Enhanced simultaneous nitrogen and phosphorus removal from low COD/TIN domestic wastewater through nitritation-denitritation coupling improved anammox process with an optimal Anaerobic/Oxic/Anoxic strategy

New insights on destruction mechanisms of waste activated sludge during simultaneous thickening and digestion process via forward osmosis membrane

This study delved into the efficacy of sludge digestion and the mechanisms involved in sludge destruction during the implementation of forward osmosis process for sludge thickening and digestion (FO-MSTD). Utilizing a lab-scale FO membrane reactor for the thickening and digestion of waste activated sludge (WAS), the investigation explored the effects of sludge thickening and digestion in FO-MSTD processes using draw solutions of varying concentrations. The findings underscored the significance of hydraulic retention time (HRT) as a pivotal parameter influencing the swift thickening or profound digestion of sludge. Consequently, tailoring the HRT to specific processing objectives emerged as a key strategy for achieving desired treatment outcomes. In the investigation, the use of a 1 M NaCl draw solution in the FO-MSTD process showcased enhanced thickening and digestion capabilities. This specific setup raised the concentration of mixed liquor suspended solids (MLSS) to over 30 g/L and achieved a 42.7% digestion efficiency of mixed liquor volatile suspended solids (MLVSS) within an operational timeframe of 18 days. Furthermore, the research unveiled distinct stages in the sludge digestion process of the FO-MSTD system, characterized by fully aerobic digestion and aerobic-local anaerobic co-existing digestion. In the fully aerobic digestion stage, the sludge digestion rate exhibited a steady increase, leading to the breakdown of sludge floc structures and the release of a substantial amount of nutrients into the sludge supernatant. The predominant microorganisms during this stage were typical functional microorganisms found in wastewater treatment systems. Transitioning into the aerobic-local anaerobic co-existing digestion stage, both MLSS concentration and MLVSS digestion efficiency continued to rise, accompanied by a decreasing dissolved oxygen (DO) concentration. More organic matter was released into the supernatant, and sludge microbial flocs tended to reaggregate. The localized anaerobic environment within the FO-MSTD reactor fostered an increase in the relative abundance of bacteria with nitrogen and phosphorus removal functions, thereby positively impacting the mitigation of total nitrogen (TN) and total phosphorus (TP) concentrations in the sludge supernatant. The results of this research enhance comprehension of the advanced FO-MSTD technology in the treatment of WAS.

Robustness of the anammox process at low temperatures and low dissolved oxygen for low C/N municipal wastewater treatment

Low water temperatures and ammonium concentrations pose challenges for anammox applications in the treatment of low C/N municipal wastewater. In this study, a 10 L-water bath sequencing batch reactor combing biofilm and suspended sludge was designed for low C/N municipal wastewater treatment. The nitrogen removal performance via partial nitrification anammox-(endogenous) denitrification anammox process was investigated with anaerobic-aerobic-anoxic mode at low temperatures and dissolved oxygen (DO). The results showed that with the decrease of temperature from 30 to 15℃, the influent and effluent nitrogen concentrations and nitrogen removal efficiencies were 73.7 ± 6.5 mg/L, 7.8 ± 2.8 mg/L, and 89.4 %, respectively, with aerobic hydraulic retention time of only 6 h and DO concentration of 0.2-0.5 mg/L. Among that, the stable anammox process compensated for the inhibitory effects of the low temperatures on the nitrification and denitrification processes. Notably, from 30 to 15℃, the anammox activity and relative abundance of the dominant Brocadia genus were increased from 39.7 to 45.5 mgN/gVSS/d and 7.3 to 12.0 %, respectively; the single gene expression level of the biofilm increased 9.0 times. The anammox bacteria showed a good adaptation to temperatures reduction. However, nitrogen removal by anammox was not improved by increasing DO (≥ 4 mg/L) at 8-4℃. Overall, the results of this study demonstrate the feasibility of the mainstream anammox process at low temperatures.

Achieving ultra-high nitrogen and phosphorus removal from real municipal wastewater in a novel continuous-flow anaerobic/aerobic/anoxic process via partial nitrification, endogenous denitrification and nitrite-type denitrifying phosphorus removal

Achieving economic and efficient removal of nutrients in mainstream wastewater treatment plants (WWTPs) continues to be a challenging research topic. In this study, a continuous-flow anaerobic/aerobic/anoxic system with sludge double recirculation (AOA-SDR), which integrated partial nitrification (PN), endogenous denitrification (ED) and nitrite-type denitrifying phosphorus removal (nDNPR), was constructed to treat real carbon-limited municipal wastewater. The average effluent concentrations of total inorganic nitrogen (TIN) and PO43--P during the stable operation period were 1.8 and 0.3 mg/L, respectively. PN was achieved with an average nitrite accumulation ratio of 90.4 % by combined strategies. Adequate storage of polyhydroxyalkanoates and glycogen in the anaerobic zone promoted the subsequent nitrogen removal capacity. In the anoxic zone, nitrite served as the main electron acceptor for the denitrifying phosphorus removal process. Mass balance analysis revealed that nDNPR contributed to 23.6 % of TIN removal and 44.7 % of PO43--P removal. The enrichment of Nitrosomonas (0.45 %) and Ellin 6067 (1.31 %), along with the washout of Nitrospira (0.15 %) provided the bacterial basis for the successful implementation of PN. Other dominant endogenous heterotrophic bacteria, such as Dechlormonas (10.81 %) and Candidatus Accumulibacter (2.96 %), ensured simultaneous nitrogen and phosphorus removal performance. The successful validation of integrating PN, ED and nDNPR for advanced nutrient removal in the AOA-SDR process provides a transformative technology for WWTPs.

Tracing and utilizing nitrogen loss in wastewater treatment: The trade-off between performance improvement, energy saving, and carbon footprint reduction

Biological nitrogen removal is widely applied to reduce the discharge of inorganic nitrogen and mitigate the eutrophication of receiving water. However, nitrogen loss is frequently observed in wastewater treatment systems, yet the underlying principle and potential enlightenment is still lacking a comprehensive discussion. With the development and application of novel biological technologies, there are increasing achievement in the deep understanding and mechanisms of nitrogen loss processes. This article reviews the potential and novel pathways of nitrogen loss, occurrence mechanisms, influential factors, and control strategies. A survey of recent literature showed that 3%∼73% of nitrogen loss beyond the nitrogen budget can be ascribed to the unintentional presence of simultaneous nitrification/denitrification, partial nitrification/anammox, and endogenous denitrification processes, under low dissolved oxygen (DO) and limited available organic carbon source at aerobic conditions. Key influential parameters, including DO, aeration strategies, solid retention time (SRT), hydraulic retention time (HRT), temperature and pH, significantly affect both the potential pathways of nitrogen loss and its quantitative contribution. Notably, the widespread and spontaneous growth of anammox bacteria is an important reason for ammonia escape at anaerobic/anoxic conditions, leading to 7%∼78% of nitrogen loss through anammox pathway. Moreover, the unwanted nitrous oxide (N2O) emission should also be considered as a key pathway in nitrogen loss. Future development of new nitrogen removal technologies is proposed to suppress the generation of harmful nitrogen losses and reduce the carbon footprint of wastewater treatment by controlling key influential parameters. Transforming "unintentional observation" to "intentional action" as high-efficiency and energy-efficient nitrogen removal process provides a new approach for the development of wastewater treatment.

Advanced nitrogen removal from municipal wastewater through partial nitrification-denitrification coupled with anammox in step-feed continuous system

Combined partial nitrification-denitrification/anammox (PN-PD/A) processes have attracted great attention from researchers in recent years to achieve high nitrogen removal from low carbon /nitrogen (C/N) municipal wastewater. In this context, a step-feed anoxic/oxic (A/O) process was conducted in this study through the combination of the partial nitrification-anammox (PN/A) and partial denitrification-anammox (PD/A) to remove N from municipal wastewater with low C/N. The enhancement of the PN-PD/A process resulted in N removal efficiency of 85.6% at C/N of 2.8. The contributions of the anammox reached 36.4 and 8.8% in the anoxic and oxic chambers, respectively. The biocarriers added to the anoxic and oxic chambers increased the relative abundance of the anammox bacteria in biofilms from 0.61% to 1.51% and 1.02%, respectively. This study demonstrated that employing the step-feed A/O process can create optimal conditions for the anammox bacteria growth, thereby ensuring advanced N removal from low C/N municipal wastewater.

Carbon-Restricted Anoxic Zone as an Overlooked Anammox Hotspot in Municipal Wastewater Treatment Plants

The anoxic zone serves as the core functional unit in municipal wastewater treatment plants (MWWTPs). Unfortunately, in most cases, the downstream range of the anoxic zone is severely lacking in available organic carbon and thus contributes little to the removal of nutrients. This undesirable range is termed the "carbon-restricted anoxic zone", representing an insurmountable drawback for traditional MWWTPs. This study uncovers a previously overlooked role for the carbon-restricted anoxic zone: a hotspot for anaerobic ammonium oxidation (anammox). In a continuous-flow pilot-scale plant treating municipal wastewater (55 m3/d), virgin biocarriers were introduced into the carbon-restricted anoxic zone (downstream 25% of the anoxic zone with BOD5 of 5.9 ± 2.3 mg/L). During the 517-day monitoring, anammox bacteria highly self-enriched within the biofilms, with absolute and relative abundance reaching up to (9.4 ± 0.1) × 109 copies/g-VSS and 6.17% (Candidatus Brocadia), respectively. 15N isotopic tracing confirmed that anammox overwhelmingly dominated nitrogen metabolism, responsible for 92.5% of nitrogen removal. Following this upgrade, the contribution ratio of the carbon-restricted anoxic zone to total nitrogen removal increased from 9.2 ± 4.1% to 19.2 ± 4.2% (P < 0.001), while its N2O emission flux decreased by 84.5% (P < 0.001). These findings challenge stereotypes about the carbon-restricted anoxic zone and highlight the multiple environmental implications of this newfound anammox hotspot.

Non-filamentous sludge bulking induced by exopolysaccharide variation in structure and properties during aerobic granulation

The forming mechanism of non-filamentous sludge bulking during aerobic granulation were investigated basing on three feeding strategies (R1 direct aeration after fast feeding, R2 anaerobic stirring after fast feeding and R3 anaerobic plug-flow slow feeding). Results showed that strong selection stress (shortening settling time) led to a sharp flocs washout and the subsequent increase of food to microorganisms (F/M) in R1 and R3 reactors, but not found in R2 due to the different strategies of feeding modes. With the increase of F/M, zeta potential and hydrophobicity of sludge surfaces significantly decreased and thus leading to an enhanced repulsive force and energy barriers for sludge aggregation. Particularly, when F/M exceeded 1.2 kgCOD/(kgMLSS·d), non-filamentous sludge bulking was ultimately triggered in R1 and R3. Further analysis showed that massive extracellular exopolysaccharide (PS) accumulated on the surfaces of non-filamentous bulking sludge due to the increased abundance of the microorganisms related to PS secretion during sludge bulking. In addition, significantly increased intracellular second messenger (c-di-GMP), a key substance regulating PS biosynthesis, was confirmed via its concentration determination as well as microbial function prediction analysis, which played a critical role in sludge bulking. Combing with the systematic detection from surface plasmon resonance system, rheometer and size-exclusion chromatography-multiangle laser light detection-refractive index system, higher molecular weight, compact conformation, higher viscosity and higher hydrophilicity was determined in sludge bulking PS relative to PS extracted from non-filamentous bulking sludge. Clearly, the changes of PS (content, structures and properties) driven by c-di-GMP are the dominant mechanism for the formation of non-filamentous sludge bulking during aerobic granulation. This work could provide theoretical support for successful start-up and application of aerobic granular sludge technology.

Changes of nitrogen and phosphorus removal pattern caused by alternating aerobic/anoxia from the perspective of microbial characteristics

The purpose of this study was to compare the impact of different numbers of alternating aerobic/anoxic (A/O) cycles on pollutant removal. Three sequential batch reactors (SBRs) with varying numbers of alternating A/O cycles were established. Under the tertiary anoxic operating conditions, the removal efficiencies of chemical oxygen demand (COD), ammonia nitrogen (NH₄⁺-N), total nitrogen (TN), and total phosphorus (TP) were 88.73%, 89.56%, 72.15%, and 77.61%, respectively. Besides, alternating A/O affected the dominant microbial community relative abundance (Proteobacteria and Bacteroidetes) and increased microbial richness and diversity. It also increased the relative abundance of aerobic denitrifying, heterotrophic nitrifying, and denitrifying phosphorus removal bacteria to change N and P removal patterns. Furthermore, the abundance of carbohydrate metabolism and amino acid metabolism was improved by alternating A/O to improve organic matter and TN removal.

Start-up and maintenance of indigenous microalgae-bacteria consortium treating toilet wastewater through partial nitrification and nitrite-type denitrification

Microalgae-bacteria consortium (MBC) provides an alternative to sustainable treatment of human toilet wastewater (TWW) and resource recovery. This study compared the conventional activated sludge system and wastewater indigenous MBC system (IMBC) for nitrogen removal in TWW through the coupled partial nitrification (PN) and nitrite-type denitrification process. PN was firstly established by alternating FA and FNA. Subsequently, the successful PN maintenance with the nitrite accumulation rate ranging between 90.1-95.3% was achieved using two strategies: light irradiation with the appropriate specific light energy density at 0.0188-0.0598 kJ/mg VSS and the timely nitrite-type denitrification with the algae-secreted organics as the carbon source, eventually resulting in the nitrite accumulation rate ranging between 90.1-95.3%. In the IMBC-PN system, bacterial metabolism contributed to 91.5% of nitrogen removal and the rest was through microalgal assimilation. This study offers a sustainable hybrid IMBC-PN process for high NH₄⁺-N strength wastewater treatment (e.g., TWW), which theoretically saves 23.5% aeration and 34.2% carbon source as well as reduces 17.0% sludge production.

Simultaneous removal of ammonia nitrogen, recovery of phosphate, and immobilization of nickel in a polyester fiber with shell powder and iron carbon spheres bioreactor: Optimization and pathways mechanism

Composite pollutants are prevalent in wastewater, whereas, the simultaneous accomplishment of efficient nitrogen removal and resources recovery remains a challenge. In this study, a bioreactor was constructed to contain Pseudomonas sp. Y1 using polyester fiber wrapped with shell powder and iron carbon spheres, achieving ammonia nitrogen (NH₄⁺-N) removal, phosphate (PO₄^3--P) recovery, and nickel (Ni²⁺) immobilization. The optimal performance of bioreactor was average removal efficiencies of NH₄⁺-N, PO₄^3--P, calcium (Ca²⁺), and Ni²⁺ as 82.42, 96.67, 76.13, and 98.29% at a hydraulic retention time (HRT) of 6 h, pH of 7.0, and influent Ca²⁺ and Ni²⁺ concentrations of 100.0 and 3.0 mg L^-1, respectively. The bioreactor could remove PO₄^3--P, Ca²⁺, and Ni²⁺ by biomineralization, co-precipitation, adsorption, and lattice substitution. Moreover, microbial community analysis suggested that Pseudomonas was the predominant genus and had possessed tolerance to Ni²⁺ toxicity in wastewater. This study presented an effective method to synchronously remove NH₄⁺-N, recover PO₄^3--P, and fix heavy metals through microbially induced carbonate precipitation (MICP) and heterotrophic nitrification and aerobic denitrification (HNAD) technology.

Limited dissolved oxygen facilitated nitrogen removal at biocathode during the hydrogenotrophic denitrification process using bioelectrochemical system

Effects of limited dissolved oxygen (DO) on hydrogenotrophic denitrification at biocathode was investigated using bioelectrochemical system. It was found that total nitrogen removal increased by 5.9%, as DO reached about 0.24 mg/L with the cathodic chamber unplugged (group R_Exposure). With the presence of limited DO, not only the nitrogen metabolic pathway was influenced, but the composition of microbial communities of ammonia-oxidizing bacteria and nitrite-oxidizing bacteria were enriched accordingly. After metagenomic analysis, enriched genes in R_Exposure were found to be associated with nearly each of nitrogen removal steps as denitrification, nitrification, DNRA, nitrate assimilation and even nitrogen fixation. Moreover, genes encoding both Complexes III and IV constituted the electron transfer chain were significantly enriched, indicating that more electrons would be orientated to the reduction of NO₂^--N, NO-N and oxygen. Therefore, enhanced nitrogen removal could be attained through the co-respiration of nitrate and oxygen by means of NH₄⁺-N oxidation.

Achieving stable anammox process and revealing shift of bacteria during the start-up in landfill leachate treatment

Partial nitrification-anammox (PN/A) is an energy-efficient technology for nitrogen removal in landfill leachate treatment. Numerous studies have reported successful implementation of the PN/A process and its stable operation under laboratory conditions. One of the primary challenges in PN/A engineering applications is the mass of the seed sludge required for start-up. This study examined the PN/A using a sequence batch reactor (SBR) inoculating a common mixture to treat landfill leachate. After 70 days of operation, the system successfully realized a one-stage PN/A process and maintained a stable ammonium NH 4 + $$ \left({NH}_4^{+}\right) $$ removal efficiency of 97.65% ± 1%, where the effluent of NH 4 + $$ {NH}_4^{+} $$ and nitrate ( NO 3 - $$ {NO}_3^{-} $$ ) were less than 4 ± 1.5 mg L^-1 and 10 mg L^-1 . In addition, the relative abundances of Ca. Kuenenia and Ca. Brocadia, which are typical anaerobic ammonia-oxidizing bacteria (AnAOB), increased from 0.08% to 3.99% (70 days) and 0.01% to 0.45%, respectively. The relative abundances of ammonia-oxidizing bacteria (AOB) Nitrosomonas and Nitrosospira increased from 0.9% to 2.89% and 0.007% to 0.1% (70 days), respectively. Both AnAOB and AOB are important niches of the system. PRACTITIONER POINTS: The research realized PN/A rapidly by inoculating common mixture sludge. The experiment successfully enriched AnAOB from 0.09% to 3.89% within 70 days. The article revealing the ecological roles of AOB and AnAOB in the landfill leachate treatment.

Robust nitrogen removal from municipal wastewater by partial nitrification anammox at ultra-low dissolved oxygen in a pure biofilm system

Efficient nitrogen removal from municipal wastewater applying a pure biofilm system has promise. In this study, a partial nitrification anammox (PNA) pure biofilm system was established in a sequencing batch reactor with anaerobic/oxic/anoxic operation; using this reactor, robust nitrogen removal from municipal wastewater at ambient temperature was achieved with a nitrogen removal efficiency (NRE) of 93.3 %. Partial nitrification with anammox could be coupled at dominant nitrite-oxidizing bacteria (NOB) abundance by controlling ultra-low dissolved oxygen (<0.1 mg/L) in the aerobic section where the contribution to nitrogen removal was 79.4 %. Microorganisms with different oxygen affinity spatially distributed on the carrier. Ammonia-oxidizing bacteria (AOB) dominated on the surface of the carrier, while anammox bacteria dominated on the interior of the carrier, with their relative abundance increasing from 0.26 % to 1.78 %. The intercalary NOB were inhibited by the restricted oxygen transfer. Overall, this study provides a new approach to realize PNA in biofilm system.

Rapid enrichment of anammox bacteria and transformation to partial denitrification/anammox with nitrification/denitrification sludge

The stable nitrite (NO₂^--N) generation and rapid startup of anammox-based process are the main bottlenecks hindering its application in mainstream municipal wastewater treatment. In this study, a Partial-Denitrification (PD) system reducing nitrate (NO₃^--N) to NO₂^--N was rapidly developed within 40 days, using the nitrification/denitrification sludge from wastewater treatment plant. The NO₃^--N to NO₂^--N transformation ratios achieved 80.6 %. Significantly, a fast self-enrichment of anammox bacteria in this system was subsequently obtained, resulting in the successful transformation to an efficient PD/Anammox (PD/A) process after 79-day operation. The total nitrogen removal efficiency increased from 12.4 % to 90.0 % with influent ammonia and nitrate of 45.9 mg N/L and 62.2 mg N/L, corresponding to the anammox activity significantly increasing to 6.0 mgNH₄⁺-N/g VSS/h without seeding anammox sludge. Abundance of anammox increased from 6.7 × 10⁸ to 2.0 × 10¹¹ copies/g dry sludge. High-throughput sequencing results showed that Candidatus Brocadia was the only known anammox genus and accounted for 1.08 % during the PD/A stage. Functional bacteria for PD, assumed to be the Thauera, was enriched from 1.99 % to 60.06 % but decreased to 32.49 % during the improvement of anammox activity. It demonstrated that the PD system with stable NO₂^--N accumulation enabled a rapid self-enrichment of anammox bacteria and sufficient nitrogen removal with ordinary nitrification/denitrification sludge. This provides new insights into the scaling application of anammox by integrating PD with shortened startup periods and improved TN removal efficiency.

Effect of pistachio shell as a carbon source to regulate C/N on simultaneous removal of nitrogen and phosphorus from wastewater

Acid-pretreated pistachio shells were used as carbon sources to investigate the effects of carbon source dosage on simultaneous nitrogen and phosphorus removal under different carbon/nitrogen (C/N) ratios (7, 9, and 11). Results showed that C/N was positively correlated with mixed liquor suspended solids (MLSS) (R² = 0.998, p < 0.01) and f value (R² = 0.975, p < 0.05). Moreover, it was negatively correlated with the sludge volume index (SVI) (R² =  - 0.959, p < 0.05). C/N was also significantly negatively related to chemical oxygen demand removal rate (R² =  - 0.986, p < 0.05) and positively related to ammonia nitrogen (NH₄⁺-N), total nitrogen (TN), and total phosphorus (TP) removal rate (p < 0.05), the correlation coefficients were 0.992, 0.990 and 0.994, respectively. In the reactor with C/N of 11, the MLSS concentration and f value were the highest, the SVI was the lowest, and the removal efficiencies of NH₄⁺-N (85.49 % ± 1.96 %), TN (84.19 % ± 1.42 %) and TP (94.10 % ± 1.67 %) were the highest. Furthermore, the relative abundance of denitrifying bacteria was the highest in the reactor. The abundance of nitrifying bacteria and phosphorus-removal bacteria was also relatively high.

Simultaneous nitrogen and phosphorus removal from municipal wastewater by Fe(III)/Fe(II) cycling mediated partial-denitrification/anammox

The efficient removal of nitrogen and phosphorus remains challenging for traditional wastewater treatment. In this study, the feasibility for enhancing the partial-denitrification and anammox process by Fe (III) reduction coupled to anammox and nitrate-dependent Fe (II) oxidation was explored using municipal wastewater. The nitrogen removal efficiency increased from 75.5 % to 83.0 % by adding Fe (III). Batch tests showed that NH₄⁺-N was first oxidized to N₂ or NO₂^--N by Fe (III), then NO₃^--N was reduced to NO₂^--N and N₂ by Fe (II), and finally, NO₂^--N was utilized by anammox. Furthermore, the performance of phosphorus removal improved by Fe addition and the removal efficiency increased to 78.7 %. High-throughput sequencing showed that the Fe-reducing bacteria Pseudomonas and Thiobacillus were successfully enriched. The abundance of anammox bacterial increased from 0.03 % to 0.22 % by multiple nitrite supply pathways. Fe addition presents a promising pathway for application in the anammox process.

Advanced nitrogen removal from municipal sewage via partial nitrification-anammox process under two typical operation modes and seasonal ambient temperatures

A novel two-stage partial nitrification-anammox (PN-A) process was developed, achieving nitrogen removal from low carbon/nitrogen ratio municipal sewage under two typical operational modes and seasonal ambient temperatures. When complete nitritation-anammox was performed at temperatures greater than 19.4 °C, the effluent concentration of total inorganic nitrogen (TIN) was 4.1 mg/L, corresponding to a nitrogen removal efficiency (NRE) of 94.3 %. In contrast, when partial nitritation-anammox was performed at temperatures below 19.4 °C, the effluent TIN was 12.3 mg/L, corresponding to a NRE of 83.6 %. The relative abundance of Nitrosomonas and Nitrosomonadaceae increased from 0.02 % to 0.28 %, while Ca. Brocadia decreased from 1.85 % to 1.30 %, with the contribution of anammox to nitrogen removal being highest under low temperatures (19.4℃ to 13.8℃), at 59.0 %. This novel two-stage PN-A process provides a new approach for the stable operation of wastewater treatment plants (WWTPs) under low ambient temperatures.

Successional dynamics of low C/N activated sludge system under salinity shock: Performance, nitrogen removal pathways, microbial community, and assembly

Limited carbon (low C/N) and salinity stress affect the stability of wastewater treatment plants. However, the effect of salinity shock on activated sludge systems with low C/N ratio wastewater remains unclear. An anaerobic/aerobic/anoxic sequencing batch reactor treating low C/N wastewater was established to investigate the effects of salinity shock on system performance, nitrogen removal pathways, microbial community, interactions, and assembly. The results showed that the effluent COD concentration could maintain a stable level, and the average COD removal efficiency was 94.9%. However, total nitrogen removal was significantly inhibited. With the addition of salinity, efficiencies of total nitrogen removal and simultaneous nitrification and denitrification decreased from 91.4 to 73.8% to 86.7 and 39.7%, respectively; however, nitrite reduction capacity increased by 25.4%. After removing salinity, ammonia oxidation capacity further deteriorated, evidenced by the increase in effluent NH₄⁺-N from 8.0 to 11.8 mg/L. During the salinity shock, partial nitrification became the main nitrogen removal pathway because of the inhibition of Nitrospira and high nitrite accumulation ratio (>99.0%). Molecular ecological network analysis indicated that increased competition, decreased total modules, and disappearance of keystone taxa were related to the deterioration of ammonia oxidation capacity and simultaneous nitrification and denitrification. Moreover, the abundant denitrification module and increased denitrifiers contributed to the increase in nitrite reduction capacity. Salinity shock under low C/N conditions resulted in a stronger stochastic community assembly. This study provided information that can help enable stable operations for treating low C/N wastewater.

Mature landfill leachate treatment using granular sludge-based reactor (GSR) via nitritation/denitritation: Process startup and optimization

Mature landfill leachate wastewater (LLW) was characterized by high ammonia, refractory chemical oxygen demand (COD) and heavy metal contents, which limits the nitrogen removal in conventional activated sludge systems. Granular sludge is known to be more resistant to toxic compounds because of its dense structure and diverse microbial community. Here, granular sludge-based reactor (GSR) was applied with nitritation/denitritation (Nit/DNit) process for effective ammonia-rich mature LLW treatment at 20 °C. After a short startup period, the efficiencies of ammonia removal and total inorganic nitrogen removal stabilized at 99 % and 93 %, respectively, under a hydraulic retention time (HRT) of 6 h. High ammonia oxidation rate (~ 0.64 g N/g VSS/d) was achieved, with ~93 % ammonia conversing to nitrite before being reduced to nitrogen gas. Microbial analysis results revealed that Nitrosomonas (ammonia oxidizing bacteria) and Thauera (denitrifiers) were the dominant bacteria with key functional genes involved in the Nit/DNit. With an increase in the LLW loading, increased ammonia oxidation rates and biomass retention were also observed. This study demonstrated that granular sludge-based technology is feasible for mature LLW treatment.

Rapid start-up and stable operation of an aerobic/oxic/anoxic simultaneous nitrification, denitrification, and phosphorus removal reactor with no sludge discharge

An anaerobic/aerobic/anoxic mode simultaneous nitrification, denitrification, and phosphorus removal system was visited for enhanced low-strength wastewater treatment and dramatic in situ sludge reduction. Results showed that rapid start-up was achieved with conventional activated sludge after 15 days, with effluent ammonia nitrogen, total nitrogen, total phosphorus, and chemical oxygen demand being 0.25, 7.89, 0.12, 24.37 mg/L, respectively. A two-stage biomass growth rate was observed with the sludge yield of 0.285 (day 1-50) and 0.017 g MLSS/g COD (day 51-110) without sludge discharge. Dynamics of bacterial community has been identified with outstanding accumulation of Candidatus_Competibacter up to 29.06 %, which contributed to both simultaneous nutrients removal and sludge reduction. Further analysis via PICRUSt2 revealed the main pathway of nitrogen metabolism, while proposed mechanism for phosphorus removal with no sludge discharge was analyzed from the intracellular and extracellular perspectives. Overall, this study provided guidance and reference for the development and application of A/O/A-SNDPR technology.

Rapidly recovering and maintaining simultaneous partial nitrification, denitrification and anammox process through hydroxylamine addition to advance nitrogen removal from domestic sewage

The collapse of simultaneous partial nitrification, denitrification and anammox (SPNDA) system, caused by the destruction of partial nitrification (PN), is the most likely phenomenon to occur. Therefore, recovering the process quickly and maintaining efficient nitrogen removal is a valuable topic for research. In the anaerobic/aerobic/anoxic operation mode, SPNDA process was used to treat domestic sewage in a sequencing batch biofilm reactor. After the deterioration of PN effect, with the addition of hydroxylamine, the activity of ammonia-oxidizing bacteria in the nitrobacteria increased (61.0-91.3 %), whereas the accumulation of nitrite quickly recovered to 90.4 % within 5 days. Meanwhile, the nitrogen removal efficiency improved (61.8-95.6 %) and the effluent TN was 2.1 mg/L. Furthermore, Candidatus Brocadia was enriched (0.50-1.82 %) in the system. The results indicated that the addition of hydroxylamine was an effective strategy to recover and economically maintain the SPNDA process for advanced nitrogen removal from domestic sewage.

A novel process for anammox pretreatment of municipal wastewater: semi-partial nitrification, biological phosphorus removal and recovery

Achieving simultaneous semi-partial nitrification and deep phosphorus removal is a preferred process technology for Anammox pretreatment. In this study, semi-partial nitrification combined with in-situ phosphorus recovery (PNPR) was used to treat municipal wastewater. The SRT conflict between the nitrification and phosphorus removal was resolved by in-situ phosphorus recovery every 20 cycles of Anaerobic/Oxid, and a supernatant with more than 10 times the influent phosphorus concentration was obtained, thus achieving bio-enhanced phosphorus removal and recovery with satisfactory semi-partial-nitrification effluent. Interestingly, the results showed that phosphorus removal and recovery process could improve the activity of AOB. The PNPR system's nitrite accumulation rate (NAR) and phosphorus removal rate (PRR) were more than 90% each, whereas the relative abundance of AOB and PAOs increased from 0.04% to 0.74% and from 0.25% to 0.70%, respectively (P < 0.01). Furthermore, on average, the NO₂^--N_eff/NH₄⁺-N_eff value was 1.96, which laid the foundation for the subsequent anammox treatment.

Advanced nitrogen removal from real municipal wastewater by multiple coupling nitritation, denitritation and endogenous denitritation with anammox in a single suspended sludge bioreactor

Achieving advanced nitrogen removal based on anammox for treating mainstream municipal wastewater in a single suspended sludge bioreactor is a challenging research topic. In this study, multiple coupling nitritation, denitritation and endogenous denitritation with anammox (PNA-(E)PDA) was simultaneously achieved in a 10 L step-feed bioreactor, which enhanced stable nitrogen removal. After 223 days of operation, the total nitrogen concentrations of the influent and effluent were 70.7 ± 6.1 and 4.3 ± 1.8 mg/L, respectively, when treating municipal wastewater even at C/N ratio of 2.24 with only 5 h of aerobic time (DO: 0.5-0.8 mg/L). After the evolution of nitritation/anammox to PNA-(E)PDA, the contribution of anammox to nitrogen removal increased to 78.6% and the anammox activity increased from 4.3 ± 0.2 to 15.2 ± 0.7 mg NH₄⁺-N/gVSS/d. qPCR results showed that the abundance of anammox bacteria increased from 4.1 × 10⁹ to 4.5 × 10¹⁰ copies/ (g VSS). High-throughput sequencing further revealed that the relative abundance of Candidatus Brocadia, the dominant anammox genus, increased from 0.09 to 0.46%. Based on the strong competitiveness of anammox on nitrite, this novel PNA-(E)PDA process provides a potential strategy for enriching anammox bacteria in municipal wastewater treatment plants.

Mutual boost of granulation and enrichment of anammox bacteria in an anaerobic/oxic/anoxic system as the temperature decreases when treating municipal wastewater

Low temperature is an important factor affecting the municipal wastewater treatment systems. The aim of this study was tracking the variations in the abundance of anammox bacteria (AnAOB) and the sludge form as the temperature decreased. Mutual boost of granulation and enrichment of AnAOB was achieved even though the temperature dropped from 20.4 °C to 12.9 °C. The average particle size of the sludge increased from 128.5 μm to 245.6 μm. With low dissolved oxygen (DO) aeration (0.2-0.5 mg/L) and short oxic hydraulic retention time (HRT) (5 h), nitritation in the anaerobic/oxic/anoxic (AOA) system was stable enough to provide NO₂^- for AnAOB. Ca. Brocadia, a type of typical AnAOB, was enriched from 0.03% to 0.24% in the suspended sludge and reached 16.09% in the granular sludge. Overall, this study presents the prospects of anammox and granule technologies when treating municipal wastewater at a low temperature.

Study on Water Purification Effect and Operation Parameters of Various Units of Wastewater Circulation

The discharge of wastewater from aquaculture ponds causes a certain degree of damage to the environment. It is necessary to continuously improve the treatment efficiency of wastewater treatment devices. The purpose of this study is to obtain an optimal ratio of wastewater circulation devices in order to obtain the best operating parameters and to reduce the discharge of polluted water. We constructed an experimental wastewater circulation device consisting of three units. The primary unit contained modified attapulgite (Al@TCAP-N), volcanic stone, and activated carbon for precipitation. The secondary and tertiary units used biological methods to enhance removal rates of nitrogen and phosphorus. Water quality indicators of total phosphorus (TP), total nitrogen (TN), ammonia (NH3-N), permanganate (CODMn), and total suspended solids (TSS) were detected. Water quality was tested under different matching ratios for three units of different hydraulic retention time (HRT) and load Results showed that the removal rate of TP, TN, NH3-N, and TSS reached 20–60%, 20%, 30–70%, and 10–80%, respectively. The average reduction efficiencies of secondary module chlorella and filler on TP, TN, NH3-N, CODMn, and TSS were 56.88%, 30.09%, 0.43%, 46.15%, and 53.70%, respectively. The best removal rate can be achieved when the matching ratio of each unit becomes 2:1:1 and the hydraulic retention time is maintained within 2 h in the high-concentration load. Finally, the average removal rates of TP, TN, NH3-N, and TSS reached 58.87%, 15.96%, 33.99%, and 28.89%, respectively. The second unit obtained the enhanced removal effect in this wastewater treatment system when adding microorganisms and activated sludge.

Advanced nitrogen removal in a single return anaerobic/aerobic/anoxic/aerobic (AnOAO) bioreactor treating municipal wastewater through hydroxylamine addition: Performance and microbial community

The NH₂OH dosing strategy for nitrogen removal was investigated in a single return continuous-flow anaerobic/aerobic/anoxic/aerobic (A_nOAO) reactor fed with real municipal wastewater. A high nitrite accumulation ratio of 98% was achieved in only two days by continuously adding 10 mg/L NH₂OH. When gradually reducing dosing frequency to one day every four days, effluent total nitrogen was as low as 4.8 ± 2.2 mg N/L with removal efficiency of 88.7 ± 5.3%, under aerobic HRT of 4.6 h, DO below 1.0 mg/L, and C/N of 2.8 without external carbon sources. Batch test showed that nitrite oxidizing bacteria (NOB) activity decreased by 81% after adding NH₂OH, while ammonia oxidizing bacteria (AOB) activity remained stable. qPCR confirmed that NOB abundance decreased, and 16S rRNA sequencing further showed that g_Nitrospira belonging to NOB decreased significantly (P < 0.001). Overall, this study provides a novel strategy for advanced nitrogen removal from municipal wastewater in continuous flow systems.

A novel control strategy to strengthen nitrogen removal from domestic wastewater through eliminating nitrite oxidizing bacteria in a plug-flow process

In this study, intermittent aeration strategy was investigated in a plug-flow reactor on real municipal wastewater. Over 200 days of operation, the total inorganic nitrogen (TIN) removal efficiency of 72.43 ± 7.56% was achieved with a total aerobic hydraulic retention time in the range 2.4-3.0 h under a low C/N ratio of 3.19. The batch tests showed that the activity of nitrite oxidizing bacteria (NOB) was effectively inhibited, and simultaneous nitrification and denitrification via nitrite were observed under double intermittent aeration mode. The Illumina MiSep sequencing revealed that the relative abundance of the Nitrospira as the only detected NOB, decreased from 2.22% (day 0) to 0.91% (day 207) at the genus level. Overall, this study provides a new strategy for NOB suppression to strengthen nitrogen removal from low C/N domestic wastewater through the continuous process.

Realization of partial nitrification and in-situ anammox in continuous-flow anaerobic/aerobic/anoxic process with side-stream sludge fermentation for real sewage

A continuous-flow anaerobic/aerobic/anoxic reactor with complete suspended activated sludge using sludge alkaline fermentation products as carbon source was utilized to strengthen nitrogen removal performance for low C/N ratio (<4) wastewater. Long-time performance indicated that the nitrite accumulation rate reached 60.40%, which strengthened the contribution of anammox. The average total inorganic nitrogen removal efficiency improved 19.40%. The abundance of ammonia oxidizing bacteria has not changed, but the abundance of nitrite oxidizing bacteria reduced from 5.79% to 0.69%. Quantitative PCR results demonstrated that the abundance of anammox bacteria has raised by 80.5 times. These results indicated that side-stream sludge alkaline fermentation promoted the mainstream partial nitrification, consequently accelerating the in-situ enrichment of anammox bacteria. No external carbon source dosing and short oxic hydraulic retention time (5.3 h) save energy and reduce consumption significantly in this system.

Enhanced Nitrogen Removal from Domestic Wastewater by Partial-Denitrification/Anammox in an Anoxic/Oxic Biofilm Reactor

A partial-denitrification coupling with anaerobic ammonium oxidation (anammox) process (PD/A) in a continuous-flow anoxic/oxic (A/O) biofilm reactor was developed to treat carbon-limited domestic wastewater (ammonia (NH4+-N) of 55 mg/L and chemical oxygen demand (COD) of 148 mg/L in average) for about 200 days operation. Satisfactory NH4+-N oxidation efficiency above 95% was achieved with rapid biofilm formation in the aerobic zone. Notably, nitrite (NO2−-N) accumulation was observed in the anoxic zone, mainly due to the insufficient electron donor for complete nitrate (NO3−-N) reduction. The nitrate-to-nitrite transformation ratio (NTR) achieved was as high as 64.4%. After the inoculation of anammox-enriched sludge to anoxic zones, total nitrogen (TN) removal was significantly improved from 37.3% to 78.0%. Anammox bacteria were effectively retained in anoxic biofilm utilizing NO2−-N produced via the PD approach and NH4+-N in domestic wastewater, with the relative abundance of 5.83% for stable operation. Anammox pathway contributed to TN removal by a high level of 38%. Overall, this study provided a promising method for mainstream nitrogen removal with low energy consumption and organic carbon demand.

Mainstream double-anammox driven by nitritation and denitratation using a one-stage step-feed bioreactor with real municipal wastewater

A novel double-anammox process for advanced mainstream nitrogen removal was established using step-feed sequencing batch reactor (SBR) system with integration of suspend sludge and biofilms. Following optimization of influent distribution ratio, the effluent total inorganic nitrogen (TIN) was < 10.2 mg N/L, with influent TIN of 43.4 mg N/L, and anammox contributed 71.4% to TIN removal. Biological processes and batch tests revealed that gradient C/N reduction promoted denitratation/anammox in anoxic stage, and simultaneous nitritation and anammox were achieved in oxic stage. Specially, anammox maintained on biofilms with abundance over 10⁹ copies/ (g dry sludge). High-throughput sequencing revealed that Thauera and Nitrosomonas were enriched in flocs. Furthermore, metagenomic sequencing confirmed that Thauera owns narG and napA (NO₃^-→NO₂^-) and Nitrosomonas owns amoA (NH₄⁺→NO₂^-), support stable NO₂^- supply for double-anammox. This mainstream anammox-dominant process could potentially be used for stable nitrogen removal in municipal wastewater treatment plants.

Rapidly achieving and optimizing simultaneous partial nitrification denitrification and anammox integrated process by hydroxylamine addition for advanced nitrogen removal from domestic wastewater

The achievement and stable maintenance of partial nitrification and partial anammox process for municipal sewage is a challenging research topic at present. In this study, a novel strategy of hydroxylamine (NH₂OH) addition under low DO condition was adopted for rapidly achieving simultaneous partial nitrification denitrification and anammox process (SPNDA) to deal with domestic wastewater, the nitrite accumulation ratio (NAR) increased from 1% to 82% in the first 4 days. After the addition of NH₂OH was stopped, the PN effect of SPNDA process remained relatively stable within 100 days. During the stable operation period with aerobic HRT of 5 h, the nitrogen removal efficiency was 87.9 ± 4.2%. Moreover, the abundance of denitrifying bacteria and Candidatus Brocadia increased from 1.79% and 0.062% to 22.49% and 0.38% respectively, which promoted nitrogen removal effect. Overall, this study provided a quickly way for achieving the cost-effective SPNDA process to enhance nitrogen removal with NH₂OH addition.

Improving performance and efficiency of partial anammox by coupling partial nitrification and partial denitrification (PN/A-PD/A) to treat municipal sewage in a step-feed reactor

Improving contribution and nitrogen removal efficiency (NRE) of partial anammox in municipal wastewater is a researching hotspot. This study developed an innovative PN/A-PD/A process with fixed biocarriers in anaerobic/anoxic chambers for actual sewage treatment in a typical step-feed reactor over 390 days. Two coupled pathways providing continuous NO₂^- (partial nitrification in oxic chambers and partial denitrification in anaerobic/anoxic chambers) for anammox were introduced to the process, achieving 47% nitrogen loss by anammox in stable phase. The influent and effluent total inorganic nitrogen (TIN) were 51.3 and 11.0 mg/L, respectively, even with chemical oxygen demand (COD)/TIN ratio of 2.9. Anammox activity improved from 6.52 to 9.68 mg NH₄⁺-N/gMLSS/d and abundance on the biocarriers raised to 3.16 × 10¹⁰ gene copies/g dry sludge. Overall, this study confirmed partial anammox, spatially coupled with partial nitrification and partial denitrification via oxic/anoxic distribution with step feed mode, as an alternative for application of mainstream anammox.

Rapidly achieving partial nitrification of municipal wastewater in enhanced biological phosphorus removal (EBPR) reactor: Effect of heterotrophs proliferation and microbial interactions

Stable nitritation is the major challenge for short-cut nitrogen removal from municipal wastewater. This paper demonstrated a rapid achievement of partial nitrification (PN) in an enhanced biological phosphorus removal (EBPR) reactor treating domestic wastewater. Polyphosphate accumulating organisms (PAOs) were enriched operated at a short aerobic HRT (2.0 h) and SRT (10 d), with satisfactory phosphorus removal efficiency (95.9%). Both of ammonia oxidizing bacteria (AOB) and nitrite oxidizing bacteria (NOB) were elutriated simultaneously. Interestingly, AOB recovered much faster than NOB by a subsequent extension of aerobic HRT and SRT, resulting in a rapid development of PN within 15 days. Ammonia oxidation rates of AOB significantly increased by 44.2%, facilitating a high nitrite accumulation rate (NAR) of 95.8%. Genus Tetrasphaera, Halomonas, Paracoccus and Candidatus_Accumulibacter belonging to PAOs accounted for 4.6%. The proliferation of heterotrophs, typically as PAOs, maximized the microbial competition against NOB by favoring AOB activity and synergy with functional bacteria.

Pre-anaerobic treatment enhanced partial nitrification start-up coupled with anammox for advanced nitrogen removal from low C/N domestic wastewater

A modified two-stage partial nitrification/anammox (PN/A) process with short aerobic HRT of 5.7 h was established and realized advanced nitrogen removal from domestic wastewater. The first process was partial nitrification in the PN-SBR, the initiation and stable maintenance of partial nitrification was achieved by pre-anaerobic treatment without inoculation or addition of inhibitor, nitrite accumulation rate was over 90% and maintained over 200 days, meanwhile pre-anaerobic improved the storage of endogenous carbon sources to enhance the nitrogen removal efficiency. The second process was anammox in the AMX-SBR, which was fed with the effluent of PN-SBR, the effluent total inorganic nitrogen was below 5 mgN/L, nitrogen removal efficiency reached 92.06%. Furthermore, Candidatus-Brocadia was enriched after treating domestic sewage with low ammonia from 0.02% to 0.23%.This study demonstrated the feasibility of starting and maintaining partial nitrification by pre-anaerobic treatment and the feasibility of nitrogen removal by coupling simultaneous-nitrification-denitrification with anammox to treat actual municipal sewage.

Novel insights into overcoming nitrite oxidation bacteria acclimatization problem in treatment of high-ammonia wastewater through partial nitrification

A partial nitrification sequencing batch reactor was operated to reveal mechanisms behind nitrite oxidation bacteria (NOB) acclimatization in high-ammonia wastewater treatment. The influent NH₄⁺-N increased stepwise from 499.7 ± 4.2 mg/L to 6994.5 ± 7.5 mg/L with initial free ammonia (FA) concentration rising from 37.9 ± 3.2 mg NH₃-N/L to 715.3 ± 47.3 mg NH₃-N/L, respectively. NOB acclimatized this FA range with NO₃^--N production increasing from 29.2 ± 2.6 mg/L to 144.1 ± 31.0 mg/L in a cycle, which was caused by the shift of dominant NOB genus from Nitrospira to Nitrolencea. Nitrosomonas as ammonia oxidation bacteria, could sustain its activity of 62.1 ± 0.1 mg NH₄⁺-N/(gVSS∙L∙h) under the same condition. Hydroxylamine addition could be implemented as an emergency measure to alleviate NOB acclimatization in short-term operation. The findings expanded knowledge about NOB acclimatization types and provided novel insights for addressing this problem in a targeted way.

Treatment efficiency and greenhouse gas emissions of non-floating and floating bed activated sludge system with acclimatized sludge treating landfill leachate

This research investigates the treatment efficiency and greenhouse gas (GHG) emissions of non-floating and floating bed AS systems with acclimatized sludge treating landfill leachate. The GHGs under study included carbon dioxide (CO₂), methane (CH₄), and nitrous oxide (N₂O). The non-floating and floating bed AS systems were operated in parallel with identical landfill leachate influent under different hydraulic retention time (HRT) conditions (24, 18, and 12 h). The experimental results showed that the treatment efficiency of organic compounds under 24 h HRT of both systems (90 - 98%) were insignificantly different, while the nutrient removal efficiency of both systems were between 54 and 98 %. The treatment efficiency of the floating bed AS system, despite shorter HRT, remained relatively unchanged due to an abundance of effective bacteria residing in the floating media. The CO₂ emissions were insignificantly different between both AS systems under all HRT conditions (22 - 26.3 μmol/cm².min). The CO₂ emissions were positively correlated with organic loading but inversely correlated with HRT. The CH₄ emissions were positively correlated with HRT (26.3 μmol/cm².min under 24 h HRT of the floating bed AS system). The N₂O emissions were positively correlated with nitrogen loading, and the N₂O emissions from the floating bed AS system were lower due to an abundance of N₂O-reducing bacteria. The floating media enhanced the biological treatment efficiency while maintaining the bacterial community in the system. However, the floating media promoted CH₄ production under anoxic conditions. The originality of this research lies in the use of floating media in the biological treatment system to mitigate GHG emissions, unlike existing research which focused primarily on enhancement of the treatment efficiency.