Elucidating performance failure in the use of an Anaerobic-Oxic-Anoxic (AOA) plug-flow system for biological nutrient removal

Multi-omics reveals mechanism of hydroxylamine-enhanced ultimate nitrogen removal in pilot-scale anaerobic/aerobic/anoxic system

Hydroxylamine (HA) dosing is an effective strategy for promoting partial nitrification (PN); however, its impact on endogenous denitrification remains underexplored. In this study, long-term continuous HA dosing (1.4 mg/L) was introduced for over 110 days in a pilot-scale anaerobic/aerobic/anoxic (AOA) system treating municipal wastewater (66.7-75 m3/d). The HA dosing significantly increased the nitrite accumulation ratio to 67.6 ± 5.0 % (p<0.001) and reduced the effluent total inorganic nitrogen concentration from 6.2 ± 2.0 to 2.4 ± 1.1 mg/L (p<0.001), achieving a nitrogen removal efficiency of 87.4 ± 4.5 % (p<0.001) at a hydraulic retention time of 8 h. During the HA dosing, aerobic nitrogen removal contribution increased from 2.4 ± 3.4 % to 25.8 ± 8.1 % (p<0.001), and the anoxic nitrogen removal rate improved from 1.63 ± 0.11 to 2.35 ± 0.13 mg N/(L·h) (p<0.001). Enhanced nitrogen removal was not only achieved through the rapid establishment of PN but also driven by the long-term impact of HA dosing on microbial community dynamics. Multi-omics analyses revealed that HA disrupted the polyphosphate (poly-P) cycle, evidenced by enhanced transcription of ppx (poly-P degradation) and suppressed ppk (poly-P synthesis), thereby reducing energy availability for phosphate-accumulating organisms (PAOs) and shifting the carbon source competition toward glycogen-accumulating organisms (GAOs), with Ca. Competibacter abundance increased from 0.16 % to 1.13 % (p < 0.001). The economic analysis demonstrated that HA reduced sludge production by 11.2 % and saved operating costs by 31.4-42.8 % compared to conventional carbon sources. These findings highlight the potential of HA dosing to achieve sustainable and highly efficient wastewater treatment.

Efficient nitrogen and phosphorus removal performance and microbial community in a pilot-scale anaerobic/anoxic/oxic (AOA) system with long sludge retention time: Significant roles of endogenous carbon source

Stringent wastewater discharge standards require wastewater treatment plants (WWTPs) to focus on enhancing nitrogen (N) and phosphorus (P) removal efficiency. Increasing sludge concentration by regulation of sludge retention time (SRT) would enhance wastewater treatment loads. However, phosphorus-accumulating organisms (PAOs) would be outcompeted by glycogen-accumulating organisms (GAOs) under long SRT, leading to a collapse of P removal. In this work, pilot-scale anaerobic-oxic-anoxic (AOA) and anaerobic-anoxic-oxic (AAO) systems with long SRT (30 d) were parallelly established for actual urban wastewater treatment. The results indicated that sludge reflux ratio, temperature, and C/N ratio significantly impact N and P removal performance of AOA and AAO systems with long SRT, and removal efficiency of AOA system significantly exceeded that of AAO system. AOA system with long SRT achieved the optimal performance at sludge reflux ratio of 200%, temperature of 25 °C, and C/N ratio of 8, with COD, NH4+-N, TN, and PO43--P removal ratio of 92.80 ± 2.24%, 97.38 ± 0.89%, 88.97 ± 2.47%, and 94.33 ± 3.27%, respectively. In addition, compared to AAO system, AOA system could save 23.08% of the aeration volume. This work highlighted that AOA system with long SRT included multiple coupled nitrogen and phosphorus removal pathways, such as autotrophic/heterotrophic nitrification, anoxic/oxic denitrification, endogenous denitrification, and denitrifying phosphorus removal. Among these, the synergistic effect of endogenous denitrification and denitrifying phosphorus removal driven by internal carbon sources contributed to satisfactory nitrogen and phosphorus removal efficiency in AOA system with long SRT.

Understanding nitrogen removal and N2O emission mechanisms in an anaerobic-swing-anoxic-oxic (ASAO) continuous plug-flow system for low C/N municipal wastewater

This study aims to investigate effects of dissolved oxygen (DO) levels and aerated hydrodynamic retention time (HRT) on nitrogen removal and nitrous oxide (N2O) emissions in a novel anaerobic-swing-anoxic-oxic (ASAO) continuous plug-flow system for treating low carbon to nitrogen ratio municipal wastewater. The swing zones had varying DO levels and volumes, deciding the aerated HRT of the ASAO system. Results showed that low DO level (0.8-1.0 mg/L) and short aerated HRT led to high nitrogen removal performance (91.4 %-96.3 %) and low N2O emission factor (2.8 %). The simultaneous nitrification and denitrification (SND) in swing zones and endogenous denitrification in anoxic zones contributed to the nitrogen removal. Meanwhile, the SND and autotrophic denitrification processes were identified as the N2O sources. Low DO level enriched ammonia-oxidizing bacteria and enhanced the SND and autotrophic denitrification pathway. These findings suggest that the ASAO system is promising for reducing carbon emissions in municipal wastewater treatment.

Enhanced nitrogen removal for low C/N wastewater via preventing futile carbon oxidation and augmenting anammox

Efficient carbon use is crucial for biological nitrogen removal. Traditional aerobic processes can waste carbon sources, exacerbating carbon deficiency. This study explores an anaerobic/oxic/anoxic system with sludge double recirculation to improve nitrogen removal in low C/N wastewater. This system integrated aerobic nitrification after the carbon intracellular storage, separating carbon and nitrogen by denitrifying glycogen-accumulating organisms (DGAOs) with endogenous partial denitrification and Anammox within the anoxic units. A significant efficiency of 91.02±7.01% chemical oxygen demand (COD) was converted into intracellular carbon in anaerobic units, significantly reducing carbon futile oxidation in the aerobic units by effectively separating COD from ammonia. Intracellular storage of carbon sources and microbial adaptation to carbon scarcity prevent futile oxidation of COD in the aerobic units even with short-term high dissolved oxygen (DO), thereby enhancing nitrogen removal under anoxic conditions with sufficient intracellular carbon source. The microbial analysis identified Candidatus Brocadia as the dominant anammox bacteria, in combination with the activity of DGAOs and other related microbial communities, accounting for 37.0% of the TN removal. Consequently, the system demonstrated remarkable nitrogen removal efficiencies, achieving 81.3±3.3% for total nitrogen (TN) and 98.5±0.9% for ammonia nitrogen while maintaining an effluent COD concentration of 17.2±9.1 mg/L, treating the low C/N of 4.18 in the influent wastewater. The findings in this study provide a sustainable and energy-saving technique for conventional WWTPs to meet strict discharge standards by avoiding futile oxidation of COD and encouraging anammox contributions.

Establishment of continuous flow partial denitrification biofilm module with short hydraulic retention time

Partial denitrification (PD) can supply essential nitrite (NO2-) and is supposed to promote the application of Anammox. However, PD-related research mainly involves sequencing batch reactors and activated sludge. Here, we proposed establishing PD in a continuous-flow submerged biofilm module (PD-BfM). Benefiting from employing anoxic starvation treatment to quickly start PD and transferring enriched functional bacteria onto biofilms in time, the preparation work of PD-BfM was completed within a quite short period of 21 days. With the hydraulic retention time adjusted to 50 min, PD-BfM demonstrated an impressive efficiency in generating NO2-, achieving a nitrate-to-nitrite transformation ratio of over 75 %, even at the influent chemical oxygen demand to nitrate ratio of 4 condition. Meanwhile, the dominant genus in the biofilms was shifted from Thauera to Flavobacterium and Comamonadaceae family members. The gradient of substrate concentrations also possibly differentiated microbial communities between the top and bottom bio-carriers.

Optimizing operation strategy to improve storage of intracellular carbon sources in anaerobic/oxic/anoxic system: Enhanced nitrogen removal by endogenous denitrification

Endogenous denitrification (ED) can make full use of the carbon sources and avoid replenishment of it. However, strengthening the storage of intracellular carbon sources is an important factor in improving ED efficiency. In this study, employed batch experiments using real domestic wastewater in the anaerobic/oxic (A/O) process. The anaerobic and oxic processes were run for 4 h under ambient conditions with the dissolved oxygen (DO) concentrations in the oxic stage controlled at 0.5, 1.0, 1.5, and 3.0 mg/L, respectively. The results showed that the content of poly-β-hydroxyalkanoates (PHA) reached its peak at 60 min (1.25 mmolC/L). And with DO concentrations of 1.5 mg/L, the contents of glycogen (Gly) were 27.74 mmolC/L. Subsequently, the AOA-SBR was established to investigate its effect on the long-term nitrogen removal performance of domestic wastewater by optimizing the anaerobic time and DO concentrations. The results showed that at an anaerobic time of 60 min and DO concentration of 1.5 mg/L, the storage of the intracellular carbon sources was highest and the total nitrogen (TN) removal efficiency increased to 82.12%. In addition, Candidatus Competibacter dominated gradually in the system as the strategy was optimized.

Inadvertently enriched cyanobacteria prompted bacterial phosphorus uptake without aeration in a conventional anaerobic/oxic reactor

The enhanced biological phosphorus removal (EBPR) process requires alternate anaerobic and aerobic conditions, which are regulated respectively by aeration off and on. Recently, in an ordinary EBPR reactor, an abnormal orthophosphate concentration (PO43--P) decline in the anaerobic stage (namely non-aerated phosphorus uptake) aroused attention. It was not occasionally but occurred in each cycle and lasted for 101 d and shared about 16.63 % in the total P uptake amount. After excluding bio-mineralization and surface re-aeration, indoor light conditions (180 to 260 lx) inducing non-aerated P uptake were confirmed. High-throughput sequencing analysis revealed that cyanobacteria could produce oxygen via photosynthesis and were inhabited inside wall biofilm. The cyanobacteria (Pantalinema and Leptolyngbya ANT.L52.2) were incubated in a feeding transparent silicone hose, entered the reactor along with influent, and outcompeted Chlorophyta, which existed in the inoculum. Eventually, this work deciphered the reason for non-aerated phosphorus uptake and indicated its potential application in reducing CO2 emissions and energy consumption via the cooperation of microalgal-bacterial and biofilm-sludge.

Rapid establishment of municipal sewage partial denitrification-anammox for nitrogen removal through inoculation with side-stream anammox biofilm without domestication

Mainstream partial denitrification anammox was achieved through inoculation of side-stream mature partial nitritation anammox biofilm without domestication. The contribution of anammox to nitrogen removal was 29.4 %. Moreover, prolonging anoxic hydraulic retention time and introducing side-stream nitrite under different carbon/nitrogen ratios enriched anammox bacteria. The abundance of anammox bacteria increased by ∼ 10 times ((2.19 ± 0.17) × 1012 copies gene / g dry sludge) with a total relative abundance of 18.51 %. During 258 days of operation, the contribution of anammox to nitrogen removal gradually increased to 68.8 %. The total nitrogen in the effluent decreased to 8.84 mg/L with a total nitrogen removal efficiency of 76.4 % under a carbon/nitrogen ratio of 3. This paper proposes a novel way to rapidly achieve mainstream partial denitrification anammox via inoculation with side-stream mature partial nitritation anammox biofilm. This method achieves advanced nitrogen removal from municipal wastewater, even under low carbon/nitrogen ratios.

Partial denitrifying phosphorus removal coupling with anammox (PDPRA) enables synergistic removal of C, N, and P nutrients from municipal wastewater: A year-round pilot-scale evaluation

Applying anaerobic ammonium oxidation (anammox) in municipal wastewater treatment plants (MWWTPs) can unlock significant energy and resource savings. However, its practical implementation encounters significant challenges, particularly due to its limited compatibility with carbon and phosphorus removal processes. This study established a pilot-scale plant featuring a modified anaerobic-anoxic-oxic (A2O) process and operated continuously for 385 days, treating municipal wastewater of 50 m3/d. For the first time, we propose a novel concept of partial denitrifying phosphorus removal coupling with anammox (PDPRA), leveraging denitrifying phosphorus-accumulating organisms (DPAOs) as NO2- suppliers for anammox. 15N stable isotope tracing revealed that the PDPRA enabled an anammox reaction rate of 6.14 ± 0.18 μmol-N/(L·h), contributing 57.4 % to total inorganic nitrogen (TIN) removal. Metagenomic sequencing and 16S rRNA amplicon sequencing unveiled the co-existence and co-prosperity of anammox bacteria and DPAOs, with Candidatus Brocadia being highly enriched in the anoxic biofilms at a relative abundance of 2.46 ± 0.52 %. Finally, the PDPRA facilitated the synergistic conversion and removal of carbon, nitrogen, and phosphorus nutrients, achieving remarkable removal efficiencies of chemical oxygen demand (COD, 83.5 ± 5.3 %), NH4+ (99.8 ± 0.7 %), TIN (77.1 ± 3.6 %), and PO43- (99.3 ± 1.6 %), even under challenging operational conditions such as low temperature of 11.7 °C. The PDPRA offers a promising solution for reconciling the mainstream anammox and the carbon and phosphorus removal, shedding fresh light on the paradigm shift of MWWTPs in the near future.

Achieving synchronous and highly efficient removal of nitrogen and phosphorus by rapid enrichment and cultivation denitrifying phosphorus accumulating organisms in anaerobic-oxic-anoxic operation mode

Realizing the quick enrichment and development of denitrifying phosphorus accumulating organisms (DPAOs) in actual household wastewater and industrial nitrate wastewater has significant research significance. In this study, a novel operation mode of anaerobic-oxic-anoxic (AOA) was adopted to successfully realize the enrichment and cultivation of DPAOs in urban domestic wastewater. Adjusting influent COD to PO43--P ratio, shortening the aerobic time and decreasing the aeration volume were conducive to select DPAOs in microbial populations. The system was operated for 180 days and the DPAOs were well enriched during the stable operation with the percentage of Dechloromonas increased to 5.1 %. Accordingly, the effluent PO43--P was < 0.3 mg P/L, the removal efficiency of phosphorus was 96.9 % and the removal efficiency of nitrate was 92.5 %. Above all, DPR can be successfully applied to AOA systems with good phosphorus removal performance.

Robust rehabilitation of anammox system by granular activated carbon under long-term starvation stress: Microbiota restoration and metabolic reinforcement

This article investigates the buffering capacity and recovery-enhancing ability of granular activated carbon (GAC) in a starved (influent total nitrogen: 20 mg/L) anaerobic ammonium oxidation (anammox) reactor. The findings revealed that anammox aggregated and sustained basal metabolism with shorter performance recovery lag (6 days) and better nitrogen removal efficiency (84.9 %) due to weak electron-repulsion and abundance redox-active groups on GAC's surface. GAC-supported enhanced extracellular polymeric substance secretion aided anammox in resisting starvation. GAC also facilitated anammox bacterial proliferation and expedited the restoration of anammox microbial community from a starved state to its initial-level. Metabolic function analyses unveiled that GAC improved the expression of genes involved in amino acid metabolism and sugar-nucleotide biosynthesis while promoted microbial cross-feeding, ultimately indicating the superior potential of GAC in stimulating more diverse metabolic networks in nutrient-depleted anammox consortia. This research sheds light on the microbial and metabolic mechanisms underlying GAC-mediated anammox system in low-substrate habitats.