Technology For Nitrogen Removal Research Articles

Key factors improving the stability and the loading capacity of nitrogen removal in a hydroxyapatite (HAP)-enhanced one-stage partial nitritation/anammox process

Anammox is attracting attention as the most promising nitrogen removal technology. Recently, the combination of the anammox process and the hydroxyapatite (HAP) granules comes with the advantages, including simultaneous removals of nitrogen and phosphorus and better settleability of the sludge. In this study, the key factors for a stable and efficient nitrogen removal in a one-stage partial nitritation/anammox (PNA) process with HAP-based granules was explored. Under a fixed hydraulic retention time of 12 h, in the case that HAP was not properly formed and phosphorus removal efficiency (PRE) was low, the highest nitrogen removal rate (NRR) achieved stably was only 1.3 ± 0.07 kg N/m3/d, while a stable nitrogen removal efficiency of 81 ± 3.3 % and a higher NRR of 2.0 ± 0.1 kg N/m3/d were obtained with the proper formation of HAP and higher PRE. The achievement of the high NRR benefited by a high microorganism concentration with improved settleability due to the formation of HAP-based granules. To achieve a high NRR in a long-term operation, a necessary phosphorus removal rate/nitrogen removal rate ratio above 0.02 was confirmed, which allowed for the desired sludge settleability and concentration. The average diameter of the formed micro-granules was 92–159 μm. The spatial distribution of the functional bacteria in the granules was analyzed with fluorescence in situ hybridization. Anammox bacteria and the ammonium oxidizing bacteria the two most abundant bacteria during the whole operation. The results of this study are evidence of the necessity and potential of proportionable HAP formation for highly-efficient nitrogen removal in the one-stage PNA process.

Metagenomic insights into the symbiotic relationship in anammox consortia at reduced temperature

Anammox as a promising biological nitrogen removal technology has attracted much attention. However, cold temperature would limit its wide application and little is known about the microbial interactions between anammox bacteria (AnAOB) and heterotrophic bacteria at cold temperature. Here, we observed reduced temperature (25–15 °C) promoted the secretion of EPS and thus stimulated bigger size of granular sludge in a laboratory-scale anammox reactor. We further combined co-occurrence network analysis and genome-centered metagenomics to explore the potential interactions between AnAOB and heterotrophic bacteria. Network analysis suggested 22 out of 25 positively related species were reported as definite heterotrophic bacteria in subnetwork of AnAOB. Genome-centered metagenomics analysis yielded 23 metagenomic assembly genomes (MAGs), and we found that Acidobacteriota-affiliated bacteria could biosynthesize most polysaccharides (PS) precursors and contain the most glycosyltransferases and transporters to facilitate exopolysaccharides biosynthesis, together with partial PS precursors produced by AnAOB. AMX1 as the only anammox genome could synthesize most amino acids and cross feed with some heterotrophs to affect the extracellular protein function. Additionally, Bacteroidota, Planctomycetota, Chloroflexota, and Proteobacteria could contribute folate and molybdopterin cofactor for AMX1 to benefit their activity and growth. Superphylum Patescibacteria could survive by cross-feeding with AnAOB and heterotrophic organisms about organic compounds (Glyceraldehyde-3P and lactate). These cross-feedings maintained the stability of anammox reactor performance and emphasize the importance of heterotrophs in anammox system at reduced temperature.

Distinct mechanisms underlying assembly processes and interactions of microbial communities in two single-stage bioreactors coupling anammox with denitrification

Coupling anammox with denitrification processes are promising technologies for total nitrogen removal from wastewater. However, the assembly processes and interactions of functional bacteria in the coupling systems are still poorly understood. Herein, two lab-scale up-flow bioreactors were separately operated to achieve the simultaneous anammox and denitrification (SAD) and the partial denitrification and anammox (PDA) systems under different substrate loading conditions. Although the two bioreactors seemed to have similar composition of functional bacteria, denitrifying bacteria Denitratisoma and Thauera exhibited higher abundances in the PDA bioreactor. In contrast, the genus Candidatus Jettenia was more abundant in the SAD bioreactor. The null model showed that stochastic processes dominantly governed the community assembly in both bioreactors across different substrate loadings. However, homogeneous selection appeared to be more important for shaping communities in the SAD system under high and moderate substrate-loading conditions. Particularly, drift was the most important process in driving the assembly of dominant denitrifying bacteria (e.g. genera Denitratisoma and Thauera), whereas anammox bacteria were primarily governed by the homogenous selection under high substrate-loading conditions. Moreover, anammox and denitrifying bacteria in the PDA bioreactor established simple and efficient networks, probably exerting impacts on the assembly processes of their co-occurring species. By contrast, the networks in the SAD bioreactor showed low modularity and interconnectivity. This study highlighted the specificity of community assembly processes and interactions in SAD and PDA systems, which would advance our understanding of the fundamental ecological laws underpinning the collaboration of anammox and denitrifying bacteria.

Deciphering the coupling of partial nitrification/anammox and sulfur autotrophic denitrification: Microbial metabolism and antibiotic resistance genes propagation

Partial nitrification/anaerobic ammonium oxidation (anammox) (PN/A) was considered as an economically beneficial and environmentally friendly biological nitrogen removal process. The elimination of nitrate produced by PN/A system was the key to improving the total nitrogen removal efficiency (TNRE) of the system under mainstream conditions. In this study, the feasibility of the coupling S(0)-driven autotrophic denitrification (SAD) with PN/A in a single sequencing moving bed biofilm reactor (SMBBR) at room temperature was explored. Additionally, biological treatment process is ideal environments for the emerge and spread of emerging pollutants antibiotic resistance genes (ARGs), therefore, the shifts of ARGs in the system were investigated. After the coupling of PN/A and SAD, SMBBR was steadily operated with high TNRE. Nitrite-oxidizing bacteria (Nitrospira and Candidatus Nitrotoga) and nitrite oxidoreductase enzyme were inhibited by SAD process. And anammox bacteria were enriched, especially oligotype GAGTTTAAT of Ca. Brocadia. Sulfurimonas became the predominant autotrophic denitrifier. Heterotrophic denitrifiers Thauera and Dechloromonas were elutriated out from the system during the coupling of PN/A and SAD process, and they might be potential hosts for ARGs, leading to the decrease of ARGs. Overall, this study proposed a new strategy that SAD could promote the TNRE of PN/A and cut the risks of ARGs propagation, and PN/A-SAD coupling system was a biological nitrogen removal technology with promising applications under mainstream conditions.

Techno-economic assessment of upfront nitrogen removal in a baseload LNG plant

As the world's energy demand grows, the LNG industry is gaining popularity as a cleaner fossil fuel source. Studies on process enhancement have proposed that retrofitting upfront nitrogen removal (UNR) technologies like the lithium cycle to the cold section, increases the process efficiency, and plant capacity and decreases power consumption. In this paper, an economic evaluation is conducted for varying percentages of UNR to study its profitability compared to the conventional cold section (base case) by calculating the grass-root capital expenditure (CGR) and cost of manufacturing (COM). The cost incurred to produce one million BTU LNG (COE) is used as a basis to compare the cases and calculate the increment in profit. While the techno-economic impact was studied for a range of UNR percentages, the specific results for 87.5 % UNR are as follows. The CGR and COM for the cold section alone decrease by 3.2% and 2.3% respectively compared to the base case. The CGR and COM of the lithium cycle increase linearly with the UNR percentage. The total annual cost (TAC) and LNG energy content increased by 12.3% and 4.5% respectively over the base process. This leads to a 7.4% COE increment over the base case for 87.5% UNR. The profit calculation for LNG price of 15$/MBTU showed a 4.27% or 101.74 million $/year increase in profit for 87.5% UNR case as compared to the base case.

The granular sludge membrane bioreactor: A new tool to enhance Anammox performance and alleviate membrane fouling

Anammox process has be widely concerned as a novel biological nitrogen removal technology because of reduced operating costs and environmental friendliness. However, the long start-up time and operational instability of Anammox process has hindered its wide applications. A new-type hybrid granular sludge membrane bioreactor (GSMBR) combining sludge granulation with membrane filtration was developed for accelerating start-up of Anammox process and stabilizing its operation as well as alleviating the membrane fouling. Anammox start-up period was shortened to 43 days, the maximum nitrogen removing rate and efficiency reached 0.83 kg N m−3 d−1 and 97 % respectively on 115 d. Moreover, the membrane fouling is relatively light. The maximum transmembrane pressure only reached 32 kPa on 115 d, when there were still several visible micropores and channels on the fouled membrane surface. The mature brownish-red Anammox granular sludge mostly sized at 0.5–1.0 mm. The specific population shift from norank Alphaproteobacteria to Candidatus Brocadia was found. Three Anammox species were present in the mature sludge and uncultured Candidatus Brocadia sp. was the predominated bacteria. GSMBR was proven to be a robust reactor for enhancing Anammox performance. This study was helpful to promote the engineering applications of Anammox process in treating the high-strength nitrogenous wastewaters.

Inorganic quantum dots - anammox consortia hybrid for stable nitrogen elimination under high-intensity solar-simulated irradiation

External stimulus such as light irradiation is able to deteriorate intracellular redox homeostasis and induce photooxidative damage to non-photogenic bacteria. Exploiting effective strategies to help bacteria resisting infaust stress is meaningful for achieving a stable operation of biological treatment system. In this work, selenium-doped carbon quantum dots (Se-CQDs) were blended into anaerobic ammonia oxidation (anammox) bacteria and an inorganic nanoparticle-microbe hybrid was successfully fabricated to evaluate its nitrogen removal performance under solar-simulated irradiation. It was found that the specific anammox activity decreased by 29.7 ± 5.2% and reactive oxygen species (ROS) content increased by 134.8 ± 4.1% under 50,000 lux light. Sludge activity could be completely recovered under the optimum dosage of 0.42 mL·(g volatile suspended solid) -1 Se-CQDs. Hydroxyl radical (·OH) and superoxide anion radical (·O2−) were identified as the leading ROS inducing lipid peroxidation and antioxidase function detriment. Also, the structure of ladderane lipids located on anammoxosome was destroyed by ROS and functional genes abundances declined accordingly. Although cell surface coated Se-CQDs could absorb ultraviolet light and partially mitigated the photoinhibition, the direct scavenging of ROS by intracellular Se-CQDs primarily contributed to the cellular redox homeostasis, antioxidase activity recovery and sludge activity improvement. The findings of this work provide in-depth understanding the metabolic response mechanism of anammox consortia to light irradiation and might be valuable for a more stable and sustainable nitrogen removal technology, i.e., algal-bacterial symbiotic system, development.

A critical review on the practical use of lithium cycle as an upfront nitrogen removal technology from natural gas

AbstractThe increase in energy demand as the world population grows, as well as the competition in the liquefied natural gas (LNG) market, force producers to work hard on developing cost‐effective production technologies. Upfront nitrogen removal (UNR) before the LNG plant's cold section is considered a promising option to save energy that would otherwise be wasted to cool down a large volume of unused nitrogen in the gas stream. In this study, the use of the lithium cycle (Li‐Cy) as a cost‐effective method for UNR is investigated. The Li‐Cy is compromised of three stages: lithium chemisorption of nitrogen (), hydrolysis of lithium nitride (), and electrowinning (Elec.‐w) of the final product to precipitate lithium metal for further reuse. The relevant chemistry, applicability, economic, and future challenges of Li‐Cy as a UNR technology from natural gas (NG) were explored and discussed. The main challenges that required further investigation to apply Li‐Cy to large‐scale applications were highlighted for future works. The literature review revealed that Li‐Cy can spontaneously remove nitrogen from NG even at low temperatures and produces ammonia as a valuable hydrogen storage material. The used lithium can be regenerated via and Elec.‐w and reused again many times. The cost of the Li‐Cy can be compensated by energy savings, the increase in production rate, and by selling the generated ammonia. Calculations showed that selling the produced ammonia from LNG plants with capacity in the range of 1–5 MTPA would not only offset the costs of Li‐Cy but would generate a net profit of $21MM to $103MM, respectively.

Short-term responses of the anammox process to Ni(II): nitrogen removal, mechanisms and inhibition recovery

Anaerobic ammonia oxidizing (anammox) has already been recognized as an innovative and economical nitrogen removal technology. However, the effect of heavy metals on anammox bacteria in aquatic ecosystem remains largely unknown. Ni(II) is a common kind of heavy metals detected in industrial wastewater and municipal sewage treatment plants. Hence, the responses of the anammox process to Ni(II) were studied here. The results showed that anammox was the dominant reaction with Ni(II) concentrations no more than 25 mg/L. 1 mg/L of Ni(II) addition promoted nitrogen removal by anammox. The higher the Ni(II) concentrations and longer exposure time, the more inhibition for anammox bacteria was gotten. The IC50 of Ni(II) to anammox was determined as 83.86 mg/L by an exponential regression equation. The inhibition of Ni(II) on anammox activity was mainly attributed to intracellular accumulation Ni(II) inhibition to HDH activity. Two times increase of IC50 after 4 times circles of domestication suggests multiple intermittent domestication can increase the tolerance of anammox bacteria to Ni(II). EDTA washing can eliminate the inhibition of anammox activity by Ni(II) with Ni(II) addition no more than 25 mg/L.

Anaerobic Ammonium Oxidation Performance in Shrimp Pond Wastewater Treatment

Intensive shrimp culture waste contains high nitrogen, reducing water quality and environmental carrying capacity. Anaerobic ammonium oxidation (anammox) is a potential technology for nitrogen removal. This research aimed to analyze nitrogen removal performance in a filter bioreactor (FtBR). Ammonium and nitrite concentrations of 70-100 mg-N/L were added to sterilized seawater as artificial wastewater and flowed to the reactor with HRT 24-hour. After 120 days of the experiment, the maximum nitrogen removal performance with parameters ACE, NRE, NRR were 82.48%, 72.58%, 0.12 kg-N/m3·d, respectively. The nitrogen stoichiometric ratio NH4+-N:NO2--N:NO3--N was 1:1,40:0,12, which was close to the stoichiometry of anammox process. The anammox process can be a new method for intensive shrimp culture wastewater treatment.

Methane simultaneous recovery from CH4/N2 stream and CH4/CO2 stream by the displacement VPSA process

Both CO2 and N2 are the common impurities in unconventional natural gas, their existence reduces the utilization efficiency of methane. Therefore, research and development of decarbonization and nitrogen removal technologies from the unconventional natural gas is more significant. In this article, the displacement VPSA process is developed to recover simultaneously methane from CH4/N2 and CH4/CO2 mixtures (two important unconventional natural gas sources). This proposed VPSA process is the integration of CH4/N2 and CH4/CO2 adsorption separation, including seven steps of pressurization, adsorption, displacement, depressurization, blowdown under vacuum pressure, two purges under vacuum pressure. After the adsorption separation by the developed process, three product and by-product gases can be obtained at the same time, namely as CH4-rich product gas acquired at the displacement step, N2-rich by-product gas got at the adsorption step, CO2-rich by-product gas obtained at the blowdown step. Experimental demonstration is carried out for methane simultaneous recovery from CH4/N2 and CH4/CO2 mixtures by the proposed process with coconut shell activated carbon adsorbents. Furthermore, the mathematical model is developed to predict the composition of three product and by-product gases, and to explore the effect of CO2 existence in CH4/N2 stream on the separation performance of the displacement VPSA process. Both the experimental results and numerical predictions testify the high-purity methane product gas (∼95.0% CH4 purity) can be acquired by the proposed process from CH4/N2 and CH4/CO2 mixtures.

Achieving Ammonium Removal Through Anammox-Derived Feammox With Low Demand of Fe(III).

Feammox-based nitrogen removal technology can reduce energy consumption by aeration and emission of carbon dioxide. However, the huge theoretical demand for Fe(III) becomes a challenge for the further development of Feammox. This study investigated an anammox-derived Feammox process with an intermittent dosage of Fe2O3 and proposed a novel approach to reduce the Fe(III) consumption. The results showed that anammox genera Candidatus Brocadia and Candidatus Kuenenia in the seed anammox sludge significantly decreased after cultivation. The formation of N2 was the dominating pathway in Feammox while that of nitrite and nitrate could be neglected. Batch tests showed that specific Feammox activity of ammonium oxidation was 1.14–9.98 mg N/(g VSS·d). The maximum removal efficiency of ammonium reached 52.3% in the bioreactor with a low dosage of Fe(III) which was only 5.8% of the theoretical demand in Feammox. The removal of ammonium was mainly achieved through Feammox, while partial nitrification/anammox also played a role due to the non-power and unintentional oxygen leakage. The super-low oxygen also responded to the low demand of Fe(III) in the bioreactor because it could trigger the cycle of Fe(III)/Fe(II) by coupling Feammox and chemical oxidation of Fe(II) to Fe(III). Therefore, anammox-derived Feammox can achieve the removal of ammonium with low Fe(III) demand at super-low oxygen.

Deciphering the concurrence of comammox, partial denitrification and anammox in a single low-oxygen mainstream nitrogen removal reactor

One-stage anammox-based autotrophic nitrogen removal technology has attracted increasing interest to sustainable biological nitrogen removal for future wastewater treatment. However, its application in mainstream municipal wastewater treatment is still challenging due to low nitrogen and high organics of raw wastewater. Herein, a novel Simultaneous Carbon Oxidation, partial Comammox, Denitratation and Anammox (SCOCDA) was firstly developed in a single sequencing batch biofilm reactor operated at a dissolved oxygen concentration of ∼0.5 mg/L for treating synthetic municipal wastewater (50 mg/L NH4+-N and 100–250 mg/L COD). The long-term operation showed that almost complete COD and nitrogen removal performance could be achieved at a carbon/nitrogen ratio (COD/NH4+-N) of 3–5 with the corresponding effluent total nitrogen (TN)＜5 mg/L. Microbial community and amoA-targeting amplicon sequencing analysis further verified that comammox Nitrospira spp., denitrifier Thauera and other aerobic/facultative heterotrophs could work synergistically with anammox bacteria, Candidatus Kuenenia. Moreover, nitrogen metabolic and inorganic carbon fixation pathways through the interaction between comammox and anammox were also revealed with the aid of Kyoto Encyclopedia of Genes and Genomes (KEGG). Lastly, potential application of proposed SCOCDA process was illustrated. This research sheds new light on advanced nitrogen removal towards limit of technology via the synergy of comammox and anammox.

Performance and bacterial community analysis of a two-stage A/O-MBBR system with multiple chambers for biological nitrogen removal

A two-stage anoxic/oxic (A/O)-moving bed biofilm reactor (MBBR) system with multiple chambers was established for municipal wastewater treatment. At the total hydraulic retention time (HRT) of 11.2 h and nitrate recycling ratio of 1, the removal efficiencies reached 83.8%, 82.5%, and 77.8% for soluble chemical oxygen demand (SCOD), 98.0%, 97.5%, and 94.9% for ammonia nitrogen (NH4+-N), and 91.8%, 92.0%, and 87.7% for total inorganic nitrogen (TIN) in summer, autumn and winter, respectively. Biofilms with functional bacterial populations were formed in the pre-anoxic reactors, the pre-oxic reactors, the post-anoxic reactors and the post-oxic reactors of the two-stage A/O-MBBR system. The highest nitrification potential was found in the last oxic reactor of the first A/O-MBBR subsystem with the highest relative abundances of the functional genes including [EC:1.14.99.39] and [EC:1.7.2.6]). The highest denitrification potential was found in the post-anoxic reactors with the highest relative abundances of the functional genes including [EC:1.7.2.1], [EC:1.7.2.5] and [EC:1.7.2.4]. This work constructed an efficient municipal biological nitrogen removal technology to achieve high effluent nitrogen standards in winter, and investigated its working mechanism to provide a basis for its design and optimization.

Recovery of Clean Water and Ammonia from Domestic Wastewater: Impacts on Embodied Energy and Greenhouse Gas Emissions.

Treatment of domestic wastewater can recover valuable resources, including clean water, energy, and ammonia. Important metrics for these systems are greenhouse gas (GHG) emissions and embodied energy, both of which are location- and technology-dependent. Here, we determine the embodied energy and GHG emissions resulting from a conventional process train, and we compare them to a nonconventional process train. The conventional train assumes freshwater conveyance from a pristine source that requires energy for pumping (US average of 0.29 kWh/m3), aerobic secondary treatment with N removal as N2, and Haber-Bosch synthesis of ammonia. Overall, we find that this process train has an embodied energy of 1.02 kWh/m3 and a GHG emission of 0.77 kg-CO2eq/m3. We compare these metrics to those of a nonconventional process train that features anaerobic secondary treatment technology followed by further purification of the effluent by reverse osmosis and air stripping for ammonia recovery. This "short-cut" process train reduces embodied energy to 0.88 kWh/m3 and GHG emissions to 0.42 kg-CO2eq/m3, while offsetting demand for ammonia from the Haber-Bosch process and decreasing reliance upon water transported over long distances. Finally, to assess the potential impacts of nonconventional nitrogen removal technology, we compared the embodied energy and GHG emissions resulting from partial nitritation/anammox coupled to anaerobic secondary treatment. The resulting process train enabled a lower embodied energy but increased GHG emissions, largely due to emissions of N2O, a potent greenhouse gas.

Model study on real-time aeration based on nitrite for effective operation of single-stage anammox

Anaerobic ammonia oxidation (Anammox) is an innovative technology for cost-efficient nitrogen removal without intensive aeration. However, effective control of the competition between nitrite oxidizing bacteria (XNOB) and Anammox bacteria (XANA) for nitrite is a key challenge for broad applications of single-stage Anammox processes in real wastewater treatment. Therefore, a real-time aeration scheme was proposed to determine dissolved oxygen (DO) based on nitrite concentration for effective control of XNOB growth while maintaining the XANA activity in a single-stage Anammox process. In this study, a non-steady state mathematical model was developed and calibrated using previously reported lab-scale Anammox results to investigate the efficiency of the proposed real-time aeration scheme in enhancing the Anammox process. Based on the calibrated model simulation results, DO of about 0.10 mg-O2/L was found to be ideal for maintaining effective nitrite creation by ammonia oxidizing bacteria (XAOB) while slowing down the growth of XNOB. If DO is too low (e.g., 0.01 mg-O2/L or lower), the overall rate of the ammonia removal is limited due to slow growth of XAOB. On the other hand, high DO (e.g., 1.0 mg-O2/L or higher) inhibits the growth of XANA, resulting in dominancy of XAOB and XNOB. According to the simulation results, nitrite concentration was found to be a rate-limiting parameter on effective nitrogen removal in single-stage Anammox processes. We also found that nitrite concentration can be used as a real-time switch for aeration in a single-stage Anammox process. A schematic aeration method based on real-time nitrite concentration was proposed and examined to control the competition between XANA and XNOB. In the model simulation, the XANA activity was successfully maintained because the schematic aeration prevented an outgrowth of XNOB, allowing energy-efficient nitrogen removal using single-stage Anammox processes.

Rapid enrichment of denitrifying methanotrophs in a series hollow-fiber membrane biofilm reactor

Denitrifying anaerobic methane oxidation (DAMO) process uses methane as electron donor to reduce nitrate/nitrite to dinitrogen, which is a potentially efficient, low-cost and clean biological nitrogen removal technology. However, slow microbial growth rate severely limits the application of this promising process. In this study, a series hollow-fiber membrane biofilm reactor (HfMBR) was operated for 90 days to achieve rapid enrichment of these denitrifying methanotrophs. Finally, the highest relative abundance of denitrifying methanotrophic archaea and bacteria (DAMO archaea and bacteria) reached 47.5% and 11.3%, respectively. And the average abundance of DAMO archaea and bacteria increased 92.9 and 136.6 times respectively during the 90-day enrichment. High growth rate of DAMO archaea with a doubling time of 11.6 days was achieved in the second HfMBR according to quantitative PCR results. The results implied that dissolved oxygen would inhibit the growth of DAMO archaea, but the series HfMBR could effectively counteract this unfavorable factor. This work provided theoretical guidance for the rapid enrichment of denitrifying methanotrophs and contributed to the application of methane-dependent denitrification process.

Efficient nitrogen removal from onsite wastewater by a novel continuous flow biofilter

Soil-based passive biofiltration system is an economically feasible technology for nitrogen removal from onsite wastewater. However, the conventional design requires a large system footprint with limited treatment capacity. In this study, a novel continuous flow biofilter (CFB) with adjustable recirculation and continuous flow pattern was developed for onsite wastewater treatment with a small footprint. Efficient total nitrogen removal (80.1–97.5%) was observed at various hydraulic loadings (0.03–0.12 m3 m−2 d−1), nitrogen loadings (1.1–8.6 g N m−2 d−1) and recycle ratios (2–3) when treating septic tank effluent (STE), with low effluent TN (0.7–13.6 mg N L−1). Nitrous oxide was observed in the denitrification effluent indicating incomplete denitrification at elevated dissolved oxygen levels (3.3–5.8 mg L−1). Nitrogen removal rate (2.9–7.0 g N m−2 d−1) and ammonium removal rate (2.4–7.2 g N m−2 d−1) were positively correlated with nitrogen loadings increase (1.1–8.6 g N m−2 d−1) but were not significantly impacted by the hydraulic loading rate change (0.08–0.12 m3 m−2 d−1). The total biomass abundance and nitrifying microorganisms decreased significantly as the nitrification columns depth increased, while homogeneous microbial distribution was observed in the denitrification columns. The abundance of ammonium oxidizing archaea (AOA) increased significantly at increased hydraulic and nitrogen loading rate, while the ammonium oxidizing bacteria (AOB) abundance remained steady. The abundance of functional genes involved in denitrification process (nirS, nirK and nosZ) responded differently when hydraulic and nitrogen loading rate changes. Collectively, this study suggested the CFB could efficiently remove nitrogen from onsite wastewater with fluctuating influent compositions and various hydraulic loadings.

A composite Fe–C/layered double oxides (Fe–C/LDO) carrier fabrication and application for enhanced removal of nitrate and phosphate from polluted water with a low carbon/nitrogen ratio

In this study, a multifunctional biofilm carrier (Fe–C/LDO) integrating Fe–C micro-electrolysis with Mg–Al layered double oxides (LDO) was developed and applied in a biofilter aimed to remove nitrogen and phosphorus simultaneously from slightly polluted water with a low carbon/nitrogen (C/N) ratio. Four types of carriers were prepared under different conditions and their performance in nitrate reduction and phosphorus removal were compared. Results showed that the composite Fe–C/LDO carrier prepared with polyethylene glycol as the pore-forming agent demonstrated the highest nitrate reduction rate and phosphorate adsorption capacity (2.09 mg PO43--P·g−1). A biofilter packed with the Fe–C/LDO carrier was investigated under heterotrophic conditions (C/N = 4.13) and complete autotrophic conditions (C/N = 0) during a long-term operation. Good removal of NH4+-N (96–98%), TN (92–96%) and PO43--P (50–58%) were achieved in the biofilter under both heterotrophic and autotrophic conditions. High throughput pyrosequencing analysis revealed that a large population of autotrophic denitrifying bacteria Thiobacillus and Thioalkalispira were enriched in the biofilm. Metagenomic analysis was performed to analyze functional genes related to nitrogen metabolism and carbon cycle, and it confirmed the presence of a whole denitrification and Calvin-Benson-Bassham carbon fixation pathway in biofilm. Finally, the possible mechanisms for nitrogen and phosphorus removal in the biofilter were proposed. The Fe–C/LDO carrier not only enriched abundant autotrophic denitrifying bacteria, but also provided numerous Fe–C galvanic cells for nitrate reduction through micro-electrolysis. In addition, the Mg–Al LDO in combination with iron hydroxides generated during micro-electrolysis may contribute greatly to phosphorus removal. Biofilter packed with the multifunctional Fe–C/LDO carrier could be a promising technology for simultaneous nitrogen and phosphorus removal from slightly polluted water.

Dynamic modelling of N2 O emissions from a full-scale granular sludge partial nitritation-anammox reactor.

Partial nitration-anammox is a resource-efficient pathway for nitrogen removal from wastewater. However, the advantages of this nitrogen removal technology may be counter-acted by the emission of N2 O, a potent greenhouse gas. In this study, mathematical modelling was applied to analyse N2 O formation and emission dynamics and to develop N2 O mitigation strategies for a one-stage partial nitritation-anammox granular sludge reactor. Dynamic model calibration for such a full-scale reactor was performed, applying a one-dimensional biofilm model and including several N2 O formation pathways. Simultaneous calibration of liquid phase concentrations and N2 O emissions leads to improved model fit compared to their consecutive calibration. The model could quantitatively predict the average N2 O emissions and qualitatively characterize the N2 O dynamics, adjusting only seven parameter values. The model was validated with N2 O data from an independent data set at different aeration conditions. Nitrifier nitrification was identified as the dominating N2 O formation pathway. Off-gas recirculation as a potential N2 O emission reduction strategy was tested by simulation and showed indeed some improvement, be it at the cost of higher aeration energy consumption.