Effects of sludge morphology on the anammox process: Analysis from the perspectives of performance, structure, and microbial community

Stability of aerobic granular sludge for treating inorganic wastewater with different nitrogen loading rates

This paper investigated the effect of nitrogen loading rates (NLRs) on the stability of aerobic granular sludge (AGS) for treating simulated ionic rare earth mine wastewater with high ammonia nitrogen and extremely low organic content. Mature AGS from a sequencing batch reactor (SBR) was seeded into five identical SBRs (R1, R2, R3, R4 and R5). The five reactors were operated with different NLRs (0.2, 0.4, 0.8, 1.2 and 1.6 kg/m3·d). After 30 days of operation, R1, R2 and R5 were dominated by broken granules, while most of the granules in R3 and R4 still maintained a complete structure. The properties of granules from R1, R2, R3, R4 and R5 deteriorated to varying degrees, while the granules from R3 and R4 showed better stability than that from R1, R2 and R5. In R1, R2, R3 and R4, the steady-state ammonia nitrogen removal efficiencies were all greater than 90%, and the steady-state removal efficiencies of total inorganic nitrogen (TIN) were approximately 30%. In R5, the removal efficiencies of ammonia nitrogen and TIN were both approximately 70%. The dominant nitrifying and denitrifying bacterial genera of the granules from the five reactors were Nitrosomonas and Thauera, respectively, and their relative abundance was much higher in granules from R3 and R4. The results demonstrated that a relative equilibrium between the growth and metabolism of nitrifying/denitrifying bacteria was achieved when NLR was between 0.8 and 1.2 kg/m3·d, which could provide technical support for the stability maintenance of AGS in the treatment of ionic rare earth mine wastewater.

Deciphering the anammox microbial community succession with humic acid exposure to optimize large anammox granules for robust nitrogen removal

The robustness of the anaerobic ammonia oxidation (anammox) process in treating wastewater with high concentrations of humic acids (HAs), including landfill leachate and sludge anaerobic digestion liquid, has been paid great attention. This study revealed that the anammox sludge granule size of 1.0-2.0 mm could be robust under the HA exposure with high concentrations. The total nitrogen removal efficiency (NRE) was 96.2% at the HA concentration of 20-100 mg/L, while the NRE was 88.5% at the HA concentration of 500 mg/L, with reduced by 7.7%. The increased extracellular polymeric substances (EPS) content which was stimulated by the HA exposure favored the formation of large granules (1.0-2.0 mm) by enveloping medium and micro granules (0.2-1.0 mm). The abundance of anammox bacteria Candidatus Brocadia was found to be higher (14.2%) in large anammox granules sized 1.0-2.0 mm, suggesting a potentially high anammox activity. However, the abundance of denitrifiers Denitratisoma increased by 4.3% in ultra-large anammox granules sized >2.0 mm, which could be attributed to the high EPS content for heterotrophic denitrifiers metabolism as organic matter. The feedback mechanism of the anammox community for maintaining the ecological function under the HA exposure resulted in a closely related microbial community, with positive and negative correlations in the ecological network increased by 64.3%. This study revealed that the HA exposure of the anammox system resulted in the anammox granules of 1.0-2.0 mm size being the dominant granules with robust nitrogen removal, providing significant guidance for the optimization of anammox granules for an efficient treatment of HA-containing wastewater in anammox applications.

Mechanistic insight into the aggregation ability of anammox microorganisms: Roles of polarity, composition and molecular structure of extracellular polymeric substances

The chemical characteristics of extracellular polymeric substances (EPS) of anammox bacteria (AnAOB) play a crucial role in the rapid enrichment of AnAOB and the stable operation of wastewater anammox processes. To clarify the influential mechanisms of sludge EPS on AnAOB aggregation, multiple parameters, including the polarity distribution, composition, and molecular structure of EPS, were selected, and their quantitative relationship with AnAOB aggregation was analyzed. Compared to typical anaerobic sludge (anaerobic floc and granular sludge), the anammox sludge EPS exhibited higher levels of tryptophan-like substances (44.82-56.52 % vs. 2.57-39.81 %), polysaccharides (40.02-53.49 mg/g VSS vs. 30.22-41.69 mg/g VSS), and protein structural units including α-helices (20.70-23.98 % vs. 16.48-19.32 %), β-sheets (37.43-42.98 % vs. 25.78-36.72 %), and protonated nitrogen (Npr) (0.065-0.122 vs. 0.017-0.061). In contrast, it had lower contents of β-turns (20.95-27.39 % vs. 28.17-39.04 %). These biopolymers were found to originate from different genera of AnAOB. Specifically, the α-helix-rich proteins were mainly derived from Candidatus Kuenenia, whereas the extracellular proteins related to tryptophan and Npr were closely associated with Candidatus Brocadia. Critically, these EPS components could drive anammox aggregation through interactions. Substantial amounts of tryptophan-like substances facilitated the formation of β-sheet structures and the exposure of internal hydrophobic clusters, which benefited the anammox aggregation. Meanwhile, extracellular proteins with high Npr content played a pivotal role in the formation of mixed protein-polysaccharide gel networks with the electronegative regions of polysaccharides, which could be regarded as the key component in the maintenance of anammox sludge stability. These findings provide a comprehensive understanding of the multifaceted roles of EPS in driving anammox aggregation and offer valuable insights into the development of EPS regulation strategies aimed at optimizing the anammox process.

Stability of aerobic granular sludge for simultaneous nitrogen and Pb(II) removal from inorganic wastewater

ABSTRACTIn this paper, we proposed a strategy for the establishment of an aerobic granular sludge (AGS) system for simultaneous nitrogen and Pb(II) removal from inorganic wastewater. AGS was stored in lead nitrate solution to select functional bacteria resistant to lead poison, and then an AGS system for ammonia nitrogen (180-270 mg/L) and Pb(II) (15-30 mg/L) removal was established based on carbon dosing and a two-stage oxic/anoxic operational mode. After storage for 40 days, the stability of AGS decreased because specific oxygen uptake rate, nitrification rate and abundance of Nitrosomonas decreased to different degrees compared with those before storage. During the first 70 days of the recovery process, AGS in R1 (the blank reactor) and R2 (the control reactor) both experienced a first breakage and then regranulation process. The main properties of AGS in reactors R1 and R2 tended to be stable after days 106 and 117, respectively, but the structure of steady-state AGS in R2 was more compact. The total inorganic nitrogen (TIN) in effluent from R1 and R2 basically remained below 25 mg/L after days 98 and 90, respectively. The Pb(II) concentration in effluent from R2 was always below 0.3 mg/L. On day 140, the relative abundance of Nitrosomonas in R2 (6.17%) was significantly lower than that in R1 (12.15%), whereas the relative abundance of denitrifying bacteria was significantly higher than that in R1 (62.44% and 46.79%). The system removed 1 kg of influent TIN only consuming approximately 1.85 kg of carbon source, demonstrating clear advantages in energy savings.

Dimorphism of Candida tropicalis and its effect on nitrogen and phosphorus removal and sludge settleability

Candida tropicalis PNY, a novel dimorphic strain with the capacity of simultaneous carbon, nitrogen and phosphorus removal in anaerobic and aerobic conditions, was isolated from activated sludge. Dimorphism of C. tropicalis PNY had effect on removing nitrogen and phosphorous and slightly affected COD removal under aerobic condition. Sample with high hypha formation rate (40 ± 5%) had more removal efficiencies of NH₄⁺-N (50 mg/L) and PO₄^3--P (10 mg/L), which could achieve 82.19% and 97.53%, respectively. High hypha cells dosage exhibited good settleability and filamentous overgrowth was not observed. According to label-free quantitative proteomics assays. Up-regulated proteins involved in the mitogen-activated protein kinase (MAPK) pathway indicated the active growth and metabolism process of sample with high hypha formation rate (40 ± 5%). And proteins concerning about glutamate synthetase and SPX domain-contain protein explain for the nutrient removal mechanism including assimilation of ammonia and polyphosphates synthesis.

Nitrogen removal performance and rapid start-up of anammox process in an electrolytic sequencing batch reactor (ESBR)

In this study, the electrolytic sequencing batch reactor (ESBR) with different current densities was constructed to investigate the nitrogen removal performance and rapid start-up of anaerobic ammonia oxidation (anammox) process. The changes of total nitrogen removal rate (TNRR), specific anammox activity (SAA) and nitrogen concentration under different current densities were analyzed, and then the effect of the optimal current density on the start-up of anammox in ESBR was explored. The results showed that ammonium nitrogen removal efficiency (92.7%), nitrite nitrogen removal efficiency (15.5%) and total nitrogen removal efficiency (28.1%) were obtained with the TNRR and SAA were 0.0118 g N L^-1 d^-1 and 0.0050 g N (g Vss d)^-1, respectively under the optimal conditions (i.e., current density = 0.10 mA cm^-2, temperature = 36 °C and pH = 7.6). In addition, the stoichiometric ratio indicated that anammox was initiated successfully for 91 days in ESBR with the current density of 0.10 mA cm^-2, which was shortened by 10 days compared with the conventional SBR without current density. These results suggest that an array of rapid start-up processes of anammox can be developed through applying current density to stimulate the activity of anammox bacteria (AnAOB).

Responses of anammox to long-term p-nitrophenol stress: From apparent and microscopic phenomena to mechanism simulation

p-Nitrophenol is usually present in ammonia-rich wastewaters produced by some chemical plants. In this work, the response of anammox process to long-term p-nitrophenol stress was investigated. The changes in the efficiency, sludge characteristics, and microorganisms of the anammox system under different levels of p-nitrophenol stress were examined, and the potential stress mechanisms of p-nitrophenol on anammox were further speculated. The results showed that 10-50 mg/L p-nitrophenol had no obvious impact on nitrogen removal efficiency, but stimulated the secretion of more extracellular polymeric substances. 60 mg/L p-nitrophenol caused the nitrogen removal efficiency to decrease by 64.5% in 5 days. Long-term exposure to p-nitrophenol led to 8.6% reduction in Candidatus_Kuenenia abundance and 18.4%-35.9% decrease in the expression level of anammox bacterial functional genes. Molecular simulation indicated that p-nitrophenol could bind to key enzymes of anammox. This study provides new insights into the treatment of wastewater containing p-nitrophenol or phenol by anammox.

A comparison between exogenous carriers enhanced aerobic granulation under low organic loading in the aspect of sludge characteristics, extracellular polymeric substances and microbial communities

In this study, polymeric ferric sulfate (PFS), aluminum sulfate (AS) and diatomite were added to enhance the aerobic granulation under low organic loading rate (OLR) of 0.6 kg·COD/(m³·d), and their effects of aerobic granule formation, extracellular polymeric substances (EPS) secretion and microbial community were investigated. The results showed that adding carriers could facilitated the growth of aerobic granules and improve the sludge settleability and biomass retention. Nutrient removal efficiencies were also enhanced. Compared with diatomite, adding PFS and AS resulted in more significant increase in EPS production, especially for the extracellular proteins. For microbial community, the dominated bacteria (Zoogloea, 18.47-23.95%) in the mature granular consortia were similar. Moreover, the introduction of PFS and diatomite contributed to the enrichment of Paracoccus, which was responsible for denitrification. Adding carriers potentially activated the functional genes related to metabolism and genetic information processing, and PFS had the most significant effects.