Ceramic nanofiltration (NF) is a newly-developed technology for water recycling, but is still limited to pilot-scale applications. Lacking efficient and eco-friendly strategies for cleaning ceramic NF membrane impedes its scaling-up in industries. Forward flush, backwash and acidic/caustic cleaning are not efficient enough. In this work, a novel oxalic acid-aided Fenton process was proposed for synergistic relaxation/oxidation of persistent Ca2+-mediated gel-like fouling of ceramic NF membrane. A reactive catalyst layer was online pre-coated on top of the membrane via a pressure-driven cross-flow pre-filtration of Fe3O4 hydrosols. The gel-like fouling was simulated by alginate in the presence of Ca2+ ions. Results show that the Fe3O4 loading could be readily tuned from 0.16 to 1.34 g m−2 by altering the permeate flux during the pre-coating. The membrane permeability loss due to the pre-coating was minimal (<10%). The combination of oxalic acid chelation and Fenton-based oxidation resulted in high flux recovery (85.07%) for the iron-oxide pre-coated membrane, whereas the single treatment by hydrogen peroxide (H2O2) or oxalic acid was inefficient. This synergistic effect was attributed to relaxation of the Ca2+-mediated gel layer via oxalic acid/Ca2+ chelation, which presumably facilitated H2O2 diffusion at the Fe3O4/foulant interface. The iron-oxide pre-coated membrane maintained stable initial normalized fluxes (83.33–90.15%) through the oxalic acid/H2O2 cleaning over five cycles, with no need of refreshing the iron-oxide pre-coat. Additionally, the leaching of iron from the iron-oxide pre-coat by oxalic acid was suppressed by the oxalic acid/H2O2 combination, owing to a reactive shielding by competitive sorption of H2O2 onto the Fe3O4 surface. Overall, the synergistic relaxation/oxidation method, demonstrated in this study, provides new insights into improving reactivity of Fenton-based processes on hybrid catalytic ceramic membranes for water treatment or fouling control.@en

Catalytic ceramic nanofiltration (NF) is a promising technology for direct wastewater reclamation, given its high separation selectivity and reactive surfaces for oxidative removal of fouling. A better understanding of the relation between fouling types and oxidative cleaning efficacy under high organic loading conditions is of practical importance for realizing stable filtration/cleaning performance in long-term water reclamation operations. In this work, Fenton cleaning, using a hydrogen peroxide solution and an iron oxychloride catalyst pre-coat layer on top of commercially available ceramic NF membranes, was studied with respect to high organic loaded fouling, simulated by a concentrated sodium alginate solution in the presence of calcium. Adsorption (in the absence of a permeate flow) and constant-pressure filtration (with a permeate flow) experiments were performed to distinguish between permeance decreases as a result of either adsorptive or cake layer fouling. The results show that the flux evolution could be divided into an initial sharp flux decline, due to rapid adsorption of the foulants, and a subsequent gradual flux decrease, resulting from progressive cake build-up on the membrane. The two-stage flux decrease was enhanced during the constant-pressure filtration experiments, because they start at a high flux with a high fouling rate, while the flux gradually decreases as fouling proceeds. During multiple adsorption/cake filtration/Fenton cleaning cycles, the cake layer fouling was sufficiently removed by Fenton cleaning in contrast to the adsorptive fouling. However, the total permeate production during ceramic NF was not influenced by the remaining adsorptive fouling (after cleaning), since the adsorptive fouling always only occurs at the beginning of each cycle. The findings provide new insights into the criteria for evaluating and optimizing the efficacy of oxidative (Fenton) cleaning during ceramic NF in water treatment.@en

Ceramic nanofiltration (NF) is a promising alternative for direct surface water treatment, but is hampered for full-scale applications by fouling and a lack of eco-friendly cleaning regimes. In this work, an innovative reactive pre-coat layer, consisting of an iron oxychloride catalyst, was constructed on top of commercial ceramic NF membranes, for segregating a large-sized colloid fraction in canal water and Fenton cleaning with a hydrogen peroxide (H2O2) solution. The large-sized colloids (3−30 μm) were identified as dominant substances fouling the TiO2 separation layer of the pristine membranes, leading to a fast increase in their filtration resistance, in contrast to the small-sized colloids (<0.04 μm) and natural organic matter (NOM). As a consequence, the catalyst pre-coat layer with a pore size of 0.1–0.5 μm was able to segregate the large-sized colloids from the TiO2 separation layer during direct filtration of the raw water. Moreover, filtration under an acceptable flux of around 23 L m−2 h−1 did not cause pore clogging in the catalyst pre-coat. In addition, Fenton oxidation initiated by the catalytic pre-coat efficiently restored the filtration resistance, whereas sole H2O2 flush of the pristine membrane was not effective. In the meantime, the TiO2 separation layer of the membrane exerted a high NOM rejection of approximately 90%, measured as dissolved organic carbon, while the catalyst pre-coat on the membrane remained active in Fenton cleaning, over five one-day cycles. The findings of this work may provide guidance on the structural and functional design of a catalytic pre-coat layer for a dual purpose of foulant segregation and oxidative removal, particularly in response to key fouling-causing substances, during membrane-based treatment of real water matrices.@en

The influence of effluent organic matter (EfOM) on phosphate polishing removal by adsorption plays an important role in determining the application potential of adsorbents. Molecular understanding of EfOM regarding its impact on adsorption is insufficient due to a lack of appropriate EfOM fractionation/characterization protocols, corresponding to a particular structure-function property of adsorbents. In this work, a combinative method of coupling DEAE/XAD fractionation with molecular characterization was proposed, targeting the versatile structure-function characters of nanocomposite, for investigating EfOM and its impact on phosphate removal by nanocomposite during long-term adsorption/regeneration runs. Zirconium-based polystyrene anion exchanger (HZO-201) was selected as a representative nanocomposite, featuring with porous networking matrix, positively charged surface and multiple adsorptive sites. The EfOM samples from three biologically treated sewage effluent sources were separated into fractions of negatively charged organic acid (OA) and hydrophobic-, transphilic-, hydrophilic-neutral/base (HPO-n/b, TPI-n/b and HPI-n/b). The combinative method effectively differentiated the charge, aromaticity, molecular weight and functionalities between the fractions, which corresponded to the multiple structural/surface characteristics of HZO-201 and favored the evaluation on the impact of the EfOM fractions. The extent of interference of the EfOM fractions on phosphate removal followed an order of OA > HPO-n/b > TPI-n/b > HPI-n/b. The OA fraction, characterized by negatively charged, aromatic functionalities and broad molecular weight distribution (1–5 kDa and 14 kDa), was recognized as the key interfering fraction, presumably due to its multiple adsorption pathways (i.e., ion exchange, π–π interactions and pore filling). Particularly, the low-molecular-weight OA moieties (1–4 kDa) progressively accumulated onto the nanocomposite via irreversible adsorption, causing a continuous phosphate-capacity loss by 32.70% over multiple cycles. We believe the combinative fractionation/characterization method may widely apply to complex water systems for identifying key influential organic matters in polishing treatment of various pollutants by adsorption.@en

The influence of effluent organic matter (EfOM) on phosphate polishing removal by adsorption plays an important role in determining the application potential of adsorbents. Molecular understanding of EfOM regarding its impact on adsorption is insufficient due to a lack of appropriate EfOM fractionation/characterization protocols, corresponding to a particular structure-function property of adsorbents. In this work, a combinative method of coupling DEAE/XAD fractionation with molecular characterization was proposed, targeting the versatile structure-function characters of nanocomposite, for investigating EfOM and its impact on phosphate removal by nanocomposite during long-term adsorption/regeneration runs. Zirconium-based polystyrene anion exchanger (HZO-201) was selected as a representative nanocomposite, featuring with porous networking matrix, positively charged surface and multiple adsorptive sites. The EfOM samples from three biologically treated sewage effluent sources were separated into fractions of negatively charged organic acid (OA) and hydrophobic-, transphilic-, hydrophilic-neutral/base (HPO-n/b, TPI-n/b and HPI-n/b). The combinative method effectively differentiated the charge, aromaticity, molecular weight and functionalities between the fractions, which corresponded to the multiple structural/surface characteristics of HZO-201 and favored the evaluation on the impact of the EfOM fractions. The extent of interference of the EfOM fractions on phosphate removal followed an order of OA > HPO-n/b > TPI-n/b > HPI-n/b. The OA fraction, characterized by negatively charged, aromatic functionalities and broad molecular weight distribution (1–5 kDa and 14 kDa), was recognized as the key interfering fraction, presumably due to its multiple adsorption pathways (i.e., ion exchange, π–π interactions and pore filling). Particularly, the low-molecular-weight OA moieties (1–4 kDa) progressively accumulated onto the nanocomposite via irreversible adsorption, causing a continuous phosphate-capacity loss by 32.70% over multiple cycles. We believe the combinative fractionation/characterization method may widely apply to complex water systems for identifying key influential organic matters in polishing treatment of various pollutants by adsorption.@en

The influence of effluent organic matter (EfOM) on phosphate polishing removal by adsorption plays an important role in determining the application potential of adsorbents. Molecular understanding of EfOM regarding its impact on adsorption is insufficient due to a lack of appropriate EfOM fractionation/characterization protocols, corresponding to a particular structure-function property of adsorbents. In this work, a combinative method of coupling DEAE/XAD fractionation with molecular characterization was proposed, targeting the versatile structure-function characters of nanocomposite, for investigating EfOM and its impact on phosphate removal by nanocomposite during long-term adsorption/regeneration runs. Zirconium-based polystyrene anion exchanger (HZO-201) was selected as a representative nanocomposite, featuring with porous networking matrix, positively charged surface and multiple adsorptive sites. The EfOM samples from three biologically treated sewage effluent sources were separated into fractions of negatively charged organic acid (OA) and hydrophobic-, transphilic-, hydrophilic-neutral/base (HPO-n/b, TPI-n/b and HPI-n/b). The combinative method effectively differentiated the charge, aromaticity, molecular weight and functionalities between the fractions, which corresponded to the multiple structural/surface characteristics of HZO-201 and favored the evaluation on the impact of the EfOM fractions. The extent of interference of the EfOM fractions on phosphate removal followed an order of OA > HPO-n/b > TPI-n/b > HPI-n/b. The OA fraction, characterized by negatively charged, aromatic functionalities and broad molecular weight distribution (1–5 kDa and 14 kDa), was recognized as the key interfering fraction, presumably due to its multiple adsorption pathways (i.e., ion exchange, π–π interactions and pore filling). Particularly, the low-molecular-weight OA moieties (1–4 kDa) progressively accumulated onto the nanocomposite via irreversible adsorption, causing a continuous phosphate-capacity loss by 32.70% over multiple cycles. We believe the combinative fractionation/characterization method may widely apply to complex water systems for identifying key influential organic matters in polishing treatment of various pollutants by adsorption.@en

Over the past decades, direct nanofiltration (NF) without pre-treatment has been widely recognized as an alternative for conventional membrane technologies in both drinking water and wastewater treatment, owing to its advantages in energy saving, low chemical usage and high permeate purity. As an alternative, ceramic NF has received growing attention in recent years, given its good robustness and stable separation capabilities as compared to polymeric NF membranes. Organic fouling of ceramic NF membranes remains the key problem affecting the performance of the membranes in water treatment. However, conventional forward flush with pure water is not effective for removing the organic fouling due to its sticky nature. Backwash with pure water has to be applied at high pressures, thereby having a risk of damaging the structure of the membranes. Therefore, chemical forward flush with strong acids, bases or chlorine is frequently required as a substitute for backwash and conventional forward flush, leading to more consumption of the chemicals. To apply innovative ceramic NF in direct surface water treatment, an eco-friendly cleaning strategy of using Fenton-based oxidation was studied in relation to the fouling characteristic of the membrane. A literature study was done to review the present knowledge on using oxidation methods for fouling mitigation of ceramic membranes. It was found that existing studies predominantly were focused on direct oxidation of organic substances in feed water of ceramic membrane filtration. This kind of oxidation strategies could mainly reduce cake layer fouling of the membranes, while in many cases aggravating pore clogging due to an oxidation-induced conversion of large-sized organic molecules into smaller ones. Additionally, there is a risk of secondary pollution by using oxidation in the feed water, since the oxidants and potentially produced oxidation by-products can penetrate the membranes into permeate. However, little knowledge is available on using oxidation for cleaning fouling layers, in particular, on a ceramic NF membrane, in terms of the impact of its fouling characteristics on the efficacy of oxidative cleaning. It was thus recommended that, investigating the efficacy and mechanisms of an oxidative cleaning method for ceramic NF membranes, should be based on an in-depth understanding of fouling of the membranes... @en