Autoclaved sludge as the ideal seed to culture anammox bacteria: Reactor performance and microbial community diversity

Metagenomic unveils the promotion of mainstream PD-anammox process at lower nZVI concentration and inhibition at higher dosage

The partial-denitrification-anammox (PdNA) process exhibits great potential in enabling the simultaneous removal of NO3--N and NH4+-N. This study delved into the impact of exogenous nano zero-valent iron (nZVI) on the PdNA process. Adding 10 mg L-1 of nZVI increased nitrogen removal efficiency up to 83.12 % and maintained higher relative abundances of certain beneficial bacteria. The maximum relative abundance of Candidatus Brocadia (1.6 %), Candidatus Kuenenia (1.5 %), Ignavibacterium (1.3 %), and Azospira (1.2 %) was observed at 10 mg L-1 of nZVI. However, the greatest relative abundance of Thauera (1.3 %) was recorded under 50 mg L-1. Moreover, applying nZVI selectively enhanced the abundance of NO3--N reductase genes. So, keeping the nZVI concentration at 10 mg L-1 or below is advisable to ensure a stable PdNA process in mainstream conditions. Considering nitrogen removal efficiency, using nZVI in the PD-anammox process could be more cost-effective in enhancing its adoption in industrial and mainstream settings.

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.

Unravelling the importance of Ca2+ and Mg2+ as essential in anammox culture medium

The mechanism and nitrogen removal performance of anammox process under different concentrations of Ca²⁺ and Mg²⁺ were explored from the perspective of molecular biology analysis based on the metabolic changes of the second messenger cyclic diguanylate (c-di-GMP). After 100-day operation, reactor with 98 mg/L Ca²⁺ and 30 mg/L Mg²⁺ achieved a higher anammox performance with an average total nitrogen (TN) removal efficiency of 85.8%. Under the Mg²⁺concentration of 30 mg/L, a higher Ca²⁺ could accelerate anammox process by promoting the amplification of Candidatus Brocadia (0.62%) and production of Diguanylate cyclase (DGC-s: 6.54 × 10⁸ copies/μL DNA) which function was to synthesize c-di-GMP. While under the Ca²⁺concentration of 49 mg/L, Mg²⁺ concentration at appropriate rang could promote the degradation process of c-di-GMP. Since Ca²⁺ had positive linear relationship with TN removal (R² = 0.96), a higher Ca²⁺ concentration is recommended in the culture medium. This study provided a potential method for optimization of anammox process.

Nano zero-valent iron improves anammox activity by promoting the activity of quorum sensing system

The addition of nano zero-valent iron (nZVI) has been proven to improve the efficiency of the anammox process, however, the mechnism is not clear. Here, the effect of nZVI on anammox microbial community was studied by metagenomic sequencing methods. It was found that 50 mg/L nZVI indeed promoted the removal of NH₄⁺ and NO₂^- of the anammox reactor and significantly improved the relative abundance of AnAOB (Ca. Brocadia) from 42.1% to 52.5%. What's more, 50 mg/L nZVI increased the abundance of c-di-GMP synthesized protein from 148 rpmr to 252 rpmr in the microbial community and decreased the abundance of c-di-GMP degradation protein from 238 rpmr to 204 rpmr, which indirectly led to the enrichment of c-di-GMP in the microbial community. The enrichment of c-di-GMP reduced the motility of microorganisms in the reactor and promoted the secretion of extracellular polymers by bacteria, which is beneficial to the formation of sludge particles in the anammox reactor. In conclusion, this research clarified the mechanism of nZVI promoting the anammox process and provided theoretical guidance for the engineering application of anammox.

A review of quorum sensing improving partial nitritation-anammox process: Functions, mechanisms and prospects

Partial nitritation-anammox (PNA) is a promising and energy-efficient process for the sustainable nitrogen removal. However, its wide applications are still limited by the long start-up period and instability of long-term operation. Quorum sensing (QS), as a way of cell-to-cell communication generally regulating various microbial behaviors, has been increasingly investigated in PNA process, because QS may substantially manipulate the metabolism of microorganisms and overcome the limitations of PNA process. This critical review provides a comprehensive analysis of QS in PNA systems, and identifies the challenges and opportunities for the optimization of PNA process based on QS. The analysis is grouped based on the configurations of PNA process, including partial nitritation, anammox and single-stage PNA systems. QS is confirmed to regulate various properties of PNA systems, including microbial activity, microbial growth rate, microbial aggregation, microbial interactions and the robustness under adverse conditions. Major challenges in the mechanisms of QS, such as QS circuits, target genes and the response to environmental inputs, are identified. Potential applications of QS, such as short-term addition of certain acyl-homoserine lactones (AHLs) or substances containing AHLs, transient unfavorable conditions to stimulate the secretion of AHLs, are also proposed. This review focuses on the theoretical and practical cognation for QS in PNA systems, and serves as a stepping stone for further QS-based strategies to enhance nitrogen removal through PNA process.

Sludge ratio affects the start-up performance and functional bacteria distribution of a hybrid CANON system

To investigate the effect of sludge ratio on the hybrid CANON system, autotrophic nitrogen removal sludge was inoculated with different granule/floc ratios to initiate the CANON system, and maintained the sludge ratio during the operation process. The start-up performances were compared, and the distribution characteristics of functional bacteria were investigated. The results show that the Equivalent system (granules:flocs = 1:1-1:1.5) successfully started-up on day 19, and the nitrogen removal rate (NRR) reached 0.299 kgN m^-3·d^-1 on day 63. At the same time, it was less affected by the load shock than High-granules and High-flocs systems. Therefore, the Equivalent system had the strongest start-up performance. The activities of the functional bacteria conformed to spatial heterogeneity, unlike the abundance. With the increased floc proportion, the difference in the activity and abundance of anaerobic ammonium-oxidizing bacteria (AAOB) between the granules and flocs increased, while there was a decrease in the difference in aerobic ammonium-oxidizing bacteria (AOB). However, the abundance of Nitrosomonas in the granules was higher than in the flocs when the proportion of flocs was higher than 50%. This study provides new ideas and insights for the fast start-up of the CANON system and can conform to the varying needs of engineering applications.

Gel immobilization: A strategy to improve the performance of anaerobic ammonium oxidation (anammox) bacteria for nitrogen-rich wastewater treatment

Anaerobic ammonium oxidation (anammox) process appears a suitable substitute to nitrification-denitrification at a lower C/N ratios. Anammox is a chemolithoautotrophic process, belong to phylum Planctomycetes, and they are slow growing bacteria. Different strategies, e.g., biofilm formation, granulation and gel immobilization, have been applied to maintain a critical mass of bacterial cells in the system by avoiding washout from the bioreactor. Gel immobilization of anammox appears the best alternative to the natural process of biofilm formation and granulation. Polyvinyl alcohol-sodium alginate, polyethylene glycol, and waterborne polyurethane are the most reported materials used for the entrapment of anammox bacteria. However, dissolution of the gel beads refrains its application for long term bioprocess. Magnetic powder could coat on the surface of the beads which may increase the mechanical strength and durability of pellets. Application and problem of immobilization technology for the commercialization of this technology also addressed.

Effects of key enzyme activities and microbial communities in a flocculent-granular hybrid complete autotrophic nitrogen removal over nitrite reactor under mainstream conditions

Recently, a flocculent-granular hybrid reactor was reported as a novel nitrogen removal system; however, the mechanisms of stable operation in the system remain unclear. In this study, the mechanisms of the stable nitrogen removal performance in a flocculent-granular hybrid system were investigated with temperature reduction. The operational period was divided into three phases with different temperatures ranges. In phase I, the nitrogen removal efficiency was stabilized at about 90% with nitrogen removal load maintained at approximately 0.28 kg N/(m³·day). In phase II, while decreasing the temperature to 20 °C, the activities of key enzymes were reduced immediately and were then maintained at a certain level. The relative abundances of aerobic ammonium-oxidizing bacteria and anaerobic ammonium-oxidizing bacteria gradually increased at this phase. In phase III, after the temperature dropped to 15 °C, the activities of key enzymes gradually increased due to adaptation to low temperature, boosting the nitrogen removal efficiency to 83%.

Roles and correlations of functional bacteria and genes in the start-up of simultaneous anammox and denitrification system for enhanced nitrogen removal

Simultaneous anammox and denitrification (SAD) is a newly developed wastewater treatment process efficient in nitrogen removal, but its underlying microbiological mechanisms during start-up remains unknown. This study investigated the changing patterns of functional bacteria and genes, as well as their correlation during the start-up (260 d) of the SAD systems in two lab-scale up-flow anaerobic sludge blanket bioreactors separately inoculated with anaerobic granular sludge (R1) and aerobic floccular sludge (R2). Results showed that high total nitrogen removal was achieved in the SAD systems of both R1 (88.25%) and R2 (89.42%). High-throughput sequencing of 16S rRNA gene amplicons revealed that Armatimonadetes phylum had a high abundance (44.34%) in R2, while was not detectable in R1 during the anammox stage. However, the SAD bioreactors retained inherent microbial community and the inoculation with different sludge showed less notable effects on their microbial composition. In the SAD systems, Candidatus Brocadia had high abundance in R1 (2.93%) and R2 (4.64%) and played important role in anammox. Network analysis indicated that Denitratisoma and Dokdonella were positively correlated with nitrite reductase genes nirS and nirK (p < 0.05), while Thermomonas and Pseudomonas showing a positive correlation with nitrate reductase gene narG (p < 0.05) were mainly responsible for the nitrate reduction in the SAD systems. Moreover, the overwhelming dominance of narG v.s. napA revealed the crucial roles of respiratory nitrate reduction in the bioreactors. The results extend our knowledge regarding the microbial ecology of the SAD system, which might be practically helpful for application of the process in wastewater treatment.

Functional characteristics and influence factors of microbial community in sewage sludge composting with inorganic bulking agent

The metabolic function of microbial community dominated organics and nutrients transformation in aerobic composting process. In this study, the metabolic characteristics of bacterial and fungal communities were evaluated in 60 days composting of sludge and pumice by using FUNGuild and PICRUSt, respectively. The results showed that microbial community structure and metabolic characteristics were distinctively different at four composting periods. Bacterial genes related to carbohydrate metabolisms decreased during the first 30 days, but bacterial sequences associated with oxidative phosphorylation and fatty acids synthesis were enhanced in curing phase. Most of fungal animal pathogen and plant pathogen disappeared after treatment, and the abundance of saprotroph fungi increased from 44.3% to 97.8%. Oxidation reduction potential (ORP) significantly increased from -28 to 175 mV through incubation. RDA analysis showed that ORP was a crucial factor on the succession of both bacterial and fungal communities in sludge composting system.