The mechanism of silica and transparent exopolymer particles (TEP) on reverse osmosis membranes fouling

Effects of calcium ions and polysaccharides type on transparent exopolymer particle formation and the related fouling mechanisms

Organics and divalent cations are the primary barriers constraining the performance of membrane technology, while the interactions between them and the detailed mechanisms of their impacts are still lacking in-depth analysis. In this study, sodium alginate and xanthan gum were selected as polysaccharides models, and the formation of transparent extracellular polymer particles (TEP) was assessed to examine the effect of Ca2+ and polysaccharides type on membrane fouling from both qualitative and quantitative perspectives. The results revealed that higher Ca2+ concentrations led to a greater abundance of TEP, and the transformation of TEP microstructure is a key factor for the membrane fouling change indicated by specific filtration resistance (SFR). TEP formed by sodium alginate underwent a transformation from amorphous-TEP (a-TEP) form to particle-TEP (p-TEP), corresponding to a unimodal pattern of SFR variation. With increasing Ca2+ concentration, the molecular interactions of xanthan gum became stronger, resulting in larger fibrous a-TEP and a continuous SFR increase. According to the extended Derjaguin-Landau-Verwey-Overbeek (XDLVO) theory, TEP formed by xanthan gum exhibited higher adhesion energy, thus causing more severe membrane fouling. The SFR variation of the TEP system can be satisfactorily explained by the conception of chemical potential change in the filtration process depicted in Flory-Huggins theory. This study is the first work to introduce models regarding chemical potential and TEP microstructure, linking the system chemical potential and TEP microstructure with membrane fouling indicated by SFR. As all, this study provided a new perspective for analyzing the polysaccharide fouling behavior via TEP determination and further enhanced the understanding through thermodynamic analysis.

Calcium silicate hydrate complex konjac glucomannan-based hydrogel selectively adsorbed phosphate in low alkalinity solution

The selective recovery of phosphate from wastewater can manage nutrients and realize the recycling of phosphorus resources. In this study, a novel konjac glucomannan/pectin/calcium silicate composite hydrogel (KP-CSH) was developed for efficient recovery of phosphate in aqueous solution. The amount of alkali released after the reaction of KP-CSH in a neutral solution was small (the pH of the solution after the reaction was < 9). In a wide initial pH range (3-10), the adsorption capacity of KP-CSH in 50 mg-P/L phosphate solution reached 39∼45 mg-P/g. Besides, even if the pH of the solution after the reaction was less than 8, it could still well adsorb phosphate. The kinetic and isothermal adsorption experiments indicated that the adsorption process of phosphate by KP-CSH was chemical adsorption, and the maximum adsorption capacity was 61.2 mg-P/g. KP-CSH preferentially adsorbed phosphate even in the presence of high concentrations of competitive ions. In the actual biogas slurry, KP-CSH also exhibited the strongest selectivity/affinity for phosphate, and its distribution coefficient (Kd) was significantly higher than that of other co-existing anions and cations. The adsorption mechanism analysis indicated that Ca was the main adsorption site of KP-CSH, and that the adsorption process of target pollutants mainly involved ligand exchange and the intra-sphere complexation. Further plant seed germination and seedling growth experiments suggested that KP-CSH after phosphate recovery did not exert a negative effect on the growth of plant seedlings, and increased the chlorophyll content of seedling leaves. These results demonstrate that KP-CSH is a potential adsorbent for efficient phosphate recovery, which can be used as a slow-release phosphate fertilizer after recovering phosphate.

Stratified structure of membrane clogs observed in full scale cross-flow tubular ultrafiltration (UF) system: Fouling characterization and a proposed two-stage formation mechanism

Membrane fouling remains a significant challenge in wastewater treatment, hindering both efficiency and lifespan. This study reports a distinct phenomenon of stratified membrane clogging observed in a full-scale cross-flow tubular ultrafiltration (UF) system treating sludge anaerobic digestion (AD) reject water. The distinct stratified structure, comprising inner and outer layers within the cake layer, has not been previously described. This research involved characterizing the filtration performance, analyzing membrane clog composition, and proposing a two-stage formation mechanism for the stratified clogs. It was revealed that higher inorganic and lower organic content in the outer layer compared to the inner layer. Acid and alkali treatments demonstrated the effectiveness of combined cleaning strategies. A mathematical model was developed to determine the critical conditions for stratified clog formation, influenced by membrane flux and cross-flow velocity (CFV). It is proposed that outer layer forms through long-term selective deposition, while the inner layer results from short-term dewatering within limited tubular space. High CFV (>2.5 m/s) prevents inner layer formation. Critical conditions for stratification occur at a flux of 18 L/m2/h with a CFV of 0.1 m/s or 65 L/m2/h with a CFV of 0.35 m/s. This study contributes a novel understanding of stratified membrane clogging, proposing a two-stage formation mechanism and identifying critical conditions, which provides insights for effective fouling control strategies and maintenance of operational efficiency for membrane systems.

Enhanced desalination with polyamide thin-film membranes using ensemble ML chemometric methods and SHAP analysis

Addressing global freshwater scarcity requires innovative technological solutions, among which desalination through thin-film composite polyamide membranes stands out. The performance of these membranes plays a vital role in desalination, necessitating advanced predictive modeling for optimization. This study harnesses machine learning (ML) algorithms, including support vector machine (SVM), neural networks (NN), linear regression (LR), and multivariate linear regression (MLR), alongside their ensemble techniques to predict and enhance average water flux (AWF) and average salt rejection (ASR) essential metrics of desalination efficiency. To ensure model interpretability and feature importance analysis, SHapley Additive exPlanations (SHAP) were employed, providing both global and local insights into feature contributions. Initially, the individual models were validated, with NN demonstrating superior performance for both AWF and ASR, achieving the lowest mean absolute error (MAE = 0.001) and root mean squared error (RMSE = 0.0111) for AWF and an MAE = 0.0107 and RMSE = 0.0982 for ASR. The accuracy of predictions improved significantly with ensemble models, as evidenced by the near-perfect Nash-Sutcliffe efficiency (NSE) values. Specifically, the NN ensemble (NN-E) and Linear Regression ensemble (LR-E) reached an MAE and RMSE of 0.001 and 0.0111, respectively, for AWF. For ASR, NN-E reduced the MAE to 0.0013 and the RMSE to 0.0089, while LR-E maintained competitive performance with an MAE of 0.0133 and an RMSE of 0.0936. SHAP analysis revealed that features such as MDP and TMC were critical drivers of performance, with MDP showing the most significant positive impact on ASR. These findings demonstrate the dominance of ensemble methods over individual algorithms in predicting key desalination parameters. The enhanced precision in estimating AWF and ASR offered by these neuro-intelligent ensembles, combined with the interpretability provided by SHAP analysis, can lead to significant environmental and operational improvements in membrane performance, optimizing resource usage and minimizing ecological impacts. This study paves the way for integrating intelligent ML ensembles and SHAP-based interpretability into the practical field of membrane technology, marking a step forward toward sustainable and efficient desalination processes.

Effect of Mn2+ on RO membrane organic fouling: Insights into the complexation and interfacial interaction

RO process is commonly used to treat and reuse manganese-containing industrial wastewater. Nevertheless, even after undergoing multi-stage treatment, the secondary biochemical effluent still exhibits a high concentration of Mn2+ coupled with organics entering the RO system, leading to membrane fouling. In this work, we systematically analyze the RO membrane organic fouling processes and mechanisms, considering the coexistence of Mn2+ with humic acid (HA), sodium alginate (SA), bovine serum albumin (BSA) and their mixtures (HBS). The impact of Mn2+ on membrane fouling was HBS > SA > HA > BSA, controlling polysaccharide pollutant concentrations should be a priority for mitigating membrane fouling. In the presence of Mn2+ with HA, SA, or HBS, membrane fouling is primarily attributed to the complexation of organics and Mn2+ and the facilitation of interfacial interaction energy. RO membrane BSA fouling was not directly affected by Mn2+, the addition of Mn2+ induced a salting-out effect, leading to the deposition of BSA in a single molecular on the membrane. Simultaneously, adhesion energy hinders the deposition of BSA on the membrane, resulting in milder membrane fouling. This study provided the theoretical basis and suggestions for RO membrane organic fouling control in the presence of Mn2+.

Microplastics affect membrane biofouling and microbial communities during gravity-driven membrane filtration of primary wastewater

Recently, gravity-driven membrane (GDM) filtration has been adopted as an alternative solution for decentralized wastewater treatment due to easy installation and maintenance, reduced energy and operation cost, and low global warming impact. This study investigated the influence of microplastic size (0.5-0.8 μm and 40-48 μm) and amount (0.1 and 0.2 g/L) on the membrane performance and microbial community in GDM systems for primary municipal wastewater treatment. The results showed that dosing microplastics in the GDM systems led to 9-54% lower permeate flux than that in the control. This was attributed to more cake formation (up to 6.4-fold) with more deposition of extracellular polymeric substances (EPS, up to 1.5-fold) and divalent cations (up to 2.1-fold) in the presence of microplastics, especially with increasing microplastic amount or size. However, the dosed microplastics promoted formation of heterogeneous cake layers with more porous nature, possibly because microplastics created void space in the cake and also tended to bind with divalent cations to reduce EPS-divalent cations interactions. In the biofilm of the GDM systems, the presence of microplastics could lower the number of total species, but it greatly enhanced the abundance of certain dominant prokaryotes (Phenylobacterium haematophilum, Planctomycetota bacterium, and Flavobacteriales bacterium), eukaryotes (Stylonychia lemnae, Halteria grandinella, and Paramicrosporidium saccamoebae), and virus (phylum Nucleocytoviricota), as well as amino acid and lipid metabolic functions. Especially, the small-size microplastics at a higher dosed amount led to more variations of microbial community structure and microbial metabolic functions.

Scaling and cleaning of silica scales on reverse osmosis membrane: Effective removal and degradation mechanisms utilizing gallic acid

Silica scaling on membranes represents one of the most important issues in industrial water systems because of its complex composition and difficulty in removal. However, there is a lack of understanding of the mechanisms for cleaning silica scales from reverse osmosis (RO) membranes. To address this research gap, this study investigated the scaling and cleaning behavior of silica on RO membrane processes, with a specific focus on the silica scale cleaning mechanism using gallic acid (GA). The membrane flux continuously decreased with operation time, even at the lowest initial silicic acid concentration, owing to silica scale blockage. The GA solution exhibited a strong efficacy in cleaning silica-scaling RO membranes. The membrane flux returned to 89.7% of the initial value by removing 81.87% of the silica scale within the first 30 min of the study period. The cleaning mechanism of GA involved its adsorption onto the surface of silica scale particles to form a surface complex and subsequently transition into a water-soluble 1:3 complex within the solution. This complex interaction facilitated the gradual decomposition of the silica scales that adhered to the membrane surface. This study has valuable implications for the development of efficient and effective silica scale cleaning solutions, providing insights into the complex interplay between GA and silica scaling mechanisms.