Characterization of stratified EPS and their role in the initial adhesion of anammox consortia

Anammox Granule Enlargement by Heterogenous Granule Self-assembly

Expansion in the size is an indispensable stage in the granular sludge life cycle, but little attention has been payed to the enlargement mechanism of granular sludge. Here, we propose a novel anammox granule enlargement mechanism by the self-assembly of heterogenous granules. Two different colors of anammox granules, dark-red granules (DR-Granules) and bright-red granules (BR-Granules) were found in an expanded granular sludge bed reactor. These two heterogenous granules were not isolated but were assembled into granules with a larger DR-Granule in the center and many smaller BR-Granules aggregated on the surface, increasing the overall granular size. Their physiochemical characteristics in terms of EPS, adherence, rheological properties, and microbial compositions, were identified and compared to elucidate the interaction between the different colors of granules. The BR-Granules created 92% more extracellular polymeric substances than the DR-Granules. This material blocked the passage of gas and substrate, leading to BR-Granules smaller size and a yield stress approximately 48% lower than that of the DR-Granules. Nevertheless, the BR-Granules had compact extracellular protein secondary structures and a high adherence rate to the surface of the DR-Granules, upon which they formed a compact adhered layer. These unique features enabled them to directionally adhere to DR-Granules in the core, that is, two heterogenous colors of granules self-assembled into large anammox granules. The enlargement mechanism was further supported by the abundance of K-strategy Ca. Kuenenia in the DR-Granules (inner layer) being higher than in the BR-Granules (outer layer; 2.9 ± 0.4% vs. 0.4 ± 0.1%; p = 0.0003) and by visualized confirmation that the larger BR-Granules wrapped around smaller DR-Granules inside. This demonstrates that heterogenous anammox granules actively self-assemble into large granules, which is an important step in the lifecycle of anammox granules.

Visual evidence for anammox granules expanding their size by aggregation of anammox micro-granules

Granular sludge is superior in sustainable wastewater treatment; however, no consensus has achieved in its formation mechanism. In this study, we provide visual and experimental evidences to reveal how the large anammox granules formed. Micro-observation of anammox granules illustrated that some special anammox granules were clearly composed of numerous micro-granules, which enveloped by transparent extracellular polymeric substances (EPS). Static culture experiment proved that anammox granules were easy to aggregate and form a larger entirety within approximately 14 days when there were no severe external disturbances (mainly hydraulic shear force). Stratified EPS extraction and selective enzymatic digestion tests further elucidated that tightly-bound EPS and extracellular proteins were the most vital constituents in maintaining the structure of anammox granules, and the minimal size of anammox micro-granules that aggregated to form large anammox granules was approximately 100-150 μm in the reactor studied herein. Our findings highlight that anammox granules could expand their size and form larger granules by the aggregation of anammox micro-granules, representing a natural but significant granule formation and enlargement mechanism. Understanding the enlargement mechanism could consummate the granulation process and help to culture large size anammox granules.

Long-term effects of Fe3O4 NPs on the granule-based anaerobic ammonium oxidation process: Performance, sludge characteristics and microbial community

The performance of anaerobic ammonium oxidation (anammox) granules were studied under long-term exposure to Fe₃O₄ NPs. The Fe₃O₄ NPs had no negative impacts on nitrogen removal performance with the addition of 2-200 mg L^-1. The specific anammox activity (SAA) slightly decreased from 287.0 ± 13.2 to -253.0 ± 9.2 mg TN g^-1VSS d^-1 with the increase in Fe₃O₄ NPs level from 2 to 60 mg L^-1, and then significantly enhanced to 381.8 ± 15.7 mg TN g^-1VSS d^-1 at 200 mg L^-1 Fe₃O₄ NPs. And the change trends of the heme c content, extracellular polymeric substance amount and settling velocity were consistent with that of SAA. The Candidatus_Kuenenia was the dominant species during the entire experiment and its relative abundance was up to 33.4 % at the end the experiment. The results provide some useful information for comprehending the impact of Fe₃O₄ NPs on the performance of wastewater biological treatment systems.

Insight into formation and biological characteristics of Aspergillus tubingensis-based aerobic granular sludge (AT-AGS) in wastewater treatment

The long start-up time and facile biomass loss of aerobic granular sludge (AGS) impede its application for actual wastewater treatment. The present study investigates a novel assist-aggregation strategy based on Aspergillus tubingensis (AT) mycelium pellets to accelerate sludge granulation, and engineered Fe₃O₄ nanoparticles (NPs) were used to further enhance flocculent sludge (FS) aggregation. The AT mycelium pellets, modified by 0.5 g/L Fe₃O₄@SiO₂-QC NPs (AT-V), had a more compact internal structure than the unmodified group (AT-I). The content of extracellular polymeric substances (EPS) and the zeta potential values were observed to increase from 39.86 mg/gVSS and -9.19 mv for AT-I to 69.64 mg/gVSS and 2.35 mv for AT-V, respectively. In optimized cultivation conditions, the aggregated sludge biomass of AT-V reached 1.54 g/g. An original AT-based AGS (AT-AGS) with a high biological activity (64.45 mgO₂/gVSS·h as specific oxygen uptake rate) and enhanced velocity (58.22 m/h) was developed in only 9 days. The removal efficiencies of total nitrogen (TN) and total phosphorus (TP) of the AT-AGS were 12.24% and 16.29% higher than those of the inoculated FS under high feeding load. Additionally, the analysis of cyclic diguanylate (c-di-GMP) and con-focal microscope images implied that polysaccharide (PS) of EPS played an important role in maintaining the stability of the AT-AGS. Finally, the dominant functional species contributing to sludge aggregation and pollutants removal of the AT-AGS showed a larger richness and diversity than those of the inoculated FS. This study might provide a novel high-efficiency strategy for the fast formation of AGS.

Denitrification strategies of strain YSF15 in response to carbon scarcity: Based on organic nitrogen, soluble microbial products and extracellular polymeric substances

This paper aims to determine the denitrification strategies of strain YSF15 in carbon scarcity condition from novel view of organic nitrogen, soluble microbial products (SMP) and extracellular polymeric substances (EPS). The batch tests demonstrated that strain YSF15 could achieve complete denitrification at C/N of 3.0. The conversion ratio of nitrogen gas accounted for 89.03%, 85.29% and 82.95% among total nitrogen in C/N systems from 3.0 to 5.0, respectively, indicating denitrification instead of assimilation was the major contribution to nitrogen removal. C/N could affect composition and content of organic nitrogen, SMP and EPS. The biodegradability of EPS was better than SMP, whereas polysaccharide (PS) likely correlated with nitrogen removal, predating the protein (PN). These results implied high biodegradability of EPS and more electron donors for denitrification both improved denitrification capacity of strain YSF15, which revealed the potential contribution of bacterium with production of biodegradable SMP or EPS in biological treatment process.

Start-up of single-stage partial nitritation-anammox micro-granules system: Performance and microbial community dynamics

By manipulating influent nitrogen load and DO concentration in bulk liquid, the start-up and performance of a new micro-granule based partial nitritation-anammox process was investigated in a continuous stirred tank reactor (CSTR). Under the condition of nitrogen loadings from 0.3 to 1.4 kgN /m³/d and DO <0.21mg/L, the single-stage partial nitritation-anammox (SPNA) system was successfully started, with a nitrogen removal of 76.2%. Meanwhile, the oxygen utilization efficiency by ammonium oxidizing bacteria (AOB) increased in the system with the increase of influent ammonia loading rate. Micro-granules with an average diameter of 0.25 mm were formed. Sludge granulation was promoted by increasing influent nitrogen load, and there was a positive correlation between nitrogen load, extracellular polymeric substances (EPS) content and sludge particle size. Ca. Kuenenia became the dominant anaerobic ammonium oxidizing bacteria (AnAOB) in the SPNA system. As the dominant AOB genera, Nitrosomonas coexist with Ca. Kuenenia in the micro-granules.

Interactions between nanoscale zero valent iron and extracellular polymeric substances of anaerobic sludge

The effects on the activity of anaerobic digestion (AD) of interactions between extracellular polymeric substances (EPS), a protective barrier of microorganisms towards toxic compounds, and nanoscale zero valent iron (nZVI) remain incompletely understood. In this work, EPS induced a dosage-dependent dispersion of nZVI clusters due to their effective accumulation on the nZVI surface. The small size of nZVI clusters and the formation of stable Fe-EPS complex promoted the dissolution of nZVI with a final increase of 15-20% H₂ yield. Further characterizations of EPS demonstrated the presence of some semiquinones, like riboflavin, which may work as a sink to accept electrons from nZVI. This likely explains the EPS dosage-related reduction of H₂ release rate in the initial stage and the possible decrease in nZVI reducibility responsible for disrupting cell integrity. Interactions between nZVI and EPS could improve the electrochemical activity of EPS, favoring microbial extracellular electron transfer. Therefore, the presence of EPS at relatively higher concentrations may 1) reduce the inhibition of nZVI to AD process by avoiding the fast accumulation of H₂ and restricting damage to cell integrity; 2) benefit the methanogenesis process by providing more exogenous H₂ from complete nZVI dissolution with higher electrochemical activity of EPS. This study provides insight into the interactions between EPS and nanoparticles with strong reducibility in biological wastewater treatment systems.

Anammox bacteria enrichment and denitrification in moving bed biofilm reactors packed with different buoyant carriers: Performances and mechanisms

Anaerobic ammonium oxidation (anammox) is recognized as the most cost-effective process for nitrogen removal from wastewater. In this study, effects of polyethylene plastics, nonwoven fabric, granular activated carbon (GAC) and polyurethane sponge as buoyant carriers were evaluated in lab-scale moving bed biofilm reactors (MBBRs). The overall performance of MBBRs with four types of carriers from priority to inferiority was noticed as, GAC, nonwoven fabrics, polyurethane sponge and polyethylene plastics under the same packing ratio of 20 v% and an average carrier size of 4 × 4 × 4 mm. The hydrophobic surface of GAC could selectively adsorb hydrophobic protein and favor anammox bacteria attachment, which contributed to achieving a total nitrogen removal rate of 0.40 kg-N/(m³·d) in 60 days. In conclusion, our results provide compelling evidence for achieving effective anammox process in an MBBR with GAC carriers and would benefit towards accomplishing a stable partial nitritation-anammox process in the future.

Quantitative determination of cavitation formation and sludge flotation in Anammox granules by using a new diffusion-reaction integrated mathematical model

The granulation of anaerobic ammonium oxidation (Anammox) biomass plays a key role in high rate performance of upflow-type Anammox reactors. However, the formation of cavitation inside granules may result in sludge flotation problem, which negatively affects the operation stability. For quantitative evaluation of the Anammox granules flotation in upflow reactors, an integrated mathematical model was formulated based on the principles that the limitation of substrate diffusion would result in bacterial starvation, lysis and subsequently aiding the formation of cavitation in the inner zone of granules. The proposed model is used to investigate the possible mechanism of cavitation formation and granules flotation. The combined modelling and experimental results showed that the model predictions matched well with the actual floating behavior of granules (R² = 0.83 for settled sludge and 0.76 for floating sludge). Based on the model results, the granule flotation could be divided into three zones namely (i) no-flotation zone (no flotation occurrence), (ii) transition zone (flotation with a part of granules), and (iii) flotation zone (inevitable flotation occurrence). The floating behavior of granules was mainly influenced by granule diameter (2.5-4.5 mm) and substrate concentration (NO₂-N, 50-250 mg/L) in the transition zone. The optimum granule diameter to avoid flotation but with excellent settling performance was identified around 2.5 mm. Additionally, the granule size is more sensitivity to flotation than substrate concentration. Hence, controlling the size of granules is more important to alleviate granule flotation in Anammox reactors' operation.