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

Iron sulfide mineral/polylactic acid mixotrophic biofilter for simultaneous nitrate and phosphate removal

Heterotrophic denitrification based on polylactic acid (PLAHD) can remove nitrate effectively, but it is expensive and can't remove phosphate. Autotrophic denitrification based on iron sulfide (ISAD) can simultaneously remove nitrate and phosphate cost-effectively, but its nitrate rate is slow. So, iron sulfide mineral/polylactic acid mixotrophic biofilter (ISPLAB) was constructed to combine advantages of ISAD and PLAHD. ISPLAB achieved nitrogen and phosphorus removal rates of 98.04 % and 94.12 %, respectively, at a hydraulic retention time (HRT) of 24 h. The study also revealed that controlling molecular weight (MW) of PLA improved the release of soluble organic matter; adding iron sulfide enhanced the hydrolysis of PLA and precipitated PO43- of Fe2+/Fe3+, thereby facilitated simultaneous nitrogen and phosphorus removal. Microbial community analysis resulted that denitrifying bacterias (Phaeodactylibacter and Methylotenera), sulfur-reducing bacterias (Hyphomicrobium), sulfur-oxidizing bacteria (Denitratisoma), iron-reducing bacteria (Romboutsia) and hydrolyzed bacterias (norank_f_norank_o_1-20 and norank_f_Caldilineaceae) coexisted in the ISPLAB system. Organics and iron sulfide drived the denitrification process in ISPLAB.

Enhanced nitrogen removal from low C/N ratio wastewater by coordination of ternary electron donors of Fe0, carbon source and sulfur: Focus on oxic/anoxic/oxic process

Insufficient organics was the major obstacle for total nitrogen (TN) removal in conventional pre-anoxic denitrification when treating low carbon to nitrogen (C/N) ratio wastewater. This study constructed a novel ternary-electron donors (Fe0, organics and S0) enhanced oxic/anoxic/oxic (O/A/O) process, integrating simultaneous nitrification and denitrification and autotrophic denitrification (ADN), and evaluated its feasibility to achieve efficient nutrient removal under organics-deficient condition. Long-term operation results showed that TN removal was lower (9.9 %) when Fe0 added individually, then raised to 27.3 %∼46.0 % in simultaneous presence of Fe0 and organics. And the highest TN removal (82.0 %) was obtained by coordination of ternary-electron donors, with 8.46 ± 0.43 mg/L TN in effluent. Meanwhile, the O/A/O process exhibited excellent total phosphorous (TP) removal (84.8 %∼98.4 %) derived from chemical precipitation by Fe0, of which the effluent was <0.76 ± 0.04 mg/L TP. Metabolic characteristics indicated that the coordination of multi-electron donors improved microbial metabolism and denitrifying enzymatic activities, thereby promoting ammonia assimilation and enhancing TN removal. And the secretion of EPS was also stimulated, which favored the bio-utilization of Fe0 and S0 and alleviated organics dependence. Besides, the notable increase in abundances of aerobic denitrifiers (23.95 %∼27.37 %), autotrophic denitrifiers (9.31 %) and denitrifying genes further verified the synergy effect of multi-electron donors on TN removal. This study revealed the enhancement mechanism of O/A/O process by coordination of ternary-electron donors, verified its cost-effectiveness and provided innovative insights on low C/N ratio wastewater remediation.

Enhanced heterotrophic denitrification in groundwater using pretreated Ginkgo biloba leaves: Optimized carbon utilization and metabolic function diversity

Ginkgo biloba leaves (Gbl), as abundant agricultural and forestry residues which contains the quercetin that plays an important role in mediating electron transfer, represent a promising heterotrophic denitrification carbon source. Nonetheless, challenges persist due to concerns over nitrate leaching. This study pioneers the application of pretreated Gbl as external carbon sources for heterotrophic denitrification, with a focus on enhancing carbon bioavailability and mitigating nitrate leaching risks. Among the pretreatment strategies employed, the extraction process effectively eliminated NO3--N leaching, while the fermentation process reduced it by 52.8%. The saturated total organic carbon (TOC) concentration per unit mass of fermented Gbl was marginally lower compared to untreated leaves, yet the secondary kinetic reaction constant increased from 10.94 to 12.91 mg/(g·h·L), indicating an accelerated organic carbon release rate. Fermentation with Eurotium cristatum disrupted the rigid lignocellulose structure, thereby enhancing carbon source bioavailability. This resulted in a significant increase in alcohols in the leaching solution, from 27.0% to 68.6%, and a substantial reduction in aromatic compounds, from 20.2% to 0.2%, which alleviated microbial toxicity. In terms of denitrification performance, fermented Ginkgo biloba leaves (Fl) outperformed Ginkgo biloba extract residue leaves (Erl), which in turn surpassed untreated Gbl. Both Fl and Erl demonstrated robust adaptability across a broad pH range of 5.0-11.0. Under neutral conditions, the Fl system exhibited the highest primary kinetic constant for nitrate removal, reaching 0.0494 h⁻1. Microbial community revealed that all three carbon sources harbored denitrification and lignocellulose degradation capabilities. Notably, the Fl and Erl systems exhibited enhanced carbohydrate transport (G), amino acid transport (E), and inorganic ion transport (P), underscoring the potential pretreatments to optimize carbon source utilization. Collectively, these findings affirm the viability of Gbl as a carbon source for heterotrophic denitrification, providing valuable insights for its application in addressing nitrate pollution in aquatic environments.

Unraveling the synergistic interplay of sulfur and wheat straw in heterotrophic-autotrophic denitrification for sustainable groundwater nitrate remediation

Nitrate pollution in groundwater is a global environmental issue that poses significant threats to human health and ecological security. This study focuses on elucidating the mechanisms of heterotrophic-autotrophic cooperative denitrification (HAD) by employing wheat straw and elemental sulfur as electron donors in varying proportions. The research initially underscores that heterotrophic denitrification (HD) accelerates the denitrification process due to its high-energy metabolism. However, as readily degradable organic matter diminished, reliance on more complex substrates such as lignocellulose posed a challenge to HD. This marks a pivotal transition towards autotrophic denitrification (AD), which, despite a slower initial rate, exhibits a more sustained denitrification performance. A low proportion of heterotrophic denitrification layer (e.g., 3:1) at the bottom facilitating efficient and sustainable denitrification. HD is capable of simultaneous removal of nitrates and nitrites, whereas AD demonstrates a higher affinity for nitrates, with nitrite accumulation reaching 100% at high influent nitrate concentrations (100 mg/L). HD not only provides the necessary alkaline environment for AD but also reduces sulfate production, whereas AD utilizes the residual organic carbon and ammonia produced by HD. The heterotrophic layer is characterized by a diverse community, whereas the autotrophic layer is predominantly composed of Thiobacillus. By delineating the interactive mechanisms and characteristics of HAD, this study highlights the importance of balancing heterotrophic and autotrophic activities for the effective remediation of groundwater nitrates.

Integrated catalytic systems for simultaneous NOx and PM reduction: a comprehensive evaluation of synergistic performance and combustion waste energy utilization

The global transition towards sustainable automotive vehicles has driven the demand for energy-efficient internal combustion engines with advanced aftertreatment systems capable of reducing nitrogen oxides (NOx) and particulate matter (PM) emissions. This comprehensive review explores the latest advancements in aftertreatment technologies, focusing on the synergistic integration of in-cylinder combustion strategies, such as low-temperature combustion (LTC), with post-combustion purification systems. Selective catalytic reduction (SCR), lean NOx traps (LNT), and diesel particulate filters (DPF) are critically examined, highlighting novel catalyst formulations and system configurations that enhance low-temperature performance and durability. The review also investigates the potential of energy conversion and recovery techniques, including thermoelectric generators and organic Rankine cycles, to harness waste heat from the exhaust and improve overall system efficiency. By analyzing the complex interactions between engine operating parameters, combustion kinetics, and emission formation, this study provides valuable insights into the optimization of integrated LTC-aftertreatment systems. Furthermore, the review emphasizes the importance of considering real-world driving conditions and transient operation in the development and evaluation of these technologies. The findings presented in this article lay the foundation for future research efforts aimed at overcoming the limitations of current aftertreatment systems and achieving superior emission reduction performance in advanced combustion engines, ultimately contributing to the development of sustainable and efficient automotive technologies.

Using rice straw-augmented ecological floating beds to enhance nitrogen removal in carbon-limited wastewater

Agricultural biomass used as solid carbon substrates in ecological floating beds (EFBs) has been proven to be applicable in nitrogen removal for carbon-limited wastewater treatment. However, the subtle interactions among plants, rhizosphere microorganisms, and supplementary carbon sources have not been thoroughly studied. This study combined rice straw mats with different aquatic macrophytes in EFBs to investigate denitrification efficiency in carbon-limited eutrophic waters. Results showed that rice straw significantly enhanced the nitrogen removal efficiency of EFBs, while enriching nitrogen-fixing and denitrifying bacteria (such as Rhizobium, Rubrivivax, and Rhodobacter, etc.). Additionally, during the denitrification process in EFBs, rice straw can release humic acid-like fraction as electron donors to support the metabolic activities of microorganisms, while aquatic macrophytes provide a more diverse range of dissolved organic matters, facilitating a sustainable denitrification process. These findings help to understand the synergistic effect of denitrification processes within wetland ecosystems using agricultural biomass.

Salinity stress results in ammonium and nitrite accumulation during the elemental sulfur-driven autotrophic denitrification process

In this study, we investigated the effects of salinity on elemental sulfur-driven autotrophic denitrification (SAD) efficiency, and microbial communities. The results revealed that when the salinity was ≤6 g/L, the nitrate removal efficiency in SAD increased with the increasing salinity reaching 95.53% at 6 g/L salinity. Above this salt concentration, the performance of SAD gradually decreased, and the nitrate removal efficiency decreased to 33.63% at 25 g/L salinity. Approximately 5 mg/L of the hazardous nitrite was detectable at 15 g/L salinity, but decreased at 25 g/L salinity, accompanied by the generation of ammonium. When the salinity was ≥15 g/L, the abundance of the salt-tolerant microorganisms, Thiobacillus and Sulfurimonas, increased, while that of other microbial species decreased. This study provides support for the practical application of elemental sulfur-driven autotrophic denitrification in saline nitrate wastewater.

Intensified denitrification in a fluidized-bed reactor with suspended sulfur autotrophic microbial fillers

Sulfur-based autotrophic denitrification (SAD) is a promising low-carbon approach to tackle nitrate pollution. However, practical SAD reactor implementation faces challenges of slow denitrification rates and prolonged start-up periods. In this work, a fluidized-bed denitrification reactor with suspended composite fillers immobilized with elemental sulfur and SAD bacteria was constructed. The reactor reaches a steady state within the first day of operation. A denitrification rate of 0.61 g N L-1 d-1 was realized, which is 2.4-fold higher than that in the packed-bed reactor. Mixotrophic denitrification prevailed during the start-up period, while the SAD process became the predominant pathway (>70%) after several days of operation. The prevailing bacteria in the fillers, notably Thiobacillus, are enriched during denitrification operations. Overall, this study highlights the impressive denitrification capabilities of the fluidized SAD reactor with microbial fillers, providing valuable insights for enhancing denitrification techniques.

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.

Response of nitrite accumulation, sludge characteristic and microbial transition to carbon source during the partial denitrification (PD) process

Partial denitrification (PD, nitrate (NO3--N) → nitrite (NO2--N)) as a novel pathway for NO2--N production has been widely concerned, but the specific conditions for highly efficient and stable nitrite maintenance are not yet fully understood. In this study, the effects of carbon sources (acetate, R1; propionate, R2; glucose, R3) on NO2--N accumulation was discussed without seeding PD sludge and the mechanism analysis related to sludge characteristic and microbial evolution were elucidated. The optimal NO2--N, nitrate-to-nitrite transformation ratio (NTR) and nitrite removal efficiency (NRE) reached up to 32.10 mg/L, 98.01 %, and 86.95 % in R1. However, due to the complex metabolic pathway of glucose, the peak time of NO2--N production delayed from 30 min to 60 min. The sludge particle size decreased from 154.2 μm (R1), 130.8 μm (R2) to 112.6 μm (R3) with the increasing extracellular polymeric substances (EPS) from 80.75-85.44 mg/gVSS, 82.68-92.75 mg/gVSS to 106.31-110.25 mg/gVSS, where the ratio of proteins/polysaccharides (PN/PS) was proved to be closely associated with NO2--N generation. For the microbial evolution, Saccharimonadales (70.42 %) dominated the glucose system, while Bacillus (7.42-21.63 %) and Terrimonas (4.24-5.71 %) were the main contributors for NO2--N accumulation in the acetate and propionate systems. The achievement of PD showed many advantages of lower carbon demand, minimal sludge production, lesser greenhouse gas emission and prominent nutrient removal, offering an economically and technically attractive alternative for NO3--N containing wastewater treatment.

Effects of salinity on nitrite and elemental sulfur accumulation in a double short-cut sulfur autotrophic denitrification process

Double short-cut sulfur autotrophic denitrification (DSSADN) coupled with Anammox is of great significance in the low-carbon treatment of nitrogen-containing wastewater. In order to achieve high salinity autotrophic nitrogen removal, the effects of different salinities on the accumulation characteristics of NO₂^--N and S⁰ and microorganisms in DSSADN process were studied. The results showed that the effect of salinity on the DSSADN process could be categorized into the stimulation, stable, and inhibition. When the salinity gradually increased to 2.5 %, the highest NO₂^--N production rate (NiPR) and S⁰ production rate (S⁰PR) of DSSADN were 0.54 kg/(m³·d) and 1.1 kg/(m³·d) respectively. NiPR and S⁰PR gradually decreased as the salinity increased to more than 3 %. However, salinity had a relatively low impact on nitrite accumulation efficiency and S⁰ accumulation efficiency, which were 80 % and 81.5 %, respectively, when the salinity reached 5 %. Salinity has a great influence on the structure and abundance of microbial communities in the system.

Sediment diffusion is feasible to simultaneously reduce nitrate discharge from recirculating aquaculture system and ammonium release from sediments in receiving intensive aquaculture pond

Nitrogen accumulation has become one of the greatest unresolved challenges restricting the development of aquaculture worldwide. In recirculating aquaculture system (RAS), lack of organic matter (OM) and sensitive organisms makes it difficult to apply efficient denitrifying technology, thus leading to a high nitrate‑nitrogen (NO₃^--N) accumulation. In contrast, excess OM accumulation in intensive aquaculture pond sediments is associated with dissolved oxygen depletion and ammonium‑nitrogen (NH₄⁺-N) accumulation in the sediments. Based on the opposing effects of OM on the nitrogen accumulation in RAS and intensive aquaculture ponds, this study assessed the feasibility of simultaneously reducing NO₃^--N discharge from RAS and controlling NH₄⁺-N accumulation in intensive aquaculture ponds by in situ diffusing RAS tailwater containing NO₃^--N into intensive aquaculture pond sediments. The results showed that NO₃^--N diffusion strategy improved the native sediment denitrification capacity, thus increasing NO₃^--N removal efficiency from RAS tailwater and significantly decreasing the NH₄⁺-N concentration in interstitial water and the total organic carbon content in intensive aquaculture pond sediments. High-throughput sequencing and quantitative real-time polymerase chain reaction (qPCR) results revealed that NO₃^--N addition significantly increased both nitrifying bacteria and denitrifying bacteria abundance. These results implied that NO₃^--N diffusion strategy could effectively stimulate microbial decomposition of OM, thus relieving the hypoxia limitation of sediment nitrification. Overall, this study offers a feasible method for simultaneous reduction of NO₃^--N from RAS tailwater and NH₄⁺-N in intensive aquaculture ponds with low cost and high efficiency.

The research progress, hotspots, challenges and outlooks of solid-phase denitrification process

Nitrogen pollution is one of the main reasons for water eutrophication. The difficulty of nitrogen removal in low-carbon wastewater poses a huge potential threat to the ecological environment and human health. As a clean biological nitrogen removal process, solid-phase denitrification (SPD) was proposed for long-term operation of low-carbon wastewater. In this paper, the progress, hotspots, and challenges of the SPD process based on different solid carbon sources (SCSs) are reviewed. Compared with synthetic SCS and natural SCS, blended SCSs have more application potential and have achieved pilot-scale application. Differences in SCSs will lead to changes in the enrichment of hydrolytic microorganisms and hydrolytic genes, which indirectly affect denitrification performance. Moreover, the denitrification performance of the SPD process is also affected by the physical and chemical properties of SCSs, pH of wastewater, hydraulic retention time, filling ratio, and temperature. In addition, the strengthening of the SPD process is an inevitable trend. The strengthening measures including SCSs modification and coupled electrochemical technology are regarded as the current research hotspots. It is worth noting that the outbreak of the COVID-19 epidemic has led to the increase of disinfection by-products and antibiotics in wastewater, which makes the SPD process face challenges. Finally, this review proposes prospects to provide a theoretical basis for promoting the efficient application of the SPD process and coping with the challenge of the COVID-19 epidemic.

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.

An efficient anoxic/aerobic/aerobic/anoxic process for domestic sewage treatment: From feasibility to application

In this paper, the anoxic/aerobic/aerobic/anoxic (AOOA) process was proposed using fixed biofilms in a continuous plug-flow multi-chamber reactor, and no sludge reflux operation was performed during the 190 days of operation. The reactor volume ratio of 1.5:2:1.5:1 (A/O/O/A) with the dissolved oxygen (DO) concentration of 2 mg L⁻¹ in the aerobic zone was the optimal condition for reactor operation. According to the results obtained from the treatment of real domestic sewage, when the hydraulic retention time (HRT) was 6 h, the effluent of the reactor could meet the discharge standard even in cold conditions (13°C). Specifically, the elemental-sulfur-based autotrophic denitrification (ESAD) process contributed the most to the removal of total inorganic nitrogen (TIN) in the reactor. In addition, the use of vibration method was helpful in removing excess sludge from the biofilms of the reactor. Overall, the AOOA process is an efficient and convenient method for treating domestic sewage.

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.

Simultaneously pollutant removal and S0 recovery from composite wastewater containing Cr(VI)-S2- based on biofilm enhancement

Bioaugmentation of extracellular polymeric substances-producing bacteria was applied in pollutant removal and S⁰ recovery from composite wastewater in a mixotrophic denitrification system. In the presence of 200 mg·L^-1 S^2- and 50 mg·L^-1 Cr(VI), the removal efficiencies of chemical oxygen demand, NO₃^-, S^2- and Cr(VI) were 86.38%, 91.82%, 95.75%, and 100.00% respectively, while S⁰ recovery efficiency reached 79.17%. Increased contents of protein and polysaccharide, especially the high ratio of protein/polysaccharide verified the structural stability of biofilm was promoted by biofilm enhancement. The widespread distribution of bacteria/extracellular polymeric substance (EPS) revealed the more obvious biofilms formation in biofilm-enhanced group. High-throughput sequencing analysis showed that EPS-producing bacteria (Flavobacterium, Thauera, Thiobacillus and Simplicispira) were dominant bacteria in the biofilm-enhanced group. Moreover, by comprehensive considering of redundancy analysis, the colonization of selected bacteria improved the robustness of the reactor and treatment performance to wastewater contained toxic pollutions.

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

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