Partial nitrification under limited dissolved oxygen conditions

Comammox rather than AOB dominated the efficient autotrophic nitrification-denitrification process in an extremely oxygen-limited environment

The discovery of complete ammonia oxidizer (comammox) has challenged the traditional understanding of the two-step nitrification process. However, their functions in the oxygen-limited autotrophic nitrification-denitrification (OLAND) process remain unclear. In this study, OLAND was achieved using comammox-dominated nitrifying bacteria in an extremely oxygen-limited environment with a dissolved oxygen concentrations of 0.05 mg/L. The ammonia removal efficiency exceeded 97 %, and the total nitrogen removal efficiency reached 71 % when sodium bicarbonate was used as the carbon source. The pseudo-first- and second-order models were found to best fit the ammonia removal processes under low and high loads, respectively, suggesting distinct ammonia removal pathways. Full-length 16S rRNA gene sequencing and metagenomic results revealed that comammox-dominated under different oxygen levels, in conjunction with anammox and heterotrophic denitrifiers. The abundance of enzymes involved in energy metabolism indicates the coexistence of anammox and autotrophic nitrification-heterotrophic denitrification pathways. The binning results showed that comammox bacteria engaged in horizontal gene transfer with nitrifiers, anammox bacteria, and denitrifiers to adapt to an obligate environments. Therefore, this study demonstrated that comammox, anammox, and heterotrophic denitrifiers play important roles in the OLAND process and provide a reference for further reducing aeration energy in the autotrophic nitrogen removal process.

Oxygen-to-ammonium-nitrogen ratio as an indicator for oxygen supply management in microoxic bioanodic ammonium oxidation

Microbial electrolysis cells (MECs) have been proven effective for oxidizing ammonium (NH4+), where the anode acts as an electron acceptor, reducing the energy input by substituting oxygen (O2). However, O2 has been proved to be essential for achieving high removal rates MECs. Thus, precise control of oxygen supply is crucial for optimizing treatment performance and minimizing energy consumption. Unlike previous studies focusing on dissolved oxygen (DO) levels, this study introduces the O2/NH4+-N ratio as a novel control parameter for balancing oxidation rates and the selectivity of NH4+ oxidation towards dinitrogen gas (N2) under limited oxygen condition. Our results demonstrated that the O2/NH4+-N ratio is a more relevant oxygen supply indicator compared to DO level. Oxygen served as a more favorable electron acceptor than the electrode, increasing NH4+ oxidation rates but also resulting in more oxidized products such as nitrate (NO3-). Additionally, nitrous oxide (N2O) and N2 production were higher with the electrode as the electron acceptor compared to oxygen alone. An O2/NH4+-N ratio of 0.5 was found to be optimal, achieving a balance between product selectivity for N2 (51.4 % ± 4.5 %) and oxidation rates (344.6 ± 14.7 mg-N/L*d), with the columbic efficiency of 30.7 % ± 2.0 %. Microbial community analysis revealed that nitrifiers and denitrifiers were the primary bacteria involved, with oxygen promoting the growth of nitrite-oxidizing bacteria, thus facilitating complete NH4+ oxidation to NO3-. Our study provides new insights and guidelines on the appropriate oxygen dosage, offering strategies into optimizing operational conditions for NH4+ removal using MECs.

Advanced treatment of coking wastewater: Recent advances and prospects

Advanced treatment of refractory industrial wastewater is still a challenge. Coking wastewater is one of coal chemical wastewater, which contains various refractory organic pollutants. To meet the more and more rigorous discharge standard and increase the reuse ratio of coking wastewater, advanced treatment process must be set for treating the biologically treated coking wastewater. To date, several advanced oxidation processes (AOPs), including Fenton, ozone, persulfate-based oxidation, and iron-carbon micro-electrolysis, have been applied for the advanced treatment of coking wastewater. However, the performance of different advanced treatment processes changed greatly, depending on the components of coking wastewater and the unique characteristics of advanced treatment processes. In this review article, the state-of-the-art advanced treatment process of coking wastewater was systematically summarized and analyzed. Firstly, the major organic pollutants in the secondary effluents of coking wastewater was briefly introduced, to better understand the characteristics of the biologically treated coking wastewater. Then, the performance of various advanced treatment processes, including physiochemical methods, biological methods, advanced oxidation methods and combined methods were discussed for the advanced treatment of coking wastewater in detail. Finally, the conclusions and remarks were provided. This review will be helpful for the proper selection of advanced treatment processes and promote the development of advanced treatment processes for coking wastewater.

Advance nitrogen removal from anaerobic sludge digestion liquor using partial nitrification and denitrification coupled with simultaneous partial nitrification, anammox, and denitrification process

A novel two-stage continuous-flow partial nitrification and denitrification coupled with simultaneous partial nitrification, anammox, and denitrification (PND-SNAD) process was developed to treat anaerobic sludge digestion liquor. During the stable phase, the total nitrogen and chemical oxygen demand (COD) removal efficiencies were 93 ± 3 % and 59 ± 7 %, respectively. Free ammonia suppression (26.2 ± 12.7 mg/L) and low dissolved oxygen (DO, 0.12 ± 0.07 mg/L) were key factors in the operation of the PND process, while the SNAD process was restricted successfully by limited oxygen (DO < 0.1 mg/L) and short solids retention time (9.7 d). The PND process was an important pretreatment process that could remove biodegradable dissolved COD by denitrification and supply ammonium-oxidizing bacteria (AOB) to the SNAD process. Nitrosomonas and Ca. Brocadia were the dominant AOB and anammox bacteria, respectively. Overall, this research presents a distinctive SNAD combined process for anaerobic sludge digestion liquor treatment.

Effect of phosphorylated polyvinyl alcohol matrix size of cell entrapment on partial nitrification of ammonia in wastewater

Partial nitrification is known as first and critical step for autotrophic nitrogen removal in high strength nitrogenous wastewater. Phosphorylated polyvinyl alcohol gel entrapment was used for suppressing oxygen to nitrite-oxidizing bacteria (NOB) in the gel matrix. The study investigated the effect of the size of gel matrix on partial nitrification. Results show that ammonia-oxidizing bacteria (AOB) proportion in the inoculum rather than the size of gel matrix governed ammonia oxidation. Nitrite oxidation depended on the size of gel matrix not the relative proportions of NOB and AOB in the inoculum. Larger size of gel matrix lead to less in situ oxygen penetration and available for NOB resulting in higher nitrite accumulation. This finding gains a better understanding of using suitable inoculum to control partial nitrification that is beneficial for the preparation of anaerobic ammonium oxidation-suited effluent.

Biochar and hematite amendments suppress emission of CH4 and NO2 in constructed wetlands

Constructed wetlands (CWs) are considered a widely used cost-effective technology for pollutant removal. However, greenhouse gas emissions are a non-negligible problem in CWs. In this study, four laboratory-scale CWs were established to evaluate the effects of gravel (CW_B), hematite (CW_Fe), biochar (CW_C), and hematite + biochar (CW_Fe-C) as substrates on pollutants removal, greenhouse gas emissions, and associated microbial characteristics. The results showed that the biochar-amended CWs (CW_C and CW_Fe-C) enhanced the removal efficiency of pollutants, with 92.53 % and 93.66 % of COD and 65.73 % and 64.41 % of TN removal, respectively. Both single and combined inputs of biochar and hematite significantly reduced CH₄ and N₂O fluxes, with the lowest average of CH₄ flux obtained in CW_C (5.99 ± 0.78 mg CH₄ m^-2 h^-1) and the least N₂O flux in CW_Fe-C (287.57 ± 44.84 μg N₂O m^-2 h^-1). The substantial reduction of global warming potentials (GWP) was obtained in the applications of CW_C (80.25 %) and CW_Fe-C (79.5 %) in biochar-amended CWs. The presence of biochar and hematite mitigated CH₄ and N₂O emissions by modifying microbial communities with higher ratios of pmoA/mcrA and nosZ genes abundances, as well as increasing the abundance of denitrifying bacteria (Dechloromona, Thauera and Azospira). This study demonstrated that biochar and the combined use of biochar and hematite could be the potential candidates as functional substrates for the efficient removal of pollutants and simultaneously reducing GWP emissions in the constructed wetlands.

Recent advances in simultaneous nitrification and denitrification for nitrogen and micropollutant removal: a review

Simultaneous Nitrification and Denitrification (SND) is a promising process for biological nitrogen removal. Compared to conventional nitrogen removal processes, SND is cost-effective due to the decreased structural footprint and low oxygen and energy requirements. This critical review summarizes the current knowledge on SND related to fundamentals, mechanisms, and influence factors. The creation of stable aerobic and anoxic conditions within the flocs, as well as the optimization of dissolved oxygen (DO), are the most significant challenges in SND. Innovative reactor configurations coupled with diversified microbial communities have achieved significant carbon and nitrogen reduction from wastewater. In addition, the review also presents the recent advances in SND for removing micropollutants. The micropollutants are exposed to various enzymes due to the microaerobic and diverse redox conditions present in the SND system, which would eventually enhance biotransformation. This review presents SND as a potential biological treatment process for carbon, nitrogen, and micropollutant removal from wastewater.

Continuous monitoring of dissolved inorganic nitrogen (DIN) transformations along the waste-vadose zone - groundwater path of an uncontrolled landfill, using multiple N-species isotopic analysis

Landfill leachates contain a heavy load of dissolved inorganic nitrogen (DIN), posing a threat to water resources. Therefore, it is highly important to understand the processes that control its evolution (speciation, accumulation, or attenuation) during the percolation of leachates through the unsaturated zone, finally affecting the groundwater. However, tracking DIN transformations in this complex and inaccessible environment is challenging, and knowledge concerning this important topic under field conditions is scarce. The presented study used a unique monitoring system that allows sampling of repetitive samples from within the waste and the unsaturated zone. An array of 8 wells penetrating the underlying aquifer completed the spatial observation. Multiple N-species isotopic approach was applied to discern the dominating N-involving processes over the continuum - from the waste mound through the unsaturated zone and the underlying aquifer. Despite the considerable heterogeneity observed throughout the profile, the results provided a cohesive and valuable reflection of the evolution of the inorganic nitrogen pool in this highly contaminated environment. Leachates inside the waste had reducing characteristics with high accumulation of ammonium (up to 360 mg/l NH₄⁺-N), and a distinct δ¹⁵N-NH₄⁺ range (-3‰ to +10‰). The upper layers of the unsaturated zone underneath the landfill margins found to be aerated, promoting N oxidation which resulted in the accumulation of nitrate in the leachates (up to 490 mg/l NO₃^-N). Exceptionally high concentrations of nitrite (up to 126 mg/l NO₂^-N) were found as oxygen levels decreased in deeper sections of the vadose zone. Enrichment of δ¹⁵N-NO₂^- compared to δ¹⁵N-NO₃^- indicated the significance of autotropic nitrite reduction, controlling the DIN composition, correlated with NO₂^- accumulation and net DIN attenuation. The δ¹⁵N: δ¹⁸O ratio implied co-occurrence of denitrification in the leachates, even in the more oxidized sections, further contributing to N-attenuation in the unsaturated zone. In the aquifer, δ¹⁵N-NH₄⁺ values and δ¹⁵N: δ¹⁸O ratio linked N contamination to the leachates source. The encounter with the oxidized groundwater promoted intensive nitrification. δ¹⁵N-NO₂^- values in the groundwater were lighter than both δ¹⁵N-NH₄⁺ and δ¹⁵N-NO₃^- by 22‰ to 62‰, implying the co-occurrence of nitrification-denitrification processes. The effect of denitrification grew with decreasing dissolved oxygen (DO) levels below 0.5 mg/l towards the center of the plume, contributing to net DIN attenuation in the plume. The findings are significant for any consideration of the risk posed by DIN, as well as remediation measures, in a landfill environment and other sites with a heavy load of degrading organic matter.

Simultaneous biological removal of nitrogen and phosphorus from secondary effluent of wastewater treatment plants by advanced treatment: A review

With the advancement of water ecological protection and water control standard, it is the general trend to upgrade the wastewater treatment plants (WWTPs). The simultaneous removal of nitrogen and phosphorus is the key to improve the water quality of secondary effluent of WWTPs to prevent the eutrophication. Therefore, it is urgent to develop the applicable technologies for simultaneous biological removal of nitrogen and phosphorus from secondary effluent. In this review, the composition of secondary effluent from municipal WWTPs were briefly introduced firstly, then the three main treatment processes for simultaneous nitrogen and phosphorus removal, i.e., the enhanced denitrifying phosphorus removal filter, the pyrite-based autotrophic denitrification and the microalgae biological treatment system were summarized, their performances and mechanisms were analyzed. The influencing factors and microbial community structure were discussed. The advanced removal of nitrogen and phosphorus by different technologies were also compared and summarized in terms of performance, operational characteristics, disadvantage and cost. Finally, the challenges and future prospects of simultaneous removal of nitrogen and phosphorus technologies for secondary effluent were proposed. This review will deepen to understand the principles and applications of the advanced removal of nitrogen and phosphorus and provide some valuable information for upgrading the treatment process of WWTPs.

Construction of Recombinant Magnetospirillum Strains for Nitrate Removal from Wastewater Based on Magnetic Adsorption

Nitrate ion (NO3−) in wastewater is a major cause of pollution in aquatic environments worldwide. Magnetospirillum gryphiswaldense (MSR-1) has a complete dissimilatory denitrification pathway, converts NO3− in water into nitrogen (N2) and simultaneously removes ammonium ions (NH4+). We investigated and confirmed direct effects of regulatory protein factors Mg2046 and MgFnr on MSR-1 denitrification pathway by EMSAs and ChIP-qPCR assays. Corresponding mutant strains were constructed. Denitrification efficiency in synthetic wastewater medium during a 12-h cell growth period was significantly higher for mutant strain Δmgfnr (0.456 mmol·L−1·h−1) than for wild-type (0.362 mmol·L−1·h−1). Presence of magnetic particles (magnetosomes) in MSR-1 greatly facilitates collection and isolation of bacterial cells (and activated sludge) by addition of a magnetic field. The easy separation of magnetotactic bacteria, such as MSR-1 and Δmgfnr, from wastewater using magnetic fields is a unique feature that makes them promising candidates for practical application in wastewater treatment and sludge pretreatment.

A critical review of various adsorbents for selective removal of nitrate from water: Structure, performance and mechanism

Nitrate is ubiquitous pollutant due to its high water solubility, usually contributing to eutrophication, and posing a threat to aquatic ecosystem and human health. Adsorption approach has been widely used for nitrate removal because of the simplicity, easy operation, and low cost. Adsorbent plays a key role in the adsorptive removal of nitrate. The adsorption performance and adsorption mechanism are determined by the structural feature of adsorbent that is dependent on the preparation method. In this review, various types of adsorbents for nitrate removal were systematically summarized, their preparation, characterization, and adsorption performance were evaluated; the factors influencing the nitrate adsorption performance were discussed; the adsorption isotherm models, kinetic models and thermodynamic parameters were examined; and the possible adsorption mechanisms responsible for nitrate adsorption were categorized; the possible correlation of adsorbent structure to adsorption performance and adsorption mechanism were explained; the potential applications of adsorbents were discussed; finally, the strategies for improving adsorption capacity and selectivity towards nitrate, the challenges and future perspectives for developing novel adsorbent were also proposed. This review will deepen the understanding of nitrate removal by adsorption process and help the development of high-performance adsorbents for selective nitrate removal from water and wastewater.

Denitrification performance and microbial community of bioreactor packed with PHBV/PLA/rice hulls composite

In this study, a novel biodegradable PHBV/PLA/rice hulls (PPRH) composite was applied and tested as biofilm attachment carrier and carbon source in two bioreactors for biological denitrification process. The denitrification performance, effect of operational conditions and microbial community structure of PPRH biofilm were evaluated. The batch experiment results showed that PPRH-packed bioreactor could completely remove 50 mg L^-1 of NO₃^--N at natural pH (ca. 7.5) and room temperature. The continuous flow experiments indicated that high NO₃^--N removal efficiency (77%-99%) was achieved with low nitrite (<0.48 mg L^-1) and ammonia (<0.81 mg L^-1) accumulation, when influent NO₃^--N concentration was 30 mg L^-1 and hydraulic retention time was 2-6 h. Furthermore, the microbial community analysis indicated that bacteria belonging to genus Diaphorobacter in phylum Proteobacteria were the most dominant and major denitrifiers in denitrification. In summary, PPRH composite was a promising carbon source for biological nitrate removal from water and wastewater.

Inhibition of ferrous iron (Fe2+) to sulfur-driven autotrophic denitrification: Insight into microbial community and functional genes

The effect of Fe²⁺ on the performance of sulfur-driven autotrophic denitrification (SDAD) using S⁰ as electron donor was evaluated. The experimental results showed that as initial Fe²⁺ concentration increased, nitrate (NO₃^-) removal rate significantly decreased. Fe²⁺ ion (0.1 mM and 1 Mm) inhibited SDAD rate (approximately 10% and 50%) and resulted in an accumulation of nitrite (NO₂^-) and nitrous oxide (N₂O). The relative abundance of Thiobacillus was positively correlated with NO₃^- removal rate, whereas negatively correlated with Fe²⁺ concentration, suggesting that Fe²⁺ inhibited the sulfur-oxidizing denitrifying bacteria. Moreover, the abundance of bacterial 16S rRNA, denitrifying genes (narG, nirS, nirK and nosZ) and sulfur-oxidizing genes (soxB and dsrA) decreased with the increase of Fe²⁺ concentration, among them nosZ and soxB were the most sensitive genes to Fe²⁺, and nosZ/narG, soxB/(bacterial 16S rRNA) and soxB/nirK had influence on NO₃^- removal rate, while nosZ/(bacterial 16S rRNA) affected N₂O accumulation rate.

Integration of a nitrification bioreactor and an anoxic biotrickling filter for simultaneous ammonium-rich water treatment and biogas desulfurization

A preliminary assessment has been carried out on the integration of an anoxic biotrickling filter and a nitrification bioreactor for the simultaneous treatment of ammonium-rich water and H₂S contained in a biogas stream. The nutrient consumption in the biotrickling filter was as follows (mol^-1 NO₃^--N): 6.3·10^-4 ± 1.2·10^-4 mol PO₄^3--P, 0.04 ± 0.05 mol NH₄⁺-N and 0.04 ± 0.03 mol K⁺-K. Furthermore, it was possible to supply a mixture of biogenic NO₃^- and NO₂^- into the biotrickling filter from the nitrification bioreactor to obtain a maximum elimination capacity of 152 gH₂S-S m^-3 h^-1. The equivalence between the two compounds was 1 mol NO₃^--N equal to 1.6 mol NO₂^--N. The biotrickling filter was also operated under a stepped variable inlet load (30-100 gH₂S-S m^-3 h^-1) and outlet H₂S concentrations of less than 150 ppm_V were obtained. It was also possible to maintain the outlet H₂S concentration close to 15 ppm_V with a feedback controller by manipulating the feed flow (in the nitrification bioreactor). Two stepped variable inlet loads were tested (60-111 and 16-102 gH₂S-S m^-3 h^-1) under this type of control. The implementation of feedback control could enable the exploitation of biogas in a fuel cell, since the H₂S concentrations were 15.1 ± 4.3 and 15.0 ± 3.4 ppm_V. Finally, the anoxic biotrickling filter experienced partial denitrification and this implied a loss of the desulfurization effectiveness related to SO₄^2- production.

Pollutant removal from municipal sewage by a microaerobic up-flow oxidation ditch coupled with micro-electrolysis

The development of efficient and low-cost wastewater treatment processes remains an important challenge. A microaerobic up-flow oxidation ditch (UOD) with micro-electrolysis by waterfall aeration was designed for treating real municipal wastewater. The effects of influential factors such as up-flow rate, waterfall height, reflux ratio, number of stages and iron dosing on pollutant removal were fully investigated, and the optimum conditions were obtained. The elimination efficiencies of chemical oxygen demand (COD), ammonia nitrogen (NH₄ ⁺-N), total nitrogen (TN) and total phosphorus (TP) reached up to 84.33 ± 2.48%, 99.91 ± 0.09%, 93.63 ± 0.60% and 89.27 ± 1.40%, respectively, while the effluent concentrations of COD, NH₄ ⁺-N, TN and TP were 20.67 ± 2.85, 0.02 ± 0.02, 1.39 ± 0.09 and 0.27 ± 0.02 mg l^-1, respectively. Phosphorous removal was achieved by iron-carbon micro-electrolysis to form an insoluble ferric phosphate precipitate. The microbial community structure indicated that carbon and nitrogen were removed via multiple mechanisms, possibly including nitrification, partial nitrification, denitrification and anammox in the UOD.

Various electron donors for biological nitrate removal: A review

Nitrate (NO₃^-) pollution in water and wastewater has become a serious global issue. Biological denitrification, which reduces NO₃^- to N₂ (nitrogen gas) by denitrifying microorganisms, is an efficient and economical process for the removal of NO₃^- from water and wastewater. During the denitrification process, electron donor is required to provide electrons for reduction of NO₃^-. A variety of electron donors, including organic and inorganic compounds, can be used for denitrification. This paper reviews the state of the art of various electron donors used for biological denitrification. Depending on the types of electron donors, denitrification can be classified into heterotrophic and autotrophic denitrification. Heterotrophic denitrification utilizes organic compounds as electron donors, including low-molecular-weight organics (e.g. acetate, methanol, glucose, benzene, methane, etc.) and high-molecular-weight organics (e.g. cellulose, polylactic acid, polycaprolactone, etc.); while autotrophic denitrification utilizes inorganic compounds as electron donors, including hydrogen (H₂), reduced sulfur compounds (e.g. sulfide, element sulfur and thiosulfate), ferrous iron (Fe²⁺), iron sulfides (e.g. FeS, Fe_1-xS and FeS₂), arsenite (As(Ш)) and manganese (Mn(II)). The biological denitrification processes and the representative denitrifying microorganisms are summarized based on different electron donors, and their denitrification performance, operating costs and environmental impacts are compared and discussed. The pilot- or full-scale applications were summarized. The concluding remarks and future prospects were provided. The biodegradable polymers mediated heterotrophic denitrification, as well as H₂ and sulfur mediated autotrophic denitrification are promising denitrification processes for NO₃^- removal from various types of water and wastewater.

Mechanistic understanding of the pollutant removal and transformation processes in the constructed wetland system

Constructed wetland systems (CWs) are biologically and physically engineered systems to mimic the natural wetlands which can potentially treat the wastewater from the various point and nonpoint sources of pollution. The present study aims to review the various mechanisms involved in the different types of CWs for wastewater treatment and to elucidate their role in the effective functioning of the CWs. Several physical, chemical, and biological processes substantially influence the pollutant removal efficiency of CWs. Plants species Phragmites australis, Typha latifolia, and Typha angustifolia are most widely used in CWs. The rate of nitrogen (N) removal is significantly affected by emergent vegetation cover and type of CWs. Hybrid CWs (HCWS) removal efficiency for nutrients, metals, pesticides, and other pollutants is higher than a single constructed wetland. The contaminant removal efficiency of the vertical subsurface flow constructed wetlands (VSSFCW) commonly used for the treatment of domestic and municipal wastewater ranges between 31% and 99%. Biochar/zeolite addition as substrate material further enhances the wastewater treatment of CWs. Innovative components (substrate materials, plant species) and factors (design parameters, climatic conditions) sustaining the long-term sink of the pollutants, such as nutrients and heavy metals in the CWs should be further investigated in the future. PRACTITIONER POINTS: Constructed wetland systems (CWs) are efficient natural treatment system for on-site contaminants removal from wastewater. Denitrification, nitrification, microbial and plant uptake, sedimentation and adsorption are crucial pollutant removal mechanisms. Phragmites australis, Typha latifolia, and Typha angustifolia are widely used emergent plants in constructed wetlands. Hydraulic retention time (HRT), water flow regimes, substrate, plant, and microbial biomass substantially affect CWs treatment performance.

Effect of two different intermediate landfill leachates on the ammonium oxidation rate of non-adapted and adapted nitrifying biomass

A widely employed approach to minimize the detrimental effect of landfill leachates (LL) on nitrifying biomass is to adapt it to these contaminated effluents prior to use. In the study reported here the impact of different intermediate landfill leachates (intermediate 1 (ILL1) and intermediate 2 (ILL2)) and synthetic medium (SM) on the nitritation rates of non-adapted and adapted nitrifying biomass were evaluated and modeled. The models, based on previously reported models (Haldane, Edwards and Aiba), considered the effect of three different heavy metals (Cu, Ni and Zn) present in both landfill leachates. The proposed models fitted well with the different biomasses. The highest specific substrate oxidation rate (q_S) of the present study (41.85 ± 1.09 mg N-NH₄⁺ g TSS^-1 h^-1) was obtained by the non-adapted biomass using SM. The non-adapted biomass was characterized by ~5- and ~28-fold higher nitritation rates on using the different ammonium sources tested (SM, ILL1 and ILL2) when compared to the other biomasses adapted to ILL1 (~9 mg N-NH₄⁺ g TSS^-1 h^-1) and ILL2 (~1.3 mg N-NH₄⁺ g TSS^-1 h^-1), respectively. The calculated inhibition constants indicate that the inhibitory effect of the heavy metals followed the order Ni>Zn>Cu. The results reported here bring into question the commonly accepted idea that an adaptation period of the biomass is required to treat landfill leachate.

Factors affecting nitrification with nitrite accumulation in treated wastewater by oxygen injection

This work provides information on nitrification with nitrite accumulation in low strength ammonia (below 50 mg L^-1 NH₄-N) and low organic matter (below 150 mg L^-1 COD) reclaimed wastewater. In the South Tenerife reclaimed wastewater pipeline (62 km long), injection of O₂ has been applied to promote a nitrification process in order to improve water quality and to avoid anaerobic conditions. Nitrification occurs, in most cases, with nitrite accumulation. The amount of oxidized nitrogen compounds produced increases with the oxygen dose applied. The nitrification process is usually favoured instead of the organic matter transformation, due to the low organic matter/ammonia nitrogen ratio of water. The influence of organic matter content on nitrification has been analysed, and a good suitability for COD has been found as an indicator for nitrification limitation (for the range of COD and NH₄-N concentrations of the system). Nitrification limitation has been observed above 85 mg L^-1 COD, and nitrification inhibition above a concentration of 105 mg L^-1. In addition, the limitation of nitrite oxidation bacteria activity (nitrite accumulation) by free ammonia and temperature has been assessed, finding that, for the range of free ammonia (0.6-2.1 mg L^-1 NH₃) and temperature (20.4-27.0°C) in the study, temperature plays a much more relevant role than free ammonia on nitrite accumulation. The lower limiting temperature for nitrite build-up in the system has been 21.0°C. Below this temperature, nitrite accumulation did not exist or was very low.

The Operating Characteristics of Partial Nitrification by Controlling pH and Alkalinity

In many experiments, a partial nitrification device is initiated with the use of highly active nitrating sludge because of the large number of nitrifying bacteria. Ammonia-oxidizing bacteria (AOB) are more adaptable to low-dissolved oxygen environments than nitrite-oxidizing bacteria (NOB). NOB activity was inhibited when the dissolved oxygen (DO) levels were decreased, causing the nitrate-nitrogen concentration to gradually decrease in the effluent and the nitrite-nitrogen concentration to gradually increase, achieving the accumulation of nitrous nitrogen. In this experiment, a sequencing batch reactor (SBR) was used to suppress NOB activity at a given pH while maintaining DO at a very low level so that the ammonia–water reaction mainly occurred in the device, and then the mud and water separated. Compared with other experiments, this approach can occur in 25 days, and it runs stably for more than two months until the device closes when the ammonia-nitrogen concentration is about 170 mg/L. This experiment also compared the difference between the pH change at the beginning of the device operation and after the device was stable. In order to increase the efficiency of bacterial appreciation, supplementing NaHCO3 increased the HCO3− concentration by 300 mg/L on the 25th day. It was found that some nitrification reactions still occurred, but they were not enough to destabilize the device. The nitrosate accumulation efficiency still gradually increased, and the average nitrite accumulation efficiency was 87.25% after NaHCO3 supplementation.

Insight into the mechanism of chemoautotrophic denitrification using pyrite (FeS2) as electron donor

In this study, denitrification was performed using pyrite as sole electron donor. The nitrate reducing rate ranged from 0.61 to 0.95 mM/d. The production of nitrous oxide (N₂O) was observed, accounting for 20% of the total nitrate reduction. The isotope fractionation indicated that N₂O production was mainly caused by the bacterial denitrification, instead of chemical denitrification by Fe(Ⅱ). Thiobacillus was the predominant genus, of which relative abundance decreased after the incubation with pyrite. Conversely, other genera belonging to Actinobacteria, like Rhodococcus, increased by more than 10 times. These Actinobacteria-like bacteria lack nitrous oxide reductase, which might be the reason for high N₂O production. Furthermore, the predicted microbial functions analysis by PICRUSt2 showed that the genes (menC/E/G) involved in the biosynthesis of electron shuttles (menaquinone-related redox-active molecule), which were remarkably enriched during the process, suggesting that the first step of pyrite oxidation might be driven by the microbial derived electron shuttles.

Active biomass estimation based on ASM1 and on-line OUR measurements for partial nitrification processes in sequencing batch reactors

The main challenge for partial nitrification is to reach stable nitrite accumulation, which strongly depends on the nitrite-oxidizing bacteria (NOB) growth in the reactor. The on-line estimation of active biomass may enhance the decision-making process to maintain a high nitrite accumulation in the reactor. In this work, we propose an active biomass estimator based on ASM1 and on-line oxygen uptake rate measurements (OUR-E) in a sequencing batch reactor. In order to validate the OUR-E, two operating scenarios were applied during 200 days of operation: unfavorable (sludge retention time (SRT) = 40 d, pH = 7.6, dissolved oxygen (DO) = 2 mg/L) and favorable for partial nitrification (SRT = 10 d, pH = 8.5, DO = 2 mg/L). Furthermore, a second estimation method based on off-line measurements of N-species concentrations (Nsp-E) was implemented to evaluate the performance of the OUR-E. The OUR-E was able to predict a reduction in the NOB active fraction from 10.3% to 1.6% with nitrite accumulation over 80% when we shifted the operating scenario. Although both estimators predicted similar results, the OUR-E showed a better prediction quality than the Nsp-E, according to Theil's coefficient of inequality.

Formation of anaerobic granules and microbial community structure analysis in anaerobic hydrolysis denitrification reactor

An anaerobic hydrolysis denitrification (AnHD) process was developed to pretreat municipal wastewater for integrating partial nitration/anammox process. The results indicated that the carbon to nitrogen (C/N) ratio of municipal wastewater changed from 4.4 ± 0.3 to 2.2 ± 0.2 after pretreatment by AnHD process, which was favorable to the partial nitration/anammox process. The influent C/N ratio had influence on the formation of anaerobic granules. Two intrinsic factors, cyclic diguanylic acid (c-di-GMP) concentration and core bacterial community, were mainly responsible for the anaerobic granular formation. The higher c-di-GMP content increased the extracellular polymeric substances and decreased the motility of the bacteria, which was beneficial for the formation of anaerobic granules. The microbial community analysis showed that the lactic acid bacteria (Lactococcus) was the core bacteria during anaerobic hydrolysis process, while the denitrifying bacteria (Denitratisoma and unclassified Comamonadaceae) were the core bacterial community during AnHD process, which were responsible for nitrogen removal and anaerobic granular formation.

Effective nitrogen removal from ammonium-depleted wastewater by partial nitritation and anammox immobilized in granular and thin layer gel carriers

This study investigates the effect of physicochemical conditions on the partial nitritation and anammox treatment by immobilized ammonia oxidizers under ammonium-deplete conditions. The impact of oxygen and temperature was studied by measuring the activity of immobilized aerobic and anaerobic ammonia-oxidizing organisms (Ammonia-oxidizing bacteria (AOB) and archaea (AOA), and Anammox bacteria) embedded in polyvinyl alcohol - sodium alginate (PVA-SA) beads and in thin layer poly-ethylene glycol hydrogels. Beads and flat hydrogels were incubated in a fluidized bed reactor (FBR) and in two flow cells, respectively. Both systems were fed with synthetic wastewater (15 mg N-NH₄⁺/L) at different temperatures (20 °C and/or 30 °C) and different dissolved oxygen (DO) concentrations (0.1, 0.3, 0.5 and/or 1 mg/L) over 152 and 207 days, respectively. The FBR system had a maximum removal rate of 1.7 g-N/m³/d at 0.1 mg O₂/L, corresponding to 80% removal efficiency, while a high aerobic ammonia-oxidizing activity but a partial oxygen inhibition of Anammox bacteria were observed at higher DO concentrations. In both flow cells, nitrogen removal efficiency was highest (80%) at 30 °C and 1 mg O₂/L while removal was less favorable at lower DO and lower temperature. Our results indicate a potential use of hydrogel beads for an energy efficient technology with reduced aeration demand for treating low ammonia wastewater, while layered hydrogels are a possible first step for biological treatments of wastewater using tangential flow. In addition, we provide blueprint drawings of the flow cells, which may be used to 3D-print the apparatus for other applications.

System Performance Corresponding to Bacterial Community Succession after a Disturbance in an Autotrophic Nitrogen Removal Bioreactor

Performance of a bioreactor is affected by complex microbial consortia that regulate system functional processes. Studies so far, however, have mainly emphasized the selective pressures imposed by operational conditions (i.e., deterministic external physicochemical variables) on the microbial community as well as system performance, but have overlooked direct effects of the microbial community on system functioning. Here, using a bioreactor with ammonium as the sole substrate under controlled operational settings as a model system, we investigated succession of the bacterial community after a disturbance and its impact on nitrification and anammox (anaerobic ammonium oxidation) processes with fine-resolution time series data. System performance was quantified as the ratio of the fed ammonium converted to anammox-derived nitrogen gas (N₂) versus nitrification-derived nitrate (npNO₃ ^-). After the disturbance, the N₂/npNO₃ ^- ratio first decreased, then recovered, and finally stabilized until the end. Importantly, the dynamics of N₂/npNO₃ ^- could not be fully explained by physicochemical variables of the system. In comparison, the proportion of variation that could be explained substantially increased (tripled) when the changes in bacterial composition were taken into account. Specifically, distinct bacterial taxa tended to dominate at different successional stages, and their relative abundances could explain up to 46% of the variation in nitrogen removal efficiency. These findings add baseline knowledge of microbial succession and emphasize the importance of monitoring the dynamics of microbial consortia for understanding the variability of system performance.IMPORTANCE Dynamics of microbial communities are believed to be associated with system functional processes in bioreactors. However, few studies have provided quantitative evidence. The difficulty of evaluating direct microbe-system relationships arises from the fact that system performance is affected by convolved effects of microbiota and bioreactor operational parameters (i.e., deterministic external physicochemical forcing). Here, using fine-resolution time series data (daily sampling for 2 months) under controlled operational settings, we performed an in-depth analysis of system performance as a function of the microbial community in the context of bioreactor physicochemical conditions. We obtained statistically evaluated results supporting the idea that monitoring microbial community dynamics could improve the ability to predict system functioning, beyond what could be explained by operational physicochemical variables. Moreover, our results suggested that considering the succession of multiple bacterial taxa would account for more system variation than focusing on any particular taxon, highlighting the need to integrate microbial community ecology for understanding system functioning.

Three-dimensional electrodes enhance electricity generation and nitrogen removal of microbial fuel cells

One of the critical problems for practical application of microbial fuel cells (MFCs) is the poor electron transfer between microbial cells and anode. Hence, good biocompatibility and high specific surface area of electrodes are indispensable for MFC scale-up. In this study, three-dimensional electrode MFC (3DEMFC) was developed by filling biochar between anode and cathode. Three types of biochar electrodes (biochar, biochar and zeolite mixture, and MgO-modified biochar) were employed, and the performance of 3DEMFCs treating nitrogen in wastewater was investigated. The results showed that the highest power density of MFCs was 4.45 ± 0.21 W m^-3 achieved by 3DEMFC filled with MgO-modified biochar, and the overall power generation of 3DEMFCs (2.40 ± 0.28 ~ 4.45 ± 0.21 W m^-3) was higher than that of MFC without biochar (1.31 ± 0.24 W m^-3). The linear sweep voltammetry (LSV) results also demonstrated biochar addition to MFC was conducive to electron transfer between microbes and anode and MgO-modified biochar presented the highest coulombs transfer ability. Moreover, the highest removal efficiencies of ammonium, total nitrogen, and COD (93.6 ± 3.2%, 84.8 ± 2%, and 91.6 ± 1.3%, respectively) were achieved by 3DEMFC containing MgO-modified biochar, and simultaneous short-cut nitrification and denitrification were observed in MFCs. Furthermore, the SEM images displayed the bacteria adhesion on biochar and the biofilm dry weights of MgO-modified biochar after experiment was the highest of 103 ± 4 mg g^-1 among three kinds of biochar electrodes. Therefore, the power generation and nitrogen removal conspicuously enhanced in 3DEMFCs and biochar exhibited excellent biocompatibility and distinct electrochemical performance for MFC practical applications in wastewater treatment.

Spatial-temporal characteristics of nitrogen degradation in typical Rivers of Taihu Lake Basin, China

In this study, we focus on the measurement of different nitrogen (N) forms and investigate the spatial-temporal variability of degradation coefficient in river channels. We aim to provide a new approach of deriving in-situ degradation coefficients of different N forms, and highlight factors that determine the spatial-temporal variability of degradation coefficients. Our results are based on a two-year field survey in 34 channels around the Taihu Lake Basin, eastern China. The derived degradation coefficients of different N forms based our newly-developed experimental device are: degradation coefficients of TN, NH₄⁺-N and NO₃-N range from 0.006-0.449 d^-1, 0.022-1.175 d^-1 and -0.096-2.402 d^-1, respectively. The degradation coefficients of N show strong dependence on N concentration and water temperature. The seasonal difference of water temperature and N concentration leads to spatial-temporal variability of degradation coefficients. The derived degradation coefficients of N are further verified through one-dimensional water quality model simulations. The degradation coefficient obtained in this study and the influencing factors of its spatial-temporal variability provide invaluable reference for studies in aquatic environment.

The effect of dissolved oxygen concentration on long-term stability of partial nitrification process

Dissolved oxygen (DO) concentration is regarded as one of the crucial factors to influence partial nitrification process. However, achieving and keeping stable partial nitrification under different DO concentrations were widely reported. The mechanism of DO concentration influencing partial nitrification is still unclear. Therefore, in this study two same sequencing batch reactors (SBRs) cultivated same seeding sludge were built up with real-time control strategy. Different DO concentrations were controlled in SBRs to explore the effect of DO concentration on the long-term stability of partial nitrification process at room temperature. It was discovered that ammonium oxidation rate (AOR) was inhibited when DO concentration decreased from 2.5 to 0.5 mg/L. The abundance of Nitrospira increased from 10^11.5 to 10^13.7 copies/g DNA, and its relative percentage increased from 0.056% to 3.2% during 190 operational cycles, causing partial nitrification gradually turning into complete nitrification process. However, when DO was 2.5 mg/L the abundance of Nitrospira was stable and AOB was always kept at 10^10.7 copies/g DNA. High AOR was maintained, and stable partial nitrification process was kept. Ammonia oxidizing bacteria (AOB) activity was significantly higher than nitrite oxidizing bacteria (NOB) activity at DO of 2.5 mg/L, which was crucial to maintain excellent nitrite accumulation performance.

Heterotrophic nitrification and aerobic denitrification by a novel Acinetobacter sp. ND7 isolated from municipal activated sludge

A novel strain was isolated from municipal activated sludge and identified as Acinetobacter sp. ND7 based on its phenotypic and phylogenetic characteristics, which had efficient capability for heterotrophic nitrification and aerobic denitrification. Strain ND7 could remove approximately 99.8% of ammonium-N (51.0 mg/L), 96.2% of nitrite-N (51.8 mg/L) and 97.18% of nitrate-N (52.1 mg/L), with the maximum specific removal rate of 5.74, 4.17 and 3.63 mg/(L h), respectively. Ammonium was manifested to be utilized preferentially during simultaneous nitrification and denitrification. The functional genes hao, napA and nirS were successfully amplified by PCR, further evidencing the heterotrophic nitrification and aerobic denitrification capability of Acinetobacter sp. ND7. The optimal conditions for nitrogen removal were temperature of 35 °C, C/N ratio of 8. Acinetobacter sp. ND7 displays superior performance for nitrogen removal, with no nitrite accumulation under aerobic condition, and thus has significant potential for practical application for nitrogen removal from wastewater.

Evaluation of the integration of P recovery, polyhydroxyalkanoate production and short cut nitrogen removal in a mainstream wastewater treatment process

Wastewater treatment systems are nowadays evolving into systems where energy and resources are recovered from wastewater. This work presents the long term operation of a demo-scale pilot plant (7.8 m³) with a novel configuration named as mainstream SCEPPHAR (ShortCut Enhanced Phosphorus and polyhydroxyalkanoate (PHA) Recovery) and based on two sequencing batch reactors (R1-HET and R2-AUT). This is the first report of an implementation at demo scale and under relevant operational conditions of the simultaneous integration of shortcut nitrification, P recovery and production of sludge with a higher PHA content than conventional activated sludge. An operating period under full nitrification mode achieved successful removal efficiencies for total N, P and COD_T (86 ± 12%, 93 ± 9% and 79 ± 6%). In the following period, nitrite shortcut (with undetectable activity of nitrite oxidising bacteria) was achieved by implementing automatic control of the aerobic phase length in R2-AUT using ammonium measurement and operating at a lower sludge retention time. Similar N, P and COD_T removal efficiencies to the full nitrification period were obtained. P-recovery from the anaerobic supernatant of R1-HET was achieved in a separate precipitator by increasing pH and dosing MgCl₂, recovering an average value of 45% of the P in the influent as struvite precipitate, with a peak up to 63%. These values are much higher than the typical values of sidestream P-recovery (12%). Regarding PHA, a percentage in the biomass in the range 6.9-9.2% (gPHA·g^-1TSS) was obtained.

Effects of different light conditions on ammonium removal in a consortium of microalgae and partial nitrifying granules

Ammonium removal by a coupling process of microalgae (Chlorella sorokiniana) with partial nitrifying granules was evaluated in batch reactors illuminated in a wide range of light intensities (0, 100, 450, and 1600 μmol photons m^-2 s^-1). Ammonium oxidation performance for different light exposure time showed that the granules had a light stress tolerance at 1600 μmol photons m^-2 s^-1 for up to 12 h, but continuous illumination induced severe inhibition on nitrifying bacteria thereafter. Ammonium removal efficiencies at the end of tests were 66%, 62%, 5%, and -10% (due to ammonification) for 0, 100, 450, and 1600 μmol photons m^-2 s^-1, respectively. The nitrogen mass balance shows co-occurrence of microalgal growth taking up 24% of fed ammonium and nitrifying bacteria oxidizing 38% of fed ammonium at 100 μmol photons m^-2 s^-1, while both nitrification and microalgal growth are inhibited at light intensity above 450 μmol photons m^-2 s^-1. In comparing results from this study with previous results, it was found that the ammonium removal pathway, i.e., nitrification or microalgal uptake, is regulated more strongly by daily average light intensity than by instantaneous light intensity. Empirical model equations to estimate the oxygen balance in consortium reactors categorized the effect of daily average light intensities on process performance as follows: (i) below 27 μmol photons m^-2 s^-1: insufficient oxygen for nitrification; (ii) 27 to 35: sufficient oxygen for nitrification via nitrite; (iii) 35 to 180: sufficient oxygen for nitrification via nitrate; (iv) above approximately 200-300: oversaturated dissolved oxygen, excess free ammonia and/or intensive light inhibitions.

Nitrite accumulation stability evaluation for low-strength ammonium wastewater by adsorption and biological desorption of zeolite under different operational temperature

How to achieve stable nitrite accumulation was still a huge challenge for low-carbon and energy-saving biological nitrogen removal of low-strength ammonium wastewater. This study proposed a new way to solve this problem with zeolite biological fixed bed (ZBFB) by cycle operation of adsorption and biological desorption. In order to evaluate nitritation performance of this reactor, the influence of operational temperature on nitrite accumulation stability was investigated by 126 cycles operation in four parallel ZBFB reactors for low-strength ammonium wastewater (50 mg/L NH₄⁺-N). It was found that higher operational temperature (i.e., 36.0 °C), rather than other temperature (i.e., 27.0 °C, 30.0 °C, 33.0 °C), could maintain stable nitrite accumulation with nitrite production rate of 0.312 kg NO₂^--N·m^-3 zeolite·day^-1 and nitrite accumulation ratio higher than 95.0% after biological desorption. High-throughput sequencing analysis results showed that bacterial structure significantly changed in ZBFB under different operational temperature, and obvious enrichment of genus Nitrosomonas (AOB) and gradually enhanced free ammonia (FA) inhibition on genus Nitrospira and Nitrobacter (NOB) were found by elevation of operational temperature, leading to different nitrite accumulation performance in ZBFB reactors. The mechanism for stable nitrite accumulation performance by ZBFB might be attributed to overwhelming growth rate of AOB than NOB, faster ammonium desorption and enhanced FA inhibition on NOB under operational temperature (i.e., 36.0 °C). All in all, keeping high temperature for biological desorption step should be extremely crucial for stable nitrite accumulation by ZBFB, which could facilitate further low-carbon and energy-saving biological nitrogen removal for low-strength ammonium wastewater treatment.

Intermittent Aeration in a Hybrid Moving Bed Biofilm Reactor for Carbon and Nutrient Biological Removal

The paper presents an experimental study on a lab scale hybrid moving bed biofilm reactor with intermittent aeration. Specifically, a comparison between two different operating conditions was analyzed: continuous and intermittent aeration. Both continuous and intermittent aeration were monitored and compared in order to get the best operational conditions. The intermittent aeration campaign was sub-divided in three phases with different duration of alternation of aerobic and anoxic times and organic and nitrogen loading rates. The efficiency of N-removal improved by 70% during the intermittent aeration. The best condition was observed with 40 min of aeration and 20 min of no-aeration, an organic loading rate of 2.2 kgCODm−3day−1 and a nitrogen loading rate of 0.25 kgNm−3day−1: under these operational conditions the removal efficiencies for carbon and nitrogen were 93% and 90%, respectively. The derived results provide the basis for WWTP upgrade in order to meet stricter effluent limits at low energy requirements.

Complex microbial nitrogen-cycling networks in three distinct anammox-inoculated wastewater treatment systems

Microbial nitrogen removal mediated by anaerobic ammonium oxidation (anammox) are cost-effective, yet it is time-consuming to accumulate the slow-growing anammox bacteria in conventional wastewater treatment plants (WWTPs). Inoculation of anammox enriched pellets is an effective way to establish anammox and achieve shortcut nitrogen removal in full-scale WWTPs. However, little is known about the complex microbial nitrogen-cycling networks in these anammox-inoculated WWTPs. Here, we applied metagenomic and metatranscriptomic tools to study the microbial nitrogen removal in three conventional WWTPs, which have been inoculated exogenous anammox pellets, representing partial-nitrification anammox (PNA) and nitrification-denitrification nitrogen removal processes. In the PNA system of Bali (BL), ammonia was partially oxidized by ammonia-oxidizing bacteria (AOB) Nitrosomonas and the oxidized nitrite and the remaining ammonium were directly converted to N₂ by anammox bacteria Ca. Brocadia and Ca. Kuenenia. In the nitrification-denitrification system of Wenshan (WS), ammonia-oxidizing archaea (AOA) Thaumarchaeota unexpectedly dominated the nitrifying community in the presence of AOB Nitrosomonas. Meanwhile, the biomass yield of Ca. Brocadia was likely inhibited by the high biodegradable organic compound input and limited by substrate competitions from AOA, AOB, complete ammonia oxidizers (comammox) Nitrospira, nitrite-oxidizing bacteria (NOB) Nitrospira, and heterotrophic denitrifiers. Unexpectedly, comammox Nitrospira was the predominant nitrifier in the presence of AOB Nitrosomonas in the organic carbon-rich nitrification-denitrification system of Linkou (LK). These results clearly showed that distinct active groups were working in concert for an effective nitrogen removal in different WWTPs. This study confirmed the feasibility of anammox application in ammonium-rich systems by direct inoculation of the exogenous anammox pellets and improved our understanding of microbial nitrogen cycling in anammox-driven conventional WWTPs from both physiochemical and omics perspectives.

Pilot study of nitrogen removal from landfill leachate by stable nitritation-denitrification based on zeolite biological aerated filter

A pilot (about 1 m³/d) process consisting of pre-denitrification and zeolite biological aerated filter (ZBAF) was established and run for nitrogen removal of landfill leachate. The results showed that stable nitritation and denitrification was achieved for landfill leachate with removal efficiency of Chemical Oxygen Demand (COD_Cr), ammonium and total nitrogen (TN) of 53.2 ± 3.0%, 93.5 ± 2.4% and 74.7 ± 9.4%, respectively. Based on the ammonium adsorption equilibrium by zeolite, stable free ammonia could be maintained for inhibition of nitrite oxidizing bacteria (NOB) and dominance of ammonia oxidizing bacteria (AOB) in ZBAF, resulting in efficient nitritation with a nitrite accumulation ratio higher than 90.0% and an average nitrite production rate of 1.387 kg NO₂^--N m^-3 day^-1. High-throughput sequencing analysis further revealed enrichment of AOB and elimination of NOB in ZBAF. Compared to two-stage anoxic-oxic process, the pilot-scale process could save approximate 5000 mg/L glucose (about 3.10 US dollar/m³) with almost similar TN removal performance. All results obtained demonstrated the feasibility of the pilot process, which might be highly promising for the nitritation and denitrification of low C/N landfill leachate in the future.

Constructed wetland microcosms as sustainable technology for domestic wastewater treatment: an overview

Constructed wetland microcosms (CWMs) are artificially designed ecosystem which utilizes both complex and ordinary interactions between supporting media, macrophytes, and microorganisms to treat almost all types of wastewater. CWMs are considered as green and sustainable techniques which require lower energy input, less operational and maintenance cost and provide critical ecological benefits such as wildlife habitat, aquaculture, groundwater recharge, flood control, recreational uses, and add aesthetic value. They are good alternatives to conventional treatment systems particularly for smaller communities as well as distant and decentralized locations. The pH, dissolved oxygen (DO), and temperature are the key controlling factors while several other parameters such as hydraulic loading rates (HLR), hydraulic retention time (HRT), diversity of macrophytes, supporting media, and water depth are critical to achieving better performance. From the literature survey, it is evaluated that the removal performance of CWMs can be improved significantly through recirculation of effluent and artificial aeration (intermittent). This review paper presents an assessment of CWMs as a sustainable option for treatment of wastewater nutrients, organics, and heavy metals from domestic wastewater. Initially, a concise note on the CWMs and their components are presented, followed by a description of treatment mechanisms, major constituents involved in the treatment process, and overall efficiency. Finally, the effects of ecological factors and challenges for their long-term operations are highlighted.

Upflow packed bed Anammox reactor used in two-stage deammonification of sludge digester effluent

The sludge digester effluent taken from a full scale municipal wastewater treatment plant (WWTP) in Istanbul, Turkey, was successfully deammonified using a laboratory scale two-stage partial nitritation (PN)/Anammox (A) process and a maximum nitrogen removal rate of 1.02 kg N/m³/d was achieved. In the PN reactor, 56.8 ± 4% of the influent NH₄-N was oxidized to NO₂-N and the effluent nitrate concentration was kept below 1 mg/L with 0.5-0.7 mg/L of dissolved oxygen and pH of 7.12 ± 12 at 24 ± 4°C. The effluent of the PN reactor was fed to an upflow packed bed Anammox reactor where high removal efficiency was achieved with NO₂-N:NH₄-N and NO₃-N:NH₄-N ratios of 1.32 ± 0.19:1 and 0.22 ± 0.10:1, respectively. The results show that NH₄-N removal efficiency up to 98.7 ± 2.4% and total nitrogen removal of 87.7 ± 6.5% were achieved.

System performance and microbial community succession in a partial nitrification biofilm reactor in response to salinity stress

The system performance and microbial community succession in a partial nitrification biofilm reactor in response to salinity stress was conducted. It was found that the NH₄⁺-N removal efficiency decreased from 98.4% to 42.0% after salinity stress increased to 20 g/L. Specific oxygen uptake rates suggested that AOB activity was more sensitive to the stress of salinity than that of NOB. Protein and polysaccharides contents showed an increasing tendency in both LB-EPS and TB-EPS after the salinity exposure. Moreover, EEM results indicated that protein-like substances were the main component in LB-EPS and TB-EPS as self-protection in response to salinity stress. Additionally, humic acid-like substances were identified as the main component in the effluent organic matter (EfOM) of partial nitrification biofilm, whereas fulvic acid-like substances were detected at 20 g/L salinity stress. Microbial community analysis found that Nitrosomonas as representative species of AOB were significantly inhibited under high salinity condition.

Nitrifiers activity and community characteristics under stress conditions in partial nitrification systems treating ammonium-rich wastewater

Long-term exposure of nitrifiers to high concentrations of free ammonia (FA) and free nitrous acid (FNA) may affect nitrifiers activity and nitrous oxide (N₂O) emission. Two sequencing batch reactors (SBRs) were operated at influent ammonium nitrogen (NH₄-N) concentrations of 800mg/L (SBR_H) and 335mg/L (SBR_L), respectively. The NH₄-N removal rates in SBR_H and SBR_L were around 2.4 and 1.0g/L/day with the nitritation efficiencies of 99.3% and 95.7%, respectively. In the simulated SBR cycle, the N₂O emission factors were 1.61% in SBR_H and 2.30% in SBR_L. N₂O emission was affected slightly by FA with the emission factor of 0.22%-0.65%, while N₂O emission increased with increasing FNA concentrations with the emission factor of 0.22%-0.96%. The dominant ammonia oxidizing bacteria (AOB) were Nitrosomonas spp. in both reactors, and their relative proportions were 38.89% in SBR_H and 13.36% in SBR_L. Within the AOB genus, a species (i.e., operational taxonomic unit [OTU] 76) that was phylogenetically identical to Nitrosomonas europaea accounted for 99.07% and 82.04% in SBR_H and SBR_L, respectively. Additionally, OTU 215, which was related to Nitrosomonas stercoris, accounted for 16.77% of the AOB in SBR_L.

Nitrogen removal via nitritation pathway for low-strength ammonium wastewater by adsorption, biological desorption and denitrification

Stable nitritation for low-strength ammonium wastewater was the key obstacle for cost-effective and low-carbon biological nitrogen removal. A zeolite biological fixed bed (ZBFB) and an anoxic sequencing batch reactor (ASBR) were successfully applied for achieving nitritation-denitrification of low-strength ammonium wastewater by adsorption, biological desorption and denitrification. Based on free ammonia inhibition on biofilm, stable nitrite accumulation could be realized with suitable operational time and aeration in biological desorption. During cycle operation, adsorption effluent NH₄⁺-N kept at 3.0-4.0 mg/L, biological desorption effluent NO₂^--N maintained at 226.8-243.2 mg/L with average nitrite accumulation ratio of 97.18%, and nitrite removal rate was about 0.628-0.672 kg NO₂^--N·m^-3·day^-1, revealing obvious feasibility of ZBFB and ASBR for low-strength ammonium wastewater treatment. High-throughput sequencing analysis results further presented significant microbial community variations happened after cycle operation, with ammonia oxidizing bacteria enrichment and nitrite oxidizing bacteria inhibition in ZBFB and dominance of denitrifiers in ASBR.

An anaerobic-aerobic sequential batch process with simultaneous methanogenesis and short-cut denitrification for the treatment of marine biofoulings

Although combination of denitritation and methanogenesis for wastewater treatment has been widely investigated, an application of this technology to solid waste treatment has been rarely studied. This study investigated an anaerobic-aerobic batch system with simultaneous denitritation-methanogenesis as an effective treatment for marine biofoulings, which is a major source of intermittently discharged organic solid wastes. Preliminary NO₂^--exposed sludge was inoculated to achieve stable methanogenesis process without NO₂^- inhibition. Both high NH₄⁺-N removal of 99.5% and high NO₂^--N accumulation of 96.4% were achieved on average during the nitritation step. Sufficient CH₄ recovery of 101 L-CH₄ kg-COD^-1 was achieved, indicating that the use of NO₂^--exposed sludge is effective to avoid NO₂^- inhibition on methanogenesis. Methanogenesis was the main COD utilization pathway when the substrate solubilization occurred actively, while denitritation was the main when solubilization was limited because of substrate shortage. The results showed a high COD removal efficiency of 96.0% and a relatively low nitrogen removal efficiency of 64.4%. Fitting equations were developed to optimize the effluent exchange ratio. The estimated results showed that the increase of effluent exchange ratio during the active solubilization period increased the nitrogen removal efficiency but decreased CH₄ content in biogas. An appropriate effluent exchange ratio with high anaerobic effluent quality below approximately 120 mg-N L^-1 as well as sufficient CH₄ gas quality which can be used as fuel for gas engine generator was achieved by daily effluent exchange of 80% during the first week and 5% during the subsequent 8 days.

Low temperature effects on nitrification and nitrifier community structure in V-ASP for decentralized wastewater treatment and its improvement by bio-augmentation

The vegetation-activated sludge process (V-ASP) has been proved to be an environment-friendly decentralized wastewater treatment system with extra esthetic function and less footprint. However, the effects of low temperature on the treatment performance of V-ASP and related improvement methods are rarely investigated, up to now. In this work, the effect of low temperature on nitrification in V-ASP was comprehensively investigated from overall nitrification performance, substrate utilization kinetics, functional enzymatic activities, and microbial community structure shift by comparison with conventional ASP. Bio-augmentation methods in terms of single-time nitrifier-enriched biomass dosage were employed to improve nitrification efficiency in bench- and full-scale systems. The experiment results demonstrated that the NH₄⁺-N removal efficiency in V-ASP system decreased when the operational temperature decreased from 30 to 15 °C, and the decreasing extent was rather smaller compared to ASP, as well as ammonium and nitrite oxidation rates and enzymatic activities, which indicated the V-ASP system possesses high resistance to low temperature. With direct dosage of 1.6 mg nitrifier/gSS sludge, the nitrification efficiency in V-ASP was enhanced dramatically from below 50% to above 90%, implying that bio-augmentation was effective for V-ASP whose enzymatic activities and microbial communities were both also improved. The feasibility and effectiveness of bio-augmentation was further confirmed in a full-scale V-ASP system after a long-term experiment which is instructive for the practical application.

Partial nitrification performance and mechanism of zeolite biological aerated filter for ammonium wastewater treatment

A zeolite biological aerated filter (ZBAF) with continuous feeding was successfully applied for achieving stable partial nitrification. Excellent nitrite accumulation (higher than 98.0%) and high nitrite/nitrate production rate (NPR) (approximately 0.760kg/m³/d) were obtained with increase influent ammonium concentration from 250 to 550mg/L within a nitrogen loading rate (NLR) of 0.854-1.200kg/m³/d. Owning to the adsorption of zeolite to ammonium, free ammonia (FA) concentration could remain at an appropriate range for inhibition of nitrite oxidizing bacteria (NOB) and dominance of ammonia-oxidizing bacteria (AOB), which should be responsible for the excellent partial nitrification realized in ZBAF. Kinetic study showed that the production of nitrite in ZBAF followed the zero-order kinetics model and high-throughput sequencing analysis further presented the enrichment of AOB and inhibition of NOB in ZBAF. All the results demonstrated that ZBAF hold a great potential in the application of partial nitrification for ammonium wastewater treatment.

A coupled system of half-nitritation and ANAMMOX for mature landfill leachate nitrogen removal

A coupled system of membrane bioreactor-nitritation (MBR-nitritation) and up-flow anaerobic sludge blanket-anaerobic ammonium oxidation (UASB-ANAMMOX) was employed to treat mature landfill leachate containing high ammonia nitrogen and low C/N. MBR-nitritation was successfully realized for undiluted mature landfill leachate with initial concentrations of 900-1500 mg/L [Formula: see text] and 2000-4000 mg/L chemical oxygen demand. The effluent [Formula: see text] concentration and the [Formula: see text] accumulation efficiency were 889 mg/L and 97% at 125 d, respectively. Half-nitritation was quickly realized by adjustment of hydraulic retention time and dissolved oxygen (DO), and a low DO control strategy could allow long-term stable operation. The UASB-ANAMMOX system showed high effective nitrogen removal at a low concentration of mature landfill leachate. The nitrogen removal efficiency was inhibited at excessive influent substrate concentration and the nitrogen removal efficiency of the system decreased as the concentration of mature landfill leachate increased. The MBR-nitritation and UASB-ANAMMOX processes were coupled for mature landfill leachate treatment and together resulted in high effective nitrogen removal. The effluent average total nitrogen concentration and removal efficiency values were 176 mg/L and 83%, respectively. However, the average nitrogen removal load decreased from 2.16 to 0.77 g/(L d) at higher concentrations of mature landfill leachate.

Qualitative and quantitative analysis of extracellular polymeric substances in partial nitrification and full nitrification reactors

In present study, two column-type sequencing batch reactors with alternative anoxic/aerobic phases were operated and compared under partial nitrification and full nitrification modes by controlling different dissolved oxygen (DO) conditions. During steady state, the characterizations of extracellular polymeric substances (EPS) from two reactors were qualitatively and quantitatively analyzed through chemical and spectroscopic approaches. Data implied that partial nitrification reactor had relatively higher total nitrogen (TN) removal efficiency and loosely bound EPS (LB-EPS) and tightly bound EPS (TB-EPS) contents. According to excitation emission matrix (EEM) spectra, LB-EPS and TB-EPS from two kinds of reactors expressed similar fluorescence peak locations but different intensities. Fluorescence regional integration (FRI) further suggested that Region IV was the main fraction in both types of EPS fractions. Moreover, TB-EPS exhibited a greater number of molecular weight fractions than those of LB-EPS. Both EPS fractions had similar functional groups, which represented the complex nature of EPS compositions.

Long-term dynamic and pseudo-state modeling of complete partial nitrification process at high nitrogen loading rates in a sequential batch reactor (SBR)

Recently, partial nitrification has been adopted widely either for the nitrite shunt process or intermediate nitrite generation step for the Anammox process. However, partial nitrification has been hindered by the complexity of maintaining stable nitrite accumulation at high nitrogen loading rates (NLR) which affect the feasibility of the process for high nitrogen content wastewater. Thus, the operational data of a lab scale SBR performing complete partial nitrification as a first step of nitrite shunt process at NLRs of 0.3-1.2kg/(m³d) have been used to calibrate and validate a process model developed using BioWin® in order to describe the long-term dynamic behavior of the SBR. Moreover, an identifiability analysis step has been introduced to the calibration protocol to eliminate the needs of the respirometric analysis for SBR models. The calibrated model was able to predict accurately the daily effluent ammonia, nitrate, nitrite, alkalinity concentrations and pH during all different operational conditions.