Oxidation and coagulation/adsorption dual effects of ferrate (VI) pretreatment on organics removal and membrane fouling alleviation in UF process during secondary effluent treatment

Gradient purification of surface water and ultrafiltration membrane fouling mitigation based on Fe(VI) multifunctional integration characteristics: Is a sedimentation unit necessary?

Ferrate (Fe(VI)) can provide oxidation and in-situ coagulation/adsorption for the removal of emerging contaminants and natural organic matter, and can be used in conjunction with ultrafiltration (UF) membrane to enhance the removal of composite contaminants and mitigate UF membrane fouling. Based on the Fe(VI) multifunctional integration characteristics, the study objectively and comprehensively verified the gradient purification of surface water and the UF membrane fouling mitigation by Fe(VI)-UF and sulfite (S(IV)) activated Fe(VI) (S(IV)/Fe(VI))-UF, elucidated the effect of sedimentation unit on the UF mechanism and the membrane fouling behaviors, and revealed the free energy changes throughout the UF process. The experimental results demonstrated that S(IV)/Fe(VI)-UF showed superior purification performance and UF membrane fouling mitigation than Fe(VI)-UF. S(IV)/Fe(VI)-UF achieved removals of sulfamethoxazole, DOC, and UV254 up to 77.73 %, 61.86 %, and 86.33 %, and was able to significantly mitigate UF membrane fouling by prolonging the transition stage and positively shifting the interfacial free energy. Innovatively, the absence of a sedimentation unit was found to adversely affect the initial stages of Fe(VI)-UF and S(IV)/Fe(VI)-UF by lowering the energy barriers, while negligibly affecting cake filtration. Additionally, the water treatment cost of S(IV)/Fe(VI) was 0.3 yuan per ton, indicating notable economic benefits and engineering potential. While deepening the understanding of Fe(VI) multifunctional integration characteristics, the above results provided theoretical and data support for S(IV)/Fe(VI)-UF and Fe(VI)-UF treatment of surface water, and enriched the application scenarios of Fe(VI)-related technologies.

Research and application of active species based on high-valent iron for the degradation of pollutants: A critical review

Fe(VI), as a new green treatment agent, has two indispensable processes in water treatment: coagulation and oxidation. Fe(VI) has a strong oxidation ability. The intermediate iron species (Fe(V) and Fe(IV)) and reactive radical species (H2O2, •OH, and O2•-) produced by decomposition and reduction reaction have strong oxidation ability, in addition, the hydrolyzed product formed in situ with core (γ-Fe2O3)-shell (γ-FeOOH) structure also has good coagulation effect. Because Fe(VI) is easy to decompose and challenging to preserve, it limits the application and sometimes significantly reduces the subsequent processing effect. How to make Fe(VI) more efficient use is a hot spot in current research. This article summarizes the distribution of active substances during the hydrolysis of Fe(VI), distinguish the differences mechanisms in the similar regulation methods, reviews the current preparation methods of Fe(VI), and finally reviews the applications of Fe(VI) in the field of environmental remediation.

Critical fractions in reclaimed water responsible for membrane fouling: Isolation, fouling characteristics, quantitative and qualitative variations in practical application

Considering the different fouling characteristics between model foulants and organic components in real reclaimed water, it is of great importance to identify the critical foulants responsible for membrane fouling. This study identified and isolated the fraction with molecular weight (MW) > 100 kDa as the critical foulant in secondary effluent by MW cut-off membrane of 100 kDa with high efficiency. This fraction accounted for 92.2% membrane fouling of raw water, including 28.7%, 29.7% and 33.8% fouling contribution by subfractions with MW between 100-300, 300-500 and > 500 kDa. Specifically, the critical fraction with MW > 100 kDa were mainly distributed in two parts: < 0.22 μm and > 0.45 μm, corresponding to 41.9% and 56.9% fouling contribution of this fraction. Furthermore, both total organic carbon (TOC) and fouling potential of fraction with MW > 100 kDa were monitored, presenting about threefold increase from September to January in next year. Membrane fouling contribution of this critical fraction in raw secondary effluent were mainly distributed in 85∼95% throughout the 5 months, demonstrating its predominant fouling propensity. Moreover, the TOC concentration of fraction with MW > 100 kDa presented distinct positive correlation with the fouling potential of raw secondary effluent (R2 = 0.947), which was promising to be a surrogate for predicting membrane fouling in practical application.

Continuation of a cleaning process: Application of MNBs-coagulation process to mitigate ultrafiltration membrane fouling

The MNBs-coagulation process as a novel and cleaning enhanced coagulation process has been demonstrated to enhance the removal efficiency of hydrophilic organics. In this study, while continuing the concept of cleaning production, the MNBs-coagulation process was first applied to the ultrafiltration process and was expected to alleviate the ultrafiltration membrane fouling. This study investigated the effect of the involvement of MNBs in coagulation-ultrafiltration process (the MC-UF process) on the fouling behaviour of ultrafiltration membrane based on the calculation of membrane resistance distribution and the fitting of membrane fouling model. In addition, the NOM removal efficiency, floc characteristics analysis and membrane hydrophilicity analysis were used to illustrate the mechanism of mitigating ultrafiltration mebrane fouling by the MC-UF process. The experimental results showed that the involvement of MNBs in the coagulation-ultrafiltration process was able to reduce the irreversible fouling and TMP by 43.1 % and 41.6 % respectively. This phenomenon could be attributed to the involvement of MNBs in the coagulation process to improve the removal efficiency of hydrophilic organics and to enhance the characteristics of flocs, thus reducing the possibility of hydrophilic organics and broken flocs entering and blocking the membrane pores. In addition, the FT-IR spectral changes before and after the floc breakage were analyzed by 2D-COS technique in this study, and it was found for the first time that the participation of MNBs in the coagulation process could change the sequence of functional group transformation within the floc, and promote the generation of hydrogen bonds between flocs by hindering the generation of hydroxyl groups (-OH), and improve the shear resistance and regrowth capacity of flocs while reducing the possibility of broken flocs entering and blocking membrane pores. In summary, the MC-UF process proposed in this study can significantly mitigate ultrafiltration membrane fouling while meeting cleaning production, providing theoretical support for the application of the process to practical engineering.

Coupling pretreatment of ultraviolet/ferrate (UV/Fe(vi)) for improving the ultrafiltration of natural surface water

Ultrafiltration (UF) is a high-potential technology for purifying natural surface water; however, the problem of membrane fouling has limited its widespread application. Herein, ultraviolet (UV)-activated ferrate (Fe(vi)) was used to purify natural surface water and improve the performance of the UF membrane. The combination of UV and Fe(vi) could generate active species (Fe(v), Fe(iv), ˙OH and O2˙-) to degrade pollutants, while the in situ produced Fe(iii) had the effect of coagulation. With the above action, pollutants were removed, and the pollution load of natural surface water was reduced. After treatment with the UV/Fe(vi) system, dissolved organic carbon was reduced by 49.38%, while UV254 was reduced by 45.00%. The removal rate was further increased to 54.88% and 51.67% after UF treatment. In addition, the fluorescent organics were reduced by 44.22%, and the molecular weight of the organics became smaller. In the stage of UF, the terminal J/J0 was increased from 0.61 to 0.92, and the membrane fouling resistance was decreased by 85.94%. The analysis of the membrane fouling mechanism indicates that the role of cake filtration was weakened among all the mechanisms. Fourier transform infrared spectroscopy showed that less pollutants were accumulated on the membrane surface, and scanning electron microscopy revealed that the membrane pore blockage was relieved. In summary, the UV/Fe(vi) co-treatment process proposed in this study can significantly improve the purification efficiency of the UF systems in natural surface water treatment.

Combination of coagulation and adsorption technologies for advanced wastewater treatment for potable water reuse: By ANN, NSGA-II, and RSM

To reuse water and reduce water pollution, such as chemical oxygen demand (COD), total suspended solids (TSS), PO4, NTU, and NO3, advanced wastewater treatment technologies (a combination of coagulation (FeCl3) and adsorption (Activated Carbon (AC))) are attractive. Considering that water reclamation can help provide an irrigation system for crops and domestic purified water, removing organic matter and nutrients prior to wastewater reuse is fundamental. In order to remove contaminants like organic matter and nutrients from wastewater, advanced wastewater treatment processes are recommended. The purpose of this paper is to investigate various doses of AC and FeCl3 in wastewater treatment and study the optimum conditions for the removal of COD, TSS, PO4, NTU, and NO3. Furthermore, the evaluated FeCl3'/AC's optimum functioning pH ranges from 6.5 to 8.0, and their optimum working times range from 2.5 to 5.5 h. The optimum concentrations of AC were 0.1-25 g/L and 0.1-5 g/L of FeCl3. The most significant COD elimination rate (98%), the highest TSS elimination efficiency (94%), NTU elimination performance (99%), PO4 elimination (99%), and NO3 elimination (67%), among the investigated FeCl3 and AC. Secondly, the effects of operational variables such as AC, FeCl3, time, and solution pH were modeled, optimized, and evaluated using response surface techniques based on the D-Optimal design. Input from the response surface approach findings was used to develop an artificial neural network-based prediction model and Non-dominated Sorting Genetic Algorithm II (NSGA-II).

Using Fe(II)/Fe(VI) activated peracetic acid as pretreatment of ultrafiltration for secondary effluent treatment: Water quality improvement and membrane fouling mitigation

Ultrafiltration (UF) is a technology commonly used to treat secondary effluents in wastewater reuse; however, it faces two main challenges: 1) membrane fouling and 2) inadequate nitrogen (N), phosphorus (P), and organic micropollutants (OMPs) removal. To address these two issues, in this study, we applied peracetic acid (PAA), Fe(VI)/PAA, and Fe(II)/PAA as UF pretreatments. The results showed that the most effective pretreatment was Fe(II)/200 μM PAA, which reduced the total fouling resistance by 90.2%. In comparison, the reduction was only 29.7% with 200 μM PAA alone and 64.3% with Fe(VI)/200 μM PAA. Fe(II)/200 μM PAA could effectively remove fluorescent components and hydrophobic organics in effluent organic matter (EfOM), and enhance the repulsive force between foulants and membrane (according to XDLVO analysis), and consequently, mitigate pore blocking and delay cake layer formation. Regarding pollutant removal, Fe(II)/200 μM PAA effectively degraded OMPs (>85%) and improved P removal by 58.2% via in-situ Fe(Ⅲ) co-precipitation. The quencher and probe experiments indicated that FeIVO2+, •OH, and CH3C(O)OO•/CH3C(O)O• all played important roles in micropollutant degradation with Fe(II)/PAA. Interestingly, PAA oxidation produced highly biodegradable products such as acetic acid, which significantly elevated the BOD5 level and increased the BOD5/total nitrogen (BOD5/TN) ratio from 0.8 to 8.6, benefiting N removal with subsequent denitrification. Overall, the Fe(II)/PAA process exhibits great potential as a UF pretreatment to control membrane fouling and improve water quality during secondary effluent treatment.

Ions play different roles in virus removal caused by different NOMs in UF process: Removal efficiency and mechanism analysis

In this study, we investigated the effect of different compositions of aquatic natural organic matter (NOM) and ions on virus removal by ultrafiltration (UF). MS2 bacteriophage was used as a surrogate. Humic acid (HA) improved the MS2 removal rate from 1.95 ± 0.09 LRV to 2.40 ± 0.03 LRV at the HA dosage of 9 mg/L through the combined mechanisms of size exclusion, electrostatic repulsion and hydrophobicity. MS2 removal rate further increased to 3.10 ± 0.05 LRV by 10 mmol/L Na⁺ dosage and 3.19 ± 0.12 LRV by Ca²⁺ 1 mmol/L in the HA-containing UF system. Size exclusion turned into the dominant virus removal mechanism according to the results of the fouling model fitting and the weakening of electrostatic repulsion and hydrophobicity. The complexation of Ca²⁺ also played a role in MS2 removal based on the analysis of interaction force. MS2 removal rate by bovine serum albumin (BSA) was poor, which was 2.07 ± 0.06 LRV at the BSA dosage of 9 mg/L. Hydrophobicity was greatly reduced and the dominant virus removal mechanisms were size exclusion and electrostatic repulsion. 10 mmol/L Na⁺ in the presence of BSA deteriorated MS2 removal rate to 2.02 ± 0.07 LRV by the weakening of electrostatic repulsion, hydrophobicity and size exclusion. Electrostatic repulsion severely decreased by 1 mmol/L Ca²⁺ and the enhanced adsorption barrier represented competitive adsorption of Ca²⁺ by BSA and MS2 contributed for MS2 removal further decline (1.99 ± 0.05 LRV). Complex components in water will have different effects on virus removal due to their properties and interactions. This study can provide references for selecting more efficient water treatment methods according to the different compositions of raw water in actual water treatment applications during the UF process. Moreover, the retention of virus by UF can be predicted based on our study results.