.70% of sulfate production. Moreover, the combined system of CW-SCW is proved be an efficient operation mode for simultaneously removing total ammonia nitrogen (TAN) and nitrate. In the SCW, the richness of the microbial community was improved and sulfur-oxidizing genera (e.g. Thiobacillus, Sulfurimonas) was selectively enriched, which affect the performance the elemental sulfur-based denitrification process. The nitrate reduction pathway was overwhelmed by denitrification and the dissimilatory nitrate reduction process. These findings offer elemental sulfur-based autotrophic denitrification constructed wetland has excellent potential to enhance nitrogen removal from carbon-limited wastewater.

A tartrate-EDTA-Fe complex mediates electron transfer and enhances ammonia recovery in a bioelectrochemical-stripping system.

Traditional bioelectrochemical systems (BESs) coupled with stripping units for ammonia recovery suffer from an insufficient supply of electron acceptors due to the low solubility of oxygen. In this study, we proposed a novel strategy to efficiently transport the oxidizing equivalent provided at the stripping unit to the cathode by introducing a highly soluble electron mediator (EM) into the catholyte. To validate this strategy, we developed a new kind of iron complex system (tartrate-EDTA-Fe) as the EM. EDTA-Fe contributed to the redox property with a midpoint potential of -0.075 V (vs. standard hydrogen electrode, SHE) at pH 10, whereas tartrate acted as a stabilizer to avoid iron precipitation under alkaline conditions. At a ratio of the catholyte recirculation rate to the anolyte flow rate (RC-A) of 12, the NH4 +-N recovery rate in the system with 50 mM tartrate-EDTA-Fe complex reached 6.9 ±â€¯0.2 g N m-2 d-1, approximately 3.8 times higher than that in the non-EM control. With the help of the complex, our system showed an NH4 +-N recovery performance comparable to that previously reported but with an extremely low RC-A (0.5 vs. 288). The strategy proposed here may guide the future of ammonia recovery BES scale-up because the introduction of an EM allows aeration to be performed only at the stripping unit instead of at every cathode, which is beneficial for the system design due to its simplicity and reliability.

Insight into the shaping of microbial communities in element sulfur-based denitrification at different temperatures.

Nitrate pollution is an important cause of eutrophication and ecological disruption. Recently, element sulfur-based denitrification (ESDeN) has attracted increasing attention because of its non-carbon source dependence, low sludge yield, and cost-effectiveness. Although the denitrification performance of sulfur autotrophic denitrifying bacteria at different temperatures has been widely studied, there are still many unknown factors about the adaptability and the shaping of microbial community. In this study, we comprehensively understood the shaping of ESDeN microbial communities under different temperature conditions. Results revealed that microbial communities cultivated at temperatures ranging from 10 °C to 35 °C could be classified as high-temperature (35 °C), middle-temperature (30, 25 and 20 °C), and low-temperature (15 and 10 °C) communities. Dissolved oxygen in water was an important factor that, in combination with temperature, shaped microbial community structure. According to network analysis, the composition of keystone taxa was different for the three groups of communities. Some bacteria that did not have sulfur compound oxidation function were identified as the "keystone species". The abundances of carbon, nitrogen, and sulfur metabolism of the three microbial communities were significantly changed, which was reflected in that the high-temperature and middle-temperature communities were dominated by dark oxidation of sulfur compounds and dark sulfide oxidation, while the low-temperature community was dominated by chemoheterotrophy and aerobic chemoheterotrophy. The fact that the number of microorganisms with dark oxidation of sulfur compounds capacity was quite higher than that of microorganisms with dark sulfur oxidation capacity suggested that the sulfur bioavailability at different temperatures, especially low temperature, was the main challenge for the development of efficient ESDeN process. This study provided a biological basis for developing a high-efficiency ESDeN process to cope with temperature changes in different seasons or regions.

Coupled sulfur and electrode-driven autotrophic denitrification for significantly enhanced nitrate removal.

Elemental sulfur (S0)-based autotrophic denitrification (SAD) has gained intensive attention in the treatment of secondary effluent for its low cost, high efficiency, and good stability. However, in practice, the supplementary addition of limestone is necessary to balance the alkalinity consumption during SAD operation, which increases water hardness and reduces the effective reaction volume. In this study, a coupled sulfur and electrode-driven autotrophic denitrification (SEAD) process was proposed with superior nitrate removal performance, less accumulation of sulfate, and self-balance of acidity-alkalinity capacity by regulating the applied voltage. The dual-channel electron supply from S0 and electrodes made the nitrate removal rate constant k in the SEAD process 3.7-5.1 and 1.4-3.5 times higher than that of the single electrode- and sulfur-driven systems, respectively. The S° contributed to 75.3%-83.1% of nitrate removal and the sulfate yield during SEAD (5.67-6.26 mg SO42-/mg NO3--N) was decreased by 17%-25% compared with SAD. The S0 particle and electrode both as active bio-carriers constructed collaborative denitrification communities and functional genes. Pseudomonas, Ralstonia and Brevundimonas were the dominant denitrifying genera in S0 particle biofilm, while Pseudomonas, Chryseobacterium, Pantoea and Comamonas became dominant denitrifying genera in the cathode biofilm. The narG/Z/H/Y/I/V, nxrA/B, napA/B, nirS/K, norB/C and nosZ were potential functional genes for efficient nitrate reduction during the SEAD process. Metagenomic sequencing indicated that S0 as an electron donor has greater potential for complete denitrification than the electrode. These findings revealed the potential of SEAD for acting as a highly efficient post denitrification process.

Thiosulfate as external electron donor accelerating denitrification at low temperature condition in S0-based autotrophic denitrification biofilter.

This study was carried out to determine the inhibition of low temperature on the performance of S0-based autotrophic denitrification (S0-SAD) biofilter, and proposed to enhance the nitrate removal efficiency with thiosulfate as external electron donor. With the decline of temperature from 30 °C to 10 °C at 0.25 h of empty bed contact time (EBCT), the nitrate removal rate presented a logarithmical drop, and the effluent nitrate dramatically increased from 9.19 mg L-1 to 15.13 mg L-1. EBCT was prolonged until 0.33 h for 20 °C, 0.66 h for 15 °C and 1.5 h for 10 °C, respectively, to maintain the effluent nitrate below 10 mg L-1. Such excessive variation of EBCT for different temperature is undoubtedly incredible for practical engineering. Thiosulfate, as the external electron donor, was adopted to compensate the efficiency loss during temperature decrease, which significantly prompted nitrate removal rate to 0.59, 0.53 and 0.31 kg N m-3 d-1 at 20 °C, 15 °C and 10 °C conditions, respectively, even at a short EBCT of 0.25 h. It not only acted as compensatory electron donor for nitrate removal, but also promoted the contribution of elemental sulfur via accelerating the DO consumption and extended larger effective volume of S0-layer for denitrification. Meanwhile, the significant enrichment of Sulfurimonas and Ferritrophicum provided biological evidences to the enhancement process. However, the incomplete consumption of thiosulfate was observed especially at EBCT of 0.25 h and 10 °C, and the thiosulfate runoff needs to be concerned in case of contaminating the effluent. Herein, approximately extending EBCT to 0.66 h and decreasing thiosulfate dosage were conducted simultaneously, thereby achieving 100% thiosulfate utilization efficiency and expected nitrate removal. This study provided a fundamental guidance to design and operate S0-SAD biofilter in response to seasonal temperature variation for practical engineering.

Mitigating nitrite accumulation during S0-based autotrophic denitrification: Balancing nitrate-nitrite reduction rate with thiosulfate as external electron donor.

This study was carried out to determine the effect of influent nitrate loading on nitrite accumulation during elemental-sulfur based denitrification process, and proposed to enhance the nitrogen removal efficiency by mitigating nitrite accumulation with thiosulfate as external electron donor. Along with increasing the nitrate influent loading (from 0.09 kg N/m3/d to 1.73 kg N/m3/d) by shortening the empty bed contact time (EBCT) (from 5 h to 0.25 h), the nitrate removal loading increased from 0.08 to 0.83 kg N/m3/d. Meanwhile, the raise of the nitrate influent loading obviously aggravated the nitrite accumulation. Herein, nitrite began to accumulate since the nitrate influent loading was over 0.86 kg N/m3/d, and a maximum nitrite accumulation of 2.39 mg/L was observed under the 0.25 h of EBCT and 15 mg/L of nitrate influent concentration condition. Thiosulfate was used as the external electron donor to accelerate the nitrite reduction rate in order to mitigate the nitrite accumulation. As a result, the nitrite accumulation significantly decreased from 2.39 mg/L to 0.17 mg/L with the thiosulfate dosage of 13.36 mg/L. However, the nitrite accumulation bounced with the on-going increase of the thiosulfate dosage, indicating that the nitrate reduction rate and nitrite reduction rate were accelerated alternatively. After dosing thiosulfate, the relative abundances of sulfurimonas and ferritrophicum grew up significantly.

Sulfur autotrophic denitrification filter and heterotrophic denitrification filter: Comparison on denitrification performance, hydrodynamic characteristics and operating cost.

Sulfur autotrophic denitrification (SAD) process, as an alternative to heterotrophic denitrification (HD) filter, receives growing interest in polishing the effluent from secondary sewage treatment. Although individual studies have indicated several advantages of SAD over HD, rare study has compared these two systems under identical condition and by using real secondary effluent. In this study, two small pilot scale filters (SAD and HD) were designed with identical configuration and operated parallelly by feeding the real secondary effluent from a WWTP. The results showed SAD filter can be started up without the addition of soluble electron donor, although the time (14 days) was about 3 times longer than that of HD filter. The nitrate removal rate of SAD filter at HRT of 1.4 h was measured as 0.268 ± 0.047 kg N/(m3∙d). Similar value was observed in HD filter with supplementing 90 mg/L COD. The COD concentration of effluent always kept lower than that of influent in SAD filter but not in HD filter. In addition, SAD filter could maintain a stable denitrification performance without backwash for 15 days, while decline of nitrate removal rate was observed in HD filter just 2 days after stopping the backwash. This different behavior was further confirmed as the SAD filter had a better hydraulic flow pattern. Analysis according to high-throughput 16S rRNA gene-based Illumina MiSeq sequencing clearly showed the microbial community evolution and differentiation among the samples of seed sludge, SAD and HD filters. Finally, the economic assessment was carried out, showing the operation cost of SAD filter was over 50% lower than that of HD filter.

Extracellular electron transfer through visible light induced excited-state outer membrane C-type cytochromes of Geobacter sulfurreducens.

Dissimilatory metal-reducing bacteria (DMRB) have a variety of c-type cytochromes (OM c-cyts) intercalated in their outer membrane, and this structure serves as the physiological basis for DMRB to carry out the extracellular electron transfer processes. Using Geobacter sulfurreducens as a model DMRB, we demonstrated that visible-light illumination could alter the electronic state of OM c-cyts from the ground state to the excited state in vivo. The existence of excited-state OM c-cyts in vivo was confirmed by spectroscopy. More importantly, excited-state OM c-cyts had a more negative potential compared to their ground-state counterparts, conferring DMRB with an extra pathway to transfer electrons to semi-conductive electron acceptors. To demonstrate this, using a TiO2-coated electrode as an electron acceptor, we showed that G. sulfurreducens could directly utilise the conduction band of TiO2 as an electron acceptor under visible-light illumination (λ > 420 nm) without causing TiO2 charge separation. When G. sulfurreducens was subject to visible-light illumination, the rate of extracellular electron transfer (EET) to TiO2 accelerated by over 8-fold compared to that observed under dark conditions. Results of additional electrochemical tests provided complementary evidence to support that G. sulfurreducens utilised excited-state OM c-cyts to enhance EET to TiO2.

Resorcinol as a highly efficient aromatic electron donor in bioelectrochemical system.

Bioelectrochemical systems (BESs) have been known as a promising technology for accelerating aromatic contaminants degradation and energy recovery. However, most existing studies concentrate on aromatics metabolized through a benzoyl-CoA pathway while those metabolized through other pathways are limited. In this work, resorcinol, a typical aromatic contaminant as well as a key central intermediate (other than benzoyl-CoA) involved in aromatics anaerobic biodegradation, was studied in BESs. Unlike the general impression of the relatively poor organic-to-current performance in the aromatics driven BESs, high efficiencies for resorcinol-fed BESs were observed with a current density and coulombic efficiency of up to 0.26 ± 0.05 mAcm-2 and 74.3 ± 10.7%, respectively. The higher performance likely correlates to the readily fermentable property of resorcinol. Analysis of microbial communities in the biofilm suggests a syntrophic interaction between resorcinol-degrading bacteria (RDB) and anode-respiring bacteria (ARB) was involved in current generation. Additional tests involving the removal of accumulated acetate through fast resorcinol feeding indicates that a mechanism based on direct utilization of resorcinol for current generation may also exist. This study extends the knowledge for the fate of aromatics in BESs and indicates that aromatics entering into the resorcinol metabolic pathway can be treated efficiently with good energy recovery efficiency in BESs.