Denitrification Reactor Research Articles

Adsorption of NOx by graphene oxide loaded with CeO2 as a catalyst for atmospheric pollution control

In this study, we utilized graphene oxide (GO) loaded with cerium oxide (CeO2) to synthesize GO-CeO2 catalysts using a hydrothermal method and high-temperature calcination. The performance of the catalyst was evaluated by characterization of the catalyst material and testing for nitrogen oxide (NOx) conversion rates and nitrogen gas adsorption isotherm in mixed flue gas. The results demonstrated that increasing the ratio of NO2 in the NO+NO2 mixture significantly enhanced the conversion rate of NOx. Notably, when the proportion of NO2 reached 0.4, the denitrified gas displayed a remarkable increase in NOx conversion rate, exceeding 98%. This finding highlighted that even low concentrations of NO2 can accelerate denitrification reactions. Moreover, increasing the oxygen concentration in the gas exhibited considerable potential to elevate the NOx conversion rate. The oxygen concentration played a vital role in the process of denitrification, and the GO-CeO2 catalyst exhibited a high oxygen storage capacity. Moreover, the GO-CeO2 catalyst maintained a high denitrification efficiency even under high-temperature conditions and demonstrated excellent NOx conversion performance. The catalyst also demonstrated good stability and strong water resistance under varying proportions and temperatures while maintaining a consistent denitrification efficiency. These findings indicated the significant adsorption effect of the graphene oxide loaded with CeO2 catalyst on NOx, suggesting promising potential for application and widespread use.

Assessment of denitrification efficiency and membrane fouling potential in a sulfur-driven membrane bioreactor treating effluent from biological treatment

In this study, any remaining nitrate not eliminated during a biological treatment process with a low C/N ratio was effectively removed using sulfur powder in an autotrophic denitrification membrane bioreactor. Moreover, particular attention was given to investigating the potential for membrane fouling in a sulfur-based denitrification reactor, with a comparative analysis to conventional activated sludge in an aerobic membrane bioreactor. At an influent nitrate concentration of 320 mg/L, the removal efficiency reached 94–98 % within a 4-hour hydraulic retention time and at a reactor temperature of 30 °C. The sulfur-based denitrification reactor exhibited soluble microbial products concentration and specific cake resistance values of 38.3 mg/L and 1.4 × 1013 m/kg, respectively, which were 5.47 and 2.26 times higher than those of conventional activated sludge. Furthermore, it was observed that the supernatant from the sulfur-based denitrification reactor resulted in more significant fouling within the membrane surface.

Uranium bioavailability in soil pore water: A long-term investigation in a contaminated soil mesocosm

Uranium in soil can exist in both (IV) and (VI) oxidation states, distributed among different soil fractions (exchangeable, carbonate, oxidizable, reducible, and residual). Following its release from these fractions, uranium enters the soil pore water, becoming bioavailable and potentially posing risks due to its radio and chemical toxicity. Given the significant health and environmental risks associated with uranium, it is crucial to understand its behaviour in contaminated soil pore water and how it changes over time, especially in response to seasonal variations. To address this issue, study was designed to investigate the temporal changes in uranium availability in soil pore water, with a special focus on effect of seasonal variations and biogeochemical reactions that govern the bioavailability of uranium in a contaminated soil mesocosm. This field investigation was carried out for two consecutive years, and revealed that, seasonal variation has a significant effect on the bioavailability of the uranium in the upper soil layers (<30 cm). The biogenic NO3− induced oxidative dissolution of uranium was found to be the predominant reaction causing the dissolution of uranium into soil pore water, followed by ion-exchange in upper layer, whereas at higher depths (30 cm < d < 70 cm) bioavailability is predominantly controlled by ion-exchange reaction. Furthermore, the study shows that at upper layers bioavailability is high during the summer, which is attributed to higher rate of biogenic denitrification and ion exchange reactions. Fast vertical migration of uranium in the soil column is attributed to formation of stable mobile species such as hydroxo‑carbonato ((UO2)xCO3(OH)y−), hydroxo (UO2)x(OH)y and carbonato (UO2CO3) complexes, identified by speciation modelling. For the first time, this study reports the process controlling uranium behaviour in soil pore water and the effect of seasonal variation on it in a contaminated soil. The findings are essential for assessing its potential radiological impact and remediation planning.

The influence of organic components in sludge on selective non-catalytic reduction denitrification.

Collaborative disposal of sludge has been applied extensively in cement industry kilns, but the influence of organic components in sludge on the selective non-catalytic reduction (SNCR) denitrification is still in highly demanding. This paper focuses the influence of ethanol, n-octane, benzene, and acetone on the SNCR denitrification, and the influence of ethanol are analyzed on SNCR denitrification under different flue gas condition. The result showed that the presence of organic components reduces the reaction temperature of SNCR. When the temperature of SNCR reaction is at 750 ~ 800 ℃, the denitrification rate of four organic components is ethanol > benzene > n-octane > acetone. The optimum denitrification rate on SNCR can reach as high as 90.377% when the reaction condition is the NH3/NO is 1; the ethanol concentration is 1000ppm; O2 concentration is 2%; and temperature is 750 ℃. The presence of carbon dioxide inhibits the denitrification reaction. With the increase of temperature, the influence of SO2 on the denitrification reaction first increases and then decreases. In this paper, the influence of organic components in sludge on SNCR denitrification will be analyzed, which provides a theoretical basis for the selection of sludge dosing method for cement kiln collaborative disposal.

The crucial role of oxygen in NO heterogeneous reduction with NH3 at high temperature

The high-temperature ammonia-injected technology is acknowledged as a promising denitrification method with great NOx reduction potential. In this work, char reduction experiments and density functional theory (DFT) calculations were conducted to investigate the heterogeneous reduction mechanism of NO by ammonia (NH3) synergism with char, focusing on the role of O2. Experimental results indicate that O2 has an inhibition effect on the NO reduction efficiency, whose detailed mechanisms can be inferred from a molecular perspective. Theoretical results reveal that both Langmuir-Hinshelwood (L-H) and Eley-Rideal (E-R) mechanisms exist in the NH3-injected denitrification reactions, presenting significant differences in N2, N2O, and H2O evolution. By comparison, the E-R mechanism governs the NH3–NO reactions, which reduces barriers by up to 14.5 %. The presence of O2 can lead to a significant increase in the energy barriers with a concomitant decrease in reaction rates within a certain temperature range, which however does not change the dominant mechanism governing the NH3 and NO interactions. Although some controversies are found in the kinetic analysis when the temperature increases, the results here shed light on the NO heterogeneous reduction mechanisms with NH3 at high temperature, which also guides the coal/NH3 co-firing investigations.

Modeling coupled nitrification–denitrification in soil with an organic hotspot

Abstract. The emission of nitrous oxide (N2O) from agricultural soils to the atmosphere is a significant contributor to anthropogenic greenhouse gas emissions. The recycling of organic nitrogen (N) in manure and crop residues may result in spatiotemporal variability in N2O production and soil efflux which is difficult to capture by process-based models. We propose a multi-species, reactive transport model to provide detailed insight into the spatiotemporal variability in nitrogen (N) transformations around such N2O hotspots, which consists of kinetic reactions of soil respiration, nitrification, nitrifier denitrification, and denitrification represented by a system of coupled partial differential equations. The model was tested with results from an incubation experiment at two different soil moisture levels (−30 and −100 hPa) and was shown to reproduce the recorded N2O and dinitrogen (N2) emissions and the dynamics of important carbon (C) and N components in soil reasonably well. The simulation indicated that the four different microbial populations developed in closely connected but separate layers, with denitrifying bacteria growing within the manure-dominated zone and nitrifying bacteria in the well-aerated soil outside the manure zone and with time also within the manure layer. The modeled N2O production within the manure zone was greatly enhanced by the combined effect of oxygen deficit, abundant carbon source, and supply of nitrogenous substrates. In the wetter soil treatment with a water potential of −30 hPa, the diffusive flux of nitrate (NO3-) across the manure–soil interface was the main source of NO3- for denitrification in the manure zone, while at a soil water potential of −100 hPa, diffusion became less dominant and overtaken by the co-occurrence of nitrification and denitrification in the manure zone. Scenarios were analyzed where the diffusive transport of dissolved organic carbon or different mineral N species was switched off, and they showed that the simultaneous diffusion of NO3-, ammonium (NH4+), and nitrite (NO2-) was crucial to simulate the dynamics of N transformations and N2O emissions in the model. Without considering solute diffusion in process-based N2O models, the rapid turnover of C and N associated with organic hotspots can not be accounted for, and it may result in the underestimation of N2O emissions from soil after manure application. The model and its parameters allow for new detailed insights into the interactions between transport and microbial transformations associated with N2O emissions in heterogeneous soil environments.

Sulfur-pyrite-limestone biological filter for simultaneous nitrogen and phosphorus removal from wastewater treatment plant effluent: Interaction mechanisms of autotrophic and heterotrophic denitrification

To efficiently remove nitrogen(N) and phosphorus(P) from wastewater treatment plant effluent (WWTPE) with insufficient carbon, sulfur-pyrite-limestone autotrophic denitrification (SPLAD) reactor and gravel heterotrophic denitrification (GHD) reactor were constructed. Results showed that SPLAD was successfully started after 8 days and was faster than GHD. Under optimal hydraulic retention time (30 min) and backwash cycle (2 d), SPLAD achieved higher removal rates for TP, TN, NO3−-N, NH4+-N at 70.49 %, 90.35 %, 92.82 %, 79.66 %, respectively, surpassing those of GHD. The N removal primarily occurred through nitrification and denitrification with a proportion of 2.42–2.51 between heterotrophic and autotrophic denitrification, while P removal predominantly relied on the combination with Fe2+/Fe3+ generated by FeS2 autotrophic denitrification. Microbial community analysis revealed that Thiobacillus (17.86 %), Thioalbus (7.30 %) and Geothrix (0.81 %), belonging to autotrophic denitrification bacteria using S0/FeS2 as electron donors to reduce NO3−-N, were existed exclusively in SPLAD. Besides, the denitrification genes, such as nitrate reduction (narG, narZ, nxrA, and narH, narY, nxrB) and nitrite reduction (nirB and nirD), were higher in PSLAD than that in GHD. High abundance of sulfur oxidation genes (soxA, soxX, soxB, soxY and soxZ) and iron oxidation gene (ccoN) for autotrophic denitrification were also found in SPLAD. This study highlights the practical engineering application potential of the sulfur/pyrite/limestone involved mixotrophic denitrification process due to its simplicity, cost-effectiveness, and efficient N and P removal.

Investigating key drivers of N2O emissions in heterogeneous riparian sediments: Reactive transport modeling and statistical analysis

Nitrous oxide (N2O) is a potent greenhouse gas that also contributes to ozone depletion. Recent studies have identified river corridors as significant sources of N2O emissions. Surface water-groundwater (hyporheic) interactions along river corridors induce flow and reactive nitrogen transport through riparian sediments, thereby generating N2O. Despite the prevalence of these processes, the controlling influence of physical and geochemical parameters on N2O emissions from coupled aerobic and anaerobic reactive transport processes in heterogeneous riparian sediments is not yet fully understood. This study presents an integrated framework that combines a flow and multi-component reactive transport model (RTM) with an uncertainty quantification and sensitivity analysis tool to determine which physical and geochemical parameters have the greatest impact on N2O emissions from riparian sediments. The framework involves the development of thousands of RTMs, followed by global sensitivity and responsive surface analyses. Results indicate that characterizing the denitrification reaction rate constant and permeability of intermediate-permeability sediments (e.g., sandy gravel) are crucial in describing coupled nitrification-denitrification reactions and the magnitude of N2O emissions. This study provides valuable insights into the factors that influence N2O emissions from riparian sediments and can help in developing strategies to control N2O emissions from river corridors.

Managing microbial sulfur disproportionation for optimal sulfur autotrophic denitrification in a pilot-scale elemental sulfur packed-bed bioreactor.

Elemental sulfur packed-bed (S0PB) bioreactors for autotrophic denitrification have gained more attention in wastewater treatment due to their organic carbon-free operation, low operating cost, and minimal carbon emissions. However, the rapid development of microbial S0-disproportionation (MS0D) in S0PB reactor during deep denitrification poses a significant drawback to this new technology. MS0D, the process in which sulfur is used as both an electron donor and acceptor by bacteria, plays a crucial role in the microbial-driven sulfur cycle but remains poorly understood in wastewater treatment setups. In this study, we induced MS0D in a pilot-scale S0PB reactor capable of denitrifying over 1000 m3/d nitrate-containing wastewater. Initially, the S0PB reactor stably removed 6.6mg-NO3--N/L nitrate at an empty bed contact time (EBCT) of 20 mins, which was designated the S0-denitrification stage. To induce MS0D, we reduced the influent nitrate concentrations to allow deep nitrate removal, resulted in the production of large quantities of sulfate and sulfide (SO42-:S2- 3.2w/w). Meanwhile, other sulfur-heterologous electron acceptors (SHEAs), e.g., nitrite and DO, were also kept at trace levels. The negative correlations between the SHEAs concentrations and the sulfide productions indicated that the absence of SHEAs was a primary inducing factor to MS0D. The microbial community drastically diverged in response to the depletion of SHEAs during the switch from S0-denitrification to S0-disproportionation. An evident enrichment of sulfur-disproportionating bacteria (SDBs) was found at the S0-disproportionation stage, accompanied by the decline of sulfur-oxidizing bacteria (SOBs). In the end, we discovered that shortening the EBCT and increasing the reflux ratio could inhibit sulfide production by reducing it from 43.9mg/L to 3.2mg/L or 25.5mg/L. In conclusion, our study highlights the importance of considering MS0D when designing and optimizing S0PB reactors for sustainable autotrophic sulfur denitrification in real-life applications.

Nitrogen-sparging assisted anoxic biological drinking water treatment system.

Existing heterotrophic denitrification reactors rely on microorganisms to consume dissolved oxygen (DO) and create conditions suitable for denitrification, but this practice leads to excessive microbial growth and increased organic carbon doses. An innovative reactor that uses nitrogen gas sparging through a contactor to strip DO was developed and tested in the lab. It reduced influent nitrate from 15 to <1 mg/L as N with nitrite accumulation <1 mg/L as N. It maintained a consistent flow rate and developed minimal headloss, making it easier to operate than the denitrifying dual-media filter that was operated in parallel. Gravel, polyvinyl chloride pieces, and no packing media were assessed as options for the nitrogen-sparged contactor, and gravel was found to support denitrification at the highest loading rate and was resilient to nitrogen-sparging shutoffs and intermittent operation. This innovative reactor appears promising for small drinking water systems.

Flow and thermal mixing characteristics of a multi-jet bypass flue with main flue baffles

The decrease of inlet flue temperature of denitrification reaction unit is a major challenge that restricts the low load operation of thermal power units. In this study, a new bypass flue heating technology based on multi-pipe and multipoint was proposed. The effect of different baffle openings (RA), directions and position distance (Db) on the mixing characteristics of multiple jets was quantitatively investigated by the experimental system. Combined with the simulation, the changes of the baffle on the multi-jet flow and mixing are revealed. The results show that the mixing temperature increases with increasing RA, but high RA leads to a significant increase in the thermal mixing temperature inhomogeneity (ζT). As RA increases from 0.4 to 0.7, ζT rises from the minimum 0.368 to the maximum 0.559. The mixing temperature and ζT of the baffle in the +x direction are higher than those in the −x direction. When Db increases from 70 mm to 130 mm, the maximum heating effect decreases from 25.97 °C to 18.89 °C, and ζT decreases gradually at y/Dh = 1.25. The findings provide guidance for bypass flue modification and low load denitrification in engineering.

Preparation of Denitrification Materials with Nickel Slag for Nitric Oxide Decomposition in Cement Kilns.

NOx emission from the cement industry have received much attention. In order to reduce the NOx emission in cement kilns, nickel slag was used to prepare the non-ammonia denitrification material, and a denitrification mechanism was proposed in this study. The results showed that the denitrification material prepared at pH 7 exhibited the best denitrification performance. At low temperature, the highest denitrification performance was achieved between 200 and 300 °C with a NO decomposition rate of approximately 40%. Then, the NO decomposition rate increased as the temperature increased, reaching over 95% above 700 °C. The physicochemical characteristics showed that the material had the highest specific surface area and the highest relative Fe content, which benefited the denitrification performance. The divalent iron of the denitrification material was considered the active site for the reaction, and trivalent iron was not conducive to denitrification performance at a low temperature range. After the denitrification reaction, the Fe3+/Fe2+ increased from 0.89 to 1.31. The proposed denitrification mechanism was the redox process between divalent iron and trivalent iron. This study not only recycles industrial waste to reduce solid waste pollution but also efficiently removes nitrogen oxides from cement kilns without ammonia.

Pilot-scale demonstration of self-enrichment of anammox bacteria in a two-stage nitrification-denitrification suspended sludge system treating municipal wastewater under extremely low nitrogen loading rate

In suspended sludge system, efficient enrichment and retention of anammox bacteria are crucial obstacles in mainstream wastewater treatment by anammox process. In this study, anammox bacteria was self-enriched in a pilot-scale suspended sludge system of two-stage nitrification–denitrification process serving municipal wastewater treatment. With the low ammonia (NH4+-N) of 9.3 mg/L, nitrate (NO3–-N) of 15.6 mg/L and COD/NO3–-N of 2.2 under extremely low nitrogen loading rate of 0.012 kg N/m3/d, anammox activity bloomed after its abundance increasing from 5.9 × 107 to 4.6 × 109 copies/g dry sludge. Significant NH4+-N removal was occurred and maintained stably in the denitrification reactor with anammox bacteria accounting for 1.13%, even under temperature decreasing to 20.0℃. The adequately anoxic environment, efficient retention with the static settlement, and NO2– production via NO3– reduction provided favorable environment for anammox bacteria. This study demonstrated the feasibility and great potential in mainstream anammox application without seeding specific sludge.

Effect of magnetic fields on simultaneous nitrification and denitrification microbial systems.

Magnetic fields positively influence the nitrogen removal efficiency in activated sludge systems. However, the structural succession pattern of microorganisms by magnetic fields still remains further explored. In this paper, a magnetic simultaneous nitrification and denitrification (MSND) reactor was constructed, and the influence of optimized magnetic field intensity (0, 10, 20 and 30 mT) on the nitrogen removal efficiency was investigated at HRT 6 h, 28.0-30.0 °C, and pH 7.0-8.0. Molecular biology was used to investigate the succession process of the dominant microbial flora and the functional gene structure of MSND systems. The results showed that the denitrification effects of the MSND system were significantly enhanced, which contributed to the lower concentration of total nitrogen in the effluent of the magnetic reactor than that of the nonmagnetic group reactor. The magnetic fields induced the succession of microbial community structure and improved the stability of microbial communities, thereby the relative abundances of nitrifying and denitrifying bacteria, and the functional genes were improved. In particular, the abundance of functional genes related to gene proliferation and transmembrane transport was increased. Therefore, the efficient nitrogen removal was achieved, which gives inspiration in the enhanced wastewater treatment by magnetic fields.

Hydrogenotrophic denitrification for treatment of nitrate-contaminated groundwater: Using a microbubble diffuser to optimize performance

Nitrate nitrogen (NO3−-N) contamination of groundwater as a drinking water resource has been detected in many countries and poses a serious threat to human health. This study optimized the performance of a hydrogenotrophic denitrification (HD) reactor for nitrate-contaminated groundwater treatment by introducing a microbubble diffuser (MD), and evaluated its performance stability. The HD reactor continuously maintained effluent NO3−-N and NO2−-N concentrations below the drinking water standard for 200 days under optimized conditions (8 h hydraulic retention time, 10 mL min−1 H2 flow rate, and 50 mg L−1 NO3−-N). The nitrogen removal rate and nitrogen removal efficiency were 142.0 gN m−3 d−1 and 98 %, respectively. Additionally, the HD reactor required 0.02 m3 of H2 for 1 g of nitrogen removal, which indicated that approximately 24 % of the supplied H2 was utilized for HD. The MD produced finer gas bubbles and showed approximately 2.3-times higher gas-dissolving efficiency than an air stone diffuser producing ordinary bubbles. The efficiency is attributed to reactor performance optimization. In addition, Azoarcus spp. was the predominant genus in the HD reactor and accounted for 42.3 % of the total bacteria, suggesting that Azoarcus spp. might be related to HD in the reactor. Functional genes for denitrification (narG and nirS) were continuously detected. The experimental results lead to the development of an effective HD reactor for nitrate-contaminated groundwater treatment—in terms of elimination of H2 supply for denitrification under safe operation—and can facilitate the improvement of water security.

The behavior of heteroatom compounds during the pyrolysis of waste composite plastics

Multilayer composite packaging plastic is commonly used in daily life. Due to the complex structures and the existence of heteroatoms, such as nitrogen and oxygen, the recycling of this kind of plastic is challenging. Recycling through the pyrolysis method may provide an efficient option with respect to the mechanical and solvent methods. An understanding of the behaviour of heteroatoms during pyrolysis is significant to optimize the operating conditions. In this work, two composite packaging plastics of PET/LDPE (Polyethylene Terephthalate/Low Density Polyethylene) and PA/LDPE (Polyamide//Low Density Polyethylene) were pyrolysed at various temperatures between 400 and 500 °C in a fixed bed reactor. Heteroatom balance and composition analysis were conducted. Especially, the composition of O- and N-containing compounds in oils was fully understood by using GC-MS (Gas Chromatograph - Mass Spectrometer) and APPI FT-ICR MS (Atmospheric Pressure Photoionization Fourier Transform-Ion Cyclotron Resonance Mass Spectrometer) instruments. At 400 °C, only 2.2 wt% oxygen was left in 53.9 wt% solid residues for PET/LDPE and 3.6 wt% nitrogen was retained in 61.0 wt% solid residues for PA6/LDPE, whereas nearly no oxygen was retained in solid residues for PA6/LDPE. So the PET/PA6 can be degraded in a much lower temperature range than LDPE indicating the possibility of separating heteroatoms by step-wise pyrolysis. A large amount of O-containing compounds were observed in oils from PET/LDPE pyrolysis, while O, N-containing compounds existed in oils from PA6/LDPE. The APPI FT-ICR MS analysis revealed that parts of O-containing compounds were generated through the interaction between O-containing functional groups and aliphatics during the PET/LDPE pyrolysis. For PA6/LDPE, the deoxygenation reaction was easier to occur than the denitrification reaction.

Application of Hydrogen-Based Denitrification: Simultaneous Removal of Nitrate and Reactive Black 5 Dye from Textile Wastewater Containing Organic Matter

In this study, a hydrogen-based denitrification (HD) reactor was used to investigate the simultaneous treatment of nitrogen and decolorization in textile wastewater contaminated with organic matter. The reactor operated in two phases: without and with organic matter. Despite the short hydraulic retention time, the HD system successfully removed all pollutants, including nitrate, nitrite, reactive black-5 dye and chemical oxygen demand. The unhindered treatment efficiency for nitrogen and decolorization in the presence of organic pollutants was observed. With the addition of organic matter, the nitrogen removal efficiency increased slightly from 85% to 90–100%, and the decolorization rate doubled from 25% to 50–60%. Organic matter played a crucial role in stimulating heterotrophic bacteria during biological denitrification and acted as a carbon source facilitating biological denitrification and azo bond cleavage during dye degradation. Despite the generation of toxic byproducts and changes in the dominant microbial community, the treatment efficiency remained stable and improved. This approach offers a promising solution for enhancing treatment efficiency in textile wastewater, providing a cost-effective and environmentally friendly option for developing countries to treat wastewater before discharge.

Multirate mass transfer simulation of denitrification in a woodchip bioreactor

Denitrifying woodchip bioreactors (DWBs) have proven to be an efficient nature-based solution for nitrate removal. Modeling DWBs is required for improving their design and operation, but is hindered by the complexity of the modeled system where numerous chemical species and model parameters are needed. Reactions inside the woodchips are different from those at the edges, causing chemical localization (i.e., apparent simultaneous occurrence of incompatible reactions). We used the Multi Rate Mass Transfer (MRMT) approach to overcome these problems when simulating reactive transport processes in a DWB located at Kiruna, Sweden. Besides denitrification, other nitrogen-cycling processes (e.g., nitrification, dissimilatory nitrate reduction to ammonium, anammox) and alternative electron donors (e.g. oxygen, sulfate) were also considered. Biomass concentration is incorporated into the biochemical reaction rates, including growth and decay, to characterize microbial catalysis. We found that the MRMT model: 1) can account for the heterogeneity of the porous woodchips; 2) was capable of reproducing the nitrogen species evolution in the DWB with kinetic parameters from the literature; and 3) allows reproducing localized biochemical reactions (e.g., aerobic reactions on the woodchip edges, near the DWB entrance and anaerobic reactions inside); and 4) reproduces the full denitrification reactions sequence, but with the different reactions occurring in different portions of the woodchip (e.g., nitrate to nitrite near the edges and nitrite to nitrous oxide further inside). The latter observation suggests that increasing woodchip size may reduce the outflow of these undesired species.

Feasibility and optimization of a novel upflow denitrification reactor using denitrifying granular sludge for nitric acid pickling wastewater treatment

Stainless steel is highly valued for its superior resistance to corrosion. However, the pickling process involved in stainless steel production generates abundant NO3−-N, causing health and environmental risks. To address this issue, this study proposed a novel solution utilizing an up-flow denitrification reactor and denitrifying granular sludge for treating NO3–-N pickling wastewater under high NO3–-N loading. It was found that, the denitrifying granular sludge exhibited stable denitrification performance with the highest denitrification rate of 2.79 gN/(gVSS·d) and average removal rates of NO3−-N and TN of 99.94% and 99.31%, respectively, under optimal operating conditions of pH 6–9, temperature 35 °C, C/N ratio 3.5, hydraulic retention time (HRT) 11.1 h and ascending flow rate 2.75 m/h. This process reduced carbon source usage by 12.5–41.7% as compared to traditional denitrification methods. These findings demonstrate the efficacy of combining granular sludge and an up-flow denitrification reactor for treating nitric acid pickling wastewater.

Microbial community structure and nitrogen conversion rate of size-fractionated granules in partial denitrification and anammox reactor

Granule-based partial denitrification and anammox (PDA) process is affected by microbial community. To reveal how the nitrogen conversion rate varies with microbial composition caused by the granule size, PDA granules were divided into five sizes (<0.2, 0.2–0.5, 0.5–0.9, 0.9–1.6, and 1.6–2.0 mm). The abundance of anammox bacteria (dominated by Candidatus Brocadia) increased with particle size and peaked at 14.6% for 0.9–1.6 mm. In contrast, the abundance of denitrifiers (dominated by Thauera) decreased from 83.3% to 23.5% when granule size increased from <0.2 mm to 0.5–0.9 mm, but increased to 48.1% with further increases in particle size to 1.6–2.0 mm. The nitrogen conversion rate was affected by the microbial composition in granules. Nitrogen was autonomously removed in the granules of 0.9–2.0 mm. However, nitrite was accumulated in granules of <0.9 mm. Nitrite cross-feeding occurred inside and between granules. The findings provide a reference for the regulation of granule-based PDA systems.