Effect of organics on sulfur-utilizing autotrophic denitrification under mixotrophic conditions

The impact of benzoic acid and lactic acid on the treatment efficiency and microbial community in the sulfur autotrophic denitrification process

Nitrate poses a potential threat to aquatic ecosystems. This study focuses on the sulfur autotrophic denitrification mechanism in the process of water culture wastewater treatment, which has been successfully applied to the degradation of nitrogen in water culture farm effluents. However, the coexistence of organic acids in the treatment process is a common environmental challenge, significantly affecting the activity of denitrifying bacteria. This paper aims to explore the effects of adding benzoic acid and lactic acid on denitrification performance, organic acid removal rate, and microbial population abundance in sulfur autotrophic denitrification systems under optimal operating conditions, sulfur deficiency, and high hydraulic load. In experiments with 50 mg·L-1 of benzoic acid or lactic acid alone, the results show that benzoic acid and lactic acid have a stimulating effect on denitrification activity, with the stimulating effect significantly greater than the inhibitory effect. Under optimal operating conditions, the average denitrification rate of the system remained above 99%; under S/N = 1.5 conditions, the average denitrification rate increased from 88.34% to 91.93% and 85.91%; under HRT = 6 h conditions, the average denitrification rate increased from 75.25% to 97.79% and 96.58%. In addition, the addition of organic acids led to a decrease in microbial population abundance. At the phylum level, Proteobacteria has always been the dominant bacterial genus, and its relative abundance significantly increased after the addition of benzoic acid, from 40.2% to 61.5% and 62.4%. At the genus level, Thiobacillus, Sulfurimonas, Chryseobacterium, and Thermomonas maintained high population abundances under different conditions. PRACTITIONER POINTS: Employing autotrophic denitrification process for treating high-nitrate wastewater. Utilizing organic acids as external carbon sources. Denitrifying bacteria demonstrate high utilization efficiency towards organic acids. Organic acids promote denitrification more than they inhibit it. The promotion is manifested in the enhancement of activity and microbial abundance.

Iron mediated autotrophic denitrification for low C/N ratio wastewater: A review

In recent years, iron mediated autotrophic denitrification has been a concern because it overcomes the absence of organic carbon and has been successfully used in denitrification for low C/N ratio wastewater. However, there is currently a lack of a more systematic summary of iron-based materials that can be used for denitrification, and no detailed overview about the mechanism of iron mediated autotrophic denitrification has been reported. In this study, the iron materials with different valence states that can be used for denitrification were summarized, and emphasized, as well as the mechanism in different interaction systems were emphasize. In addition, the contribution of various microorganisms in nitrate reduction were analyzed and the effects of operating conditions and water quality were evaluated. Finally, the challenges and shortcomings of the denitrification process were discussed aiming to find better practical engineering applications of iron-based denitrification.

Efficient nitrogen removal from leachate by coupling Anammox and sulfur-siderite-driven denitrification

High concentrations of nitrate can be generated during anaerobic ammonium oxidation (Anammox) wastewater treatment processes. Addition of sulfur to Anammox reactors stimulates the growth of sulfur-driven denitrifying (SADN) bacteria that can reduce nitrate to nitrogen gas. However, protons released during the SADN process lower the pH of the system and inhibit Anammox activity. The system will keep stable when pH is in the range of 7.5-8.5. This study showed that addition of siderite stabilized the reactor system and significantly improved the nitrogen removal process. In fact, even when concentrations of total nitrogen were 477.15 ± 16.84 mg/L, the sulfur/siderite reactor maintained nitrogen removal efficiencies >90%, while efficiencies in the sulfur reactor were < 80%. Anammox accounted for 31% of the bacterial sequences in the sulfur/siderite reactor compared to only 14% in the sulfur reactor with the majority of sequences clustering with Ca. Brocadia. An abundance of c-type cytochromes in anammox aggregates in the sulfur-siderite reactor also indicated that anammox activity was higher in this system.

Simultaneous heterotrophic and FeS2-based ferrous autotrophic denitrification process for low-C/N ratio wastewater treatment: Nitrate removal performance and microbial community analysis

Heterotrophic-autotrophic denitrification reduces the cost of wastewater treatment and the risk of excess chemical oxygen demanded (COD) in the effluent. A mixotrophic denitrification system involving mixed heterotrophic and ferrous autotrophic bacteria was investigated to treat low-C/N ratio (C/N, defined as chemical oxygen demand (COD)/total nitrogen (TN)) wastewater with pyrite and organic carbon as electron donors. The system yielded effluent total nitrogen (TN) of 0.38 mg/L in 48 h due to a synergistic effect when the C/N ratio was 0.5 and influent nitrate nitrogen (NO₃^--N) was 20 mg/L; this TN value was significantly lower than those of the heterotrophic system (14.08 mg/L) and ferrous autotrophic system (12.00 mg/L). The highest abundance of the narG gene was observed in the mixotrophic denitrification system, along with more abundant microbial species. The dominant denitrification bacteria in each system included Thaurea, Ferritrophicum, Pseudomonas, and Thiobacillus, which varied with the initial inoculum source and the environment. Nevertheless, the abundance of the heterotrophic bacteria Thaurea decreased with prolonged operation of the systems. Together, these results implied that the simultaneous heterotrophic and FeS₂-based ferrous autotrophic denitrification process can be an alternative approach for the treatment of low-C/N ratio wastewater.

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

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

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

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

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

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

Denitrification performance of sulfur-based autotrophic denitrification and biomass‑sulfur-based mixotrophic denitrification in solid-phase denitrifying reactors using novel composite filters

Both the elemental sulfur-based autotrophic denitrification (ESAD) and the biomass‑sulfur-based mixotrophic (simultaneous autotrophic and heterotrophic) denitrification processes (BSMD) are efficient methods for removing nitrate from wastewater. However, a comparative analysis of the denitrification capacity of the BSMD and ESAD in the packed bed reactors is still lacking. In this paper, corncob powder was selected as the biomass source to prepare biomass‑sulfur-based composite filter (BSCF) for the BSMD process. The denitrification performances of the three identical lab-scale bioreactors packed with varying elemental sulfur-based composite filters (ESCFs) were compared under varying loading conditions, and the optimal ESCF of the ESAD system was 2:1 by weight ratio of sulfur powder to shell powder. In pilot-scale experiments, the results showed that BSCF could decrease the sulfate productivity and gave better denitrification performance than the ESCF with the optimal nitrate removal rate (NRR) of 504 ± 12.3 mg NO₃^--N·L^-1·d^-1. In addition, the two-stage flushing strategy (for the removal of aged sludge) can effectively improve the denitrification capacity, while the denitrification will be inhibited when the influent dissolved oxygen concentration was higher than 4.5 mg L^-1. Moreover, the heterotrophs and autotrophs were abundant in the reactors. Over time, the abundance of autotrophs increased while that of heterotrophs decreased. Overall, BSCF could be a promising and economic technology to improve the effluent quality.

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

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

Mixotrophic denitrification processes based on composite filler for low carbon/nitrogen wastewater treatment

Removal of nitrogen from wastewater with low carbon/nitrogen ratio was treated by using a denitrification packed bed reactor. Composite fillers with both autotrophic and heterotrophic denitrification capacity were prepared by mixing melted polycaprolactone and elemental sulfur at various alkalinity ratios (heterotrophic to autotrophic ratios of 1:2, 1:1, 3:2, and 2:1). Optimum denitrification was achieved at a ratio of 2:1. The diversity of the microbial community in the biofilm on the surface of the composite fillers showed that the increase of the elemental sulfur in the composite fillers has led to the increase of the microbial abundance. Furthermore, biofilm composition developed from a single dominant species to multiple species, and genes related to sulfur metabolism increased while those related to denitrification decreased slightly.

Effect of hydraulic retention time on performance of autotrophic, heterotrophic, and split-mixotrophic denitrification systems supported by polycaprolactone/pyrite: Difference and potential explanation

Biological denitrification is still the most important pathway to purifying nitrate-containing wastewater. In this study, pyrite (FeS₂ ) and polycaprolactone (PCL) were used as electron donors to construct sole or combined denitrification systems, that is, pyrite-based autotrophic denitrification (PAD) system, PCL-supported heterotrophic denitrification (PHD) system, and split-mixotrophic denitrification system (combined PAD + PHD), all of which were operated under five different hydraulic retention times (HRTs) for 150 days. The results showed that the removal rates (RE) of nitrate (NO₃ ^- -N) and inorganic phosphorus (PO₄ ^3- -P) by PAD were 91% and 94%, respectively, but the effluent sulfate (SO₄ ^2- ) concentration was as high as 168.2 mg/L; the removal rate of NO₃ ^- -N by PHD was higher than 99%, but the PO₄ ^3- -P could not be removed ideally; the removal rates of NO₃ ^- -N and PO₄ ^3- -P by PAD + PHD were higher than 95% and 99%, respectively, and the effluent SO₄ ^2- concentration was only 7.2 mg/L. Through the analysis of the surface scanning electron microscope (SEM) images of the two kinds of media before and after use, it was found that the coupled mode of PAD + PHD was more favorable for biofilm formation than the sole PAD or PHD process, and the microorganisms in the PAD + PHD mode made more full use of electron donors. Moreover, the biomass of the PAD + PHD mode was lower than that of the PAD or PHD process, but the denitrification efficiency of the coupled mode was more efficient, indicating that the functional microorganisms in the PAD + PHD mode might have a certain synergistic effect. PRACTITIONER POINTS: Removal rates of NO₃ -, PO₄ ³ -, and SO₄ ² - by PAD were 91%, 94%, and -233%, respectively. Removal rate of NO₃ - by PHD exceeded 99%, but PO₄ ³ - could not be removed ideally. Removal rates of NO₃ -, PO₄ ³ -, and SO₄ ² - by PAD + PHD were 95%, 99%, and 86%, respectively. The coupled mode was more favorable for biofilm formation than the sole PAD or PHD. The coupled mode had lower biomass but got more excellent denitrification efficiency.

Mixotrophic bacteria for environmental detoxification of contaminated waste and wastewater

Mixotrophic bacteria provide a desirable alternative to the use of classical heterotrophic or chemolithoautotrophic bacteria in environmental technology, particularly under limiting nutrients conditions. Their bi-modal ability of adapting to inorganic or organic carbon feed and sulfur, nitrogen, or even heavy metal stress conditions are attractive features to achieve efficient bacterial activity and favorable operation conditions for the environmental detoxification or remediation of contaminated waste and wastewater. This review provides an overview on the state of the art and summarizes the metabolic traits of the most promising and emerging non-model mixotrophic bacteria for the environmental detoxification of contaminated wastewater and waste containing excess amounts of limiting nutrients. Although mixotrophic bacteria usually function with low organic carbon sources, the unusual capabilities of mixotrophic electroactive exoelectrogens and electrotrophs in bioelectrochemical systems and in microbial electrosynthesis for accelerating simultaneous metabolism of inorganic or organic C and N, S or heavy metals are reviewed. The identification of the mixotrophic properties of electroactive bacteria and their capability to drive mono- or bidirectional electron transfer processes are highly exciting and promising aspects. These aspects provide an appealing potential for unearthing new mixotrophic exoelectrogens and electrotrophs, and thus inspire the next generation of microbial electrochemical technology and mixotrophic bacterial metabolic engineering. KEY POINTS: • Mixotrophic bacteria efficiently and simultaneously remove C and N, S or heavy metals. • Exoelectrogens and electrotrophs accelerate metabolism of C and N, S or heavy metals. • New mixotrophic exoelectrogens and electrotrophs should be discovered and exploited.

Activity and electron donor preference of two denitrifying bacterial strains identified by Raman gas spectroscopy

Human activities have greatly increased the input of reactive nitrogen species into the environment and disturbed the balance of the global N cycle. This imbalance may be offset by bacterial denitrification, an important process in maintaining the ecological balance of nitrogen. However, our understanding of the activity of mixotrophic denitrifying bacteria is not complete, as most research has focused on heterotrophic denitrification. The aim of this study was to investigate substrate preferences for two mixotrophic denitrifying bacterial strains, Acidovorax delafieldii and Hydrogenophaga taeniospiralis, under heterotrophic, autotrophic or mixotrophic conditions. This complex analysis was achieved by simultaneous identification and quantification of H₂, O₂, CO₂, ¹⁴N₂, ¹⁵N₂ and ¹⁵N₂O in course of the denitrification process with help of cavity-enhanced Raman spectroscopic (CERS) multi-gas analysis. To disentangle electron donor preferences for both bacterial strains, microcosm-based incubation experiments under varying substrate conditions were conducted. We found that Acidovorax delafieldii preferentially performed heterotrophic denitrification in the mixotrophic sub-experiments, while Hydrogenophaga taeniospiralis preferred autotrophic denitrification in the mixotrophic incubation. These observations were supported by stoichiometric calculations. The results demonstrate the prowess of advanced Raman multi-gas analysis to study substrate use and electron donor preferences in denitrification, based on the comprehensive quantification of complex microbial gas exchange processes.

Coupling transformation of carbon, nitrogen and sulfur in a long-term operated full-scale constructed wetland

The coupling transformation of carbon, nitrogen and sulfur compounds has been studied in lab-scale and pilot-scale constructed wetlands (CWs), but few studies investigated full-scale CW. In this study, we used batch experiments to investigate the potentials of carbon, nitrogen and sulfur transformation in a long-term operated, full-scale horizontal subsurface flow wetland. The sediments collected from the HSFW were incubated for 48 h in the laboratory with supplying various dosages of carbon, nitrogen and sulfur compounds. The results showed that heterotrophic denitrification was the main pathway. At the same time, the sulfide (S^2-)-based autotrophic denitrification was also present. Increasing TOC concentration or NO₃^- concentration could promote heterotrophic denitrification but did not inhibit the sulfide-based autotrophic denitrification. In our experiment, the highest NO₃^- removal via autotrophic denitrification was 25.23% while that via heterotrophic denitrification was 73.66%, leading to the total NO₃^- removal of 98.89%. The results also demonstrated that NO₃^- rather than NO₂^- was the preferable electron acceptor for both heterotrophic and sulfide-based autotrophic denitrification in the CW. Increasing S^2- concentrations promote NO₃^- removal from 12.99% to 25.23% without organic carbon, but varying NO₃^- or NO₂^- has no effects. These results indicated that concentrations of S^2-, instead of NO₃^- or NO₂^-, was the limiting factor for sulfide-based autotrophic denitrification in the studied CW. The microbial community analysis and correlation analysis between the transformation of carbon, nitrogen and sulfur compounds and relative abundance of bacteria further confirmed that in the CW, the key pathways coupling transformation were heterotrophic denitrification and sulfide-based autotrophic denitrification. Overall, the current study will enhance understanding of carbon, nitrogen, and sulfur transformation in CW and support better design and treatment efficiency.

Wood and sulfur-based cyclic denitrification filters for treatment of saline wastewaters

This study investigated the performance and microbiome of cyclic denitrification filters (CDFs) for wood and sulfur heterotrophic-autotrophic denitrification (WSHAD) of saline wastewater. Wood-sulfur CDFs integrated into two pilot-scale marine recirculating aquaculture systems achieved high denitrification rates (103 ± 8.5 g N/(m³·d)). The combined use of pine wood and sulfur resulted in lower SO₄^2- accumulation compared with prior saline wastewater denitrification studies with sulfur alone. Although fish tank water quality parameters, including ammonia, nitrite, nitrate and sulfide, were below the inhibitory levels for marine fish production, lower survival rates of Poecilia sphenops were observed compared with prior studies. Heterotrophic denitrification was the dominant removal mechanism during the early operational stages, while sulfur autotrophic denitrification increased as readily biodegradable organic carbon released from wood chips decreased over time. 16S rRNA-based analysis of the CDF microbiome revealed that Sulfurimonas, Thioalbus, Defluviimonas, and Ornatilinea as notable genera that contributed to denitrification performance.

Enhancing nitrate removal from wastewater by integrating heterotrophic and autotrophic denitrification coupled manganese oxidation process (IHAD-MnO): Internal carbon utilization performance

Due to cause the deterioration of water quality and can produce toxic nitrite, the nitrate constituted of great threatens to human health and eco-systematic safety. Among most well-known biotechnology to remove nitrate, the integrated heterotrophic and autotrophic denitrification (IHAD) process is promising, especially for the organic-limited polluted water. In this work, the IHAD coupled manganese oxidation (IHAD-MnO) process was developed by using Pseudomonas sp. SZF15 (Gram negative strain, and rod-shaped morphology with 2.3 μm in length) in the glass serum bottles. It was found that limited organic content could accelerate nitrate removal rate, and manganese oxidation efficiency can reach up to 60.08%. To further explain carbon conversion characteristics of the process, pure heterotrophic condition assays were conducted, the results confirmed that inorganic carbon will be generated by organic carbon metabolism in heterotrophic condition, the maximum accumulation content of inorganic carbon was 142.21 mg/L (when the initial organic carbon level was 293 mg-C/L). Subsequently, since the consumption of organic carbon, biogenic inorganic carbon can be further utilized by microorganisms to support autotrophic denitrification (AuDN). Besides, X-ray photoelectron spectroscopy (XPS) was employed to analyze precipitation products produced from the process. The magnified Mn 2p spectra results showed that a typical characteristic peak of manganese dioxide was observed with the intense peak at 641.8 eV and a satellite peak at 653.7 eV, respectively. This showed that Mn(II) was oxidized to manganese dioxide by the process, which may be a functional material with adsorption properties. The process posed a highly efficient and cost effective solution with less carbon consumption and less greenhouse gas emission for sustainable water treatment technologies.

Sulfur-based Mixotrophic Vanadium (V) Bio-reduction towards Lower Organic Requirement and Sulfate Accumulation

Although remediation of toxic vanadium (V) [V(V)] pollution can be achieved through either heterotrophic or sulfur-based autotrophic microbial reduction, these processes would require a large amount of organic carbons or generate excessive sulfate. This study reported that by using mixotrophic V(V) bio-reduction with acetate and elemental sulfur [S(0)] as joint electron donors, V(V) removal performance was enhanced due to cooccurrence of heterotrophic and autotrophic activities. Deposited vanadium (IV) was identified as the main reduction product by scanning electron microscopy, energy-dispersive X-ray spectroscopy, X-ray diffraction and X-ray photoelectron spectroscopy. Based on 16S rRNA gene amplicon sequencing, qPCR and genus-specific reverse transcription qPCR, it was observed that V(V) was likely detoxified by heterotrophic V(V) reducers (e.g., Syntrophobacter, Spirochaeta and Geobacter). Cytochrome c, intracellular nicotinamide adenine dinucleotide and extracellular polymeric substances were involved in V(V) reduction and binding. Organic metabolites synthesized by autotrophs (e.g., Thioclava) with energy from S(0) oxidation might compensate electron donors for heterotrophic V(V) and sulfate reducers. Less sulfate was accumulated presumably due to activities of sulfur-respiring genera (e.g., Desulfurella). This study demonstrates mixotrophic microbial V(V) reduction can save organic dosage and avoid excessive sulfate accumulation, which will be beneficial to bioremediation of V(V) contamination.

Autotrophic denitrification in constructed wetlands: Achievements and challenges

The use of constructed wetlands for wastewater treatment is rapidly increasing worldwide due to their advantages of low operating and maintenance costs. Denitrification in constructed wetlands is dependent on the presence of organic carbon sources, and the shortage of organic carbon is the primary hurdle for nitrate removal. Therefore, the use of inorganic electronic donors has emerged as an alternative. This paper provides a comprehensive review of nitrate removal pathways using various inorganic electron donors and the performance and development of autotrophic denitrification in constructed wetlands. The main environmental parameters and operating conditions for nitrate removal in wetlands are discussed, and the challenges currently faced in the application of enhanced autotrophic denitrification wetlands are emphasized. Overall, this review illustrates the need for a deep understanding of the complex interrelationships among environmental and operational parameters and wetland substrates for improving the wastewater treatment performance of constructed wetlands.

Performance and mechanism of synchronous nitrate and phosphorus removal in constructed pyrite-based mixotrophic denitrification system from secondary effluent

The performance and process of the constructed pyrite-based mixotrophic denitrification (POMD) system using pyrite and residual organic matters as the co-electron donors were investigated for simultaneous removal of N and P from secondary effluent. After the batch experiments, 61.80 ± 3.26% of phosphate and 99.99 ± 0.01% of nitrate were removed, and the obtained nitrate removal rate constant can reach 2.09 days^-1 in POMD system, which was significantly superior to that reported (0.95 day^-1) in pyrite-based autotrophic denitrification (PAD) system. PO₄^3--P removal was mainly achieved via chemical precipitation as FePO₄ with iron, and it was irrelevant with the initial nitrate and ammonium concentrations. High-throughput 16S rRNA gene sequencing analysis showed the coexistence of heterotrophic and autotrophic denitrifiers in the mixotrophic environment. The denitrification process could be divided into two stages according to the carbon balance and calculation of sulfate accumulation: (a) nitrate was mainly reduced heterotrophically during 12-36 h and (b) nitrate was reduced autotrophically after 36 h. The calculated proportion of heterotrophic denitrification was 58.17 ± 3.78%, which was promoted by a higher ammonium concentration. These findings are likely to be useful in understanding the mixotrophic denitrification process and developing a cost-effective technology to simultaneously remove N and P from secondary effluent. Graphical abstract.

Distinct microbial communities and their networks in an anammox coupled with sulfur autotrophic/mixotrophic denitrification system

Organ carbon are often used to enhance denitrification in wastewater treatment. However, their possible effects on microbial interactions are very limited. In this work, an anaerobic ammonium oxidation (anammox) coupled with sulfur autotrophic/mixotrophic denitrification (SAD/SMD) system was used to investigate the changes in microbial interactions among the microbial communities under different nutrient condition. The removal efficiency of total nitrogen increased from 70% (SAD) to 97% (SMD). The Illumina sequencing analysis indicated that Planctomycetes was the most dominant bacterial phylum in anammox system. Thiobacillus and Sulfurimonas, two typical autotrophic denitrifiers, decreased significantly from 31.9% to 17.7%-12.2% and 9.3%, when the nutrient condition changed from SAD to SMD (P < 0.05). Meanwhile, some heterotrophic or mixotrophic denitrifying bacteria, including Gemmobacter, Pseudomonas and Thauera increased significantly (P < 0.05). Molecular ecological network (MEN) analysis showed that the addition of organic carbon substantially altered the overall architecture of the network. Compared with SAD, the SMD had shorter path lengths, indicating higher transfer efficiencies of information and materials among different microorganism. The addition of organic carbon increased the microbial interaction complexity of Proteobacteria. The links of Thiobacillus, which was a typical sulfur-oxidizing autotrophic denitrifying bacteria, significantly reduced (P < 0.05) with the addition of organic carbon, while the links of the heterotrophic bacteria Geobacter significantly increased (P < 0.05). This study provided new insights into our understanding of the shifts in the bacteria community and their microbial interactions under different nutrient conditions (SAD and SMD) in sulfur-supported denitrification system.

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.

Hydrilla verticillata-Sulfur-Based Heterotrophic and Autotrophic Denitrification Process for Nitrate-Rich Agricultural Runoff Treatment

Hydrilla verticillata-sulfur-based heterotrophic and autotrophic denitrification (HSHAD) process was developed in free water surface constructed wetland mesocosms for the treatment of nitrate-rich agricultural runoff with low chemical oxygen demand/total nitrogen (C/N) ratio, whose feasibility and mechanism were extensively studied and compared with those of H. verticillata heterotrophic denitrification (HHD) mesocosms through a 273-day operation. The results showed that the heterotrophic and autotrophic denitrification can be combined successfully in HSHAD mesocosms, and achieve satisfactory nitrate removal performance. The average NO₃^--N removal efficiency and denitrification rate of HSHAD were 94.4% and 1.3 g NO₃^--N m^-3·d^-1 in steady phase II (7-118 d). Most nitrate was reduced by heterotrophic denitrification with sufficient organic carbon in phase I (0-6 d) and II, i.e., the C/N ratio exceeded 4.0, and no significant difference of nitrate removal capacity was observed between HSHAD and HHD mesocosms. During phase III (119-273 d), sulfur autotrophic denitrification gradually dominated the HSHAD process with the C/N ratio less than 4.0, and HSHAD mesocosms obtained higher NO₃^--N removal efficiency and denitrification rate (79.1% and 1.1 g NO₃^--N m^-3·d^-1) than HHD mesocosms (65.3% and 1.0 g NO₃^--N m^-3·d^-1). As a whole, HSHAD mesocosms removed 58.8 mg NO₃^--N more than HHD mesocosms. pH fluctuated between 6.9-9.0 without any pH buffer. In general, HSHAD mesocosms were more stable and efficient than HHD mesocosms for NO₃^--N removal from agricultural runoff during long-term operation. The denitrificans containing narG (1.67 × 10⁸ ± 1.28 × 10⁷ copies g^-1 mixture-soil^-1), nirS (8.25 × 10⁷ ± 8.95 × 10⁶ copies g^-1 mixture-soil^-1), and nosZ (1.56 × 10⁶ ± 1.60 × 10⁵ copies g^-1 mixture-soil^-1) of litter bags and bottoms in HSHAD were higher than those in HHD, which indicated that the combined heterotrophic and autotrophic denitrification can increase the abundance of denitrificans containing narG, nirS, and nosZ, thus leading to better denitrification performance.

A novel start-up strategy for mixotrophic denitrification biofilters by rhamnolipid and its performance on denitrification of low C/N wastewater

A novel start-up strategy for sulfur-based mixotrophic denitrification biofilters (mDNBFs) by rhamnolipid was investigated for the first time. Rhamnolipid with gradient concentrations (0-120 mg/L) was added into five lab-scale mDNBFs. Results showed that rhamnolipid could promote biomass yield and nitrogen removal rate (NRR) by 71.7% and 68.7%, respectively, while its effect on EPS and adhesion force was concentration-dependent. The spatial distribution characteristics of microbial communities demonstrated the enrichment of main heterotrophic denitrifying bacteria outcompeted that of the autotrophs, with a more pronounced difference in high concentration rhamnolipid-treated mDNBFs. Furthermore, highest abundance of napA, narG, nirK and nosZ genes was observed in 80 mg/L rhamnolipid-treated mDNBF. Interfacial processes including solubilizing effect and hydration repulse and variations of organics were discussed to explicate the underlying mechanism. The study enlightened that an appropriate concentration (∼80 mg/L) of rhamnolipid may be a good solution for accelerating biofilm formation and enriching denitrifying bacteria to promote denitrification performance of mDNBFs treating low C/N wastewater.

Nitrogen removal from food waste digestate using partial nitritation-anammox process: Effect of different aeration strategies on performance and microbial community dynamics

The feasibility of employing anammox and partial nitritation-anammox (PN/A) processes for nitrogen removal from food waste (FW) digestate was investigated in this study. The effects of different aeration strategies on the microbial community were also investigated. To achieve this, after anammox enrichment (Phase 1), the reactor was fed with digestate supplemented with nitrite (Phase 2), and subsequently different aeration strategies were evaluated to establish PN/A. Aeration strategies with high anoxic periods (30 and 45 min) in relation to aerobic periods (15 min) coupled with low air flow rates (0.026 L  min^-1. L_reator^-1) were found to be better for establishing PN/A, as coefficients of produced nitrate/removed ammonium were closer to those reported previously (0.17 and 0.21). Aeration conditions considerably altered the microbial community. Candidatus Brocadia was replaced by Candidatus Jettenia, after the first aeration strategies. These results support the feasibility of FW digestate treatment using anammox and PN/A processes and provide a better understanding of the effect of aeration on microbial dynamics in PN/A reactors.

Efficient Nitrogen Removal of Reject Water Generated from Anaerobic Digester Treating Sewage Sludge and Livestock Manure by Combining Anammox and Autotrophic Sulfur Denitrification Processes

The reject water from anaerobic digestion with high (Total Nitrogen) TN concentration was treated by a demonstration plant combining the anammox process and SOD (SOD®; Sulfur Oxidation Denitrification) process. The anaerobic digestion was a co-digestion of livestock wastewater, food waste water, and sewage sludge so that the TN concentration and conductivity of the reject water were very high. This anammox plant was the first anammox demonstration plant in South Korea. The maximum TN removal efficiency of 80% was achieved for the anammox reactor under nitrogen loading rate (NLR) of 0.45 kg-N/m3·d. As a result of decreasing the dilution of the reject water, the influent conductivity and NLR values were increased to 7.8 mS/cm and 0.7 kg/m3·d, causing a rapid decrease in the TN removal efficiency. The sludge concentration from the hydro-cyclone overflow was about 40 mg-MLVSS/L in which small sized anammox granules were detected. It was proven that the increase in (Mixed Liquor Volatile Suspended Solids) MLVSS concentration in the anammox reactor was not easy under high influent conductivity and NLR. 97% of NO2−-N+NO3−-N generated from the anammox process could be treated successfully by the SOD reactor. A TN removal efficiency of 35% under poor annamox treatment could increase to 67% by applying the SOD reactor post treatment for the removal of NO3−-N. The dominant anammox bacteria in the anammox reactor was identified as Brocadia fulgida and 9.3% (genus level) of the bacteria out of the total bacteria were anammox bacteria.

Nitrate removal from drinking water with a focus on biological methods: a review

This article summarizes several developed and industrial technologies for nitrate removal from drinking water, including physicochemical and biological techniques, with a focus on autotrophic nitrate removal. Approaches are primarily classified into separation-based and elimination-based methods according to the fate of the nitrate in water treatment. Biological denitrification as a cost-effective and promising method of biological nitrate elimination is reviewed in terms of its removal process, applicability, efficiency, and associated disadvantages. The various pathways during biological nitrate removal, including assimilatory and dissimilatory nitrate reduction, are also explained. A comparative study was carried out to provide a better understanding of the advantages and disadvantages of autotrophic and heterotrophic denitrification. Sulfur-based and hydrogen-based denitrifications, which are the most common autotrophic processes of nitrate removal, are reviewed with the aim of presenting the salient features of hydrogenotrophic denitrification along with some drawbacks of the technology and research areas in which it could be used but currently is not. The application of algae-based water treatment is also introduced as a nature-inspired approach that may broaden future horizons of nitrate removal technology.

Comparison of sulphide and nitrate removal from synthetic wastewater by pure and mixed cultures of nitrate-reducing, sulphide-oxidizing bacteria

In this study, the activities of hydrogen sulphide (H₂S) oxidation and nitrate (N-NO₃^-) reduction by three pure and mixed strains of nitrate-reducing, sulphide oxidizing bacteria (NR-SOB) were determined. Batch experiments were performed at 35 °C and pH 7.0-8.0 with initial H₂S concentrations of 650-900 ppm_v and N-NO₃^- concentrations of ∼120 mg/L. The strains MAL 1HM19, TPN 1HM1 and TPN 3HM1 were capable of removing 100% gas-phase H₂S. The co-cultures showed better performance for H₂S and N-NO₃^- removal. The mixed NR-SOB strains showed a higher H₂S oxidation rate (143 ± 18 ppm_v/h), while the highest N-NO₃^- removal rate (5.5 ± 0 and 5.1 ± 0.6 N-NO₃^- mg/L·h) was obtained by a mixture of two NR-SOB strains. The 16S rDNA sequence analysis revealed that all strains belonged to the sub-class Alphaproteobacteria and are closely related to Paracoccus sp. (>99%).

Nitrogen Removal by Sulfur-Based Carriers in a Membrane Bioreactor (MBR)

Sulfur-based carriers were examined to enhance the nitrogen removal efficiency in a mixed anoxic⁻anaerobic-membrane bioreactor system, in which sulfur from the carrier acts as an electron donor for the conversion of nitrate to nitrogen gas through the autotrophic denitrification process. A total nitrogen removal efficiency of 63% was observed in the system with carriers, which showed an increase in the removal efficiency of around 20%, compared to the system without carriers. The results also indicated that the carriers had no adverse effect on biological treatment for the organic matter and total phosphorus. The removal efficiencies for chemical oxygen demand (COD) and total phosphorus (TP) were 98% and 37% in both systems, respectively. The generation of sulfate ions was a major disadvantage of using sulfur-based carriers, and resulted in pH drop. The ratio of sulfate in the effluent to nitrate removed in the system ranged from 0.86 to 1.97 mgSO₄^2-/mgNO₃^--N, which was lower than the theoretical value and could be regarded as due to the occurrence of simultaneous heterotrophic and autotrophic denitrification.

Biological denitrification in marine aquaculture systems: A multiple electron donor microcosm study

There is a lack of information on denitrification of saline wastewaters, such as those from marine recirculating aquaculture systems (RAS), ion exchange brines and wastewater in areas where sea water is used for toilet flushing. In this study, side-by-side microcosms were used to compare methanol, fish waste (FW), wood chips, elemental sulfur (S⁰) and a combination of wood chips and sulfur for saline wastewater denitrification. The highest denitrification rate was obtained with methanol (23.4 g N/(m³·d)), followed by FW (4.5 g N/(m³·d)), S⁰ (3.5 g N/(m³·d)), eucalyptus mulch (2.6 g N/(m³·d)), and eucalyptus mulch with sulfur (2.2 g N/(m³·d)). Significant differences were observed in denitrification rate for different wood species (pine > oak ≫ eucalyptus) due to differences in readily biodegradable organic carbon released. A pine wood-sulfur heterotrophic-autotrophic denitrification (P-WSHAD) process provided a high denitrification rate (7.2-11.9 g N/(m³·d)), with lower alkalinity consumption and sulfate generation than sulfur alone.

Nitrate removal from aquaculture effluents using woodchip bioreactors improved by adding sulfur granules and crushed seashells

This study examined the effects on nitrate removal when adding sulfur granules and crushed seashells to a woodchip bioreactor treating aquaculture effluents. Using a central composite design, the two components were added at three levels (0.000, 0.125 and 0.250 m³/m³ bioreactor volume) to 13 laboratory-scale woodchip bioreactors, and a response surface method was applied to find and model the optimal mixture ratios with respect to reactor performance. Adding 0.125 m³/m³ sulfur granules improved the total N removal rate from 3.27 ± 0.38 to 8.12 ± 0.49 g N/m³/d compared to pure woodchips. Furthermore, the inclusion of crushed seashells together with sulfur granules helped to maintain the pH above 7.4 and prevent a production (i.e., release) of nitrite. According to the modeled response surfaces, a sulfur granule:crushed seashell:woodchip mixture ratio containing about 0.2 m³ sulfur granules and 0.1 m³ crushed seashells per m³ reactor volume would give the best results with respect to high N removal and minimal nitrite release. In conclusion, the study showed that N removal in woodchip bioreactors may be improved by adding sulfur granules and seashells, contributing to the optimization of woodchip performance in treating aquaculture effluents.

Coupling autotrophic denitrification with partial nitritation-anammox (PNA) for efficient total inorganic nitrogen removal

The performance of and the microbial ecology in an integrated lab scale set up comprising of a PN/A bioreactor and an elemental sulfur-supported packed bed autotrophic denitrification (ESSAD) are reported. The PN/A reactor exhibited an average removal rate of 0.56±0.103kgNm^-3d^-1, whereas the ESSAD reactor removed an average of 0.0018kg NO₃^--Nm^-3d^-1. The combined average removal rate was 0.6kgNm^-3d^-1, yielding an overall total inorganic nitrogen efficiency of 97%. Based on 16S rRNA gene clone libraries from the ESSAD reactor, the extracted Operational Taxonomic Units (OTUs) formed a clade with Thiobacillus denitrificans sp. indicating a common ancestral relationship. High throughput amplicon sequencing targeting V3 region of 16S rRNA gene for the biofilm in the ESSAD also revealed an abundance of the Thiobacillus genus. Additionally, 16s rRNA amplicon sequencing of the genomic DNA from the PN/A reactor reflected a dominance of the Planctomycetes phylum.

Nitrate-rich agricultural runoff treatment by Vallisneria-sulfur based mixotrophic denitrification process

Vallisneria-sulfur based mixotrophic denitrification (VSMD) process was put forward for the treatment of nitrate-rich agricultural runoff with low COD/TN (C/N) ratio in free water surface constructed wetland mesocosms, whose feasibility and mechanism were thoroughly studied through 273-day operation. The results showed that the average NO₃^--N removal efficiency and denitrification rate of VSMD mesocosms were 97.7% and 1.5gNO₃^--Nm^-3d^-1 under 5.0 or higher C/N ratio conditions in phase II (7-117d), which were similar with those of Vallisneria packed heterotrophic denitrification (VHD) mesocosms. However, VSMD mesocosms with 2.0 average C/N ratio in phase III (118-273d) were more stable and efficient than VHD mesocosms. More than 49.4mg NO₃^--N was reduced by VSMD mesocosms than that by VHD mesocosms throughout the operation. NO₂^--N accumulation in phase I (0-6d) had no influence on denitrification performance of VSMD mesocosms. In phase II and III, effluent COD, NH₄⁺-N and NO₂^--N could meet the Class II standard of Environmental quality for surface water (GB3838-2002) if the experiment was carried out in batch mode. pH in VSMD mesocosms fluctuated between 7.0 and 8.9 throughout the operation without any pH buffer. The abundance of three denitrifying genes coding for nitrate (narG), nitrite (nirS), and nitrous oxide (nosZ) reductases in bottom soil and mixture from litter bags was quantified. VSMD could supply more favorable circumstances for the growth of denitrificans containing narG (3.1×10⁸±7.9×10⁷copiesg^-1mixture^-1) and nirS (2.1×10⁸±2.0×10⁶copiesg^-1mixture^-1) in litter bags than VHD, i.e., 8.7×10⁷±1.4×10⁷ and 1.4×10⁸±1.5×10⁷copiesg^-1mixture^-1 for narG and nirS respectively. Sulfur addition in VSMD mesocosms might increase the abundance of denitrificans containing narG and nirS, thus led to better denitrification performance.

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

Elemental sulfur (S(o)) can serve as an electron donor for denitrification. However, the mechanisms and rates of S(o)-based denitrification, which depend on a biofilm development on a solid S(o) surface, are not well understood. We used completely-mixed reactors packed with S(o) chips to systematically explore the behavior of S(o)-based denitrification as a function of the bulk nitrate (NO3(-)) concentration and biofilm development. High-purity (99.5%) and agricultural-grade (90% purity) S(o) chips were tested to explore differences in performance. NO3(-) fluxes followed a Monod-type relationship with the bulk NO3(-) concentration. For high-purity S(o), the maximum NO3(-) flux increased from 0.4 gN/m(2)-d at 21 days to 0.9 g N/m(2)-d at around 100 days, but then decreased to 0.65 gN/m(2)-d at 161 days. The apparent (extant) half-saturation constant for NO3(-) KSapp, based on the bulk NO3(-) concentration and NO3(-) fluxes into the biofilm, increased from 0.1 mgN/L at 21 days to 0.8 mgN/L at 161 days, reflecting the increasing mass transfer resistance as the biofilm thickness increased. Nitrite (NO2(-)) accumulation became significant at bulk NO3(-) concentration above 0.2 mgN/L. The behavior of the agricultural-grade S(o) was very similar to the high-purity S(o). The kinetic behavior of S(o)-based denitrification was consistent with substrate counter-diffusion, where the soluble sulfur species diffuse from the S(o) particle into the base of the biofilm, while NO3(-) diffuses into the biofilm from the bulk. Initially, the fluxes were low due to biomass limitation (thin biofilms). As the biofilm thickness increased with time, the fluxes first increased, stabilized, and then decreased. The decrease was probably due to increasing diffusional resistance in the thick biofilm. Results suggest that fluxes comparable to heterotrophic biofilm processes can be achieved, but careful management of biofilm accumulation is important to maintain high fluxes.

Comparative investigation on integrated vertical-flow biofilters applying sulfur-based and pyrite-based autotrophic denitrification for domestic wastewater treatment

Two parallel biofilters applying sulfur/pyrite-based autotrophic denitrification were investigated for removing COD, TP and TN by a coordinated process. Results demonstrated good performance by removing 86.32% vs 87.14% COD and 92.56% vs 89.65% NH4(+)-N. Biofilter with sulfur (BS) was superior on nitrate (89.74% vs 80.72%) and TN removal (83.18% vs 70.42%) while biofilter with pyrite (BP) was better on TP removal (82.58% vs 77.40%) and maintaining sulfate (27.56mgL(-1) vs 41.55mgL(-1)) and pH (7.13 vs 6.31). Water-permeable adsorbents lowered clogging risk and buffered loading. Clone library revealed reasons of diversities, pH variation and sulfate accumulation of both biofilters. Sulfur was efficient on denitrification but whose byproducts were troublesome, pyrite produced less byproduct but which was sensitive to organics. This research was the first attempt to systematically compare two promising alternatives and their merits/demerits for rural wastewater on-site treatment.

Woodchip-sulfur based heterotrophic and autotrophic denitrification (WSHAD) process for nitrate contaminated water remediation

Nitrate contaminated water can be effectively treated by simultaneous heterotrophic and autotrophic denitrification (HAD). In the present study, woodchips and elemental sulfur were used as co-electron donors for HAD. It was found that ammonium salts could enhance the denitrifying activity of the Thiobacillus bacteria, which utilize the ammonium that is produced by the dissimilatory nitrate reduction to ammonium (DNRA) in the woodchip-sulfur based heterotrophic and autotrophic denitrification (WSHAD) process. The denitrification performance of the WSHAD process (reaction constants range from 0.05485 h(-1) to 0.06637 h(-1)) is better than that of sulfur-based autotrophic denitrification (reaction constants range from 0.01029 h(-1) to 0.01379 h(-1)), and the optimized ratio of woodchips to sulfur is 1:1 (w/w). No sulfate accumulation is observed in the WSHAD process and the alkalinity generated in the heterotrophic denitrification can compensate for alkalinity consumption by the sulfur-based autotrophic denitrification. The symbiotic relationship between the autotrophic and the heterotrophic denitrification processes play a vital role in the mixotrophic environment.

Sulfur-based mixotrophic denitrification corresponding to different electron donors and microbial profiling in anoxic fluidized-bed membrane bioreactors

Sulfur-based mixotrophic denitrifying anoxic fluidized bed membrane bioreactors (AnFB-MBR) were developed for the treatment of nitrate-contaminated groundwater with minimized sulfate production. The nitrate removal rates obtained in the methanol- and ethanol-fed mixotrophic denitrifying AnFB-MBRs reached 1.44-3.84 g NO3 -N/L reactor d at a hydraulic retention time of 0.5 h, which were significantly superior to those reported in packed bed reactors. Compared to methanol, ethanol was found to be a more effective external carbon source for sulfur-based mixotrophic denitrification due to lower sulfate and total organic carbon concentrations in the effluent. Using pyrosequencing, the phylotypes of primary microbial groups in the reactor, including sulfur-oxidizing autotrophic denitrifiers, methanol- or ethanol-supported heterotrophic denitrifiers, were investigated in response to changes in electron donors. Principal component and heatmap analyses indicated that selection of electron donating substrates largely determined the microbial community structure. The abundance of Thiobacillus decreased from 45.1% in the sulfur-oxidizing autotrophic denitrifying reactor to 12.0% and 14.2% in sulfur-based methanol- and ethanol-fed mixotrophic denitrifying bioreactors, respectively. Heterotrophic Methyloversatilis and Thauera bacteria became more dominant in the mixotrophic denitrifying bioreactors, which were possibly responsible for the observed methanol- and ethanol-associated denitrification.

Characterization of the anaerobic denitrification bacterium Acinetobacter sp. SZ28 and its application for groundwater treatment

Acinetobacter sp. SZ28 exhibited efficient autotrophic denitrification ability using Mn(2+) as an electron donor. Sequence amplification identified the presence of the nirS gene. Meteorological chromatography analysis showed that N2 was produced as an end product. Response surface methodology experiments showed that the maximum removal of nitrate occurred under the following conditions: Mn(2+) concentration of 143.56 mg/L, C/N ratio of 6.82, initial pH of 5.17, and temperature of 34.26 °C, where the initial Mn(2+) concentration produced the largest effect. In the groundwater experiment, nitrate levels decreased from 1.63 mg/L to 0 mg/L. Three-dimensional fluorescence analysis showed a decrease in the peak intensity of the original humus. Humus and the small-molecule amino acid tryptophan were detected. These results demonstrated that strain SZ28 is a suitable candidate for the simultaneous removal of nitrogen and Mn(2+) in groundwater treatment.

Heterotrophic-autotrophic sequential system for reductive nitrate and perchlorate removal

Nitrate and perchlorate were identified as significant water contaminants all over the world. This study aims at evaluating the performances of the heterotrophic-autotrophic sequential denitrification process for reductive nitrate and perchlorate removal from drinking water. The reduced nitrate concentration in the heterotrophic reactor increased with increasing methanol concentrations and the remaining nitrate/nitrite was further removed in the following autotrophic denitrifying process. The performances of the sequential process were studied under varying nitrate loads of [Formula: see text] at a fixed hydraulic retention time of 2 h. The C/N ratio in the heterotrophic reactor varied between 1.24 and 2.77 throughout the study. Nitrate and perchlorate reduced completely with maximum initial concentrations of [Formula: see text] and 1000 µg/L, respectively. The maximum denitrification rate for the heterotrophic reactor was [Formula: see text] when the bioreactor was fed with [Formula: see text] and 277 mg/L methanol. For the autotrophic reactor, the highest denitrification rate was [Formula: see text] in the first period when the heterotrophic reactor performance was low. Perchlorate reduction was initiated in the heterotrophic reactor, but completed in the following autotrophic process. Effluent sulphate concentration was below the drinking water standard level of 250 mg/L and pH was in the neutral level.

Investigation on thiosulfate-involved organics and nitrogen removal by a sulfur cycle-based biological wastewater treatment process

Thiosulfate, as an intermediate of biological sulfate/sulfite reduction, can significantly improve nitrogen removal potential in a biological sulfur cycle-based process, namely the Sulfate reduction-Autotrophic denitrification-Nitrification Integrated (SANI(®)) process. However, the related thiosulfate bio-activities coupled with organics and nitrogen removal in wastewater treatment lacked detailed examinations and reports. In this study, S2O3(2-) transformation during biological SO4(2-)/SO3(2-) co-reduction coupled with organics removal as well as S2O3(2-) oxidation coupled with chemolithotrophic denitrification were extensively evaluated under different experimental conditions. Thiosulfate is produced from the co-reduction of sulfate and sulfite through biological pathway at an optimum pH of 7.5 for organics removal. And the produced S2O3(2-) may disproportionate to sulfide and sulfate during both biological S2O3(2-) reduction and oxidation most possibly carried out by Desulfovibrio-like species. Dosing the same amount of nitrate, pH was found to be the more direct factor influencing the denitritation activity than free nitrous acid (FNA) and the optimal pH for denitratation (7.0) and denitritation (8.0) activities were different. Spiking organics significantly improved both denitratation and denitritation activities while minimizing sulfide inhibition of NO3(-) reduction during thiosulfate-based denitrification. These findings in this study can improve the understanding of mechanisms of thiosulfate on organics and nitrogen removal in biological sulfur cycle-based wastewater treatment.

The heterotrophic-combined-with-autotrophic denitrification process: performance and interaction mechanisms

In this work, the interaction mechanisms between an autotrophic denitrification (AD) and heterotrophic denitrification (HD) process in a heterotrophic-autotrophic denitrification (HAD) system were investigated, and the performance of the HAD system under different S/Ac(-) molar ratios was also evaluated. The results demonstrated that the heterotrophic-combined-with-autotrophic denitrification process is a promising technology which can remove chemical oxygen demand (COD), sulfide and nitrate simultaneously. The reduction rate of NO(3)(-) to NO(2)(-) by the HD process was much faster than that of reducing NO(2)(-) to N₂, while the reduction rate of NO(3)(-) to NO(2)(-) by the AD process was slower than that of NO(2)(-) to N₂. Therefore, the AD process could use the surplus NO(2)(-) produced by the HD process. This could alleviate the NO(2)(-)-N accumulation and increase the denitrification rate. In addition, the inhibition effects of acetate on AD bacteria and sulfide on HD were observed, and the inhibition was compensated by the promotion effects on NO(2)(-). Therefore, the processes of AD and HD seem to react in parallel, without disturbing each other, in our HAD system.

Kinetics of autotrophic denitrification process and the impact of sulphur/limestone ratio on the process performance

Kinetics of sulphur-limestone autotrophic denitrification process in batch assays and the impact of sulphur/limestone ratio on the process performance in long-term operated packed-bed bioreactors were evaluated. The specific nitrate and nitrite reduction rates increased almost linearly with the increasing initial nitrate and nitrite concentrations, respectively. The process performance was evaluated in three parallel packed-bed bioreactors filled with different sulphur/limestone ratios (1:1, 2:1 and 3:1, v/v). Performances of the bioreactors were studied under varying nitrate loadings (0.05 - 0.80 gNO(-)(3) - NL⁻¹ d⁻¹) and hydraulic retention times (3-12 h). The maximum nitrate reduction rate of 0.66 g L⁻¹ d⁻¹ was observed at the loading rate of 0.80 g NO(-)(3) - N L⁻¹ d⁻¹ in the reactor with sulphur/limestone ratio of 3:1. Throughout the study, nitrite concentrations remained quite low (i.e. below 0.5 mg L⁻¹ NO(-)(2) -N. The reactor performance increased in the order of sulphur/limestone ratio of 3:1, 2:1 and 1:1. Denaturing gradient gel electrophoresis analysis of 16S rRNA genes showed quite stable communities in the reactors with the presence of Methylo virgulaligni, Sulfurimonas autotrophica, Sulfurovum lithotrophicum, Thiobacillus aquaesulis and Sulfurimonas autotrophica related species.

A Tire-Sulfur Hybrid Adsorption Denitrification (T-SHAD) process for decentralized wastewater treatment

Nitrogen discharges from decentralized wastewater treatment (DWT) systems contribute to surface and groundwater contamination. However, the high variability in loading rates, long idle periods and lack of regular maintenance presents a challenge for biological nitrogen removal in DWT. A Tire-Sulfur Hybrid Adsorption Denitrification (T-SHAD) process was developed that combines nitrate (NO3(-)) adsorption to scrap tire chips with sulfur-oxidizing denitrification. This allows the tire chips to adsorb NO3(-) when the influent loading exceeds the denitrification capacity of the biofilm and release it when NO3(-) loading rates are low (e.g. at night). Three waste products, scrap tire chips, elemental sulfur pellets and crushed oyster shells, were used as a medium in adsorption, leaching, microcosm and up-flow packed bed bioreactor studies of NO3(-) removal from synthetic nitrified DWT wastewater. Adsorption isotherms showed that scrap tire chips have an adsorption capacity of 0.66 g NO3(-)-N kg(-1) of scrap tires. Leaching and microcosm studies showed that scrap tires leach bioavailable organic carbon that can support mixotrophic metabolism, resulting in lower effluent SO4(2-) concentrations than sulfur oxidizing denitrification alone. In column studies, the T-SHAD process achieved high NO3(-)-N removal efficiencies under steady state (90%), variable flow (89%) and variable concentration (94%) conditions.

Pilot and full scale applications of sulfur-based autotrophic denitrification process for nitrate removal from activated sludge process effluent

Sulfur-based autotrophic denitrification of nitrified activated sludge process effluent was studied in pilot and full scale column bioreactors. Three identical pilot scale column bioreactors packed with varying sulfur/lime-stone ratios (1/1-3/1) were setup in a local wastewater treatment plant and the performances were compared under varying loading conditions for long-term operation. Complete denitrification was obtained in all pilot bioreactors even at nitrate loading of 10 mg NO3(-)-N/(L.h). When the temperature decreased to 10 °C during the winter time at loading of 18 mg NO3(-)-N/(L.h), denitrification efficiency decreased to 60-70% and the bioreactor with S/L ratio of 1/1 gave slightly better performance. A full scale sulfur-based autotrophic denitrification process with a S/L ratio of 1/1 was set up for the denitrification of an activated sludge process effluent with a flow rate of 40 m(3)/d. Almost complete denitrification was attained with a nitrate loading rate of 6.25 mg NO3(-)-N/(L.h).

A soil infiltration system incorporated with sulfur-utilizing autotrophic denitrification (SISSAD) for domestic wastewater treatment

To enhance the denitrification performance of soil infiltration, a soil infiltration system incorporated with sulfur-utilizing autotrophic denitrification (SISSAD) for domestic wastewater treatment was developed, and the SISSAD performance was evaluated using synthetic domestic wastewater in this study. The aerobic respiration and nitrification were mainly taken place in the upper aerobic stage (AES), removed 88.44% COD and 89.99% NH4(+)-N. Moreover, autotrophic denitrification occurred in the bottom anaerobic stage (ANS), using the CO2 produced from AES as inorganic carbon source. Results demonstrated that the SISSAD showed a remarkable performance on COD removal efficiency of 95.09%, 84.86% for NO3(-)-N, 95.25% for NH4(+)-N and 93.15% for TP. This research revealed the developed system exhibits a promising application prospect for domestic wastewater in the future.

Heterotrophic and elemental-sulfur-based autotrophic denitrification processes for simultaneous nitrate and Cr(VI) reduction

Nitrate and chromate can be present together in water resources as nitrate is a common co-contaminant in surface and ground waters. This study aims at comparatively evaluating simultaneous chromate and nitrate reduction in heterotrophic and sulfur-based autotrophic denitrifying column bioreactors. In sulfur-based autotrophic denitrification process, elemental sulfur and nitrate act as an electron donor and an acceptor, respectively, without requirement of organic supplementation. Autotrophic denitrification was complete and not adversely affected by chromate up to 0.5 mg/L. Effluent chromate concentration was <50 μg/L provided that influent chromate concentration was ≤0.5 mg/L. Heterotrophic denitrification performance was not adversely affected even at 20 mg/L chromate and complete chromate reduction was attained up to 10 mg/L. Although autotrophic denitrification rate was much lower compared with heterotrophic one, it may be preferred in drinking water treatment due to the elimination of organic supplementation and the risk of treated effluent contamination.