Effect of calcinated pyrite on simultaneous ammonia, nitrate and phosphorus removal in the BAF system and the Fe2+ regulatory mechanisms: Electron transfer and biofilm properties

Towards low carbon demand and highly efficient nutrient removal: Establishing denitrifying phosphorus removal in anaerobic/anoxic/oxic + nitrification system

A two-sludge anaerobic/anoxic/oxic + nitrification system with simultaneous nitrogen and phosphorus removal was studied for enhanced low-strength wastewater treatment. After 158 days of operation, excellent NH4+-N, chemical oxygen demand (COD) and PO43--P removal (99.0 %, 90.0 % and 92.0 %, respectively) were attained under a low carbon/nitrogen ratio of 5, resulting in effluent NH4+-N, COD and PO43--P concentrations of 0.3, 30.0 and 0.5 mg/L, respectively. The results demonstrate that the anaerobic/anoxic/oxic sequencing batch reactor (A2-SBR) and nitrification sequencing batch reactor (N-SBR) had favorable denitrifying phosphorus removal and nitrification performance, respectively. High-throughput sequencing results indicate that the phosphate-accumulating organisms Dechloromonas (1.1 %) and Tetrasphaera (1.2 %) were enriched in the A2-SBR, while the ammonia-oxidizing bacteria Nitrosomonas (7.8 %) and the nitrite-oxidizing bacteria Nitrospira (18.1 %) showed excellent accumulation in the N-SBR. Further analysis via functional prediction revealed that denitrification is the primary pathway of nitrogen metabolism throughout the system. Overall, the system achieved low carbon and high efficiency nutrient removal.

Effect of copper, arsenic and nickel on pyrite-based autotrophic denitrification

Pyritic minerals generally occur in nature together with other trace metals as impurities, that can be released during the ore oxidation. To investigate the role of such impurities, the presence of copper (Cu(II)), arsenic (As(III)) and nickel (Ni(II)) during pyrite mediated autotrophic denitrification has been explored in this study at 30 °C with a specialized microbial community of denitrifiers as inoculum. The three metal(loid)s were supplemented at an initial concentration of 2, 5, and 7.5 ppm and only Cu(II) had an inhibitory effect on the autotrophic denitrification. The presence of As(III) and Ni(II) enhanced the nitrate removal efficiency with autotrophic denitrification rates between 3.3 [7.5 ppm As(III)] and 1.6 [7.5 ppm Ni(II)] times faster than the experiment without any metal(loid) supplementation. The Cu(II) batches, instead, decreased the denitrification kinetics with 16, 40 and 28% compared to the no-metal(loid) control for the 2, 5 and 7.5 ppm incubations, respectively. The kinetic study revealed that autotrophic denitrification with pyrite as electron donor, also with Cu(II) and Ni(II) additions, fits better a zero-order model, while the As(III) incubation followed first-order kinetic. The investigation of the extracellular polymeric substances content and composition showed more abundance of proteins, fulvic and humic acids in the metal(loid) exposed biomass.

Effect of ibuprofen (IBU) on the sulfur-based and calcined pyrite-based autotrophic denitrification (SCPAD) systems with two filling modes: Performance and toxic response mechanism

To investigate the effect of ibuprofen (IBU) on the sulfur-based and calcined pyrite-based autotrophic denitrification (SCPAD) systems, two individual reactors with the layered filling (L-SCPAD) and mixed filling (M-SCPAD) systems were established via sulfur and calcined pyrite. Effluent NO3--N concentration of the L-SCPAD and M-SCPAD systems was first increased to 6.44, 0.93 mg/L under 0.5 mg/L IBU exposure and gradually decreased to 1.66 mg/L, 0 mg/L under 4.0 mg/L IBU exposure, indicating that NO3--N removal performance of the M-SCPAD system was better than that of the L-SCPAD system. The variation of extracellular polymeric substances (EPS) characteristics demonstrated that more EPS was secreted in the M-SCPAD system compared to the L-SCPAD system, which contributed to forming a more stable biofilm structure and protecting microorganisms against the toxicity of IBU in the M-SCPAD system. Moreover, the increased electron transfer impedance and decreased cytochrome c implied that IBU inhibited the electron transfer efficiency of the L-SCPAD and M-SCPAD systems. The decreased adenosine triphosphate (ATP) and electron transfer system activity (ETSA) content showed that IBU inhibited metabolic activity, but the M-SCPAD system exhibited higher metabolic activity compared to the L-SCPAD system. In addition, the analysis of the bacterial community indicated a more stable abundance of nitrogen removal function bacteria (Bacillus) in the M-SCPAD system compared to the L-SCPAD system, which was conducive to maintaining a stable denitrification performance. The toxic response mechanism based on the biogeobattery effect was proposed in the SCPAD systems under IBU exposure. This study provided an important reference for the long-term toxic effect of IBU on the SCPAD systems.

Effect mechanism of micron-scale zero-valent iron enhanced pyrite-driven denitrification biofilter for nitrogen and phosphorus removal

This study aims to explore the effect mechanism of micron-scale zero-valent iron (mZVI) to improve nitrogen and phosphorus removal in a pyrite (FeS2)-driven denitrification biofilter (DNBF) for the secondary effluent treatment. Two similar DNBFs (DNBF-A with FeS2 as fillers and DNBF-B with the mixture mZVI and FeS2 as carrier) were developed. The results showed that NO3--N, total nitrogen (TN) and PO43--P removal efficiencies were up to 91.64%, 67.44% and 80.26% in DNBF-B, which were obviously higher than those of DNBF-A (with NO3--N, TN and PO43--P removal efficiencies of 38.39%, 44.89% and 53.02%, respectively). Kinetic analysis of both PO43--P and NO3--N showed an increase in the rate constant (K) for DNBF-B compared to DNBF-A. The addition of mZVI not only improved the electron transport system activity (ETSA), but also achieved system Fe(II)/Fe(III) redox cycle in DNBF-B. In addition, the high-throughput sequencing analysis indicated that the addition of mZVI could obviously stimulate the enrichment of functional bacteria, such as Thiobacillus (11.99%), Mesotoga (7.50%), JGI-0000079D21 (6.37%), norank_f__Bacteroidetes_vadinHA17 (6.19%), Aquimonas (5.93%) and Arenimonas (3.97%). These genus played the important role in nitrogen and phosphorus removal in DNBF-B. Addition mZVI in the FeS2-driven denitrification biofilter is highly promising for TN and TP removal during secondary effluent treatment.

The enrichment of more functional microbes induced by the increasing hydraulic retention time accounts for the increment of autotrophic denitrification performance

With pyrite (FeS2) and polycaprolactone (PCL) as electron donors, three denitrification systems, namely FeS2-based autotrophic denitrification (PAD) system, PCL-supported heterotrophic denitrification (PHD) system and split-mixotrophic denitrification (PPMD) system, were constructed and operated under varying hydraulic retention times (HRT, 1-48 h). Compared with PAD or PHD, the PPMD system could achieve higher removals of NO3--N and PO43--P, and the effluent SO42- concentration was greatly reduced to 7.28 mg/L. Similarly, the abundance of the dominant genera involved in the PAD (Thiobacillus, Sulfurimonas, and Ferritrophicum, etc.) or PHD (Syntrophomonas, Desulfomicrobium, and Desulfovibrio, etc.) process all increased in the PPMD system. Gene prediction completed by PICRUSt2 showed that the abundance of the functional genes involved in denitrification and sulfur oxidation all increased with the increase of HRT. This also accounted for the increased contribution of autotrophic denitrification to total nitrogen removal in the PPMD system. In addition, the analysis of metabolic pathways disclosed the specific conversion mechanisms of nitrogen and sulfur inside the reactor.

Coupled pyrite and sulfur autotrophic denitrification for simultaneous removal of nitrogen and phosphorus from secondary effluent: feasibility, performance and mechanisms

The discharge standards of nitrogen (N) and phosphorus (P) in wastewater treatment plants (WWTPs) have become increasingly strict to reduce water eutrophication. Further reducing N and P in effluent from municipal WWTPs need to be achieved effectively and eco-friendly. In this study, a carbon independent pyrite and sulfur autotrophic denitrification (PSAD) system using pyrite and sulfur as electron donor was developed and compared with pyrite autotrophic denitrification (PAD) and sulfur autotrophic denitrification (SAD) systems through batch and continuous flow biofilter experiments. Compare to PAD and SAD, PSAD was more effective in simultaneous removal in N and P. At hydraulic retention time (HRT) 3 h, average effluent concentrations of total nitrogen (TN) and total phosphate (TP) of 1.40 ± 0.03 and 0.19 ± 0.02 mg/L were achieved when treating real secondary effluent with 20.65 ± 0.24 mg/L TN and 1.00 ± 0.24 mg/L TP. The improvement in simultaneous removal of N and P was attributed to the coupling of PAD and SAD in enhancing the transformation of sulfur and iron and enlarging the reaction zone in the pyrite and sulfur autotrophic denitrification biofilter (PSADB) system. Therefore, more biomass was accumulated and the microbial denitrification functional stability, including electrons transfer and consumption was enhanced on the surface of pyrite and sulfur particles in the PSADB system. Moreover, autotrophic denitrifiers (Thiobacillus and Ferritrophicum), sulfate-reducing bacteria (Desulfocapsa) and iron reducing bacteria (Geothrix), acting as contributors to microbial nitrogen, sulfur and iron cycle, were specially enriched. In addition, the leaching of iron ions was promoted, which facilitated the removal of phosphate in the form of Fe3(PO4)2·8H2O and Fe3PO4. PSADB has proven to be an efficient technology for simultaneous removal of N and P, which could meet increasingly stringent discharge standards effectively and eco-friendly.

Effect of ibuprofen on the sulfur autotrophic denitrification process and microbial toxic response mechanism

The effect of ibuprofen (IBU) on the sulfur autotrophic denitrification (SAD) process and microbial toxic response mechanism were investigated. Nitrate removal performance was inhibited by high IBU concentrations (10 and 50 mg/L), and the effect of low IBU concentrations (1 mg/L) on nitrate removal performance was negligible. The low IBU concentration induced basal oxidative stress for microbial self-protection, while the high IBU concentration induced high-intensity oxidative stress to damage the microbial cell membrane structure. Electrochemical characterization showed that the low IBU concentration stimulated the electron transfer efficiency, which was inhibited at the high IBU concentration. Moreover, the variation content of nicotinamide adenine dinucleotide (NADH) and nitrate reductase showed that metabolic activity increased at low IBU concentrations and decreased at high IBU concentrations during the sulfur autotrophic nitrate reduction process. This study proposed the hormesis toxic response mechanism of the SAD process to IBU exposure.

Responses of anammox and sulfur/pyrite autotrophic denitrification in one-stage system to high nitrogen load: Performance, metabolic and bacterial community

To remove residual nitrate from anammox process and achieve efficient nitrogen removal, a two-stage system (TAS) with the two individual reactors and a one-stage system (OAS) with the spatial functional areas in one reactor were established via anammox coupling sulfur autotrophic denitrification. The total nitrogen removal efficiency (TNRE) of OAS system (97.85 ± 1.92%) was higher than that of TAS system (93.63 ± 1.87%) under the influent NH₄⁺-N and NO₂^--N of 227 and 300 mg/L. Meanwhile, the responses of microbial metabolism to high nitrogen load were investigated in term of microbial metabolites, electron transfer and metabolic activity. Microbial metabolites characteristics demonstrated that the OAS system secreted more EPS with lower protein (PN)/polysaccharide (PS) ratio than that in the TAS system, which was beneficial to protect bacteria from high nitrogen load. Electrochemical analysis suggested that the secretion of electron conductive substance (such as PN, PS) and redox active substances (such as flavin mononucleotide, the binding of flavins and cytochrome c on the outer membrane) were increased in the OAS system, which promoted the electron transfer efficiency. Moreover, the electron transport system activity (ETSA) values and ATP contents in OAS system were higher than that in the TAS system, which indicated that metabolic activity was improved in OAS system under the stimulation of high nitrogen load. Additionally, the bacterial community analysis indicated that the functional bacteria of Candidatus_Kuenenia and Armatimonadetes_gp5 had higher abundance in the OAS system than that in the TAS system, which was beneficial to realize the stable nitrogen removal performance. Overall, the responses mechanism of the OAS system was established to explain the resistant to high nitrogen load.

Start-up phase optimization of pyrite-intensified hybrid sequencing batch biofilm reactor (PIHSBBR): Mixotrophic denitrification performance and mechanism

Pyrite-based autotrophic denitrification (PAD) is an emerging biological process to diminish nitrate pollution, but the relatively low NO₃^--N removal rate limits its practical application. In this research, a pyrite-intensified hybrid sequencing batch biofilm reactor (PIHSBBR) was designed to treat low C/N ratio domestic wastewater. The results showed that PIHSBBR could achieve optimal removal of COD, NH₄⁺-N, and TN under the aeration rate of 1.0 L/L∙min and the hydraulic retention time (HRT) of 8 h, with removal rates of 69.67 ± 4.37%, 77.04 ± 4.84%, and 63.92 ± 6.66%, respectively. The PAD efficiency in PIHSBBR during the stable operation was not high (13.05-31.01%), and the main nitrogen removal pathway in PIHSBBR, especially in the aerobic zone, was simultaneous nitrification and denitrification (SND). High-throughput sequencing analysis unraveled that Planctomycetota (3.65%) had a high abundance in the anoxic zone of PIHSBBR, implying that anaerobic ammonium oxidation (anammox) might have occurred in the anoxic zone. In addition, the nitrogen cycle function gene with the highest abundance was nirBD, indicating the possible presence of dissimilatory nitrate reduction to ammonium (DNRA) within the system (aerobic and anoxic zones). Our research can provide useful information for the improvement and future application of PIHSBBR.

Enhancement and mechanisms of micron-pyrite driven autotrophic denitrification with different pretreatments for treating organic-limited waters

Pyrite-driven autotrophic denitrification (PAD) represents a cheap and promising way for nitrogen removal from organic-limited wastewater, which has obtained increasing attention in recent years. However, the limited denitrification rate and unclear mechanism underlying the process have hindered the engineered application of PAD. This study aims to shed light on the impacts of different pretreatments (i.e., ultrasonication, acid-washing and calcination) on micron-pyrite surface characteristics, denitrification performance and biofilm formation during PAD in batch reactors. A series of solid-phase analyses revealed that all pretreatments could significantly promote biofilm attachment on pyrite granules, but impacted the proportion, distribution and chemical oxidation state of sulfur (S) and iron (Fe) at varying degrees. Batch tests showed that ultrasonication and acid-washing could enhance the total nitrogen reduction rate by 14% and 99%, and decrease the sulfate production rate by 51% and 42%, respectively, when compared with untreated pyrite. Microbial community analysis indicated that Thiobacillus and Rhodanobacter dominated in PAD systems. Two types of indirect mechanisms (i.e., contact and non-contact) for pyrite leaching may co-occur in PAD system, resulting in ferrous iron (Fe²⁺), thiosulfate (S₂O₃^2-) and sulfide (S^2-) as the main electron donors for denitrification. A PAD mechanism model was proposed to describe the PAD electron transfer pathway with the aim to optimize the engineered application of PAD for nitrogen removal.

New insights on simultaneous nitrate and phosphorus removal in pyrite-involved mixotrophic denitrification biofilter for a long-term operation: Performance change and its underlying mechanism

Simultaneous nitrate and phosphorus removal can be completed by pyrite- and influent organics-involved mixotrophic denitrification and chemical phosphorus removal via iron precipitation. However, so far, how their removal performances change with iron precipitation accumulation remains unclear. In this study, the differences in nitrate and phosphorus removal from municipal tailwater between volcanic and pyrite supported biofilters (V-BF, P-BF) for a long-term operation were investigated, as well as the underlying mechanism for these differences. The nitrate removal efficiencies (NREs) in P-BF were greater than those in V-BF due to the synergistic effect of influent organic and pyrite, as evidenced by comparable TOC consumption and Fe²⁺/SO₄^2- production. The NREs in P-BF were gradually lower than in V-BF as a result of bacterial cell-iron encrustation observed in TEM images, which would deteriorate microbial activity. However, the phosphorus removal efficiencies (PREs) in P-BF remained consistently higher than in V-BF, resulting from chemical phosphorus removal which was confirmed that P, Fe and O elements dominated on the pyrite surface after use by SEM-EDS. The dominant denitrifying bacteria differed significantly, autotrophic and heterotrophic denitrifying microorganisms coexisted in P-BF. The relative abundances of the narG coding gene in P-BF were higher than that in V-BF, which was consistent with the total relative abundances of identified denitrifying bacteria. Besides, the mechanism of simultaneous nitrogen and phosphorus removal in the pyrite-involved mixotrophic denitrification process has been deduced. This work has significant implications for the practical application of a pyrite-involved mixotrophic denitrification process for low C/N wastewater treatment.

Nitrogen removal performance and microbial characteristics during simultaneous chemical phosphorus removal process using Fe3

To evaluate the effect of Fe³⁺ on nitrogen (N) removal and associated microbial characteristics during simultaneous chemical phosphorus (P) removal, a sequencing batch reactor was used to analyze the changes in the microbial community and metabolic pathways caused by Fe³⁺ addition. Results demonstrated that Fe³⁺ promoted ammonia nitrogen (NH₄⁺-N) removal and inhibited denitrification process, and increased the sludge particles (D50) and the biomass per sludge particle size. Furthermore, the abundances of denitrifying bacteria (Haliangium and Terrimonas) and biological phosphorus removing bacteria (Halaingium, norank_f_Saprospiraceae and SM1A02) were decreased. On the contrary, the increase of nitrifying bacteria abundance and the coding genes of nitrification-related enzymes confirmed the promotion for nitrification with Fe³⁺ addition. Besides, Fe³⁺ inhibited the interspecific relationship between denitrifying bacteria genera and other genera to reduce denitrification efficiency.

Effect of electric current intensity on performance of polycaprolactone/FeS2-based mixotrophic biofilm-electrode reactor

In this study, a bioelectrochemical system consisting of pyrite-based autotrophic denitrification (PAD) and heterotrophic denitrification (HD) was established to polish nitrate wastewater. The loading of electric current (EC) could stimulate the dissolution of pyrite. Appropriate EC (I ≤ 30 mA) was conducive to nitrate removal, too high EC (I = 40 mA) would inhibit nitrate removal and lead to an obvious accumulation of NO₂^--N and NH₄⁺-N. Microbial analysis revealed that the increase of EC could inhibit the diversity of heterotrophic microbes, but appropriate EC (I = 10 mA) could increase the diversity of autotrophic microbes. The EC loading was conducive to the enrichment of iron autotrophic denitrifiers (Ferritrophicum), pyrite-oxidizing bacteria (Thiobacillus, Sulfurimonas), and sulfur autotrophic denitrifiers (Dechloromonas, Thiobacillus, and Arenimonas). The EC loading enlarged the contribution of PAD, making PAD a dominant pathway in denitrification.

Enhanced simultaneous partial nitrification and denitrification performance of aerobic granular sludge via tapered aeration in sequencing batch reactor for treating low strength and low COD/TN ratio municipal wastewater

The aerobic granular sludge simultaneous partial nitrification, denitrification and phosphorus removal (AGS-SPNDPR) process was carried out via tapered aeration in sequencing batch reactor (SBR) for treating low strength and low COD/TN ratio municipal wastewater. The results showed that aerobic granular sludge was successfully cultivated with good sedimentation performance when treating the municipal wastewater. Meanwhile, the median granule size increased to 270 (R1) and 257 (R2) μm on day 80. The excellent removal performance of COD (92%) and NH₄⁺-N (95%) were achieved under different aeration modes, while the higher TN removal efficiency (76%) was achieved by tapered aeration. The accumulation of NO₂^--N in R2 indicated that the tapered aeration was beneficial to achieve simultaneously partial nitrification and denitrification. Meanwhile, the high-efficiency phosphorus (95%) removal was realized via additional carbon source, and SPNDPR process was formed under tapered aeration. The bacterial community analysis indicated denitrifying glycogen-accumulating organisms (DGAOs) Candidatus_Competibacter and ammonia-oxidizing bacteria (AOB) Nitrosomonas were more effectively enriched via tapered aeration, while phosphorus-accumulating organisms (PAOs) Candidatus_Accumulibacter were effectively enriched under additional organic carbon. AOB, denitrifying bacteria and PAOs were simultaneously enriched by tapered aeration and additional carbon source, which was beneficial to nutrients removal. This study might be conducive to the application of AGS-SPNDPR system for treating low strength and low COD/TN ratio municipal wastewater under tapered aeration.

Pyrite-assisted denitrification in recirculated biofilter tolerates pH lower than 5

This study demonstrates the feasibility of pyrite-assisted denitrification to treat mildly acidic wastewaters and proposes a convenient solution for nitrogen removal from acidic mining waters. Pyrite is a highly available and low-cost electron donor for autotrophic denitrification, whereas mining waters represent a source of nitrogen (N) contamination due to the use of N-containing explosives during mining operations and cyanide (CN)-based compounds for ore processing. In this study, the denitrification performance of a recirculated pyrite-packed biofilter (RPPB) treating simulated N-contaminated acidic mining waters is evaluated under decreasing pH conditions. The effect of feed organic carbon on the autotrophic process has been also investigated. N removal efficiencies (NRE) > 60% could be achieved at feed pH ≥ 4.5 and HRT ≥ 5 h. Decrease of influent pH to 3.0 reduced the NRE to 20%. The addition of 10 and 20 mg/L of ethanol to the influent did not significantly impact NRE of the RPPB. PRACTITIONER POINTS: Pyrite-assisted denitrification is proposed for treating acidic wastewaters. Nitrogen removal >60% was maintained at feed pH as low as 4.7. Decrease of feed pH to 3 strongly inhibited denitrification. The presence of organic carbon in the feed did not affect the autotrophic process.