Deciphering the spatial distribution along the upflow anammox reactor: Sludge characteristics and interspecies interactions

Polyphenolic compounds mitigate the oxidative damage of anammox sludge under long-term light irradiation

Continuous high-intensity light exposure can inhibit anaerobic ammonium oxidation (anammox) bacteria activity, though the specific impacts on anammox reactor performance remain unclear. This study investigates the effects of long-term light stress on anammox sludge reactors and explores the use of tea polyphenols as an engineering interventions to mitigate photo oxidation damage. The results showed that the nitrogen removal efficiency (NRE) of the reactor rapidly deteriorated to 41.4 % under 10,000 lx light conditions. However, reactors supplemented with 1 mg·L-1 and 5 mg·L-1 tea polyphenols sustained NREs of 75.2 % and 82.5 %, respectively. The addition of tea polyphenols alleviated oxidative stress by scavenging reactive oxygen species such as ·OH and H2O2, and by enhancing the activities of antioxidant enzymes including total superoxide dismutase and glutathione peroxidase. Candidatus Kuenenia was negatively impacted by light, while unclassified_f__Brocadiaceae thrived under light stress. These findings provide insights for the development of stable nitrogen removal systems under light exposure.

Start-up of Anammox-HAP in IC reactors: Revelation of sludge characteristics and microbial community structure

The scarcity of seed sludge poses a significant barrier to the advancement of anaerobic ammonia oxidation (anammox) process. In this investigation, two alternative sludge (anaerobic granular sludge (AGS) and activated flocculent sludge (AFS)) were employed to start up the anammox process in internal circulation (IC) reactors with the hydroxyapatite (HAP) strategy. Both reactors achieved rapid start-up on days 83 and 53, respectively. Subsequently, a nitrogen removal rate (NRR) of 1.34 gN/L/d was attained at a nitrogen loading rate (NLR) of 1.39 gN/L/d on days 107 and 81 correspondingly. The analysis of granular properties revealed that the anammox granular sludge (AMXGS) transformed from AGS exhibited superior granular size distribution and settling performance. Furthermore, the assessment of microbial community structure demonstrated that inoculating AFS was capable of enriching anammox bacteria (AnAOB) in a shorter time. Last but most importantly, this study provides a comprehensive analysis of the distinct granulation routes of AGS and AFS. AGS predominantly underwent a "broken-adsorption-granulation" process, whereas AFS exhibited not only a typical "adsorption-granulation" process but also a "biofilm growth-granulation" cycle process. The findings of this study offer a novel approach for quickly initiating anammox process when inoculating alternative sludge.

Sulfide promotes nitrogen removal in anammox low-strength ammonium wastewater treatment system

Anammox has been widely used for denitrification from different wastewaters due to its low energy and carbon sources consumption. Nevertheless, the presence of nitrate in the effluent has been found to impede the enhancement of total nitrogen removal efficiency (TNRE). In this study, anammox was employed in conjunction with sulfur autotrophic denitrification (SAD) in order to enhance TNRE. During a long-term test in an UASB reactor with 30 mg L-1 ammonium influent concentration, it was observed that sulfide facilitated both nitrate removal efficiency (NRE) and TNRE. Specifically, compared with 0 mg L-1 sulfide addition, the NRE and TNRE were enhanced from 92.55% and 74.56% to 94.10% and 89.51%, respectively with 10 mg L-1 sulfide. However, with sulfide concentration increased to 20 mg L-1, 81.13% of TNRE was only found. Notably, the anammox performance was observed to enhancement when the sulfide level was reduced to 0 mg L-1 again, the result indicated that the inhibitory effect of 20 mg L-1 sulfides can be reversed. Further exploration revealed that sulfide not only suppressed the activity of nitrifying bacteria but also acted as electron donor, aiding the conversion of NO3--N to N2 through SAD process, this made the nitrate in effluence as low as 2.31 mg L-1. The results of microbiological analysis demonstrated that the population of microorganisms associated with SAD increased in response to the addition of sulfide. The findings suggested a potential equilibrium and collaboration between SAD and anammox to further increase TNRE in low-strength ammonium wastewater treatment.

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.

Enhanced-nitrogen removal through Fe(III)-triggered partial dissimilatory nitrate reduction to ammonium coupling with anammox in anammox bioreactor

Anammox is recognized as a prospective alternative for future biological nitrogen removal technologies. However, the nitrate by-products produced by anammox bacteria limit its overall nitrogen removal efficiency below 88 %. This study introduced Fe(III) into the anammox bioreactor to enhance the nitrogen removal efficiency to approximately 95 %, surpassing the biochemical limit of 88 % imposed by anammox stoichiometry. Anammox sludge was demonstrated to utilize extracellular polymeric substances to reduce Fe(III) into Fe(II), and this process promoted the dominance of Ca. Brocadia. The iron addition improved the abundance of narGHI genes and facilitated the partial dissimilatory nitrate reduction to ammonium, with nitrite as the end product. The accumulated nitrite was then eliminated through the anammox pathway, along with the excess ammonium (30 mg/L) in the influent. Overall, this study deepens our understanding of the enhanced nitrogen removal triggered by Fe(III) in anammox sludge and offers an effective approach to boost anammox process.

Enhancing anammox process with granular activated carbon: A study on Microbial Extracellular Secretions (MESs)

Granular activated carbon (GAC), a porous carbon-based material, provides increased attachment space for functional microorganisms and enhances nitrogen removal by facilitating extracellular electron transfer in the anammox process. This study investigates the effects of GAC on the biosynthesis of microbial extracellular secretions (MESs) and explores the roles of these secretions in anammox activities. Four lab-scale reactors were operated: two downstream UASB reactors (D1 and D2) receiving effluents from the upstream UASB reactors (U1: no-GAC, U2: yes-GAC). Our results indicate that MESs were enhanced with the addition of GAC. The effluent from U2 exhibited a 59.62 % higher amino acid content than that from U1. These secretions contributed to an increase in the nitrogen loading rate (NLR) in the downstream reactors. Specifically, NLR in D1 increased from 130.5 to 142.7 g N/m3/day, and in D2, it escalated from 137.5 to 202.8 g N/m3/day, likely through acting as cross-feeding substrates or vital nutrients. D2 also showed increased anammox bacterial activity, enriched Ca. Brocadia population and hao gene abundance. Furthermore, this study revealed that D2 sludge has significantly higher extracellular polymeric substances (EPS) (48.71 mg/g VSS) and a larger average granule size (1.201 ± 0.119 mm) compared to D1 sludge. Overall, GAC-stimulated MESs may have contributed to the enhanced performance of the anammox process.

Distinct microbial characteristics of the robust single-stage coupling system during the conversion from anammox-denitritation to anammox-denitratation patterns

Simultaneous anammox-denitrification is effectively operated in two types, i.e., the anammox-denitritation (SAD pattern) and the anammox-denitratation (PDA pattern). The nitrate derived from inevitable nitrite oxidization likely determines the practical operational pattern of the coupling system, while little information is available regarding the microbial characteristics during the pattern conversion. Here, the single-stage bioreactor coupling anammox with denitrification was operated under conditions with a changed ratio of influent nitrite and nitrate. Results showed that the bioreactor exhibited a robust performance during the conversion from SAD to PDA patterns, corresponding with the total nitrogen removal efficiency ranging from 89.5% to 92.4%. Distinct community structures were observed in two patterns, while functional bacteria including the genera Denitratisoma, Thauera, Candidatus Brocadia, and Ca. Jettenia steadily co-existed. Meanwhile, the high transcription of hydrazine synthase genes demonstrated a stable anammox process, while the up-regulated transcription of nitrite and nitrous oxide reductase genes indicated that the complete denitrification process was enhanced for total nitrogen removal during the PDA pattern. Ecologically, stochastic processes dominantly governed the community assembly in two patterns. The PDA pattern improved the interconnectivity of communities, especially for the cooperative behaviors between dominant denitrifying bacteria and low-abundant species. These findings deepen our understanding of the microbial mechanism underlying the different patterns of the coupling system and potentially expand its engineering application.

Spatial difference in nitrogen removal pathways and microbial functional diversity in an EGSB reactor during the start-up of PD/Anammox

The start-up of a relatively high nitrogen load PD/Anammox in an EGSB reactor was achieved through strategies of bioaugmentation, mass transfer enhancement, and COD/NO3--N control, with NRR of 5.2 g N/L/d. Longitudinal heterogeneity in EGSB reactor induced divergent nitrogen conversion pathways and enriched different functional microorganisms between stratified sludge. Along the elevation of the reactor, the proportion of removed nitrogen through anammox increased continuously from bottom, middle and up, which were 65.0 %, 79.8 %, and 84.1 %, respectively, consistent with the trend of ex-situ activities calculated with Gompertz model. The bottom zone played a role in mixed nitrogen conversion to provide NO2--N accumulation and nitrogen removal, with higher abundance of Thauera, Denitratisoma and Ignavibacterium. The middle part was enriched Candidatus_Kuenenia (12.51 %), and up inhibited completed denitrification, together forming the anammox dominant zone. The proposed functional zones in the EGSB reactor provided approaches for the optimisation of high-load PD/Anammox systems.

Granulation of partial denitrification sludge: Advances in mechanism understanding, technologies development and perspectives

The high-rate and stably efficient nitrite generation is vital and still challenges the wide application of partial denitrification (PD) and anammox technology. Increasing attention has been drawn to the granulation of PD biomass. However, the knowledge of PD granular sludge is still limited in terms of granules characterization and mechanisms of biomass aggregation for high nitrite accumulation. This work reviewed the performance and granulation of PD biomass for high nitrite accumulation via nitrate reduction, including the system start-up, influential factors, granular characteristics, hypothetical mechanism, challenges and perspectives in future application. The physiochemical characterization and key influential factors were summarized in view of nitrite production, morphology analysis, extracellular polymer substance structure, as well as microbial mechanisms. The PD granules exhibit potential advantages of a high biomass density, good settleability, high hydraulic loading rates, and strong shock resistance. A novel granular sludge-based PD combined with anammox process was proposed to enhance the capability of nitrogen removal. In the future, PD granules utilizing different electron donors is a promising way to broaden the application of anammox technology in both municipal and industrial wastewater treatment.