Membranes are widely recognized as promising technologies for addressing the growing demand for freshwater driven by rapid population growth and industrialization. Ceramic membranes, in particular, offer advantages such as high mechanical strength, chemical resistance, and long lifespan compared to polymeric membranes. However, permeability loss during filtration is one of the challenges. The specific contributions of concentration polarization (CP) and fouling to this decline remain insufficiently understood. Fouling is a major obstacle in ceramic membrane operation, and the removal of organic micropollutants (OMPs)—increasingly detected in different water bodies—poses an additional challenge due to the relatively large pore sizes of ceramic membranes, being mostly in the range of microfiltration and ultrafiltration (UF). To address these challenges, various modification strategies have been developed to improve membrane performance in water treatment. One promising approach is to modify ceramic membranes with catalysts, enabling them to perform both separation and catalytic functions. However, currently, developed catalytic ceramic membranes often suffer from severe flux decline after deposition, especially when coated with a dense or thick coating. In addition, catalytic membranes coupled with advanced oxidation processes (AOPs) have typically been achieved by large dosages of oxidating agenting during filtration. The excessive loading of catalysts and the overdosage of chemicals not only increase operational costs but also limit the practical application of catalytic ceramic membranes.
This thesis aims to enhance the performance of ceramic membranes for water treatment, focusing on the challenges of both fouling and OMPs’ removal. First a method was proposed to determine the main reason for flux decline, suggesting that both CP and fouling had impact on flux decline. Then catalytic ceramic ultrafiltration membranes, modified with CuFe2O4 and palladium, were employed to be coupled with H2O2 and peroxymonosulfate (PMS) based AOPs, respectively. The catalytic ceramic membranes not only exhibited a high flux after coating but were also effective in fouling removal and OMPs degradation.
High flux loss in membrane filtration can result from both CP and fouling, and a high CP level may further exacerbate fouling. However, the traditional CP model is unable to qualify their individual contributions. To better understand flux decline, a practical strategy was developed to distinguish the effects of CP and fouling by measuring pure water flux before and after the filtration of nano-sized colloids by ceramic nanofiltration (NF) membrane. The results indicated that colloidal CP could account for 43% to 95% of the total flux decline, with the remainder attributed to fouling. The CP values, calculated by a modified model, showed that the colloidal CP was in the range of 7-460, which is considerably higher than the CP (typically 1-2) caused by ions in spiral-wound reverse osmosis or NF. The highest CP level, i.e., 460, was observed for larger silica colloids, likely due to their slower diffusion. Although an increased crossflow was found to mitigate CP, high CP levels, i.e., values of around 250, were still observed.
To address membrane fouling, CuFe2O4-coated ceramic UF membranes were fabricated. The catalytic membranes with a minor flux loss after coating were then combined with Fenton-like backwash to enhance fouling removal. A low cleaning efficacy (1%–14%) was found in conventional hydraulic backwash. In contrast, due to the strong radicals induced by H2O2-based AOPs, the cleaning efficacy for removing alginate fouling from the catalytic membranes was improved to approximately 70% over multiple cycles. The backwash pressure or flux, rather than duration, was found to govern the AOP-enhanced cleaning performance. This is attributed to the increased residence time of H2O2 at low backwash pressure or flux. The presence of calcium (Ca) can form the rigid alginate-Ca clusters, not only negatively influencing the flux but also limiting the transport of radicals to the internal structure to break down the fouling. Besides, the fragments of alginate can reattach to the membrane surface by binding with excess Ca, thus reducing Fenton-like backwash efficacy. During seven-cycle filtration of concentrated alginate feedwater, the catalytic membranes restored 83%-94% flux after Fenton-like backwash. The leaching of catalysts gradually ceased over time, with negligible leaching in NaClO or NaOH.
Building upon this success, the catalytic ceramic membranes were further explored with a low loading of catalyst. Therefore, atomic layer deposition (ALD) was used to achieve a precise and low loading of Pd, ensuring minimal impact on membrane flux. The catalytic membranes modified with 30 ALD cycles were coupled with PMS for in-situ AOP degradation of OMPs during filtration. The coupled system achieved nearly 100% OMP removal at flux below 100 L/(m2·h) and maintained a high degradation efficacy (76% to 96%) even at a higher flux of 200 L/(m2·h). The OMPs’ degradation was enhanced by Pd deposited within the membrane pores, improving degradation kinetics by up to three orders of magnitude due to nanoconfinement effects, compared to the effect of Pd deposited on the membrane surface. The contribution of different reactive species (RS) to OMPs degradation was found to depend on the compound. Although ions and natural organic matter had minimal impact, harsh feedwater conditions, such as high salinity of brine water and pH at 2.5 or 11, reduced the degradation of certain OMPs, likely due to inhibited PMS activation.
Although the AOPs-enhanced removal of fouling and OMPs have widely been studied, little is known about the effect of fouling on OMPs’ degradation. Therefore, Pd-deposited ceramic ultrafiltration membranes with PMS were used to treat feedwater containing alginate and OMPs. The results showed that the Pd-coated membranes effectively mitigated fouling and achieved a high degradation efficacy of OMPs, even under severe cake fouling or pore blocking. Fouling was found to influence permeability and changed fouling mechanisms (cake fouling and pore blocking) depending on the PMS concentration, flux, foulant type, and Ca concentration, but its effect on OMP degradation was minimal. This is attributed to the synergy between membrane separation of foulants and nanoconfinement, which prevents the deactivation of catalytic sites and enriches RS and OMPs within the membrane pores. Although the governed RS for OMPs degradation almost remained consistent under fouling and non-fouling conditions, fouling altered their relative contributions to OMP degradation. However, different fouling is likely to alter the dominant oxidation pathway during OMP degradation. ... Read more

Monitoring the microparticle transfer process in wastewater treatment systems is crucial for improving treatment performance. Supervised deep learning methods show high performance to automatically detect particles, but they rely on vast amounts of labeled data for training. To overcome this issue, we proposed a semi-supervised learning (SSL) method based on the Simple framework for Contrastive Learning of visual Representations (SimCLR), to detect microparticles free from sludge and attached to sludge. First, we pre-trained a ResNet50 backbone by SimCLR, to extract features from much unlabeled data (1,000 images). Then, we constructed a Mask R-CNN architecture based on the pre-trained ResNet50, and fine-tuned it on a small quantity of labeled data (≈200 images with ≈600 annotated particles) in supervised learning fashion. We showcased its performance and practical applicability for microscopy images obtained from the water lab of TU Delft. The results demonstrate that the SSL methods obtain a significant improvement in mean average precision of up to 5% compared to the conventional supervised learning method, when a limited amount of labeled data is available (e.g., 91 labeled images). Furthermore, these methods improve the average precision for detecting attached particles by over 12%. With the detection results from the SSL methods, we measured the attachment efficiency of microparticles to sludge under varying mixed liquor suspended solids concentration and aeration intensity. The precise measurements demonstrate the effectiveness and practical applicability of the SSL method in facilitating long-term monitoring of particle transfer processes in biological wastewater treatment systems. ... Read more

Fouling remains a critical challenge for ceramic ultrafiltration membranes, limiting their long-term performance for water treatment. Fenton-like reactions have been widely used for fouling removal due to the formation of strong radicals. Integrating these reactions into backwash offers a promising strategy for fouling control. However, it has been unclear how Fenton-like backwash is influenced by operational parameters and fouling structures. Here we reveal the key factors influencing Fenton-like backwash by systematically studying its performance under varying conditions, such as backwash pressure (0.3–1 bar), duration (18–36 min), fouling structure (caused by 1–5 mM Ca) and the long-term operation, to provide an effective and practical cleaning. CuFe₂O₄ was grown on ceramic ultrafiltration membranes due to its stability and high catalytic efficiency in activating Fenton-like reactions. We found that Fenton-like backwash achieved the highest cleaning efficacy of approximately 70 % over three cycles at a low backwash pressure of 0.3 bar, while hydraulic backwash remained ineffective under all conditions. Backwash pressure, rather than duration, was identified as the dominant factor governing the Fenton-like cleaning, due to its impact on the residence time of Fenton-like agents (H₂O₂). The presence of a high Ca concentration (3 and 5 mM) altered the fouling behaviour, and reduced the cleaning efficacy of Fenton-like backwash. This reduction was attributed to the formation of rigid alginate clusters that were resistant to Fenton-like reactions. The contribution of •OH to the enhanced Fenton-like backwash was confirmed by the quenching experiments. Furthermore, the CuFe₂O₄-coated membranes exhibited stable flux recovery (83 %–94 %) in the long-term treatment of a concentrated alginate (800 mg/L), showed low or negligible leaching in hash environments (30 mM H₂O₂, 0.1 % NaClO or 10 mM NaOH), and maintained comparable performance after 96 h aging by 30 mM H₂O₂. This study clarifies the factors governing Fenton-like backwash, and demonstrates that a robust and effective strategy for fouling removal can be achieved by coupling this cleaning method with catalytic ceramic membranes. ... Read more

A large decrease in permeability is often observed during the filtration of nano-sized colloids, while fouling is widely regarded as the main explanation for this phenomenon. The osmotic pressure or concentration polarization (CP) of colloids can also contribute to the flux decline. However, the contribution of CP to flux loss cannot be determined by the traditional CP model. In this study, the effect of fouling and CP/osmotic pressure on flux was distinguished. The CP values of polyethylene glycol (PEG) and silica-colloids were determined by the osmotic pressures near the membrane surface and in the feed. The CP induced by colloids accounted for 43–95% of the flux loss in our experiments. Silica exhibited higher CP values (127–460), compared to 7–71 for PEG. This was attributed to the slower back diffusion caused by the larger colloids, as evidenced by the diffusion coefficients of 4.30 × 10⁻¹¹ m²/s for silica (10 nm) and 1.45 × 10⁻¹⁰ m²/s for PEG (2.9 nm). Although the CP was mitigated by increasing the cross-flow velocity, CP values of 31 and 250 were observed for PEG and silica at high Reynolds number of 7317, respectively. The experimentally obtained CP values were also compared with those calculated by the film diffusion model. ... Read more

This work focuses on the photocatalytic removal of recalcitrant organic pollutants in water treatment. Based on facile precipitation reaction, we fabricated a photocatalyst (PbCrO₄) in single crystals that present evident response to visible light and employed the catalyst in the photocatalytic decomposition of microcystin-LR (MC-LR). In the degradation test using the nanorods with prepared PbCrO₄ photocatalyst, a 100% removal efficiency (27 min reaction) and a kinetics constant of 0.1356 min⁻¹ were achieved. Such a high performance of PbCrO₄ in photocatalytic conversion of MC-LR was ascribed to its high carrier separation efficiency, positive valence band (VB) position, and good delocalization of VB and conduction band (CB). The test of electron spin-resonance resonance (ESR) demonstrated that excessive free [rad]OH radicals were produced during the PbCrO₄ photocatalysis of MC-LR. The density functional theory (DFT) and LC/MS/MS technology were employed to ascertain the intermediates during the MC-LR photocatalytic degradation. The major intermediates were resulted from the attack of hydroxyl radicals to the ADDA side chains of MC-LR structure. This study provides a proof-of-concept strategy to develop effective photocatalysts to efficiently produce [rad]OH radicals for the visible-light induced photocatalytic degradation of MC-LR in water. ... Read more