Nitrate Removal Rate Research Articles

Manganese sulfide-sulfur and limestone autotrophic denitrification system for deep and efficient nitrate removal: Feasibility, performance and mechanism

Despite the great potential of sulfur-based autotrophic denitrification, an improvement in nitrate removal rate is still needed. This study used the desulfurized products of Mn ore to develop the MnS-S0-limestone autotrophic denitrification system (MSLAD). The feasibility of MSLAD for denitrification was explored and the possible mechanism was proposed. The nitrate (100 mg/L) was almost removed within 24 h in batch experiment in MSLAD. Also, an average TN removal of 98 % (472.0 mg/L/d) at hydraulic retention time of 1.5 h in column experiment (30 mg/L) was achieved. MnS and S0 could act as coupled electron donors and show synergistic effects for nitrate removal. γ-MnS with smaller particle size and lower crystallinity was more readily utilized by the bacterium and had higher nitrate removal efficiency than that of α-MnS. Thiobacillus and Sulfurimonas were the core functional bacterium in denitrification. Therefore, MnS-S0-limestone bio-denitrification provides an efficient alternative method for nitrate removal in wastewater.

Enhanced Nitrate Nitrogen Removal from Constructed Wetland via Fe3O4/Granular Activated Carbon Anode Microbial Electrolysis Cell under Low C/N Ratio

In this study, a constructed wetland–Fe3O4/granular activated carbon anode microbial electrolysis cell (CW-FMEC) was constructed to enhance denitrification in low COD/N ratio wastewater. The introduction of Fe3O4 boosted the expression of functional genes involved in the denitrification pathway, and the abundance of narG, nirS, and nosZ increased by 99.29%, 70.54%, and 132.18%, respectively, compared to CW. In addition, the content of c-type cytochromes (c-Cyts) and EPS were also enhanced in the CW-FMEC. The microbial communities study displayed that Thauera, Dechloromonas, and Arenimonas became the main genera for denitrification. The denitrification performance at different COD/N ratios was investigated in depth. Under optimal working circumstances, the CW-FMEC had an excellent nitrate removal rate (88.9% ± 1.12%) while accumulating nearly no NO2−-N or NH4+-N in the effluent. This study provides a new direction for the development of CW-MEC and accelerates its implementation.

Effect of pyrite particle size on the denitrification performance of autotrophic or split-mixotrophic bioreactors supported by pyrite/polycaprolactone.

In this study, a pyrite-based autotrophic denitrification (PAD) system, a polycaprolactone (PCL)-supported heterotrophic denitrification (PHD) system, and a pyrite+PCL-based split-mixotrophic denitrification (PPMD) system were constructed. The pyrite particle size was controlled in 1-3, 3-5, or 5-8mm in both the PAD and PPMD systems to investigate the effect of pyrite particle size on the denitrification performance of autotrophic or split-mixotrophic bioreactors. It was found that the PAD system achieved the best denitrification efficiency with an average removal rate of 98.98% in the treatment of 1- to 3-mm particle size, whereas it was only 19.24% in the treatment of 5- to 8-mm particle size. At different phases of the whole experiment, the nitrate removal rates of both the PHD and PPMD systems remained stable at a high level (>94%). Compared with the PAD or PHD system, the PPMD system reduced the concentrations of sulfate and chemical oxygen demand in the final effluent efficiently. The interconnection network diagram explained the intrinsic metabolic pathways of nitrogen, sulfur, and carbon in the three denitrification systems at different phases. In addition, the microbial community analysis showed that the PPMD system was beneficial for the enrichment of Firmicutes. Finally, the impact mechanism of pyrite particle size on the performance of the PPMD system was proposed. PRACTITIONER POINTS: The reduction of pyrite particle size was beneficial for improving the efficiency of the PAD process. The change in particle size had an effect on NO2 --N accumulation in the PAD system. The accumulation of NH4 +-N in the PPMD system increased with the decrease in particle size. The reduction of pyrite particle size increased the production of SO4 2- in the PAD and PPMD systems. The correlations among the effluent indicators of the PAD and PPMD systems could be well explained.

Microbial interactions facilitating efficient methane driven denitrification via in-situ utilization of short chain fatty acids

High nitrate pollution in agriculture and industry poses a challenge to emerging methane oxidation coupled denitrification. In this study, an efficient nitrate removal efficiency of 100 % was achieved at an influent loading rate of 400 mg-N/L·d, accompanied by the production of short chain fatty acids (SCFAs) with a maximum value of 80.9 mg/L. Batch tests confirmed that methane was initially converted to acetate, which then served as a carbon source for denitrification. Microbial community characterization revealed the dominance of heterotrophic denitrifiers, including Simplicispira (22.8 %), Stappia (4.9 %), and the high‑nitrogen-tolerant heterotrophic denitrifier Diaphorobacter (19.0 %), at the nitrate removal rate of 400 mg-N/L·d. Notably, the low abundance of methanotrophs ranging from 0.24 % to 3.75 % across all operational stages does not fully align with the abundance of pmoA genes, suggesting the presence of other functional microorganisms capable of methane oxidation and SCFAs production. These findings could facilitate highly efficient denitrification driven by methane and contributed to the development of denitrification using methane as an electron donor.

The interplay of hematite and photic biofilm triggers the acceleration of biotic nitrate removal

The soil-water interface is replete with photic biofilm and iron minerals; however, the potential of how iron minerals promote biotic nitrate removal is still unknown. This study investigates the physiological and ecological responses of photic biofilm to hematite (Fe2O3), in order to explore a practically feasible approach for in-situ nitrate removal. The nitrate removal by photic biofilm was significantly higher in the presence of Fe2O3 (92.5%) compared to the control (82.8%). Results show that the presence of Fe2O3 changed the microbial community composition of the photic biofilm, facilitates the thriving of Magnetospirillum and Pseudomonas, and promotes the growth of photic biofilm represented by the extracellular polymeric substance (EPS) and the content of chlorophyll. The presence of Fe2O3 also induces oxidative stress (•O2−) in the photic biofilm, which was demonstrated by electron spin resonance spectrometry. However, the photic biofilm could improve the EPS productivity to prevent the entrance of Fe2O3 to cells in the biofilm matrix and mitigate oxidative stress. The Fe2O3 then promoted the relative abundance of Magnetospirillum and Pseudomonas and the activity of nitrate reductase, which accelerates nitrate reduction by the photic biofilm. This study provides an insight into the interaction between iron minerals and photic biofilm and demonstrates the possibility of combining biotic and abiotic methods to improve the in-situ nitrate removal rate.

Effect of excess CO2 on semi-continuous microalgae systems: Carbon biofixation

Microalgae-based CO2 fixation is a promising solution for achieving carbon neutrality. However, the low rate of carbon fixation is a significant issue due to gas transfer. A closed cultivation system was employed to overcome this challenge, incorporating headspace micro-positive pressure and elevated total CO2 (TCO2) levels to minimize CO2 loss and improve CO2 mass transfer. Microalgae could thrive and maintain a higher level of system pH within the TCO2 range of 0.12–0.49 mol CO2/L absorbent, and high TCO2 levels over 0.36 mol CO2/L absorbent were the optimal range. The semi-continuous microalgae system removed up to 8.72 ± 0.85 g CO2/L/day. Simultaneously, it exhibited phosphate and nitrate removal rates of 50.95 ± 10.71 and 182.14 ± 30.71 mg/L/day, respectively. The study found that 80% of the removed carbon was fixed as organic biomass. An increased level of TCO2 resulted in the accumulation of more lipids within the cells. Moreover, microalgae released carbon fixation products into the extracellular environment, and the dissolved organic carbon fixation rate reached as high as 14.58 ± 1.89 mmol/L/day. These findings suggest that microalgae use multiple pathways for carbon fixation, including biomass organic carbon accumulation, extracellular organic carbon secretion, and inorganic carbon precipitation. Improving carbon fixation rates may benefit the industrial application of microalgae technology, which aims to mitigate carbon emissions.

Advancing towards electro-bioremediation scaling-up: On-site pilot plant for successful nitrate-contaminated groundwater treatment

The potential of nitrate electro-bioremediation has been fully demonstrated at the laboratory scale, although it has not yet been fully implemented due to the challenges associated with scaling-up bioelectrochemical reactors and their on-site operation. This study describes the initial start-up and subsequent stable operation of an electro-bioremediation pilot plant for the treatment of nitrate-contaminated groundwater on-site (Navata site, Spain). The pilot plant was operated under continuous flow mode for 3 months, producing an effluent suitable for drinking water in terms of nitrates and nitrites (<50 mg NO3- L-1; 0 mg NO2- L-1). A maximum nitrate removal rate of 0.9 ± 0.1 kg NO3- m-3 d-1 (efficiency 82 ± 18 %) was achieved at a cathodic hydraulic retention time (HRTcat) of 2.0 h with a competitive energy consumption of 4.3 ± 0.4 kWh kg-1 NO3-. Under these conditions, the techno-economic analysis estimated an operational cost of 0.40 € m-3. Simultaneously, microbiological analyses revealed structural heterogeneity in the reactor, with denitrification functionality concentrated predominantly from the centre to the upper section of the reactor. The most abundant groups were Pseudomonadaceae, Rhizobiaceae, Gallionellaceae, and Xanthomonadaceae. In conclusion, this pilot plant represents a significant advancement in implementing this technology on a larger scale, validating its effectiveness in terms of nitrate removal and cost-effectiveness. Moreover, the results validate the electro-bioremediation in a real environment and encourage further investigation of its potential as a water treatment.

Efficient Electrochemical Nitrate Removal by Ordered Ultrasmall Intermetallic AuCu3 via Enhancing Nitrate Adsorption.

Developing a high-performance electrocatalyst for synthesizing ammonia from nitrate represents a promising solution for addressing wastewater pollution and achieving sustainable ammonia production. However, it remains a formidable challenge. Herein, an intermetallic AuCu3 electrocatalyst with high-density active sites is designed and prepared for an efficient nitrate electroreduction to generate ammonia. Remarkably, the Faraday efficiency and yield rate of ammonia at -0.9 V are 97.6% and 75.9 mg h-1 cm-2, respectively. More importantly, after 10 cycles of testing, the removal rate of nitrate can still reach 95.2%. Electrochemical in situ Fourier transform infrared analysis indicates that AuCu3 IM can promote the adsorption of nitrate and enhance ammonia production from nitrate. *NH3, *NO, and *NO2 have been proven to be active intermediates. Theoretical and experimental studies show that the Au site can provide a large amount of *H for nitrate reduction, and the Cu site is conducive to the reduction of nitrate to produce nitrogen-containing products. Meanwhile, AuCu3 intermetallic compounds (AuCu3 IM) can inhibit the dimerization of *H. The power density and ammonia yield of the assembled Zn-nitrate battery reached 2.17 mW cm-2 and 71.2 mg h-1 cm-2, respectively.

Algicidal activity synchronized with nitrogen removal by actinomycetes: Algicidal mechanism, stress response of algal cells, denitrification performance, and indigenous bacterial community co-occurrence

The harmful algal blooms (HABs) can damage the ecological equilibrium of aquatic ecosystems and threaten human health. The bio-degradation of algal by algicidal bacteria is an environmentally friendly and economical approach to control HABs. This study applied an aerobic denitrification synchronization algicidal strain Streptomyces sp. LJH-12–1 (L1) to control HABs. The cell-free filtrate of the strain L1 showed a great algolytic effect on bloom-forming cyanobacterium, Microcystis aeruginosa (M. aeruginosa). The optimal algicidal property of strain L1 was indirect light-dependent algicidal with an algicidal rate of 85.0%. The functional metabolism, light-trapping, light-transfer efficiency, the content of pigments, and inhibition of photosynthesis of M. aeruginosa decreased after the addition of the supernatant of the strain L1 due to oxidative stress. Moreover, 96.05% nitrate removal rate synchronized with algicidal activity was achieved with the strain L1. The relative abundance of N cycling functional genes significantly increased during the strain L1 effect on M. aeruginosa. The algicidal efficiency of the strain L1 in the raw water was 76.70% with nitrate removal efficiency of 81.4%. Overall, this study provides a novel route to apply bacterial strain with the property of denitrification coupled with algicidal activity in treating micro-polluted water bodies.

Unraveling the effect mechanism of sulfamethoxazole on nitrogen biotransformation in a PHBV supported denitrifying granular sludge

Sulfamethoxazole (SMX) is prevalent as one of the most frequently detected antibiotics in rivers worldwide, and is listed as one of the priority antibiotics in China based on the persistence, bioaccumulation, toxicity and resistance. Recently, the effect of SMX on aerobic granular sludge has attracted increasing interest. However, how SMX influences the biodegradable polymer supported denitrifying granular sludge is still obscure. Herein, two groups with or without exposure to 1 mg/L SMX with initial nitrate concentrations of 15, 30, and 50 mg/L were constructed. Results showed that there was a negligible difference on nitrate removal efficiencies between two groups under stable period (all exceeded 95 %) in batch bioreactors with an HRT of 24 h, supported by an insignificance difference (p > 0.05). However, the decline in the first-order decay kinetic constants of nitrate removal rate, NAR and NIR activities verified a slight inhibition of SMX on denitrification. Meanwhile, SMX also had an impact on the chemical and microbial properties on granular sludge. SMX boosted the EPS secretion with more humic acid, a delicate rise in PN/PS ratios and a decrease in zeta potential of EPS all favored the microbes' survival under SMX stress. SMX shifted the microbial compositions, namely, the microbes directly involving in PHBV degradation (Caldimonas, denitrificans Comamonas/Diaphorobacter) were enriched instead of some denitrifying microorganism that could not directly utilize PHBV (Denitratisoma, Azospira). Additionally, even though sul1 and sul2 were enriched, which was not readily accessible to receiving waters due to a good settling properties of granular sludge. This work contributes to a deeper understanding of the impact of SMX on this system, and is of great significance for the operation of granular sludge system for nitrate removal under antibiotic's stress.

Synchronous in-situ sludge reduction and enhanced denitrification through improving electron transfer during endogenous metabolisms with Fe(Ⅱ) addition

The creation of large amounts of excess sludge and residual nitrogen are critical issues in wastewater biotreatment. This study introduced Fe(II) into an oligotrophic anaerobic reactor (OARFe) that was implemented to modify an anoxic-oxic process to motivate in-situ sludge reduction and enhance denitrification under an effective electron shuttle among organic matter, nitrogen, and Fe. The addition of 15 mg L−1 Fe(II) resulted in a sludge reduction efficiency reached 32.0% with a decreased effluent nitrate concentration of 33.3%. This was mostly attributed to the electron transfer from Fe(II) to organic matters and nitrogen species in OARFe. The participation of Fe(II) led to the upregulation of Geothrix and Terrimonas, which caused active organic matter hydrolysis and cell lysis to stimulate the release of extracellular polymeric substances (EPS) and substance transfer between each layer of EPS. The higher utilization of released bioavailable dissolved organic matter improved endogenous denitrification, which can be combined with iron autotrophic denitrification to realize multiple electron donor-based nitrogen removal pathways, resulting in an increased nitrate removal rate of 58.2% in the absence of external carbon sources. These functional bacteria associated with the transformation of nitrogen and carbon and cycling between ferrous and ferric ions were enriched in OARFe, which contributed to efficient electron transport occurred both inside and outside the cell and increased 2,3,5-triphenyltetrazolium chloride electronic transport system activity by 46.9%. This contributed to the potential operational costs of chemical addition and sludge disposal of Fe-AO being 1.9 times lower than those of conventional A2O processes. These results imply that the addition of ferrous ions to an oligotrophic anaerobic zone for wastewater treatment has the potential for low-cost pollution control.

Optimization of nitrogen removal and microbial mechanism of a hydrogen-based membrane biofilm reactor

ABSTRACT The hydrogen-based membrane biofilm reactor (H2-MBfR) is an emerging biological nitrogen removal technology characterized by high efficiency, energy-saving capability, and environmental friendliness. The technology achieves denitrification and denitrogenation of microorganisms by passing hydrogen as an electron donor from inside to outside through the hollow fibre membrane module, and eventually the hydrogen reachs the biofilm attached to the surface of the fibre membrane. H2-MBfR has obtained favourable outcomes in the treatment of secondary biochemical effluent and low concentration nitrogen polluted water source. The experiment was optimized by s single-factor testing and response surface methodology-based optimization (RSM), and the optimal operational conditions were obtained as follows: an influent flow rate of 2 mL/min, hydrogen pressure of 0.04 MPa, and influent nitrate concentration of 24.29 mg/L. Under these conditions, a high nitrate removal rate of 98.25% was achieved. In addition, Proteobacteria and Bacteroidetes were the dominant bacteria in all stages, and the genus Hydrogenophaga was sufficiently enriched, occurring at 13.0%–49.0% throughout the reactor operation. Furthermore, the Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway for nitrate reduction and inorganic carbon utilization by microorganisms in the H2-MBfR was explored through comparison with the KEGG database. The results provided a mechanistic explanation for the denitrification and carbon sequestration capacity of the H2-MBfR.

Enhancement of nitrates removal in pilot-scale treatment wetland systems

ABSTRACT The primary objective of the present study is to enhance the denitrification process in constructed wetlands (CWs), employed as an ecological and natural technology for wastewater treatment. Eco-friendly approaches have been tested to improve the nitrate removal efficiency in pilot-scale subsurface flow CWs. Initially, the system’s performance was enhanced by using a natural substrate, namely cork, as a filter medium. Another approach is bio-inoculation into the rhizosphere of a bacterial strain with the potential capacity for nitrate removal, thereby reinforcing the synergy between the plant and microorganisms. Finally, the approaches were combined, and nitrate removal was assessed. The main results showed that the most effective nitrate removal from wastewater was achieved with cork (80%) compared to gravel (60%). Bio-inoculation into the rhizosphere of denitrifying bacteria significantly improved nitrate removal. Indeed, the nitrate removal rate by the inoculated CW filled with gravel, was higher than the non-inoculated system, with rates of 78% and 60%, respectively. Moreover, the highest removal rate of 99.77% was obtained with the inoculated system filled with cork.

Insights into the Acceleration Mechanism of Intracellular N and Fe Co-doped Carbon Dots on Anaerobic Denitrification Using Proteomics and Metabolomics Techniques.

Bulk carbon-based materials can enhance anaerobic biodenitrification when they are present in extracellular matrices. However, little information is available on the effect of nitrogen and iron co-doped carbon dots (N, Fe-CDs) with sizes below 10 nm on this process. This work demonstrated that Fe-NX formed in N, Fe-CDs and their low surface potentials facilitated electron transfer. N, Fe-CDs exhibited good biocompatibility and were effectively absorbed by Pseudomonas stutzeri ATCC 17588. Intracellular N, Fe-CDs played a dominant role in enhancing anaerobic denitrification. During this process, the nitrate removal rate was significantly increased by 40.60% at 11 h with little nitrite and N2O accumulation, which was attributed to the enhanced activities of the electron transport system and various denitrifying reductases. Based on proteomics and metabolomic analysis, N, Fe-CDs effectively regulated carbon/nitrogen/sulfur metabolism to induce more electron generation, less nitrite/N2O accumulation, and higher levels of nitrogen removal. This work reveals the mechanism by which N, Fe-CDs enhance anaerobic denitrification and broaden their potential application in nitrogen removal.

Treatment of nitrogen and phosphorus from sewage tailwater in paddy rice wetlands: concept and environmental benefits.

Domestic sewage tailwater (DSTW) reuse for crop irrigation is considered a promising practice to reduce water demand, mitigate water pollution, and substitute chemical fertilization. The level of the above environmental benefits of this water reuse strategy, especially when applied to paddy wetlands, remains unclear. In this study, soil column experiments were conducted to investigate the nitrogen and phosphorus fate in paddy wetlands subjected to different tailwater irrigation and drainage strategies, specifically, (i) TW1 and TW2 for regular or enhanced irrigation-drainage without N fertilization, (ii) TW3 and TW4 for regular irrigation with base or tillering N fertilizer, (iii) conventional fertilization N210, and (iv) no-fertilization controls N0. The results showed that the total nitrogen (TN), nitrate (NO3-), and total phosphorus (TP) removal rates from the paddies irrigated by DSTW ranged between 51.92 and 59.34%, 68.1 and 83.42%, and 85.69 and 86.98% respectively. Ammonia emissions from the DSTW-irrigated treatments were reduced by 14.6~47.2% compared to those paddies subjected to conventional fertilization (N210), similarly for TN emissions, with the exception of the TW2 treatment. Overall, it is established that the paddy wetland could effectively remove residual N and P from surface water runoffs, while the partial substitution of chemical fertilization by DSTW could be confirmed. The outcome of this study demonstrates that DSTW irrigation is a promising strategy for sustainable rice production with a minimized environmental impact.

Electrochemical removal of nitrate in high-salt wastewater with low-cost iron electrode modified by phosphate

Nitrate (NO3–) is a widespread pollutant in high-salt wastewater and causes serious harm to human health. Although electrochemical removal of nitrate has been demonstrated to be a promising treatment method, the development of low-cost electro-catalysts is still challenging. In this work, a phosphate modified iron (P-Fe) cathode was prepared for electrochemical removal of nitrate in high-salt wastewater. The phosphate modification greatly improved the activity of iron, and the removal rate of nitrate on P-Fe was three times higher than that on Fe electrode. Further experiments and density functional theory (DFT) calculations demonstrated that the modification of phosphoric acid improved the stability and the activity of the zero-valent iron electrode effectively for NO3– removal. The nitrate was firstly electrochemically reduced to ammonium, and then reacted with the anodic generated hypochlorite to N2. In this study, a strategy was developed to improve the activity and stability of metal electrode for NO3– removal, which opened up a new field for the efficient reduction of NO3– removal by metal electrode materials.

Response of sulfide autotrophic denitrification process and microbial community to oxytetracycline stress

The coexistence of antibiotics with sulfide and nitrate is common in sewage. Thus, this study explored the removal performance of nitrate and sulfide, and the response of extracellular polymer substances (EPS) and the microbial community to the sulfide autotrophic denitrification (SAD) process under oxytetracycline (OTC) stress. In Phase Ⅰ, the SAD system showed favouranle performance (nitrate removal rate ＞ 92.57%, sulfide removal rate ＞ 97.75%). However, in Phase Ⅳ, at OTC concentrations of 10, 15, and 20 mg/L, the NRE decreased to 76.13%, 40.71%, 11.37%, respectively, and the SRE decreased to 97.58%, 97.09%, 92.84%, respectively. At OTC concentrations of 0, 10, 15, and 20 mg/L, the EPS content were 1.62, 1.75, 2.03, and 1.42 mg/gVSS, respectively. The results showed that SAD performance gradually deteriorated under OTC stress. In particular, when the OTC concentration was 20 mg/L, the EPS content was lower than that of the control test, which could be attributed to the occurrence of microbial death. Finally, high-throughput sequencing results showed that OTC exposure led to gradual domination by heterotrophic denitrifying bacteria.

Synergistic effect of LDHs/loofah composites for in-situ remediation of nitrate in contaminated groundwater

Permeable reaction barrier (PRB) is an innovative technology for in-situ remediation of nitrate-contaminated groundwater. In this study, layered double hydroxides (LDHs)/loofah composites were prepared and used as active fillers for PRB to facilitate nitrate removal from groundwater. Under the conditions of initial nitrate concentration of 5 mg/L, a hydraulic retention time (HRT) of 4 h, and without additional carbon source, the nitrate removal rates of the natural loofah, MgAl-LDHs, and CaMgAl-LDHs composite were 69.31 %, 85.35 %, and 97.82 %, respectively. The CaMgAl-LDHs composite exhibits the highest denitrification capacity, attributed to the presence of Ca2+and Mg2+. Ca2+ and Mg2+is an essential element for the structural integrity of microbial cells, which demonstrated that Ca2+ reduces the surface charge of bacteria, leading to hydrophobicity and facilitating biofilm formation. Electrochemical tests demonstrate that LDHs with distinctive two-dimensional layered structure, contribute to the augmentation of electron transfer during denitrification. LDHs can also promote the formation of highly active biofilms and the secretion of extracellular polymeric substances (EPS) on the surface of loofah, thereby enhancing the metabolic activity of microorganisms. Metagenomic analysis revealed that LDHs enhance the enrichment of denitrifying bacteria and the activity of key enzymes involved in denitrification. Therefore, the adsorption-biological coupled degradation of the LDHs/loofah composites have great potential to enhance the efficiency of biological denitrification, making it promising filling materials for PRB technology applied in groundwater remediation.

Batch-Mode Denitrifying Woodchip Bioreactors for Expanded Treatment Flexibility

Denitrifying woodchip bioreactors successfully remove nitrates from reverse osmosis desalinization brine. On-farm desalination plants only operate for several hours per day in batch mode, meaning bioreactors should also operate in batch cycles, although this type of bioreactor operation is relatively unstudied. This study compared two tests of three cycles of 24 h per week with two treatments each (Test 1 8 vs. 24 h, and Test 2 8 vs. 12 h). Cylindrical pilot-scale bioreactors were filled with 130 kg of citrus woodchips and an average of 322 L of brine. The results show that the treatments with longer saturation periods of 24 and 12 h exhibited higher removal rates under operational conditions (i.e., 8 h flooding based on a 24 h cycle) than the 8 h treatment. However, the nitrate removal rates of the 8 h treatment were higher under fill cycle conditions (i.e., 8 h flooding based on an 8 h cycle). Dissolved organic carbon liberated from the woodchips was greater in treatments with longer drying periods (i.e., treatments with shorter saturation periods). Batch bioreactors should be considered under applicable conditions to increase nitrate removal rates.

New insights into the effect of EDTA on pyrite oxidation and N2O emission during pyrite autotrophic denitrification

Pyrite autotrophic denitrification (PAD) is crucial for removing nitrate in groundwater and advanced wastewater treatment. While disulfide is typically seen as the electron donor, the role of iron in pyrite oxidation is often ignored. In this study, Ethylene Diamine Tetraacetic Acid (EDTA) had been proven to have the potential to facilitate pyrite oxidation by promoting the solubility of iron in solution, thereby enhancing the kinetics of pyrite autotrophic denitrification. Firstly, EDTA could improve PAD performance, i.e. the nitrate removal rate at EDTA addition of 5 mM was 0.69 mg N/L•h and twice that of the control. However, high concentrations of EDTA inhibited the activity of the nitrous oxide reductase, resulting in the maximum accumulation of N2O reaching 13.26 mg N/L with an EDTA concentration of 15 mM. Furthermore, the X-ray Photoelectron Spectroscopy (XPS) result indicated that the EDTA addition promoted the production of bioavailable sulfur, with the concentration of S0 at 15 mM EDTA being more than twice higher than that of the control. Transmission electron microscopy (TEM) analysis indicated that EDTA could relieve the damage of iron encrustation on cell activity because it was difficult for EDTA-chelated ferrous iron to penetrate the cell membrane. Microbial community analyses showed an increase abundance of iron oxidizing bacteria, suggesting that EDTA addition promoted the bioavailability of iron for Fe(II) autotrophic denitrification. Overall, this study will provide insights into the understanding of oxidation mechanism of PAD process and the practical application for efficient nitrogen removal and N2O mitigation.