Kinetic model of autotrophic denitrification in sulphur packed-bed reactors

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.

Adapting to seasonal temperature variations: A dynamic multi-subunit strategy for sulfur autotrophic denitrification bioreactors

Elemental sulfur autotrophic denitrification (S0AD) processes are temperature-sensitive, presenting a substantial challenge for the practical implementation of S0AD bioreactors. In this study, a comprehensive methodology for designing and operating S0AD bioreactors was developed, effectively managing fluctuations in nitrogen removal efficiency caused by seasonal temperature variations. Initially, the nitrate removal rate was correlated with simulated on-site temperature and nitrate loading, revealing correlation coefficients of k1, k2, k3, and A as 5.42×10-4, -0.41, 0.04, and 0.13, respectively, to establish a mathematical model for predicting S0AD efficiency. Subsequently, by considering influence factors such as dissolved oxygen and dynamic sulfur consumption, the model was employed to accurately design a pilot-scale S0AD bioreactor for a case study. By utilizing an alternative multi-subunit operation, a stable effluent nitrate concentration of less than 8 mg-N/L was maintained throughout the year. Importantly, this approach resulted in a substantial reduction of 76.8% in excessive nitrate removal, sulfur consumption, and sulfate production. This study aims to provide an optimal design and operation strategy for the practical application of S0AD bioreactors, thereby enhancing reliability and cost-effectiveness in the face of seasonal temperature changes.

Efficient nitrite accumulation in partial sulfide autotrophic denitrification (PSAD) system: insights of S/N ratio, pH and temperature

To provide the necessary nitrite for the Anaerobic Ammonium Oxidation (ANAMMOX) process, the effect of nitrite accumulation in the partial sulfide autotrophic denitrification (PSAD) process was investigated using an SBR reactor. The results revealed that the effectiveness of nitrate removal was unsatisfactory when the S/N ratio (mol/mol) fell below 0.6. The optimal conditions for nitrate removal and nitrite accumulation were achieved within the S/N ratio range of 0.7-0.8, resulting in an average Nitrate Removal Efficiency (NRE) of 95.84%±4.89% and a Nitrite Accumulation Rate (NAR) of 75.31%±6.61%, respectively. It was observed that the nitrate reduction rate was three times faster than that of nitrite reduction during a typical cycle test. Furthermore, batch tests were conducted to assess the influence of pH and temperature conditions. In the pH tests, it became evident that the PSAD process performed more effectively in alkaline environment. The highest levels of nitrate removal and nitrite accumulation were achieved at an initial pH of 8.5, resulting in a NRE of 98.30%±1.93% and a NAR of 85.83%±0.47%, respectively. In the temperature tests, the most favourable outcomes for nitrate removal and nitrite accumulation were observed at 22±1 ℃, with a NRE of 100.00% and a NAR of 81.03%±1.64%, respectively. Moreover, a comparative analysis of 16S rRNA sequencing results between the raw sludge and the sulfide-enriched culture sludge sample showed that Proteobacteria (49.51%) remained the dominant phylum, with Thiobacillus (24.72%), Prosthecobacter (2.55%), Brevundimonas (2.31%) and Ignavibacterium (2.04%) emerging as the dominant genera, assuming the good nitrogen performance of the system.

Effects of inorganic ions on autotrophic denitrification by Thiobacillus denitrificans and on heterotrophic denitrification by an enrichment culture

Salinity of nitrate-laden wastewaters, such as those produced by metal industries, tanneries, and wet flue gas cleaning systems may affect their treatment by denitrification. Salt inhibition of denitrification has been reported, while impacts of individual ions remain poorly understood whilst being relevant for wastewaters where often the concentration of a single ion rather than the salts varies. The aim of this study was to determine the inhibition by inorganic ions (Na+, Cl-, SO42- and K+) commonly present in saline wastewaters on denitrification and reveal its potential for the treatment of such waste streams, like those produced by NOx-SOx removal scrubbers. The inhibitory effects were investigated for both heterotrophic (enrichment culture) and autotrophic (T. denitrificans) denitrification in batch assays, by using NaCl, Na2SO4, KCl and K2SO4 salts at increasing concentrations. The half inhibition concentrations (IC50) of Na+ (as NaCl), Na+ (as Na2SO4) and Cl- (as KCl) were: 4.3 ± 0.3, 7.9 ± 0.5 and 5.2 ± 0.3 g/L for heterotrophic, and 1-2.5, 2.5-5 and 4.1 ± 0.3 g/L for autotrophic denitrification, respectively. Heterotrophic denitrification was completely inhibited at 20 g/L Na+ (as NaCl), 30 g/L Na+ (as Na2SO4) and 30 g/L Cl- (as KCl), while autotrophic at 8 g/L Na+ (as NaCl), 10 g/L Na+ (as Na2SO4) and 15 g/L Cl- (as KCl). In both cases, Cl- addition had the most important role in decreasing denitrification rate, while Na+ at 1 g/L stimulated autotrophic denitrification but rapidly inhibited the rate at higher concentrations. Nitrite reduction was less inhibited by the ions than nitrate reduction and both the osmotic pressure and the toxicity of the single ions played key roles in the overall inhibition of denitrification. Eventually, both autotrophic and heterotrophic denitrification showed potential for the treatment of a saline wastewater from a NOx-SO2 removal scrubber from a pulp mill.

Nitrate removal in iron sulfide-driven autotrophic denitrification biofilter: Biochemical and chemical transformation pathways and its underlying microbial mechanism

Iron sulfides-based autotrophic denitrification (IAD) is effective for treating nitrate-contaminated wastewater. However, the complex nitrate transformation pathways coupled with sulfur and iron cycles in IADs are still unclear. In this study, two columns (abiotic vs biotic) with iron sulfides (FeS) as the packing materials were constructed and operated continuously. In the abiotic column, FeS chemically reduced nitrate to ammonium under the ambient condition; this chemical reduction reaction pathway was spontaneous and has been overlooked in IAD reactors. In the biotic column (IAD biofilter), the complex nitrogen-transformation network was composed of chemical reduction, autotrophic denitrification, dissimilatory nitrate reduction to ammonium (DNRA) and sulfate reducing ammonium oxidation (Sulfammox). Metagenomic analysis and XPS characterization of the IAD biofilter further validated the roles of functional microbial communities (e.g., Acidovorax, Diaphorobacter, Desulfuromonas) in nitrate reduction process coupled with iron and sulfur cycles. This study gives an in-depth insight into the nitrogen transformations in IAD system and provides fundamental evidence about the underlying microbial mechanism for its further application in biological nitrogen removal.

Advance in the sulfur-based electron donor autotrophic denitrification for nitrate nitrogen removal from wastewater

In the field of wastewater treatment, nitrate nitrogen (NO3--N) is one of the significant contaminants of concern. Sulfur autotrophic denitrification technology, which uses a variety of sulfur-based electron donors to reduce NO3--N to nitrogen (N2) through sulfur autotrophic denitrification bacteria, has emerged as a novel nitrogen removal technology to replace heterotrophic denitrification in the field of wastewater treatment due to its low cost, environmental friendliness, and high nitrogen removal efficiency. This paper reviews the advance of reduced sulfur compounds (such as elemental sulfur, sulfide, and thiosulfate) and iron sulfides (such as ferrous sulfide, pyrrhotite, and pyrite) electron donors for treating NO3--N in wastewater by sulfur autotrophic denitrification technology, including the dominant bacteria types and the sulfur autotrophic denitrification process based on various electron donors are introduced in detail, and their operating costs, nitrogen removal performance and impacts on the ecological environment are analyzed and compared. Moreover, the engineering applications of sulfur-based electron donor autotrophic denitrification technology were comprehensively summarized. According to the literature review, the focus of future industry research were discussed from several aspects as well, which would provide ideas for the application and optimization of the sulfur autotrophic denitrification process for deep and efficient removal of NO3--N in wastewater.

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.

Engineering-scale application of sulfur-driven autotrophic denitrification wetland for advanced treatment of municipal tailwater

An engineering-scale sulfur driven autotrophic denitrification vertical-flow constructed wetland (SADN-VFCW) was established to treat low C/N ratio tailwater from municipal wastewater treatment plants (MWTPs). One-year stable operation results indicated that the addition of sulfur prominently enhanced TN, NO₃^--N and TP removal with efficiencies higher than 68.9%, 69.2% and 45.5%, respectively. Higher nitrogen and phosphorus removal rates were achieved in summer than that in other seasons. Furthermore, the microbial analysis revealed the structure of the microbial community changed significantly after sulfur addition, which proved that sulfur promoted the enrichment of autotrophic (Thiobacillus, Sulfurimonas, Ferritrophicum) and heterotrophic (Denitratisoma, Anaerolineaae, Simplicispira) functional bacteria, thus facilitating pollutants removal. Function prediction analysis results also indicated the abundance of nitrate removal/sulfur metabolism functions was significantly strengthened. This study achieved reliable engineering-scale application of SADN-VFCW and offered great potential for simultaneous in-depth treatment of N and P in municipal tailwater by SADN system.

Integrated thiosulfate-driven denitrification, partial nitrification and anammox process in membrane-aerated biofilm reactor for low-carbon, energy-efficient biological nitrogen removal

Combining multiple bioprocesses in a single membrane-aerated biofilm reactor (MABR) unit for wastewater treatment is an emerging research focus. This study investigated the feasibility of coupling thiosulfate-driven denitrification (TDD) with partial nitrification and anammox (PNA) in a MABR for the treatment of ammonium-containing wastewater. The integrated bioprocess was tested over a continuous operation period (>130 d) in two MABRs: one with a polyvinylidene fluoride membrane (MABR-1), and the other with micro-porous aeration tubes covered with non-wovenpolyester fabrics (MABR-2). After start-up, the MABR-1 and MABR-2 based on the TDD-PNA process achieved satisfactory total nitrogen removal efficiencies of 63% and 76%, with maximum oxygen utilisation efficiencies of up to 66% and 80% and nitrogen removal fluxes of 1.3 and 4.7 gN/(m²·d), respectively. Predictions from the AQUASIM-model verified the integrated bioprocess. These lab scale findings confirmed the applicability of MABR technology for simultaneous sulfur and nitrogen removal, promising for pilot-scale application.

Insights into the Superior Bioavailability of Biogenic Sulfur from the View of Its Unique Properties: The Key Role of Trace Organic Substances

Elemental sulfur (S0) is widely utilized in environmental pollution control, while its low bioavailability has become a bottleneck for S0-based biotechnologies. Biogenic sulfur (bio-S0) has been demonstrated to have superior bioavailability, while little is known about its mechanisms thus far. This study investigated the bioavailability and relevant properties of bio-S0 based on the denitrifying activity of Thiobacillus denitrificans with chemical sulfur (chem-S0) as the control. It was found that the conversion rate and removal efficiency of nitrate in the bio-S0 system were 2.23 and 2.04 times those of the chem-S0 system. Bio-S0 was not pure orthorhombic sulfur [S: 96.88 ± 0.25% (w/w)]. Trace organic substances detected on the bio-S0 surface were revealed to contribute to its hydrophilicity, resulting in better dispersibility in the aqueous liquid. In addition, the adhesion force of T. denitrificans on bio-S0 was 1.54 times that of chem-S0, endowing a higher bacterial adhesion efficiency on the sulfur particle. The weaker intermolecular binding force due to the low crystallinity of bio-S0 led to enhanced cellular uptake by attached bacteria. The mechanisms for the superior bioavailability of bio-S0 were further proposed. This study provides a comprehensive view of the superior bioavailability of bio-S0 and is beneficial to developing high-quality sulfur resources.

Investigating the effects of physical properties of sulphur-based carriers on autotrophic denitrification

In this study, four sulphur-based carriers (C1-C4) which have different mass ratio of sodium silicate to carrier from 30% to 50% (C1-C3) and the existence of water (C4) were prepared in order to evaluate the effect of the different physical properties on denitrification in sulphur-based autotrophic processes. While the apparent density and the compressive strength decreased as the proportion of sodium silicate increased and water was added in the carriers, the average pore size and the porosity increased from 0.43 to 3.13 µm and from 38% to 67%, respectively. The treatment system using the carrier C4 with the highest surface area was stabilized most rapidly and achieved the highest nitrogen removal efficiency of 85.6 ± 5.0% during a relatively short HRT of 3 h. The efficiency of nitrate removal was enhanced by 36.9% due to the increase of the ratio of sodium silicate in the carriers from C1 to C3, and more 4.8% point of removal rate increased in the carrier C4 by adding water to the carrier C3. The sum of Thiobacillus and Sulfurimonas was obtained up to 65.90% among the microbial community in the carrier C4 which has the highest distribution (38.35%) of pore size above 20 µm considered to be favourable for retaining autotrophic denitrifiers. From the above results, it is obvious that the physical properties of the sulphur-based carrier and its ability of denitrification can be influenced significantly by the composition of the carrier.

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.

Sulfammox forwarding thiosulfate-driven denitrification and anammox process for nitrogen removal

The coupled process of thiosulfate-driven denitrification (NO₃^-→NO₂^-) and Anammox (TDDA) was a promising process for the treatment of wastewater containing NH₄⁺-N and NO₃^--N. However, the high concentration of SO₄^2- production limited its application, which needs to be alleviated by an economical and effective way to promote the application of TDDA process. In this study, TDDA process was started in a relatively short time by stepwise replacing nitrite with nitrate and operated continuously for 146 days. Results presented that the average total nitrogen removal efficiency of 82.18% can be acquired at a high loading rate of 1.98 kg N/(m³·d) with maximum nitrogen removal efficiency up to 87.04%. It was observed that the increase of S/N ratio improved the denitrification efficiency and slightly inhibit the Anammox process. Batch tests showed that Sulfammox process appeared in TDDA process under certain conditions, further contributing 2.59% nitrogen removal and 10.46% sulfur removal (14.42 mg/L NH₄⁺-N and 37.68 mg/L SO₄^2--S were removed). This finding was mainly attributed to the reduction of sulfate in TDDA system to elemental S⁰ or HS^-, which subsequently was used as an electron donor to realize the recycling of sulfate (SO₄^2--S) pollutants and promote the sulfur-nitrogen (S-N) cycle. High-throughput analysis displayed that Anammox bacteria (Candidatus_Kuenenia), Sulfur-oxidizing bacteria (Thiobacillus) with relatively high abundance of 5.37%, 7.74%, respectively, guaranteeing the excellent nitrogen and sulfate removal performance in the reactor. The enrichment of phyla Chloroflexi (31.79%), Proteobacteria (31.82%), class Ignavibacteriales (10.55%), genus Planctomycetes (13.57%) further verified the exitence of Sulfammox process in the TDDA reactor. This study provides a new perspective for the practical application of TDDA in terms of reducing the production of high concentration SO₄^2- and saving operational cost and strengthening deeply nitrogen removal.

The mixed/mixotrophic nitrogen removal for the effective and sustainable treatment of wastewater: From treatment process to microbial mechanism

Biological nitrogen removal (BNR) is one of the most important environmental concerns in the field of wastewater treatment. The conventional BNR process based on heterotrophic nitrogen removal (HeNR) is suffering from several limitations, including external carbon source dependence, excessive sludge production, and greenhouse gas emissions. Through the mediation of autotrophic nitrogen removal (AuNR), mixed/mixotrophic nitrogen removal (MixNR) offers a viable solution to the optimization of the BNR process. Here, the recent advance and characteristics of MixNR process guided by sulfur-driven autotrophic denitrification (SDAD) and anammox are summarized in this review. Additionally, we discuss the functional microorganisms in different MixNR systems, shedding light on metabolic mechanisms and microbial interactions. The significance of MixNR for carbon reduction in the BNR process has also been noted. The knowledge gaps and the future research directions that may facilitate the practical application of the MixNR process are highlighted. Overall, the prospect of the MixNR process is attractive, and this review will provide guidance for the future implementation of MixNR process as well as deciphering the microbially metabolic mechanisms.

Coupling sulfur-based denitrification with anammox for effective and stable nitrogen removal: A review

Anoxic ammonium oxidation (anammox) is an energy-efficient nitrogen removal process for wastewater treatment. However, the unstable nitrite supply and residual nitrate in the anammox process have limited its wide application. Recent studies have proven coupling of sulfur-based denitrification with anammox (SDA) can achieve an effective nitrogen removal, owing to stable provision of substrate nitrite from the sulfur-based denitrification, thus making its process control more efficient in comparison with that of partial nitrification and anammox process. Meanwhile, the anammox-produced nitrate can be eliminated through sulfur-based denitrification, thereby enhancing SDA's overall nitrogen removal efficiency. Nonetheless, this process is governed by a complex microbial system that involves both complicated sulfur and nitrogen metabolisms as well as multiple interactions among sulfur-oxidising bacteria and anammox bacteria. A comprehensive understanding of the principles of the SDA process is the key to facilitating the development and application of this novel process. Hence, this review is conducted to systematically summarise various findings on the SDA process, including its associated biochemistry, biokinetic reactions, reactor performance, and application. The dominant functional bacteria and microbial interactions in the SDA process are further discussed. Finally, the advantages, challenges, and future research perspectives of SDA are outlined. Overall, this work gives an in-depth insight into the coupling mechanism of SDA and its potential application in biological nitrogen removal.

A novel sulfide-driven denitrification methane oxidation (SDMO) system: Operational performance and metabolic mechanisms

Microbial denitrification is a crucial biological process for the treatment of nitrogen-polluted water. Traditional denitrification process consumes external organic carbon leading to an increase in treatment costs. We developed a novel sulfide-driven denitrification methane oxidation (SDMO) system that integrates autotrophic denitrification (AD) and denitrification anaerobic methane oxidation (DAMO) for cost-effective denitrification and biogas utilization in situ. Two SDMO systems were operated for 735 days, with nitrate and nitrite serving as electron acceptors, to explore the performance of sewage denitrification and characterize metabolic mechanisms. Results showed SDMO system could reach as high as 100% efficiency of nitrogen removal and biogas desulfurization without an external carbon source when HRT was 10 days and inflow nitrogen concentrations were 50-100 mgN·L^-1. Besides, nitrate was a preferable electron acceptor for SDMO system. Biogas not only enhanced nitrogen removal but also intensified the DAMO, nitrogen removed through DAMO contribution doubled as original period from 2.9 mgN·(L·d)^-1 to 6.2 mgN·(L·d)^-1, and the ratio of nitrate removal through AD to DAMO was 1.2:1 with nitrate as electron acceptor. While nitrogen removed almost all through AD contribution and DAMO was weaken as before, the ratio of nitrate removal through AD to DAMO was 21.2:1 with nitrite as electron acceptor. Biogas introduced into SDMO system with nitrate inspired the growth of DAMO bacteria Candidatus Methylomirabilis from 0.3% to 19.6% and motivated its potentiality to remove nitrate without ANME archaea participation accompanying with gene mfnE upregulating ∼100 times. According to the reconstructed genome from binning analysis, the dramatically upregulated gene mfnE was derived from Candidatus Methylomirabilis, which may represent a novel metabolism pathway for DAMO bacteria to replace the role of archaea for nitrate reduction.

Denitrification Performance in Packed-Bed Reactors Using Novel Carbon-Sulfur-Based Composite Filters for Treatment of Synthetic Wastewater and Anaerobic Ammonia Oxidation Effluent

To avoid nitrate pollution in water bodies, two low-cost and abundant natural organic carbon sources were added to make up the solid-phase denitrification filters. This study compared four novel solid-phase carbon-sulfur-based composite filters, and their denitrification abilities were investigated in laboratory-scale bioreactors. The filter F₄ (mixture of elemental sulfur powder, shell powder, and peanut hull powder with a mass ratio of 6:2.5:1.5) achieved the highest denitrification ability, with an optimal nitrate removal rate (NRR) of 723 ± 14.2 mg NO₃^–-N⋅L^–1⋅d^–1 when the hydraulic retention time (HRT) was 1 h. The HRT considerably impacted effluent quality after coupling of anaerobic ammonium oxidation (ANAMMOX) and solid-phase-based mixotrophic denitrification process (SMDP). The concentration of suspended solids (SS) of the ANAMMOX effluent may affect the performance of the coupled system. Autotrophs and heterotrophs were abundant and co-existed in all reactors; over time, the abundance of heterotrophs decreased while that of autotrophs increased. Overall, the SMDP process showed good denitrification performance and reduced the sulfate productivity in effluent compared to the sulfur-based autotrophic denitrification (SAD) process.

Removal of nitrogen from municipal wastewater by denitrification using a sulfur-based carrier: A pilot-scale study

In the present study, to improve nitrate removal rate, a sulfur-based carrier was applied for autotrophic denitrification, and the removal rate was evaluated for advanced wastewater treatment without adding any external organic carbon source. Based on the results, an increased PAC concentration affected the removal efficiency of NO₃^--N, and the optimal concentration of PAC was at 15 wt%. During the 60 d operation of a pilot process with a capacity of 1 m³/d, the removal of T-N was 81.2% and 50.2% in reactors with and without sulfur-based carrier, respectively. The removal efficiency of NO₃^--N exhibited a similar trend to that of T-N. According to the results, the removal of T-N and NO₃^--N was noticeably enhanced to approximately 30% by adding a sulfur-based carrier to the A₂O pilot system. In addition, microbial community in both reactors was dominated by Thiobacillus, which is an autotrophic microorganism, displaying a dominant denitrification status. The present study compared the relative efficiencies of nitrate removal in A₂O pilot reactors with and without sulfur-based carriers for its successful application in real-scale autotrophic denitrification.

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.

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.

Exploration and verification of the feasibility of the sulfur-based autotrophic denitrification integrated biomass-based heterotrophic denitrification systems for wastewater treatment: From feasibility to application

The sulfur-based autotrophic denitrification (SAD) and the solid organic carbon-based denitrification processes are both efficient techniques to remove nitrate from wastewater, and the hydrogen ions generated by the SAD process would be consumed in the heterotrophic denitrification process. Therefore, it is possible to improve the denitrification capacity when the solid organic carbon was added into a SAD reactor. In this study, corncob powder and sawdust powder were selected as solid organic carbon sources, and the sulfur-based autotrophic denitrification integrated biomass-based heterotrophic denitrification system was formed (SBD). The laboratory and field experiments showed that SBD could shorten the start-up period, decrease the sulfate productivity, and maintain a good denitrification performance when treated wastewater. According to the field experiment results, when the HRT was 1 h, the effluent total nitrogen (TN) concentration was always lower than 15 mg L^-1. In addition, nitrite inhibition was observed when the concentration of nitrite in the reactors reached above 30 mg L^-1. The mixture of elemental sulfur powder, shell powder, corncob powder, and sawdust powder with a mass ratio of 6:2:1:1 was the optimal filter for the SBD system, with an average nitrate reduction rate (NAR) of 420 mg NO₃^-N·L^-1·d^-1 obtained at the end of the study. During the whole operation, the major autotrophs in the SBD systems were Thermomonas, Ferritrophicum, and Thiobacillus, while the major heterotrophs were Saprospiraceae, Ferruginibacter, Dokdonella, and Simplicispira. Overall, the SBD system was a feasible and practically favorable way to remove nitrate from wastewater.

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.

Metagenomics reveals functional species and microbial mechanisms of an enriched thiosulfate-driven denitratation consortia

In this study, thiosulfate-driven denitratation (TDD) system was successfully established under optimal S/N molar ratio of 1.00, with nitrite accumulation efficiency (NAE) of 82.24 ± 17.09%. This work highlighted that thiosulfate significantly preferred the reduction of nitrate than nitrite. However, after the depletion of thiosulfate, the in-situ formed intermediate product element sulfur (S⁰) served as the main electron donor, and significantly favored the reduction of nitrite than nitrate, which constrained nitrite accumulation and nitrate removal. In addition, metagenomic sequencing revealed that the functional denitratation species might be Thiobacillus_sp._65-29, but the occurrence of Nir-annotated species would decrease nitrite accumulation. Under S/N ratio of 1.00, the decreased abundant Nir-annotated species (e.g., Thiobacillus_sp.), as well as the down-regulated quorum sensing interactions between Nar- and Nir-annotated species were key microbial metabolisms of high NAE in the TDD system. Overall, this work provides new sight into the metagenome-base functional species and metabolic potential of thiosulfate-driven denitratation.

Denitrification performance and mechanism of biofilter constructed with sulfur autotrophic denitrification composite filler in engineering application

Sulfur autotrophic denitrification (SAD) is a promising technology due to its low cost and low sludge production. Based on previous studies on SAD materials as well as the denitrification mechanism of SAD technology, this study constructed two biofilters with a sulfur autotrophic denitrification composite filler (SADCF) to investigate the application potential of SAD technology. The feasibility of a SADCF-based biofilter was demonstrated, with a maximum nitrate volume load of 0.75 kg N/(m³·d) and low accumulation of nitrite and ammonium. In addition, an improved backwashing method (air-water backwashing) was obtained by comparing two different backwashing methods. Furthermore, some iron reducing bacteria (0.4% Geothrix) along with a rapid proliferation of the main sulfur-oxidizing bacteria (23.0% Thiobacillus and 27.7% Ferritrophicum) were found under real-world operating conditions. Overall, the results of this study provide a case reference for the operation of SADCF-based biofilters and the application of SAD technology in engineering.

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.

Using a sulfur autotrophic fluidized bed reactor for simultaneous perchlorate and nitrate removal from water: S disproportionation prediction and system optimization

The sulfur autotrophic reduction (SAR) process is promising in co-reduction of perchlorate and nitrate from aqueous solution. To further understand the reaction process, we developed a sulfur autotrophic fluidized bed reactor where the proceeding extent of sulfur (S) disproportionation was predicted by Response surface methodology (RSM) for the first time. Three fundamental reaction parameters including the hydraulic retention time (HRT), co-existing nitrate concentration ([Formula: see text]) and recirculation ratio (R) were considered for reactor optimization. The results demonstrated that S disproportionation was promoted by long HRT and high R, whereas was inhibited by high [Formula: see text]. Also, the optimal HRT, [Formula: see text] and R were 0.50 h, 10.00 mg/L and 14, respectively, the bioreactor can achieve high reduction efficiency of perchlorate and nitrate (> 98.45%), and generate less sulfate (236.07 mg/L). High-throughput sequencing showed that Chlorobaculum was related to S disproportionation, and Sulfurovum was associated with nitrate/perchlorate reducing. All results indicate that the sulfur autotrophic fluidized bed reactor is a promising candidate for the treatment of perchlorate and nitrate wastewater in future practical applications.

Effects of coarse and fine bubble aeration on performances of membrane filtration and denitrification in moving bed membrane bioreactors

In this study, two lab-scale Moving Bed Membrane Bioreactors (MBMBR) were setup and operated in parallel to study the effect of coarse and fine bubble aeration on the performances of membrane filtration and denitrification treating domestic wastewater. The bacterial populations in the two MBMBRs were further analyzed to investigate the mechanisms involved in the different denitrification performances. The results showed that coarse bubble aeration could effectively mitigate membrane fouling by decreasing the formation of cake layer, although smaller sizes of bio-flocs were induced. In addition, coarse bubble aeration could also maintain dissolved oxygen (DO) at a relatively lower level without compromising the moving of bio-carriers, which achieved 10% higher total nitrogen removal rate due to anoxic zone created at inner layers of biofilms on bio-carriers. Accumulation of denitrifier (Thiobacillus denitrificans) on the bio-carriers was found under the coarse bubble aeration system, which can explain its superior denitrification performance.

Enrichment of Autotrophic Denitrifiers From Anaerobic Sludge Using Sulfurous Electron Donors

This study compared the rates and microbial community development in batch bioassays on autotrophic denitrification using elemental sulfur (S0), pyrite (FeS2), thiosulfate (S2O3 2-), and sulfide (S2-) as electron donor. The performance of two inocula was compared: digested sludge (DS) from a wastewater treatment plant of a dairy industry and anaerobic granular sludge (GS) from a UASB reactor treating dairy wastewater. All electron donors supported the development of a microbial community with predominance of autotrophic denitrifiers during the enrichments, except for sulfide. For the first time, pyrite revealed to be a suitable substrate for the growth of autotrophic denitrifiers developing a microbial community with predominance of the genera Thiobacillus, Thioprofundum, and Ignavibacterium. Thiosulfate gave the highest denitrification rates removing 10.94 mM NO3 - day-1 and 8.98 mM NO3 - day-1 by DS and GS, respectively. This was 1.5 and 6 times faster than elemental sulfur and pyrite, respectively. Despite the highest denitrification rates observed in thiosulfate-fed enrichments, an evaluation of the most relevant parameters for a technological application revealed elemental sulfur as the best electron donor for autotrophic denitrification with a total cost of 0.38 € per m3 of wastewater treated.

Achieving rapid thiosulfate-driven denitrification (TDD) in a granular sludge system

Sulfur-oxidizing bacteria (SOB) can drive a high level of autotrophic denitrification (AD) activity with thiosulfate (S₂O₃^2-) as the electron donor. However, the slow growth of SOB results in a low biomass concentration in the AD reactor and unsatisfactory biological nitrogen removal (BNR). In this study, our goal was to establish a high-rate thiosulfate-driven denitrification (TDD) system via sludge granulation. Granular sludge was successfully cultivated by increasing the nitrogen loading rate stepwise in thiosulfate-oxidizing/nitrate-reducing conditions in an upflow anaerobic blanket reactor. In the mature-granular-sludge reactor, a nitrate removal rate of 280 mg N/L/h was achieved with a nitrate removal efficiency of 97.7%±1.0% at a hydraulic retention time of only 15 minutes, with no nitrite detected in the effluent. Extracellular polymeric substance (EPS) analysis indicated that the proteins in loosely bound and tightly bound EPS were responsible for maintaining the compact structure of the TDD granular sludge. The dynamics of the microbial-community shift were identified by 16S rRNA high-throughput pyrosequencing analysis. The Sulfurimonas genus was found to be enriched at 74.1% of total community and may play the most critical role in the high-rate BNR. The batch assay results reveal that no nitrite accumulation occurred during nitrate reduction because the nitrate reduction rate (75.90±0.67 mg N/g MLVSS/h) was almost equal to the nitrite reduction rate (66.06±1.28 mg N/g MLVSS/h) in the thiosulfate-driven granular sludge reactor. The results of this study provide support for the establishment of a high-rate BNR system that maintains its stability with a low sludge yield.

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.

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.

An efficient way to achieve stable and high-rate ferrous ion-dependent nitrate removal (FeNiR): Batch sludge replacement

Ferrous ion can be used as electron donor for denitrification in the ferrous ion-dependent nitrate removal (FeNiR). To prevent the FeNiR performance decrease caused by iron encrustation, a modified FeNiR process with batch sludge replacement was developed. Based on the decay kinetics of sludge mass and sludge activity, the sludge retention time (SRT) was determined as 40 days in the modified FeNiR process. To keep the FeNiR rate at 0.70 kg-N/(m³·d), the sludge replacement amount was 25% of total sludge every 10 days. The FeNiR efficiency stabilized around 70%. The batch sludge replacement could be an effective method to offset the active sludge decay caused by iron encrustation, and therefore led to the good FeNiR performance. The wasted FeNiR sludge was found to adsorb phosphate at a rate of 0.9 mg-P/(g VS min). The modified FeNiR process was proposed to be coupled with phosphate removal, achieving the co-removal of nitrate and phosphate. The coupled technology is promising due to the less consumption of resources and energy, as well as the less production of excessive sludge.

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.

Iron as electron donor for denitrification: The efficiency, toxicity and mechanism

For the treatment of low C/N wastewaters, methanol or acetate is usually dosed as electron donor for denitrification but such organics makes the process costly. To decrease the cost, iron which is the fourth most abundant element in lithosphere is suggested as the substitution of methanol and acetate. The peak volumetric removal rate (VRR) of nitrate nitrogen in the ferrous iron-dependent nitrate removal (FeNiR) reactor was 0.70 ± 0.04 kg-N/(m³·d), and the corresponding removal efficiency was 98%. Iron showed toxicity to cells by decreasing the live cell amount (dropped 56%) and the live cell activity (dropped 70%). The toxicity of iron was mainly expressed by the formation of iron encrustation. From microbial community data analysis, heterotrophs (Paracocccus, Thauera and Azoarcus) faded away while the facultative chemolithotrophs (Hyphomicrobium and Anaerolineaceae_uncultured) dominated in the reactor after replacing acetate with ferrous iron in the influent. Through scanning electron microscope (SEM) and transmission electron microscope (TEM), two iron oxidation sites in FeNiR cells were observed and accordingly two FeNiR mechanisms were proposed: 1) extracellular FeNiR in which ferrous iron was bio-oxidized extracellularly; and 2) intracellular FeNiR in which ferrous iron was chemically oxidized in periplasm. Bio-oxidation (extracellular FeNiR) and chemical oxidation (intracellular FeNiR) of ferrous iron coexisted in FeNiR reactor, but the former one predominated. Comparing with the control group without electron donor in the influent, FeNiR reactor showed 2 times higher and stable nitrate removal rate, suggesting iron could be used as electron donor for denitrification. However, further research works are still needed for the practical application of FeNiR in wastewater treatment.

Pilot-scale application of sulfur-limestone autotrophic denitrification biofilter for municipal tailwater treatment: Performance and microbial community structure

This work aimed to study a pilot-scale sulfur-limestone autotrophic denitrification biofilter (SLADB) to remove nitrogen from municipal tailwater. The capacity of nitrogen removal and spatial distribution of microbial community at low temperature condition were analyzed. Low temperature inhibits nitrogen removal; while prolonging hydraulic retention time (HRT) increased nitrogen removal efficiency. TN and NO₃^--N removal efficiency reached 81.1% and 85.3%, respectively, with HRT of 18 h at the temperature ranging from 6.4 to 9.8 °C. Proteobacteria and Chloroflexi were two dominant phyla. Along the reactor, class β-proteobacteria and ε-proteobacteria decreased, while γ-proteobacteria and Acidobacteria increased. For genus classification, Thiobacillus, Sulfurimonas, and Ferritrophicum which promote sulfur autotrophic denitrification, decreased significantly. While Anaerolineae promoting heterotrophic denitrification increased obviously. Sphingobacteriia coexisted in SLADB and were beneficial to nitrogen removal. Microbial community spatial distribution patterns were related to nitrogen removal. This study achieved reliable pilot-scale application of SLADB under low temperature for municipal tailwater.

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.

Coupling of sulfur(thiosulfate)-driven denitratation and anammox process to treat nitrate and ammonium contained wastewater

This study investigated the feasibility of a new biological nitrogen removal process that integrates sulfur-driven autotrophic denitratation (NO₃^-→NO₂^-) and anaerobic ammonium oxidation (Anammox) for simultaneous removal of nitrate and ammonium from industrial wastewater. The proposed sulfur(thiosulfate)-driven denitratation and Anammox process was developed in two phases: First, the thiosulfate-driven denitratation was established in the UASB inoculated with activated sludge and fed with ammonium, nitrate and thiosulfate for 52 days until the nitrite level in the effluent reached 32.1 mg N/L. Second, enriched Anammox biomass was introduced to the UASB to develop the integrated thiosulfate-driven denitratation and Anammox (TDDA) bioprocess (53-212 d). Results showed that nitrate and ammonium could be efficiently removed from synthetic wastewater by the integrated TDDA system at a total nitrogen (TN) removal efficiency of 82.5 ± 1.8% with an influent NH₄⁺-N of 101.2 ± 2.2 mgN/L, NO₃^--N of 101.1 ± 1.5 mgN/L and thiosulfate of 202.5 ± 3.2 mg S/L. It was estimated that Anammox and autotrophic denitritation (NO₂^-→N₂) contributed to about 90% and 10% of the TN removal respectively at stable operation. The established TDDA system was further supported by high-throughput sequencing analysis that sulfur-oxidizing bacteria (e.g., Thiobacillus and Sulfurimonas) coexisted with Anammox bacteria (e.g., Ca. Kuenenia and Ca. Anammoxoglobus) in this syntrophic biocenosis. Additionally, batch experiments were conducted to reveal the kinetic rates and to reconcile the stoichiometry of the electron donor/acceptor couples of the TDDA process. The results unraveled the mechanisms in the new bioprocess: i) sulfite and elemental sulfur (S⁰) were initially generated from branched thiosulfate; ii) oxidation of sulfite and elemental sulfur coupled with fast and slow denitratation; iii) nitrite produced from denitratation together with ammonium were effectively converted to dinitrogen gas via Anammox.

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.

Biological perchlorate reduction: which electron donor we can choose?

Biological reduction is an effective method for removal of perchlorate (ClO₄^-), where perchlorate is transformed into chloride by perchlorate-reducing bacteria (PRB). An external electron donor is required for autotrophic and heterotrophic reduction of perchlorate. Therefore, plenty of suitable electron donors including organic (e.g., acetate, ethanol, carbohydrate, glycerol, methane) and inorganic (e.g., hydrogen, zero-valent iron, element sulfur, anthrahydroquinone) as well as the cathode have been used in biological reduction of perchlorate. This paper reviews the application of various electron donors in biological perchlorate reduction and their influences on treatment efficiency of perchlorate and biological activity of PRB. We discussed the criteria for selection of appropriate electron donor to provide a flexible strategy of electron donor choice for the bioremediation of perchlorate-contaminated water.

Nutrient removal through pyrrhotite autotrophic denitrification: Implications for eutrophication control

This study investigated nutrient removal kinetics and main influencing factors of natural pyrrhotite autotrophic denitrification (PAD), and nutrient removal performance and the microbial community in the PAD biofilter (PADB). Results demonstrated that both NO₃^- and PO₄^3- in wastewater were nearly completely removed, and biological N removal and chemical P removal took place simultaneously in the PAD process. NO₃^- removal kinetics of the PAD can be described with half-order kinetics. The PAD was effective across a wide temperature range of 11-34 °C, initial NO₃^--N range of 13-52 mg·L^-1 and PO₄^3--P below 60 mg·L^-1. Both NO₃^- and PO₄^3- decreased gradually with wastewater flowing along the PADB. The PADB operation results show that at 12 h HRT, when the PADB treated wastewater containing 30.95 ± 0.97 mg·L^-1 of NO₃^- and 3.02 ± 0.10 mg·L^-1 of PO₄^3--P, the effluent contained 1.15 ± 2.08 and 0.09 ± 0.11 mg·L^-1 of NO₃^--N and PO₄^3--P on average, respectively. In the PADB the dominant bacteria were Thiobacillus and Sulfurimonas, which used pyrrhotite as the electron donor to reduce NO₃^-. The relative abundance of Thiobacillus at the bottom of the PADB increased from 0.81% to 58.65% and that of Sulfurimonas decreased from 97.22% to 12.30%, with exposure to pyrrhotite. From the bottom to the top of the PADB, the relative abundance of Thiobacillus increased from 58.65% to 86.23% and Sulfurimonas decreased from 12.30% to 0.52%. Technologies based on the PAD are promising ways to control eutrophication.

Sulfur disproportionation tendencies in a sulfur packed bed reactor for perchlorate bio-autotrophic reduction at different temperatures and spatial distribution of microbial communities

This study investigates the sulfur (S) disproportionation tendencies in a sulfur packed bed reactor for perchlorate bio-autotrophic reduction at different temperatures. The reactor was operated with over 99% efficiency for 21.00 ± 1.40 mg L^-1 perchlorate removal when the hydraulic retention time (HRT) ranged from 12.00 h to 0.75 h at 27 ± 2 °C. When HRT was controlled at 1.00 h, the perchlorate removal efficiency was only 8 ± 1% as the temperature dropped to 6 ± 1 °C. The half-order model fit both perchlorate removal and S disproportionation reaction well. Compared with S disproportionation, the decrease of temperature had a greater influence on perchlorate reduction. As the temperature dropped from 27 ± 2 °C to 6 ± 1 °C, the 1/2K_1/2v,R for perchlorate reduction decreased from 7.37 mg^1/2 L^-1/2 h^-1 to 0.19 mg^1/2 L^-1/2 h^-1. Meanwhile, the 1/2K_1/2v,S for S disproportionation decreased from 3.04 mg^1/2 L^-1/2 h^-1 to 1.96 mg^1/2 L^-1/2 h^-1. The reaction activation energy of perchlorate reduction and S disproportionation was 120.28 kJ mol^-1 and 13.44 kJ mol^-1, respectively. The S disproportionation reaction proceeded remarkably at the beginning of the reduction, a longer HRT and higher temperature promoted S disproportionation, resulting in excessive sulfate generation and alkalinity consumption. Besides, the spatial distribution of the microbial communities and the dominant bacteria function under different HRTs was analyzed using high-throughput sequencing.

Bio-mixotrophic perchlorate reduction to control sulfate production in a step-feed sulfur-based reactor: A study of kinetics, ORP and bacterial community structure

Excess sulfate production and low concentration of perchlorate removal are the main problems for sulfur-based perchlorate reduction reactor. In this study, the problems were firstly solved by step-feeding under mixotrophic conditions. The performances of step-feed sulfur-based reactor (SFSBR) and up-flow sulfur-based reactor (UFSBR) are compared. At perchlorate of 194 mg/L, acetate of 28.8 mg/L and hydraulic retention time of 0.9 h, the Half-order reaction rate constant and the sulfate production of SFSBR were 29.7 mg^1/2/L^1/2·h and 171 mg/L, respectively, which were superior to those of UFSBR. The oxidation-reduction potential values of SFSBR were lower than that of UFSBR. Meanwhile, the biodiversity along the height of the reactor was decreased by step-feeding. Principal component analysis showed significant interrelations existed among the bacterial community composition and the operational/environmental conditions in each treatment zone. Consequently, the SFSBR provides an effectively alteration for the removal of high perchlorate concentration and control sulfate.

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.

Microbial Photoelectrotrophic Denitrification as a Sustainable and Efficient Way for Reducing Nitrate to Nitrogen

Biological removal of nitrate, a highly concerning contaminant, is limited when the aqueous environment lacks bioavailable electron donors. In this study, we demonstrated, for the first time, that bacteria can directly use the electrons originated from the photoelectrochemical process to carry out the denitrification. In such photoelectrotrophic denitrification (PEDeN) systems (denitrification biocathode coupling with TiO₂ photoanode), nitrogen removal was verified solely relying on the illumination dosing without consuming additional chemical reductant or electric power. Under the UV illumination (30 mW·cm^-2, wavelength at 380 ± 20 nm), nitrate reduction in PEDeN apparently followed the first-order kinetics with a constant of 0.13 ± 0.023 h^-1. Nitrate was found to be almost completely converted to nitrogen gas at the end of batch test. Compared to the electrotrophic denitrification systems driven by organics (OEDeN, biocathode/acetate consuming bioanode) or electricity (EEDeN, biocathode/abiotic anode), the denitrification rate in PEDeN equaled that in OEDeN with a COD/N ratio of 9.0 or that in EEDeN with an applied voltage at 2.0 V. This study provides a sustainable technical approach for eliminating nitrate from water. PEDeN as a novel microbial metabolism may shed further light onto the role of sunlight played in the nitrogen cycling in certain semiconductive and conductive minerals-enriched aqueous environment.