Mathematical modeling of autotrophic denitrification (AD) process with sulphide as electron donor

Isotope analysis of nitrogen removal pathways and N2O production potential in the SDAD-anammox system under different N/S ratios

This study explored the impact of varying nitrate to sulfide (N/S) ratios on nitrogen removal efficiency (NRE) in the sulfide-driven autotrophic denitrification and anammox (SDAD-anammox) system. Optimal nitrogen removal was observed at N/S ratios between 1.5 and 2.0. Isotope tracing results showed that the contribution of anammox to nitrogen removal was enhanced with increasing N/S ratios, reaching up to 37 % at the N/S ratio of 2.5. Additionally, complex nitrogen pathways were identified, including dissimilatory nitrate reduction to ammonium (DNRA). Furthermore, isotope tracing was innovatively applied to investigate N2O emissions, demonstrating that higher N/S ratios significantly reduced N2O emissions, with the lowest emissions at N/S ratio of 2.5. Gene expression analysis indicated that nitrogen and sulfide transformation genes decreased with increasing N/S ratios, while anammox-related genes first increased and then decreased, reflecting the system's microbial dynamics. These findings offer insights into nitrogen transformation pathways and N2O production mechanisms in the SDAD-anammox process.

Integrated control strategy for dual sludge ages in the high-concentration powder carrier bio-fluidized bed (HPB) technology: Enhancing municipal wastewater treatment efficiency

The high-concentration powder carrier bio-fluidized bed (HPB) technology is an emerging approach that enables on-site upgrading of wastewater treatment plants (WWTPs). HPB technology promotes the formation of biofilm sludge with micron-scale composite powder carriers as the core and suspended sludge mainly composed of flocs surrounding the biofilm sludge. This study proposed a novel integrated strategy for assessing and controlling the sludge ages in suspended/bio-film activated sludge supported by micron-scale composite powder carrier. Utilizing the cyclone unit and the corresponding theoretical model, the proposed strategy effectively addresses the sludge ages contradiction between denitrifying bacteria and polyphosphate-accumulating organisms (PAOs), thereby enhancing the efficiency of municipal wastewater treatment. The sludge age of the suspended (25 d) and bio-film (99 d) sludge, calculated using the model, contribute to the simultaneous removal of nitrogen and phosphorus. Meanwhile, the model further estimates distinct contributions of suspended and bio-film sludge to chemical oxygen demand (COD) and total nitrogen (TN), which are 55% and 42% for COD, 20% and 57% for TN of suspended sludge and bio-film sludge, respectively. This suggests that the contribution of suspended sludge and bio-film sludge to COD and TN removal efficiency can be determined and controlled by the operational conditions of the cyclone unit. Additionally, the simulation values for COD, ammonia nitrogen (NH4+-N), TN and total phosphorus (TP) closely align with the actual values of WWTPs over 70 days (p < 0.001) with the correlation coefficients (R2) of 0.9809, 0.9932, 0.9825, and 0.837, respectively. These results support the theoretical foundation of HPB technology for simultaneous nitrogen and phosphorus removal in sewage treatment plants. Therefore, this model serves as a valuable tool to guide the operation, design, and carrier addition in HPB technology implementation.

Sulphate reduction, mixed sulphide- and thiosulphate-driven Autotrophic denitrification, NItrification, and Anammox (SANIA) integrated process for sustainable wastewater treatment

This study proposes the Sulphate reduction, mixed sulphide- and thiosulphate-driven Autotrophic denitrification, Nitrification, and Anammox integrated (SANIA) process for sustainable treatment of mainstream wastewater after organics capture. Three moving-bed biofilm reactors (MBBRs) were applied for developing sulphate reduction (SR), mixed sulphide- and thiosulphate-driven partial denitrification and Anammox (MSPDA), and NItrification (N), respectively. Typical mainstream wastewater after organics capture (e.g., chemically enhanced primary treatment, CEPT) was synthesized with chemical oxygen demand (COD) of 110 mg/L, sulphate of 50 mg S/L, ammonium of 30 mgN/L. The feasibility of SANIA was investigated with mimic nitrifying effluent supplied in MSPDA-MBBR (Period I), followed by the examination of the applicability of SANIA process with N-MBBR integrated (Period II), under moderate temperatures (25-27 ℃). In Period I, SANIA process was established with both SR- and MSPDA-MBBR continuously operated for over 300 days (no Anammox biomass inoculation). Specifically, in MSPDA-MBBR, high rates of denitratation (2.7 gN/(m2·d)) and Anammox (2.8 gN/(m2·d)) were achieved with Anammox contributing to 81 % of the total inorganic nitrogen removal. In Period II, the integrated SANIA system was continuously operated for over 130 days, achieving up to 90 % of COD, 93 % of ammonium, and 61 % of total inorganic nitrogen (TIN) removal, with effluent concentrations lower than 10 mg COD/L, 3 mg NH4+-N/L, and 13 mg TIN-N/L. The implementation of SANIA can ultimately reduce 75 % and 40 % of organics and aeration energy for biological nitrogen removal. Considering the combination of SANIA with CEPT for carbon capture and sludge digestion/incineration for energy recovery, the new integrated wastewater technology can be a promising strategy for sustainable wastewater treatment.

Machine learning-based model construction and identification of dominant factor for simultaneous sulfide and nitrate removal process

Accurate water quality prediction models are essential for the successful implementation of the simultaneous sulfide and nitrate removal process (SSNR). Traditional models, such as regression and analysis of variance, do not provide accurate predictions due to the complexity of microbial metabolic pathways. In contrast, Back Propagation Neural Networks (BPNN) has emerged as superior tool for simulating wastewater treatment processes. In this study, a generalized BPNN model was developed to simulate and predict sulfide removal, nitrate removal, element sulfur production, and nitrogen gas production in SSNR. Remarkable results were obtained, indicating the strong predictive performance of the model and its superiority over traditional mathematical models for accurately predicting the effluent quality. Furthermore, this study also identified the crucial influencing factors for the process optimization and control. By incorporating artificial intelligence into wastewater treatment modeling, the study highlights the potential to significantly enhance the efficiency and effectiveness of meeting water quality standards.

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.

Integrating thermodynamics and mathematical modelling to investigate the stoichiometry and kinetics of sulphide oxidation-nitrate reduction with a special focus on partial autotrophic denitrification

In the present study, the stoichiometry of the Sulphur Oxidizing-Nitrate Reducing (SO-NR) process, with a focus on Partial Autotrophic Denitrification (PAD), has been evaluated through a thermodynamic-based study whereas a model-based approach has been adopted to assess process kinetics. Experimental data on process performance and biomass yields were available from a previous work achieving efficient PAD, where a biomass yield of 0.113 gVSS/gS was estimated. First, the free Gibbs energy dissipation method has been implemented, in order to provide a theoretical framework exploring the boundaries for sulphur oxidizing biomass yields. Second, a screening of available mathematical models describing SO-NR process was conducted and five published models were selected, in order to assess the most suitable model structure for describing the observed PAD kinetics. To the best of our knowledge, none of reported biomass yields are estimated in systems operating PAD as the main process and, analogously, none of the proposed models have been applied to case studies aiming at partial denitrification only. The work showed that the very low biomass yield of 0.117 ± 0.007 gVSS/gS, observed in a PAD system in our previous work, suggests that the conditions applied to achieve partial denitrification resulted in a high energy-dissipating metabolism compared to complete denitrification applications. Models' analysis revealed that nitrite accumulation can be described by a classical Monod kinetics if different μmax are adopted for each intermediate reaction, with Theil Inequality Coefficient values lower than 0.21 for both NO3- and NO2-. Nonetheless, adopting Haldane-type kinetics for nitrite uptake inferred higher identifiability to the model structure, resulting in confidence intervals below ±10% for all the parametric estimations. The thermodynamic and modelling outcomes support the experimental results obtained in the reference study and the critical comparison of model suitability to represent PAD process is believed pivotal to pave the way to its real-scale implementation.

Physics-informed neural network-based serial hybrid model capturing the hidden kinetics for sulfur-driven autotrophic denitrification process

Sulfur-driven autotrophic denitrification (SdAD) is a biological process that can remove nitrate from low carbon/nitrogen (C/N) ratio wastewater. Although this process has been intensively researched, the mechanism whereby its intermediates (i.e., elemental sulfur and nitrite ions) are generated and accumulated remains elusive. Existing mathematical models developed for SdAD cannot accurately predict the intermediates in SdAD because of the incomplete knowledge of process kinetic resulting from changes in the environmental conditions and electron competition during SdAD. To address this limitation, we proposed a novel serial hybrid model structure based on a physics-informed neural network (PINN) to capture the dynamics of the process kinetics and predict the substrate concentrations in SdAD. In this study, we evaluated the model through numerical experiments and applied it to real case studies involving batch and continuous-flow reactor scenarios. By leveraging the PINN approach, the hybrid model yielded accurate predictions at both the state (i.e. substrate concentration) and kinetic levels in the numerical experiments and performed better than both mechanistic and purely data-driven models in the case studies. Furthermore, we used the trained hybrid model to design control strategies for SdAD and a novel integrated process involving SdAD and anammox for energy-efficient nitrogen removal. Finally, we discuss the advantages and application scope of the PINN-based hybrid model.

Decryption for nitrogen removal in Anammox-based coupled systems: Nitrite-induced mechanisms

This study investigated the effects of NO₂^- on synergetic interactions between Anammox bacteria (AnAOB) and sulfur-oxidizing bacteria (SOB) in an autotrophic denitrification-Anammox system. The presence of NO₂^- (0-75 mg-N/L) was shown to significantly enhance NH₄⁺ and NO₃^- conversion rates, achieving intensified synergy between AnAOB and SOB. However, once NO₂^- exceed a threshold concentration (100 mg-N/L), both NH₄⁺ and NO₃^- conversion rates decreased with increased NO₂^- consumption via autotrophic denitrification. The cooperation between AnAOB and SOB was decoupled due to the NO₂^- inhibition. Improved system reliability and nitrogen removal performance was achieved in a long-term reactor operation with NO₂^- in the influent; reverse transcription-quantitative polymerase chain reaction analysis showed elevated hydrazine synthase gene transcription levels (5.00-fold), comparing to these in the reactor without NO₂^-. This study elucidated the mechanism of NO₂^- induced synergetic interactions between AnAOB and SOB, providing theoretical guidance for engineering applications of Anammox-based coupled systems.

The efficient utilization of thiocyanate on simultaneous removal of ammonium and nitrate through thiosulfate-driven autotrophic denitrifiers and anammox

The efficient utilization of thiocyanate remain be an important bottleneck in the low-cost nitrogen removal for wastewaters containing thiocyanate. The study aimed to investigate the feasibility of thiocyanate in removal of nitrate and ammonium through anammox (AN) and thiosulfate-driven autotrophic denitrifiers (TSAD). The results showed that removal of nitrate and ammonium were achieved rapidly utilizing thiocyanate, which was attributed to degradation of thiocyanate by TSAD and cooperation with AN. The utilization efficiency of thiocyanate in nitrogen removal was increased by 250% due to the microbial cooperation. Excess thiocyanate and ammonium did not influence the nitrogen removal amount. However, the nitrogen removal were affected obviously by the biomass ratio (X_AN/X_TSAD) between AN and TSAD Moreover, the dynamics related to removal of pollutants was described successfully by a modified Monod model with time constraints. These findings offer an insight for efficient utilization of thiocyanate in nitrogen removal via microbial cooperation.

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.

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.

Fe(Ⅱ) improving sulfurized Anammox coupled with autotrophic denitrification performance: Based on interspecies and intracellular electron transfer

Insufficient nitrite supply and slow metabolism of Anammox bacteria (AnAOB) impeded the application of Anammox process in low level ammonia (LLA) (≤50 mg/L) wastewater. At the initial concentration of 50 mg/L NH4+-N and 75 mg/L NO3--N, Fe(Ⅱ) (10 mg/L) promoted the total nitrogen removal efficiency from 80.79 to 94.92 % by core-shell sulfurized AnAOB coupled with sulfur oxidizing bacteria (S0@AnAOB + SOB). AnAOB outcompeted SOB for nitrite, because the addition of Fe(Ⅱ) not only increased the nitrate reductase activity (37.54 %), but also enhanced the metabolism and electron capture ability of AnAOB, which was highly related with energy metabolic process: hydrazine dehydrogenase activity increased to 139.00 %. Particularly, Fe(Ⅱ) accelerated the interspecies electron transfer (INET) (from SOB to AnAOB) by stimulating the secretion of redox species and electron hopping in EPS. This study shed light on the mechanism of Fe(Ⅱ) promoting electron transfer in S0@AnAOB + SOB system, and provided basis for engineering practice.

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.

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.

Response of simultaneous sulfide and nitrate removal process on acute toxicity of substrate concentration and salinity: Single toxicity and combined toxicity

Simultaneous sulfide and nitrate removal process has performed excellent to treat nitrogen and sulfur pollutants in wastewater treatment. A high salinity stress poses a great challenge to the treatment of highly saline wastewater containing nitrate and sulfide. In addition, sulfide and nitrates are also toxic for the process, and their high concentration would inhibit the process. Therefore, the current work explores the single acute toxic effect and combined toxic effect of salinity and substrate concentration on the performance of the process from the perspective of toxicology. Considering sulfide and nitrate removal performance as an indicator, the IC₅₀ values of sulfide were 293.20 mg S/L and 572.30 mg S/L, respectively; while those of salinity were 6.14% wt (91.78 mS/cm) and 6.63% wt (98.73 mS/cm), respectively. High substrate concentration or salinity resulted in elemental sulfur generation. The molar ratio of generated elemental sulfur to consumed sulfide(R-Sulfate) was close to 1. The response of nitrate reduction product to the elevating substrate concentration was not obvious, while its response to increasing salinity was on the contrary. With the increasing salinity (1.2% wt to 9.6% wt), molar ratio of generated nitrogen gas to consumed nitrate (R-Nitrogen gas) increased from 0.58 to 1, while molar ratio of generated nitrite to consumed nitrate (R-Nitrite) decreased from 0.43 to 0. Factorial analysis test revealed that the combined acute toxicity of substrate and salinity on sulfide oxidization and nitrate reduction were both antagonistic effects.

Modeling of sulfur-driven autotrophic denitrification coupled with Anammox process

While sulfur-driven autotrophic denitrification (SDAD) occurring in the anoxic reactor of the sulfate reduction, autotrophic denitrification and nitrification integrated (SANI) system has been regarded as the main nitrogen removal bioprocess, little is known about the accompanying Anammox bacteria whose presence is made possible by the co-existence of NH₄⁺ and NO₂^-. Therefore, this work firstly developed an integrated SDAD-Anammox model to describe the interactions between sulfur-oxidizing bacteria and Anammox bacteria. The model was subsequently used to explore the impacts of influent conditions on the reactor performance and microbial community structure of the anoxic reactor. The results revealed that at a relatively low ratio of <1.5 mg S/mg N, Anammox bacteria could survive and even take a dominant position (up to 58.9%). Finally, a modified SANI system configuration based on the effective collaboration between SDAD and Anammox processes was proposed to improve the efficiency of the treatment of sulfate-rich saline sewage.

Efficient nitrite accumulation and elemental sulfur recovery in partial sulfide autotrophic denitrification system: Insights of seeding sludge, S/N ratio and flocculation strategy

Partial sulfide autotrophic denitrification (PSAD) has been proposed as a promising process to achieve elemental sulfur recovery and nitrite accumulation, which is required for anaerobic ammonium oxidation reaction. This study investigated the effect of seeding sludge on the start-up performance of PSAD process, with different sludge taken from the oxidation zone (S-o) of wastewater treatment plants, partial denitrification reactor (S-_PD), and anoxic/oxic reactor (S-_A/O). The results showed that the PSAD process could be achieved rapidly in three systems on day 22, 29 and 26, respectively. In particular, the S-_O system completed the start-up in the shortest time of 22 d, with NO₃^--N and S^2- removal efficiency of 85.3% and 99.3%, respectively. Selected the S-_O system to operate long term, the nitrite (NO₂^--N) and biological elemental sulfur (S⁰) accumulation efficiencies were systematically investigated under different S/N ratios (in a range of 0.71-1.2). The maximum NO₂^--N and S⁰ accumulation efficiencies were 85.2% and 73.5%, respectively, at the S/N ratio of 1.1. In addition, the separation and recovery of S⁰ in effluent was achieved by employing polyaluminum chloride (PAC) as a flocculant. Using 2D Gaussian function as quadratic model for the maximizing of S⁰ flocculant efficiency (SFR), an optimal condition of PAC dosage 7.92 mL/L and pH 5.14 was obtained, and the SFR reached 94.1%, under such conditions. The findings offered useful information to facilitate the application of the PSAD process.

Cultivation of sulfide-driven partial denitrification granules for efficient nitrite generation from nitrate-sulfide-laden wastewater

Sulfide partial denitrification (SPD) is an alternative pathway for nitrite production accompanied with elemental sulfur (S⁰) production for nitrate removal from wastewater with anammox. In this study, the SPD granular sludge was cultivated for the first time in an upflow anaerobic sludge bed (UASB) reactor to reach the efficacy of maximum nitrate-to-nitrite transformation ratio of 92% and an in-situ maximum NO₃^--N reduction rate of 2.46 kg-N/m³-d, both much higher than literature results. Mature granules had an average particle size of 2.52 mm and hold smooth surface with excess rod bacteria. The elements Ca and S, and proteins in extracellular polymeric substances contributed to granule structure's stability. Enriched Thiobacillus genus was proposed to accumulate nitrite at moderate HRT (2-6 h). The immobilized functional strains assist efficient partial nitrification reactions to be realized with formed S⁰ as byproduct.

Sulfidation forwarding high-strength Anammox process using nitrate as electron acceptor via thiosulfate-driven nitrate denitratation

A single up-flow thiosulfate-driven nitrate denitratation coupled with the sulfurized Anammox (TDSA) with the core-shell structure (S⁰@ Anammox granules) provided a chemical/energy-saving way for the removal of high-content ammonium with nitrate as electron acceptor. Approximately 83.66% total nitrogen removal efficiency (TNRE) could be achieved by the sulfurized Anammox encrusted by S⁰/S_n^2- at a high loading rate (2.6 kg-N/(m³·d)) via resisting high concentration of free ammonia (FA) (22.35 mg/L), mainly through S₂O₃^2-, S⁰/S_n^2- -driven partial denitrification-Anammox (PDN-Anammox) process. Moreover, S⁰/S_n^2--PDN-Anammox was largely restricted when intermittently aerated, but still resulting in 74.47% TNRE due to the partial nitrification-Anammox (PN-Anammox). The sequencing analysis revealed that Anammox bacterium (Candidatus_Kuenenia) and sulfur-oxidizing bacterium (Thiobacillus) coexisted, in which Anammox process occurred mainly via NO instead of NH₂OH. This study provided a new perspective for high concentration nitrogen wastewater removal in engineering applications.

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.

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.

Development of a kinetic model to evaluate thiosulfate-driven denitrification and anammox (TDDA) process

Recently, the integration of sulfur-driven denitrification and anammox process has been extensively studied as a promising alternative nitrogen removal technology. Most of these studies investigated the process feasibility and monitored the community dynamics. However, an in-depth understanding of this new sulfur-nitrogen cycle bioprocess based on mathematical modeling and elucidation of complex interactions among different microorganisms has not yet been achieved. To fill this gap, we developed a kinetic model (with 7 bioprocesses, 12 variables, and 19 parameters) to assess the sulfur(thiosulfate)-driven denitrification and anammox (TDDA) process in a single reactor. The parameters used in this process were separately estimated by fitting the data obtained from the experiments. Then, the model was further validated under different conditions, and the results demonstrated that the developed model could describe the dynamic behaviors of nitrogen and sulfur conversions in the TDDA system. The newly developed branched thiosulfate oxidation model was also verified by conducting a metagenomics analysis. Using the developed model, we i) examined the interactions between sulfur-oxidizing bacteria and anammox bacteria at steady-state conditions with varying substrates to demonstrate the reliability of TDDA, and ii) evaluated the feasibility and operation of the TDDA process in terms of practical implementation. Our results will benefit further exploration of the significance of this novel S-N cycle bioprocess and guide its future applications.

Exploration and verification of the feasibility of sulfide-driven partial denitrification coupled with anammox for wastewater treatment

Anaerobic ammonia oxidation (anammox) is a well-developed biotechnology for treating high-strength ammonium wastewaters. Recently, partial denitrification has been considered as an alternative to supply anammox with the required nitrite. In this study, a process of sulfide-driven partial denitrification and anammox (SPDA) was developed and operated continuously in an upflow anaerobic sludge blanket (UASB) reactor for 392 days. This reactor was fed with synthetic wastewater containing 100 mgN/L nitrate, 80 mgN/L ammonium and 20-80 mgS/L sulfide. After 160 days of operation, the reactor reached stable performance, and the nitrogen removal efficiency and rate were maintained at 80% and 0.29 kgN/(m³•d), respectively. The estimated nitrogen removal via anammox and sulfide-driven denitrification were 87.2% and 12.8%. Additional batch experiments were conducted to investigate the effects of sulfide on anammox and the mechanisms of nitrogen removal in the SPDA system. The following results were obtained: (1) sulfide had an inhibitory effect on the specific anammox activity with IC₅₀ of 9.7 mgS-H₂S/L. (2) The rapid oxidation of sulfide by sulfur-oxidizing bacteria (SOB) could relieve the toxic effects of sulfide on the anammox in the SPDA system. (3) Sulfide bio-oxidation was a two-step reaction with biologically produced elemental sulfur (BPS⁰) as the intermediate, and the second step using BPS⁰ as the electron donor, can efficiently produce nitrite via partial denitrification (NO₃^- → NO₂^-) as a supply for anammox. Finally, a high-throughput sequencing analysis identified Thiobacillus and Sulfurimonas as the dominant genera of SOB in the SPDA system, and Candidatus Kuenenia as the dominant anammox bacteria. Overall, this research gives the foundation for the practical application of sulfide-driven partial denitrification and anammox process in the future.

Sulfur transformation in sulfur autotrophic denitrification using thiosulfate as electron donor

Thiosulfate is frequently used as an energy source and electron donor in autotrophic denitrification (AD) for removing nitrate from wastewater. However, transforming pathways of S₂O₃^2- in this process is unclear. Herein, the aim of this study is to explore possible transforming pathways of sulfur compounds in thiosulfate-based AD process. After measuring the variation of NO₃^-, NO₂^-, and various sulfur compounds such as S⁰, SO₄^2-, S₂O₃^2-, acid volatile sulfide (AVS), and S^2- in the presence and absence of S₂O₃^2-, the variation process of S₂O₃^2- and the contribution of various sulfur compounds were analyzed. The results indicated that S⁰, AVS, and S^2- were the intermediate products when S₂O₃^2- was applied as an electron donor. All S₂O₃^2-, S⁰, AVS, and S^2- could act as electron donors in the nitrate removal process with the final products of SO₄^2-. The utilization priority of these four sulfur sources was presumed in the following order: S^2- > S₂O₃^2- > AVS ≈ S⁰. Furthermore, sulfur transformation and balance in nitrate removal process was also investigated. This suggests the transforming pathways of sulfur compounds in denitrification process. Nitrogen removal and sulfur conversion process are dependent on the presence of microorganisms in the sludge.

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 nickel on the comparative performance of inverse fluidized bed and continuously stirred tank reactors for biogenic sulphur-driven autotrophic denitrification

The comparative performance of an inverse fluidized bed reactor (IFBR) having high density polyethylene beads as carrier materials for biofilm formation and a continuous stirred tank reactor (CSTR), both maintaining autotrophic denitrification using biogenic sulphur (ADBIOS) in the absence and presence of nickel (Ni²⁺), was studied. The reactors were compared in terms of NO₃^--N and NO₂^--N removal and SO₄^2--S production throughout the study. A simulated wastewater with an inlet NO₃^--N concentration of 225 mg/L and a decreasing concentration of biogenic sulphur (bio-S) from 1.5 to 0.375 g/L was used. Both reactors were operated at a hydraulic retention time (HRT) of 48 h for 140 days and at an HRT of 42 h for the following 68 days. A more efficient ADBIOS was observed in the CSTR than IFBR throughout the study due to a better mixing of the feed wastewater in the bulk liquid and a higher availability of bio-S to the suspended cells. The NO₃^--N removal efficiency in the IFBR decreased by approximately 41% when the feed bio-S was reduced to 0.375 g/L, while it remained unaffected in the CSTR. Conversely, the presence of Ni²⁺ did not significantly affect NO₃^--N removal in both reactors even at a feed Ni²⁺ concentration of 120 mg/L. The highest NO₃^--N removal rates achieved were 86 and 108 mg NO₃^--N/(L·day) in the IFBR and CSTR, respectively, in the presence of 120 mg/L of feed Ni²⁺ at an HRT of 42 h. Batch studies conducted with acclimatized biomass showed that the continuous-flow operation mode in both reactors played a major role in helping the autotrophic denitrifiers to tolerate Ni²⁺ toxicity.

Effects of loading rates and N/S ratios in the sulfide-dependent autotrophic denitrification (SDAD) and Anammox coupling system

This study investigated the shock resistance and the stability of a novel sulfide-dependent autotrophic denitrification (SDAD) and anaerobic ammonium oxidation (Anammox) coupling process for simultaneous removal of sulfide and nitrogen-containing wastewater in a single reactor. Results show that the total nitrogen (TN) removal efficiency reached 86.7% at a nitrogen loading rate (NLR) of 1.52 kgN m^-3 d^-1. Sulfide was fully oxidized, achieving the removal efficiency of 100% throughout the whole process. Batch tests suggest that Anammox remained dominant with the cooperation of partial SDAD (PSDAD) and could always compete over short-cut SDAD (SSDAD) for nitrite. High-throughput sequencing analysis revealed that Anammox bacteria remained active despite a relatively lower abundance and diversity than denitrifying bacteria. Candidatus Kuenenia might be the main contributor to Anammox, while Thiobacillus and Sulfurimonas were closely related to SDAD.

Application of activated sludge for odor control in wastewater treatment plants: Approaches, advances and outlooks

Odors from wastewater treatment plants (WWTPs) have attracted extensive attention and stringent environmental standards are more widely adopted to reduce odor emissions. Biological odor treatment methods have broader applications than the physical and chemical counterparts as they are environment-friendly, cost-effective and generate low secondary wastes. The aqueous activated sludge (AS) processes are among the most promising approaches for the prevention or end-of-pipe removal of odor emissions and have the potential to simultaneously treat odor and wastewater. However, AS deodorization biotechnologies in WWTPs still need to be further systematically summarized and categorized while in-depth discussions on the characteristics and underlying mechanisms of AS deodorization process are still lacking. Recently, considerable studies have been reported to elucidate the microbial metabolisms in odor control and wastewater treatment. This paper reviews the fundamentals, characteristics, advances and field experiences of three AS biotechnologies for odor treatment in WWTPs, i.e., AS recycling, microaeration in AS digester and AS diffusion. The underlying deodorization mechanisms of typical odors have been revealed through the summary of recent advances on multi-element conversions, metabolic interactions of bacteria, microscopic characterization and identification of functional microorganisms. Future research aspects to advance the emerging deodorization AS process, such as deodorization mechanisms, simultaneous odor and water treatment, synergistic treatment with other air emissions, are discussed.

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.

Potential applications of endogenous sulfide for enhanced denitrification of low C/N domestic wastewater in anodic mixotrophic denitrification microbial fuel cell: The mechanism of electrons transfer and microbial community

Anodic mixotrophic denitrification microbial fuel cell (MFC) was developed for pollutants removal and electricity generation in treatment of low C/N domestic wastewater. The experimental results show that the MFC achieved up to 100% of acetate, 100% of sulfide, and more than 91% of nitrate removal efficiency in all the MFCs. Particularly, thiosulfate was generated as the main intermediate of sulfide oxidation, and the sulfate generation ratio ranged from 66.93% to 73.76%. Those electrons produced during the acetate and sulfide oxidation were mainly used for denitrification and electricity generation. The microbial community analysis revealed that heterotrophic denitrifying bacteria (HDB) and sulfide-based autotrophic denitrifying bacteria (SADB) were the dominant bacteria for pollutants removal, and those facultative autotrophic bacterium (FAB) were key functional genera for high sulfate generation under both low and high sulfide concentrations. Meanwhile, the microbial functional prediction revealed that sulfide oxidation gene of Sqr and Sox were highly expressed. Moreover, a preliminary sulfide-based autotrophic denitrification (SAD) potential estimation indicated that the sulfide generated in the WWTPs had great potential for denitrification.

Performance enhancement of H2S-based autotrophic denitrification with bio-gaseous CO2 as sole carbon source through new pH adjustment materials

H₂S-based denitrification could achieve synchronous removal of nitrate and H₂S and had been regarded as an efficient way for biogas desulfurization and wastewater denitrification. Using CO₂ in biogas as carbon source had a potential of saving cost further, but the performance deteriorated due to the drop in pH. Two kinds of nature ore, medical stone and phosphate ore, were added as new pH adjustment materials in this study, and feasibility of using CO₂ as sole carbon source for H₂S-based denitrification was investigated. As a result, both materials could increase the pH from 4.5 to above 6.0. Compared with medical stone, higher level of pH (up to 6.39) and nitrate removal efficiency (99.1%) were obtained with phosphate ore. In addition, ATP increased more rapidly than the control, reflecting improvement on microbial activities. Therefore, phosphate ore as the pH adjustment material could improve H₂S-based denitrification performance obviously.

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.

Recent advances in partial denitrification in biological nitrogen removal: From enrichment to application

To maximize energy recovery, carbon capture followed by shortcut nitrogen removal is becoming the most promising route in biological wastewater treatment. As the intermediate of microbial denitrification, nitrite could serve as a substrate for anammox bacteria, while N₂O is a combustion promoter that can increase 37% energy release from CH₄ than O₂. Therefore, the important advances in partial denitrification (PD) that produces nitrite or N₂O as the main product using inorganic or organic electron donors were critically reviewed. Specifically, the enrichment strategies of PD microorganisms were obtained by analyzing the selection pressures, metabolism, physiology, and microbiology of these microorganisms. Furthermore, some prospective and promising processes integrating PD microorganisms and the bottlenecks of current applications were discussed. The obtained knowledge would provide new insights into the upgrading of current WWTPs involving commitment to achieve nitrogen removal from wastewaters more economically and environmentally friendly.

Long term performance and dynamics of microbial biofilm communities performing sulfur-oxidizing autotrophic denitrification in a moving-bed biofilm reactor

Sulfide-oxidizing autotrophic denitrification (SOAD) implemented in a moving-bed biofilm reactor (MBBR) is a promising alternative to conventional heterotrophic denitrification in mainstream biological nitrogen removal. The sulfide-oxidation intermediate - elemental sulfur - is crucial for the kinetic and microbial properties of the sulfur-oxidizing bacterial communities, but its role is yet to be studied in depth. Hence, to investigate the performance and microbial communities of the aforementioned new biosystem, we operated for a long term a laboratory-scale (700 d) SOAD MBBR to treat synthetic saline domestic sewage, with an increase of the surface loading rate from 8 to 50 mg N/(m²·h) achieved by shortening the hydraulic retention time from 12 h to 2 h. The specific reaction rates of the reactor were eventually increased up to 0.37 kg N/(m³·d) and 0.73 kg S/(m³·d) for nitrate reduction and sulfide oxidation with no significant sulfur elemental accumulation. Two sulfur-oxidizing bacterial (SOB) clades, Sox-independent SOB (SOB_I) and Sox-dependent SOB (SOB_II), were responsible for indirect two-step sulfur oxidation (S^2-→S⁰→SO₄^2-) and direct one-step sulfur oxidation (S^2-→SO₄^2-), respectively. The SOB_II biomass-specific electron transfer capacity could be around 2.5 times greater than that of SOB_I (38 mmol e^-/(gSOB_II·d) versus 15 mmol e^-/(gSOB_I·d)), possibly resulting in the selection of SOB_II over SOB_I under stress conditions (such as a shorter HRT). Further studies on the methods and mechanism of selecting of SOB_II over SOB_I in biofilm reactors are recommended. Overall, the findings shed light on the design and operation of MBBR-based SOAD processes for mainstream biological denitrification.

Elucidating the biofilm properties and biokinetics of a sulfur-oxidizing moving-bed biofilm for mainstream nitrogen removal

The sulfide-oxidizing autotrophic denitrification (SOAD) process offers a feasible alternative to mainstream heterotrophic denitrification in treating domestic sewage with insufficient organics. Previously SOAD has been successfully applied in a moving-bed biofilm reactor (MBBR). However, the biofilm properties and biokinetics are still not thoroughly understood. The present study was therefore designed to investigate these features of sulfur-oxidizing biofilms (SOBfs) cultivated in a lab-scale MBBR under stable operation for over a year. The biofilms developed were 160 μm thick, had an uneven and porous surface on which elemental sulfur (S⁰) accumulated, and the SOB biomass was highly diverse. The bioprocess kinetics were evaluated through 12 batch experiments. The results were interpreted by adopting a two-step sulfide oxidation model (sulfide→S⁰ and S⁰→ sulfate) with all specific rates having a linear regression coefficient of R² > 0.9. Moreover, the inhibitory kinetic analysis revealed that 1) the maximum treatment capacity (about 480 mg S/(m²·h) and 80 mg N/(m²·h)) was observed at low sulfide level (40 mg S/L), while higher sulfide level (60-150 mg S/L) showed increasing inhibition on the oxidation of both sulfide and sulfur and denitrification. 2) The denitritation activity decreased by up to 43% when free nitrous acid reached a maximum of 8.6 μg N/L, whereas the oxidation of sulfide and sulfur did not have any significant effect. Interestingly, two physiologically diverse SOB groups were found in this special biofilm. The mechanisms of the cooperation and competition for electron donors and acceptors between these two SOB clades are proposed. The results of this study greatly enhance our understanding of the design and optimization of SOAD-MBBR for mainstream nitrogen removal.

Modelling of simultaneous nitrogen and thiocyanate removal through coupling thiocyanate-based denitrification with anaerobic ammonium oxidation

Thiocyanate (SCN^-)-based autotrophic denitrification (AD) has recently been demonstrated as a promising technology that could be integrated with anaerobic ammonium oxidation (Anammox) to achieve simultaneous removal of nitrogen and SCN^-. However, there is still a lack of a complete SCN^--based AD model, and the potential microbial competition/synergy between AD bacteria and Anammox bacteria under different operating conditions remains unknown, which significantly hinders the possible application of coupling SCN^--based AD with Anammox. To this end, a complete SCN^--based AD model was firstly developed and reliably calibrated/validated using experimental datasets. The obtained SCN^--based AD model was then integrated with the well-established Anammox model and satisfactorily verified with experimental data from a system coupling AD with Anammox. The integrated model was lastly applied to investigate the impacts of influent NH₄⁺-N/NO₂^--N ratio and SCN^- concentration on the steady-state microbial composition as well as the removal of nitrogen and SCN^-. The results showed that the NH₄⁺-N/NO₂^--N ratio in the presence of a certain SCN^- level should be controlled at a proper value so that the maximum synergy between AD bacteria and Anammox bacteria could be achieved while their competition for NO₂^- would be minimized. For the simultaneous maximum removal (>95%) of nitrogen and SCN^-, there existed a negative relationship between the influent SCN^- concentration and the optimal NH₄⁺-N/NO₂^--N ratio needed.

Investigation of multiple polymers in a denitrifying sulfur conversion-EBPR system: The structural dynamics and storage states

Polyhydroxyalkanoates (PHAs), polyphosphate (poly-P) and polysulfide or elemental sulfur (poly-S) are the key functionally relevant polymers involved in the recently reported Denitrifying Sulfur conversion-associated Enhanced Biological Phosphorus Removal (DS-EBPR) process. However, little is known about the structural dynamics and storage states of these polymers. In particular, investigating the poly-S generated in this process is quite a superior challenge. This study was thus aimed at simultaneously qualitative-quantitative investigating poly-S and associated poly-P and PHAs through the integrated chemical analysis and Raman micro-spectroscopy coupled with multiple microscopic methods (i.e. optical microscopy, confocal laser scanning microscopy, and differential interference contrast microscopy). The chemical analytical results displayed a stable DS-EBPR phenotype in terms of sulfur conversion, P release/uptake and the dynamics of relevant polymers. The multiple microscopic images and Raman spectrum profiles further clearly demonstrated the existence of the polymers and their dynamic changes under alternating anaerobic-anoxic conditions, consistent with the chemical analytical results. In particular, Raman analysis for the first time unraveled the co-existence of S⁰/S_n^2- species stored either intracellularly or extracellularly; and the dynamic conversions between S⁰/S_n^2- and other sulfur species suggest that there might be a universal pool of bioavailable sulfur. The results reveal the mechanisms underlying the structural dynamics and changes in storage states of the relevant polymers that are functionally relevant to the carbon/phosphorus/sulfur-cycles during different metabolic phases. These mechanisms would otherwise not be obtained only using a traditional chemical analysis-based approach.

Integrated sulfur- and iron-based autotrophic denitrification process and microbial profiling in an anoxic fluidized-bed membrane bioreactor

The integrated sulfur- and Fe⁰-based autotrophic denitrification process in an anoxic fluidized-bed membrane bioreactor (AnFB-MBR) was developed for the nitrate-contaminated water treatment in order to control sulfate generation and avoid alkalinity supplement. The nitrate removal rate of the AnFB-MBR reached 1.22 g NO₃^--N L^-1d^-1 with NO₃^--N ranging 40-200 mg L^-1 at hydraulic retention times of 1.0-5.0 h. The denitrification in the integrated system was simultaneously carried out by sulfur- and Fe⁰-oxidizing autotrophic denitrifiers. The effluent sulfate generation was decreased by 29.3-70.3% and 31.2-50.9% due to the functional role of Fe⁰-based denitrification in the integrated system. Alkalinity produced by Fe⁰-oxidizing autotrophic denitrification could compensate for the alkalinity consumption by sulfur-based autotrophic denitrification. The sulfur- and Fe⁰-oxidizing autotrophic denitrifying bacterial consortium was composed mainly of bacteria from Thiobacillus, Sulfurimonas, and Geothrix genera. The integrated modes leads to a harmonious co-existence of sulfur- and Fe⁰-oxidizing denitrifying microbes, which may make a difference to the functional performance of the bioreactor. Overall, the integrated sulfur- and Fe⁰-based autotrophic denitrification could overcome the shortcomings of excess sulfate generation and external alkalinity supplementation compared to the sole sulfur-based autotrophic denitrification, indicating further potential for the technology in practice.

Kinetics simulation of Cu and Cd removal and the microbial community adaptation in a periphytic biofilm reactor

Periphytic biofilm reactor (PBfR) shows great potential in pollutants removal. However, few studies were focused on mathematical model of pollutants removal in PBfR. A three-step PBfR was designed and a new model was developed to simulate the kinetics of Cu and Cd removal from simulated wastewater. The results show that the PBfR could remove 99.0% Cu and 99.7% Cd from liquid wastewater. The experiment data could be well fitted with a high correlation coefficients both for Cu and Cd. The microbial community in the PBfR could be self-adjusted to tolerate the toxicities of Cu and Cd, resulting in sustainable and high decontamination efficiencies. The eukaryote in the PBfR played a vital role in Cu and Cd removal. The prokaryote showed negative effect on Cu and Cd removal, though it had more diversity than eukaryote. This study provides a new approach for Cu and Cd removal and their kinetics simulation in photoautotrophic bioreactor.

Optimization of methane-dependent oxygenic denitrification in sequencing batch reactors by insights into the microbial interactions

Methane-dependent oxygenic denitrification (O2DN) is a promising technology used for reducing greenhouse gas emissions of nitrous oxide (N₂O) during wastewater treatment. Heterotrophic bacteria are associated with methane-dependent O2DN bacteria, and it has been proposed that metabolic cross-feeding occurs between the two populations above. In this study, a mathematical model was developed to describe the microbial processes and interactions between methane-dependent O2DN bacteria and associated heterotrophic bacteria in a sequencing batch reactor (SBR). A growth factor-dependent decoupling of metabolism and growth of methane-dependent O2DN bacteria was introduced into the model. Effects of influent substrates, operating parameters, and initial biomass on microbial community and reactor performance were then investigated, and the above parameters were optimized using the model. Results surprisingly show that organic matter in the influent greatly stimulated the growth of methane-dependent O2DN bacteria but slightly limited the increase of heterotrophic bacteria. This effect could be explained by the increased excretion of growth factors by heterotrophic bacteria and the intensified competition for nitrite when methane-dependent O2DN bacteria increased. These results will assist in providing a new understanding of microbial interactions in methane-dependent O2DN systems and offer a new and efficient strategy for operating methane-dependent O2DN reactors.

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.

Simultaneous removal of thiocyanate and nitrogen from wastewater by autotrophic denitritation process

Pollutants containing sulfur as electron donors will play an important role in the energy-saving denitritation process when organic carbon source was insufficient in wastewater. However, thiocyanate (SCN^-), a hazardous pollutant, has not been characterized in denitritation. In this study, the effects of key environmental factors on removal of thiocyanate and nitrogen were investigated in denitritation. The results showed that the maximum removal efficiency of nitrogen was observed in complete removal of thiocyanate and nitrite. The elemental sulfur was observed prior to complete depletion of thiocyanate. The efficiency of denitritation was promoted by NaHCO₃ and weakly-alkaline environment. In the sludge containing dominant Thiobacillus genus, nitrite was reduced in the conversion of thiocyanate into elemental sulfur and further into sulfate. The stoichiometric ratio of NO₂^--N to SCN^--N was close to 2.0 when thiocyanate was converted completely into sulfate, which verified complete removal of thiocyanate and nitrite at the NO₂^--N/SCN^--N ratio of 2.0.

Elemental sulfur-based autotrophic denitrification and denitritation: microbially catalyzed sulfur hydrolysis and nitrogen conversions

The hydrolysis of elemental sulfur (S⁰) coupled to S⁰-based denitrification and denitritation was investigated in batch bioassays by microbiological and modeling approaches. In the denitrification experiments, the highest obtained NO₃^--N removal rate was 20.9 mg/l·d. In the experiments with the biomass enriched on NO₂^-, a NO₂^--N removal rate of 10.7 mg/l·d was achieved even at a NO₂^--N concentration as high as 240 mg/l. The Helicobacteraceae family was only observed in the biofilm attached onto the chemically-synthesized S⁰ particles with a relative abundance up to 37.1%, suggesting it was the hydrolytic biomass capable of S⁰ solubilization in the novel surface-based model. S⁰-driven denitrification was modeled as a two-step process in order to explicitly account for the sequential reduction of NO₃^- to NO₂^- and then to N₂ by denitrifying bacteria.

Simultaneous nitrite and ammonium production in an autotrophic partial denitrification and ammonification of wastewaters containing thiocyanate

Various products are observed in biological oxidation and reduction of molecules containing elements of variable valence. The variability is caused by the diversity of microorganisms and their metabolic enzymes, which may develop into novel processes in wastewater treatment. The study aimed to develop a novel denitrification process forming nitrite and ammonium in wastewaters containing thiocyanate. High-efficiency nitrite and ammonium production was observed due to autotrophic partial denitrification and ammonification as a result of nitrate and thiocyanate removal. Nitrite, ammonium and sulfate were observed as the ultimate products. The increased NO₃^--N/SCN^--N ratio in the treated wastewater resulted in the decreased removal efficiency of nitrate, and the increased nitrate-to-nitrite transformation ratio and the ratio of NO₂^--N to NH₄⁺-N. Thiocyanate sulfur was oxidized to sulfate via intermediate elementary sulfur providing electron to nitrate or nitrite. The Thiobacillus genus dominated in the sludge providing ammonium and nitrite as substrate for the potentially anammox process.

Assessing the electron transfer and oxygen mass transfer of the oxygen reduction reaction using a new electrode kinetic equation

A new kinetic equation describing the full-scale polarizations, provides a facile approach for assessing ORR performance, highlighting oxygen-mass transfer evaluation.

Mathematical modeling of simultaneous carbon-nitrogen-sulfur removal from industrial wastewater

A mathematical model of carbon, nitrogen and sulfur removal (C-N-S) from industrial wastewater was constructed considering the interactions of sulfate-reducing bacteria (SRB), sulfide-oxidizing bacteria (SOB), nitrate-reducing bacteria (NRB), facultative bacteria (FB), and methane producing archaea (MPA). For the kinetic network, the bioconversion of C-N by heterotrophic denitrifiers (NO₃^-→NO₂^-→N₂), and that of C-S by SRB (SO₄^2-→S^2-) and SOB (S^2-→S⁰) was proposed and calibrated based on batch experimental data. The model closely predicted the profiles of nitrate, nitrite, sulfate, sulfide, lactate, acetate, methane and oxygen under both anaerobic and micro-aerobic conditions. The best-fit kinetic parameters had small 95% confidence regions with mean values approximately at the center. The model was further validated using independent data sets generated under different operating conditions. This work was the first successful mathematical modeling of simultaneous C-N-S removal from industrial wastewater and more importantly, the proposed model was proven feasible to simulate other relevant processes, such as sulfate-reducing, sulfide-oxidizing process (SR-SO) and denitrifying sulfide removal (DSR) process. The model developed is expected to enhance our ability to predict the treatment of carbon-nitrogen-sulfur contaminated industrial wastewater.

Evaluation of Nitrous Oxide Emission from Sulfide- and Sulfur-Based Autotrophic Denitrification Processes

Recent studies have shown that sulfide- and sulfur-based autotrophic denitrification (AD) processes play an important role in contributing to nitrous oxide (N2O) production and emissions. However, N2O production is not recognized in the current AD models, limiting their ability to predict N2O accumulation during AD. In this work, a mathematical model is developed to describe N2O dynamics during sulfide- and sulfur-based AD processes for the first time. The model is successfully calibrated and validated using N2O data from two independent experimental systems with sulfide or sulfur as electron donors for AD. The model satisfactorily describes nitrogen reductions, sulfide/sulfur oxidation, and N2O accumulation in both systems. Modeling results revealed substantial N2O accumulation due to the relatively low N2O reduction rate during both sulfide- and sulfur-based AD processes. Application of the model to simulate long-term operations of activated sludge systems performing sulfide- and sulfur-based AD processes indicates longer sludge retention time reduced N2O emission. For sulfide-based AD process, higher initial S/N ratio also decreased N2O emission but with a higher operational cost. This model can be a useful tool to support process operation optimization for N2O mitigation during AD with sulfide or sulfur as electron donor.