Nitrate Removal Rate Research Articles

Two polarity reversal modes lead to different nitrate reduction pathways in bioelectrochemical systems

Polarity reversal is one of the effective strategies to rapidly start up denitrifying BESs，but the long-term performances of the denitrifying BESs operated under polarity reversal receive little attention. This study investigated the effects of periodic polarity reversal (PPR) and polarity reversal once only (PRO) on the long-term performances of denitrifying BESs. Repeatable oxidative and reductive currents were observed in the BESs obtained by PPR (PPR-BESs). The peak reductive currents of the PPR-BESs reached 0.95 A/m2, and nitrate was mainly removed by dissimilatory nitrate reduction to ammonium pathway with removal rates higher than 95 %. In contrast, the peak reductive currents of the BESs obtained by PRO (PRO-BESs) progressively decreased from 1.01 A/m2 to 0.12 A/m2. The nitrate removal rates of the PRO-BESs were <50 %, and the product of nitrate reduction turned to N2 instead of ammonium. 16S rDNA sequencing and metatranscriptomic analysis revealed that Geobacter capable of bidirectional extracellular electron transfer (EET) and Afipia capable of autotrophic growth were the dominant genera in the two types of BESs. Outer membrane cytochrome c and formate dehydrogenase were potentially involved in the cathodic electron uptake. These findings contribute to a better understanding of the EET mechanisms of electroautotrophic denitrifiers.

Insights into the relationship between denitrification and organic carbon release of solid-phase denitrification systems: Mechanism and microbial characteristics

Solid-phase denitrification is a promising alternative denitrification technology when facing a shortage of carbon sources. Nevertheless, it is still unclear whether there is a certain interaction between the denitrification process and the carbon release process in a solid-phase denitrification system. In this study, the concept of “Self-adaptation” was proposed for the relationship between denitrification and carbon release. At various influent nitrate loads, the PCL-supported denitrification system achieved an average nitrate removal rate of over 90.59 ± 7.01 % and a maximum denitrification rate of 0.67 ± 0.06 gN/(L·d). Microorganisms can spontaneously regulate the carbon release rate of PCL in response to changes in influent nitrate load, demonstrating “self-adaptation” of the PCL-supported solid-phase denitrification system. Regulation of carbon release rate via the “Self-adaptation” was achieved by changes in extracellular depolymerase activity. Acidovorax_sp. played a key role in “Self-adaptation”, for its function of both denitrification and PCL degradation.

Sunlight-hematite promoted denitrification by Pseudomonas aeruginosa: A little-known form of nitrogen-cycling enhancement

Microorganism-mineral interactions are widely involved in material circulation and energy flow processes in the Earth's surface system and have attracted extensive attention in recent years. Here, sunlight–hematite-promoted denitrification of Pseudomonas aeruginosa PAO1 was observed. Under illumination, the nitrate removal rate reached 90.49 ± 4.40%, which is 1.3-times higher than that in darkness (69.36 ± 1.96%). Moreover, different concentration of P. aeruginosa PAO1 and hematite showed good synergy, suggesting that enhanced denitrification processes are common. Amperometric I–t curves indicate that P. aeruginosa PAO1 generated the highest current density (3.4 ± 0.23 μA/cm2) with hematite under illumination, while electrochemical impedance spectroscopy showed that the charge transfer resistance RP increased from 144 Ω to 162 Ω when nitrate was added, indicating that there is another electron transfer pathway in the light–hematite–P. aeruginosa PAO1 system. To better understand this phenomenon in nature, experiments were conducted with red soil in place of hematite. A similarly enhanced nitrate removal rate (89.17%) was observed, confirming that this synergistic process occurs in nature. This research demonstrates a neglected denitrification model in a sunlight–semiconducting mineral–microorganism system, which expands our understanding of nitrogen cycling in natural environments.

Intelligent control of the electrochemical nitrate removal basing on artificial neural network (ANN)

Artificial intelligence technologies were confirmed as a useful tool for wastewater treatment, but its application in the electrochemical nitrate removal had less been reported. In this work, the artificial neural network in machine learning was used to construct the model, which combines the methods of electrochemistry and artificial intelligence to achieve the prediction and intelligent control of nitrate removal. The control system consists of a prediction module with an artificial neural network (ANN) algorithm model and a control module. First, initial nitrate concentration, pH, time and current density were considered as input. An ANN algorithm using 7 hidden layers and a negative feedback regulation mechanism was developed to optimize the model and predict the nitrate removal rate. Results indicates that the proposed prediction model (4–7–1) yields a better coefficient of determination and lower root mean square error. The optimal set-points of the current density in the electrochemical process can be obtained according to the water quality change and qualified effluent quality using the ANN model. Also, the proposed intelligent control strategy can eliminate the influence of water quality change on nitrate removal and reduce energy consumption by 15.0 % compared to the post strategy. This work demonstrated the potential of artificial intelligence in the electrochemical process of nitrate removal.

Coupling methanotrophic denitrification to anammox in a moving bed biofilm reactor for nitrogen removal under hypoxic conditions

Simultaneous removal of ammonium and nitrate was achieved in a methane-fed moving bed biofilm reactor (MBBR). In the reactor, methanotrophic microorganisms oxidized methane under hypoxic conditions likely to methanol, hence providing an electron donor to denitrifiers to reduce nitrate to nitrite that then allowed anaerobic ammonium oxidizing bacteria (Anammox) to remove excess ammonium as N2. The ammonium and nitrate removal rates reached 72.09 ± 5.81 mgNH4+–N/L/d and 62.61 ± 4.17 mgNO3−–N/L/d when the MBBR was operated in continuous mode. Nitrate removal by the methane-fed mixed consortia was confirmed in a batch test revealing a CH4/NO3− molar removal ratio of 1.15. The functional populations were unveiled by FISH analysis and 16S rRNA gene sequencing, which showed that the biofilm was dominated by Anammox bacteria (Candidatus Kuenenia) and diverse taxa associated with the capacity for denitrification: aerobic methanotrophs (Methylobacter, Methylomonas, and unclassified Methylococcaceae), methylotrophic denitrifiers (Opitutaceae and Methylophilaceae), and other heterotrophic denitrifiers (Ignavibacteriaceae, Anaerolineaceae, Comamonadaceae, Rhodocyclaceae and Thauera). Neither DAMO archaea nor DAMO bacteria were found in the sequencing analysis, indicating that more unknown community members possess the metabolic capacity of methanotrophic denitrification.

Enhanced electrokinetic remediation of heterogeneous aquifer co-contaminated with Cr(VI) and nitrate by rhamnolipids

Electrokinetic remediation is an in situ remediation technique that is gaining increasing attention in the field of groundwater contaminant removal. Aquifers co-contaminated with hexavalent chromium (Cr(VI)) and nitrate are becoming more common and research on the remediation of this pollutant is attracting worldwide attention owing to the considerable human health hazards of Cr(VI) and nitrate, including carcinogenicity, genotoxicity, and cytotoxicity. Electrokinetic remediation technology has significant advantages for the treatment of heterogeneous aquifers. This is the first study on the electrokinetic remediation of an aquifer co-contaminated with toxic heavy metals and inorganic salts. This study investigated the feasibility of electrokinetic remediation for layered heterogeneous aquifers co-contaminated with Cr(VI) and nitrate in the presence of groundwater flow, and it also assessed the ability of the anionic biosurfactant rhamnolipid to enhance the remediation effect. Changes in the pH, electrical conductivity, Cr(VI), nitrate, electric current, and the distribution of energy consumption and the voltage contour were analyzed through seven sets of experiments. The highest removal rates of Cr(VI) and nitrate were 97.30% and 87.08%, respectively. Rhamnolipids greatly enhanced the electrokinetic remediation process, especially in medium with moderately low permeability: 1) rhamnolipids improved Cr(VI) removal(from 1.62% of EK3 to 85.57% of EK6 and from 48.42% of EK4 to 94.18% of EK7) through migration and reduction; and 2) rhamnolipids improved nitrate removal (from 13.03% of EK3 to 71.72% of EK6 and from 23.69% of EK4 to 78.43% of EK7) through enhanced migration.

Rice husk-intensified cathode driving bioelectrochemical reactor for remediating nitrate-contaminated groundwater.

To achieve economical and eco-friendly denitrification, rice husk-intensified cathode driving bioelectrochemical reactor (RCBER) was constructed with rice husk as solid-phase carbon source and microbial carrier. Results demonstrated that the application of current improved the utilization of rice husk and enhanced the denitrification, and the quenching of anodic hydroxyl radicals by rice husk also improved the microbial resistance to current. The highest nitrate removal rate as 0.34 mg-N/(L∙d), higher economic benefits, i.e., current efficiency as 31.6% and energy consumption as 2.43 kWh/g NO3--N, and the highest environmental benefit, i.e., hydrogenotrophic denitrification contribution as 37.9%, were obtained at 200 mA/m2. The best performance at 200 mA/m2 was related to its better microenvironment, such as lower accumulation of anodic by-products and higher bioavailability of rice husks, as well as higher microbial metabolic activity, such as stable extracellular polymeric substance, the maximum electron transport system activity as 11.63 ± 0.14 μg O2·g-1·min-1·mg protein-1 and the highest activity of nitrate reductase (3.15-fold that of control check). The application of current realized the coexistence of heterotrophic and hydrogenotrophic denitrifiers, and multiple functional bacteria such as anaerobic denitrifiers Flavobacterium, aerobic denitrifiers Comamonas, hydrogenotrophic denitrifiers Thermomonas and electron transfer-related Enterobacter coexisted at 200 mA/m2, thereby improving RCBER's adaptability to the complex microenvironment. This study provides the theoretical basis for realizing a win-win situation of environmental pollution remediation and agricultural waste disposal.

Bamboo Chopstick Biochar Electrodes and Enhanced Nitrate Removal from Groundwater

The nitrate pollution of groundwater can cause serious harm to human health. Biochar electrodes, combined with adsorption and electroreduction, have great potential in nitrate removal from groundwater. In this study, bamboo chopsticks were used as feedstocks for biochar preparation. The bamboo chopstick biochar (BCBC), prepared by pyrolysis at 600 °C for 2 h, had a specific surface area of 179.2 m2/g and an electrical conductivity of 8869.2 μS/cm, which was an ideal biochar electrode material. The maximum nitrate adsorption capacity of BCBC-600-2 reached 16.39 mg/g. With an applied voltage of 4 V and hydraulic retention time of 4 h, the nitrate removal efficiency (NRE) reached 75.8%. In comparison, the NRE was only 32.9% without voltage and 25.7% with graphite cathode. Meanwhile, the average nitrate removal rate of biochar electrode was also higher than that of graphite cathode under the same conditions. Therefore, biochar electrode can provide full play to the coupling effect of adsorption and electroreduction processes and obtain more powerful nitrate removal ability. Moreover, the biochar electrode could inhibit the accumulation of nitrite and improve the selectivity of electrochemical reduction. This study not only provides a high-quality biochar electrode material, but also provides a new idea for nitrate removal in groundwater.

Enhancement of nitrate-dependent anaerobic methane oxidation via granular activated carbon

Denitrifying anaerobic methane oxidation (DAMO) is a bioprocess utilizing methane as the electron source to remove nitrate or nitrite, but denitrification rate especially for nitrate-dependent DAMO is usually limited due to the low methane mass transfer efficiency. In this research, granular active carbon (GAC) was added to enhance the nitrate-dependent DAMO process. The results showed that the maximum nitrate removal rate of GAC assisted DAMO system reached as high as 61.17 mg L−1 d−1, 8 times higher than that of non-amended control SBR. The porous structure of GAC can not only adsorb methane, but also keep the internal DAMO archaea from being washed out, and thus benefits for DAMO archaea enrichment. The relative abundance of DAMO archaea accounted for 96.3% in GAC-SBR, which was significantly higher than that of non-amended control SBR system (29.9%). Furthermore, GAC amendment up-regulated metabolic status of denitrification and methane oxidation based on gene sequence composition. The absolute abundances of function genes (NC10 pmoA and ANME mcrA) in GAC-SBR were almost 20 times higher than that of non-amended control SBR. This study provides a novel technique to stimulate the nitrate-dependent DAMO process.

Study on performance of carbon source released from fruit shells and the effect on biological denitrification in the advanced treatment

In order to solve the problem of shortage of carbon source for biological denitrification in advanced treatment of the effluent from secondary treatment of sewage, five kinds of fruit shells (pistachio shell, peanut shell, ginkgo shell, walnut shell and hazelnut shell) were preliminarily selected from eight kinds of fruit shells for experiments of static carbon release and denitrification. The carbon release performance (amount and law of carbon release and biodegradability of released carbon) and denitrification performance of different shells were investigated. Results showed that the peanut shell had the largest amount of carbon release (0.88 mg chemical oxygen demand [COD] g−1) and the highest removal rate of nitrate (NO3−-N) (76.48% ± 4.06%). However, the released carbon could not be fully utilized by denitrifying bacteria, resulting in a (205.90% ± 59.49%) increase in effluent COD compared with influent. The amounts of carbon release of ginkgo nut shell, walnut shell, and hazelnut shell were low (0.45, 0.41, and 0.43 mg COD g−1, respectively). The released carbon could not be used easily by microorganisms. Meanwhile, the contents of degradable aromatic protein and protein-like in dissolved organic matter (DOM) were low. Even the fulvic acid–like with low biodegradability also appeared in the soaking solution of the hazelnut shell. The NO3−-N and total nitrogen aveage removal rates were low in these three fruit shells and showed the removals within the 54.10–57.25% range and 52.21%–54.24% range, respectively. The amount of carbon release of pistachio shell was lower than that of peanut shell. However, the released carbon of the former was more biodegradable than that of the latter. Moreover, the relative molecular mass of DOM was small, and the contents of aromatic protein and protein-like were much higher than those of the four other kinds of fruit shells. The NO3−-N removal rate (71.48% ± 0.98%) of pistachio shell was only slightly lower than that of peanut shell. In conclusion, pistachio shell was the best carbon source for biological denitrification in the advanced treatment.

Improvement of Onsite Wastewater Systems Performance: Experimental and Numerical Investigation

Population growth and the associated increase in the use of Onsite Wastewater Treatment Systems (OWTS) in the Black Hills have been a reason for interest in nitrate contamination within the public water supply over the past few years. The main concern for the Black Hills is the presence of karst formation that all OWTS for wastewater travel faster, limiting the natural attenuation of wastewater contaminants. The treatment performance of common soils in the Black Hills and wood-based media was evaluated using soil column experiments and a numerical model, HYDRUS 2D. Nitrate treatment performances were evaluated using alluvial and cedar soils collected from the Black Hills, sand, woodchips (loose and dense), and biochar. This research investigated hydraulic and reaction parameters through a combination of experimental and inverse modeling approaches. A good agreement was obtained between the measured and model-predicted soil moisture content, with R2 values ranging from 0.57 to 0.99. The model was calibrated using flow data and nitrate concentration data measured from leachate collected at the bottom of the experimental columns. Nitrate removal rates varied from 32.3% to 70%, with the highest removal rate in loose woodchips, followed by dense woodchip and biochar, and the lowest removal rate in alluvial materials. The biochar and loose woodchips removed an additional 20% compared to common soils, attributable to the enhanced denitrification rate due to higher water content and organic content. The use of woodchips and biochar should be implemented in OWTS, where there are known karst formations.

Performance of Paracoccus pantotrophus MA3 in heterotrophic nitrification-anaerobic denitrification using formic acid as a carbon source.

Excess amount of nitrogen in wastewater has caused serious concerns, such as water eutrophication. Paracoccus pantotrophus MA3, a novel isolated strain of heterotrophic nitrification-anaerobic denitrification bacteria, was evaluated for nitrogen removal using formic acid as the sole carbon source. The results showed that the maximum ammonium removal efficiency was observed under the optimum conditions of 26.25 carbon to nitrogen ratio, 3.39% (v/v) inoculation amount, 34.64°C temperature, and at 180rpm shaking speed, respectively. In addition, quantitative real-time PCR technique analysis assured that the gene expression level of formate dehydrogenase, formate tetrahydrofolate ligase, 5,10-methylenetetrahydrofolate dehydrogenase, serine hydroxymethyltransferase, respiratory nitrate reductase beta subunit, L-glutamine synthetase, glutamate dehydrogenase, and glutamate synthase were up-regulated compared to the control group, and combined with nitrogen mass balance analysis to conclude that most of the ammonium was removed by assimilation. A small amount of nitrate and nearly no nitrite were accumulated during heterotrophic nitrification. MA3 exhibited significant denitrification potential under anaerobic conditions with a maximum nitrate removal rate of 4.39mg/L/h, and the only gas produced was N2. Additionally, 11.50 ± 0.06mg/L/h of NH4+-N removal rate from biogas slurry was achieved.

Rotten banana powder: A waste-recycling alternative for external carbon source

Although rotten banana (RB) has been proved as a superior carbon source for denitrification, how to maintain its effectiveness while reduce the depressing transportation expenses deserves for more considerations. In this study, a modified carbon source-RB powder (RBP) was prepared and utilized for treating nitrate in wastewater. Results indicated that RBP, equipped with rapid organic release and low residual properties, could achieve a maximum nitrate removal rate of 96.79% and inapparent nitrite accumulation with a denitrification rate higher than RB slurry. Supplementing RBP improved the long-term stability of sludge in the denitrification system while the dosage served as a promoting factor towards sludge dewatering performance but a limiting factor towards system stability. The breeding of unique bacteria was easily altered by increasing nitrate load, contributing to the combined denitrification via various microorganisms. Meanwhile, the phyla and genera related to denitrification and organic fermentation were effectively enriched by improving RBP dosage. Compared with RB slurry and sodium acetate, less denitrification cost would be obtained by RBP when the usage amount respectively exceeded 7.31 tons and 228.18–555.56 tons. All the results demonstrated that RBP would be selected as a more cost-efficient choice for denitrification in view of practical applications.

Role of iron(II) sulfide in autotrophic denitrification under tetracycline stress: Substrate and detoxification effect

Autotrophic denitrification using inorganic compounds as electron donors has gained increasing attention in the field of wastewater treatment due to its numerous advantages, such as no need for exogenous organic carbon, low energy input, and low sludge production. Tetracycline (TC), a refractory contaminant, is often found coexisting with nutrients (NO3− and PO43−) in wastewater, which can negatively affect the biological nutrient removal process because of its biological toxicity. However, the performance of autotrophic denitrification under TC stress has rarely been reported. In this study, the effects of TC on autotrophic denitrification with thiosulfate (Na2S2O3) and iron (II) sulfide (FeS) as the electron donors were investigated. With Na2S2O3 as the electron donor, TC slowed down the nitrate removal rate, which decreased from 1.32 to 0.18 d−1, when TC concentration increased from 0 mg/L to 50 mg/L. When TC concentration was higher than 2 mg/L, nitrite reduction was seriously inhibited, leading to nitrite accumulation. With FeS as the electron donor, nitrate removal was much more efficient under TC-stressed conditions, and no distinct nitrite accumulation was observed when the initial TC concentration was as high as 10 mg/L, indicating the effective detoxification of FeS. The detoxification effects in the FeS autotrophic denitrification system mainly resulted from the rapid adsorption of TC by FeS and effective degradation of TC, as proven by a relatively higher living biomass area. This study offers new insights into the response of sulfur-based autotrophic denitrifiers to TC stress and demonstrates that the FeS-based autotrophic denitrification process is a promising technology for the treatment of wastewater containing emerging contaminants and nutrients.

Response of denitrifying anaerobic methane oxidation enrichment to salinity stress: Process and microbiology

Denitrifying anaerobic methane oxidation (DAMO) is a novel biological process which could decrease nitrogen pollution and methane emission simultaneously in wastewater treatment. Salinity as a key environmental factor has important effects on microbial community and activity, however, it remains unclear for DAMO microorganisms. In this study, response of the enrichment of DAMO archaea and bacteria to different salinity was investigated from the aspect of process and microbiology. The results showed that the increasing salinity from 0.14% to 25% evidently deteriorated DAMO process, with the average removal rate of nitrate and methane decreased from 1.91 mg N/(L·d) to 0.07 mg N/(L·d) and 3.22 μmol/d to 0.59 μmol/d, respectively. The observed IC50 value of salinity on the DAMO culture was 1.73%. Further microbial analyses at the gene level suggested that the relative abundance of DAMO archaea in the enrichment decreased to 46%, 39%, 38% and 33% of the initial value. However, DAMO bacteria suffered less impact with the relative abundance maintaining over 75% of the initial value (except 1% salinity). In functional genes of DAMO bacteria, pmoA, decreased gradually from 100% to 86%, 43%, 15% and 2%, while mcrA (DAMO archaea) maintained at 67%–97%. This difference probably indicated DAMO bacteria appeared functional inhibition prior to community inhibition, which was opposite for the DAMO archaea. Results above-mentioned concluded that, though the process of nitrate-dependent anaerobic methane oxidation was driven by the couple of DAMO archaea and bacteria, they individually featured different response to high salinity stress. These findings could be helpful for the application of DAMO-based process in high salinity wastewater treatment, and also the understanding to DAMO microorganisms.

Influence of Hydraulic Conditions on Reclaimed Water Polishing Using Soil and Sand Columns

The removal of residual pollutants from a synthetic effluent with a composition similar to that of urban effluent from secondary treatment was evaluated in vertical downflow columns. These were filled with soil, the fine fraction of the soil, and sand, and operated in discontinuous and continuous mode. The results showed high removal rates of organic matter, ammonium, nitrate and phosphate in the discontinuous and continuous experiments, especially for the fine fraction. Therefore, the soil is suitable for removing organic matter and nutrients (N-NH4, N-NO3, and P-PO4), and can be used for polishing wastewater before its infiltration.&#x0D; Keywords: wastewater reuse, organics removal, nutrient removal, residual soil, river sand

Single-chamber microbial electrosynthesis reactor for nitrate reduction from waters with a low-electron donors’ concentration: from design and set-up to the optimal operating potential

Microbial electrosynthesis is a promising solution for removing nitrate from water with a low concentration of electron donors. Three single-chamber microbial electrosynthesis reactors were constructed and operated for almost 2 years. The single-chamber reactor design saves on construction costs, and the pH of the solute is more stable than that in the case of a two-chamber reactor. Nitrate reduction started at the working electrode potential of −756 mV versus standard hydrogen electrode (SHE), and subsequently, the working electrode potential could be increased without hindering the process. The optimal potential was −656 mV versus SHE, where the highest Faradaic efficiency of 71% and the nitrate removal rate of 3.8 ± 1.2 mgN-NO3/(L×day) were registered. The abundances of nitrite reductase and nitrous oxide reductase genes were significantly higher on the working electrode compared to the counter electrode, indicating that the process was driven by denitrification. Therefore, a microbial electrosynthesis reactor was successfully applied to remove nitrate and can be utilized for purifying water when adding organic compounds as electron donors is not feasible, that is, groundwater. In addition, at the lower working electrode potentials, the dissimilatory nitrate reduction to ammonium was observed.

Photocatalytic reduction of nitrate pollutants by novel Z-scheme ZnSe/BiVO4 heterostructures with high N2 selectivity

Eliminating nitrate pollutants in water efficiently and environmentally friendly has attracted widespread attention. As a promising technology to solve problems of environmental pollution and energy shortage, photocatalysis also displayed great potential in nitrate removal. In this work, a novel Z-scheme ZnSe/BiVO4 heterostructures were synthesized by in-situ growth of ZnSe nanospheres on BiVO4, and employed for photocatalytic reduction of nitrate in water. Experiments demonstrated that ZnSe/BiVO4 composites obtained improved photocatalytic activity compared with pristine ZnSe and BiVO4, and an optimal performance with nitrate removal rate as 89.84% and N2 selectivity as 91.03% in 50 min was achieved when initial nitrate was 100 mgN/L. The enhancements may be due to the boosted light harvesting capability and stronger redox ability of photogenerated carriers provided by Z-scheme ZnSe/BiVO4 heterostructures. Moreover, HCOOH played a better role as hole scavenger than CH3COOH and C2H5OH. The HCOOH dosage and initial solution pH greatly affected the photocatalytic removal of nitrate. Effects of impurity anions in water such as Cl−, SO42− and CO32− on nitrate removal were also investigated. The charge transfer pathway as well as photocatalytic mechanism for nitrate removal over ZnSe/BiVO4 heterostructures were proposed at the end.

The global significance of abiotic factors affecting nitrate removal in woodchip bioreactors

Woodchip bioreactor (WBR) is one of the green infrastructures in the agriculture system to reduce nitrate from agricultural drainage and stormwater. A lot of abiotic factors have been reported that affect nitrate removal lacking a comprehensive understanding. In this study, we studied eight important abiotic factors, including media age, hydraulic retention time (HRT), influent nitrate concentration (Cin), temperature, dissolved organic carbon (DOC), dissolved oxygen (DO), pH, and effective porosity (ρe) of WBR-filling materials. Based on a database that included 10,179 sets of data from 63 peer-reviewed articles, the nitrate removal rate (NRR) and nitrate removal efficiency (NRE) corresponding to the eight abiotic factors by different categories were comprehensively reported. According to this database, this study found the optimal range of abiotic factors for NRR and NRE in WBR were different. Regarding NRR, the optimal range of media age, HRT, Cin, temperature, effluent DOC, DO, pH, and ρe were in the first year, 0–5 h, 10–20 mg L−1, 20–25 °C, 0–5 mg L−1, 0–0.5 mg L−1, 7–8, and 0.6–0.7, respectively. For NRE, the optimal range of media age, HRT, Cin, temperature, effluent DOC, DO, pH, and ρe were in the first year, 500–3000 h, 0–10 mg L−1, 10–15 °C, >50 mg L−1, 0–0.5 mg L−1, 4–5, and 0.4–0.5, respectively. Moreover, the principal component analysis (PCA) indicated the field studies' principal components were different from laboratory studies. Furthermore, the structural equation modeling (SEM) also revealed the causal relationship of the eight abiotic factors on NRR and NRE is totally different. Lessons from this study can be incorporated into DNBR designs, especially improving nitrate removal rates by optimizing different abiotic factors. It also provides insights regarding the contributions of different abiotic factors for NRR and NRE independently and comprehensively.

Biological treatment of ironworks wastewater with high-concentration nitrate using a nitrogen gas aerated anaerobic membrane bioreactor

Difficulty in treating ironworks wastewater with high concentrations of nitrate is a major issue in steel manufacturing. We developed an N2 gas aerated anaerobic membrane bioreactor (AnMBR) and further assessed its nitrate removal capacity at the bench (0.026 m3) and pilot (4 m3) scales using a comprehensive microbiome survey. Both scale AnMBRs were fed with 5,300–8,000-mg/L nitrate-concentrated ironworks wastewater and methanol. At an early operation phase, nitrate and total organic carbon (TOC) increased in treated wastewater, which coincided with dynamic shift of the sludge microbiome. Thereafter, the accumulated nitrate was degraded in the absence of nitrite, in parallel with the TOC decrease and/or depletion, implying the presence of denitrification with methanol oxidation. At this phase, Hyphomicrobium nitrativorans capable of completely reducing nitrate predominated the microbiome. The increase in hydrolyzing and fermentative bacteria suggested their involvement in degradation of the decayed sludge biomass. H. nitrativorans, collaborating with high-molecular compound metabolizers, contributed importantly to the achievement of the nitrate removal rate of 1.1 ± 0.1 kg NO3-N/m3/day for pilot scale. Compared based on the complete (e.g., >99.9 %) removal, the achieved rate was comparable with the previous records (2.0–2.5 kg NO3-N/m3/day). Nevertheless, the supplied nitrate concentrations were remarkably higher than the previous ones (650–1,500 mg/L). These highlight that the N2 gas aerated AnMBR is widely applicable for the effective treatment of the nitrate-concentrated industrial wastewater with the considerably small footprint.