Wastewater Nitrogen Removal Research Articles

Enhanced nitrogen removal from secondary effluent of municipal wastewater using denitrification filter: Feasibility of refractory organics as a carbon source

Conventional advanced nitrogen removal in municipal wastewater is hindered by the limited availability of carbon sources in the secondary effluent. However, refractory organics present in it had the potential to serve as intrinsic carbon sources after hydrolysis for nitrogen removal via simultaneous denitrification and partial-denitrification anammox (PDA) processes. To assess this potential, a denitrification filter was set up in this study to evaluate its feasibility of concurrent processes. Results showed that increasing influent ammonium (NH4+-N) from 1.0 to 7.0 mg/L increased total nitrogen (TN) removal from 52.4 % to 89.9 %. Simultaneous occurrence of PDA and denitrification process were confirmed by the actual chemical oxygen demand (COD) consumption (0.8–1.2 mg/mg TN removal) from non-fluorescent organics. The presence of the anammox, hydrolytic and denitrifying bacteria further supported the achievement of nitrogen removal through PDA and denitrification processes by utilizing hydrolytic products biodegraded from refractory organics.

Investigation of Nitrogen Removal in Flue Gas Desulfurization and Denitrification Wastewater Utilizing Halophilic Activated Sludge.

In the process of flue gas desulfurization and denitrification, the generation of high-sulfate wastewater containing nitrogen is a significant challenge for biological wastewater treatment. In this study, halophilic activated sludge was inoculated in a Sequencing Batch Reactor to remove nitrogen from wastewater with a high sulfate concentration (60 g/L). With the influent concentration of 180 mg/L, the removal rate of total nitrogen was more than 96.7%. The effluent ammonium nitrogen concentration was lower than 1.94 mg/L, and the effluent nitrate nitrogen and nitrite nitrogen concentrations were even lower than 0.77 mg/L. The salt tolerance of activated sludge is mainly related to the increase in the content of ectoine in microbial cells. The Specific Nitrite Oxidation Rate is quite low, while the Specific Nitrite Reduction Rate and Specific Nitrate Reduction Rate are relatively strong. In the system, there are various nitrogen metabolic processes, including aerobic nitrification, anaerobic denitrification, and simultaneous nitrification-denitrification processes. By analyzing the nitrogen metabolic mechanisms and microbial community structure of the reaction system, dominate bacteria can be identified, such as Azoarcus, Thauera, and Halomonas, which have significant nitrogen removal capabilities.

Enhancement of the reactivation process of long-term starved anammox granular sludge with gravel balls: Microbial succession and metabolic impact

Anaerobic ammonium oxidation (Anammox) process is an economical and energy-efficient method of wastewater nitrogen removal. However, they are highly susceptible to starvation stress caused by sudden environmental changes. Rapid reactivation of starved anammox sludge is a crucial method to address seed sludge shortages and expand practical applications. This study investigated the impact of gravel balls on the reactivation of long-term starved anammox granular sludge (628 days). The results showed that gravel balls enhanced the recovery of nitrogen removal performance in starved anammox sludge, with nitrogen removal efficiency being 19.88% higher than the control group at the end of the recovery phase. The gravel balls also increased extracellular polymeric substance (EPS) secretion, contributing to the stability of the anammox system. Furthermore, the gravel balls promoted the proliferation of anammox bacteria, with the relative abundance of anammox bacteria reaching 38.25% on the 80th day. The analyses of microbial functions indicated that gravel balls facilitated cross-feeding and co-metabolism among microbes, while enhancing quorum sensing associated with anammox bacteria, forming a multifunctional community network centered on anammox bacteria. This indicates that gravel balls can effectively accelerate the reactivation process of long-term starved anammox sludge, aiding the reutilization of long-term starved anammox sludge.

Enhancing nitrogen removal in low C/N wastewater with recycled sludge-derived biochar: A sustainable solution

Denitrification is an important biological process in wastewater treatment plants (WWTPs). However, a low carbon-to-nitrogen (C/N) ratio limits the availability of organic carbon, potentially reducing denitrification efficiency. This study investigates the impact of sludge-derived biochar on the nitrogen removal of activated sludge for low C/N ratio municipal wastewater. Sludge-based biochar was characterized by its physicochemical properties, revealing that biochar prepared at 400 °C exhibited the highest specific surface area and the most favorable surface functional groups for electron transfer. The results from batch tests showed that adding 4 g/L of biochar dosage enhanced denitrification rates and total nitrogen (TN) removal efficiency the most. Sequencing batch reactors (SBRs) experiments further confirmed that biochar dosgae improved the removal efficiencies of COD, NH4+-N, and TN, achieving stable values of 97.2 ± 1.2 %, 99.2 ± 0.6 %, and 83.8 ± 2.4 %, respectively. Metabolic and electrochemical analyses revealed that biochar addition enhanced the activity of denitrification enzymes, increasing the ammonia oxidation rate by 12.9 ± 0.7 %, nitrite oxidation rate by 14.7 ± 1.2 %, nitrate reduction rate by 36.9 ± 1.5 %, and nitrite reduction rate by 16.4 ± 0.8 %. The relative abundance of denitrification functional genes (amoA, nirS, nirK, narG, nosZ) increased, and the activities of the corresponding enzymes (AMO, NXR, NAP, NIR) rose by 23±6 %, 53±5 %, 260±15 %, and 55±7 %, respectively. This increase in enzyme activity suggested enhanced denitrification processes, which was further supported by the 60.1 ± 3.7 % increase in electron transfer system activity (ETSA), indicating that biochar acted as an electron shuttle. This study proposes a potential sustainable approach for sludge recycling and enhanced wastewater nitrogen removal under low C/N conditions.

Insight into simultaneous urea hydrolysis and total nitrogen removal in textile printing wastewater: Focus on the impact of sodium sulfate salinity

The textile printing industry discharges large volumes of effluent containing high concentrations of urea and nitrogenous compounds. Anoxic-oxic (AO) treatment is a promising method for treating printing wastewater. However, the effect of sodium sulfate (Na2SO4) salinity on the urea hydrolysis and nitrogen removal simultaneously in the AO process has received little attention. In this study, five batch reactors were used to treat synthetic printing wastewater with high urea and nitrogen concentrations. A strategy was applied to increase the Na2SO4 concentration from 0 to 19 g/L in the anoxic stage of each reactor. The effect of Na2SO4 on urea hydrolysis, total nitrogen removal and COD removal, sludge characteristics, and bacterial community structure were investigated. The findings showed that urea hydrolysis increased with increasing Na2SO4 concentration. The main mechanism of urea removal was intracellular hydrolysis, with a urea removal efficiency (URE%) of approximately 98% in all batch reactors. In addition, under the stress of Na2SO4, the total nitrogen and COD removal performances were partially inhibited. The most significant removal performances after AO treatment were observed at 0 g/L Na2SO4, with nitrogen and COD removal efficiencies of 88% and 95%, respectively. When Na2SO4 concentration reached 19 g/L, the sludge settling performance and compactness were enhanced. The extracellular polymeric substance (EPS) components in the sludge were dependent on their ability of removing organics. Bacterial community diversity analysis revealed that the enrichment of the Proteobacteria, Firmicutes, and Gemmatimonadota phyla in the anoxic stages of batch reactors was related to intracellular urea hydrolysis. Bacteriodota and Chloroflexi were responsible for total nitrogen removal in all anoxic and oxic stages. This research will develop the understanding of Na2SO4 salinity impact on simultaneous urea hydrolysis and nitrogen removal during AO treatment process.

Process Performance and Nutrient Removal in Fish Processing Wastewater Using a Membrane Bioreactor (MBR) Unit for Reuse for Irrigation

This study investigated the removal of nitrogen and phosphates in fish processing wastewater through process optimization of a membrane bioreactor (MBR) treatment unit. Raw process wastewater was collected from the Makindi fish farm and was pre-filtered through a mesh of 0.8 mm. The experiment was conducted at a lab scale using commercial Polyethersulfone (PES) membranes submerged in the MBR unit. The pre-filtered wastewater sample was added into a denitrification (anoxic) tank. The process was conducted through continuous recirculation between the aeration and the anoxic tank to enhance the removal of nitrogenous compounds. The recirculation flow rate was 10L/h. A hydraulic residence time (HRT) of approximately 22-25h was maintained, and the aeration rate in the aerobic tank was 100L/min. Aluminum sulphate AI2(SO4)318H2O was added continually into the anoxic tank and with constant stirring to enhance the removal of phosphates. A removal rate of 85±2% for NH4+-N, 84±1% for NO3- -N, and 69±3% for PO43--P was obtained. The nutrient concentration in the effluent was successfully reduced to an acceptable level with both nitrogenous compounds and phosphate concentrations obtained within the range of < 30 mg/L and ≤ 5 mg/L as per the WHO guidelines for wastewater reuse for irrigation. The results showed a successful process optimization that can be applied to ensure optimized performance of the MBR unit thus improve its effectiveness for the treatment of fish processing wastewater. The study therefore recommends process optimization as a tool for effective removal of nutrients in the MBR system thus making it a potential recycling approach for treatment of fish processing wastewater for reuse for irrigation purposes.

Genome-resolved metagenomics reveals the nitrifiers enrichment and species succession in activated sludge under extremely low dissolved oxygen

Nitrification, a process carried out by aerobic microorganisms that oxidizes ammonia to nitrate via nitrite, is an indispensable step in wastewater nitrogen removal. To facilitate energy and carbon savings, applying low dissolved oxygen (DO) is suggested to shortcut the conventional biological nitrogen removal pathway, however, the impact of low DO on nitrifying communities within activated sludge is not fully understood. This study used genome-resolved metagenomics to compare nitrifying communities under extremely low- and high-DO. Two bioreactors were parallelly operated to perform nitrification and DO was respectively provided by limited gas-liquid mass transfer from the atmosphere (AN reactor, DO < 0.1 mg/L) and by sufficient aeration (AE reactor, DO > 5.0 mg/L). Low DO was thought to limit nitrifiers growth; however, we demonstrated that complete nitrification could still be achieved under the extremely low-DO conditions, but with no nitrite accumulation observed. Kinetic analysis showed that after long-term exposure to low DO, nitrifiers had a higher oxygen affinity constant and could maintain a relatively high nitrification rate, particularly at low levels of DO (<0.2 mg/L). Community-level gene analysis indicated that low DO promoted enrichment of nitrifiers (the genera Nitrosomonas and Nitrospira, increased by 2.3- to 4.3-fold), and also harbored with 2.3 to 5.3 times higher of nitrification functional genes. Moreover, 46 high-quality (>90 % completeness and <5 % contamination) with 3 most abundant medium-quality metagenome-assembled genomes (MAGs) were retrieved using binning methods. Genome-level phylogenetic analysis revealed the species succession within nitrifying populations. Surprisingly, compared to DO-rich conditions, low-DO conditions were found to efficiently suppressed the ordinary heterotrophic microorganisms (e.g., the families Anaerolineales, Phycisphaerales, and Chitinophagales), but selected for the specific candidate denitrifiers (within phylum Bacteroidota). This study provides new microbial insights to demonstrate that low-DO favors the enrichment of autotrophic nitrifiers over heterotrophs with species-level successions, which would facilitate the optimization of energy and carbon management in wastewater treatment.

Metabolic evolution and bottleneck insights into simultaneous autotroph-heterotroph anammox system for real municipal wastewater nitrogen removal

When biological nitrogen removal (BNR) systems shifted from treating simulated wastewater to real wastewater, a microbial succession occurred, often resulting in a decline in efficacy. Notably, despite their high nitrogen removal efficiency for real wastewater, anammox coupled systems operating without or with minimal carbon sources also exhibited a certain degree of performance reduction. The underlying reasons and metabolic shifts within these systems remained elusive. In this study, the simultaneous autotrophic/heterotrophic anammox system demonstrated remarkable metabolic resilience upon exposure to real municipal wastewater, achieving a nitrogen removal efficiency (NRE) of 82.83 ± 2.29 %. This resilience was attributed to the successful microbial succession and the complementary metabolic functions of heterotrophic microorganisms, which fostered a resilient microbial community. The system's ability to harness multiple electron sources, including NADH oxidation, the TCA cycle, and organics metabolism, allowed it to establish a stable and efficient electron transfer chain, ensuring effective nitrogen removal. Despite the denitrification channel's nitrite supply capability, the analysis of the interspecies correlation network revealed that the synergistic metabolism between AOB and AnAOB was not fully restored, resulting in selective functional bacterial and genetic interactions and the system's PN/A performance declined. Additionally, the enhanced electron affinity of PD increased interconversion of NO3−-N and NO2−-N, limiting the efficient utilization of electrons and thereby constraining nitrogen removal performance. This study elucidated the metabolic mechanism of nitrogen removal limitations in anammox-based systems treating real municipal wastewater, enhancing our understanding of the metabolic functions and electron transfer within the symbiotic bacterial community.

A comparison study on membrane fouling in A/O-MBR and A/A-MBR at different mixed liquor-suspended solids concentrations

ABSTRACT Membrane fouling leads to decreased membrane flux, increases the frequency of membrane tissue replacement and membrane cleaning, and increases the operating cost of membrane bioreactor. In this study, the pollutant removal effects, membrane fouling differences and microbial characteristics of anaerobic/aerobic MBR (A/O-MBR) and anaerobic/anoxic MBR (A/A-MBR) were investigated at different mixed liquor suspended solids (MLSS) concentrations. The results showed that the chemical cleaning cycle of membrane contamination was 12, 28, 44 h and 24, 40, 104 h, respectively, and the cycle was prolonged with the increase of MLSS concentration (from 6000 to 9000 mg L−1). A/O-MBR was 1.4–2.4 times the rate of membrane fouling of A/A-MBR. In irreversible resistance, extracellular polymer substances (EPS) were the most significant contributors to membrane fouling. EPS concentration in A/A-MBR (118.33, 73.75, 54.26 mg/gMLSS) was lower than that in A/O-MBR (171.68, 91.92, 62.33 mg/gMLSS). Therefore, increasing MLSS concentration could mitigate membrane fouling. 16S rRNA high-throughput sequencing demonstrated that filamentous bacteria was the primary reason for the membrane fouling difference. Filamentous bacteria were more likely to be attached to the surface of the membrane, causing membrane fouling. The abundance percentage of filamentous bacteria in A/A-MBR was smaller than that in A/O-MBR. In summary, The excellent performance of A/A-MBR in membrane fouling behaviour, resistance analysis, EPS and microorganisms proved that A/A-MBR is more promising than A/O-MBR in wastewater nitrogen and phosphorus removal. This study can provide a theoretical basis for the application of MBR in the field of sewage treatment.

3D printing for constructing biocarriers using sodium alginate/ε-poly-l-lysine ink: Enhancing microbial enrichment for efficient nitrogen removal in wastewater

The microbial enrichment of traditional biocarriers is limited due to the inadequate consideration of spatial structure and surface charging characteristics. Here, capitalizing on the ability of 3D printing technology to fabricate high-resolution materials, we further designed a positively charged sodium alginate/ε-poly-l-lysine (SA/ε-PL) printing ink, and the 3D printed biocarriers with ideal pore structure and rich positive charge were constructed to enhance the microbial enrichment. The rheological and mechanical tests confirmed that the developed SA/ε-PL ink could simultaneously satisfy the smooth extrusion for printing process and the maintenance of 3D structure. The utilization of the ε-PL secondary cross-linking strategy reinforced the 3D mechanical structure and imparted the requisite physical properties for its application as a biocarrier. Compared with traditional sponge carriers, 3D printed biocarrier had a faster initial attachment rate and a higher biomass of 14.58 ± 1.18 VS/cm3, and the nitrogen removal efficiency increased by 53.9 %. Besides, due to the superior electrochemical properties and biocompatibility, the 3D printed biocarriers effectively enriched the electroactive denitrifying bacteria genus Trichococcus, thus supporting its excellent denitrification performance. This study provided novel insights into the development of new functional biocarriers in the wastewater treatment, thereby providing scientific guidance for practical engineering.

Influence and mechanism of typical transition metal ions on the denitrification performance of heterotrophic nitrification-aerobic denitrification bacteria

To investigate the inhibitory effects of various transition metal ions on nitrogen removal and their underlying mechanisms, the single and combined effects of Cu2+ Ni2+ and Zn2+ on Heterotrophic nitrification-aerobic denitrification (HN-AD) bacteria Acinetobacter sp. TAC-1 were studied in a batch experiment system. The results revealed that increasing concentrations of Cu2+ and Ni2+ had a detrimental effect on the removal of ammonium nitrogen (NH4+-N) and total nitrogen (TN). Specifically, Cu2+ concentration of 10 mg/L, the TN degradation rate was 55.09%, compared to 77.60% in the control group. Cu2+ exhibited a pronounced inhibitory effect. In contrast, Zn2+ showed no apparent inhibitory effect on NH4+-N removal and even enhanced TN removal at lower concentrations. However, when the mixed ion concentration of Zn2++Ni2+ exceeded 5 mg/L, the removal rates of NH4+-N and TN were significantly reduced. Moreover, transition metal ions did not significantly impact the removal rates of chemical oxygen demand (COD). The inhibition model fitting results indicated that the inhibition sequence was Cu2+ > Zn2+ > Ni2+. Transcriptome analysis demonstrated that metal ions influence TAC-1 activity by modulating the expression of pivotal genes, including zinc ABC transporter substrate binding protein (znuA), ribosomal protein (rpsM), and chromosome replication initiation protein (dnaA) and DNA replication of TAC-1 under metal ion stress, leading to disruptions in transcription, translation, and cell membrane structure. Finally, a conceptual model was proposed by us to summarize the inhibition mechanism and possible response strategies of TAC-1 bacteria under metal ion stress, and to address the lack of understanding regarding the influence mechanism of TAC-1 on nitrogen removal in wastewater co-polluted by metal and ammonia nitrogen. The results provided practical guidance for the management of transition metal and ammonia nitrogen co-polluted water bodies, as well as the removal of high nitrogen.

Coupling effects of aureomycin and zinc ion on nitrification process and nitrification enzymes during the biological nitrogen removal

Aureomycin and Zinc ion (Zn2+) are common antibiotics and heavy metals that exist in livestock wastewater. The coupling effects of Aureomycin and Zn2+ on the nitrification process and nitrification function enzymes are crucial for controlling nitrogen removal in livestock wastewater. However, rare studies were focused on the coupling effects of Aureomycin and Zn2+ on nitrification. This study employed a direct equipartition ray method to investigate the coupling effects of Aureomycin and Zn2+ on nitrification. The results suggested three different ratios of Aureomycin and Zn2+ affected nitrification performance differently. Ratio 1 and Ratio 2 exhibited a promotion effect with low concentrations and an inhibition effect with high concentrations on nitrification performance. The critical concentration for Ratio 1 and Ratio 2 were 5.00 mg L−1 and 1.90 mg L−1, respectively. Ratio 3 exhibited both time-dependent and concentration-dependent inhibitory effects on nitrification performance. The maximum inhibitory efficiency on nitrification performance was 90.0%, with a concentration of 34.5 mg L−1 at 96.0 h. The effects of binary mixture on nitrogen removal performance were attributed to the effects of binary mixture on nitrite oxidase activity. The qualitative evaluation of the concentration addition model and independent action model indicated Aureomycin and Zn2+ showed a synergistic effect with strong concentration-dependent and time-dependent in the whole concentration area. The synergistic effect of Aureomycin and Zn2+ on nitrite oxidase activity mainly depended on the concentration of Aureomycin. This study offers new insights into the effects of antibiotics and heavy metals on the biological nitrogen removal process.

Combined effect of zinc and cadmium ions on nitrification performance during the biological nitrogen removal of simulated livestock breeding wastewater.

Zinc and cadmium ions are usually found in livestock breeding wastewater, and the mixed ions will have an impact on the biological nitrogen removal. Nitrification performance plays an important role in biological nitrogen removal. In order to investigate the combined effect of zinc and cadmium ions on nitrification performance and to reveal the interactions between zinc and cadmium ions, three concentration ratios of zinc and cadmium ions, as well as 18 different concentration gradients were designed with the direct equipartition ray and the dilution factor method. The effect of pollutants on the nitrification performance of biological nitrogen removal was analyzed by the nonlinear regression equation, and the concentration-addition model was conducted to probe into the relationship between the mixed pollutants and the nitrification performance. The results showed that the effect on nitrification performance increased significantly with the increase of reaction duration and pollutant concentration, which indicated that the effects are concentration-dependent and time-dependent. The concentration-addition model suggested that the interactions between zinc and cadmium ions with different concentration ratios were mainly antagonistic, and as the percentage of cadmium ions in the mixtures increased, the antagonism between the mixtures became stronger. This study will provide a relevant theoretical basis for the regulation of the ratios and concentrations of heavy metal ions during the biological treatment of livestock breeding wastewater.

Carbon-ferrous-sulfur mixotrophic denitrification driven by polyferric sulfate (PFS)-conditioned recovered sludge fermentation liquor enhanced nitrogen removal in woolen wastewater

Reducing pollution and carbon emissions synergistically in wastewater treatment is an important part of carbon neutralization. Recycling carbon sources (CSs) in sludge to refeed denitrification has been considered a promising sustainable pathway, but it is limited by CSs purification and recovery. This study evaluated the effects of polyferric sulfate (PFS), polyaluminum chloride (PAC), and polyacrylamide (PAM) conditioning on fermentation CSs recovery and denitrification efficiency. Results indicated PFS-conditioned recovered fermentation liquor showed higher denitrification efficiency for woolen wastewater. CSs recovery test showed that PFS conditioning improved the purity of high-bioavailability CSs by depleting fewer short-chain fatty acids (SCFAs) and removing more total nitrogen (TN), total phosphorus (TP), and humic acids. Under the 0.05 g/gTSS PFS conditioning and recovery, the preservation of soluble chemical oxygen demand (SCOD) and SCFAs were 71.00 % and 84.36 %, respectively, and the nitrate degradation rate achieved 3.19 mgNO3−·h−1·gVSS−1. Importantly, Fe(III) dissociated from PFS oxidized the S2− from woolen wastewater, and generated Fe(II) and S0, which composed a synergistic electron donor system together with SCFAs. They constituted a mixotrophic denitrification dominated by heterotrophic denitrification and reinforced by ferrous and sulfur-autotrophic denitrifications, which contributed 69.56, 17.72, and 12.72 % to nitrogen removal, respectively. In addition, the refeeding of PFS-conditioned recovered fermentation liquor promoted the proliferation of microorganisms involved in iron/sulfur autotrophic denitrification and the abundance of denitrification functional genes, especially the napA, nirS, cnorB, qnorB, and nosZ, forming an efficient Carbon-Ferrous-Sulfur hybrid denitrifiers community to enhanced nitrogen removal from woolen wastewater.

The combination of nanobubble aeration and iron-based multi-carbon source composites achieves efficient aquaculture wastewater nitrogen removal

The primary constraints for efficient nitrogen removal in aquaculture ponds by indigenous aerobic denitrifiers are oxygen depletion in sediment and the shortage of electron donors. This study innovatively combined nanobubble aeration with iron-based multi-carbon source composites to stimulate aerobic denitrifiers, significantly improving the sediment's oxygen level and supplementing the electron donors of the system. Compared with the systems that incorporated multi-carbon source composites (MCSC) or iron-based multi-carbon source composites (IMCSC) under diffused air aeration conditions, the system that employed nanobubble aeration and iron-based multi-carbon source composites (NIMCSC) demonstrated an increase in total nitrogen (TN) elimination rate by 14.07% and 7.46%, respectively, reaching 89.75%. Microbial community analysisrevealed that the NIMCSC systemenriched the mixotrophic aerobic denitrifiers and fostered a symbiotic relationship among the bacterial communities. Moreover, compared with the MCSC and IMCSC systems, the NIMCSC system highlighted a significant increase in the average abundance of key indigenous aerobic denitrifiers. Meanwhile, the production and transfer process of electrons were stimulated, providing more energy and electron donors and assisting various abundant key functional enzymes to achieve efficient mixotrophic aerobic denitrification in the NIMCSC system. This study presenteda novel approach to achieve efficient aquaculture wastewater nitrogen removal.

Strategies for efficient enrichment of anaerobic ammonia oxidizing bacteria in activated sludge

Anaerobic ammonia oxidation (Anammox) is an economical and sustainable wastewater nitrogen removal technology, and its application in the mainstream process is the inevitable trend of the development of Anammox. However, how to effectively enriching Anammox bacteria from the activated sludge remains challenging and restricts its extensive applications. In this study, the rapid and efficient enrichment of Anammox bacteria was achieved by raising the reflux ratio and nitrogen loading rate (NLR) using conventional activated sludge as the inoculant. In the screening phase (days 1–90), the reflux ratio was increased to discharge partial floc sludge, resulting in the relative abundance of Candidatus Brocadiaceae increased from 0.04% to 22.54%, which effectively reduced the matrix and spatial competition between other microorganisms and Anammox bacteria. On day 90, the stoichiometric ratio of the Anammox process closely approached the theoretical value of 1:1.32:0.26, indicating that the Anammox reaction was the primary nitrogen removal process in the system. In the enrichment phase (days 91–238), the NLR increased from 0.43 to 1.20 kgN/(m3·d) and removal efficiency was 71.89%, resulting in the relative abundance of Candidatus Brocadiaceae increased to 61.27% on day 180. The reactor operated steadily from days 444 to 498, maintaining the nitrogen removal rate (NRR) of 3.00 kgN/(m3·d) and achieving successful sludge granulation with the particle size of 392.4 µm. In short, this study provided a simple and efficient approach for enriching Anammox bacteria from the activated sludge, supporting to start an Anammox process efficiently.

Influence of temperature on performance and mechanism of advanced synergistic nitrogen removal in lab-scale denitrifying filter with biogenic manganese oxides

Temperature is a significant operational parameter of denitrifying filter (DF), which affects the microbial activity and the pollutants removal efficiency. This study investigated the influence of temperature on performance of advanced synergistic nitrogen removal (ASNR) of partial-denitrification anammox (PDA) and denitrification, consuming the hydrolytic and oxidation products of refractory organics in the actual secondary effluent (SE) as carbon source. When the test water temperature (TWT) was around 25, 20, 15 and 10 °C, the filtered effluent total nitrogen (TN) was 1.47, 1.70, 2.79 and 5.52 mg/L with the removal rate of 93.38%, 92.25%, 87.33% and 74.87%, and the effluent CODcr was 8.12, 8.45, 10.86 and 12.29 mg/L with the removal rate of 72.41%, 66.17%, 57.35% and 51.87%, respectively. The contribution rate of PDA to TN removal was 60.44%∼66.48%, and 0.77–0.96 mg chemical oxygen demand (CODcr) was actually consumed to remove 1 mg TN. The identified functional bacteria, such as anammox bacteria, manganese oxidizing bacteria (MnOB), hydrolytic bacteria and denitrifying bacteria, demonstrated that TN was removed by the ASNR, and the variation of the functional bacteria along the DF layer revealed the mechanism of the TWT affecting the efficiency of the ASNR. This technique presented a strong adaptability to the variation of the TWT, therefore, it has broad application prospect and superlative application value in advanced nitrogen removal of municipal wastewater.

Using rice straw-augmented ecological floating beds to enhance nitrogen removal in carbon-limited wastewater

Agricultural biomass used as solid carbon substrates in ecological floating beds (EFBs) has been proven to be applicable in nitrogen removal for carbon-limited wastewater treatment. However, the subtle interactions among plants, rhizosphere microorganisms, and supplementary carbon sources have not been thoroughly studied. This study combined rice straw mats with different aquatic macrophytes in EFBs to investigate denitrification efficiency in carbon-limited eutrophic waters. Results showed that rice straw significantly enhanced the nitrogen removal efficiency of EFBs, while enriching nitrogen-fixing and denitrifying bacteria (such as Rhizobium, Rubrivivax, and Rhodobacter, etc.). Additionally, during the denitrification process in EFBs, rice straw can release humic acid-like fraction as electron donors to support the metabolic activities of microorganisms, while aquatic macrophytes provide a more diverse range of dissolved organic matters, facilitating a sustainable denitrification process. These findings help to understand the synergistic effect of denitrification processes within wetland ecosystems using agricultural biomass.

Irons differently modulate bacterial guilds for leading to varied efficiencies in simultaneous nitrification and denitrification (SND) within four aerobic bioreactors

As a novel biological wastewater nitrogen removal technology, simultaneous nitrification and denitrification (SND) has gained increasing attention. Iron, serving as a viable material, has been shown to influence nitrogen removal. However, the precise impact of iron on the SND process and microbiome remains unclear. In this study, bioreactors amended with iron of varying valences were evaluated for total nitrogen (TN) removal efficiencies under aerobic conditions. The acclimated control reactor without iron addition (NCR) exhibited high ammonia nitrogen (AN) removal efficiency (98.9%), but relatively low TN removal (78.6%) due to limited denitrification. The reactor containing zero-valent iron (Fe0R) demonstrated the highest SND rate of 92.3% with enhanced aerobic denitrification, albeit with lower AN removal (84.1%). Significantly lower SND efficiencies were observed in reactors with ferrous (Fe2R, 66.3%) and ferric (Fe3R, 58.2%) iron. Distinct bacterial communities involved in nitrogen metabolisms were detected in these bioreactors. The presence of complete ammonium oxidation (comammox) genus Nitrospira and anammox bacteria Candidatus Brocadia characterized efficient AN removal in NCR. The relatively low abundance of aerobic denitrifiers in NCR hindered denitrification. Fe0R exhibited highly abundant but low-efficiency methanotrophic ammonium oxidizers, Methylomonas and Methyloparacoccus, along with diverse aerobic denitrifiers, resulting in lower AN removal but an efficient SND process. Conversely, the presence of Fe2+/Fe3+ constrained the denitrifying community, contributing to lower TN removal efficiency via inefficient denitrification. Therefore, different valent irons modulated the strength of nitrification and denitrification through the assembly of key microbial communities, providing insight for microbiome modulation in nitrogen-rich wastewater treatment.

Sustainable biochar application in anammox process: Unveiling novel pathways for enhanced nitrogen removal and efficient start-up at low temperature

This study explored the use of biochar to accelerate the establishment of anaerobic ammonium oxidation (anammox) reactors operating at 15 ± 1℃. Incorporating 10 g/L bamboo charcoal in S1 accelerated the start-up of anammox in 87 days, which was significantly shorter than 103 days in S0 (without biochar). After 140 days, S1 exhibited a 10.9 % increase in nitrogen removal efficiency due to a 28.9 % elevation in extracellular polymeric substances, bolstering anammox bacterial resilience. Predominant anammox bacteria (Cadidatus Brocadia and Cadidatus Jettenia) showed relative abundances of 3.19 % and 0.38 % in S1, respectively, which were significantly higher than 0.40 % and 0.05 % in S0. Biochar provides favorable habitats for the enrichment of anammox bacteria and accelerates the establishment of anammox at low temperatures. This finding holds promise for enhancing the efficiency of anammox in cold climates and advancing sustainable wastewater nitrogen removal.