Determination of the response characteristics of anaerobic ammonium oxidation bioreactor disturbed by temperature change with the spectral fingerprint

Optimizing anaerobic digestion: Benefits of mild temperature transition from thermophilic to mesophilic conditions

Anaerobic digestion (AD) plays a significant role in renewable energy recovery. Upgrading AD from thermophilic (50-57 °C) to mesophilic (30-38 °C) conditions to enhance process stability and reduce energy input remains challenging due to the high sensitivity of thermophilic microbiomes to temperature fluctuations. Here we compare the effects of two decreasing-temperature modes from 55 to 35 °C on cell viability, microbial dynamics, and interspecies interactions. A sharp transition (ST) is a one-step transition by 20 °C d-1, while a mild transition (MT) is a stepwise transition by 1 °C d-1. We find a greater decrease in methane production with ST (88.8%) compared to MT (38.9%) during the transition period. ST mode overproduced reactive oxygen species by 1.6-fold, increased membrane permeability by 2.2-fold, and downregulated microbial energy metabolism by 25.1%, leading to increased apoptosis of anaerobes by 1.9-fold and release of intracellular substances by 2.9-fold, further constraining methanogenesis. The higher (1.6 vs. 1.1 copies per gyrA) metabolic activity of acetate-dependent methanogenesis implied more efficient methane production in a steady mesophilic, MT-mediated system. Metagenomic binning and network analyses indicated that ST induced dysbiosis in keystone species and greatly enhanced microbial functional redundancy, causing loss of microbial syntrophic interactions and redundant metabolic pathways. In contrast, the greater microbial interconnections (average degrees 44.9 vs. 22.1) in MT at a steady mesophilic state suggested that MT could better maintain necessary system functionality and stability through microbial syntrophy or specialized pathways. Adopting MT to transform thermophilic digesters into mesophilic digesters is feasible and could potentially enhance the further optimization and broader application of practical anaerobic engineering.

Starvation resilience in anammox-based bioreactors: A stable nitrogen removal route on partial denitrification/anammox (PD/A)

This study investigates the performance, resilience and microbial community dynamics of two anaerobic processes, i.e. pure anammox (R1) and partial denitrification/anammox (PD/A) (R2), following a 30-day starvation period. The tolerance to starvation was assessed by comparing nitrogen removal efficiency and microbial activity across both reactors. Results show that the PD/A process recovery to pre-starvation performance levels within just one day, as compared to the pure anammox process. Notably, although the activity of anammox bacteria decreased in both processes during starvation, the decay rate in R1 was 69.59 % higher than in R2, potentially explaining the quicker recovery of R2. Furthermore, enhanced secretion of extracellular polymeric substance (EPS) during starvation served as a protective mechanism. The potential functions and genes in microorganisms, as well as the pathway of nitrogen cycling, were demonstrated through analyses using the KEGG database. This research reveals essential mechanistic insights and strategic guidance for the effective implementation of anammox-based biological nitrogen removal processes.

A novel alginate-embedded magnetic biochar-anoxygenic photosynthetic bacteria composite microspheres for multipollutant removal: Mechanisms of photo-bioelectrochemical enhancement and excellent reusability performance

Existing wastewater treatment technologies face the key challenge of simultaneously removing emerging contaminants and nutrients from wastewater efficiently, with a simplified technological process and minimized operational costs. In this study, a novel alginate-embedded magnetic biochar-anoxygenic photosynthetic bacteria composite microspheres (CA-MBC-PSB microspheres) was prepared for efficient, cost-effective and one-step removal of antibiotics and NH4+-N from wastewater. Our results demonstrated that the CA-MBC-PSB microspheres removed 97.23% of sulfadiazine (SDZ) within 7 h and 91% of NH4+-N within 12 h, which were 21.23% and 38% higher than those achieved by pure calcium alginate-Rhodopseudomonas palustris microspheres (53% and 45.7%), respectively. The enhanced SDZ and NH4+-N removal were attributed to the enhanced photoheterotrophic metabolism and excretion of extracellular photosensitive active substances from R. Palustris through the photo-bioelectrochemical interaction between R. Palustris and magnetic biochar. The long-term pollutants removal performance of the CA-MBC-PSB microspheres was not deteriorated but continuously improved with increasing ruse cycles with a simultaneous removal efficiency of 99% for SDZ and 92% for NH4+-N after three cycles. The excellent stability and reusability were due to the fact that calcium alginate acts as an encapsulating agent preventing the loss and contamination of R. palustris biomass. The CA-MBC-PSB microspheres also exhibited excellent performance for simultaneous removal of SDZ (89% in 7 h) and NH4+-N (90.7% in 12 h) from the secondary effluent of wastewater treatment plant, indicating the stable and efficient performance of CA-MBC-PSB microspheres in practical wastewater treatment.

Roles of extracellular polymeric substances in the adsorption and removal of norfloxacin during hydrothermal treatment of sewage sludge

Hydrothermal treatment (HT) is promising to remove antimicrobials from sewage sludge (SS); however, the mechanism of antimicrobial degradation during the HT of SS is not fully understood. In this study, the roles of extracellular polymeric substances (EPS) in the removal and transformation of norfloxacin (NOR) during the HT of SS at temperatures of 100 and 160 °C were investigated. The results indicated that the degradation of NOR increased with increasing HT temperature, with maximum NOR removal (52%) achieved at 160 °C. Furthermore, the NOR in sludge showed higher degradation efficiencies than the control as HT temperature was higher than 120 °C. Evident promotion effects of bound-EPS (B-EPS) in sludge were observed on the NOR degradation as HT temperature was higher than 120 °C, leading to the mineralization and deamination of protein-like components in EPS during HT. Beside, the adsorption capacity of NOR during the HT of SS decreased at temperatures higher than 120 °C. The evolution of the spatial structure of B-EPS was predominantly responsible for the adsorption of antimicrobials, a spontaneous process driven mainly by hydrophilic interactions. With the hydrothermal conversion of B-EPS, the electron transfer, and reactive species (3EPS* and ·OH) derived from B-EPS could facilitate the degradation of NOR. In particular, hydrogen bonds between B-EPS and NOR increased the apparent yield of ·OH and accelerated the decarboxylation of NOR during HT at temperatures higher than 120 °C. A toxicity evaluation suggested that HT for NOR degradation could attenuate toxicity, whereas deep oxidation or mineralization would be needed to promote ecosystem safety. These findings provide new insights into the hydrothermal activation of EPS and the interrelated hydrothermal fate of antimicrobials and other toxic pollutants in sludge.

Light-Driven Electron Uptake from Nonfermentative Organic Matter to Expedite Nitrogen Dissimilation by Chemolithotrophic Anammox Consortia

Nonphotosynthetic microorganisms are typically unable to directly utilize light energy, but light might change the metabolic pathway of these bacteria indirectly by forming intermediates such as reactive oxygen species (ROS). This work investigated the role of light on nitrogen conversion by anaerobic ammonium oxidation (anammox) consortia. The results showed that high intensity light (>20000 lx) caused ca. 50% inhibition of anammox activity, and total ROS reached 167% at 60,000 lx. Surprisingly, 200 lx light was found to induce unexpected promotion of the nitrogen conversion rate, and ultraviolet light (<420 nm) was identified as the main contributor. Metagenomic and metatranscriptomic analyses revealed that the gene encoding cytochrome c peroxidase was highly expressed only under 200 lx light. 15N isotope tracing, gene abundance quantification, and external H2O2 addition experiments showed that photoinduced trace H2O2 triggered cytochrome c peroxidase expression to take up electrons from extracellular nonfermentative organics to synthesize NADH and ATP, thereby expediting nitrogen dissimulation of anammox consortia. External supplying reduced humic acid into a low-intensity light exposure system would result in a maximal 1.7-fold increase in the nitrogen conversion rate. These interesting findings may provide insight into the niche differentiation and widespread nature of anammox bacteria in natural ecotopes.

The role of light wavelengths in regulating algal-bacterial granules formation, protein and lipid accumulation, and microbial functions

High value-added products recovery from algal-bacterial granular sludge (ABGS) has received great attention recently. This study aimed to explore the role of different light wavelengths in regulating granule formation, protein and lipid production, and microbial functions. Bacterial granular sludge (BGS, R0) was most conducive to forming ABGS under blue (R2) light with the highest chlorophyll a (10.2 mg/g-VSS) and diameter (1800 μm), followed by red (R1) and white (R3) lights. R0-R3 acquired high protein contents (>164.8 mg/g-VSS) with essential amino acids above 44.4%, all of which were suitable for recycling, but R2 was the best. Also, blue light significantly increased total lipid production, while red light promoted the accumulation of some unsaturated fatty acids (C18:2 and C18:3). Some unique algae and dominant bacteria (e.g., Stigeoclonium, Chlamydomonas, and Flavobacteria) enrichment and some key functions (e.g., amino acid, fatty acid, and lipid biosynthesis) up-regulation in R2 might help to improve proteins and lipids quality. Combined, this study provides valuable guidance for protein and lipid recovery from ABGS.

Towards advanced simultaneous nitrogen removal and phosphorus recovery from digestion effluent based on anammox-hydroxyapatite (HAP) process: Focusing on a solution perspective

In this paper, the state-of-the-art information on the anammox-HAP process is summarized. The mechanism of this process is systematically expounded, the enhancement of anammox retention by HAP precipitation and the upgrade of phosphorus recovery by anammox process are clarified. However, this process still faces several challenges, especially how to deal with the ∼ 11% nitrogen residues and to purify the recovered HAP. For the first time, an anaerobic fermentation (AF) combined with partial denitrification (PD) and anammox-HAP (AF-PD-Anammox-HAP) process is proposed to overcome the challenges. By AF of the organic impurities of the anammox-HAP granular sludge, organic acid is produced to be used as carbon source for PD to remove the nitrogen residues. Simultaneously, pH of the solution drops, which promotes the dissolution of some inorganic purities such as CaCO₃. In this way, not only the inorganic impurities are removed, but the inorganic carbon is supplied for anammox bacteria.

Oxidative stress and antioxidant mechanisms of obligate anaerobes involved in biological waste treatment processes: A review

In-depth understanding of the molecular mechanisms and physiological consequences of oxidative stress is still limited for anaerobes. Anaerobic biotechnology has become widely accepted by the wastewater/sludge industry as a better alternative to more conventional but costly aerobic processes. However, the functional anaerobic microorganisms used in anaerobic biotechnology are frequently hampered by reactive oxygen/nitrogen species (ROS/RNS)-mediated oxidative stress caused by exposure to stressful factors (e.g., oxygen and heavy metals), which negatively impact treatment performance. Thus, identifying stressful factors and understanding antioxidative defense mechanisms of functional obligate anaerobes are crucial for the optimization of anaerobic bioprocesses. Herein, we present a comprehensive overview of oxidative stress and antioxidant mechanisms of obligate anaerobes involved in anaerobic bioprocesses; as examples, we focus on anaerobic ammonium oxidation bacteria and methanogenic archaea. We summarize the primary stress factors in anaerobic bioprocesses and the cellular antioxidant defense systems of functional anaerobes, a consortia of enzymatic and nonenzymatic mechanisms. The dual role of ROS/RNS in cellular processes is elaborated; at low concentrations, they have vital cell signaling functions, but at high concentrations, they cause oxidative damage. Finally, we highlight gaps in knowledge and future work to uncover antioxidant and damage repair mechanisms in obligate anaerobes. This review provides in-depth insights and guidance for future research on oxidative stress of obligate anaerobes to boost the accurate regulation of anaerobic bioprocesses in challenging and changing operating conditions.

The response of anaerobic ammonium oxidation process to bisphenol-A: Linking reactor performance to microbial community and functional gene

As a typical endocrine disruptor, bisphenol A (BPA) has been widely detected in various water bodies. Although the influence of BPA on traditional biological treatment system has been investigated, it is not clear whether it has potential impact on anaerobic ammonium oxidation (anammox) process. The short- and long-term influences of BPA on reactor operational performance, sludge characteristics and microbial community were investigated in this study. Results revealed that 1 and 3 mg L^-1 BPA exhibited a limited adverse impact on granular sludge reactor performance. However, exposure of sludge under 10 mg L^-1 BPA would cause an obvious inhibition on nitrogen removal rate from 10.3 ± 0.2 to 7.6 ± 0.4 kg N m^-3 d^-1. BPA would affect granular sludge metabolic substance excretion and lead to effluent dissolved organic content increase. Both the microbial community and redundancy analysis showed that BPA exhibited a negative influence on Ca. Kuenenia but a positive correlation with SBR1031. Low BPA concentration appeared a limited impact on functional genes while 10 mg L^-1 BPA would cause decline of hzsA and hdh abundances. The results of this work might be valuable for in-depth understanding the potential influence of endocrine disruptor on anammox sludge.

Molecular spectroscopy and docking simulation revealed the binding mechanism of phenol onto anammox sludge extracellular polymeric substances

The rapid development of chemical industry has induced to the large amount of phenolic wastewater production. When the promising anaerobic ammonium oxidation (anammox) was employed to treat the industrial wastewater, phenolic compounds would possibly inhibit the microbial performance. Extracellular polymeric substances (EPSs) play an essential role in protecting cells from being intoxicated by phenolic compound while the distinct mechanism remains elusive. In this work, the interaction of phenol with anammox sludge EPSs and transmembrane ammonium transport (Amt) domain was explored at molecular level by using spectral method and molecular docking simulation. It was found that phenol statically quenched the fluorescent components of EPSs and the protein component dominated the interaction between EPSs and phenol. The overall interaction was an entropy-driven process with hydrophobic interaction as the main driving force, and the CO vibration responded preferentially. As phenol continued to penetrate into the cell surface, there were hydrogen bond, hydrophobic interaction force and π-π base-stacking forces between the Amt domain and phenol. The interaction between phenol and amino acid residues of the Amt domain would interfere the NH₄⁺ transport and further affect the activity of anammox sludge. This work is beneficial for in-depth understanding the role of EPSs in protecting anammox sludge from inhibiting by phenolic pollutants.

Nitrogen removal performance of anammox immobilized fillers in response to seasonal temperature variations and different operating modes: Substrate utilization and microbial community analysis

Four anaerobic ammonium oxidation (anammox) immobilized filler reactors (R1: 33 °C-normal, R2: seasonal temperature-normal, R3: seasonal temperature-feast, R4: seasonal temperature-starvation) were established to study the response of anammox immobilized fillers to seasonal temperature changes and different operating modes. The results showed that the anammox immobilized filler could better adapt to the seasonal temperature drop and maintain the activity potential by adjusting the hydraulic retention time (HRT). During the temperature rise phase, R2 activity increased rapidly with the highest nitrogen removal rate reaching 1.26 kgN·(m³·d)^-1, which was equivalent to control sample R1 (1.33 kgN·(m³·d)^-1). However, feasting and famine conditions severely impaired anammox performance and changed stoichiometric ratios; feasting, in particular, significantly lowered the nitrogen removal potential of R3. The specific anammox activity of R2, R3 and R4 was 92.2%, 52.6% and 67.9%, respectively, that of R1, respectively, where the accumulation of functional bacteria was the reason for the higher activity of R2. Degradation kinetics and NO₂^--N inhibition curves showed that R3 was less sensitive to high concentrations of NH₄⁺-N, while R4 responded earlier to low concentrations of NH₄⁺-N, and the reduction of IC50 at low temperature was the reason for the inhibition of R3 activity. Furthermore, seasonal temperature fluctuations had little effect on the microbial community structure but had a considerable impact on bacteria abundance. The anammox functional bacteria Candidatus Kuenenia was found to be the dominant genus in R1-R4; however, the relative abundance of most bacteria, including anammox bacteria, decreased in R3, while the proportion of fermentation bacteria and denitrifying bacteria increased in R4. These findings highlight the necessity of rational regulation of HRT for the adaptation of anammox immobilized fillers to seasonal temperature changes, which could enhance our understanding of the synergistic effect of seasonal temperature changes and different operating modes on nitrogen removal.

A review on characterizing the metabolite property of anammox sludge by spectroscopy

As one of the most promising autotrophic biological nitrogen removal technology, anaerobic ammonia oxidation (anammox) has gained intense attention for the past decades and several full-scale facilities have been implemented worldwide. However, anammox bacteria are easily affected by disturbed external environmental factors, which commonly leads to the fluctuations in reactor performance. The response of anammox sludge to external stress results in changes in components and structural characteristics of intracellular and extracellular polymer substances. Real-time and convenient spectral analysis of anammox sludge metabolites can give early warning of performance deterioration under external stresses, which is of great significance to the stable operation of bioreactor. This review summarized the research progress on characterizing the intracellular and extracellular metabolites of anammox sludge through spectroscopic techniques. The correlation between anammox sludge activity and its key metabolites was analyzed. Also, the limitations and future prospects of applying spectral analytical techniques for anammox bioreactor monitoring were discussed and outlooked. This review may provide valuable information for both scientific study and engineering application of anammox based nitrogen removal technology.

Molecular Insight into the Binding Property and Mechanism of Sulfamethoxazole to Extracellular Proteins of Anammox Sludge

Antibiotics are widely found in nitrogen-containing wastewater, which may affect the operation stability of anaerobic ammonium oxidation (anammox)-based biological treatment systems. Extracellular polymeric substances (EPSs) of anammox sludge play a pivotal role in combining with antibiotics; however, the exact role and how the structure of the leading component of EPSs (i.e., extracellular proteins) changes under antibiotic stress remain to be elucidated. Here, the interaction between sulfamethoxazole and the extracellular proteins of anammox sludge was investigated via multiple spectra and molecular simulation. Results showed that sulfamethoxazole statically quenched the fluorescent components of EPSs, and the quenching constant of the aromatic proteins was the largest, with a value of 1.73 × 10⁴ M^-1. The overall binding was an enthalpy-driven process, with ΔH = -75.15 kJ mol^-1, ΔS = -0.175 kJ mol^-1 K^-1, and ΔG = -21.10 kJ mol^-1 at 35 °C. The O-P-O and C═O groups responded first under the disturbance of sulfamethoxazole. Excessive sulfamethoxazole (20 mg L^-1) would decrease the ratio of α-helix/(β-sheet + random coil) of extracellular proteins, resulting in a loose structure. Molecular docking and dynamic simulation revealed that extracellular proteins would provide abundant sites to bind with sulfamethoxazole, through hydrogen bond and Pi-Akyl hydrophobic interaction forces. Once sulfamethoxazole penetrates into the cell surface and combines with the transmembrane ammonium transport domain, it may inhibit the NH₄⁺ transport. Our findings enhance the understanding on the interaction of extracellular proteins and sulfamethoxazole, which may be valuable for deciphering the response property of anammox sludge under the antibiotic stress.

Photochemical Behavior of Microbial Extracellular Polymeric Substances in the Aquatic Environment

Microbially derived extracellular polymeric substances (EPSs) occupy a large portion of dissolved organic matter (DOM) in surface waters, but the understanding of the photochemical behaviors of EPS is still very limited. In this study, the photochemical characteristics of EPS from different microbial sources (Shewanella oneidensis, Escherichia coli, and sewage sludge flocs) were investigated in terms of the production of reactive species (RS), such as triplet intermediates (³EPS*), hydroxyl radicals (^•OH), and singlet oxygen (¹O₂). The steady-state concentrations of ^•OH, ³EPS*, and ¹O₂ varied in the ranges of 2.55-8.73 × 10^-17, 3.01-4.56 × 10^-15, and 2.08-2.66 × 10^-13 M, respectively, which were within the range reported for DOM from other sources. The steady-state concentrations of RS varied among different EPS isolates due to the diversity of their composition. A strong photochemical degradation of the protein-like components in EPS isolates was identified by excitation emission matrix fluorescence with parallel factor analysis, but relatively, humic-like components remained stable. Fourier-transform ion cyclotron resonance mass spectrometry further revealed that the aliphatic portion of EPS was resistant to irradiation, while other portions with lower H/C ratios and higher O/C ratios were more susceptible to photolysis, leading to the phototransformation of EPS to higher saturation and lower aromaticity. With the phototransformation of EPS, the RS derived from EPS could effectively promote the degradation of antibiotic tetracycline. The findings of this study provide new insights into the photoinduced self-evolution of EPS and the interrelated photochemical fate of contaminants in the aquatic environment.

Architecture of HAP-anammox granules contributed to high capacity and robustness of nitrogen removal under 7°C

The anaerobic ammonium oxidation (anammox) process is an autotrophic nitrogen removal process with great potential as a cost-effective and highly efficient technology in the wastewater treatment field. The main challenges yet to be overcome in this new frontier technology are operating at lower temperatures and achieving a high and stable nitrogen removal efficiency. In this study, an up-flow expanded bed reactor with hydroxyapatite (HAP)-anammox granules was operated for more than 200 days at 7°C. The nitrogen loading rate (NLR) was improved from 1.0 g-N/L/d to 3.6 g-N/L/d, together with a high-level nitrogen removal efficiency of 84-92%, which is the highest to date at extremely low temperatures in a continuous experiment. Candidatus Kuenenia was revealed to be the only dominant anammox genus, with a relative abundance of 35.3-37.5%. The optimal operational temperature was around 35°C and the apparent activation energy (E_a) was calculated as 78.37 kJ/mol. The three-layers architecture and architectural evolution of HAP-anammox granules into HAP-cores and peeling biofilms with outstanding settling performance were characterized. Under 7°C, the high capacity of nitrogen removal with robust removal efficiency using HAP-anammox granules was achieved.