Direct Determination of Absolute Radical Quantum Yields in Hydroxyl and Sulfate Radical-Based Treatment Processes.

The absolute radical quantum yield (Φ) is a critical parameter to evaluate the efficiency of radical-based processes in engineered water treatment. However, measuring Φ is fraught with challenges, as current quantification methods lack selectivity, specificity, and anti-interference capabilities, resulting in significant error propagation. Herein, we report a direct and reliable time-resolved technique to determine Φ at pH 7.0 for commonly used radical precursors in advanced oxidation processes. For H2O2 and peroxydisulfate (PDS), the values of Φ•OH and ΦSO4•- at 266 nm were measured to be 1.10 ± 0.01 and 1.46 ± 0.05, respectively. For peroxymonosulfate (PMS), we developed a new approach to determine Φ•OHPMS with terephthalic acid as a trap-and-trigger probe in the nonsteady state system. For the first time, the Φ•OHPMS value was measured to be 0.56 by the direct method, which is stoichiometrically equal to ΦSO4•-PMS (0.57 ± 0.02). Additionally, radical formation mechanisms were elucidated by density functional theory (DFT) calculations. The theoretical results showed that the highest occupied molecular orbitals of the radical precursors are O-O antibonding orbitals, facilitating the destabilization of the peroxy bond for radical formation. Electronic structures of these precursors were compared, aiming to rationalize the tendency of the Φ values we observed. Overall, this time-resolved technique with specific probes can be used as a reliable tool to determine Φ, serving as a scientific basis for the accurate performance evaluation of diverse radical-based treatment processes.

Electrochemically selective ammonium recovery from wastewater via coupling hydrogen bonding and charge storage.

Electrochemical ammonium (NH4+) storage (EAS) has been established as an efficient technology for NH4+ recovery from wastewater. However, there are scientific difficulties unsolved regarding low storage capacity and selectivity, restricting its extensive engineering applications. In this work, electrochemically selective NH4+ recovery from wastewater was achieved by coupling hydrogen bonding and charge storage with self-assembled bi-layer composite electrode (GO/V2O5). The NH4+ storage was as high as 234.7 mg N g-1 (> 102 times higher than conventional activated carbon). Three chains of proof were furnished to elucidate the intrinsic mechanisms for such superior performance. Density functional theory (DFT) showed that an excellent electron-donating ability for NH4+ (0.08) and decrease of diffusion barrier (22.3 %) facilitated NH4+ diffusion onto electrode interface. Physio- and electro-chemical results indicated that an increase of interlamellar spacing (14.3 %) and electrochemical active surface area (ECSA, 388.9 %) after the introduction of GO were responsible for providing greater channels and sites toward NH4+ insertion. Both non-ionic chemical-bonding (V5+=O‧‧‧H, hydrogen-bonding) and charge storage were contributed to the higher capacity and selectivity for NH4+. This work offers underlying guideline for exploitation a storage manner for NH4+ recovery from wastewater.

Characterization of the microplastic photoaging under the action of typical salt ions of biological nitrogen removal processes.

Microplastics (MPs) are one of the most prevalent and diverse contaminants, and wastewater treatment plants are significant MP aggregators. Controlling the pollution caused by microplastics requires an understanding of how they age. The properties of the MPs photoaging process under the influence of salt ions typical of biological nitrogen elimination processes were disclosed in this work. The aging process of polyvinyl chloride microplastics (PVC-MPs) was greatly slowed down by greater HCO3- and NO2- concentrations, according to a comparison of the carbonyl index changes that occurred during photoaging. The carbonyl index had a negative correlation with the thermal stability of the photo-aged PVC-MPs, and aging accelerated the elimination of chlorine from the water. The samples were aged by UV radiation after 36 h at 40 °C, and the amount of chlorine eliminated was 10.13 times greater than that of the original MPs samples. It was discovered that the leachate concentration of aged MPs dramatically increased with decreasing particle size and was positively connected with the level of aging by comparing the concentration of leachate for two particle sizes (1 mm and 100 m). Photoaging caused MPs to become rougher, which in turn improved the NO3--N, NH4+-N, and NO2--N adsorption by PVC-MPs.

Dosing with pyrite significantly increases anammox performance: Its role in the electron transfer enhancement and the functions of the Fe-N-S cycle.

Anaerobic ammonium oxidation (anammox) represents an energy-efficient process for biological nitrogen removal from ammonium-rich wastewater. However, there are mechanistic issues unsolved regarding the low microbial electron transfer and undesired accumulation of nitrate in treated water, limiting its widespread engineering applications. We found that the addition of pyrite (1 g L-1 reactor), an earth-abundant iron-bearing sulfide mineral, to the anammox system significantly improved the nitrogen removal rate by 52% in long-term operation at a high substrate shock loading (3.86 kg N m-3 d-1). Two lines of evidence were presented to unravel the underlying mechanisms of the pyrite-induced enhancement. Physiochemical evidence indicated that an increase of cytochromes c and Fe-S protein was responsible for the accelerated electron transfer among metabolic enzymes. Multi-omics evidence showed that the depletion of nitrate was attributed to the Fe-N-S cycle driven by nitrate-dependent Fe(II) oxidation and S-based denitrification. This study deepens our understanding of the roles of electron transfer and the Fe-N-S cycle in anammox systems, providing a fundamental basis for the development of mediators in the anammox process for practical implications.

[Removal Characteristics of Four Typical Antibiotics in Denitrification System].

Wastewater treatment plants are an important gathering place for antibiotics, where denitrification plays a vital role in biological nitrogen removal. In order to explore the removal characteristics of antibiotics in a denitrifying sludge system, norfloxacin (NOR), oxytetracycline (OTC), sulfamethoxazole (SMX), and trimethoprim (TMP) were selected to investigate their transformation under different carbon source conditions in a denitrification system. The contribution of adsorption and biodegradation for antibiotic removal was also evaluated in this study. The results showed that a certain proportion of NOR, OTC, and TMP could be removed by denitrification, whereas NOR and OTC could act as the sole carbon source for denitrification. The removal of NOR and OTC showed a rapid adsorption and then slow biodegradation trend in the denitrification system. The contributions of adsorption were recorded as 83.5% and 58.9% for NOR and OTC removal, respectively. More than 40% were adsorbed by extracellular polymer substances (EPS), whereas the P450 enzyme played an important role in the OTC biodegradation process, with a contribution of 20%.

Understanding the slight inhibition of high As(III) stress on nitritation process: Insights from arsenic speciation and microbial community analyses.

Nitritation process with ammonia-oxidizing bacteria frequently suffers inhibition from heavy metals in industrial wastewater treatment. However, As(III), one of the most toxic metalloids, showed slight inhibition though the arsenic accumulation content in the sludge reached 91.8 mg L-1 in this study. Here, we combined long-term reactor operation with microbiological analyses to explore the slight inhibition mechanisms of As(III) on nitritation consortia. The results showed that no obvious changes induced by As(III) occurred in apparent characteristics and morphology of the nitritation consortia, whereas dosing As(III) induced shifts in the arsenic speciation and microbial community. 84.1% of As(III) was oxidized to As(V) in the acclimated sludge, decreasing the toxicity of As(III) to nitritation consortia. Insight to the microbial community, the relative abundances of Thermaceae and Phycisphaeraceae responsible for As(III) oxidation were increased to 7.4% and 6.6% under the stress of high-concentration As(III), respectively. Further, these increased arsenite-oxidizing bacteria probably accepted electron acceptor NO2- from ammonia-oxidizing bacteria to oxidize As(III). Our results indicated that microbial As(III) oxidation was the dominant detoxification pathway, providing new insights into nitritation characteristics under long-term As(III) stress.

Distinct membrane fouling characteristics of anammox MBR with low NO2--N/NH4+-N ratio.

The bacterial growth and death, and extracellular polymeric substances (EPS) and soluble microbial products (SMP) in aerobic membrane bioreactor (MBR) cause severe membrane fouling. Anammox bacteria grow slowly but produce much EPS and SMP. Therefore, the membrane fouling characteristic of anammox MBR is still indistinct. A NO2--N/NH4+-N < 1.0 into in the influent of an anammox MBR applies to investigate: 1) the slowest growing anammox bacteria (Candidatus Jettenia) could be enriched or not; 2) its membrane fouling characteristic. Results showed that Candidatus Jettenia successfully accumulated from 0.01% to 26.19%. The fouling characteristic of anammox MBR was entirely different from other MBRs. Firstly, obvious low transmembrane pressure (<4 KPa, 125 days) and low amount of foulants (0.22 gVSS/m2) might result from N2 production and the slow-growing Candidatus Jettenia. Secondly, the analysis of the components of membrane foulants indicated that polysaccharides of SMP in the gel layer and pore foulants were the key factors affecting membrane fouling. Finally, the large particle size of foulants (200 µm) might be caused by anammox bacteria living inside the foulants under anaerobic conditions. This study provides systematic insights into membrane characteristics of anammox MBR and a basis for the enrichment of anammox bacteria by MBR.

Hydroxylamine addition enhances fast recovery of anammox activity suffering Cr(VI) inhibition.

Hydroxylamine (NH2OH), one of the most important intermediates of anammox was employed to test the recovery performance because of its stimulation to anammox bacteria. Batch test indicated simultaneous addition of 1.83 ~ 9.17 mg N /L NH2OH relieved Cr(VI) inhibition because of extracellular reduction to Cr(III). The recovery efficiency (RE) was over 166%, with the effluent Cr(VI) and Cr(III) below 0.25 and 0.12 mg/L, respectively. Anammox activity after Cr(VI) inhibition was effectively recovered by 8 mg N/L NH2OH with RE at 218%. The long-term operation showed 1 ~ 2 mg N/L NH2OH accelerated the recover speed of nitrogen removal rate with 2.84 folds, as well as improving NH4+ conversion ratio and reducing NO3- production. After 55 days recovery, extracellular polymeric substance concentration, anammox activity and heme content recovered better with NH2OH addition. This study will provide the theoretical basis for rapid recovery of anammox activity by NH2OH when suffering Cr(VI) inhibition.

Development and simulation of a struvite crystallization fluidized bed reactor with enhanced external recirculation for phosphorous and ammonium recovery.

Recovering nitrogen and phosphorus from waste water in the form of struvite is an effective way to recycle resources. The insufficient purity of the resulting struvite and the large loss of nitrogen and phosphorus are the challenges at present. Therefore, it is urgent to develop innovative method in struvite crystallization process for efficient nitrogen and phosphorus recovery. This study proposed a crystallization method to reduce the loss of nitrogen and phosphorus by a struvite fluidized bed reactor (FBR) with optimized structure and operation conditions. The properties of struvite obtained under various conditions in the reactor were studied, and the internal operating conditions of the reactor were simulated with COMSOL Multiphysics to verify the effectiveness of the reactor optimization. This reactor achieved stable operation under the conditions of N/P = 1:1 and pH = 9.0. The purity of struvite obtained reached 98.5%, the conversion rate of ammonia nitrogen reached 97.2%, and struvite crystals could grow to 84 µm within 24 h. The simulation results showed that the Venturi tubes installed at multiple locations increased the turbulent energy to 4 × 10-4 m2/s2, which greatly improved the mass transfer efficiency. The trajectory of the crystal particles was consistent with the fluid flow field, which promoted the purification and growth of the crystal. In general, the new FBR with enhanced external recirculation would be a very feasible way to improve crystal growth and crystal purification of struvite, and it could enhance the recovery efficiency of nitrogen and phosphorus with reduced cost.

Performance, microbial community and inhibition kinetics of long-term Cu2+ stress on an air-lift nitritation reactor with self-recirculation.

Biological nitrogen removal process could be affected due to the presence of heavy metals owing to their toxicity and accumulation in the sludge. In this study, the impact of Cu2+ shock on a long-term nitritation operation was investigated in an air-lift reactor with self-recirculation. Both the dynamics of microbial community and inhibition kinetics under Cu2+ stress were ascertained. The results showed that Cu2+ exerted severe inhibition on nitritation performance of an air-lift reactor (ALR) at 25 mg/L. The corresponding NH4+-N removal efficiency decreased to below 50%, which was mainly due to the variation of microbial community structure, especially the inhibition of nitrifiers like Nitrosomonas (the relative abundance decreased from 30% to 1% after Cu2+ inhibition). Kinetic parameters were obtained and compared after fitting the Haldane model. The long-term Cu2+ stress on the ALR aggravated the ammonium affinity and the resistance to substrate self-inhibition of the nitritation sludge, but reduced the resistance to Cu2+ inhibition. Furthermore, Cu2+ acted as uncompetitive inhibitor on nitritation process. Our results provide new insights into the nitritation characteristics under long-term Cu2+ stress.

Long-term domestication to Mn stresses alleviates the inhibition on anammox process.

Heavy metals such as Mn2+ are common contaminants in ammonium-rich wastewater. The information of Mn2+ effect on anammox process needs further investigation. The short- and long-term effects of Mn2+ on anammox were explored by anammox granular sludge. Batch tests showed that the half inhibition value (IC50 ) of Mn2+ was 4.83 mg/L. The anammox activity was severely inhibited in 0.5 hr under 15 mg/L Mn2+ . However, after long-term domestication by increasing the concentration of Mn2+ , both the low-load reactor (R1) and the high-load reactor (R2) performed well, achieving volumetric nitrogen removal rate of 6.36 kg/(m3 ·d) and 13.99 kg/(m3 ·d), respectively. The average ammonium and nitrite removal efficiency of both reactors under 200 mg/L Mn still maintained above 90%. The results from long-term reactors' operation showed that the serious inhibition effect indicated by the batch test was significantly exaggerated. The granules became dispersed after long-term operation in the high-load reactor (R2) which might be correlated to the high osmotic pressure caused by high Mn2+ load, and the mechanism needs to be investigated further. PRACTITIONER POINTS: The half inhibition value of Mn2+ on anammox sludge was 4.83 mg/L in batch experiment. 200 mg/L Mn2+ did not cause any inhibition on anammox process during long-term operation. Granular sludge is finer under high nitrogen loads with 200 mg/L Mn stress.

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.

Synthesis of core-shell UiO-66-poly(m-phenylenediamine) composites for removal of hexavalent chromium.

The present research developed a direct in situ heterogeneous method to synthesize UiO-66-poly(m-phenylenediamine) core-shell nanostructures by inducing assembly of m-phenylenediamine radical on UiO-66 surfaces. The strong interaction between negative charged UiO-66 and positive radical from the oxidation of monomer is the major driving force. The produced UiO-66-poly(m-phenylenediamine) composites exhibited a distinct core-shell morphology with controllable surface features. The UiO-661-PmPD0.5 showed a uniform PmPD shell with a thickness of 40-60 nm and the nanocomposite exhibited a high specific surface area of 319.77 m2 g-1. Moreover, the Cr(VI) adsorption amount of the polymeric shell in the nanocomposites can reach as high as 745 mg g-1, far beyond the performance of the original PmPD. The adsorption tends to be equilibrium within 300 min. This research opens a hopeful window for facile and large-scale fabrication of core-shell nanostructures with controllable core-shell configuration, exhibiting high prospect in heavy metal removal from water.

Elucidating the effect of mixing technologies on dynamics of microbial communities and their correlations with granular sludge properties in a high-rate sulfidogenic anaerobic bioreactor for saline wastewater treatment.

In this study, three lab-scale anaerobic sulfidogenic bioreactors were operated independently using three different mixing modes (hydraulic, mechanical and pneumatic). One-way ANOVA test indicated various performance parameters (e.g. sulfate reduction and sulfide production) and granular sludge properties (e.g. EPS and particle size) statistically different in three mixing modes. Principal component analysis (PCA) and OTUs-based network demonstrated bacterial composition greatly varied among the three mixing modes. The phylum Proteobacteria was predominant among the bacterial communities, and the genus Desulfobacter (35.1% in hydraulic, 31.1% in mechanical and 27.4% in pneumatic sample) was the most dominant SRB. The PCA/Pearson's correlation analysis confirmed SRB had significant positive relationship with sludge properties (e.g. particle size). PICRUSt data highlighted that bacterial communities contained diverse predicted functions including sulfur metabolism enzymes (sulfite reductase and adenylylsulfate reductase). The findings of this research could be helpful for selection of an appropriate mixing technology for anaerobic sulfidogenic or similar bioprocess.

Effect of scrubbing by NaClO backwashing on membrane fouling in anammox MBR.

NaClO based chemically enhanced backwash (CEB) is often administered to maintain membrane permeability during the operation of MBR. However, the effect and working mechanism of NaClO concentrations in CEB were rarely investigated. The current investigation examined the changes in membrane resistance, permeate production and membrane morphology with or without CEB in an anammox MBR to reveal the scrubbing effect of different NaClO concentrations (0-596 mg/L). Good cleaning effect indicated by membrane fouling rate of 1.98-2.26 kPa/day and membrane permeate production of 80-88 L was observed when NaClO concentration of 149-596 mg/L was used. The best cleaning effect was observed when 298 mg/L of NaClO was used. To explore the mechanism of CEB action, backwashing foulants were also analyzed. Insoluble EPS transformed into soluble forms like S-EPS or SMP after the sludge was exposed to NaClO. The NaClO based CEB removed 112-675 mg of polysaccharide (PS)/m2 in foulants at NaClO concentration of 149-596 mg/L, which was significantly higher than the value obtained by pure water (35 mg PS/m2). The possible mechanisms behind the detachment of soluble PS seemed as oxidation and sterilization by NaClO. The current investigation provides useful guidelines on NaClO concentrations applied during CEB for anammox MBR.

The long-term effects of hexavalent chromium on anaerobic ammonium oxidation process: Performance inhibition, hexavalent chromium reduction and unexpected nitrite oxidation.

The toxicity of hexavalent chromium (Cr(VI)) is one of the challenges in implementing Anammox process to ammonium-rich wastewater treatment. However, the response of Anammox process to Cr(VI) stress and the inhibition mechanism remain unclear. Here, two Anammox UASB reactors were operated for 285 days under different Cr(VI) stresses. The results showed Anammox performance was not affected at low Cr(VI) concentration (i.e., 0-0.5 mg L-1), but was severely inhibited at 0.8 mg L-1. Attempts to domesticate Anammox process to higher Cr(VI) by lowering nitrogen loading rate were failed. Examination of Cr(VI) fate showed the occurrence of extracellular and intracellular Cr(VI) reduction to Cr(III). The inhibition was ascribed to the significant intracellular Cr(VI) reduction, accounting for 99.78% of the total Cr(VI) reduction. Moreover, under long-term Cr(VI) exposure, most nitrite was oxidized to nitrate. But microbial community showed no enrichment of Cr(VI) reducing bacteria and other nitrogen transformation-related bacteria.

The application of aged refuse in nitrification biofilter: Process performance and characterization.

High adsorption capacity, good biocompatibility and low cost are highly demanded for biofilter used in ammonium-rich wastewater treatment. In this study, we used SEM, BET, XRD and 16S rRNA to document the evidence for good performance in adsorption and biodegradation in aged refuse. Parallel experiment between raw and inert refuse showed ammonium adsorption occurred at the initial week, with the highest ammonium removal efficiency of 90.36%, but saturated during the subsequent long-term operation. Meanwhile, over 6months' operation of an aged refuse biofilter was conducted to confirm that nitrification was the main pathway of ammonium conversion. The maximum nitrogen loading rate could reach up to as high as 1.28kg/m3/d, with ammonium removal efficiency at 99%. Further, high nitrifier biodiversity were detected with 'Nitrosomonas' and 'Nitrospira' in domination in the refuse. However, Nitrospira would outcompete Nitrosomonas under the oxygen limiting condition and resulted in the failure of partial nitrification. The physicochemical and biological analysis show that biodegradation is the main ammonium conversion pathway, which is the critical finding of this work. This investigation would help to accelerate the application of the aged refuse process in ammonium-rich wastewater treatment.

Mechanistic Study on the Role of Soluble Microbial Products in Sulfate Radical-Mediated Degradation of Pharmaceuticals.

The role of soluble microbial products (SMP), the most important component of effluent organic matter from municipal wastewater treatment plants, in sulfate radical (SO4•-)-based advanced oxidation technologies (AOTs) remains substantially unclear. In this study, we first utilized a suite of macro- and microanalytical techniques to characterize the SMP from a membrane bioreactor for its fundamental molecular, spectroscopic, and reactivity properties. The degradation kinetics of three representative pharmaceuticals (i.e., naproxen, gemfibrozil, and sulfadiazine) in the presence of SMP was significantly reduced as compared to in its absence. Possible mechanisms for the interference by SMP in degrading these target compounds (TCs) were investigated. The low percentage of bound TCs to SMP ruled out the cage effect. The measurement of steady-state 1O2 concentration indicated that formation of 1O2 upon UV irradiation on SMP was not primarily responsible for the degradation of TCs. However, the comparative and quenching results reveal that SMP absorbs UV light acting as an inner filter toward the TCs, and meanwhile scavenges SO4•- with a high second-order rate constant of 2.48 × 108 MC-1 s-1.

Insights into the role of extracellular polymeric substances in Zn2+ adsorption in different biological sludge systems.

The adsorption behavior of Zn2+ in four different biological sludge systems, i.e. activated sludge, denitrification sludge, short-cut nitrification sludge, and anammox granules, was investigated. The results indicated that all sludge samples possessed considerable potential for Zn2+ adsorption. Short-cut nitrification sludge possessed the highest Zn2+ maximum adsorption capacity (qm) of 36.4 mg g SS-1, which was much higher than other sludges applied (12.8-14.7 mg g SS-1). Besides, the adsorption rate for short-cut nitrification sludge was fastest among the four types of sludge after fitting with a pseudo-second-order rate equation. Comparing with the physicochemical properties of the four sludges, the soluble extracellular polymeric substances (EPS), especially soluble polysaccharide (PS), played a prior role in binding metal cations (i.e., Zn). The present study also showed that with less than 30% of Zn2+ trapped by EPS, 61.6-71.9% of Zn2+could be harvested directly by cells, indicating that the protecting capability by EPS was limited. Therefore, it is important to remove metal ions as early as possible if the activated sludge processes encountered high stress of heavy metal. Graphical abstract ᅟ.

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.