Ceramic Membranes for Fouling and Organic Micropollutant Control

Abstract

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.