Identifying active concentrations of biopolymers for enhancing membrane nanofiltration performance: From bench-scale tests to real production considerations

In the last decades, membrane-based nanofiltration (NF) technique has been widely applied for safe and high-quality drinking water production worldwide. NF membrane fouling has become one of the main obstacles in its application due to high operation cost, and thus numerous efforts have been made. However, there is still a large disconnect between academic findings and their applications. Hence, novel approaches for further exploitation and application are required based on feasibility of implementation. In this work, an optimized design of membrane-based NF plants was proposed, inspired by natural biopolymers present in feed water of NF unit. Specifically, we found beneficial functions of biopolymers, including NF membrane fouling alleviation and effluent quality improvement; these advantages could only be "activated" under a certain concentration range of biopolymers (0-1 mg C/L here), and less or more is not acceptable. This indicated that a NF unit is better to follow a microfiltration (MF) (instead of ultrafiltration (UF) which removes biopolymers) process during which natural biopolymers could be remained; also, this approach is suggested to be valid across different seasons when biopolymers' concentrations could be controlled within an "activated" range by mixing MF and UF permeates. Furthermore, three representative reference biopolymers with different, confirmed spatial structures and molecular weight (MW) were used to elucidate the micro-level functions of natural biopolymers on NF membranes, suggesting that cake layer structures shaped by various biopolymers determine the resulting NF performance. Overall, this innovative proposal is expected to be considered and adopted towards more energy-efficient NF technology for drinking water supply.

Performance and properties of coking nanofiltration concentrate treatment and membrane fouling mitigation by an Fe(ii)/persulfate-coagulation-ultrafiltration process

Coking nanofiltration (NF) concentrates, as typical wastewater with high salinity and refractory organics, have become one of the greatest challenges for "near-zero emission" processes. In our study, an advanced oxidation technology based on ferrous iron/persulfate (Fe(ii)/PS) and polyferric sulfate (PFS) coagulation coupled with ultrafiltration (UF) was used to treat NF concentrates and mitigate membrane fouling. Based on batch experiments, the optimal parameters of Fe(ii)/PS were obtained, during which we discovered that the slow reaction stage of total organic carbon (TOC) removal followed first-order degradation kinetics. Under the optimal reaction conditions, Fe(ii)/PS could efficiently mineralize 69% of organics in coking NF concentrates. In order to eliminate the iron floc generated in the Fe(ii)/PS step, a small amount of PFS (0.05 mM) was added to coagulate the iron floc, which could further improve the effluent quality so that the turbidity, iron content and TOC were significantly reduced by 79.18%, 98% and 21.79% respectively. Gas chromatography coupled with time-of-flight mass spectrometry (GC × GC-TOFMS) and fluorescence excitation-emission matrix spectrometry (EEM) were performed to characterize the removal of phenols, PAHs, quinolines and humic acids in NF concentrates which were responsible for UF membrane fouling. Moreover, scanning electronic microscopy (SEM) and atomic force microscopy (AFM) were conducted to study the surface of the UF membrane after treatment of NF concentrates. The result exhibited that the organic pollutants deposited on the UF membrane surface were reduced by Fe(ii)/PS-PFS pretreatment, and UF membrane flux was thus enhanced. Our results show the potential of the approach of applying Fe(ii)/PS-PFS-UF in NF concentrate treatment.

Using fluorescence surrogates to track algogenic dissolved organic matter (AOM) during growth and coagulation/flocculation processes of green algae

Tracking the variation of the algogenic organic matter (AOM) released during the proliferation of green algae and subsequent treatment processes is crucial for constructing and optimizing control strategies. In this study, the potential of the spectroscopic tool was fully explored as a surrogate of AOM upon the cultivation of green algae and subsequent coagulation/flocculation (C/F) treatment processes using ZrCl₄ and Al₂(SO₄)₃. Fluorescence excitation emission matrix coupled with parallel factor analysis (EEM-PARAFAC) identified the presence of three independent fluorescent components in AOM, including protein-like (C1), fulvic-like (C2) and humic-like components (C3). Size exclusion chromatography (SEC) revealed that C1 in AOM was composed of large-sized proteins and aromatic amino acids. The individual components exhibited their unique characteristics with respect to the dynamic changes. C1 showed the highest correlation with AOM concentrations (R² = 0.843) upon the C/F processes. C1 could also be suggested as an optical predictor for the formation of trihalomethanes upon the C/F processes. This study sheds a light for the potential application of the protein-like component (C1) as a practical surrogate to track the evolution of AOM in water treatment or wastewater reclamation systems involving Chlorella vulgaris green algae.

Coagulation removal of fluoride by zirconium tetrachloride: Performance evaluation and mechanism analysis

Fluoride (F^-) pollution is a worldwide issue. Coagulation with aluminum (Al) salts is an efficient and economical method for the removal of F^-. However, due to the strong complexation between Al³⁺ and F^-, the residual F^- and Al after coagulation usually exceed the limits. Zirconium (Zr) coagulants have drawn increasing attention due to their excellent flocculation ability for organic matter. In this work, the performance and mechanism of ZrCl₄ coagulation for F^- removal were investigated with the widely used Al₂(SO₄)₃ as a reference. The optimum pH range is 4.0-6.0 for ZrCl₄ and 8.0-10.0 for Al₂(SO₄)₃. ZrCl₄ was superior to Al₂(SO₄)₃ for F^- removal as the initial F^- concentration was less than 30.0 mg L^-1. Coexisting substances at environmental concentration levels showed negligible effects on F^- removal by ZrCl₄. Besides the better F^- removal, another advantage of ZrCl₄ over Al₂(SO₄)₃ was the much lower residual metal concentration in the pH range of 4.0-11.0. The hydrolysis of Al₂(SO₄)₃ was significantly inhibited due to the formation of Al-F complexes while the hydrolysis of ZrCl₄ was not influenced even under strongly acidic conditions. Therefore, F^- removal by Al₂(SO₄)₃ was mainly achieved by preliminary complexation between Al³⁺ and F^- and subsequent hydrolysis and polymerization of these complexes, while adsorption onto hydrolysates and ion exchange with surface hydroxyl groups were the main ways of F^- removal by ZrCl₄. The work here provides a new method for F^- removal and may shed light on the application of Zr coagulants for other pollutants.