Effect of dissolved oxygen on high C/N wastewater treatment in moving bed biofilm reactors based on heterotrophic nitrification and aerobic denitrification: Nitrogen removal performance and potential mechanisms

Denitrification efficiency and biofilm community succession in a bidirectional alternating influent biofilter

Biofilters are widely used for nitrogen removal in wastewater treatment. This study developed a bidirectional alternating-influent biofilter to reduce clogging and enhance nitrogen removal. Alternating influent utilized biofilm on the media as a denitrification carbon source. With initial ammonium, nitrate, and total nitrogen concentrations of 8.49±0.30, 12.52±0.20, and 19.89±0.79 mg/L, the forward influent achieved ammonium, nitrate, and total nitrogen removal efficiencies of 81.6%, 66.8%, and 71.2%, increasing by 13.3%, 3.0%, and 4.8% at the effluent. Reverse influent further boosted nitrate and total nitrogen removal by 14.0% and 5.5%. The natural DO gradient under conventional influent conditions was simulated, and the nitrogen removal mechanism and treatment effect, mainly nitrification and denitrification, were discussed. Microbial analysis showed that endogenous carbon in the biofilm, derived from decaying cells and EPS, reduced clogging risk. Significant changes in bacterial count, EPS content, and microbial abundance were observed across influent directions, with Proteobacteria, Bacteroidetes, and Pseudomonas increasing under reverse flow. These results indicate that bidirectional alternating influent can significantly improve nitrogen removal and reduce clogging, offering an effective optimization for wastewater treatment.

Wastewater sample storage for physicochemical and microbiological analysis

Wastewater-based epidemiology (WBE) is a crucial tool for health and environmental monitoring, providing real-time data on public health indicators by analysis of sewage samples. Ensuring the integrity of these samples from collection to analysis is paramount. This study investigates the effects of different cold-storage conditions on the integrity of wastewater samples, focusing on both microbiological markers (such as extractable nucleic acids, SARS-CoV-2, and crAssphage) and physicochemical parameters (including ammonium, orthophosphate, pH, conductivity, and turbidity). Composite samples from the combined raw wastewater influent from five wastewater treatment works in South Wales, UK, were stored at 4°C, -20°C, and -80°C, and subjected to up to six freeze-thaw cycles over one year. The study found significant effects of storage temperature on the preservation of certain WBE markers, with the best yield most frequently seen in samples stored at -80°C. However, the majority of WBE markers showed no significant difference between storage at -80°C or at 4°C, demonstrating that it may not always be necessary to archive wastewater samples at ultra-low temperatures, thus reducing CO2 emissions and laboratory energy costs. These findings underscore the importance of optimized storage conditions to maintain sample integrity, while ensuring accurate and reliable WBE data for public health and environmental monitoring.

Fulvic acid mediated highly efficient heterotrophic nitrification-aerobic denitrification by Paracoccus denitrificans XW11 with reduced C/N ratio

Reducing the C/N ratio requirements for heterotrophic nitrification-aerobic denitrification (HNAD) is crucial for its practical application; however, it remains underexplored. In this study, a highly efficient HNAD bacterium, Paracoccus denitrificans XW11, was isolated. The HNAD characteristics of XW11 were studied, and the redox mediator fulvic acid (FA) was used to reduce the C/N requirements. Whole-genome sequencing revealed multiple denitrification genes in XW11; however, nitrification genes were not identified, because heterotrophic nitrification-related gene sequences were not included in the database. However, the nitrogen removal related enzyme activity test revealed complete nitrification and denitrification pathways. Reverse transcription PCR showed that the membrane-bound nitrate reductase (NarG), rather than the periplasmic nitrate reductase, was responsible for aerobic denitrification. The conventional nitrite reductase (NirS) also does not mediate nitrite denitrification. When the C/N ratio was 10, the ammonia removal efficiency of the Control was 71.71 % and the addition of FA increased it to 86.12 %. Transcriptomic analysis indicated electron flow from the carbon source to FA without proton transmembrane transport, and the presence of FA constructs another electron transfer system. The redox potential of oxidized FA/reduced FA is 0.3679 V, avoiding competition for electrons from Complex III. Thus, ammonia monooxygenase obtains electrons more easily, thereby promoting nitrification. The enzyme activity test of the nitrification process confirmed this view. In addition, NarG expression increased, and the denitrification process was enhanced. Overall, FA improved HNAD efficiency by facilitating electron transfer to the nitrogen dissimilation process, offering a novel approach to reduce the C/N requirement of HNAD.

Novel adaptive activated sludge process leverages flow fluctuations for simultaneous nitrification and denitrification in rural sewage treatment

The fluctuating characteristics of rural sewage flow pose a significant challenge for wastewater treatment plants, leading to poor effluent quality. This study establishes a novel adaptive activated sludge (AAS) process specifically designed to address this challenge. By dynamically adjusting to fluctuating water flow in situ, the AAS maintains system stability and promotes efficient pollutant removal. The core strategy of AAS leverages the inherent dissolved oxygen (DO) variations caused by flow fluctuations to establish an alternating anoxic-aerobic environment within the system. This alternating operation mode fosters the growth of aerobic denitrifiers, enabling the simultaneous nitrification and denitrification (SND) process. Over a 284-day operational period, the AAS achieved consistently high removal efficiencies, reaching 94 % for COD and 62.8 % for TN. Metagenomics sequencing revealed HN-AD bacteria as the dominant population, with the characteristic nap gene exhibiting a high relative abundance of 0.008 %, 0.010 %, 0.014 %, and 0.015 % in the anaerobic, anoxic, dynamic, and oxic zones, respectively. Overall, the AAS process demonstrates efficient pollutant removal and low-carbon treatment of rural sewage by transforming the disadvantage of flow fluctuation into an advantage for robust DO regulation. Thus, AAS offers a promising model for SND in rural sewage treatment.

Non-filamentous bulking of activated sludge induced by graphene oxide: Insights from extracellular polymeric substances

Widespread use of nanomaterials raises concerns. The underlying mechanism by which graphene oxide (GO) nanoparticles causes poor settleability of activated sludge remains unclear. To explore this mechanism, three reactors with different GO concentrations were established. Extended Derjaguin-Landau-Verwey-Overbeek theory indicated that GO destroyed the property of extracellular polymeric substances (EPS), increasing the energy barrier between bacteria. Low levels of uronic acid and hydrogen bonding in exopolysaccharide weakened the EPS gelation increasing aggregation repulsion. Lager amounts of hydrophilic amino acid and looser structure of extracellular proteins for exposing inner hydrophilic groups significantly contributed to the hydrophilicity of EPS. Both changes implied deterioration in EPS structure under GO stress. Metagenome demonstrated a decrease in genes responsible for capsular polysaccharide colonization and genes regulated the translocation of loose proteins were increased, which increased repulsion between bacteria. This study elucidated that changes in EPS secretion under GO exposure are the underlying causes of poor settleability.

Biological performance and membrane fouling of a microalgal-bacterial membrane photobioreactor for wastewater treatment without external aeration and carbonation

Biological nutrient removal processes involving the use of activated sludge (AS) to treat municipal wastewater normally result in high aeration energy consumption and significant greenhouse gas (GHG) emissions. Therefore, developing cost-efficient and environmentally friendly processes for wastewater treatment is vital. In this work, a novel non-aerated microalgal-bacterial membrane photobioreactor (MB-MPBR) was proposed, and its feasibility for organic contaminant and nutrient removals was evaluated, for the first time. The effects of inoculation ratio (microalgae to bacteria (M/B)) on the biological performance and membrane fouling were systematically investigated. The results showed that 95.9% of the chemical oxygen demand (COD), 74.5% of total nitrogen (TN), 98.5% of NH4+-N and 42.0% of total phosphorus (TP) were removed at an inoculation M/B ratio of 3:2 at steady state, representing a significant improvement compared to the M/B inoculation ratio of 1:3. Additionally, the higher inoculation M/B ratio (3:2) significantly promoted the biomass production owing to the favorable mutual exchange of oxygen and carbon dioxide between microalgae and bacteria. Cake layer formation was the primary fouling mechanism owing to the absence of aeration scouring on the membrane surface. The membrane fouling rate was slightly higher at the higher inoculation ratio (M/B = 3:2) owing to the increased biomass and extracellular polymeric substances (EPS) productions, despite the larger particle size. These results demonstrated that the non-aerated MB-MPBR could achieve superior biological performance, of which the inoculation M/B ratio was of critical importance for the initiation and maintenance of microalgal-bacterial symbiotic system, yet possibly caused severer membrane fouling in the absence of external aeration and carbonation. This study provides a new perspective for further optimizing and applying non-aerated MB-MPBR to enhance municipal wastewater treatment.

The enrichment characterisation of Nitrospira under high DO conditions

Nitrite-oxidizing bacteria (NOB) are crucial to nitrification and nitrogen elimination in wastewater treatment. Mass reports exist on the links between NOB and other microorganisms, for instance, ammonia-oxidizing bacteria (AOB). However, a few studies exist on the enrichment characterisation of NOB under high dissolved oxygen (DO) conditions. In this study, NOB was designed to be enriched individually under high DO conditions in a continuous aeration sequencing batch reactor (SBR), and the kinetic characterisation of NOB was evaluated. The analysis revealed that the average NO2--N removal rate was steady above 98%, with DO and NO2--N being 3-5 mg L-1 and 50-450 mg L-1, respectively. The NO2--N removal efficiency of the system was significantly enhanced and better than in other studies. The high-throughput sequencing suggested that Parcubacteria_ genera_incertae_sedis was the first dominant genus (21.99%), which often appeared in the NOB biological community with Nitrospira. However, the dominant genus NOB was Nitrospira rather than Nitrobacter (8.49%). This result suggested that Nitrospira was capable of higher NO2--N removal. But lower relative abundance indicated that excessive NO2--N had an adverse effect on the enrichment and activity of Nitrospira. In addition, the nitrite half-saturation constant (KNO2) and the oxygen half-saturation constant (KO) were 1.71 ± 0.19 mg L-1 and 0.95 ± 0.10 mg L-1, respectively. These results showed that the enriched Nitrospira bacteria had different characteristics at the strain level, which can be used as a theoretical basis for wastewater treatment plant design and optimisation.

Lumen air pressure regulated multifunctional microbiotas in membrane-aerated biofilm reactors for simultaneous nitrogen removal and antibiotic elimination from aquaculture wastewater

In this study, two membrane-aerated biofilm reactors (MABRs) were constructed: one solely utilizing biofilm and another hybrid MABR (HMABR) incorporating both suspended-sludge and biofilm to treat low C/N aquaculture wastewater under varying lumen air pressure (LAP). Both HMABR and MABR demonstrated superior nitrogen removal than conventional aeration reactors. Reducing LAP from 10 kPa to 2 kPa could enhance denitrification processes without severely compromising nitrification, resulting in an increase in total inorganic nitrogen (TIN) removal from 50.2±3.1 % to 71.6±1.0 %. The HMABR exhibited better denitrification efficacy than MABR, underscoring its potential for advanced nitrogen removal applications. A decline in LAP led to decreased extracellular polymeric substance (EPS) production, which could potentially augment reactor performance by minimizing mass transfer resistance while maintaining microbial matrix stability and function. Gene-centric metagenomics analysis revealed decreasing LAP impacted nitrogen metabolic potentials and electron flow pathways. The enrichment of napAB at higher LAP and the presence of complete ammonia oxidation (Comammox) Nitrospira at lower LAP indicated aerobic denitrification and Comammox processes in nitrogen removal. Multifunctional microbial communities developed under LAP regulation, diversifying the mechanisms for simultaneous nitrification-denitrification. Increased denitrifying gene pool (narGHI, nirK, norB) and enzymatic activity at a low LAP can amplify denitrification by promoting denitrifying genes and electron flow towards denitrifying enzymes. Sulfamethoxazole (SMX) was simultaneously removed with efficiency up to 80.2 ± 3.7 %, mainly via biodegradation, while antibiotic resistome and mobilome were propagated. Collectively, these findings could improve our understanding of nitrogen and antibiotic removal mechanisms under LAP regulation, offering valuable insights for the effective design and operation of MABR systems in aquaculture wastewater treatment.

Nitrogen removal efficiency and mechanisms of an improved anaerobic-anoxic-oxic system for decentralized sewage treatment

The unstable operation and poor effluent quality often associated with decentralized sewage treatment systems due to fluctuating water flows have garnered significant attention. In this study, a novel integrated process combining anoxic denitrification and simultaneous nitrification and denitrification was developed to address these challenges. The improved anaerobic-anoxic-aerobic system achieved average effluent concentrations of 20.83 mg/L and 4.63 mg/L for chemical oxygen demand and NH4+-N, with average removal rates of 91 % and 68 %, respectively. Moreover, the aerobic zone demonstrated an impressive efficiency of 40.8 % for simultaneous nitrification and denitrification. The key bacteria groups driving the system's performance were heterotrophic and aerobic nitrifying bacteria, which dominated the microbial populations. Overall, the system optimizes the traditional anaerobic-anoxic-aerobic process, providing an effective solution for fluctuating wastewater flows. It establishes a successful coexistence model for multiple microbial populations, highlighting its applicability for superior nitrogen removal performance, and reference for optimizing rural sewage treatment. TAKE HOME MESSAGE SENTENCE: The improved anaerobic-anoxic-aerobic system for fluctuating wastewater treatment has superior nitrogen removal performance depending on multiple microbial populations.

Feast/famine ratio regulates the succession of heterotrophic nitrification-aerobic denitrification and autotrophic ammonia oxidizing bacteria in halophilic aerobic granular sludge treating saline wastewater

Heterotrophic nitrification-aerobic denitrification (HN-AD) shows innovation potential of wastewater treatment process in a single tank. However, how to enrich HN-AD bacteria in activated sludge to enhance their contribution remained unknown. This study explored the impact of the feast/famine (F/F) ratio on the succession of autotrophic ammonia oxidizing bacteria (AOB) and HN-AD bacteria in a halophilic aerobic granular sludge (HAGS) system. As the F/F ratio decreased from 1/9 to 1/15, the total inorganic nitrogen (TIN) removal performance significantly decreased. The proportion of heterotrophic bacteria was dropped from 79.0 % to 33 %. Accordingly, the relative abundance of Paracoccus decreased from 70.8 % to 25.4 %, and the copy number of the napA gene was reduced from 2.2 × 1010 copies/g HAGS to 8.1 × 109 copies/g HAGS. It found the F/F ratio regulated the population succession of autotrophic AOB and HN-AD bacteria, thereby providing a solution to achieve the enrichment of HN-AD bacteria in HAGS.

Optimization of moving bed biofilm reactors for the treatment of municipal wastewater

The properties of biocarriers significantly influence the performance of a moving bed-biofilm reactor (MBBR). This study aimed to assess the impact of media type, filling ratio, and hydraulic retention time (HRT) on biofilm formation and MBBR performance in both batch and continuous setups using real municipal wastewater. Two different media, high-density polyethylene (HDPE) and polypropylene (PPE), with varying surface area and properties were used. Biofilm growth and MBBR performance were monitored and optimized using response surface methodology. The effect of different media was investigated for three filling ratios of 20%, 40% and 60% and HRT of 4, 6 and 8 h. Results depicted a better biofilm growth on HDPE media in comparison to PPE carriers due to difference in media structure and surface properties. At all the conditions tested, HDPE media showed comparatively better performance for the removal of organic matter and nutrients than PPE media. The maximum organic matter removal efficiency was found as 77% and 75% at an HRT of 6 h and filling ratio of 40% for HDPE and PPE media, respectively. The ammonia removal was also found better for HDPE media due to its geometry and structure favoring the anoxic conditions with maximum removal of 89% achieved at 6-h HRT and 40% filling ratio. Overall, the system with HDPE media indicated more stability in terms of reactor performance than PPE carriers with variations in the operating conditions.

Achieving partial nitrification: A strategy for washing NOB out under high DO condition

This study investigated the effect of high DO concentrations on PN. The experimental setup involved operating at high DO concentrations (1.5-2.5 mg/L) and environmental temperatures (15-20 °C) over a period of 180 days. Through a sludge enrichment process, the kinetic parameters of ammonia-oxidizing bacteria (AOB) and nitrite-oxidizing bacteria (NOB) were determined. Surprisingly, contrary to conventional reports, it was observed that NOB exhibited a stronger affinity for DO compared to AOB. As a result, high DO concentrations were necessary to provide favorable conditions for the growth of AOB. In order to establish PN, strategies including intermittent aeration, free ammonia (FA), and controlled sludge retention time (SRT) were employed. The successful PN was achieved with a specific ammonia oxidation rate of 24 mg N/g MLVSS/h and a specific nitrite oxidation rate below 0.10 mg N/g MLVSS/h. The nitrite accumulation rate (NAR) was maintained at 100% during stable operation. The abundance of Nitrosomonas, a typical genus of AOB, was found to be 68.62%, which surpasses previous studies in similar research. A slightly higher DO concentrations may increase energy consumption, but achieve higher efficiency and stability in PN. This study provided new insights into the application of PN in wastewater treatment.

Effect of dissolved oxygen concentration on performance and mechanism of simultaneous nitrification and denitrification in integrated fixed-film activated sludge sequencing batch reactors

Integrated fixed-film activated sludge (IFAS) is a superior system for achieving simultaneous nitrification and denitrification (SND), however, the impact of dissolved oxygen (DO) has not been fully elucidated. Therefore, this study investigated the effect of DO concentration on performance and mechanism of SND in IFAS system. Results showed that IFAS outperformed control systems and achieved optimal SND performance at a DO concentration of 0.5 mg/L, with an SND efficiency of 88.51% and total nitrogen removal efficiency of 82.78%. Typical cycles analysis demonstrated limited-DO promoted SND performance. Further analysis implied biofilms exhibited high biomass and denitrification activity with decreasing DO. Microbial community analysis revealed low DO concentrations were responsible for abundant functional groups and genes associated with SND and promoted unconventional nitrogen removal pathways. Moreover, co-occurrence network analysis elucidated microbial interactions, responses to DO, and keystone genera. This study helps understanding the roles of DO for enhanced SND in IFAS.

Enhancement strategies of aerobic denitrification for efficient nitrogen removal from low carbon-to-nitrogen ratio shale oil wastewater

The strategy of high reflux ratio and long solids retention time was adopted to realize efficient nitrogen removal from real shale oil wastewater. This was undertaken with a low chemical oxygen demand to total nitrogen (COD/TN) ratio by strengthening aerobic denitrification in an anoxic/aerobic membrane bioreactor (A/O-MBR). The TN removal load climbed from 22 to 25 g N/(kg MLSS·d) as the COD/TN ratio declined from 8 to 3. The abundance of heterotrophic nitrifying and aerobic denitrifying (HNAD) bacteria increased by 13.8 times to 42.5%, displacing anoxic denitrifying bacteria as the predominant bacteria. The abundance of genes involved in denitrification (napAB, narGHI, norBC, nosZ) increased, however the genes related to assimilatory nitrate reduction (nirA, narB, nasC) decreased. The capacity of the dominant HNAD bacteria in an A/O-MBR to efficiently utilize a carbon source is the key to efficient nitrogen removal from shale oil wastewater with a low COD/TN ratio.

Spatiotemporal evolution of dissolved organic matter (DOM) and its response to environmental factors and human activities

The South-to-North Water Diversion East Project (SNWDP-E) is an effective way to realize the optimal allocation of water resources in China. The North Dasha River (NDR) is the reverse recharge section that receives water from the Yufu River to the Wohushan Reservoir transfer project line in the SNWDP. However, the dissolved organic matter (DOM) evolution mechanism of seasonal water transfer projects on tributary waters has not been fully elucidated. In this paper, the NDR is the main object, and the changes in the composition and distribution of spectral characteristics during the winter water transfer period (WT) as well as during the summer non-water transfer period (NWT) are investigated by parallel factor analysis (PARAFAC). The results showed that the water connectivity caused by water transfer reduces the environmental heterogeneity of waters in the basin, as evidenced by the ammonia nitrogen (NH4+-N) and total phosphorus (TP) in the water body were significantly lower (p<0.05, p<0.01) during the water transfer period than the non-water transfer period. In addition, the fluorescence intensity of DOM was significantly lower in the WT than the NWT (p<0.05) and was mainly composed of humic substances generated from endogenous sources with high stability. While the NWT was disturbed by anthropogenic activities leading to significant differences in DOM composition in different functional areas. Based on the redundancy analysis (RDA) and multiple regression analysis, it was found that the evolution of the protein-like components is dominated by chemical oxygen demand (COD) and NH4+-N factors during the WT. While the NWT is mainly dominated by total nitrogen (TN) and TP factors for the evolution of the humic-like components. This study helps to elucidate the impact of water transfer projects on the trunk basin and contribute to the regulation and management of inter-basin water transfer projects.

Nitrogen removal performance of aerobic denitrifying bacteria enhanced by an iron-anode pulsed electric field

Pulsed electric field (PEF) technology has attracted considerable attention because it can efficiently treat pollutants that are difficult to degrade. In this study, a PEF system using iron as the electrode was constructed to investigate the effect of PEF-Fe on the growth and metabolism of aerobic denitrifying bacteria and the effectiveness of wastewater nitrogen removal. The chemical oxygen demand, NO3--N and nitrate removal rates were 98.93%, 97.60% and 24.40 mg·L-1·h-1, respectively, under optimal conditions. As confirmed in this study, PEF-Fe could improve the key enzyme activities of W207-14. Scanning electron microscopy revealed that the surface of PEF-Fe-treated W207-14 was intact and smooth without any irreversible deformation. Flow cytometry combined with fluorescence staining analysis also confirmed reversible electroporation on the cell membrane surface of PEF-Fe-treated W207-14. Differentially expressed gene enrichment analysis showed that PEF-Fe activated the transmembrane transport function of ATP-binding cassette transporte (ABC) transport proteins and enhanced the cell membrane permeability of aerobic denitrifying bacteria. The significant differential expression of iron-sulphur cluster proteins facilitated the regulation of electron transport and maintenance of the dynamic balance of iron ions within the PEF-Fe system.

Acceleration of nitrogen removal performance in a biofilm reactor augmented with Pseudomonas sp. using polycaprolactone as carbon source for treating low carbon to nitrogen wastewater

Heterotrophic nitrification-aerobic denitrification (HN-AD) process was achieved in a moving bed biofilm reactor after 180-days acclimation using PCL as carbon source for low C/N wastewater treatment. A novel HN-AD strain, JQ-H3, with ability of PCL degradation was augmented to improve nitrogen removal. TN removal efficiencies of 82.31%, 90.05%, and 93.16% were achieved in the augmented reactor (R2), at different HRTs of 24 h, 20 h, and 16 h, while in the control reactor (R1), the TN removal efficiencies were 59.24%, 74.61%, and 76.68%. The effluent COD in R2 was 10.17 mg/L, much lower than that of 42.45 mg/L in R1. Microbial community analysis revealed that JQ-H3 has successfully proliferated with a relative abundance of 4.79%. Relative abundances of functional enzymes of nitrogen cycling remarkably increased due to bioaugmentation based on the analysis of PICRUSt2. This study provides a new approach for enhancing nitrogen removal in low C/N sewage treatment via the HN-AD process.

Enhanced aquaculture wastewater treatment in a biofilm reactor filled with sponge/ferrous oxalate/biochar composite (Sponge-C2FeO4@NBC) biocarriers: Performance and mechanism

Fabricating low-cost and efficient biofilm carriers for moving bed biofilm reactors in wastewater treatment is crucial for achieving environmental sustainability. Herein, a novel sponge biocarrier doped with NaOH-loaded biochar and nano ferrous oxalate (sponge-C₂FeO₄@NBC) was prepared and evaluated for nitrogenous compounds removal from recirculating aquaculture systems (RAS) wastewater by stepwise increasing ammonium nitrogen (NH₄⁺-N) loading rates. The prepared NBC, sponge-C₂FeO₄@NBC, and matured biofilms were characterized using SEM, FTIR, BET, and N₂ adsorption-desorption techniques. The results reveal that the highest removal rates of NH₄⁺-N reached 99.28 ± 1.3% was yielded by the bioreactor filled with sponge-C₂FeO₄@NBC, with no obvious nitrite (NO₂^--N) accumulation in the final phase. The reactor packed with sponge-C₂FeO₄@NBC biocarrier had the highest relative abundance of functional microorganisms responsible for nitrogen metabolism than in the control reactor, confirmed from 16S rRNA gene sequencing analysis. Our study provides new insights into the newly developed biocarriers for enhancing RAS biofilters treatment performance in keeping water quality within the acceptable level for the rearing of aquatic species.

Enhanced nitrogen removal in immersed rotating self-aerated biofilm reactor: nitrogen removal pathway and microbial mechanism

To achieve energy-efficient treatment of the rural wastewater with satisfying performance, a novel immersed rotating self-aerated biofilm reactor (iRSABR) was proposed in this study. The iRSABR system showed better biofilm renewal performance and higher microbial activity. The effect of different regulation strategies on the iRSABR system was investigated in this study. The 70% immersion ratio and 4 r/min rotation speed (stage III) exhibited the best performance, with a total nitrogen removal efficiency of 86% and a simultaneous nitrification-denitrification (SND) rate of 76%, along with the highest electron transport system activity. The nitrogen removal pathway revealed that the SND was achieved through autotrophic/heterotrophic nitrification and aerobic/anoxic denitrification. The regulation strategy in the iRSABR system established a synergistic microbial community with main functional bacteria of nitrification (Nitrosomonas), anoxic denitrification (Flavobacterium, Pseudoxanthomonas), and aerobic denitrification (Thauera). This study highlighted the feasibility and adaptability of the iRSABR system toward energy-efficient rural wastewater treatment.

Nitrogen removal by algal-bacterial consortium during mainstream wastewater treatment: Transformation mechanisms and potential N2O mitigation

This work investigated nitrogen transformation pathways of the algal-bacterial consortium as well as its potential in reducing nitrous oxide (N₂O) emission in enclosed, open and aerated reactors. The results confirmed the superior ammonium removal performance of the algal-bacterial consortium relative to the single algae (Chlorella vulgaris) or the activated sludge, achieving the highest efficiency at 100% and the highest rate of 7.34 mg N g MLSS^-1 h^-1 in the open reactor with glucose. Enhanced total nitrogen (TN) removal (to 74.6%) by the algal-bacterial consortium was achieved via mixotrophic algal assimilation and bacterial denitrification under oxygen-limited and glucose-sufficient conditions. Nitrogen distribution indicated that ammonia oxidation (∼41.8%) and algal assimilation (∼43.5%) were the main pathways to remove ammonium by the algal-bacterial consortium. TN removal by the algal-bacterial consortium was primarily achieved by algal assimilation (28.1-40.8%), followed by bacterial denitrification (2.9-26.5%). Furthermore, the algal-bacterial consortium contributed to N₂O mitigation compared with the activated sludge, reducing N₂O production by 35.5-55.0% via autotrophic pathways and by 81.0-93.6% via mixotrophic pathways. Nitrogen assimilation by algae was boosted with the addition of glucose and thus largely restrained N₂O production from nitrification and denitrification. The synergism between algae and bacteria was also conducive to an enhanced N₂O reduction by denitrification and reduced direct/indirect carbon emissions.