Reactivation of effluent granular sludge from a high-rate Anammox reactor after storage

Roles of red mud-based biochar carriers in the recovery of anammox activity: characteristics and mechanisms

Anaerobic ammonium oxidation (anammox) sludge is easily deactivated in the process of treating ammonia-laden wastewater. To investigate an effective recovery method, red mud-based biochar carriers (RMBC) were prepared and added to a deactivated anammox reactor; the operation of this reactor had been interrupted for 6 months with starvation and low temperature. The deactivated sludge with added RMBC was recovered rapidly after 31 days, with the specific anammox activity rapidly increasing to 0.84 g N/(g VSS∙day), and the recovery efficiency of nitrogen removal rate increased by four times compared to the unadded control. The granulation degree and extracellular polymeric substances secretion of the anammox sludge with the added RMBC were significantly higher than that of the control group. In addition, a large number of spherical anammox bacteria were observed moored at the porous channels of RMBC, and the copy numbers of functional genes of anammox bacteria were approximately twice that of the control group. Hence, RMBC is a potential sludge activator, and it can provide a "house" to protect anammox bacteria, enhance the metabolic activity and the agglomerative growth of anammox bacteria, and synergistically achieve rapid recovery of deactivated anammox sludge.

Characteristics variations of size-fractionated anammox granules and identification of the potential effects on these evolutions

Anaerobic ammonium oxidation (anammox) granulation which contributed to system stabilization and performance improvement has great potential in the field of wastewater nitrogen removal. The researchers fractionated anammox granules into small-size (0.5-0.9 mm), medium-size (1.8-2.2 mm), and large-size (2.8-3.5 mm) categories to examine their properties and mechanisms. Various analyses, including high-throughput sequencing, determination of inorganic elements and extracellular polymeric substances (EPS), and microbial function prediction, were conducted to characterize these granules and understand their impact. The results revealed distinct characteristics among the different-sized granules. Medium-size granules exhibited the highest sphericity, EPS content, and anammox abundance. In contrast, large-size granules had the highest specific surface area, heme c content, specific anammox activity, biodiversity, and abundance of filamentous bacteria. Furthermore, the precipitates within the granules were identified as CaCO3 and MgCO3, with the highest inorganic element content found in the large-size granules. Microbial community and function annotation also varied with granule size. Based on systematic analysis, the researchers concluded that cell growth, chemical precipitation, EPS secretion, and interspecies interaction all played a role in granulation. Small-size granules were primarily formed through cell growth and biofilm formation. As granule size increased, EPS secretion and chemical precipitation became more influential in the granulation process. In the large-size granules, chemical precipitation and interspecies interaction, including synergistic effects with nitrifying, denitrifying, and filamentous bacteria, as well as metabolic cross-feeding, played significant roles in aggregation. This interplay ultimately contributed to higher anammox activity in the large-size granules. By fully understanding the mechanisms involved in granulation, this study provides valuable insights for the acclimation of anammox granules with optimal sizes under different operational conditions.

Feasibility of anaerobic ammonium oxidation process for treatment of pretreated printed circuit board wastewater

The treatment of pretreated printed circuit board (PCB) wastewater by anaerobic ammonium oxidation (anammox) process has been rarely reported. This study sought to investigate the performance of the anammox process during various phases of pretreated PCB wastewater treatment. The nitrogen removal efficiency (NRE) reached 90 ± 10% at a Cu2+ concentration of 2.5 mg·L-1, but declined to 22 ± 11% as the Cu2+ level increased to 10.3  mg·L-1. During phase III, there was a 38% increase in the relative abundance of Candidatus Kuenenia compared to phase I. By adjusting the substrate concentration and introducing synthetic wastewater into the reactor, the anammox performance was nearly restored to that of phase I. These findings underscore the potential of the anammox process for treating pretreated PCB wastewater and expanding its practical applications to industrial wastewater treatment.

Metabolomic pathway regulation to achieve optimal control of inorganic carbon in anammox process

The significance of inorganic carbon (IC) for anaerobic ammonium oxidation (anammox) bacteria has been verified. However, the regulation of metabolic pathways under IC stress is not clear, limiting the optimization of IC supply. In this study, the regulatory pathways at IC concentration of 5-150 mg/L were explored to achieve optimal control of IC. The results show that the changes of metabolic pathway under IC stress determined anammox characteristics. At IC concentration of 5 mg/L, the anammox activity distinctly decreased due to the guanosine tetraphosphate (ppGpp) -mediated regulation under IC limitation. With less than 15 mg/L of IC, the decrease of carbon fixation limited the biosynthesis of gluconeogenesis and amino acids, causing the decline of extracellular polymeric substance synthesis. With more than 50 mg/L of IC, the improvement of purine and pyrimidine metabolism enhanced the electron transport capacity and growth potential of anammox bacteria. This study provides metabolic insights into IC influence on anammox consortia and a novel method of IC concentration optimization using metabolomics analysis.

Anammox biofilter with denitrification sludge as seed in treating low nitrogen strength wastewater

Deficient seed sludge, low substrate concentrations are recognized as the major barriers for the application of anaerobic ammonia oxidation (Anammox) to treat mainstream wastewater. In this work, anammox biofilter (A-BF) was started up by inoculating denitrification sludge at low nitrogen strength at 25 °C. The total nitrogen removal efficiency (TNRE) and nitrogen removal rate (NRR) reached 74.8 ± 3.4% and 0.81 kg-N m^-3 d^-1 under nitrogen loading rate (NLR) of 1.20 kg-N m^-3 d^-1 with 7.00 mg-NH₄⁺-N L^-1 and 10.00 mg-NO₂^--N L^-1 as influent. 1.00-2.00 mg-DO L^-1 negatively impacted effluent, but the total nitrogen of effluent (TN_eff) was 10.65 ± 2.76 mg L^-1, in limit of the standard of Class 1A for municipal WWTP discharge (GB18918-2002). The abundance of Planctomycetes increased from 0.6% to 1.4-2.6%, in which, Candidatus_Brocadia was the dominant genera. The results establish the application feasibility of A-BFs as advanced nitrogen removal technique in treating mainstream wastewater.

Nitrogen contribution and microbial community of size-fractionated anammox sludge in continuous stirred-tank reactors

In this study, the microbial diversity of size-fractionated anammox sludge in a well-mixed system and their contribution to nitrogen transformation were investigated. Results showed that small granules (0.2-1.0 mm) contributed to the major part of the nitrogen removal rate (56 %) due to its largest mixed liquor volatile suspended solids (1240 ± 80 mg·L^-1). However, large granules (>1.0 mm) possessed the highest relative abundances of Ca. Kuenenia stuttgartiensis and specific anammox activity, representing 49.34 % and 24.45 ± 0.01 mg-N·g^-1-mixed liquor volatile suspended solids·h^-1, respectively. The microbial diversity decreased as the increase of granular size, resulting in microbial community shifting to a simpler model. Metagenomic analysis showed that fine sludge might be the potential major for NO/N₂O production in the mature well-mixed system under inorganic conditions. This study provides guidance for the evaluation of nitrogen contribution by anammox size-fractionated sludge and the inhibition of the potential NO/N₂O emission in anammox processes.

Inorganic quantum dots - anammox consortia hybrid for stable nitrogen elimination under high-intensity solar-simulated irradiation

External stimulus such as light irradiation is able to deteriorate intracellular redox homeostasis and induce photooxidative damage to non-photogenic bacteria. Exploiting effective strategies to help bacteria resisting infaust stress is meaningful for achieving a stable operation of biological treatment system. In this work, selenium-doped carbon quantum dots (Se-CQDs) were blended into anaerobic ammonia oxidation (anammox) bacteria and an inorganic nanoparticle-microbe hybrid was successfully fabricated to evaluate its nitrogen removal performance under solar-simulated irradiation. It was found that the specific anammox activity decreased by 29.7 ± 5.2% and reactive oxygen species (ROS) content increased by 134.8 ± 4.1% under 50,000 lux light. Sludge activity could be completely recovered under the optimum dosage of 0.42 mL·(g volatile suspended solid) ^-1 Se-CQDs. Hydroxyl radical (·OH) and superoxide anion radical (·O₂^-) were identified as the leading ROS inducing lipid peroxidation and antioxidase function detriment. Also, the structure of ladderane lipids located on anammoxosome was destroyed by ROS and functional genes abundances declined accordingly. Although cell surface coated Se-CQDs could absorb ultraviolet light and partially mitigated the photoinhibition, the direct scavenging of ROS by intracellular Se-CQDs primarily contributed to the cellular redox homeostasis, antioxidase activity recovery and sludge activity improvement. The findings of this work provide in-depth understanding the metabolic response mechanism of anammox consortia to light irradiation and might be valuable for a more stable and sustainable nitrogen removal technology, i.e., algal-bacterial symbiotic system, development.

Recovery of anammox process performance after substrate inhibition: Reactor performance, sludge morphology, and microbial community

Most of the current studies have focused on the inhibition of anaerobic ammonium oxidation (anammox) by substrates, however, little attention has been paid to the recovery process of the reactor after inhibition. Therefore, we investigated the changes in reactor performance, granular sludge structure, and microbial community during the recovery phase after being inhibited by a high nitrogen load for 15 d. The nitrogen removal rate of the reactorwasrestored to pre-inhibition levels after 75 d of recovery, and the stoichiometric ratio converged to the theoretical value. The surface of the granular sludge developed into a broccoli-like structure, and the Ca and P contents of the granules increased from 6.88% and 4.39% to 24.42% and 13.88%, respectively. The abundance of the anammox bacterium Candidatus brocadia increased from 5.86% to 12.10%, and network analysis indicated that SMA102 and SBR1031 were positively correlated with the occurrence of Candidatus brocadia.

Accelerating anammox nitrogen removal in low intensity ultrasound-assisted ASBBR: Performance optimization, EPS characterization and microbial community analysis

Efficient enrichment of slow-growing anammox species is essential for rapid start-up and stable operation of high-rate anammox reactors. Herein, a low intensity ultrasound (LIU) was introduced into anaerobic sequencing batch biofilm reactors (ASBBRs) to enhance anammox nitrogen removal from nitrogen-rich wastewater. Operation results demonstrated that the maximum total nitrogen (TN) removal efficiency of 91.5% were achieved under the optimal ultrasonic parameters (32.7 °C water temperature, 0.18 W/cm² ultrasonic intensity and 25.7 min ultrasonication time). Moreover, significant increases of extracellular polymeric substances (EPS) components and contents were observed via the ultrasonication stimulation. A close correlation between nitrogen removal and shifts in transformation and intensity of spectrum peaks was also verified by three-dimensional excitation-emission matrix spectroscopy (3D-EEM) analysis. High-throughput sequencing revealed that the relative abundance of Candidatus Kuenenia as the key anammox consortium significantly increased after applying optimal ultrasonication condition. Furthermore, enhancement mechanisms and future prospect of the LIU-assisted anammox process was elucidated and discussed. This research provides a viable and promising acceleration strategy for anammox-based process in practice.

Mesophilic condition favors simultaneous partial nitrification and denitrification (SPND) and anammox for carbon and nitrogen removal from anaerobic digestate food waste effluent

Three simultaneous partial nitrification and denitrification (SPND) bioreactors were established on ambient (30 °C), mesophilic (40 °C) and thermophilic condition (50 °C) at high dissolved oxygen levels (2-7 mg L^-1) to remove nitrogen and carbon from anaerobic digestate food waste effluent (ADFE). The bioreactor performed best under mesophilic condition, with TN and COD removal efficiency of 96.3 ± 0.1% and 91.7 ± 0.1%, respectively. Free ammonia (FA) and free nitrous acid (FNA) alternately ensured selective inhibition of nitrite-oxidizing bacteria (NOB) in long-term operation of SPND systems. Candidatus Brocadia, known as anammox bacteria, was observed unexpectedly in the bioreactors. The analysis of microbial community and metabolic pathways revealed that mesophilic strategy stimulated SPND and anammox process. Mesophilic condition helped autotropic microbes resist the competitive pressure from heterotrophic bacteria, improving the balance between nitrifiers, anammox bacteria and other co-existing heterotrophs. Overall, this study offers new insights into the linkage among temperature, pollutant removals (carbon and nitrogen) and metabolic potential in the SPND bioreactors.

Enhancing anammox resistance to low operating temperatures with the use of PVA gel beads

Low temperatures, or a sudden decrease in operating temperature, can seriously inhibit anammox activity, it is, therefore, important to maintain anammox activities at a low temperature. In this study, the use of gel beads to enhance the resistance of anammox biomass to a low temperature was investigated. The performance of three reactors: R1 without gel beads; R2 with polyvinyl alcohol/chitosan (PVA/CS); R3 with PVA/CS/Fe, was studied and compared in a temperature transition from 35 to 8 °C. When the operating temperature was ≥25 °C, there was little difference in nitrogen removal among the three reactors. Decreasing the temperature to < 25 °C created obvious difference between R1 and R2/R3. R1 had a nitrogen removal efficiency (NRE) of 33.1 ± 25.3% at 10 °C, significantly lower than that of R2 (90.5 ± 2.5%) or R3 (87.7 ± 11.1%). Unclassified Candidatus Brocadiaceae was the dominant genus at 10 °C, with an abundance of 44.4, 56.5 and 58.7% in R1, R2 and R3, respectively. These differences were attributed to the use of gel beads, which promoted the granulation of both the non-immobilized sludge and the immobilized biomass, resulting in higher anammox activities in R2/R3. The non-immobilized sludge of R1 was dominated by small particles (<300 μm) at 10 °C, while in R2 and R3 large particles (1000-2000 μm) were the main components. Furthermore, the immobilized biomass on gel beads exhibited much higher anammox activity and maintained a relatively high level of nitrate reductase and nitrite reductase in response to the temperature decrease. The Fe²⁺/Fe³⁺ in the PVA/CS/Fe gel beads further promoted microbial aggregation and led to an improved performance in R3 compared to R2. The results of this study demonstrate an effective approach to increase anammox resistance at low operating temperatures.

Multiple electron acceptor-mediated sulfur autotrophic denitrification: Nitrogen source competition, long-term performance and microbial community evolution

Sulfur-driven autotrophic denitrification (SDAD) is feasible for the treatment of low-C/N-ratio and sulfur-laden wastewaters. The nitrite accumulated in SDAD will affect the performance and stability of the system but can be a potential electron acceptor. Thus, single- and multiple-electron acceptor-mediated SDAD systems were investigated. Batch assays revealed that nitrite and nitrate were the preferential options in the SDAD system with single and multiple electron acceptors, respectively. Synchronous nitrogen and sulfur removal was successfully achieved in continuous flow experiments with multiple electron acceptors, and the system could adapt well to high concentrations of sulfide, nitrate and nitrite (i.e., 720, 108 and 64.8 mg L^-1, respectively), with the predominant genera shifting from Thiobacillus (48.88%) at the initial stage to unclassified_p_Firmicute (34.24%) and Syner-01 (12.31%) at the last stage. This work provides a fundamental basis for applying and regulating SDAD with multiple electron acceptors for the remediation of nitrogen- and sulfide- laden wastewaters.

Effects of biotin on promoting anammox bacterial activity

Anaerobic ammonium oxidation (anammox) bacteria significantly improve the efficiency and reduce cost of nitrogen removal in wastewater treatment plants. However, their slow growth and vulnerable activity limit the application of anammox technology. In this paper, the enhancement of biotin on the nitrogen removal activity of anammox bacteria in short-term batch experiments was studied. We found that biotin played a significant role in promoting anammox activity within a biotin concentration range of 0.1-1.5 mg/L. At a biotin concentration of 1.0 mg/L, the total nitrogen removal rate (NRR) increased by 112%, extracellular polymeric substance (EPS) secretion and heme production significantly improved, and anammox bacterial biomass increased to maximum levels. Moreover, the predominant genus of anammox bacteria was Candidatus Brocadia.

A critical review on the effects of antibiotics on anammox process in wastewater

Anaerobic ammonium oxidation (anammox) has recently become of significant interest due to its capability for cost-effective nitrogen elimination from wastewater. However, anaerobic ammonia-oxidizing bacteria (AnAOB) are sensitive to environmental changes and toxic substances. In particular, the presence of antibiotics in wastewater, which is considered unfavorable to the anammox process, has become a growing concern. Therefore, it is necessary to evaluate the effects of these inhibitors to acquire information on the applicability of the anammox process. Hence, this review summarizes our knowledge of the effects of commonly detected antibiotics in water matrices, including fluoroquinolone, macrolide, β-lactam, chloramphenicol, tetracycline, sulfonamide, glycopeptide, and aminoglycoside, on the anammox process. According to the literature, the presence of antibiotics in wastewater could partially or completely inhibit anammox reactions, in which antibiotics targeting protein synthesis or DNA replication (excluding aminoglycoside) were the most effective against the AnAOB strains.

A novel reactor combining anammox and Fenton-like reactions for the simultaneous removal of organic carbon and nitrogen at different organic carbon to nitrogen ratios

A Fenton-like reaction and anaerobic ammonium oxidation (anammox) were combined to construct a novel process named FenTaMox for removing nitrogen (N) and organic carbon (measured as chemical oxidation demand (COD)). Two columns were packed with iron-manganese-sepiolite, a catalyst that uses hydrogen peroxide (H₂O₂) to catalyze Fenton-like reactions, and inoculated with marine anammox bacteria. During the start-up, marine anammox medium was fed into both columns to acclimate the marine anammox bacteria to iron-manganese-sepiolite. Batch experiments revealed that the marine anammox bacteria were not affected by 60 mgL^-1 of H₂O₂. Next, medium containing glucose and H₂O₂ was fed into one column as the FenTaMox treatment, while medium containing glucose but no H₂O₂ was fed into the other column as the control. At a COD/N of 4, FenTaMox exhibited higher removal efficiencies of N and COD compared with that of the control, suggesting the application of FenTaMox for organic carbon- and N- removal.

Granule-based immobilization and activity enhancement of anammox biomass via PVA/CS and PVA/CS/Fe gel beads

Granule-based immobilization of anammox biomass assisted by polyvinyl alcohol/chitosan (PVA/CS) and PVA/CS/Fe gel beads was studied, via the operation of three identical up-flow reactors (R1 without gel beads, R2 with PVA/CS, R3 with PVA/CS/Fe) for 203 days. In the end, the nitrogen removal rates (NRR) were 5.3 ± 0.4, 10.0 ± 0.3 and 13.9 ± 0.5 kg-N m^-3 d^-1 for R1, R2 and R3, respectively. The porous PVA/CS and PVA/CS/Fe created a suitable eco-niche for anammox bacteria to grow and attach, thus being retained in the reactor. The EPS entangles newly grown cells within the gel beads, resulting in compact aggregation. The interaction between Fe ions added to PVA/CS/Fe gel beads and negatively charged EPS groups strongly promoted granule strength and compactness. The immobilization method proposed by this study was found to effectively improve biomass retention in the reactors, which is promising for advanced anammox process applications.

A novel strategy for anammox consortia preservation: Transformation into anoxic sulfide oxidation consortia

The preservation of anaerobic ammonia oxidation (anammox) consortia is crucial for the rapid start-up and the process stability of the anammox based bioreactor. This work proposed and evaluated the feasibility of an anammox consortia preservation strategy, in which the anammox sludge was transformed into intermediate anoxic sulfide oxidation (ASO) functional microorganisms. Initially, the ASO process was successfully started up by inoculating anammox sludge and the overall sulfide and nitrate removal rates stabilized at 57.5 ± 0.22 and 10.0 ± 0.18 kg m^-3 day^-1, respectively. Then, the bioreactor function was reversely transformed into anammox, whose nitrogen removal rate reached 1.68 kg m^-3 day^-1. Granule characteristics analysis revealed that both biomass and extracellular polymeric substance content returned to their original states after the reverse start-up. Although the population of Candidatus_Kuenenia was greatly declined during ASO process, its richness was successfully recovered after the reverse start-up of the anammox process. The inferred metagenomes analysis demonstrated that the shifts in functional microorganisms were related to variation in the main metabolic pathways. The specific activities of anammox and ASO both are regarded as key indicators for the successful start-up of bioreactor. This work revealed a novel technique for the preservation of anammox consortia and might be a potential strategy for overcoming the drawback of long start-up time.

Deciphering the microbial community and functional genes response of anammox sludge to sulfide stress

Sulfide has attracted increasing attention due to its odor nuisance, toxicity and corrosion. Although variations in the nitrogen removal performance of anammox under sulfide stress have been reported previously, understanding the microorganisms at the molecular level is of greater significance. This study first deciphered the microbial community and functional gene response of anammox sludge to sulfide stress. Results showed that 20 mg L^-1 sulfide could reduce specific anammox activity by 61.7%. The protein-like substances within extracellular polymeric substances were quenched at the end of the experiment. Moreover, the relative abundance of Candidatus Kuenenia significantly decreased from 28.7% to 6.4% while Thiobacillus increased from 0 to 7.2% due to sulfide stress. Furthermore, the abundances of functional genes (hzsA, hdh, nirK and nirS) significantly decreased when the sulfide concentration reached 20 mg L^-1. These findings provide a further theoretical basis for the anammox process for nitrogen removal from wastewater containing sulfide.

Effects of biomass and environmental factors on nitrogen removal performance and community structure of an anammox immobilized filler

In order to reduce the loss of anaerobic ammonia oxidation (anammox) sludge and stabilize the reaction microenvironment, polyvinyl alcohol - polypropylene (PVA-PP) was used to encapsulate anammox bacteria on a filler. The influence of different inoculation amounts (2, 4, 6 and 8%) on the overall nitrogen removal process was first compared and then the anammox characteristics of the immobilized filler under the influence of different environmental factors were evaluated through batch experiments. The results show that the biomass only affected the growth rate of the activity during the logarithmic phase, while the total nitrogen removal rate (NRR) tended to be similar after 99 d of culture. The NRR reached 1.83 kg·(m³·d)^-1 on day 140, which was 9.4 times that of suspended sludge before encapsulation, and the structure of embedding filler was complete without shedding. Scanning electron microscopy (SEM) showed that the internal porous network structure formed channels and a large number of anammox bacteria were observed around. Microbial community analysis of the 16S rDNA gene showed that the diversity was maintained in the entrapped carrier. Furthermore, the effective enrichment of the anammox functional bacteria Candidatus Kuenenia (AF375995.1) increarsed from 11.06% to 32.55%. The PVA-PP immobilized filler fit well with the biological nitrogen removal kinetic model and could also achieve coupling of anammox and denitrification. The inhibition effect of the organic carbon source interference and starvation on anammox bacteria was significantly weakened.

Investigation of the process stability of different anammox configurations and assessment of the simulation validity of various anammox-based kinetic models

Over the last 30 years, the successful implementation of the anammox process has attracted research interest from all over the world. Various reactor configurations were investigated for the anammox process. However, the construction of the anammox process is a delicate topic in regards to the high sensitivity of the biological reaction. To better understand the effects of configurations on the anammox performance, process-kinetic models and activity kinetic models were critically overviewed, respectively. A significant difference in the denitrification capabilities was observed even with similar dominated functional species of anammox with different configurations. Although the kinetic analysis gained insight into the feasibility of both batch and continuous processes, most models were often applied to match the kinetic data in an unsuitable manner. The validity assessment illustrated that the Grau second-order model and Stover–Kincannon model were the most appropriate and shareable reactor-kinetic models for different anammox configurations. This review plays an important role in the anammox process performance assessment and augmentation of the process control.

Over the last 30 years, the successful implementation of the anammox process has attracted research interest from all over the world.

The performance and microbial community in response to MnO2 nanoparticles in anammox granular sludge

The response of anaerobic ammonium oxidation (anammox) process and granular sludge in the upflow anaerobic sludge blanket reactor was analyzed under long-term exposure to MnO₂ nanoparticles (NPs) (1-200 mg L^-1). The addition of 200 mg L^-1 MnO₂ NPs had a significantly positive effect on nitrogen removal and this system exhibited excellent performance, with a total nitrogen removal efficiency of 93.1%. Moreover, the specific anammox activity enhanced with increasing concentrations of MnO₂ NPs up to the maximum value of 657.3 ± 9.3 mg TN g^-1 VSS d^-1 under MnO₂ NPs concentration of 200 mg L^-1. This value was approximately 1.6-fold higher than that of the reactor in the absence of MnO₂ NPs. The extracellular polymeric substances and settling velocity were both increased with MnO₂ NPs addition. Meanwhile, the high-throughput sequencing results revealed that MnO₂ NPs increased the relative abundance of dominant bacteria (Candidatus Kuenenia) from 17.3% at the absence of MnO₂ NPs to 23.9% at 200 mg L^-1 MnO₂ NPs, which resulted in a higher efficiency of biological nitrogen removal on the anammox system. These results indicated that MnO₂ NPs enhanced nitrogen removal performance of anammox process.

Adaption and restoration of anammox biomass to Cd(II) stress: Performance, extracellular polymeric substance and microbial community

Cadmium (Cd) can cause the deterioration of biological systems through inhibiting the enzymes activity and disturbing the microbial metabolism. Although the influence of Cd on conventional wastewater treatment process has been studied, the response of anammox to Cd exposure still remains unclear. This study firstly investigated the adaption and restoration of anammox biomass to Cd(II) stress. Results showed that long-term exposure of anammox bacteria to 2 mg L^-1 Cd(II) was beneficial for the reactor performance, while 5 mg L^-1 Cd(II) would cause the decline of SAA, extracellular polymeric substance content and relative abundance of Candidatus kuenenia by 40%, 25% and 31%, respectively. Furthermore, these indexes could approximately recover to the initial status after withdrawing Cd(II) from the influent. Overall, the anammox biomass exhibited a certain adaption and restoration ability to the suppression of Cd(II). This study may provide key valuable information for the biological treatment of wastewater containing Cd(II).

PVA/CS and PVA/CS/Fe gel beads' synthesis mechanism and their performance in cultivating anaerobic granular sludge

Biomass washout from high-speed anaerobic suspended bed bio-reactors is still a challenge to their stable operation. Preserving active biomass to efficiently retain biomass in the reactor is one of the solutions to this problem. Herein, two carriers (polyvinyl alcohol/chitosan (PVA/CS) and PVA/CS/Fe gel beads) were prepared using the cross-linking method. The fourier transform infrared (FTIR) and ¹³C nuclear magnetic resonance (¹³C NMR) analyses showed that PVA/CS gel beads formed mainly through hydrogen-bonds (NH₂OH^-). Furthermore, FTIR, ¹³C NMR, energy dispersive spectrum (EDS), X-ray diffractometer (XRD) and X-ray photoelectron spectroscopy (XPS) analyses showed that PVA/CS/Fe gel beads formed mainly through chelate bond (NH₂-Fe^M+OH^-). The scanning electron microscope (SEM) results affirmed that the gel beads had rough and well-developed porous structure for the attachment of microbes. Furthermore, the abilities of gel beads on the cultivation of granular sludge in an up-flow anaerobic sludge bed (UASB) reactor were effectively demonstrated while treating wastewater polluted with glucose and alkali lignin. The results showed that the gel beads-assisted reactors had a higher performance than those without the gel beads. The cultivation of granules in these reactors was accelerated, while the granules became bigger and exhibited better settling velocities. The reactor with gel beads was easier to withstand a higher organic loading rate due to dense microbial aggregates, which were caused by more humic-like substance. Particularly, the reactor with PVA/CS/Fe gel beads was able to improve the overall robustness of the system due to stronger mechanical properties of gel beads, and also prevented cells detachment.

Physico-chemical and microbial characterization of compartment-wise profiles in an anammox baffled reactor

In this study, compartment-wise investigation of an anammox baffled reactor (AnBR) was performed. The AnBR achieved steady-state conditions after a start-up period of ∼50 days and achieved NH₄ and NO₂ conversion percentages of 88.5 and 99.3%, respectively. Examination of the nitrogen mass balance revealed that an AnBR with a two-compartment configuration was sufficient for nitrogen loading rates (NLRs) ranging from 0.125 to 1.975 kg N/m³/d and resulted in a nitrogen removal efficiency (NRE) of 86.7-93.7%. Higher NLRs (4.04-5.05 kg N/m³/d) required four compartments to achieve an NRE of 82.2-87.1%. Further, an overall NLR increase of up to 5.93 ± 0.23 kg N/m³/d resulted in complete AnBR failure. The maximum nitrogen removal rate was consistently recorded in the 1^st compartment for all NLRs examined; as a result, this compartment exhibited the highest bacterial activity. Biomass concentration, specific anammox activity, extracellular polymeric substances, and average granule diameter in the 1^st compartment with an overall NLR of 0.05 kg N/m³/d were estimated to be 11.2 g_VSS/L, 0.03 mg N/g_VSS/h, 84.3 mg/g_VSS, and 0.65 mm, respectively. These values increased to 26.1 g_VSS/L, 11.80 mg N/g_VSS/h, 242.1 mg/g_VSS, and 2.31 mm, respectively, when the overall NLR was incremented to 4.04 kg N/m³/d. However, a gradual reduction in bacterial activity was observed from the 1^st to the 5^th compartment. The microbial community analysis indicated that the dominant phyla in the 1^st compartment (NLR of 0.252 kg N/m³/d) with the highest nitrogen removal were Chloroflexi (38.13%), Planctomycetes (22.62%), and Proteobacteria (14.75%).

Insight into the short- and long-term effects of quinoline on anammox granules: Inhibition and acclimatization

The short- and long-term influence of quinoline on the properties of anaerobic ammonium oxidation (anammox) biogranules was evaluated. During batch tests, the bioactivity of anammox granules in the presence of different quinoline concentrations was monitored, and the IC₅₀ of quinoline was calculated to be 13.1 mg L^-1 using a non-competitive inhibition model. The response of anammox granules to pre-exposure to quinoline was dependent on metabolic status, and the presence of both quinoline and NO₂^--N had a rapid detrimental effect, resulting in a 64.5% decrease within 12 h. During continuous-flow experiments, the nitrogen removal rate (NRR) of the reactor decreased sharply within 3 days in the presence of 10 mg L^-1 quinoline and then was restored to 2.6 kg N m^-3 d^-1. In the presence of quinoline-induced stress, the specific anammox activity and levels of extracellular polymeric substance and heme c were decreased, while settling velocity persistently increased. After cultivation and acclimation obtained by adding a medium level of quinoline to the influent, the anammox granule sludge was able to tolerate 10 mg L^-1 quinoline in 178 days.

Fast start-up of anammox process with mixed activated sludge and settling option

In this study, successful start-up of the anaerobic ammonium oxidation (anammox) process in a sequencing batch reactor (SBR) was achieved by seeding mixed activated sludge which included aerobic sludge, anaerobic sludge, simultaneous partial nitrification, anammox and denitrification (SNAD) sludge, and anammox sludge with low activity at a 2200:2100:5:2 volume ratio. On day 15, the effective anammox activity was attained in SBR, with the specific total nitrogen removal rate (SRR) of 0.214 gNg^-1 VSSd^-1. The total nitrogen removal rate (NRR) increased to 230 gNm^-3 d^-1 by gradually reducing the setting time to 10 min. With the nitrogen loading rate (NLR) up to 506 gNm^-3 d^-1, the total NRR of the SBR reached 433 gNm^-3 d^-1 during stationary phase. Candidatus Brocadia was detected as predominant functional microbes in the anammox SBR. The results demonstrated the feasibility of seeding mixed activated sludge to start-up an anammox SBR by settling option.

Roles of MnO2 on performance, sludge characteristics and microbial community in anammox system

The long-term impacts of MnO₂ on performance, sludge characteristics and microbial community of biogranule-based anaerobic ammonium oxidation (anammox) process were evaluated in an up-flow anaerobic sludge blanket reactor. It was found that the total nitrogen removal efficiency of reactor was fluctuated between 90%-93% at 1-200mgL^-1 MnO₂. Notably, the specific anammox activity was increased to maximum value of 657.3±10.6mgTNg^-1VSSd^-1 at 50mgL^-1 MnO₂ and then slightly decreased, but still higher than that achieved at 0-15mgL^-1 MnO₂, which had similar variation trends to the content of heme c and extracellular polymeric substances in anammox granules. High throughput sequencing indicated that MnO₂ could improve the microbial richness and diversity of anammox granules and Candidatus Kuenenia was always the dominant species, and its abundance continued to increase to 21.3% at the end of operational experiment. Therefore, MnO₂ could be applied to enhance the anammox process and the optimal influent MnO₂ concentration was lower than 50mgL^-1 in view of the reactor performance and cost issues.

Effect of chromium on granule-based anammox processes

In this study, the feasibility of application of anaerobic ammonium oxidation (anammox) to teat wastewater containing chromium was assessed. Anammox granule activity decreased with increasing Cr(VI) concentration and pre-exposure time in batch tests, and the 50% inhibition concentration of Cr(VI) on anammox biomass was 296.5 mg L^-1. Approximately 93.9% chromium was absorbed by loosely bound-extracellular polymeric substances and tightly bound-extracellular polymeric substances when less than 60 mg L^-1 Cr(VI) was added. During long-term operation in up-flow anaerobic sludge blanket reactor, significant inhibitory effects anammox performance were observed for Cr(VI) concentrations up to 2 mg L^-1. The nitrogen removal rate (NRR) rapidly decreased to 1.49 ± 0.89 kg N m^-3 d^-1, whereas the NRR was 11.37 ± 1.30 kg N m^-3 d^-1 in a control reactor. Compared with initial levels, specific anammox granule activity was 22%. The tolerance of the anammox process to Cr(VI) can be enhanced after a long-term adaptive phase.

Short-term effects of nanoscale Zero-Valent Iron (nZVI) and hydraulic shock during high-rate anammox wastewater treatment

The stability and resilience of an anaerobic ammonium oxidation (anammox) system under transient nanoscale Zero-Valent Iron (nZVI) (50, 75 and 100 mg L^-1), hydraulic shock (2-fold increase in flow rate) and their combination were studied in an up-flow anaerobic sludge blanket reactor. The response to the shock loads can be divided into three phases i.e. shock, inertial and recovery periods. The effects of the shock loads were directly proportional to the shock intensity. The effluent quality was gradually deteriorated after exposure to high nZVI level (100 mg L^-1) for 2 h. The higher effluent sensitivity index and response caused by unit intensity of shock was observed under hydraulic and combined shocks. Notably, the specific anammox activity and the content of heme c were considerably reduced during the shock phase and the maximum loss rates were about 30.5% and 24.8%, respectively. Nevertheless, the extracellular polymeric substance amount in the shock phase was enhanced in varying degrees and variation tendency was disparate at all the tested shock loads. These results suggested that robustness of the anammox system was dependent on the magnitude shocks applied and the reactor resistance can be improved by reducing hydraulic retention time with the increase of nZVI concentration under these circumstances.

Comparison of inoculum sources for long-term process performance and fate of ANAMMOX bacteria niche in poly(vinyl alcohol)/sodium alginate gel beads

The process performance and microbial niche of anaerobic ammonia oxidation (ANAMMOX) bacteria were compared in two identical bioreactors inoculated with different inoculum sources (i.e., pre-cultured ANAMMOX bacteria: PAB and activated sludge: AS) entrapped in poly(vinyl alcohol)/sodium alginate (PVA/SA) gel beads for a long-term period (i.e., 1.5 years). The start-up period with AS was longer than that with PAB; however, both bioreactors were successfully operated over the long-term with stable ANAMMOX activity. After long-term operation, the 16S rRNA gene concentration of ANAMMOX bacteria in both bioreactors was significantly increased, and thereby became comparable. In addition, Candidatus Jettenia sp. became the dominant ANAMMOX species in both bioreactors. Our results suggested that the ANAMMOX performance and microbial niche of ANAMMOX bacteria became nearly identical during long-term operation despite the use of different inoculum sources. Therefore, the use of PVA/SA gel beads entrapping AS appears to be a relevant option for constructing an ANAMMOX process in places where a full-scale ANAMMOX process has never been done previously.

Nitrogen removal performance and operation strategy of anammox process under temperature shock

Sequencing batch reactors were used to study anaerobic ammonium oxidation (anammox) process under temperature shock. Both long-term (15-35 °C) and short-term (10-50 °C) temperature effects on nitrogen removal performance were performed. In reactor operation test, the results indicated that ammonium removal rate decreased from 0.35 kg/(m³ day) gradually to 0.059 kg/(m³ day) when temperature dropped from 35 to 15 °C. Although bacteria morphology was not modified, sludge settling velocity decreased with decreasing temperature. In batch test, apparent activation energy (E_a) increased with decreasing temperature, which suggested the activity decrease of anaerobic ammonium oxidizing bacteria (AAOB). Low temperature inhibited AAOB and weakened nitrogen removal performance. The cardinal temperature model with inflection was first used to describe temperature effect on anammox process. Simulated results revealed that anammox reaction could occur at 10.52-50.15 °C with maximum specific anammox activity of 0.50 kg/(kg day) at 36.72 °C. The cold acclimatization of AAOB could be achieved and glycine betaine could slightly improve nitrogen removal performance at low temperature.

Sludge reduction using aquatic worms under different aeration regimes

Adding aquatic worms to a wastewater treatment system can reduce sludge production through predation. The aeration level is crucial for success. To evaluate aeration impacts on sludge reduction and determine an optimal aeration regime, this study investigated the processes of in-situ sludge reduction, using aquatic worms exposed to different aeration levels. The experiment also compared treatment results between a conventional reactor and an aquatic worm reactor (WR). Results indicated that the recommended concentration of dissolved oxygen (DO) was 2.5 mg L^-1. The removal rate of chemical oxygen demand remained steady at 80% when the DO concentration was higher than 2.5 mg L^-1, while the removal rate of ammonia nitrogen continued to moderately increase. Increasing the DO concentration to 5 mg L^-1 did not improve sludge reduction, and consumed more power. With a DO concentration of 2.5 mg L^-1 and a power of 0.19 kWh t^-1 water, the absolute sludge reduction and relative sludge reduction rates in the WR were 60.0% and 45.7%, respectively, and the daily aquatic worm growth rate was 0.150 d^-1 during the 17-d test. Therefore, at the recommended aeration regime, aquatic worms reduced the sludge without increasing the power consumption or deteriorating the effluent.

Effects of thiocyanate on granule-based anammox process and implications for regulation

The feasibility of using anaerobic ammonium oxidation (anammox) process to treat industrial wastewater containing thiocyanate (SCN^-) was examined in this study. Anammox activity decreased with increasing thiocyanate concentration and pre-exposure time in batch tests. A typical noncompetitive model was used to fit the data for thiocyanate inhibition, and the 50% inhibition concentration (IC₅₀) of thiocyanate on anammox was 620.4mgL^-1 at 200mgL^-1 total nitrogen level. The influent thiocyanate concentration of test reactor (R₁) in phase II gradually increased from 10 to 120mgL^-1, and the average nitrogen removal efficiency (NRE) of R₁ was maintained at 83.0±7.82%. This robustness was attributed to the self-adaptation ability of anammox biomass through long-term acclimatization. The NRE was decreased to 57.1% under 130mgL^-1 thiocyanate within two days. However, the NRE of control reactor (R₀) in absence of thiocyanate was 91.23±4.11% in this phase. Under thiocyanate stress, the specific anammox activity, settling velocity and heme c content of the granules significantly decreased, and the extracellular polymeric substances content slightly increased. The short- and long-term performance inhibition could be reversed in the presence of 10mgL^-1 Fe(III).

Roles of EDTA washing and Ca²⁺ regulation on the restoration of anammox granules inhibited by copper(II)

We investigated the feasibility of using ethylene diamine tetraacetic acid (EDTA) washing followed by Ca(2+) enhancement for the recovery of anammox reactors inhibited by Cu(II). Kinetic experiments and batch activity assays were employed to determine the optimal concentration of EDTA and washing time; and the performance and physiological dynamics were tracked by continuous-flow monitoring to evaluate the long-term effects. The two-step desorption process revealed that the Cu in anammox granules was primarily introduced via adsorption (approximately, 80.5%), and the portion of Cu in the dispersible layer was predominant (accounting for 71.1%). Afterwards, the Cu internalized in the cells (approximately, 14.7%) could diffuse out of the cells and be gradually washed out of the reactor over the next 20 days. The Ca(2+) addition that followed led to an accelerated nitrogen removal rate recovery slope (0.1491 kgN m(-3) d(-2)) and a normal biomass growth rate (0.054 d(-1)). The nitrogen removal rate returned to normal levels within 90 days and gradual improvements in granular characteristics were also achieved. Therefore, this study provides a new insight that externally removing the adsorbed heavy metals followed by internally repairing the metabolic system may represent an optimal restoration strategy for anammox consortium damaged by heavy metals.

Evaluation of the inhibitory effects of heavy metals on anammox activity: A batch test study

This study evaluated the interactive effect of Cu(II) and Zn(II) on anaerobic ammonium oxidation (anammox) activity using response surface methodology with a central composite design. A regression model equation was developed and validated to predict the normalized anammox activity (NAA) of anammox granules exposed to various heavy metal concentrations. The joint inhibitory effect tended to exacerbate initially and reversed as the concentrations increased and then moderated again. The most severe inhibition, resulting in a NAA of 20.1%, occurred at Cu(II) and Zn(II) concentrations of 16.3 and 20.0mgL(-1), respectively. Notably, the cumulative toxicity was mitigated with the aid of intermittent exposure acclimatization. Additionally, pre-exposure to Cu(II) in the absence of substrates strongly inhibited anammox activity. However, the presence of NO2(-) significantly enhanced Cu(II) inhibition. Therefore, such conditions should be avoided to minimize the disturbance of the anammox process.

Behavior and fate of copper ions in an anammox granular sludge reactor and strategies for remediation

In this study, the behavior, distribution and form dynamics of overloaded Cu(II) in anaerobic ammonium oxidation (anammox) granular sludge reactors were investigated. The performance and physiological characteristics were tracked by continuous-flow monitoring to evaluate the long-term effects. High Cu loading (0.24 g L(-1)d(-1)) exceeded sludge bearing capacity, and precipitation dominated the removal pathway. The Cu distribution migrated from the extracellular polymeric substances-bound to the cell-associated Cu and the Cu forms shifted from the weakly bound to strongly bound fractions over time. Pearson correlation and fluorescence spectra analyses showed that the increase in protein concentrations in the EPS was a clear self-defense response to Cu(II) stress. Two remediation strategies, i.e., ethylenediamine tetraacetic acid (EDTA) washing and ultrasound-enhanced EDTA washing, weakened the equilibrium metal partition coefficient from 5.8 to 0.45 and 0.34 L mg(-1)SS, respectively, thereby accelerating the external diffusion of the Cu that had accumulated in the anammox granules.

Anaerobic ammonium-oxidizing bacteria gain antibiotic resistance during long-term acclimatization

Three broad-spectrum antibiotics, amoxicillin (AMX), florfenicol (FF) and sulfamethazine (SMZ), that inhibit bacteria via different target sites, were selected to evaluate the acute toxicity and long-term effects on anaerobic ammonium oxidation (anammox) granules. The specific anammox activity (SAA) levels reduced by approximately half within the first 3 days in the presence of antibiotics but no nitrite accumulation was observed in continuous-flow experiments. However, the SAA levels and heme c content gradually recovered as the antibiotic concentrations increased. Extracellular polymeric substances (EPS) analysis suggested that anaerobic ammonium-oxidizing bacteria gradually developed a better survival strategy during long-term acclimatization, which reduced the antibiotic stress via increased EPS secretion that provided a protective 'cocoon.' In terms of nitrogen removal efficiency, anammox granules could resist 60 mg-AMX L(-1), 10 mg-FF L(-1) and 100 mg-SMZ L(-1). This study supported the feasibility of using anammox granules to treat antibiotic-containing wastewater.

Anaerobic ammonium oxidation (anammox) under realistic seasonal temperature variations: Characteristics of biogranules and process performance

In this study, the effects of realistic seasonal temperatures on the nitrogen removal performance of anaerobic ammonium oxidation (anammox) and the properties of the anammox granules were comparatively investigated for 330 days. The results demonstrated that the nitrogen removal efficiency (NRE), nitrogen loading rate (NLR) and nitrogen removal rate (NRR) were decreased dramatically, as the temperature decreased from 31.2 to 2.5 °C. However, the nitrogen removal performance recovered andante as the temperature increased gradually. After low temperature exposure, the settleability tended to worsen, and granules appeared to be more irregular with a smaller average granule diameter, and the extracellular polymeric substances (EPS) content increased slightly, while the specific anammox activity (SAA) decreased obviously. This realistic seasonal temperatures based research was an illation of the actual operation, and could be potentially implemented to maintain stability for the application of anammox technology.

Enhancement of anammox performance by Cu(II), Ni(II) and Fe(III) supplementation

This study explored the influence of metal ion addition on specific anaerobic ammonium oxidation activity (SAA). Batch assays were used to demonstrate the enhancement of the SAA upon the addition of Cu2+, Ni2+ and Fe3+. The SAA was enhanced by 41.0% when the Cu2+ concentration was below 1 mg L−1, while it was improved by 63.5% at Ni2+ concentrations below 1.74 mg L−1. An enhancement of 533.2% was obtained when 3.68 mg L−1 Fe3+ was supplied. The effects of Fe3+, Cu2+ and Ni2+ on the SAA were analyzed and optimized by a response surface methodology, which demonstrated that the interaction between Fe3+ and Cu2+ was significant and that 6.61 mg Fe3+ L−1, 1.18 mg Cu2+ L−1 and 1.11 mg Ni2+ L−1 were the optimal values for metal dosing. Subsequently, an Fe3+–Cu2+–Ni2+ continuous test was carried out under optimal conditions and revealed that the addition of Fe3+, Cu2+ and Ni2+ could stimulate the reactor potential at ambient temperature. The maximum nitrogen removal rate (NRR) of the test reactor was 52.8% higher than that of the control reactor (8.1 versus 5.3 kg N m−3 d−1). Moreover, a continuous test conducted by adding Fe3+ achieved an average nitrogen removal efficiency and maximum NRR of 67.4% and 4.9 kg N m−3 d−1, respectively, while the corresponding values of the control test were 64.7% and 4.1 kg N m−3 d−1, respectively. Altogether, appropriate dosages of Cu2+, Ni2+ and Fe3+ can significantly enhance the SAA and improve the reactor capacity at ambient temperature.

Optimization of process performance in a granule-based anaerobic ammonium oxidation (anammox) upflow anaerobic sludge blanket (UASB) reactor

In this study, the individual and interactive effects of influent substrate concentration (TNinf), hydraulic retention time (HRT) and upflow velocity (Vup) on the performance of anaerobic ammonium oxidation (anammox) in a granule-based upflow anaerobic sludge blanket (UASB) reactor were investigated by employing response surface methodology (RSM) with a central composite design. The purpose of this work was to identify the optimal combination of TNinf, HRT and Vup with respect to the nitrogen removal efficiency (NRE) and nitrogen removal rate (NRR). The reduced cubic models developed for the responses indicated that the optimal conditions corresponded to a TNinf content of 644-728mgNL(-1), an HRT of 0.90-1.25h, and a Vup of 0.60-1.79mh(-1). The results of confirmation trials were similar to the predictions of the developed models. These results provide useful information for improving the nitrogen removal performance of the anammox process in a UASB reactor.

Performance and microbial response during the fast reactivation of Anammox system by hydrodynamic stress control

Anaerobic ammonium oxidation (Anammox) has become a promising method for biological nitrogen removal. However, this biotechnology application is always limited due to the low growth rate and biomass yield of Anammox bacteria. This study investigated the process of fast reactivation of an Anammox consortium idled for 2 years via hydrodynamic stress control. The results showed that the Anammox system was efficiently and quickly reactivated by shortening of the hydraulic retention time (HRT) of the reactor from 12 to 6 hr within 68 days of operation. Moreover, at a 4-hr HRT with an influent total nitrogen loading rate of 1.2kg N/(m(3)·day), the reactor maintained high biological performance with an ammonium removal loading rate of 0.52kg N/(m(3)·day) and a nitrite removal rate of 0.59kg N/(m(3)·day). In the reactivated Anammox reaction, the stoichiometric coefficients of NH4(+)-N to NO2(-)-N and NH4(+)-N to NO3(-)-N were 1:1.04±0.08 and 1:0.31±0.03, respectively. The specific Anammox activity and hydrazine oxidoreductase activity, both of which represent the degree of Anammox bacteria present, increased as the hydrodynamic stress increased and were maximally (125.38±3.01mg N/(g VSS·day) and 339.42±6.83μmol/(min·g VSS), respectively) at 4-hr HRT. Microbial response analysis showed that the dominant microbial community was obviously shifted and the dominance of Anammox bacteria was enhanced during the hydrodynamic selection.