Sulfur-based denitrification: Effect of biofilm development on denitrification fluxes

Nitrogen and sulfur cycling and their coupling mechanisms in eutrophic lake sediment microbiomes

Microorganisms play important roles in the biogeochemical cycles of lake sediment. However, the integrated metabolic mechanisms governing nitrogen (N) and sulfur (S) cycling in eutrophic lakes remain poorly understood. Here, metagenomic analysis of field and bioreactor enriched sediment samples from a typical eutrophic lake were applied to elucidate the metabolic coupling of N and S cycling. Our results showed significant diverse genes involved in the pathways of dissimilatory sulfur metabolism, denitrification and dissimilatory nitrate reduction to ammonium (DNRA). The N and S associated functional genes and microbial groups generally showed significant correlation with the concentrations of NH4+, NO2- and SO42, while with relatively low effects from other environmental factors. The gene-based co-occurrence network indicated clear cooperative interactions between N and S cycling in the sediment. Additionally, our analysis identified key metabolic processes, including the coupled dissimilatory sulfur oxidation (DSO) and DNRA as well as the association of thiosulfate oxidation complex (SOX systems) with denitrification pathway. However, the enriched N removal microorganisms in the bioreactor ecosystem demonstrated an additional electron donor, incorporating both the SOX systems and DSO processes. Metagenome-assembled genomes-based ecological model indicated that carbohydrate metabolism is the key linking factor for the coupling of N and S cycling. Our findings uncover the coupling mechanisms of microbial N and S metabolism, involving both inorganic and organic respiration pathways in lake sediment. This study will enhance our understanding of coupled biogeochemical cycles in lake ecosystems.

Revealing sulfur-iron coupling mechanism for enhanced autotrophic denitrification in ecological floating beds

A sulfur-iron coupled ecological floating bed (EFB-SFe) was developed to enhance the denitrification capability of sulfur-based ecological floating beds (EFB-S). The denitrification performance, kinetic process and microbial community composition were explored. Results showed that sulfur-iron coupling effectively enhanced the denitrification performance of EFB, surpassing the sum of their individual effects. The average total nitrogen removal rate ranged from 1.56 to 4.56 g·m-2·d-1, with a removal efficiency of 22-84 %. The k value for the S + Fe group increased from 0.04 to 0.18 d-1 to 0.40-0.46 d-1 relative to the S group. The sulfur-iron coupling promoted the enrichment of denitrifying bacteria (Thiobacillus and Ferritrophicum). The denitrification genes in EFB-SFe were upregulated, being 12-22 times more abundant than in EFB-S. Sulfur and iron autotrophic denitrification were identified as the main nitrogen removal processes in EFB-SFe. Overall, sulfur-iron coupling showed the potential to enhance the denitrification capacity of EFB-S for treating low-pollution water.

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.

Effects of temperature on nitrifying membrane-aerated biofilms: An experimental and modeling study

Temperature is known to have an important effect on the morphology and removal fluxes of conventional, co-diffusional biofilms. However, much less is known about the effects of temperature on membrane-aerated biofilm reactors (MABRs). Experiments and modeling were used to determine the effects of temperature on the removal fluxes, biofilm thickness and morphology, and biofilm microbial community structure of nitrifying MABRs. Steady state tests were carried out at 10 °C and 30 °C. MABRs grown at 30 °C had higher ammonium removal fluxes (5.5 ± 0.9 g-N/m2/day at 20 mgN/L) than those grown at 10 °C (3.4 ± 0.2 g-N/m2/day at 20 mgN/L). The 30 °C biofilms were thinner and rougher, with a lower protein to polysaccharides ratio (PN/PS) in their extracellular polymeric substance (EPS) matrix and greater amounts of biofilm detachment. Based on fluorescent in-situ hybridization (FISH), there was a higher relative abundance of nitrifying bacteria at 30 °C than at 10 °C, and the ratio of AOB to total nitrifiers (AOB + NOB) was higher at 30 °C (95.1 ± 2.3%) than at 10 °C (77.2 ± 8.6 %). Anammox bacteria were more abundant at 30 °C (16.6 ± 3.7 %) than at 10 °C (6.5 ± 2.4 %). Modeling suggested that higher temperatures increase ammonium oxidation fluxes when the biofilm is limited by ammonium. However, fluxes decrease when oxygen becomes limited, i.e., when the bulk ammonium concentrations are high, due to decreased oxygen solubility. Consistent with the experimental results, the model predicted that the percentage of AOB to total nitrifiers at 30 °C was higher than at 10 °C. To investigate the effects of temperature on biofilm diffusivity and O2 solubility, without longer-term changes in the microbial community, MABR biofilms were grown to steady state at 20 °C, then the temperature changed to 10 °C or 30 °C overnight. Higher ammonium oxidation fluxes were obtained at higher temperatures: 1.91 ± 0.24 g-N/m2/day at 10 °C and 3.19 ± 0.40 g-N/m2/day at 30 °C. Overall, this work provides detailed insights into the effect of temperature on nitrifying MABRs, which can be used to better understand MABR behavior and manage MABR reactors.

Optimized start-up strategies for elemental sulfur packing bioreactor achieving effective autotrophic denitrification

The start-up efficiency of the elemental sulfur packing bioreactor (S0PB) is constrained by the slow growth kinetics of autotrophic microorganisms, which is essentially optimized. This study aims to optimize start-up procedures and offer scientific guidance for the practical applications of S0PB. Through comparing the start-up efficiencies under various conditions related to inoculation, backwashing, and EBCT, it was found that these conditions did not significantly influence start-up time, but they did impact denitrification performance in detail. Using activated sludge as the inoculum was not recommended as the 2.5 ± 0.2 mg-N/L higher nitrite accumulation and 26.0 ± 5.1 % lower TN removal rate, compared to self-enrichment. Starting with a long-to-short EBCT (1 → 0.33 h) achieved higher nitrate removal of 11.5 ± 0.6 mg-N/L and eliminated nitrite accumulation compared to constantly short EBCT (0.33 h) conditions. Daily and postponed backwashing were suggested for long-to-short EBCT and constantly short EBCT start-up, respectively. Enrichment of Sulfurimonas was beneficial for the effective nitrite reduction process.

The effect of organic sources on the electron distribution and N2O emission in sulfur-driven autotrophic denitrification biofilters

Sulfur-driven autotrophic denitrification (SAD) is considered as an effective alternative to traditional heterotrophic denitrification (HD) due to its cheap, low sludge production and non-toxicity. Nitrous oxide (N2O) as an intermediate product inevitably was generated at the limited supply of electron donor or unbalanced electron distribution condition during the denitrification process. Recently, autotrophic denitrification biofilters were conclusively implemented for advanced nitrogen removal in wastewater treatment plant (WWTP). However, residual organic sources after wastewater treatment could affect the electron distribution among denitrifying reductases and few studies are known about this issue. In this study, several lab-scale biofilters packed with elemental sulfur slices were applied to explore the electron distribution characteristics of autotrophic denitrification through the combination of different nitrogen oxides (NOx). The results clearly delineated that the different combination of nitrogen oxides had a remarkable effect on the electron distribution. In any case, the electrons likely flow toward nitrate reductase (Nar) under a single nitrogen oxide combination, followed by nitrite reductase (Nir) and nitrous oxide reductase (Nos). The concurrent presence of multiple electron acceptors resulted in most electrons flowing toward Nar, and least toward Nos. Compared to traditional SAD, the reduction rate of nitrogen oxide in the sulfur-driven autotrophic denitrification with influent of organic source (OSAD) was greatly improved. The maximum value of the true specific rates of NO3- in OSAD process was 9.43 mg-N/g-VSS/h. It was increased by 8.26 folds higher than that in traditional SAD. The electrons were more easily distributed to Nos with the addition of sodium acetate, which further promoted the N2O reduction. This study will provide theoretical support for controlling N2O release in SAD biofilters.

Monitoring biofilm thickness using the membrane aerated biofilm reactor (MABR) fingerprint soft sensor to optimize nitrogen removal

The ongoing commercialization and installation of full-scale membrane aerated biofilm reactors (MABRs) stimulate the increasing need to monitor biofilm development. Biofilm thickness in MABRs can be assessed indirectly by plotting the exhaust oxygen purity versus bulk ammonia concentration, defined here as the MABR fingerprint soft sensor. Dynamic simulations with diurnal flow variations of an MABR unit model were implemented over a broad range of biofilm thicknesses and influent conditions consisting of variable C/N ratios and applied ammonia fluxes to assess the utility of the MABR fingerprint. Results show that the continuously decreasing trend of the MABR fingerprint plot slopes can be employed as a useful signal for biofilm thickness control in nitrogen removal processes. This technique is useful in a wide range of influent conditions and is helpful for MABR operators and designers to arrange biofilm thickness control events efficiently and determine where in an overall treatment process the technique can be applied to control biofilm thickness and optimize process performance. PRACTITIONER POINTS: The linear relationship between exhaust oxygen purity and bulk ammonia concentration is defined as the MABR fingerprint plot. MABR fingerprint plots are generated for a given biofilm thickness with diurnal flow or short-term loading variations implemented. Continuously decreasing trends of the MABR fingerprint plot slopes are useful signals for biofilm control in nitrogen removal. The MABR fingerprint is useful over a wide range of influent conditions regarding C/N ratios and applied ammonia fluxes. MABR practitioners can use the fingerprint plots to determine when biofilm control measures should be taken.

Elemental sulfur-driven autotrophic denitrification process for effective removal of nitrate in mariculture wastewater: Performance, kinetics and microbial community

To date, there is a lack of systematic investigation on the elemental sulfur-driven autotrophic denitrification (SDAD) process for removing nitrate (NO3--N) from mariculture wastewater deficient in organic carbon sources. Therefore, a packed-bed reactor was established and continuously operated for 230 days to investigate the operation performance, kinetic characteristics and microbial community of SDAD biofilm process. Results indicate that the NO3--N removal efficiencies and rates varied with the operational conditions including HRT (1-4 h), influent concentrations of NO3--N (25-100 mg L-1) and DO (0.2-7.0 mg L-1), and temperature (10oC-30 °C), in the ranges of 51.4%-98.6% and 0.054-0.546 g L-1 d-1, respectively. Limestone could partially neutralize the produced acidity. Small portions of NO3--N were converted to nitrite (<4.5%) and ammonia (<2.8%) in the reactor. Operational conditions also influenced the production of acidity, nitrite and ammonia as well as sulfate. Shortening HRT and increasing influent NO3--N concentration turned the optimal fitting model depicting the NO3--N removal along the reactor from half-order to zero-order. Furthermore, the NO3--N removal was accelerated by a higher temperature and influent NO3--N concentration and a lower HRT and influent DO concentration. Microbial richness, evenness and diversity gradually decreased during the autotrophic denitrifier enrichment cultivation and the reactor start-up and operation. Sulfurimonas constituted the predominate genus and the primary functional bacteria in the reactor. This study highlights the SDAD as a promising way to control the coastal eutrophication associated with mariculture wastewater discharge.

Effects of acidifiers on soil greenhouse gas emissions in calcareous soils in a semi-arid area

In most agricultural fields, when soil pH is high, elemental sulfur or sulfuric acid are used to reduce soil pH and increase the availability of macro and micronutrients for optimum crop yield. However, how these inputs impact soil greenhouse gas emissions is unknown. This study aimed to measure the amount of greenhouse gas emissions and pH after the application of various doses of elemental sulfur (ES) and sulfuric acid (SA). Using static chambers, this study quantifies soil greenhouse gas emissions (CO2, N2O, and CH4) for 12 months after the application of ES (200, 400, 600, 800, and 1000 kg ha−1) and SA (20, 40, 60, 80 and 100 kg ha−1) to a calcareous soil (pH 8.1) in Zanjan, Iran. Also, in order to simulate rainfed and dryland farming which are common practices in this area, this study was conducted with and without sprinkler irrigation. Application of ES slowly decreased soil pH (more than half a unit) over the year whereas application of SA temporarily reduced the pH (less than a half unit) for a few weeks. CO2 and N2O emissions and CH4 uptake were maximum during summer and lowest in winter. Cumulative CO2 fluxes ranged from 1859.2 kg−1 CO2-C ha−1 year−1 in the control treatment to 2269.6 kg CO2-C ha−1 year−1 in the 1000 kg ha−1 ES treatment. Cumulative fluxes for N2O-N were 2.5 and 3.7 kg N2O-N ha−1 year−1 and cumulative CH4 uptakes were 0.2 and 2.3 kg CH4-C ha−1 year−1 in the same treatments. Irrigation significantly increased CO2 and N2O emissions and, depending on the amount of ES applied, decreased or increased CH4 uptake. SA application had a negligible effect on GHGs emissions in this experiment and only the highest amount of SA altered GHGs emissions.

Improved sulfur autotrophic denitrification using supplementary bovine serum albumin

Excess nitrate presented in natural water body and drinking water has been a challenge for maintaining safe ecosystem and human health. Sulfur autotrophic denitrification is proved a feasible technology to remove nitrate from water environment. However, comparatively low rate of sulfur autotrophic denitrification needs to be addressed before wide application of this technology, which is a result of the low solubility of elemental sulfur. Therefore, this study employed bovine serum albumin (BSA) as a supplementary material to modify the elemental sulfur for improved sulfur autotrophic denitrification rate. Artificial biofilm of Thiobacillus denitrificans was prepared and employed in experiments. By testing different amount of BSA applied in both elemental sulfur and the biofilm, including 1 %, 2 % and 4 % mass ratios, it was found that larger employment of BSA had significant effect in increasing the denitrification rate. Particularly when 4 % BSA was added into elemental sulfur, the highest denitrification rate reached 26.8 mg-N/(L·d), 3.7 times of the control group. Meanwhile, the largest reaction rate constant was achieved, 4.13 mg^0.5/(L^0.5·d), 2.78 times of the control group. This effect was attributed to promoted conversion of elemental sulfur to polysulfide that was easily utilized by sulfur-oxidizing bacteria. A long-term operation (14 days) of packed bed reactor filled with sulfur particles and 1 % BSA delivered a much faster start-up than the control and outperformed it with better denitrification performance all-through the experiment. This result evidenced again that BSA could make a highly effective supplement in sulfur autotrophic denitrification.

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 (ω_e, 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.

Dissimilatory and Cytoplasmic Antimonate Reductions in a Hydrogen-Based Membrane Biofilm Reactor

A hydrogen-based membrane biofilm reactor (H₂-MBfR) was operated to investigate the bioreduction of antimonate [Sb(V)] in terms of Sb(V) removal, the fate of Sb, and the pathways of reduction metabolism. The MBfR achieved up to 80% Sb(V) removal and an Sb(V) removal flux of 0.55 g/m²·day. Sb(V) was reduced to Sb(III), which mainly formed Sb₂O₃ precipitates in the biofilm matrix, although some Sb(III) was retained intracellularly. High Sb(V) loading caused stress that deteriorated performance that was not recovered when the high Sb(V) loading was removed. The biofilm community consisted of DSbRB (dissimilatory Sb-reduction bacteria), SbRB (Sb-resistant bacteria), and DIRB (dissimilatory iron-reducing bacteria). Dissimilatory antimonate reduction, mediated by the respiratory arsenate reductase ArrAB, was the main reduction route, but respiratory reduction coexisted with cytoplasmic Sb(V)-reduction mediated by arsenate reductase ArsC. Increasing Sb(V) loading caused stress that led to increases in the expression of arsC gene and intracellular accumulation of Sb(III). By illuminating the roles of the dissimilatory and cytoplasmic Sb(V) reduction mechanism in the biofilms of the H₂-MBfR, this study reveals that the Sb(V) loading should be controlled to avoid stress that deteriorates Sb(V) reduction.

Extracellular DNA plays a key role in the structural stability of sulfide-based denitrifying biofilms

Sulfide-based biofilm processes are increasingly used for wastewater denitrification, yet little is known about the extracellular polymeric substance (EPS) composition of sulfide-oxidizing biofilms. This can have an important impact on biofilm mechanical strength and stability. In this research, the properties and roles of EPS components in biofilm stability were investigated. Weak biofilm stability characterized by high roughness and numerous "needle" structures was visualized by optical coherence tomography (OCT) and microscopy. A high abundance of extracellular DNA (eDNA) and a low protein to polysaccharide ratio were found in the biofilm. The roles of eDNA, protein and polysaccharide in biofilm cohesion and adhesion were identified through enzyme treatment and atomic force microscopy (AFM). The enzymatic hydrolysis of eDNA increased the elastic modulus of biofilms by 57 times and reduced the adhesion energy by 96%. The hydrolysis of proteins led to an increase of elastic modulus by 27 times and a loss of adhesion energy by 95.5%. The enzymatic hydrolysis of polysaccharides caused minimal changes in elastic modulus and adhesion energy. These results suggest that eDNA was the key EPS component for biofilm cohesion and adhesion, possibly because it provided special binding sites and can form strong cross-linking with magnesium or other multivalent cations. This study provided new insights into the role of eDNA in biofilm stability and shed light on the development of sulfide-based denitrifying biofilms.

A new inoculation method of sulfur autotrophic denitrification reactor for accelerated start-up and better low-temperature adaption

Elemental sulfur (S⁰) autotrophic denitrification (SAD) has been proved feasible for nitrate removal from aquatic environments. The long start-up period up to weeks of the SAD reactor impedes its industrial application. To accelerate the start-up process, this study employed S⁰ powder packed sequencing batch reactor operated for 10 days to obtain a seed biofilm, which was inoculated into a regular S⁰ flake packed bed reactor afterwards. Merely two days after inoculation, the reactor inoculated with seed biofilm was well started up and outperformed the control reactor, which was inoculated with regular anaerobic sludge and operated for more than 10 days, delivering much increased denitrification rate of 126 ± 0.68 mg N/(L·d) and a high nitrate removal efficiency of 93.0%. Batch tests during the start-up period showed that the seed biofilm developed well on S⁰ flakes and delivered improved nitrate removal performance than the control. Extracellular polymeric substance (EPS) analysis revealed an abundant content of protein in tightly bound EPS in the biofilm developed from the seed biofilm, which was recognized as a major contributor to facilitate the biofilm's attachment and growth onto S⁰ flakes. After operating under moderate temperature, the reactors were tested at a reduced temperature of 15 °C. Results indicated that the reactor inoculated with seed biofilm showed stronger adaptation ability towards low temperature and sustained better denitrification performance than the control, which was attributed to increased protein content in tightly bound EPS produced by the microbes against low-temperature. Determination of the microbial communities in tested reactors when the whole experiment was closing found that sulfur-related genera were dominating in the packed-bed reactor inculcated with seed biofilm, which played an important role in the S⁰-based denitrification process.

Metagenomic analysis of microbial community and metabolic pathway of simultaneous sulfide and nitrite removal process exposed to divergent hydraulic retention times

The role of hydraulic retention time (HRT) on S⁰ production was assessed through metagenomics analyses. Considering comprehensive performance for the tested HRTs (0.25-13.33 h), the optimal HRT was 1 h, while respective sulfide and nitrite loading rate could reach 6.84 kg S/(m³·d) and 1.95 kg N/(m³·d), and total S⁰ yield was 0.36 kg S/(kg (VSS)·d). Bacterial community richness decreased along the shortening of HRT. Microbacterium, Sulfurimonas, Sulfurovum, Paracoccus and Thauera were highly abundant bacteria. During sulfur metabolism, high expression of sqr gene was the main reason of maintaining high desulfurization load, while lacking soxB caused the continuous increase of S⁰. Regarding nitrogen metabolism, the rapid decrease of nitrite transporter prevented nitrite to enter in cells, which caused a rapid decrease of nitrite removal under extreme HRT. Adjusting HRT is an effective way to enhance S⁰ production for the application of the simultaneous sulfide and nitrite removal process.

Modelling the effect of SMP production and external carbon addition on S-driven autotrophic denitrification

The aim of this study was to develop a mathematical model to assess the effect of soluble microbial products production and external carbon source addition on the performance of a sulfur-driven autotrophic denitrification (SdAD) process. During SdAD, the growth of autotrophic biomass (AUT) was accompanied by the proliferation of heterotrophic biomass mainly consisting of heterotrophic denitrifiers (HD) and sulfate-reducing bacteria (SRB), which are able to grow on both the SMP derived from the microbial activities and on an external carbon source. The process was supposed to occur in a sequencing batch reactor to investigate the effects of the COD injection on both heterotrophic species and to enhance the production and consumption of SMP. The mathematical model was built on mass balance considerations and consists of a system of nonlinear impulsive differential equations, which have been solved numerically. Different simulation scenarios have been investigated by varying the main operational parameters: cycle duration, day of COD injection and quantity of COD injected. For cycle durations of more than 15 days and a COD injection after the half-cycle duration, SdAD represents the prevailing process and the SRB represent the main heterotrophic family. For shorter cycle duration and COD injections earlier than the middle of the cycle, the same performance can be achieved increasing the quantity of COD added, which results in an increased activity of HD. In all the performed simulation even in the case of COD addition, AUT remain the prevailing microbial family in the reactor.

Advances in elemental sulfur-driven bioprocesses for wastewater treatment: From metabolic study to application

Elemental sulfur (S0) is known to be an abundant, non-toxic material with a wide range of redox states (-2 to +6) and may serve as an excellent electron carrier in wastewater treatment. In turn, S0-driven bioprocesses, which employ S0 as electron donor or acceptor, have recently established themselves as cost-effective therefore attractive solutions for wastewater treatment. Numerous related processes have, to date, been developed from laboratory experiments into full-scale applications, including S0-driven autotrophic denitrification for nitrate removal and S0-reducing organic removal. Compared to the conventional activated sludge process, these bioprocesses require only a small amount of organic matter and produce very little sludge. There have been great efforts to characterize chemical and biogenic S0 and related functional microorganisms in order to identify the biochemical pathways, upgrade the bioprocesses, and assess the impact of the operating factors on process performance, ultimately aiming to better understand and to optimize the processes. This paper is therefore a comprehensive overview of emerging S0-driven biotechnologies, including the development of S0-driven autotrophic denitrification and S0-based sulfidogenesis, as well as the associated microbiology and biochemistry. Also reviewed here are the physicochemical characteristics of S0 and the effects that environmental factors such as pH, influent sulfur/nitrate ratio, temperature, S0 particle size and reactor configurations have on the process. Research gaps, challenges of process applications and potential areas for future research are further proposed and discussed.

Hybrid zeolite-based ion-exchange and sulfur oxidizing denitrification for advanced slaughterhouse wastewater treatment

The discharge of slaughterhouse wastewater (SWW) is increasing and its wastewater has to be treated thoroughly to avoid the eutrophication. The hybrid zeolite-based ion-exchange and sulfur autotrophic denitrification (IX-AD) process was developed to advanced treat SWW after traditional secondary biological process. Compared with traditional sulfur oxidizing denitrification (SOD), this study found that IX-AD column showed: (1) stronger ability to resist NO₃^- pollution load, (2) lower SO₄^2- productivity, and (3) higher microbial diversity and richness. Liaoning zeolites addition guaranteed not only the standard discharge of NH₄⁺-N, but also the denitrification performance and effluent TN. Especially, when the ahead secondary biological treatment process run at the ultra-high load, NO₃^--N removal efficiency for IX-AD column was still ~100%, whereas only 64.2% for control SOD column. The corresponding average effluent TN concentrations for IX-AD and SOD columns were 5.89 and 65.55 mg/L, respectively. Therefore, IX-AD is a promising technology for advanced SWW treatment and should be widely researched and popularized.

Alkalinity regulation in a sulfur autotrophic denitrifying filter substantially reduced total dissolved solids and sulfate in effluent

Sulfur autotrophic denitrification (SAD) filters are considered a promising technology due to their stable and excellent performance in nitrogen removal, affordable costs, and operational advantages. In this work, a novel operational strategy that employed sodium bicarbonate as an alkalinity source in the autotrophic denitrification filter (S-SAD) was established. With the sufficient supply of alkalinity, the S-SAD reached an excellent denitrification performance (98.01%±0.43%) with a nitrate concentration of 10 mg/L in influent and hydraulic retention time of 3 hrs. The total dissolved solids increment and sulfate concentration in effluent were significantly reduced by one-third, compared with that of the traditional SAD process under the same conditions. The analysis of microbial community indicated that Thiobacilhus, typical species with the functions of simultaneous sulfur oxidation and denitrification, was evidently enriched in the S-SAD. Thus, this present work demonstrated a feasible, relatively cost-effective and environmentally friendly approach to operate SAD towards further application.

Biofilm carrier type affects biogenic sulfur-driven denitrification performance and microbial community dynamics in moving-bed biofilm reactors

Autotrophic denitrification with biosulfur (ADBIOS) provides a sustainable technological solution for biological nitrogen removal from wastewater driven by biogenic S⁰, derived from biogas desulfurization. In this study, the effect of different biofilm carriers (conventional AnoxK™ 1 and Z-200 with a pre-defined maximum biofilm thickness) on ADBIOS performance and microbiomics was investigated in duplicate moving bed-biofilm reactors (MBBRs). The MBBRs were operated parallelly in continuous mode for 309 days, whilst gradually decreasing the hydraulic retention time (HRT) from 72 to 21 h, and biosulfur was either pumped in suspension (days 92-223) or supplied in powder form. Highest nitrate removal rates were approximately 225 (±11) mg/L·d and 180 (±7) mg NO₃^--N/L·d in the MBBRs operated with K1 and Z-200 carriers, respectively. Despite having the same protected surface area for biofilm development in each MBBR, the biomass attached onto the K1 carrier was 4.8-fold more than that on the Z-200 carrier, with part of the biogenic S⁰ kept in the biofilm. The microbial communities of K1 and Z-200 biofilms could also be considered similar at cDNA level in terms of abundance (R = 0.953 with p = 0.042). A relatively stable microbial community was formed on K1 carriers, while the active portion of the microbial community varied significantly over time in the MBBRs using Z-200 carriers.

Sulfur transformation and bacterial community dynamics in both desulfurization-denitrification biofilm and suspended activated sludge

Types of microbial aggregates have essential effects on bacterial communities' characteristics, thus affecting the pollutants removal. An up-flow biofilm reactor was used to study the different performances of S^2-/NO₂^- removal and functional genes in suspended sludge and biofilms. The metabolic pathways of sulfurous and nitrogenous pollutants in the desulfurization-denitrification process were proposed. The results showed that S⁰ formation dominated the reactor with a high S^2- concentration. Autotrophic Sulfurovum responsible for S^2-/S⁰ oxidation was the only dominant bacteria in suspended sludge. Heterotrophic Desulfocapsa responsible for SO₄^2- reduction coexisted with Sulfurovum and dominated in biofilms. S^2- oxidation to S⁰ was catalyzed via fccA/B and sqr genes in suspended sludge. S₃^2-/S⁰ oxidation to SO₄^2- was catalyzed via dsrA/B gene in biofilms. SO₄^2- and NO₂^- were removed via the dissimilatory sulfate reduction and denitrification pathway, respectively. This work provides a fundamental and practical basis for optimizing suspended sludge/biofilm systems for S^2-/NO₂^- removal.

Sulfur-based mixotrophic denitrification with the stoichiometric S0/N ratio and methanol supplementation: effect of the C/N ratio on the process

The impact of the organic carbon to nitrate ratio (C/N ratio) on mixotrophic denitrification rate has been scarcely studied. Thus, this work aims to investigate the effect of the C/N ratio on the mixotrophic denitrification when methanol is used as a source of organic matter and elemental sulfur as an electron donor for autotrophic denitrification. For this, two initial concentrations of NO₃^--N (50 and 25 mg/L) at a stoichiometric ratio of S⁰/N, and four initial C/N ratios (0, 0.6, 1.2, and 1.9 mg CH₃OH/mg NO₃^- -N) were used at 25 (±2) °C. The results showed that when using a C/N ratio of 0.6, the highest total nitrogen removal was obtained and the accumulation of nitrites was reduced, compared to an autotrophic system. The most significant contribution to nitrate consumption was through autotrophic denitrification (AuDeN) for a C/N ratio of 0.6 and 1.2, while for C/N = 1.9 the most significant contribution of nitrate consumption was through heterotrophic denitrification (HD). Finally, organic supplementation (methanol) served to increase the specific nitrate removal rate at high and low initial concentrations of substrate. Therefore, the best C/N ratio was 0.6 since it allowed for increasing the removal efficiency and the denitrification rate.

Sulfur-driven autotrophic denitrification of nitric oxide for efficient nitrous oxide recovery

Nitrous oxide (N₂ O) was previously deemed as a potent greenhouse gas but is actually an untapped energy source, which can accumulate during the microbial denitrification of nitric oxide (NO). Compared with the organic electron donor required in heterotrophic denitrification, elemental sulfur (S⁰ ) is a promising electron donor alternative due to its cheap cost and low biomass yield in sulfur-driven autotrophic denitrification. However, no effort has been made to test N₂ O recovery from sulfur-driven denitrification of NO so far. Therefore, in this study, batch and continuous experiments were carried out to investigate the NO removal performance and N₂ O recovery potential via sulfur-driven NO-based denitrification under various Fe(II)EDTA-NO concentrations. Efficient energy recovery was achieved, as up to 35.5%-40.9% of NO was converted to N₂ O under various NO concentrations. N₂ O recovery from Fe(II)EDTA-NO could be enhanced by the low bioavailability of sulfur and the acid environment caused by sulfur oxidation. The NO reductase (NOR) and N₂ O reductase (N₂ OR) were inhibited distinctively at relatively low NO levels, leading to efficient N₂ O accumulation, but were suppressed irreversibly at NO level beyond 15 mM in continuous experiments. Such results indicated that the regulation of NO at a relatively low level would benefit the system stability and NO removal capacity during long-term system operation. The continuous operation of the sulfur-driven Fe(II)EDTA-NO-based denitrification reduced the overall microbial diversity but enriched several key microbial community. Thauera, Thermomonas, and Arenimonas that are able to carry out sulfur-driven autotrophic denitrification became the dominant organisms with their relative abundance increased from 25.8% to 68.3%, collectively.

Sequential sulfur-based denitrification/denitritation and nanofiltration processes for drinking water treatment

Efficient and cost-effective solutions for nitrogen removal are necessary to ensure the availability of safe drinking water. This study proposes a combined treatment for nitrogen-contaminated groundwater by sequential autotrophic nitrogen removal in a sulfur-packed bed reactor (SPBR) and excess sulfate rejection via nanofiltration (NF). Autotrophic nitrogen removal in the SPBR was investigated under both denitrification and denitritation conditions under different NO₃^- and NO₂^- loading rates (LRs) and feeding strategies (NO₃^- only, NO₂^- only, or both NO₃^- and NO₂^- in the feed). Batch activity tests were carried out during SPBR operation to evaluate the effect of different feeding conditions on nitrogen removal activity by the SPBR biofilm. Bacteria responsible for nitrogen removal in the bioreactor were identified via Illumina sequencing. Dead-end filtration tests were performed with NF membranes to investigate the elimination of excess sulfate from the SPBR effluent. This study demonstrates that the combined process results in effective groundwater treatment and evidences that an adequately high nitrogen LR should be maintained to avoid the generation of excess sulfide.

Elemental sulfur as electron donor and/or acceptor: Mechanisms, applications and perspectives for biological water and wastewater treatment

Biochemical oxidation and reduction are the principle of biological water and wastewater treatment, in which electron donor and/or acceptor shall be provided. Elemental sulfur (S⁰) as a non-toxic and easily available material with low price, possesses both reductive and oxidative characteristics, suggesting that it is a suitable material for water and wastewater treatment. Recent advanced understanding of S⁰-respiring microorganisms and their metabolism further stimulated the development of S⁰-based technologies. As such, S⁰-based biotechnologies have emerged as cost-effective and attractive alternatives to conventional biological methods for water and wastewater treatment. For instance, S⁰-driven autotrophic denitrification substantially lower the operational cost for nitrogen removal from water and wastewater, compared to the conventional process with exogenous carbon source supplementation. The introduction of S⁰ can also avoid secondary pollution commonly caused by overdose of organic carbon. S⁰ reduction processes cost-effectively mineralize organic matter with low sludge production. Biological sulfide production using S⁰ as electron acceptor is also an attractive technology for metal-laden wastewater treatment, e.g. acid mine drainage. This paper outlines an overview of the fundamentals, characteristics and advances of the S⁰-based biotechnologies and highlights the functional S⁰-related microorganisms. In particular, the mechanisms of microorganisms accessing insoluble S⁰ and feasibility to improve S⁰ bio-utilization efficiency are critically discussed. Additionally, the research knowledge gaps, current process limitations, and required further developments are identified and discussed.

Simultaneous denitrification and desulfurization-S0 recovery of wastewater in trickling filters by bioaugmentation intervention based on avoiding collapse critical points

In order to better achieve efficiently simultaneous desulfurization and denitrification/S⁰ recovery of wastewater, the intervention of sulfur oxidizing bacteria (SOB) and denitrifying bacteria (DNB) was employed to avoid the collapse critical points (the dramatically decrease of S/N removal efficiency) under the fluctuated load. With the assistance of DNB and SOB, collapse critical point of trickling filter (TF) was delayed from the P₈ (105-114 d) to P₁₀ stage (129-138 d). The treatment efficiency of nitrogen and sulfur was the highest with the S/N ratio of 3:1. The bioaugmentation of DNB and SOB at collapse critical point could effectively regulated collapse situation, which further increased the maximum system utilization/elimination capacity to 4.50 kg S m^-3·h^-1 and 0.90 kg N m^-3·h^-1 (increased by 56.89% and 65.56% in comparison to control). High-throughput sequencing analysis indicated that Proteobacteria (average 78.59%) and Bacteroidetes (average 9.30%) were dominant bacteria in the reactor at all stages. As the reaction proceeds, the microbial community was gradually dominated by some functional genera such as Chryseobacterium (average 2.97%), Halothiobacillus (average 22.71%), Rhodanobacter (average 14.02%), Thiobacillus (average 9.01%), Thiomonas (average 16.70%) and Metallibacterium (average 21.63%), which could remove nitrate or sulfide. Both of Principal Component Analysis (PCA) and Canonical Correlation Analysis (CCA) demonstrated the important role of DNB/SOB during the long-term run in the trickling filters (TFs).

Enhanced tetrabromobisphenol A debromination by nanoscale zero valent iron particles sulfidated with S0 dissolved in ethanol

Modification of nanoscale zero-valent iron (nZVI) with reducing sulfur compounds has proven to improve the reactivity of nZVI towards recalcitrant halogenated organic contaminants. In this study, we develop a novel method for the preparation of sulfidated nZVI (S-nZVI) with S0 (a low cost and available reducing sulfur agent) dissolved in ethanol under mild conditions and apply it for the transformation of tetrabromobisphenol A (TBBPA), a potential persistent organic pollutant. Surface analysis shows that S0 dissolved in ethanol has been successfully doped into nZVI via a reaction with Fe0 to form a relatively homogeneous layer of FeS/FeS2 on the nZVI surface. The H2 production test and the electrochemical analysis show that the FeS/FeS2 layer not only slows the H2 evolution reaction but also enhances the electron transfer. Debromination kinetics indicate that the resulting S-nZVI with a S/Fe ratio of 0.015-0.05 possesses higher debromination activity for TBBPA and its debromination products (i.e., tri-BBPA, di-BBPA, mono-BBPA and BPA) in comparison with nZVI. Among them, S-nZVI at a S/Fe of 0.025 (S-nZVIS-0.025) has the greatest debromination rate constant (kobs) of 1.19 ± 0.071 h-1 for TBBPA. It debrominates TBBPA at a faster rate than other conventional S-nZVI made from Na2S and Na2S2O4 and has been successfully applied in the treatment of TBBPA-spiked environmental water samples (including river water, groundwater, and tap water). The results suggest that the modification of nZVI with S0 dissolved in ethanol is a simple, safe, inexpensive, and effective sulfidation technique, which can be applied for the large-scale production of S-nZVI for treating contaminated water.

Organic carbon coupling with sulfur reducer boosts sulfur based denitrification by Thiobacillus denitrificans

Sulfur autotrophic denitrification utilizes elemental sulfur as the electron donor for nitrate removal from aquatic environments. Organic carbon could stimulate the conversion of sulfur and facilitates the S⁰-based denitrification process in the mix-trophic. In this study, the co-cultured system of sulfur reducer (Geobacter sulfurreducens) and Thiobacillus denitrificans was used to investigate that how organic carbon could boost the S⁰-based denitrification. The results showed that the rate of S⁰-based denitrification was improved with C/N ratio of 0.13 and this improvement continued even after the acetate was exhausted. Sulfur probe test and Raman analysis suggested that reduced sulfur species (S_x^2-) were formed with the addition of organic carbon. The S_x^2- could recombine with element sulfur and the bioavailability of S⁰ would be improved, as a result, the rate of S⁰-based denitrification increased as well. Nitrate reduction rate could further increase with the C/N ratio of 0.88, but it would decrease significantly when the C/N ratio increased to 1.50 as the high concentration of generated S^2-. Our results provided explanations that why organic carbon addition would improve the bioavailability of S⁰ which could further promote the S⁰-dominant denitrification process.

High-rate nitrogen removal from carbon limited wastewater using sulfur-based constructed wetland: Impact of sulfur sources

This study aims to explore the application of sulfur-based constructed wetlands (CWs) for effective nitrogen (N) removal from wastewater. Two solid sulfur sources namely elemental sulfur (S⁰) and pyrite (FeS₂) were used as substrates in two CWs, i.e. S-CW and P-CW, respectively. The CWs were vegetated with a common wetland plant Iris pseudacorus, and were operated to investigate the effects of hydraulic retention time (HRT) and temperature on N removal. The use of S⁰ resulted in the highest denitrification rate (19.0 ± 7.5 g m^-2 d^-1), whereas up to 20 times slower total inorganic nitrogen (TIN) removal was observed with FeS₂. Different sulfur sources had negligible effects on the growth of I. pseudacorus, but the element contents (e.g., N, S, and P) within the plant tissues were different. Iris roots in S-CW had higher S content compared with those in P-CW, which resulted in the difference in shoots colors. The characteristics of rhizospheric microbial communities were closely related to the sulfur and nitrogen sources. Briefly, denitrifying and sulfur-oxidizing genera (e.g., Denitratisoma, Sulfurimonas, Thiobacillus) were dominating in the S-CW, suggesting the occurrence of both autotrophic and heterotrophic denitrification processes in the wetland. On the other hand, nitrifying bacteria were more abundant (e.g. Nitrospira, Piscinibacter) in the P-CW. S⁰ layer and rhizosphere accounted for 99.3% of nitrogen removal and the former part most likely played important roles with a decrease in HRT. Low temperature strongly affected the rate and efficiency of denitrification but recovered to 49.2 ± 25.8% when added with 30 mg L^-1 sodium acetate. This study broadens the applications of sulfur-based CWs and provides a promising management strategy for denitrification at low temperatures.

Effect of influent feeding pattern on municipal tailwater treatment during a sulfur-based denitrification constructed wetland

This work studied three parallel pilot-scale constructed wetlands based on sulfur-based autotrophic denitrification (SAD-CWs) with horizontal, vertical-horizontal and integrated vertical inflow for nitrogen removal of municipal tailwater. SAD system played the predominant role for nitrate removal and the integrated vertical inflow pattern was the most efficient pattern with 96.1% NO₃^--N and 44.3% total phosphorus (TP) removal efficiency, respectively, at the condition of 3.5 h hydraulic retention time (HRT) and 18.5-23.5 °C. Although no great and serious change for microbial community structure was observed among these systems, the diversity in term of abundance of microbes and certain function species was observed. Proteobacteria, Ignavibacterae and Chloroflexi were the dominant phyla and accounted for over 59.1%, 7.5%, and 6.0% in SAD-CWs, respectively. Moreover, the richness and diversity of denitrifies in SAD-CWs with integrated vertical inflow were both higher than that in the other two reactors, especially sulfur autotrophic denitrifying bacteria.

A novel coupling process with partial nitritation-anammox and short-cut sulfur autotrophic denitrification in a single reactor for the treatment of high ammonium-containing wastewater

In this study, a novel coupling process with partial nitritation-anaerobic ammonium oxidation (anammox) (PNA) and sulfur autotrophic denitrification (SAD) was studied using an upflow biofilm reactor with mechanical vibration. At a lower dissolved oxygen (DO) concentration (0.40 ± 0.20 mg L^-1), ammonia could be efficiently removed from synthetic wastewater by the coupling system with a total nitrogen removal efficiency (NRE) of 98% and an influent NH₄⁺-N concentration of 600 mg L^-1. In this system, the nitrate, which was produced during the anammox reaction, could be timely reduced by the SAD reaction. Compared with the conventional PNA and SAD processes, coupling the PNA and SAD processes in a single reactor prevented nitrite accumulation in the SAD reaction and reduced the total sulfate production by 59%. The high-throughput sequencing analysis supported that the SAD bacteria (Thiobacillus) and anammox bacteria (Candidatus Kuenenia) could coexist on the elemental sulfur stone. Additionally, sulfur consumption and sulfate production were increased under a high DO concentration. The sulfate production/nitrate reduction ratio and changing profile of the substrate suggested that the short-cut SAD process mainly occurred in this coupling system. Otherwise, batch experiments also suggested that the nitrite removal rate in the anammox process was 34.5 times higher than that in the SAD process. The outcomes of these experiments revealed that most of the nitrite, as an intermediate product in the SAD reaction, served as an electron acceptor for the anammox reaction. A stoichiometric calculation of this coupling process indicated that the novel reaction scheme with a high NRE was successfully achieved. Under an ideal short-cut SAD process, almost 55% of the sulfur consumption could be reduced in this coupling system. The coupling system provides a new perspective for nitrogen removal in a single reactor and further promotes anammox and SAD performance in wastewater treatment processes.

Elemental sulfur-based autotrophic denitrification in stoichiometric S0/N ratio: Calibration and validation of a kinetic model

The inclusion of S⁰ hydrolysis in a kinetic model of autotrophic denitrification has been recently proposed; however the model has not been calibrated or validated yet. Thus, a new methodology was developed and applied to calibrate and validate this kinetic model for the first time. An inoculum adapted from a poultry wastewater treatment plant at stoichiometric S⁰/NO₃^- ratio was used. The model was calibrated with batch data (initial nitrate concentrations of 50 and 6.25 mg NO₃^--N/L) at an S⁰/N ratio = 2.29 mg S/mg N and validated with seven different batch data. The sensitivity analysis showed that the most sensitive parameters are related to S⁰ hydrolysis. The kinetic model was successfully calibrated with the new methodology and validated, with Theil inequality coefficient values lower than 0.21. Thus, the proposed model and methodology were proved to be well suited for the simulation of elemental sulfur-based autotrophic denitrification in batch systems.

Combined heterotrophic and autotrophic system for advanced denitrification of municipal secondary effluent in full-scale plant and bacterial community analysis

Total nitrogen (TN) removal is the major technical challenge for wastewater treatment plants to meet the more stringent discharge standard. In this study, lab- (0.05 m³/d), pilot- (1000 m³/d) and full-scale (10,000 m³/d) combined heterotrophic and autotrophic denitrification reactors (HARs) were designed and operated to treat municipal secondary effluent. During the 110-day stable operation, the effluent TN was reduced below 2.5 mg/L without secondary pollution causing by the excessive addition of organics, close to Class IV of Environmental Quality Standards for Surface Water. The bacterial richness and diversity increased with the expansion of reactor scale. Denitrifying bacteria (DB) dominated in all reactors, however, Thiomonas (12.42%), Methylotenera (6.35%), Thiobacillus (20.62%), Methyloverstatilis (5.44%) and Thauera (8.21%) were the main genera in lab-, pilot- and full-scale reactors respectively. The denitrification efficiency temporarily deteriorated at the later stage, and redundancy analysis (RDA) indicated the obviously increased sulfate reducing bacteria (SRB) and sulfide were main contributors. Sludge supplement rapidly recovered the reactors performance in five days. This study suggests that HARs could be a promising technique for advanced denitrification of the municipal secondary effluent.

Feasibility test of autotrophic denitrification of industrial wastewater in sequencing batch and static granular bed reactors

In order to evaluate the efficacy of using reduced sulfur species in lieu of conventional substrates, a sequencing batch reactor (SBR) was used to develop an autotrophic denitrifying culture which in turn was used to seed a static granular bed reactor (SGBR) for continuous flow treatment. Both bioreactors were able to quickly acclimate to the anoxic environment and achieve stable autotrophic denitrification within several weeks of being placed in operation. The seed for the SBR was obtained from operating basins at the Cedar Rapids plant. MiSeq analysis showed the presence of the autotrophic denitrifier Thiobacillus in the seed from the sulfur oxidation basin; however, Shinella and Sulfurovum became the dominant autotrophic denitrifiers in the SBR. Both the SBR and SGBR achieved excellent nitrate removal (i.e., >95%) with stoichiometric amounts of thiosulfate added to the synthetic influent. The results of this feasibility study suggest that anaerobic granules from the UASB at the plant serve as good seed biomass for autotrophic denitrification when augmented by sulfur oxidation basin and sulfide scrubber biomass, and that reduced sulfur species at the plant (or augmented with an external sulfur source) can serve as electron donors for nearly complete denitrification. PRACTITIONER POINTS: Autotrophic denitrification of industrial wastewater was investigated to evaluate reduced sulfur species as electron donor for nitrogen removal. An autotrophic denitrifying culture was cultivated in an SBR, and continuous autotrophic denitrification was accomplished in an SGBR. No increase in head loss was observed in the SGBR, and it was able to operate without the need for backwashing in more than 200 days of operation. Reduced sulfur was demonstrated to be a sufficient electron donor for nearly complete denitrification. MiSeq analysis resolved primary species responsible for autotrophic denitrification in this study.

Denitrification with non-organic electron donor for treating low C/N ratio wastewaters

Denitrification with non-organic electron donors for treating low C/N ratio wastewater has attracted growing interests. Hydrogen, reduced sulfur compounds and ferrous ions are mainly used in autotrophic denitrification, holding promise for achieving practical applications. Recently, the development of autotrophic denitrification-based processes, such as bioelectrochemically-supported hydrogenotrophic denitrification and sulfur-/iron-based denitrification assisted multi-contaminant removal, provide opportunities for applying these processes in wastewater treatment. Exploration of the autotrophic denitrification process in terms of contaminant removal mechanism, interaction among functional microorganisms, and potential full-scale applications is thus of great importance. Here, an overview of the commonly used non-organic electron donors, e.g., hydrogen, reduced sulfur compounds and ferrous ions, in denitrification for treating low C/N ratio wastewater is provided. Also, the feasibility of applying the combined processes based on autotrophic denitrification with the compounds is discussed. Furthermore, challenges and future possibilities as well as concerns about the practical applications are envisaged in this review.

Paenibacillus polymyxa biofertilizer application in a tea plantation reduces soil N2O by changing denitrifier communities

Increasing the use of nitrogen fertilizers in tea orchards has led to intense nitrous oxide (N₂O) emissions. Foliar application of Paenibacillus polymyxa biofertilizer has been proven to be beneficial for organic tea production. In this study, tea yield and quality were significantly improved after application of P. polymyxa biofertilizer compared with the control but were not significantly different from chemical fertilizer treatments. However, the average N₂O fluxes in tea fields treated with chemical fertilizers and biofertilizers (225 kg N·ha^-1·year^-1 for both) were 50.6-973.7 and 0.6-29.1 times higher than those in the control treatment, respectively. Pot experiments conducted to explore the mechanism of N₂O reduction induced by P. polymyxa biofertilizer showed that applying P. polymyxa in addition to urea could reduce N₂O fluxes by 36.5%-73.1%. Quantitative PCR analysis suggested that a significant increase in the quantity of nirK and nosZ genes was linked to the reduction of N₂O, and high-throughput sequencing of nosZ revealed active and potentially efficient denitrifiers in different treatments. Our findings suggest that P. polymyxa biofertilizer is in line with the requirements of modern agriculture, which aims to increase product yield and quality while reducing negative environmental impacts.

Overlooked pathways of denitrification in a sulfur-based denitrification system with organic supplementation

Elemental sulfur-driven autotrophic denitrification (SADN) is a cost-effective approach for treating secondary effluent from wastewater treatment plants (WWTPs). Additional organics are generally supplemented to promote total nitrogen (TN) removal, reduce nitrite accumulation and sulfate production, and balance the pH decrease induced by SADN. However, understanding of the impacts of organic supplementation on microbial communities, nitrogen metabolism, denitrifier activity, and SADN rates in sulfur-based denitrification reactors is still limited. Here, a sulfur-based denitrification reactor was continuously operated for 272 days during which six different C/N ratios were tested successively (2.7, 1.5, 0.7, 0.5, 0.25, and 0). Organic supplementation improved TN removal and decreased NO₂^- accumulation, but reduced the relative abundance of denitrifiers and the contribution of autotrophic nitrate-reducing bacteria (aNRB) to TN removal during the long-term operation of reactor. Predictive functional profiling showed that nitrogen metabolism potential increased with decreasing C/N ratios. SADN was the predominant removal process when the C/N ratio was ≤0.7 (achieving 60% contribution when C/N = 0.7). Although organic supplementation weakened the dominant role of aNRB in denitrification, batch tests for the first time demonstrated that it could accelerate the SADN rate, attributed to the improvement of sulfur bioavailability, likely via the formation of polysulfide. A possible nitrogen removal pathway with multiple electron donors (i.e., sulfur, organics, sulfide, and polysulfide) in a sulfur-based denitrification reactor with organic supplementation was therefore proposed. However, supplementation with a high level of organics could increase the operational cost and effluent concentrations of sulfide and organics as well as enrich heterotrophic denitrifiers. Moreover, microbial community had substantial changes at C/N ratios of >0.5. Accordingly, an optimal C/N ratio of 0.25-0.5 was suggested, which could simultaneously minimize the additional operating cost associated with organic supplementation and maximize TN removal and SADN rates.

DL-cysteine and L-cystine formation and their enhancement effects during sulfur autotrophic denitrification

Sulfur autotrophic denitrification has been proved feasible for nitrate removal from aquatic environments and it utilizes elemental sulfur as the electron donor. A maximum denitrification rate of 194.57 mg N/L·d was achieved with biogenic sulfur as electron donor in a mixed culture collected from sulfur packed bed reactors; this rate was considerably higher than that delivered by α-S₈ or μ-S in the same mixed culture. The elemental sulfur was also tested in the pure culture of Thiobacillus denitrificans, while a lower denitrification rate was noted than in the mixed culture, bio-S (4.86 mg N/L·d) again outperformed other two elemental sulfur's. X-ray absorption near edge structure spectra were collected to examine possible metabolic intermediates during the sulfur autotrophic denitrification process. The analysis revealed the existence of two major intermediates: DL-cysteine and L-cystine. They were found to not only provide electrons but also play a critical role in promoting the elemental sulfur-mediated sulfur autotrophic denitrification process. In general, we investigated the formation and enhancement effects of sulfur intermediates in the sulfur autotrophic denitrification process.

Simultaneous reduction of perchlorate and nitrate in a combined heterotrophic-sulfur-autotrophic system: Secondary pollution control, pH balance and microbial community analysis

A combined heterotrophic-sulfur-autotrophic system (CHSAS) was established to simultaneously reduce perchlorate and nitrate in water. In this system, the OH^- produced by the acetate heterotrophic part (H-part) could be neutralized with the H⁺ produced by the sulfur autotrophic part (S-part); thus, the pH of the final effluent could keep neutral. In addition, the S-part could further reduce the pollutants and residual carbon from the H-part to achieve a high performance. For 19.62 ± 0.30 mg/L ClO₄^- and 21.56 ± 0.83 mg/L NO₃^--N in the influent, the operating parameters were optimal at a hydraulic retention time (HRT) of 1.0 h and an acetate concentration of 70 mg/L. The removal efficiency of ClO₄^- and NO₃^- reached 95.43% and 99.23%, without secondary pollution caused by residual organic carbon. It was also revealed that sulfur (S⁰) disproportionation can be inhibited by shortening the HRT and reducing the acetate dosage. The dominant heterotrophic and autotrophic bacteria were Thauera and Ferritrophicum, respectively, while Chlorobaculum was related to S⁰ disproportionation.

Sulfur-siderite autotrophic denitrification system for simultaneous nitrate and phosphate removal: From feasibility to pilot experiments

Siderite (FeCO₃) is one kind of abundant and cheap carbonate minerals, but it has never been used as inorganic carbon sources and pH buffer in the sulfur autotrophic denitrification before. In this study, sulfur-siderite autotrophic denitrification (SSAD) system was formed. Batch, column and pilot experiments of the SSAD system showed that siderite could provide inorganic sources and pH buffer for sulfur autotrophic denitrification. The optimal volume ratio of sulfur and siderite was 1:3 for the SSAD system. Siderite could not be used as an electron donor by the sulfur autotrophic denitrifiers. The SSAD column removed 28 mg/L NO₃^--N and 3.1 mg/L PO₄^3--P completely at 12 h HRT. The SSAD pilot biofilter during treating secondary effluent obtained stable NO₃^- and PO₄^3- removal, and controlled effluent NO₃^--N and PO₄^3--P around 4 and 0.2 mg/L, respectively, at 4 h HRT, and no blocking occurred in operation of 401 days. In the SSAD system, the main bacteria were Thiobacillus (67.8%), Sulfurimonas (20.0%), and Simplicispira (3.5%); and Sulfurimonas (29.4%), Ferritrophicum (15.2%), and Thiobacillus (10.3%) during treating synthetic wastewater and secondary effluent, respectively. PO₄^3- was removed through iron phosphate precipitate. The SSAD system was a promising way to remove NO₃^- and PO₄^3- simultaneously from wastewater lack of organic carbon sources.

pH control of an upflow pyrite-oxidizing denitrifying bioreactor via electrohydrogenesis

Maintenance of stable pH during pyrite-oxidizing denitrification process is important. Here, we demonstrated effective pH control (7.80 ± 0.20-8.40 ± 0.30) in an electrochemical-H₂ and pyrite-oxidizing denitrifying bioreactor (HPR) through in situ electrohydrogenesis. HPR achieved a higher nitrate removal activity (maximum:19.66 ± 0.63 mg NO₃^--N/(L·h)) with excellent resistance to high nitrate loading (up to 400 mg/L NO₃^--N) compared to that of the control groups. Nitrate removal rate of HPR fitted the Michaelis-Menten kinetic model (R² = 0.98, p < 0.01) well, and the denitrification followed the zero-order rate law. The results of the biofilm community analyses suggested that Thauera was the dominant bacteria in the cathode biofilm of HPR and may prefer hydrogen as an electron donor for autotrophic denitrification, while the relative abundance of Pseudomonas were similar in the cathode biofilm and pyrite biofilm. This study provides a new alternative for effective pH control in denitrifying bioreactors with pyrite as a packing material.

Coupled Sulfur and Iron(II) Carbonate-Driven Autotrophic Denitrification for Significantly Enhanced Nitrate Removal

Sulfur-based denitrification process has attracted increasing attentions because it does not rely on the external addition of organics and avoids the risk of COD exceeding the limit. Traditionally, limestone is commonly employed to maintain a neutral condition (SLAD process), but it may reduce the efficiency as the occupied zone by limestone cannot directly contribute to the denitrification. In this study, we propose a novel sulfur-based denitrification process by coupling with iron(II) carbonate ore (SICAD system). The ore was demonstrated to play roles as the buffer agent and additional electron donor. Moreover, the acid produced through sulfur driven denitrification was found to promote the Fe(II) leaching from the ore and likely extend the reaction zone from the surface to the liquid. As a result, more biomass was accumulated in the SICAD system compared with the controls (sulfur, iron(II) carbonate ore and SLAD systems). Owing to these synergistic effects of sulfur and iron(II) carbonate on denitrification, SICAD system showed much higher denitrification rate (up to 720.35 g·N/m³·d) and less accumulation of intermediates (NO₂^- and N₂O) than the controls. Additionally, sulfate production in SICAD system was reduced. These findings offer great potential of SICAD system for practical use as a highly efficient postdenitrification process.

Biokinetics of microbial consortia using biogenic sulfur as a novel electron donor for sustainable denitrification

In this study, the biokinetics of autotrophic denitrification with biogenic S⁰ (ADBIOS) for the treatment of nitrogen pollution in wastewaters were investigated. The used biogenic S⁰, a by-product of gas desulfurization, was an elemental microcrystalline orthorhombic sulfur with a median size of 4.69 µm and a specific surface area of 3.38 m²/g, which made S⁰ particularly reactive and bioavailable. During denitritation, the biomass enriched on nitrite (NO₂^-) was capable of degrading up to 240 mg/l NO₂^--N with a denitritation activity of 339.5 mg NO₂^--N/g VSS·d. The use of biogenic S⁰ induced a low NO₂^--N accumulation, hindering the NO₂^--N negative impact on the denitrifying consortia and resulting in a specific denitrification activity of 223.0 mg NO₃^--N/g VSS·d. Besides Thiobacillus being the most abundant genus, Moheibacter and Thermomonas were predominantly selected for denitrification and denitritation, respectively.

Sensitivity analysis for an elemental sulfur-based two-step denitrification model

A local sensitivity analysis was performed for a chemically synthesized elemental sulfur (S⁰)-based two-step denitrification model, accounting for nitrite (NO₂ ^-) accumulation, biomass growth and S⁰ hydrolysis. The sensitivity analysis was aimed at verifying the model stability, understanding the model structure and individuating the model parameters to be further optimized. The mass specific area of the sulfur particles (a*) and hydrolysis kinetic constant (k₁) were identified as the dominant parameters on the model outputs, i.e. nitrate (NO₃ ^-), NO₂ ^- and sulfate (SO₄ ^2-) concentrations, confirming that the microbially catalyzed S⁰ hydrolysis is the rate-limiting step during S⁰-driven denitrification. Additionally, the maximum growth rates of the denitrifying biomass on NO₃ ^- and NO₂ ^- were detected as the most sensitive kinetic parameters.

Autotrophic Vanadium(V) Bioreduction in Groundwater by Elemental Sulfur and Zerovalent Iron

Vanadium (V) is an emerging contaminant in groundwater that can adversely affect human health. Although bioremediation has been shown effective, little is known on autotrophic V(V) bioreduction in the context of oligotrophic characteristics of groundwater. In this study, we demonstrate that efficient V(V) bioreductions can be coupled with bio-oxidation of elemental sulfur (S(0)) or zerovalent iron (Fe(0)), and the V(V) removal efficiencies reached 97.5 ± 1.2% and 86.6 ± 2.5% within 120 h using S(0) and Fe(0), respectively. V(IV) is the main reduction product and precipitates naturally in near-neutral conditions. Microbial community, functional gene, and metabolites analyses reveal that synthetic metabolisms among autotrophs and heterotrophs played major roles in V(V) reduction using S(0) and Fe(0). These results demonstrate a new approach for V(V) contaminated groundwater remediation.

Nitrogen removal by thiosulfate-driven denitrification and plant uptake in enhanced floating treatment wetland

This study investigated the potential of thiosulfate-driven autotrophic enhanced floating treatment wetland (AEFTW) in removing nitrogen from the secondary effluent at the relatively short hydraulic retention times and low S/N ratios. Simultaneous autotrophic and heterotrophic denitrification was observed in AEFTW. The peak TN removal rate (15.3gm^-2d^-1) exceeded most of the reported floating treatment wetlands. Based on the kinetic model results, low mean temperature coefficient and high k₂₀ verified that the excellent performance in AEFTW diminished the microbial dependence on temperature. Nitrogen removal performance of enhanced floating treatment wetland (EFTW) and floating treatment wetland (FTW) were similar and highly sensitive to temperature. The interaction of sulfur transformation on the nitrogen, carbon uptake of plants was studied. Thiosulfate addition significantly raised sulfur content in the shoots and further enhanced the uptake of nitrogen and carbon, and increased the plant biomass at the same time. Higher composition of autotrophic and heterotrophic denitrifiers in AEFTW interpreted the occurrence of mixotrophic denitrification during summer. Thiosulfate induced mutual promotion of nitrogen removal by plant uptake and microbial denitrification in AEFTW.